University of Groningen
Levulinic acid from lignocellulosic biomass
Girisuta, Buana
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Levulinic Acid from
Lignocellulosic Biomass
Buana Girisuta
The author thanks the University of Groningen for the financial support through
an Ubbo Emmius Scholarship.
RIJKSUNIVERSITEIT GRONINGEN
Levulinic Acid from
Lignocellulosic Biomass
Proefschrift
ter verkrijging van het doctoraat in de
Wiskunde en Natuurwetenschappen
aan de Rijksuniversiteit Groningen
op gezag van de
Rector Magnificus, dr. F. Zwarts,
in het openbaar te verdedigen op
maandag 5 november 2007
om 14.45 uur
door
Buana Girisuta
geboren op 15 augustus 1975
te Bandung, Indonesië
Promotores
: Prof. dr. ir. H. J. Heeres
Prof. dr. ir. L. P. B. M. Janssen
Beoordelingscommissie
: Prof. dr. A. A. Broekhuis
Prof. dr. J. G. de Vries
Prof. ir. G. J. Harmsen
ISBN 978-90-367-3228-4
ISBN 978-90-367-3229-1 (electronic version)
To Rina and my beloved parents
Table of Contents
1 Introduction
1.1
1.2
1.3
1.4
Biomass applications for energy generation and chemicals production
Biomass: definitions, composition and sources
Biomass valorisation using the biorefinery concept
Conversion of biomass to levulinic acid
1.4.1 Preparation methods of LA
1.4.2 Continuous production of LA
1.4.3 Mechanistic studies
1.5 Potential applications of LA and its derivatives
1.5.1 Reactions involving the carboxylic group
1.5.2 Reactions involving the carbonyl group
1.5.3 Reactions involving the methyl group
1.5.4 Oxidation reactions
1.5.5 Reduction reactions
1.6 Thesis Outline
1.7 References
2
3
6
7
8
10
13
15
15
15
16
17
18
18
19
2 Exploratory Catalyst-Screening Studies on the Conversion of 5Hydroxymethylfurfural and Glucose to Levulinic Acid
2.1 Introduction
2.2 Materials and methods
2.2.1 Chemicals
2.2.2 Solid acid catalysts
2.2.3 Experimental procedure for the acid-catalysed decompositions
of glucose and HMF
2.2.4 Adsorption phenomena of LA and FA on the ZSM-5 zeolites
2.2.5 Analytical methods
2.2.6 Definitions
2.3 Results and discussions
2.3.1 Acid-catalysed hydration reaction of HMF to LA
2.3.1.1 Homogeneous Brønsted acid catalysts
2.3.1.2 Solid acid catalysts
2.3.2 Acid-catalysed dehydration reaction of glucose to LA
2.3.2.1 Homogeneous Brønsted acid catalysts
2.3.2.2 Solid acid catalysts
2.4 Conclusions
2.5 Nomenclature
2.6 References
28
29
29
29
30
31
31
31
32
32
32
34
37
37
38
38
39
40
3 A Kinetic Study on the Decomposition of 5-Hydroxymethylfurfural
into Levulinic Acid
3.1 Introduction
3.2 Experimental
3.2.1 Experimental procedure
3.2.2 Analytical methods
3.2.3 Heat transfer experiments
3.2.4 Determination of the kinetic parameters
3.3 Results and discussions
3.3.1 Acid screening
3.3.2 Reaction products
3.3.3 Effects of temperature, acid concentration and initial HMF
concentration on HMF conversions and LA yields
3.3.4 Development of a kinetic model
3.3.4.1 Modelling results
3.3.4.2 Alternative models
3.4 Application of the kinetic model
3.4.1 Comparisons with literature models
3.4.2 Batch simulation and optimisation
3.5 Conclusions
3.6 Nomenclature
3.7 References
44
45
45
46
47
48
49
49
49
51
52
54
56
57
57
58
59
60
61
4 A Kinetic Study on the Conversion of Glucose to Levulinic Acid
4.1 Introduction
4.2 Experimental
4.2.1 Experimental procedure
4.2.2 Analytical methods
4.2.3 Heat transfer experiments
4.2.4 Determination of the kinetic parameters
4.3 Results and discussions
4.3.1 Effects of process variables on the decomposition reaction of
glucose
4.3.2 Development of a kinetic model for glucose decomposition to
levulinic acid
4.3.3 Modelling results
4.4 Application of the kinetic model
4.4.1 Batch simulation and optimisation
4.4.2 Optimisation of continuous reactor systems
4.5 Conclusions
4.6 Nomenclature
4.7 References
viii
64
66
66
67
67
69
69
69
71
76
78
78
79
81
81
83
5 A Kinetic Study on the Acid Catalysed Hydrolysis of Cellulose to
Levulinic Acid
5.1 Introduction
5.2 Materials and methods
5.2.1 Chemicals
5.2.2 Experimental procedures
5.2.2.1 Kinetic experiments
5.2.2.2 Heat-transfer experiments
5.2.3 Method of analyses
5.2.4 Determination of the kinetic parameters
5.3 Results and discussions
5.3.1 Reaction products
5.3.2 Effects of process variables on the yield of LA
5.3.3 Kinetic modelling
5.3.3.1 Development of a kinetic model
5.3.3.2 Modelling results
5.3.3.3 Evaluation of mass-transfer effects
5.3.3.4 Model implications
5.3.3.5 Comparisons with previous kinetic studies
5.4 Applications of the kinetic model for reactor optimisation
5.4.1 Optimisation of LA production in a batch reactor
5.4.2 Optimisation of LA production in continuous reactors
5.5 Conclusions
5.6 Nomenclature
5.7 References
86
88
88
88
88
89
90
90
91
91
93
96
96
99
101
103
103
104
104
106
108
108
111
6 Experimental and Kinetic Modelling Studies on the Acid-Catalysed
Hydrolysis of the Water Hyacinth Plant to Levulinic Acid
6.1 Introduction
6.2 Materials and methods
6.2.1 Water hyacinth
6.2.2 Chemicals
6.2.3 Experimental procedures
6.2.3.1 Water hyacinth characterisation
6.2.3.2 Kinetic experiments
6.2.4 Analytical equipment
6.2.5 Modelling techniques and software
6.2.6 Definitions of LA yield
6.3 Results and discussions
6.3.1 Determination of the water hyacinth composition
6.3.2 Exploratory experiments
6.3.2.1 Results for 1.0 M sulphuric acid
6.3.2.2 Results for 0.1 M sulphuric acid
6.3.3 Optimisation experiments
114
115
115
116
116
116
116
117
117
118
118
118
120
120
123
125
ix
6.4 Development of a kinetic model for the acid-catalysed hydrolysis
of water hyacinth to LA
6.5 Conclusions
6.6 Nomenclature
6.7 References
128
132
134
135
Summary
139
Samenvatting (Dutch Summary)
143
Acknowledgements
147
List of Publications
149
x
Chapter 1
Introduction
Abstract
In this chapter, a general overview of the use of biomass for chemical production
and energy generation will be provided. Valorisation of biomass using the
biorefinery concept will be introduced. It will be shown that levulinic acid (LA) is
an interesting biomass-derived chemical. The conversion technology developed
for the conversion of biomass to LA is reviewed. Subsequently, an extensive
overview of the reactions of LA to interesting derivatives is provided, categorised
according to functional group transformations. Finally, an outline of this thesis is
provided.
Chapter 1
1.1 Biomass applications for energy generation and chemicals
production
World Consumption of Fossil Resources
15
(Quadrillion (10 ) Btu)
The world is highly dependent on the utilization of fossil resources (e.g.,
petroleum, natural gas and coal) to fulfil its energy needs. Furthermore, a wide
range of modern products like polymers, resins, textiles, lubricants, fertilizers, etc.
are also derived from fossil resources. The consumption rate of fossil resources in
the world has increased 50% in the period 1980–2004, and it is projected that the
world needs 600×1015 Btu of fossil resources in 2030 as shown in Figure 1.1.
Petroleum
Coal
Natural Gas
Projection
(reference case)
History
600
500
400
300
200
100
0
1980
1985
1990
1995
2000
2005
2010
2015
2020
2025
2030
Year
Figure 1.1 World consumption of fossil resources 1980–2030 (taken from [1]).
However, fossil resources are not-renewable and its availability is irrevocably
decreasing. Due to high demands, a dramatic increase in the oil price was
observed in the last decade (Figure 1.2). CO2 emissions from burning of fossil
resources have resulted in a major increase in the CO2 concentration in the earth
atmosphere. There is more and more evidence available that this will have a major
impact on our global climate. These issues have stimulated the development of
alternative renewable resources to substitute fossils. Biomass is a prime candidate
because it is the only renewable resource of fixed carbon , which is essential for the
production of conventional hydrocarbon liquid transportation fuel [2-7] and
petrochemicals products [8,9].
To stimulate the transition from a fossil-based economy to renewable
alternatives, active government participation is required. For example, the U.S.
Department of Energy has set goals to replace 20% of the liquid petroleum
transportation fuel with biofuels and to replace 25% of industrial organic
chemicals with biomass-derived chemicals by 2025 [10]. Meanwhile, the European
Union has targeted 2% of all petrol and diesel transport fuels to be biomassderived by December 2005 and 5.75% by December 2010 [11].
2
Introduction
90
80
Oil Price
(US$ / barrel)
70
60
50
40
30
20
10
0
Dec-97 Dec-98 Dec-99 Dec-00 Dec-01 Dec-02 Nov-03 Nov-04 Oct-05 Oct-06
Figure 1.2 NYMEX Light Sweet Crude Oil price in the period 1997–2006 (taken
from [12]).
1.2 Biomass: definitions, composition and sources
The term biomass is defined as any organic matter that is available on a
renewable basis, including dedicated energy crops and trees, agricultural food and
feed crop residues, aquatic plants, wood and wood residues, animal wastes and
other waste materials [13]. The annual production of biomass is about 1.7–2.0×1011
tons [14]; however, only 6×109 tons are currently used for food and non-food
applications. Food applications are by far the most important (96.5−97%). The
remainder is used in non-food applications, for example as a feedstock for the
chemical industry.
Hemicellulose
23-32%
Lignin
15-25%
Cellulose
38-50%
Figure 1.3 Distribution of important organic constituents in biomass (taken from
[15]).
The chemical composition of biomass depends strongly on its source. Generally
biomass consists of 38–50% of cellulose, 23–32% hemicellulose and 15–25% lignin
(see Figure 1.3). Cellulose is a non-branched water-insoluble polysaccharide
consisting of several hundred up to tens of thousands of glucose units. Cellulose is
3
Chapter 1
the most abundant biopolymer synthesised by nature, its amount is estimated at
approximately 2×109 tons year-1 [16]. Hemicellulose is a polymeric material,
although lower in molecular weight than cellulose, consisting of C6-sugars
(glucose, mannose and galactose) and C5-sugars (mainly arabinose and xylose).
The third component (lignin) is a highly cross-linked polymer made from
substituted phenylpropene units (see Figure 1.4). It acts as glue, holding together
the cellulose and hemicellulose fibres.
CH2OH
CH2OH
CH2OH
OMe
MeO
OMe
OH
OH
OH
p-coumaryl
alcohol
p-coniferyl
alcohol
p-sinapyl
alcohol
Figure 1.4 Molecular structures of the building blocks of lignin [17].
A wide variety of biomass sources is available for further conversion and
utilisation. Selection of the biomass feedstock is of paramount importance from
both techno- and socio-economical points of view. For ethical reasons, the biomass
feedstock should not compete with the food chain. Waste streams with a low or
even negative value, such as agricultural waste are preferred. Furthermore, it is
also advantageous to select sources that are not prone to diseases, only require a
limited amount of fertiliser, have a high growth rate per ha per year and are
preferably available throughout the year. Based on these criteria, the water
hyacinth could be an excellent biomass feedstock for further conversions and
utilisation and has been selected as the biomass feedstock of choice for this thesis.
Water hyacinth (Eichhornia crassipes) is a free-floating aquatic plant originating
from the Amazon River basin in South America. It was brought from its native
habitat into the United States at the World's Industrial and Cotton Centennial
Exposition of 1884−1885 in New Orleans, Louisiana [18,19]. Owing to its beautiful
lavender flower, the water hyacinth was subsequently introduced to various
countries as an ornamental plant and has spread to more than 50 countries on five
continents [20-22]. The plant can be cultivated in various places (e.g., shallow
temporary ponds, wetlands, marshes, sluggish flowing waters, large lakes,
reservoirs and rivers) because it can tolerate extremes in water level fluctuations,
seasonal variations in flow velocity and extremes with respect to nutrient
availability, pH, temperature and toxic substances [20]. It can even tolerate salinity
levels up to 0.24% as was shown in Indonesia [23].
The water hyacinth leaves (about 10−20 cm in diameter) are thick, waxy,
rounded and glossy, and rise well above the water surface on stalks. The stalk is
about 50 cm long, and carries about 8−15 flowers at the top (see Figure 1.5). The
4
Introduction
flowers have six petals, purplish blue or lavender to pinkish, the uppermost petal
with a yellow, blue-bordered central splotch. About 50% of the water hyacinth’s
biomass consists of the roots, which are fibrous (10−300 cm in length), feathery
and purplish black. A typical chemical composition of water hyacinth is given in
Table 1.1.
Figure 1.5 Water hyacinth (Eichhornia crassipes) (taken from [24]).
Table 1.1 Typical chemical composition of water hyacinth. a
Component
wt %
Moisture
85−95
Organic matter (dry basis)
Cellulose
18–31
Hemicelluloses
18–43
Lignin
7–26
Ash (dry basis)
15–26
Elemental composition (dry basis)
a
C
41.1– 43.7
H
5.3–6.4
O
27.5–28.8
N
1.5–4.3
Values are collected from [25-29,2].
The water hyacinth reproduces itself vegetatively through stolons and sexually
by seeds [30]. Vegetative reproduction is more important than sexual reproduction
and allows the plant to quickly cover large areas of water in relatively short
periods of time. It doubles its population every 6 to 18 days, depending on the
location and time of year. Extremely high growth rates of up to 100–140 ton dry
material ha-1 year-1 have been reported [31,32], depending on the location and time
5
Chapter 1
of the year. This enormous growth rate is among the highest reported for a wide
range of biomass sources [31].
The coverage of waterways by water hyacinth has created various problems.
Examples are destructions of ecosystems (Victoria Lake in Africa), irrigation
problems and an increase in mosquito populations. These negative effects have
pinpointed the water hyacinth as one of the world's worst weeds [33] and
stimulated the search for control measures. Chemical control of the water hyacinth
using herbicides is very effective but the long-term effects of these chemical
substances on the environment are unknown. Furthermore, the sprayed plants are
left to rot in the water, leading to pollution and eutrophication. So far, control by
manual and mechanical harvesting has been practised widely in countries
suffering from the water hyacinth. However, as removal of the weed by both
means is extremely costly, the interest in valorisation of harvested water hyacinth
plants has grown rapidly. Commercial utilization of the water hyacinth as a whole
or partly is considered as a suitable method to reduce the cost of the removal.
1.3 Biomass valorisation using the biorefinery concept
The utilisation of biomass for the production of non-food products has fostered
research and development activities in various countries. To steer the research and
development activities and to enhance market introduction, a novel concept was
introduced: biorefining [34-36,13]. According to the American National Renewable
Energy Laboratory (NREL) a biorefinery is a facility that integrates biomass
conversion processes and equipment to produce fuels, power and chemicals from
biomass [37].
Biorefining aims for a complete valorisation of the biomass source by
performing the overall processes with a minimum loss of energy and mass and to
maximize the overall value of the production chain. It consists of an efficient
fractionation of the biomass into various value-added products and energy using
physical separation processes in combination with (bio-) chemical and thermochemical conversion steps. In that sense, the biorefinery concept has similar
objectives as today’s petroleum refineries.
Large-scale biorefinery systems are already operational; however, these
existing systems deliver predominantly food products such as soy oil and soy
protein, wheat starch and gluten, potato starch and protein. With the biorefinery
concept, these existing biomass based production processes may be optimised and
novel processes may be developed that are more energy and cost effectively so
that they can also be applied for non-food uses.
Typically, three stages may be defined in a biorefinery:
1. Separation of the biomass into its components (cellulose, hemicellulose,
lignin, proteins, amino acids, pure plant oil (PPO), minerals, fine chemicals
and pharmaceutical compounds) in a primary fractionation/depolymerization unit. Typical technologies applied in this stage are
6
Introduction
traditional separation processes like filtration, solvent extraction and
distillation. However, novel concepts like supercritical CO2 extractions and
catalytic de-polymerization may also be explored.
2. Conversion of the intermediate fractions to valuable end products (e.g., biofuels) and chemical intermediates is performed in a secondary refinery
process. Examples of chemical intermediates are conventional
intermediates, such as alcohols or acids, and platform chemicals (vide infra)
like levulinic acid, lactic acid or phenolic compounds. The secondary
conversion processes may be distinguished into thermo-chemical processes
(e.g., gasification, liquefaction) and biochemical processes (e.g.,
fermentation).
3. Further (catalytic) processing of the chemical intermediates to high added
value end-products.
The residues of all process steps are applied for the production of power and
heat. A schematic representation of a biorefinery process is given in Figure 1.6.
Figure 1.6 A simplified scheme of a biorefinery concept.
1.4 Conversion of biomass to levulinic acid
Researchers from NREL and PNNL (Pacific Northwest National Laboratory)
have recently conducted an extensive study to identify valuable sugar-based
building blocks for lignocellulosic biomass [38]. Of 300 initially selected
7
Chapter 1
candidates, a long list of thirty-interesting-chemicals was obtained through an
iterative process. The list was further reduced to twelve by evaluating the
potential markets of the building blocks and their derivatives and the complexity
of the synthetic pathways. One of these promising top-twelve building blocks is
levulinic acid that is accessible from lignocellulosic biomass using an acid catalyst.
The conversion of a typical lignocellulosic biomass to LA is shown in Figure 1.7.
Cellulose
Levulinic acid
+ Formic acid
Glucose
Mannose
Galactose
Hexose
Biomass
5-Hydroxymethylfurfural
Glucose
Hemicellulose
Xylose
Pentose
Furfural
Arabinose
Lignin
Acid-soluble products
Figure 1.7 Simplified reaction scheme for the conversion of lignocellulosic biomass
to LA.
Levulinic acid (1, LA), also known as 4-oxopentanoic acid or γ-ketovaleric acid,
is a C5-chemical with a ketone and a carboxylic group. The presence of both
groups results in interesting reactivity patterns [16]. LA is readily soluble in water,
ethanol, diethyl ether, acetone and many other organic solvents. The dissociation
constant (pKa) of LA is 4.59 [39], which is comparable with low molecular weight
aliphatic carboxylic acids. Some selected physical properties of LA are given in
Table 1.2.
O
OH
H3 C
O
1
Table 1.2 Selected physical properties of LA [39].
8
Physical properties
Values
pKa
4.59
Melting point
37 °C
Boiling point
246 °C
Density
1.14
Refractive index (20 °C)
1.1447
Surface tension (25 °C)
39.7 dyne cm-1
Heat of vaporisation (150 °C)
0.58 kJ mol-1
Heat of fusion
79.8 kJ mol-1
Introduction
1.4.1
Preparation methods of LA
The first study on the preparation of LA was reported in the 1840s by the
Dutch professor G. J. Mulder [40], who prepared LA by heating sucrose with
mineral acids at high temperature (equation 1.1) [Note: Unfortunately, details on
the reaction conditions and the LA yield are unknown].
O
HCl
C12H22O11
2
O
OH
H 3C
+
(1.1)
2
H
OH
O
The controlled degradation of hexose (C6-sugars) by acids is still the most
widely used approach to prepare LA from lignocellulosic biomass. The theoretical
yield of LA from C6-sugars is 100 mol %, or 64.5 wt % due to the co-production of
formic acid [41]. Commonly, LA yields of about two thirds (or even less) than the
theoretical value are attained. These lower yields are due to the formation of
undesired black insoluble-materials called humins. Another possible by-product
of biomass hydrolysis is furfural, formed by the decomposition reactions of C5sugars. Table 1.3 gives an overview of LA synthesis using various types of
feedstock and acid catalysts.
Table 1.3 Overview of acid catalysed production methods for LA.
Feedstock
C0 (wt %) a
Acid
Cacid (wt %) T (°C)
t (h) YLA (wt %) b Ref.
Cane sugar
28
HCl
18
100
24
15
[42]
Glucose
32
HCl
20
R.T.
24
15
[43]
Corn starch
29
HCl
6.5
162
1
26
[44]
Sucrose
29
HCl
6.5
162
1
29
[44]
Glucose
29
HCl
6.5
162
1
24
[44]
Fructose
29
HCl
6.5
162
1
25
[44]
Hydrol d
42
HCl
7.4
R.T. c
22
25
[45]
Corn starch
33
HCl
1.8
200
0.5
35
[46]
Starch
26.5
HCl
5.2
R.T.
24
19
[47]
Rice hulls
14
HCl
1
160
3
10.3
[48]
Rice straw
14
HCl
1
160
3
5.5
[48]
Corn stalks
c
c
14
HCl
1
160
3
7.5
[48]
Cotton linters 14
HCl
1
160
3
7.4
[48]
Sucrose
6
H2SO4
9
125
16
30
[49]
Sucrose
6
HCl
9.7
125
16
43
[49]
Sucrose
6
HBr
9
125
16
50
[49]
Sucrose
27
Amberlite IR-120 19
R.T.
41
15.6
[50]
Fructose
27
Amberlite IR-120 19
R.T.
27
23.5
[50]
124
5.8
Glucose
27
Amberlite IR-120 19
R.T.
Glucose
5-20
H2SO4
160−240 f(T) e 35.4
0.1−4
[50]
[51]
9
Chapter 1
Table 1.3 (continued)
Feedstock
C0 (wt %) a
Acid
Cacid (wt %) T (°C)
t (h) YLA (wt %) b Ref.
Pulp slurry
10
HCl
6
160
1
40.5
[52]
Glucose
10
HCl
6
160
0.25 41.4
[52]
Cotton stems
n.a.
H2SO4
5
180−190 2
f
6.13
[53]
Wood sawdust
20
HCl
1.5
190
0.5
9
[54]
Oakwood
n.a.
H2SO4
3
180
3
17.5
[55]
Bagasse
9
H2SO4
1.3
25−195
2
17.5
[56]
Fructose
4.5-18
HCl
2−7.5
100
24
52
[57]
Sucrose
20
Resin-Dowex
6.25
100
24
17
[58]
Sawdust
n.a.
HCl
8
n.a.
n.a. 6.9
[59]
Shredded paper
n.a.
HCl
8
n.a.
n.a. 17.2
[59]
Fructose
50
LZY-zeolite
50
140
15
43.2
[60]
Glucose
12
Clay-catalyst g
3
150
24
12
[61]
Glucose
12
HY-zeolite
3
150
24
6
[62]
Cellulose
10
H2SO4
3
250
2
25.2
[63]
Various woods h 10-20
H2SO4
5
200−240 2−4 13−18
[63]
Cellulose
10
H2SO4
1–5
150−250 2−7 ≤ 25.2
[64]
Cellulose
10
HCl
1–5
150−250 2−7 ≤ 28.8
[64]
Cellulose
10
HBr
1–5
150−250 2−7 ≤ 26.9
[64]
Aspen wood
10
H2SO4
1–5
150−250 2−7 ≤ 15.5
[64]
Aspen wood
10
HCl
1–5
150−250 2−7 ≤ 12.4
[64]
Aspen wood
10
HBr
1–5
150−250 2−7 ≤ 13.0
[64]
Newspaper
30
H2SO4
10
150
8
12.8
[65]
Sorghum grain
10
H2SO4
8
200
0.67 32.6
[66]
Extruded starch
25
H2SO4
4
200
0.67 47.5
[67]
Wheat straw
6.4
H2SO4
3.5
209.3
0.63 19.8
[68]
a C is the initial concentration of feedstock and defined as the ratio between the mass of feedstock
0
and the total mass; b YLA is defined as the ratio between the mass of LA and the mass of feedstock; c
R.T. = Refluxed Temperature; d Mother liquor of crystalline corn starch; e Time is a function of
temperature; f n.a. = data is not available; g Fe-pillared montmorillonite; h Types of wood are beech,
aspen, pine and spruce.
Other starting materials and reagents have also been applied. Examples are the
hydrolysis of acetyl succinate ester [69], the acid hydrolysis of furfuryl alcohol
[70,71] and the oxidation of ketones [72-74]. LA can also be prepared by a Pdcatalysed carbonylation of ketones [75] and by the alkylation of nitroalkanes [76].
However, all these methods result in relatively high amounts of various by
products and require expensive feedstocks.
1.4.2
Continuous production of LA
Most of the studies provided in Table 1.3 were exploratory in nature and
carried out in typical laboratory batch reactors. Evidently, continuous processing
10
Introduction
for larger scale applications is advantageous. A continuous process for the
production of LA from corncob furfural residue at atmospheric pressure was
proposed by Dunlop and Wells (1957) [77] and is shown schematically in Figure
1.8. In this process, the carbohydrate feedstock (corncob furfural residue) is mixed
with sulphuric acid and water to reach a concentration of corncob furfural residue
of 21 wt % and an acid concentration of 3 wt %. Subsequently, the mixture is
continuously passed through a reactor maintained at an elevated temperature (169
°C). Typical residence times are 2 h. The insoluble humins are separated from the
product mixture in a filter unit. The aqueous mixture containing the acid catalyst
and LA is then contacted with a water-immiscible solvent (methyl isobutyl ketone)
to obtain an extract containing LA and an aqueous solution containing the acid
catalyst. The latter is recycled to the mixer prior to the reactor. In an evaporator,
the extraction solvent is separated from the LA and is recycled to the extraction
column. Further concentration and purification of LA is carried out in a
fractionation unit (vacuum distillation). With this process, a LA yield of 19.9 wt %
based on the weight of the dry feedstock charged to the process was obtained.
Figure 1.8 Continuous process for producing LA from corncob furfural residue
[77].
Ghorpade and Hanna (1999) [78] proposed a concept based on reactive
extrusion for the continuous production of LA. A simplified diagram of the
process is shown in Figure 1.9. Corn starch, water and sulphuric acid are mixed in
a pre-conditioner unit, and the slurry is then fed to a twin-screw extruder with a
variable temperature profile of 80–100 °C, 120–150 °C and 150 °C. The product is
filtered to separate the LA from the humins. The filtrate is then fed to a vacuum
distillation unit to purify the LA. By feeding 820 kg h-1 of corn starch, 40 kg h-1 of
sulphuric acid 5 wt % and 290 kg h-1 of water, the yield of LA was about 48 wt %.
11
Chapter 1
Figure 1.9 Production of LA using reactive extrusion [78].
Figure 1.10 shows the Biofine technology for the continuous production of LA
[79], and typical yields of LA for the Biofine technology at different reaction
conditions and intakes are given in Table 1.4. Carbohydrate feedstock and
sulphuric acid catalyst solution are mixed, and the slurry is supplied continuously
to a tubular reactor. This reactor is operated at a typical temperature of 210–220 °C
and a residence time of only 12 s in order to hydrolyse the carbohydrate
polysaccharide into their soluble monomers (hexose and pentose). This hydrolysis
reaction is rapid according to their invention. The outflow of the first reactor is fed
to a continuously stirred tank reactor operated at a lower temperature (190–200
°C) but with a longer residence time of 20 min. LA is removed by drawing-off
liquid from the second reactor. The reaction conditions in the second reactor are
chosen as such to vaporise formic acid and furfural, and the vapour is externally
condensed to collect these side products. Solid by-products are removed from the
LA solution in a filter-press unit.
E-1 : Pre-mixer
E-2 : Tubular reactor
E-3 : CISTR
E-4 : Filter-press
Formic acid
Furfural
E-3
Carbohydrate
feedstock
Levulinic acid
E-2
E-4
E-1
Acid catalyst
solution
Lignin
Humins
Steam
Figure 1.10 Continuous production of LA by the Biofine technology [79].
12
Introduction
Table 1.4 Typical conditions for the Biofine process.
Feedstock
(wt %)
Feed
(L
a
min–1)
C0 b
CH2SO4
T1 c
τ1c
T2 c
τ2c
LA outflow YLA d
(wt %)
(wt %)
(°C)
(s)
(°C)
(min)
(kg min–1)
(wt %)
A (44 wt %)
0.945
4
3.5
232
14
196
30
0.0088
23
A (44 wt %)
0.96
2
1.9
215
14
200
20
0.0061
32
A (44 wt %)
0.32
10
3
232
23.3
206
29.8
0.0048
15
B (80 wt %)
1.02
1
1.15
220
14
200
20
0.0040
39
B (80 wt %)
1.04
2
1.5
215
14
200
25
0.0085
41
C (42 wt %)
0.70
10
5
220
15.7
210
20
0.0121
17
Feedstock A is a sludge of bleached kraft paper that contains 44 wt % of cellulose, feedstock B is a
sludge of partially or non-bleached kraft paper that contains 80 wt % of cellulose and feedstock C is
raw wood flour that contains 42 wt % of cellulose. b C0 is the initial concentration of feedstock and
defined as the ratio between the mass of the feedstock and the total mass. c T1, τ 1, T2 andτ 2 are the
temperature in the 1st reactor, the residence time of the 1st reactor, the temperature in the 2nd
reactor and the residence time of the 2nd reactor, respectively. d YLA is LA yield and defined as the
ratio between the mass of LA and the mass of feedstock.
a
To the best of our knowledge, the first commercial-scale plant for the
conversion of lignocellulosic biomass to LA has been built in Caserta, Italy [80,81].
This unit will process 3,000 ton of feedstock per year, originating from local
tobacco bagasse and paper mill sludge. The plant is applying the Biofine
technology and the major products will be LA and ethyl levulinate, the latter to be
used as a fuel additive.
1.4.3
Mechanistic studies
The acid catalysed degradation of hexoses into LA has been extensively
studied; however, only a limited amount of information is available on the
underlying reaction mechanism [82-89]. The available information implies that
hexose sugars initially dehydrate to form the intermediate product 5hydroxymethylfurfural (HMF, 2), which is subsequently hydrated to give the final
product LA. Scheme 1.1 shows the proposed mechanism for the conversion of
hexose sugars, such as D-glucose (3), D-mannose (4) or D-fructose (5) to HMF. The
conversion of HMF into LA is the result of water addition to the C2 – C3 bond of
the furan ring to give the final products LA and formic acid (6) (see Scheme 1.2).
13
Chapter 1
OH
CH2OH
CHO
CHO
O
HO
HO
HO
HO
OH
OH
OH
OH
OH
OH
CH2OH
CH2OH
CH2OH
3
4
5
- H2O
CHO
OH
CH
OH
OH
CH2OH
- H2O
CHO
CHO
HOH2C
HO
OH
O
CH
CH
CH
CH
CHOH
O
- H2O
OH
OH
CH2OH
CH2OH
HOH2C
CHO
O
2
Scheme 1.1 Dehydration reactions of hexose sugars to HMF [87,90].
HOH2 C
O
OH
OH
H2O, H +
HOH2 C
CHO
O
2
CHO
H2 C
O
CHO
OH
OH
OH
H3 C
H
H3 C
CHO
OO
O
H3 C
CHO
CHO
OO
O
6
O
OH
H3 C
CH(OH)2
O
OH
H3 C
O
1
Scheme 1.2 Proposed reaction mechanism for the conversion of HMF to LA [86].
14
Introduction
1.5 Potential applications of LA and its derivatives
LA has been identified as a platform chemical for various interesting
derivatives [41,39]. The applications of LA and its derivatives have been reviewed
extensively [91,59,92,93]. In this overview, a number of interesting chemical
reactions of LA will be provided. Functional group transformations involving the
carboxylic-, carbonyl- and methyl-group as well as typical oxidation and reduction
reactions will be reported
1.5.1
Reactions involving the carboxylic group
One of important reactions involving the carboxylic group of LA is
esterification to produce various esters of LA. To obtain high yields of levulinate
esters, the reaction is usually carried out in the presence of an acid catalyst, for
example sulphuric, polyphosphoric acid or p-toluenesulfonic acid [94,95].
Recently, a novel technique to produce levulinate esters has been proposed. It
involves the reaction of LA with an organic alcohol in a reactive-extraction mode.
Here, the organic alcohol phase acts both as the esterifying agent and the
extractant phase [96].
Ethyl levulinate, made by esterifying LA with ethanol, can be used as an
oxygenate additive. A research by Tecaxo and Biofine Inc. showed that a mixture
of 20% ethyl levulinate, 1% of co-additive and 79% diesel can be used in regular
diesel engines [97]. Ethyl levulinate is also used in the flavouring and fragrance
industries. Levulinate esters from high boiling alcohols can be used as plasticizer
for cellulose plastics [98-100]. Another potential application of levulinate esters is
to replace kerosene as a fuel for the direct firing of gas turbines [101].
1.5.2
Reactions involving the carbonyl group
A wide range of interesting LA derivatives is available by nucleophilic
additions to the carbonyl group. As an example, reactions of LA with nitrogencontaining nucleophiles give either amides (7) or the cyclodehydration products of
the amides, depending on whether the carboxyl group of LA is protected or not
[102,103] (see Scheme 1.3). The amides formed by the reaction of LA with various
amines are attractive because of their biological activity. In the presence of a metal
catalyst and hydrogen gas, LA can react with ammonia or ammonium hydroxide
to give 5-methyl-2-pyrrolidone (8) [104], which is a useful intermediate for the
pharmaceutical industry.
15
Chapter 1
NHR2
R1 = Et
O
OR1
H3 C
OEt
H3 C
O
7
H2, cat., R2NH2
R2
O
R1 = H
N
H3 C
O
8
R1 = H, Et.
R2 = H, Me, Ar.
Scheme 1.3 Reductive amination of LA.
The carbonyl group of LA can also undergo through an acid-catalysed
condensation reaction with aromatic or heterocyclic alcohols to give 4,4-diarylsubstituted valeric acids [105-107]. A typical example is diphenolic acid (10) (or
4,4-bis-(4’-hydroxyphenyl)pentanoic acid) that is prepared by reacting one mol of
LA with two moles of phenol (9) (see Scheme 1.4).
HO
O
HO
OH
H3 C
H+
+ 2
O
R
R
H3 C
R
9
1
OH
10
OH
O
R = H, Me.
Scheme 1.4 Preparation of diphenolic acid.
Diphenolic acid 10 has several uses, such as in the production of various
polymers [108-111], lubricants [112], fire-retardant materials [113] and paints [107].
It can copolymerise with bisphenol A (BPA, 11) [114] or directly replaces BPA in
the production of polycarbonates, epoxy resins and other polymers [93,38,115]. In
contrast to BPA, diphenolic acid contains a carboxyl group, which offers more
functionality in the polymer synthesis. A recent study has estimated that
diphenolic acid could capture a market of 4.5×104 tons year-1 as a BPA replacement
and also another 2.3×103 tons year-1 as a coating material [93].
CH3
HO
OH
CH3
11
1.5.3
Reactions involving the methyl group
The methyl group of LA can be easily halogenated using bromide or chloride
to yield organic halides. For example, 5-bromolevulinic acid (12) may be obtained
by the bromination of LA in methanol [116,117] (see Scheme 1.5). 5-Bromolevulinic
acid is a precursor for δ-aminolevulinic acid (DALA). The latter is an active
16
Introduction
ingredient of a biodegradable herbicide, with a projected market of 9.1–13.6×104
tons year-1 [38]. In the pharmaceutical industry, DALA has also been used in
limited quantities as an active component in photodynamic cancer treatment [118].
O
O
Br2, MeOH
OH
H3 C
OH
O
O
Br
1
12
Scheme 1.5 Bromination reactions for LA.
DALA (13) is prepared by reacting 5-bromolevulinic acid with nitrogencontaining nucleophiles, such as sodium azide [117] or potassium phtalimide
[119]. Recently, the use of sodium diformyalmide (14) [120] as the N-nucleophile
to give an intermediate product (15) has been reported (see Scheme 1.6). Using 14,
DALA was obtained in high yields (> 80 mol %) and purity (> 90%) [121].
OHC
N-Na
O
OH
OH
O
N
O
Br
O
OHC
14
OHC
12
CHO
15
H+
O
OH
O
NH2
O
+
OH
H
6
13
Scheme 1.6 Synthesis of DALA.
1.5.4
Oxidation reactions
LA can be oxidised to various interesting derivatives. The chemoselectivity is
highly depending on the type of oxidant. High-temperature (365–390 °C)
oxidations of LA using oxygen in the presence of a V2O5 catalyst give succinic acid
(16) with a typical yield of about 80% [122]. Succinic acid (1,4-butanedioic acid) is a
versatile compound, and its production uses and reactions are reviewed in the
literature [123]. In 2004, the market potential for products based on succinic acid is
estimated to be 2.7×105 tons year-1 [124].
O
OH
HO
16
O
2 H2
H2
O
17
O
OH
HO
18
- H2 O
O
19
Scheme 1.7 Conversion of succinic acid into several derivatives.
17
Chapter 1
Several well-known derivatives of succinic acid (see Scheme 1.7) are γbutyrolactone (GBL, 17), 1,4-butanediol (BDO, 18) and tetrahydrofuran (THF, 19).
Hydrogenation of succinic acid leads to the formation of GBL that is used as an
intermediate for agrochemicals and pharmaceuticals [115]. BDO is a compound of
great interest as a starting material for the production of important polymers such
as polyesters, polyurethanes and polyethers [125]. A major BDO-based polymer is
polybutylene terephthalate, which is mainly used for engineering plastics, fibers,
films and adhesives. Other relevant BDO applications are in the formation of THF,
which is obtained by homogeneous dehydration of BDO. THF is a solvent for
poly-(vinyl chloride) (PVC) and is used as a monomer in the manufacture of
polytetramethylene glycol, which is used as an intermediate for Spandex fibers
and polyurethanes [125].
1.5.5
Reduction reactions
LA may be reduced by catalytic hydrogenation to γ-valerolactone (GVL, 20).
Typical catalysts are platinum oxide [126], Raney nickel [127,128], copper-chromite
[128], rhenium catalysts [129], rhodium complexes [130] and ruthenium complexes
[131-133]. GVL is used as a solvent for lacquers, insecticides and adhesives [134].
O
OH
H3 C
O
1
H2, cat
H2, cat
H3 C
O
20
O
H3 C
O
21
Scheme 1.8 Reduction of LA to MTHF through GVL as the intermediate.
One of the important derivatives of GVL is methyltetrahydrofuran (MTHF, 21),
which has potential as a gasoline oxgenate and has a predicted [93] market
potential as high as 2.6×105 m3 year-1. Yields as high as 83 mol % (63 wt %) were
reported [135].
1.6 Thesis Outline
The primary objective of this thesis is to define optimum catalysts, reaction
conditions and reactor configurations for the conversion of water hyacinth to LA.
The conversion of the C6-sugars present in the water hyacinth plant to LA
involves several reactions that together form a complex reaction network. A
number of side reactions giving rise to by-products formation (e.g., insoluble
humin compounds) significantly complicate the catalyst selection and the
development of kinetic models. Therefore, a stepwise approach was applied by
investigating the individual reactions in the overall reaction scheme separately.
18
Introduction
In Chapter 2, the results of an experimental screening study using a wide
variety of acid catalysts for the conversion of glucose and HMF to LA is reported.
The optimum catalyst was selected and applied in subsequent studies.
Chapter 3 describes an experimental and modelling study on the conversion of
HMF to LA. The kinetics of the main and side reactions were determined and
optimum reaction conditions were defined on the basis of the kinetic model.
The results of Chapter 3 together with additional experiments were applied to
determine the kinetics for the desired and side-reactions for the reaction of glucose
to LA. The resulting kinetic models and the implication for reactor design are
provided in Chapter 4.
Extensions of the kinetic schemes when using cellulose as the reactant are
reported in Chapter 5. Combined with the result of Chapter 3 and 4, this has led to
kinetic expressions for all main and side reactions taking place when converting
cellulose to LA. The kinetic expressions are used to determine the optimum
reaction conditions and reactor configurations for the conversion of cellulose to
LA.
An experimental and modelling study on the acid catalysed hydrolysis of the
water hyacinth plant to LA is reported in Chapter 6. The kinetic models described
in Chapters 3-5 are applied to determine the optimum conditions to obtain high
LA yields.
1.7 References
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19
Chapter 1
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20
Introduction
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21
Chapter 1
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and other lignocellulosic materials. US patent 3,701,789, 1972.
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24
Introduction
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25
Chapter 2
Exploratory Catalyst-Screening Studies
on the Conversion of 5-Hydroxymethylfurfural and Glucose to Levulinic Acid
Abstract
A catalyst-screening study on the decomposition reaction of glucose and 5hydroxymethylfurfural (HMF) to levulinic acid (LA), an interesting green
platform chemical, is reported. The catalytic activities of various types of acids
(homogeneous and heterogeneous Brønsted acids) were tested to determine the
optimal acid catalyst with respect to activity and selectivity towards LA. When
using HMF (CHMF,0 = 0.1 M, T = 98 °C, Cacid = 1.0 M and t = 60 min), H2SO4, HCl
and HBr showed the highest catalytic activities and LA yields of all the
homogeneous Brønsted acids tested. The HMF conversion and the LA yield are
correlated with the concentration of H+ in solutions, indicating the absence of
anion effects. When using glucose (CGLC,0 = 0.1 M, T = 141 °C, Cacid = 1.0 M and t =
60 min), H2SO4 and HCl showed the highest catalytic activities and LA yields. It
was proven that the products LA and formic acid do not auto-catalyse the
decomposition reactions of glucose or HMF to LA. Of a range of solid acid
catalysts, ZSM−5 gave very promising result for the conversion of HMF to LA.
Yields of LA of 70 mol % at 93 mol % conversion were obtained (CHMF,0 = 0.02 M, T
= 116 °C, Cacid = 5 wt % and t = 120 min). However, when using ZSM−5 for the
conversion of glucose to LA, the LA yields were very low. Other solid Brønsted
acids, like Nafion® SAC-13 or Ferrierite were less active, and they profoundly
catalysed the decomposition reactions of HMF and glucose to the undesired
humins by-product.
Keywords: Brønsted acids, levulinic acid, zeolites, dehydration reaction of glucose.
Chapter 2
2.1 Introduction
Research efforts to identify attractive chemical transformations for the
conversion of biomass into alternative fuels and useful bulk chemicals have
intensified considerably in the last decade [1,2]. A well-known example is the
hydrolysis of lignocellulosic biomass, which is typically catalysed by enzymes or
by mineral acids, to give glucose as the intermediate product. Glucose can be
converted to bio-ethanol as an alternative fuel or to various other organic (bulk)
chemicals. An attractive option is the conversion of glucose into levulinic acid (4oxopentanoic acid) by acid treatment. Levulinic acid (LA) is a versatile building
block for the synthesis of various organic compounds, as shown in Figure 2.1 [3,4].
Several reviews [5-9] have described the properties and the potential industrial
applications of LA and its derivatives.
O
O
O
O
OH
H2 C
O
O
Acrylic acid
2-Methyl-THF
γ-Valerolactone
O
CH3
CH3
CH3
O
δ-Amino levulinic acid
O
α-Angelica lactone
OH
OH
H3 C
OH
Levulinic acid
OH
H3 C
OH
H2 N
O
HO
1,4-Pentanediol
O
O
O
H3 C
OH
H3 C
R
OH
CH3
4,4-Bis-(4-hydroxyphenyl) O
valeric acid
O
O
β-Acetylacrylic acid
Levulinate esters
Figure 2.1 Potentially interesting derivatives of LA [3,4].
The acid-catalysed dehydration reaction of glucose follows a complex reaction
pathway with several intermediates (Scheme 2.1). In the main reaction, glucose (1)
reacts to give 5-hydroxymethylfurfural (HMF, 2). In the presence of acid, HMF is
subsequently hydrated to give LA (3) and formic acid (FA, 4) in a 1:1 mol ratio.
Unfortunately, both glucose and HMF also decompose in a parallel reaction mode
to produce insoluble compounds known as humins.
H
OH
O
HO
H
H
HO
OH
H
- 3 H2O
O
OH
H+
O
HO
O
H
H+
H+
Humins
Humins
+
H
OH
O
3
H+
Scheme 2.1 Acid-catalysed decomposition of glucose to LA.
28
OH
H 3C
2
1
O
+ 2 H2O
4
Exploratory Catalyst-Screening Studies …
A number of papers have appeared on the decomposition reaction of glucose
using various Brønsted acid-catalysts, which can be categorized either as
homogeneous catalysts (e.g., H2SO4, HCl, H3PO4, HNO3) [10-16] or solid acid
catalysts [17-19]. The applications of homogeneous catalysts on the decomposition
reaction of the intermediate HMF to LA have also been reported [20-24]. However,
the use of solid acids for the latter reaction is unknown.
We here report an exploratory catalyst-screening study on the decomposition
of HMF to LA and the decomposition of glucose to LA. Initial studies were
performed with HMF, as this compound is an intermediate in the conversion of
glucose to LA (Scheme 2.1). As such, the number of possible reaction pathways is
limited, simplifying the discussion of the results considerably. Subsequently, the
best catalysts for HMF were also tested for glucose.
2.2 Materials and methods
2.2.1
Chemicals
All chemicals used in this study were of analytical grade and used without
purification. Glucose, FA and various organic acids were purchased from Merck
GmbH (Darmstadt, Germany); LA 98 wt % and HMF were purchased from Acros
Organics (Geel, Belgium). Deionised water was used to prepare various solutions.
2.2.2
Solid acid catalysts
Nafion® SAC−13, which is a poly-(tetrafluoro-ethylene)-sulfonic acid resin
dispersed within amorphous silica, was purchased from Sigma-Aldrich. The
catalyst strudates were ground into fine powder (< 250 mm) prior to the reaction
to minimize the diffusional effects. Several commercial zeolites were also tested
after accommodation in the H−form. Table 2.1 summarises the main characteristics
of these materials. They consist of highly acidic molecular sieves with Si/Al ratio
close to 10. They were acquired in the ammonium form and activated in air at 550
°C for 6 h in air. In case of Ferrierite (Na/K form), it was exchanged twice with
NH4NO3 (0.5 M) and calcined afterwards at the same conditions stated above. The
degree of NH4−exchange (Na and K content in Table 2.1) was confirmed by
inductively coupled plasma spectroscopy (ICP). As shown, the degree of exchange
was successful with remaining loadings of ca. 10 and 82 ppm for Na+ and K+
respectively. ICP was also employed to measure the actual silicon-aluminium ratio
of the sample specimen employed in the catalytic tests. As these reactions are
sensitive to metals, the Fe content on the samples was also measured. It was found
that the ZSM−5 samples contained traces of Fe (< 500 ppm), likely from the steel
templates used in industrial synthesis.
29
Chapter 2
Table 2.1 Specifications of the commercial zeolites employed in this study.
Product
Cation Type
Si/Al / mol
a
Ferrierite (TOSOH)
Beta Zeolyst CP8145
720KOA
ZSM-5 27 Alsi Penta
K / Na
NH4
NH4
8.9 (9.2)
12.5 (11.1)
12-14 (12.5)
Na2O / wt %
1.2 (10
0.05
< 0.03
K2O / wt %
5.7 (82 ppm) b
—
—
Fe2O3 / wt %
—
—
< 0.05
Remaining Fe (by ICP) / ppm
102
224
468
ppm) b
values between parentheses are the Si/Al ratio determined by ICP;
exchange with NH4NO3.
a
b
ppms of Na/K after
In order to check the effect of the mesoporosity, the ZSM−5 sample was treated
with 0.2 M NaOH (80 °C for 120 min). For details of the protocol the reader is
referred elsewhere [25]. This sample is labelled as ZSM−5 AW−120. Different
characterization techniques such as SEM, elemental analysis (ICP) and N2 sorption
experiments were applied. The reader is referred to previous work for details [25].
2.2.3
Experimental procedure for the acid-catalysed decompositions of glucose
and HMF
The reactions were carried out in two types of glass ampoules with a wall
thickness of 1.5 mm and a length of 15 cm, differing in internal diameter (3 and 5
mm). The reactions catalysed by homogeneous Brønsted acids were carried out in
the smaller ampoules (3 mm i.d.); the larger ampoules (5 mm i.d.) were used for
the reactions catalysed by solid acids.
For reactions with homogeneous Brønsted acids, an ampoule was filled with a
certain amount of reaction mixture (between 0.2–0.5 cm3), consisting of reactant
(either glucose or HMF) and catalyst at a certain concentration. For the reactions
with the solid acid catalyst, an ampoule was loaded with a predetermined amount
of solid acid (typically 5 wt %). Subsequently, about 1 cm3 of either glucose or
HMF solution in water was added. The glass ampoules were then sealed using a
torch and placed in a constant-temperature oven (± 1 °C). To enhance the mixing
process between the reactant and the solid acid catalysts, the large ampoules were
arranged in a rotating aluminium plate (10 rpm). At various reaction times,
ampoules were taken from the oven and quenched in an ice-water bath (4 °C) to
stop the reaction. The ampoule was opened, and the liquid was separated from the
solids using a micro-centrifuge (Omnilabo International BV) for approximately
15−20 min at 1200 rpm. A certain amount of the clear solution was taken (100−200
µL) and diluted with water (2 cm3). The composition of the solution was
determined using high performance liquid chromatography (HPLC).
30
Exploratory Catalyst-Screening Studies …
2.2.4
Adsorption phenomena of LA and FA on the ZSM−5 zeolites
Standard solutions of LA and FA in water were prepared at a certain
concentration. Several large ampoules were loaded with a predetermined amount
of ZSM−5 zeolite catalysts (typically 5 wt %). About 1 cm3 of either LA or FA
standard solution was added into these ampoules, which were then sealed using a
torch. Subsequently, the same experimental procedure described in subsection
2.2.3 was applied. The concentrations of LA and FA in the bulk-liquid phase at
various reaction times were measured using HPLC and were used to calculate the
partitioning coefficients of LA (yLA) and FA (yFA) using the following equations:
y LA =
nLA,solid ntotal − nLA,bulk
n
=
= total − 1
nLA,bulk
nLA,bulk
nLA,bulk
(2.1)
y FA =
nFA,solid ntotal − nFA, bulk
n
=
= total − 1
nFA, bulk
nFA, bulk
nFA, bulk
(2.2)
In equations (2.1)−(2.2), nLA,solid, nLA,bulk, nFA,solid, nFA,bulk and ntotal represent the
moles of LA adsorbed on the solid surface, the moles of LA in the bulk-liquid
phase, the moles of FA adsorbed on the solid surface, the moles of FA in the bulkliquid phase and total moles in standard solutions, respectively.
2.2.5
Analytical methods
The composition of the liquid phase was determined using an HPLC system
consisting of a Hewlett Packard 1050 pump, a Bio-Rad organic acids column
Aminex HPX-87H and a Waters 410 refractive index detector. The mobile phase
was a diluted solution of sulphuric acid (5 mM) at a flow rate of 0.55 cm3 min-1.
The column was operated at 60 °C. A typical chromatogram is given in previous
reports [26,27]. The concentrations of each compound in the liquid-phase mixture
were determined using calibration curves obtained by analysing standard
solutions with known concentrations.
2.2.6
Definitions
Catalyst performance for the HMF hydration reaction to LA was quantified in
terms of conversion of HMF (XHMF), the yields of LA (YLA/HMF) and FA (YFA/HMF):
X HMF = 1 −
YLA/HMF =
YFA/HMF =
C HMF
C HMF,0
C LA
C HMF,0
C FA
C HMF,0
(2.3)
(2.4)
(2.5)
31
Chapter 2
In equations (2.3)–(2.5), CHMF, CHMF,0, CLA and CFA are the concentration of HMF,
the initial concentration of HMF, the concentration of LA and the concentration of
FA at a certain reaction time, respectively.
Similar definitions were applied for the dehydration reaction of glucose to LA:
X GLC = 1 −
YHMF =
C GLC
C GLC,0
(2.6)
C HMF
C GLC,0
YLA/GLC =
YFA/GLC =
(2.7)
C LA
(2.8)
C GLC,0
C FA
(2.9)
C GLC,0
Here, CGLC and CGLC,0 are the concentration of glucose after a certain reaction time
and the initial concentration of glucose, respectively.
2.3 Results and discussions
2.3.1
2.3.1.1
Acid-catalysed hydration reaction of HMF to LA
Homogeneous Brønsted acid catalysts
A range of Brønsted acids varying in acid strengths were tested at a CHMF,0 of
0.1 M, a reaction temperature (T) of 98 °C, an acid concentration (Cacid) of 1 M and
a reaction time (t) of 60 min. The results are provided in Table 2.2.
Table 2.2 Catalytic activity of Brønsted acids on the HMF hydration reaction to
LA. a
Acid catalyst
LA, C5H8O3
XHMF
YLA/HMF
YFA/HMF
(mol %)
(mol %)
(mol %)
0
0
0
Acidity b
pKa = 4.59 b
FA, HCOOH
1
0
0
pKa = 3.74
H3PO4
6
5
6
pKa1 = 2.15, pKa2 = 7.2, pKa3 = 12.4
Oxalic acid, (COOH)2
24
9
9
pKa1 = 1.25, pKa2 = 3.77
HNO3
46
0
12
pKa < - 2
HI
76
10
46
pKa < - 2
HCl
52
48
49
pKa < - 2
HBr
54
51
51
pKa < - 2
H2SO4
57
53
54
pKa,1 < - 2, pKa2 = 1.96
CHMF,0 = 0.1 M, T = 98 °C, Cacid = 1.0 M and t = 60 min; b All acid ionisation constants were taken
from [28], except for LA [29,30].
a
32
Exploratory Catalyst-Screening Studies …
Acids with a pKa > 2 (H3PO4, LA and FA) are poor catalysts for the reaction and
the XHMF for all these catalysts was less than 6 mol %. The near absence of catalytic
activity at this condition for both LA and FA imply that the reaction of HMF to LA
is not auto-catalysed by the products. Oxalic acid (pKa1 = 1.25) was considerably
more active and the XHMF was 24 mol %. However, the selectively to LA formation
was low (9 mol %) and significant amounts of insoluble humins were formed.
Significant higher activities were found for catalysts with pKa values below 1.
For common mineral acids like HCl, HBr and H2SO4, the HMF conversions (52-57
mol %) and LA yields (48-53 mol %) were in the same range. These data indicate
that the reaction is highly selective under these conditions and that insoluble
humins by-products are formed in only very minor amounts. LA and FA are also
formed in the theoretical 1 : 1 molar ratio.
The highest HMF conversion was obtained with HI (76 mol %); however, the
LA yield was only 10 mol %. LA and FA were not present in the reaction mixture
after reaction in the theoretical 1 : 1 molar ratio. Instead, the FA concentration is
consistently higher and even in the same range as for HCl, HBr and H2SO4. This
suggests that the LA formed may be subsequently decomposed by HI, supported
by the presence of several other peaks in the HPLC chromatograms.
For HNO3, a different reactivity pattern was observed. The conversion of HMF
was close to the values of HCl, HBr and H2SO4. However, the yield of LA was
close to zero, and that of FA was only 12 mol %. The main products in this case are
unidentified gaseous products and insoluble humins. On the basis of this data, we
can conclude that amongst the homogeneous Brønsted acids tested, HCl, HBr and
H2SO4 are the best catalyst for LA formation with respect to activity and
selectivity.
The question arises whether only the proton concentration in solution or also
the anion plays a role and affects the activity-selectivity patterns. To study this
effect, additional experiments with all acids, except HNO3 and HI, were
performed at different acid concentrations (0.5 and 2 M). The concentration of H+
in solution was calculated for each experiment using the pKa values given in Table
2.2. The HMF conversions and LA yields were determined and the results are
provided in Figure 2.2. It clearly shows that both the XHMF and YLA/HMF are
directly proportional to the CH+. It implies that the anion does not play an
important role and that activity/selectivity is solely depending on the CH+.
33
100
100
80
80
YLA/HMF / %-mol
XHMF / %-mol
Chapter 2
60
H2SO4
HCl
HBr
H3PO4
40
(COOH)2
20
FA
LA
60
H2SO4
HCl
HBr
H3PO4
40
(COOH)2
20
FA
LA
0
0
0.0
0.5
1.0
1.5
CH / M
+
2.0
2.5
0.0
0.5
1.0
1.5
2.0
2.5
CH / M
+
(a)
(b)
Figure 2.2 Effects of CH+ on the XHMF (a) and YLA/HMF (b) for the acid-catalysed
decomposition of HMF to LA (CHMF,0 = 0.1 M, T = 98 °C and t = 60 min).
2.3.1.2
Solid acid catalysts
Several solid acid catalysts (Table 2.3) were tested for the conversion of HMF to
LA. Their catalytic activities were evaluated at CHMF,0 = 0.02 M , T = 116 °C and t =
120 min, and were compared with sulphuric acid. The solid acid concentration
was 5 wt % for all experiments and is defined as the ratio between the mass of
solid acid catalyst and the total mass of the reaction mixture. The experimental
results are given in Table 2.4.
Nafion SAC-13®, a strong solid acid catalyst based on (tetrafluoro-ethylene)sulfonic polymer with an acid strength comparable to sulphuric acid, shows poor
performance and only 16 mol % conversion was observed (c.f. 88 mol % for
sulphuric acid under these conditions). Additional experiments were performed at
longer reaction times, and after 18 h XHMF and YLA/HMF were 75 and 32 mol %,
respectively. Thus, the catalyst is stable under the reaction conditions; however,
catalytic activity is reduced considerably compared to sulphuric acid. In addition,
the yield is also considerably lower as a result of excessive humins formation.
Speculatively, these results could either be due to the lower acid concentration in
the reaction mixture compared to H2SO4 or intra-particle diffusion limitations of
HMF. For parallel reactions (Scheme 2.1), such mass transfer effects generally
lower the selectivity and the yield of the desired products [31].
Disappointing results were obtained with Ferrierite, Beta and ZSM−5 AW−120
zeolites. Although the conversions were higher than for Nafion SAC-13®, and for
ZSM−5 AW−120 even higher than for sulphuric acid, the product yield was low (<
8 mol %). Insights in the factors related to the low LA yields may be obtained by
comparing the mol ratio of LA and FA after reaction. For zeolites Beta and ZSM-5,
the FA content was considerably higher than LA, indicating excessive LA
decomposition or strong adsorption of the LA formed on the zeolites (vide infra).
34
Exploratory Catalyst-Screening Studies …
Table 2.3 Properties of the solid-acid catalysts tested.
Sample
Si/Al a
Grain size / µm b
SBET / m2 g-1
Vtotal / cm3 g-1
Vµpores / cm3 g-1
Nafion SAC-13®
0.15 c
n.a. d
200 e
0.70 e
Ferrierite
9.2
1-3
331
0.23
−
0.12
Beta
11.1
< 0.1
636
0.89
0.18
ZSM-5-AW120 f
10.9
1-2
364 (135) g
0.38
0.09
ZSM-5
12.5
3-5
405 (35)
0.21
0.16
g
determined by ICP; b visualized by SEM; c acidity density as meq H+ per gram; d this material
consists of nano particles of H-nafion resins with quadrulobe-extruded silica matrix (5-10 mm
length); e from [32]; f samples derived by applying controlled alkaline desilication; g values between
brackets are mesopores and are derived from t-plot analysis.
a
Table 2.4 Catalytic activity of solid-acid catalyst on the HMF hydration reaction to
LA. a
Solid-acid catalyst
XHMF (mol %)
YLA/HMF (mol %)
YFA/HMF (mol %)
16
4
6
Ferrierite
43
0
0
Beta
85
0
7
ZSM-5 AW-120
90
8
29
ZSM-5
93
22
52
ZSM-5 corrected for adsorption
93
70
71
Sulphuric acid
88
77
79
Nafion
a
SAC-13®
CHMF,0 = 0.02 M, T = 116 °C, Cacid = 5 wt % and t = 120 min
The most promising results were obtained using ZSM−5 zeolite, which gave
the highest YLA/HMF among all solid-acid catalysts. However, again the LA and FA
were not formed in the theoretical 1 to 1 mol ratio. To gain more insights in this
phenomenon, the reaction was followed in time between 0 and 180 min (Figure
2.3).
Clearly, throughout the reaction, the CFA is always higher than the CLA.
Furthermore, the amount of HMF drops rapidly at the start of the reaction,
without a concurrent increase in product formation. This suggests that HMF
adsorbs strongly to the ZSM−5 zeolite. Such adsorption phenomena could also
explain the deviation of the molar ratio of LA to FA from the theoretical value of 1.
35
Chapter 2
0.025
CFA
CLA
0.020
CHMF
C/M
0.015
0.010
0.005
0.000
0
30
60
90
120
150
180
t / min
Figure 2.3 Concentration profiles for the ZSM-5-zeolite-catalysed reaction of HMF
(CHMF,0 = 0.023 M, T = 116 °C, CZSM-5 = 5 wt %).
To gain insights in the extent of adsorption of LA and FA on ZSM−5, a number
of adsorption experiments were performed using the products LA and FA and the
ZSM−5 zeolite. Standard solutions of LA and FA (0.023 M) were mixed with
ZSM−5 zeolite (5 wt %) at T = 116 °C. The concentrations of LA and FA were
measured at various reaction times between 0 and 180 min. By substituting the
concentration values of LA and FA into equations (2.1)−(2.2), the partition
coefficients of LA (yLA) and FA (yFA) during the reaction course were obtained and
are given in Figure 2.4. It is clearly shown in Figure 2.4 that LA has higher
partition coefficients, and thus LA adsorbs more strongly to the ZSM−5 zeolite
than FA.
yLA
yFA
yLA, yFA / mol mol
-1
1
0.1
0
30
60
90
120
150
180
t / min
Figure 2.4 Partition coefficients of LA and FA on ZSM-5 zeolite as function of
reaction time (CLA,0 = CFA,0 = 0.023 M, T = 116 °C, CZSM-5 = 5 wt %).
36
Exploratory Catalyst-Screening Studies …
The amounts of LA (nLA,corrected) and FA (nFA,corrected) formed by chemical
reaction distribute between the bulk-liquid phase and the solid-phase and may be
calculated using the following equations:
nLA,corrected = nLA,bulk + nLA,solid = nLA,bulk + y LA nLA,bulk = nLA,bulk (1 + y LA )
(2.10)
n FA, corrected = n FA, bulk + n FA,solid = n FA, bulk + y FA n FA, bulk = n FA, bulk (1 + y FA )
(2.11)
By combining the experimental results given in Figures 2.3−2.4 and equation
(2.10)−(2.11), the amount of LA and FA formed by chemical reactions can be
calculated and the results are shown in Figure 2.5. With this information, the
yields may be recalculated and are 70 mol % for YLA/HMF and 71 mol % for YFA/HMF
after 180 min reaction time. These values are only slightly lower than for sulphuric
acid. Thus the relatively low yields and the deviation from a 1 : 1 mol ratio of LA
and FA are due to preferential adsorption of LA on ZSM−5.
These results imply that ZSM−5 is a promising solid acid catalyst for the
conversion of HMF to LA. Further optimisation studies will be required to assess
the full potential and to identify whether it could be a replacement for sulphuric
acid. With ZSM−5, catalyst recycle may be more facile than with sulphuric acids,
leading to simplified catalyst recycle strategies.
0.025
nFA
nLA
nLA, nFA / mmol
0.020
0.015
0.010
0.005
0.000
0
30
60
90
120
150
180
t / min
Figure 2.5 The total amount of LA and FA formed in the HMF hydration reaction
using ZSM-5 zeolite as catalyst (CHMF,0 = 0.023 M, T = 116 °C, CZSM-5 = 5 wt %).
2.3.2
2.3.2.1
Acid-catalysed dehydration reaction of glucose to LA
Homogeneous Brønsted acid catalysts
The decomposition of glucose to LA using a variety of homogeneous Brønsted
acid catalysts has been studied extensively [10-16]. To investigate possible autocatalytic effects, both LA and FA were tested as catalysts for the reaction and the
results were compared with strong acids like sulphuric acid and hydrochloric
acid. All the experiments were conducted at a temperature of 141 °C and a
37
Chapter 2
reaction time of 60 min. The initial concentration of glucose and the acid
concentration were kept constant at 0.1 M and 1 M, respectively. The experimental
results are provided in Table 2.5.
Table 2.5 Catalytic activities of Brønsted acid catalysts on the glucose dehydration
reaction to LA. a
a
Acid catalyst
XGLC (mol %)
YHMF (mol %)
YLA/GLC (mol %) YFA/GLC (mol %)
No catalysts
5
0
0
0
LA
6
3
0
0
FA
8
0
0
0
HCl
67
1
45
48
H2SO4
69
1
45
44
CGLC,0 = 0.1 M, T = 141 °C, Cacid = 1 M and t = 60 min.
Both LA and FA gave low catalytic-activities and the results were not
significantly different from the blank/un-catalysed experiment. Based on these
results, it can be concluded that both LA and FA do not auto-catalyse the reaction.
2.3.2.2
Solid acid catalysts
For the acid-catalysed HMF hydration reaction to LA, positive results were
only obtained for ZSM−5; therefore this catalyst was tested for the dehydration of
glucose to LA. The reaction was conducted at CGLC,0 = 0.05 M, T = 141°C and Cacid
= 1 wt %. The reaction was followed in time (between 0 and 420 min), and the
results were compared with that of sulphuric acid (see Figure 2.6).
60
60
ZSM-5
H2SO4
50
50
40
YLA/GLC / mol %
40
XGLC / mol %
ZSM-5
H2SO4
30
20
10
30
20
10
0
0
0
60
120
180
240
t / min
300
360
420
0
60
120
180
240
300
360
420
t / min
(a)
(b)
Figure 2.6 Comparison of the catalytic activities of ZSM-5 and sulphuric acid
expressed as XGLC (a) and as YLA/GLC (b) for the acid-catalysed dehydration
reaction of glucose to LA (CGLC,0 = 0.05 M, T = 141 °C and CZSM−5 = 1 wt %).
38
Exploratory Catalyst-Screening Studies …
ZSM−5 is considerably less active than sulphuric acid and the selectivity to LA
is very low as shown in Figure 2.6 (b). The yields of LA were considerably higher
when using HMF instead of glucose as the reactant (vide supra). When considering
the proposed kinetic pathway for the conversion of glucose to LA (Scheme 2.1), it
appears that the reaction of glucose to HMF is not favoured by ZSM-5, resulting in
the formation of insoluble humins, whereas the subsequent step (HMF to LA) is
well possible with ZSM−5. It is well possible that glucose diffusion into the small
pores of ZSM−5 is hindered considerably and has a negative effect on the
selectivity and activity. Further experiments with acidic meso-porous materials are
in progress to test this hypothesis and will be reported in due course.
2.4 Conclusions
Homogeneous and heterogeneous Brønsted acids have been tested as catalysts
for the reaction of HMF or glucose to LA. From the screening results with liquid
Brønsted acids, it may be concluded that inorganic strong acids like sulphuric acid
or hydrochloric acid give the best performance. The products LA and FA do not
auto-catalyse the decomposition reactions of glucose or HMF to LA.
Of a range of solid acid catalysts tested, ZSM−5 gave very promising result for
the conversion of HMF to LA, though further optimisation studies will be required
to identify whether it could be used as a replacement for sulphuric acid catalyst.
With ZSM−5, catalyst recycle may be more facile than with sulphuric acid, leading
to simplified catalyst recycle strategies.
2.5 Nomenclature
CGLC
: Concentration of glucose (M)
CGLC,0
: Initial concentration of glucose (M)
CHMF
: Concentration of HMF (M)
CHMF,0
: Initial concentration of HMF (M)
CFA
: Concentration of FA (M)
CLA
: Concentration of LA (M)
nFA,bulk
: Moles of FA in the bulk-liquid phase (mol)
nFA,corrected : Total moles of FA both in bulk-liquid and on the solid surface (mol)
nFA,solid
: Moles of FA adsorbed on the solid surface (mol)
nLA,bulk
: Moles of LA in the bulk-liquid phase (mol)
nLA,corrected : Total moles of LA both in bulk-liquid and on the solid surface (mol)
nLA,solid
: Moles of LA adsorbed on the solid surface (mol)
ntotal
t
: Total moles in the standard solutions (mol)
XGLC
: Conversion of glucose (mol %)
XHMF
: Conversion of HMF (mol %)
yFA
: Partitioning coefficient of FA on the solid surface of ZSM–5 zeolites (−)
yLA
: Partitioning coefficient of LA on the solid surface of ZSM–5 zeolites (−)
: Time (min)
39
Chapter 2
YFA/GLC
: Yield of FA from glucose (mol %)
YFA/HMF
: Yield of FA from HMF (mol %)
YHMF
: Yield of HMF from glucose (mol %)
YLA/GLC
: Yield of LA from glucose (mol %)
YLA/HMF
: Yield of LA from HMF (mol %)
2.6 References
[1]
Klass, D. L., Biomass for Renewable Energy, Fuels, and Chemicals. Academic Press: New York,
1998.
[2]
Kamm, B.; Kamm, M.; Gruber, P. R.; Kromus, S., Biorefinery Systems - An Overview. In
Biorefineries - Industrial Processes and Products: Status Quo and Future Directions Volume 1,
Kamm, B.; Gruber, P. R.; Kamm, M., Eds. Wiley-VCH: Weinheim, 2006; pp 3-40.
[3]
Bozell, J. J.; Moens, L.; Elliott, D. C.; Wang, Y.; Neuenscwander, G. G.; Fitzpatrick, S. W.;
Bilski, R. J.; Jarnefeld, J. L., Production of levulinic acid and use as a platform chemical for
derived products. Resour. Conserv. Recycl. 2000, 28, 227-239.
[4]
Werpy, T.; Petersen, G. Top Value Added Chemicals from Biomass Volume I-Results of Screening
for Potential Candidates from Sugars and Synthesis Gas.; NREL/TP-510-35523; National
Renewable Energy Laboratory (NREL): 2004.
[5]
Leonard, R. H., Levulinic Acid As A Basic Chemical Raw Material. Ind. Eng. Chem. 1956, 48,
1331-1341.
[6]
Kitano, M.; Tanimoto, F.; Okabayashi, M., Levulinic acid, a new chemical raw material. Its
chemistry and use. Chem. Econ. Eng. Rev. 1975, 7, 25-29.
[7]
Thomas, J. J.; Barile, R. G. In Conversion of cellulose hydrolysis products to fuels and chemical
feedstocks., 8th Symposium on Energy from Biomass and Wastes, Lake Buena Vista, FL, USA,
1984; pp 1461-1494.
[8]
Ghorpade, V. M.; Hanna, M. A. In Industrial applications for levulinic acid., International
Conference on Cereals: Novel Uses and Processes, Campbell, G. M.; Webb, C.; McKee, S. L.,
Eds. Plenum: New York, NY, 1997; pp 49-55.
[9]
Timokhin, B. V.; Baransky, V. A.; Eliseeva, G. D., Levulinic acid in organic synthesis. Russ.
Chem. Rev. 1999, 68, 80-93.
[10] Harris, E. E.; Lang, B. G., Hydrolysis of Wood Cellulose and Decomposition of Sugar in
Dilute Phosphoric Acid. J. Phys. Colloid Chem. 1947, 51, 1430-1441.
[11] Sowden, J. C., The Action of Hydrobromic Acid on 1-C-14-D-Glucose. J. Am. Chem. Soc. 1949,
71, 3568.
[12] Mednick, M. L., Acid-Base-Catalyzed Conversion of Aldohexose Into 5-(Hydroxymethyl)-2Furfural. J. Org. Chem. 1962, 27, 398.
[13] Smith, P. C.; Grethlein, H. E.; Converse, A. O., Glucose Decomposition at High-Temperature,
Mild Acid, and Short Residence Times. Sol. Energy 1982, 28, 41-48.
[14] Bienkowski, P. R.; Ladisch, M. R.; Narayan, R.; Tsao, G. T.; Eckert, R., Correlation of Glucose
(Dextrose) Degradation at 90 to 190-Degrees-C in 0.4 to 20-Percent Acid. Chem. Eng. Commun.
1987, 51, 179-192.
[15] Mosier, N. S.; Ladisch, C. M.; Ladisch, M. R., Characterization of acid catalytic domains for
cellulose hydrolysis and glucose degradation. Biotechnol. Bioeng. 2002, 79, 610-618.
40
Exploratory Catalyst-Screening Studies …
[16] Xiang, Q.; Lee, Y. Y.; Torget, R. W., Kinetics of glucose decomposition during dilute-acid
hydrolysis of lignocellulosic biomass. Appl. Biochem. Biotechnol. 2004, 113-16, 1127-1138.
[17] Lourvanij, K.; Rorrer, G. L., Reactions of Aqueous Glucose Solutions Over Solid-Acid YZeolite Catalyst at 110-160 Degrees-C. Ind. Eng. Chem. Res. 1993, 32, 11-19.
[18] Lourvanij, K.; Rorrer, G. L., Dehydration of Glucose to Organic-Acids in Microporous
Pillared Clay Catalysts. Appl. Catal. A-Gen. 1994, 109, 147-165.
[19] Lourvanij, K.; Rorrer, G. L., Reaction rates for the partial dehydration of glucose to organic
acids in solid-acid, molecular-sieving catalyst powders. J. Chem. Technol. Biotechnol. 1997, 69,
35-44.
[20] Teunissen, H. P., Velocity measurements on the opening of the furane ring in
hydroxymethylfurfuraldehyde. Recl. Trav. Chim. Pays-Bas 1930, 49, 784-826.
[21] Heimlich, K. R.; Martin, A. N., A Kinetic Study of Glucose Degradation in Acid Solution. J.
Am. Pharmaceut. Assoc. 1960, 49, 592-597.
[22] McKibbins, S. W.; Harris, J. F.; Saeman, J. F., Kinetics of the acid-catalyzed conversion of Dglucose to 5-hydroxymethyl-2-furaldehyde and levulinic acid. For. Prod. J. 1962, 12, 17-23.
[23] Kuster, B. F. M.; van der Baan, H. S., Dehydration of D-Fructose (Formation of 5Hydroxymethyl-2-Furaldehyde and Levulinic Acid) .2. Influence of Initial and Catalyst
Concentrations on Dehydration of D-Fructose. Carbohydr. Res. 1977, 54, 165-176.
[24] Baugh, K. D.; Mccarty, P. L., Thermochemical Pretreatment of Lignocellulose to Enhance
Methane Fermentation .1. Monosaccharide and Furfurals Hydrothermal Decomposition and
Product Formation Rates. Biotechnol. Bioeng. 1988, 31, 50-61.
[25] Melian-Cabrera, I.; Espinosa, S.; Groen, J. C.; van de Linden, B.; Kapteijn, F.; Moulijn, J. A.,
Utilizing full-exchange capacity of zeolites by alkaline leaching: Preparation of Fe-ZSM5 and
application in N2O decomposition. J. Catal. 2006, 238, 250-259.
[26] Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., A kinetic study on the conversion of glucose to
levulinic acid. Chem. Eng. Res. Des. 2006, 84, 339-349.
[27] Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., A Kinetic Study on the Decomposition of 5Hydroxymethylfurfural into Levulinic Acid. Green Chem. 2006, 8, 701-709.
[28] Petrucci, R. H.; Harwood, W. S.; Herring, F. G., General Chemistry: Principles and Modern
Application. 8th ed. ed.; Prentice-Hall, Inc.: Upper Saddle River, New Jersey, 2002.
[29] Beale, S. I., Biosynthesis and metabolism of δ-aminolevulinic acid in Chlorella. Plant Physiol.
1971, 48, 316-319.
[30] Sasser, D. E. Process for the distillative purification of citral. US Patent 5,094,720, 1992.
[31] Westerterp, K. R.; van Swaaij, W. P. M.; Beenackers, A. A. C. M., Chemical Reactor Design and
Operation. 2nd ed.; John Wiley & Sons Ltd.: 1984; p 84-87.
[32] Harmer, M. A.; Sun, Q., Solid acid catalysis using ion-exchange resins. Appl. Catal. A-Gen.
2001, 221, 45-62.
41
Chapter 3
A Kinetic Study on the Decomposition
of 5-Hydroxymethylfurfural into
Levulinic Acid*
Abstract
Levulinic acid (LA), accessible by the acid-catalysed degradation of biomass, is
potentially a very versatile building-block for the synthesis of various (bulk)
chemicals for applications like fuel additives, polymer and resin precursors. We
here report a kinetic study on one of the key steps in the conversion of biomass to
LA, i.e., the reaction of 5-hydroxymethylfurfural (HMF) to LA. The kinetic
experiments were performed in a temperature window of 98–181 °C, acid
concentrations between 0.05 and 1 M and initial HMF concentrations between 0.1
and 1 M. The highest LA yield was 94 mol %, obtained at an initial HMF
concentration of 0.1 M and a sulphuric acid concentration of 1 M. The yield at full
HMF conversion is independent of the temperature. An empirical rate expression
for the main reaction as well as the side reaction to undesired humins was
developed using the power-law approach. Agreement between experimental and
model data is good. The rate expressions were applied to gain insights into the
optimum process conditions for batch processing.
Based on: Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., A kinetic study on the decomposition of
5-hydroxymethylfurfural into levulinic acid. Green Chem. 2006, 8, 701–709.
*
Chapter 3
3.1 Introduction
Biomass has been identified as an important source for biofuels and chemical
products [1]. Biomass is abundantly available, for instance in the form of waste
from agricultural, forest and industrial activities (e.g., paper industry). A
substantial amount of research activities is currently undertaken worldwide to
identify attractive chemical transformations to convert biomass into organic
chemicals and to develop economically feasible processes for these
transformations on a commercial scale. An attractive option is the conversion of
lignocellulosic biomass into levulinic acid (4-oxopentanoic acid) by acid treatment
[2-6].
Levulinic acid (LA) is a very versatile building block for the synthesis of (bulk)chemicals for applications like fuel additives, polymer and resin precursors [7].
Several reviews have been published describing the properties and potential
industrial applications of LA and its derivatives [8-12].
On a molecular level, the conversion of a typical lignocellulosic biomass like
wood or straw to LA follows a complicated reaction pathway [13], involving
several intermediate products (see Figure 3.1). The simplified reaction scheme
given in Figure 3.1 does not explicitly show the reactions leading to undesired
black insoluble polymeric materials also known as humins. As part of a larger
project to develop efficient reactor configurations for the conversion of biomass to
LA, we have initiated a study to determine the kinetics of all steps involved
(Figure 3.1). A stepwise approach was followed, starting with the conversion of 5hydroxymethylfurfural (HMF) to LA.
Cellulose
Glucose
Hexose
Biomass
5-Hydroxymethylfurfural
Levulinic acid
+ Formic acid
Glucose
Mannose
Galactose
Hemicellulose
Pentose
Lignin
Xylose
Furfural
Arabinose
Acid-soluble products
Figure 3.1 Simplified reaction scheme for the conversion of lignocellulosic biomass
into LA.
A number of experimental studies have been reported on the kinetics of the
acid-catalysed HMF decomposition to LA. The first study was carried out by
Teunissen in 1930 [14]. Reactions were carried out at 100 °C using various acid
catalysts with acid concentrations ranging between 0.1 and 0.5 N. Heimlich and
Martin [15] studied the reaction in a temperature range of 100−140 °C using
44
A Kinetic Study on the Decomposition of 5-Hydroxymethylfurfural into Levulinic Acid
hydrochloric acid (0.35 N) as catalyst. McKibbins and co-workers [16] investigated
the influence of sulphuric acid concentration (0.025−0.4 N) and temperature
(160−220 °C) on the decomposition rate of HMF to LA. In all these studies, the
effect of the initial concentration of HMF was not determined and first order
kinetics was assumed. Kuster and van der Baan [17] studied the influence of the
initial HMF concentration on the kinetics of HMF decomposition at 95 °C using
various concentrations (0.5−2.0 N) of hydrochloric acid. The most recent kinetic
study was reported by Baugh and McCarty [18], who used dilute acid as catalyst
at variable pH (2−4) and temperature (170−230 °C). Table 3.1 summarises the
results from previous kinetic studies on the acid-catalysed reaction of HMF to LA.
On the basis of the data given in Table 3.1, it may be concluded that a general
kinetic expression for a broad range of temperatures, catalyst concentration and
initial HMF concentrations is lacking. In addition, all earlier studies focus on the
overall decomposition of HMF without discriminating between the rates of the
main reaction to LA and formic acid (FA) and the side reaction to humins. In this
chapter, the kinetics of the acid-catalysed decomposition of HMF in a broad range
of process conditions will be reported, including the kinetics of the reactions
leading to humins. The results will be applied to gain insights into the optimum
process conditions to reduce humins formation and to achieve the highest LA
yield. Furthermore, the results will also be used as input for a full kinetic model
for the acid-catalysed hydrolysis of biomass to LA.
Table 3.1 Literature overview of the kinetic constants for the acid-catalysed
decomposition of HMF.
T
100 °C
Cacid
CH2SO4 = 0.1–0.5 N
CHCl = 0.1–0.5 N
100–140 °C CHCl = 0.35 M
CHMF,0
RHMF / mol L-1 min-1
Ref.
0.08–0.09 M
R HMF = 6.8 × 10 −3 C H + C HMF
[14]
not available
⎛ 96 ,000 ⎞
RHMF = 1.1 × 10 11 exp⎜ −
⎟C HMF
RT ⎠
⎝
[15]
96 ,800 ⎞
160–220 °C CH2SO4 = 0.025–0.4 N 0.061–0.139 M RHMF = 2.4 × 10 11 α HC A exp⎛⎜ −
⎟C HMF
95 °C
CHCl = 0.5–2.0 N
170–230 °C pH = 2–4
a
(
)
1.1
a
RT ⎠
⎝
0.25–1 M
R HMF = 0.01 C H+
0.024 M
⎛ 55 ,900 ⎞
RHMF = 1,300 + 4.1 × 10 6 C H + exp⎜ −
⎟C HMF
RT ⎠
⎝
C HMF
(
[16]
[17]
)
[18]
αH represents the correction factor and CA is expressed in normality (N).
3.2 Experimental
3.2.1
Experimental procedure
All chemicals used in this study were of analytical grade and used without
purification. HMF was obtained from Fisher Scientific BV (Netherlands). All acid
catalysts were purchased from Merck GmbH (Darmstadt, Germany). Milli-Q
water was applied to prepare the various solutions.
45
Chapter 3
The reactions were carried out in glass ampoules (inside diameter of 3 mm,
wall thickness of 1.5 mm and length of 15 cm). The ampoules were filled with
approximately 0.5 cm3 of reaction mixture and sealed using a torch. The sealed
ampoules were placed in a special rack that can hold up to 20 ampoules, and
placed in a constant-temperature oven (± 1 °C). At different reaction times,
ampoules were taken from the oven and quenched into an ice-water bath (4 °C) to
stop the reaction. The reaction mixture was taken out of the ampoule and diluted
with water to 10 cm3. Insoluble humins were separated using a 0.2 µm celluloseacetate filter (Schleicher & Schuell MicroScience GmbH, Dassel, Germany). The
particle-free solution was subsequently analysed using high performance liquid
chromatography (HPLC).
3.2.2
Analytical methods
The composition of the liquid phase was determined using an HPLC system
consisting of a Hewlett Packard 1050 pump, a Bio-Rad organic acids column
Aminex HPX-87H and a Waters 410 differential refractive index detector. The
mobile phase consisted of an aqueous solution of sulphuric acid (5 mM) at flow
rate of 0.55 cm3 min-1. The column was operated at 60 °C. The analysis for a
sample was complete within 40 minutes. A typical chromatogram is shown in
Figure 3.2. The concentrations of each compound in the product mixture were
determined using calibration curves obtained by analysing standard solutions
with known concentrations.
The gas composition was analysed with gas chromatography (Varian Micro GC
CP-2003) equipped with a TCD cell using a Porapak Q column operated at 75 °C.
Helium was used as the carrier gas. Humins particles were analysed using field
emission scanning electron microscope (FESEM) on a JEOL 6320F. C and H
elemental analyses were performed at the Analytical Department of the University
of Groningen using an automated Euro EA3000 CHNS analyser.
Levulinic Acid
HMF
Formic
Acid
0
5
10
15
20
25
30
35
40
t / min
Figure 3.2 Typical HPLC chromatogram for the HMF decomposition.
46
A Kinetic Study on the Decomposition of 5-Hydroxymethylfurfural into Levulinic Acid
3.2.3
Heat transfer experiments
At the start-up of the reaction, the reaction takes place non-isothermally due to
the heating-up of the contents of the ampoule from room temperature to the oven
temperature. To gain insights into the time required to heat up the reaction
mixture and to compensate for this effect in the kinetic modelling studies, the
temperature inside the ampoule as a function of the time during the heat-up
process was determined experimentally. For this purpose, an ampoule equipped
with a thermocouple was filled with a representative reaction mixture (1 M
solution of HMF in water without acid). The ampoule was then closed tightly
using a special bolt-and-screw system to prevent evaporation of the liquid. The
ampoule was subsequently placed in the oven at a specified temperature and the
temperature of the reaction mixture was followed in time. Before and after an
experiment, the amount of liquid inside the ampoule was measured to ensure that
evaporation of the liquid did not occur.
The experimental profiles at different temperatures were modelled using a heat
balance for the contents in an ampoule:
(
d MC pT
dt
) = UA (T
t
oven
−T)
(3.1)
When assuming that the heat capacity of reaction mixture is constant and not a
function of temperature, rearrangement of equation (3.1) will give:
dT UAt
(Toven − T ) = h(Toven − T )
=
dt MC p
(3.2)
Solving the ordinary differential equation (3.2) with an initial value T = Ti at t =
0 leads to:
T = Toven − (Toven − Ti ) exp − ht
(3.3)
The value of h was determined by fitting all experimental data at different oven
temperatures (100−160 °C) using a non-linear regression method and was found to
be 0.596 min-1. Figure 3.3 shows an experimental and modelled temperature
profile performed at an oven temperature of 100 °C. Equation (3.3) was
incorporated in the kinetic model to describe the non-isothermal behaviour of the
system at the start-up of the reaction.
The effect of chemical reaction on the heating profiles was modelled using the
mass and energy balance (equation (3.1) with an additional term for the chemical
reaction) for a batch reactor. The heating profiles did not change significantly
when taking into account an additional term for chemical reaction. Therefore, the
heating profiles were not compensated for the occurrence of chemical reaction.
47
Chapter 3
120
Experimental data
Model according to equation (3.3)
100
o
T/ C
80
60
40
20
0
2
4
6
8
10
12
14
16
t / min
Figure 3.3 Heating profile of the reaction mixture at Toven = 100 °C.
3.2.4
Determination of the kinetic parameters
The kinetic parameters were estimated using a maximum-likelihood approach,
which is based on the minimization of errors between the experimental data and
the kinetic model. Details about this procedure can be found in the literatures
[19,20]. Minimization of the objective function is initiated by providing initial
guesses for each kinetic parameter. The best estimates were obtained using the
MATLAB toolbox fminsearch, which is based on the Nelder-Mead optimisation
method.
The concentrations of HMF and LA vary considerably from experiment to
experiment and within an experimental run. As a result, the high concentrations
will dominate the error calculation when minimizing the objective function. To
solve this problem, the concentrations of HMF and LA were scaled and
transformed to the HMF conversion and the LA yield, respectively. By definition
[21], the HMF conversion (XHMF) and LA yield (YLA) vary between 0−1 and are
expressed as:
X HMF =
YLA =
48
(C
(C
HMF,0
− C HMF )
C HMF,0
LA
− C LA,0 )
C HMF,0
(3.4)
(3.5)
A Kinetic Study on the Decomposition of 5-Hydroxymethylfurfural into Levulinic Acid
3.3 Results and discussions
3.3.1
Acid screening
In the beginning of this research, a number of acid catalysts were screened
(H3PO4, oxalic acid, HCl, H2SO4 and HI) to determine the preferred acid for
further studies. All screening experiments were conducted at 98 °C and 1 hour
reaction time using CHMF,0 of 0.1 M and acid concentrations of 1 M. The results are
shown in Figure 3.4. H3PO4 and oxalic acid gave very low HMF conversions (< 25
mol %). In addition, the LA yields were also very low (5−9 mol %). The application
of HI resulted in very high HMF conversion, unfortunately accompanied with
very low LA yields. Major by-products were humins and some yet unidentified
soluble products. Of all acids screened, HCl and H2SO4 gave the best results.
Conversions of HMF were ranged between 52 and 57 mol %, and the LA yields
between 48 and 53 mol %. H2SO4 showed a slightly better performance than HCl
and was used in subsequent experiments.
100
100
76%
60
80
57%
52%
YLA / mol %
XHMF / mol %
80
40
60
53%
48%
40
24%
20
20
6%
9%
5%
0
9%
0
H3PO4
(COOH)2
H3PO4 (COOH)2
HCl
HCl
H2SO4
H2SO4
HI
HI
H3PO4
(COOH)2
H3PO4 (COOH)2
HCl
HCl
H2SO4
H2SO4
HI
HI
(a)
(b)
Figure 3.4 Effects of acid type on HMF conversion (a) and LA yield (b).
3.3.2
Reaction products
The acid-catalysed decomposition of HMF (1) to LA (2) and FA (3) is presented
in Scheme 3.1.
O
HOH2 C
O
CHO
+
2 H2O
OH
H3 C
H+,
O
k2H
+
OH
H
2
1
HOH2 C
O
H+, k1H
O
3
Humins
CHO
Scheme 3.1 Acid-catalysed decomposition of HMF to LA.
49
Chapter 3
A typical reaction profile of the acid-catalysed HMF decomposition reaction is
given in Figure 3.5. In line with the reaction stoichiometry, the LA and FA were
always produced in a 1 : 1 molar ratio. This implies that both LA and FA are stable
under the reaction conditions employed and do not decompose to other products
(vide infra).
0.12
CFA
CLA
Concentration / M
0.10
CHMF
0.08
0.06
0.04
0.02
0.00
0
30
60
90
120
150
180
t / min
Figure 3.5 Typical concentration profile of HMF decomposition reaction (T = 98
°C, CHMF,0 = 0.1 M, CH2SO4 = 1 M).
Possible by-products other than FA are insoluble black-polymeric substances,
known as humins and gas-phase components due to thermal degradation of
reactants/products. Humins were formed in all experiments. The elemental
composition of a typical humins sample was determined and contained 61.2 wt %
of carbon and 4.5 wt % of hydrogen. These values are close to the elemental
composition given in the literature [22] (C, 63.1; H, 4.2) for the humins obtained by
reacting HMF with 0.3 wt % oxalic acid at 130 °C for 3 hours. To gain insights into
the average particle size and the particle morphology, a number of humins
samples were analysed using SEM. A typical example is given in Figure 3.6. The
humins appear as round shaped, agglomerated particles with a diameter between
5–10 µm.
The gas-phase composition after reaction was analysed using GC. Only CO2
could be detected. However, the amount of CO2 formed was always less than 2 wt
% of the HMF intake, implying that this is only a minor reaction pathway under
these conditions.
50
A Kinetic Study on the Decomposition of 5-Hydroxymethylfurfural into Levulinic Acid
Figure 3.6 Scanning electron microscope image of the insoluble humins product.
3.3.3
Effects of temperature, acid concentration and initial HMF concentration
on HMF conversions and LA yields
A total of 11 batch experiments were performed in a broad range of reaction
conditions (T = 98−181 °C, CHMF,0 = 0.1−1 M) using sulphuric acid as the catalyst
(0.05−1 M). The reaction rate is very sensitive to the temperature. For instance,
essential quantitative HMF conversion (XHMF) can be achieved in 10 minutes at 181
°C (CH2SO4 = 0.1 M). However the rate is reduced dramatically at lower
temperatures and a 10 h reaction time was required to obtain XHMF = 80 mol % at
98 °C (CH2SO4 = 0.25 M, see Figure 3.7).
100
T = 98 °C, CH SO = 0.25 M
2
4
T = 141 °C, CH SO = 0.25 M
2
XHMF / mol %
80
4
T = 181 °C, CH SO = 0.1 M
2
4
60
40
20
0
1E-3
0.01
0.1
1
10
100
1000
t / min
Figure 3.7 Effect of temperature on HMF conversion (CHMF,0 = 0.1 M).
51
100
100
80
80
60
60
YLA / mol %
XHMF / mol %
Chapter 3
40
CH SO = 0.25 M
20
2
40
CH SO = 0.25 M
20
4
2
CH SO = 1 M
2
0
0
5
10
15
20
4
25
4
CH SO = 1 M
2
30
t / min
(a)
0
0
5
10
15
20
4
25
30
t / min
(b)
Figure 3.8 Effects of acid concentration on XHMF (a) and YLA (b) at T = 141 °C and
CHMF,0 = 1 M.
The effect of the CH2SO4 on HMF conversion and LA yield is graphically
provided in Figure 3.8. Evidently, higher acid concentrations result in higher
reaction rates (Figure 3.8 (a)). At 181 °C, the highest temperature in our study,
only dilute solution of sulphuric acid could be applied. Due to the occurrence of
very fast reactions at this temperature, regular sampling to obtain concentrationtime profiles proved not possible. At similar conversion levels, the LA yield is
slightly improved when using higher acid concentrations (Figure 3.8 (b)).
A number of experiments were performed with variable initial concentrations
of HMF (0.1−1.7 M) at T = 100 °C and CH2SO4 = 1 M. The conversion of HMF is only
slightly depending on the initial concentration of HMF (Figure 3.9 (a)), an
indication that the reaction order in HMF is close to 1. The initial concentration of
HMF has a dramatic effect on the LA yield (Figure 3.9 (b)). The LA yield was
significantly higher when using low initial concentration of HMF (84 mol % versus
50 mol %).
3.3.4
Development of a kinetic model
The kinetic model is based on the equations given in Scheme 3.1. It is assumed
that HMF decomposes to LA and humins in a parallel-reaction mode [16,17]. It
cannot be excluded a priori that LA and FA are also a source for humins and
decompose under the reaction conditions employed. A number of experiments
were conducted using pure LA and FA (at 100 °C and CH2SO4 = 1 M).
Decomposition of both compounds did not occur under these conditions,
implying that HMF is the sole source of humins.
52
A Kinetic Study on the Decomposition of 5-Hydroxymethylfurfural into Levulinic Acid
100
100
CHMF,0 = 0.1 M
CHMF,0 = 0.1 M
CHMF,0 = 1 M
CHMF,0 = 1 M
CHMF,0 = 1.7 M
60
40
60
40
20
20
0
CHMF,0 = 1.7 M
80
YLA / mol %
XHMF / mol %
80
0
30
60
90
120
150
0
180
0
30
60
t / min
90
120
150
180
t / min
(a)
(b)
Figure 3.9 Influence of initial concentration of HMF on XHMF (a) and YLA (b) at T =
98 °C and CH2SO4 = 1 M.
Both FA and LA are acidic compounds that potentially could also catalyse the
decomposition of HMF. To investigate possible autocatalytic effects of the reaction
products, a number of experiments were performed using FA or LA as catalysts
(Cacids = 1 M) to probe this possibility. The results are given in Figure 3.10. It may
be concluded that both LA and FA do not catalyse the decomposition of HMF,
excluding the autocatalytic effects in the kinetic scheme. Apparently, the pKa of
both acids (FA = 3.74 and LA = 4.59) are too low to catalyse the reaction.
0.14
FA-catalyst
LA-catalyst
H2SO4-catalyst
0.12
CHMF / M
0.10
0.08
0.06
0.04
0.02
0.00
0
30
60
90
120
t / min
Figure 3.10 Concentration profile of HMF using FA, LA and sulphuric acid as
catalyst (T = 98 °C, CHMF,0 = 0.1 M and Cacids = 1 M).
53
Chapter 3
When applying the kinetic scheme as given in Scheme 3.1 and applying the
power-law approach instead of the first-order approach to express the rate
equations, the following relations hold:
R1 = k1H (C HMF )aH
(3.6)
R2 = k 2H (C HMF )bH
(3.7)
The temperature dependencies of the kinetic rate constants are defined in term of
modified Arrhenius equations:
k1H = (C H + ) k1RH exp
αH
k 2H = (C H + ) k 2RH exp
βH
⎡ E1H ⎛ T −TR
⎜⎜
⎢
⎣⎢ R ⎝ TR T
⎞⎤
⎟⎟ ⎥
⎠ ⎦⎥
⎡ E2H ⎛ T −TR
⎜⎜
⎢
⎣⎢ R ⎝ TRT
⎞⎤
⎟⎟ ⎥
⎠ ⎦⎥
(3.8)
(3.9)
where TR is the reference temperature, set at 140 °C.
In a batch system, the concentrations of HMF and LA as a function of time are
represented by the following differential equations:
dC HMF
= − (R1 + R 2 )
dt
(3.10)
dC LA
= R1
dt
(3.11)
3.3.4.1
Modelling results
A total of 11 batch experiments gave 106 sets of experimental data, where each
set consists of the concentrations of HMF and LA at certain reaction times. The
best estimates of the kinetic parameters and their standard deviations were
determined using a MATLAB optimisation routine and the results are given in
Table 3.2. A good fit between experimental data and the kinetic model was
observed, as shown in Figure 3.11. This is confirmed by a parity plot (Figure 3.12).
Table 3.2 Kinetic parameters of HMF decomposition using H2SO4 as catalyst.
Parameter
k1RH
(M1-aH-αH
E1H (kJ
min-1) a
0.340 ± 0.010
110.5 ± 0.7
mol-1)
k2RH (M1-bH-βH
a
Estimate
min-1) a
0.117 ± 0.008
E2H (kJ mol-1)
111 ± 2.0
aH (−)
0.88 ± 0.01
bH (−)
1.23 ± 0.03
αH (−)
βH (−)
1.38 ± 0.02
TR = 140°C
54
1.07 ± 0.04
1.2
0.12
0.10
1.0
0.10
0.8
0.08
0.08
0.06
0.04
0.02
0.00
Concentration / M
0.12
Concentration / M
Concentration / M
A Kinetic Study on the Decomposition of 5-Hydroxymethylfurfural into Levulinic Acid
0.6
0.4
0.04
0.02
0.2
0
30
60
90
120
150
0.0
180
0
100
200
(a) CH SO = 1 M, T = 98 °C
2
300
400
0.00
500
2
2
4
0.10
1.0
0.8
0.08
0.8
0.6
0.4
0.2
Concentration / M
1.0
Concentration / M
1.2
0.06
0.04
0.02
5
10
6
15
20
0.00
25
8
10
12
4
0.6
0.4
0.2
0
2
4
6
t / min
(d) CH SO = 0.25 M, T = 141 °C
2
4
(c) CH SO = 1 M, T = 141 °C
(b) CH SO = 0.25 M, T = 98 °C
4
0.12
0
2
t / min
1.2
0.0
0
t / min
t / min
Concentration / M
0.06
10
12
14
0.0
0
2
4
6
8
10
12
t / min
t / min
(e) CH SO = 0.1 M, T = 181 °C
(f) CH SO = 0.05 M, T = 181 °C
2
4
8
4
2
14
16
4
Figure 3.11 Comparison of experimental data ({: CHMF, …: CLA) and kinetic model
(solid lines).
100
XHMF
YLA
Predicted Value / mol %
80
60
40
20
0
0
20
40
60
80
100
Experimental Value / mol %
Figure 3.12 Parity plot for all experimental and model point.
With the model available, it is possible to gain quantitative information on the
effects of the reaction conditions and input variables on the selectivity of the
reaction. For this purpose, it is convenient to use the rate selectivity parameter (S)
55
Chapter 3
[23], which is defined as the ratio between the rate of the desired reaction and the
rate of undesired reaction.
⎛ rate of LA formation ⎞ R1
S=⎜
⎟=
⎝ rate of humin formation ⎠ R2
(3.12)
Substitution of the rate expressions and kinetic constants equations as given in
equations (3.6−3.9) gives:
⎡ ( E1H − E2H ) ⎛ T −TR
⎜⎜ T T
R
⎝ R
⎢
k
S = 1RH exp ⎢⎣
k 2RH
⎞⎤
⎟⎟ ⎥
⎠ ⎥⎦
(C HMF )a
H − bH
(C )
H+
α H −βH
(3.13)
Using equation (3.13), it is possible to maximise S by the selection of CHMF,
CH2SO4 and temperature. The activation energies of the main reaction (E1H = 110.5
kJ mol-1) and the side reaction (E2H = 111 kJ mol-1) are similar (see Table 3.2). This
means that the selectivity of the reaction is independent on the temperature. Thus,
to achieve high conversion rates in combination with high selectivity, it is
attractive to perform the reaction at high temperatures.
Higher acid concentrations will speed up both the main and side reactions. The
reaction order in acid of the main reaction (αH = 1.38) is higher than that of the
side reaction (βH = 1.07), which means that higher acid concentrations will have a
positive effect on the selectivity of the reaction. Hence, both from a conversion and
selectivity point of view, it is advantageous to work at high acid concentrations.
Equation (3.13) predicts that the selectivity will be higher when working at low
CHMF because the order in HMF is negative (aH−bH = −0.35). Here, a compromise
between a high reaction rates (high HMF concentration favoured) and a good
selectivity (low HMF concentration favoured) needs to be established (vide infra).
3.3.4.2
Alternative models
We have applied the power-law approach to define the reaction rates of the
two reactions (Scheme 3.1). With the experimental data set available, it is also
possible to test other reaction models and particularly those models where all
reactant orders are set to 1. To compare the quality of the models, the goodness-offit approach was applied. The goodness-of-fit for a response of a model can be
represented by fit-percentages such as:
(
)
ˆ
⎡ norm C HMF − C
⎤
HMF
%FITHMF = ⎢1 −
⎥ × 100%
(
)
C
−
C
norm
HMF
HMF
⎣
⎦
(
)
ˆ
⎤
⎡ norm C LA − C
LA
%FITLA = ⎢1 −
⎥ × 100%
⎣ norm (C LA − CLA )⎦
(3.14)
(3.15)
Table 3.3 shows the results for a number of possible models. It is clear that the
power law model described in this study including humins formation shows the
highest goodness-of-fit.
56
A Kinetic Study on the Decomposition of 5-Hydroxymethylfurfural into Levulinic Acid
Table 3.3 Goodness-of-fit of several kinetic models.
Main reaction
R 1 = k 1HC HMF C H +
R1 = k1H (C HMF )
0.97
R 1 = k 1HC HMF C H +
R1 = k 1H (C HMF )
0.88
(C )
1.33
H+
Side reaction
%FITHMF
%FITLA
−
53%
41%
−
58%
48%
70%
62%
89%
87%
R2 = k2HC HMFC H +
(C )
R2 = k 2H (C HMF )
1.38
1.23
H+
(C )
1.07
H+
3.4 Application of the kinetic model
3.4.1
Comparisons with literature models
Various kinetic models for the sulphuric acid-catalysed decomposition of HMF
have been reported in the literature (Table 3.1). To demonstrate the broad
applicability of the model presented in this paper, the predicted HMF reaction
rates according to this model were compared to the various literature models. For
this purpose, a set of reaction conditions (T, CH2SO4 and CHMF) was selected within
the validity range of our model (100 °C < T < 180 °C, 0.05 M < CH2SO4 < 1 M, 0.1 M
< CHMF,0 < 1 M). The reaction rates of HMF (RHMF,power) at various reaction
conditions were calculated using equations (3.6–3.7), by taking into account that
RHMF,power = R1 + R2. Similarly, the RHMF for the literature models (RHMF,lit) were
calculated using the data provided in Table 3.1. The RHMF,lit were compared with
RHMF,power and the results are given in Figure 3.13. A good fit between the RHMF,lit
and RHMF,power was observed, indicting the broad applicability of our power-law
model.
0
Teunissen (1930) [14]
Heimlich & Martin (1960) [15]
McKibbins et al. (1962) [16]
Kuster & van der Baan (1977) [17]
Baugh & McCarthy (1988) [18]
-1
10
-1
RHMF,power / mol L min
-1
10
-2
10
-3
10
-4
10
-5
10
-5
10
-4
10
-3
10
-2
-1
10
-1
10
0
10
-1
RHMF,lit / mol L min
Figure 3.13 Comparison between kinetic model provided here and previous
kinetic studies.
57
Chapter 3
3.4.2
Batch simulation and optimisation
With the model available, it is possible to calculate the XHMF and YLA as a
function of the batch time and process conditions. As an example, the modelled
batch time required for XHMF = 90 mol % at various temperatures and acid
concentration is given in Figure 3.14.
10000
CH SO = 0.05 M
2
4
CH SO = 0.5 M
1000
2
4
CH SO = 1 M
Batch time / min
2
4
100
10
1
0.1
0.01
100
120
140
160
180
200
T / °C
Figure 3.14 Batch time for XHMF = 90 mol % as a function of temperature and acid
concentration (CHMF,0 = 0.1 M).
The kinetic model also allows determination of the optimum reaction condition
to achieve the highest YLA. For this purpose, equation (3.4) is differentiated to give:
dX HMF = −
dC HMF
C HMF,0
(3.16)
Combination of equations (3.10–3.11) and equation (3.16) leads to the following
expressions:
dC HMF
= −C HMF,0
dX HMF
(3.17)
dC LA
R1
=
C HMF,0
dX HMF R1 + R2
(3.18)
Equations (3.17–3.18) were solved using the numerical integration toolbox
ode45 in MATLAB software package from 0 to 90 mol % HMF conversion. The LA
yield is subsequently calculated using equation (3.5). Figure 3.15 shows the LA
yield as a function of CHMF,0 and CH+ at T = 180 °C and XHMF = 90 mol %. It is
evident that the LA yield is highest at high acid concentrations and low initial
HMF concentrations, in line with the experimental results (vide supra).
58
A Kinetic Study on the Decomposition of 5-Hydroxymethylfurfural into Levulinic Acid
Figure 3.15 Effects of CHMF,0 and CH+ on YLA (T = 180 °C and XHMF = 90 mol %).
3.5 Conclusions
This study describes an in-depth experimental and modelling study on the
acid-catalysed decomposition of HMF into LA and FA and humins by-products in
a batch reactor. Acid screening studies show that H2SO4 and HCl are the catalysts
of choice with respect to LA yield. The LA yield is highest at high acid
concentrations and low initial HMF concentrations and is essentially independent
of the temperature.
A broadly applicable kinetic model for the acid-catalysed HMF decomposition
at sulphuric acid concentrations between 0.05 and 1 M, initial concentrations of
HMF between 0.1 and 1 M and a temperature window of 100−181 °C using a
power-law approach has been developed. The reaction rates for the main reaction
to LA and FA and the side reaction to humins were modelled as a function of the
CHMF,0, the CH+ and the T. A maximum-likelihood approach has been applied to
estimate the kinetic parameters. A good fit between experimental data and
modelling results was obtained. The highest LA yield at a short batch times is
obtained at high temperature, a low initial HMF concentration and a high acid
concentration.
59
Chapter 3
3.6 Nomenclature
aH
:
Reaction order of HMF in the main reaction to LA and FA (−)
αH
:
Reaction order of H+ in the main reaction to LA and FA (−)
At
:
Heat transfer area (m2)
bH
:
Reaction order of HMF in the side reaction to humins (−)
βH
:
Reaction order of H+ in the side reaction to humins (−)
CH+
:
Concentration of H+ (M)
CHMF
:
Concentration of HMF (M)
CHMF,0
:
Initial concentration of HMF (M)
Cp
:
Heat capacity of reaction mixture (J g-1 K-1)
CLA
:
Concentration of LA (M)
CLA,0
:
Initial concentration of LA (M)
E1H
:
Activation energy of the main reaction to LA and FA (kJ mol-1)
E2H
h
:
Activation energy of the side reaction to humins (kJ mol-1)
:
Heat transfer coefficient from the oven to the reaction mixture (min-1)
k1H
:
Reaction rate constant of HMF for the main reaction (M1-aH min-1)
k1RH
:
Reaction rate constant k1H at reference temperature (M1-aH-αH min-1)
k2H
:
Reaction rate constant of HMF for the side reaction to humins (M1-bH min-1)
k2RH
:
Reaction rate constant k2H at reference temperature (M1-bH-βH min-1)
M
:
Mass of the reaction mixture (g)
R
:
Universal gas constant, 8.3144 J mol-1 K-1
R1
:
Reaction rate of HMF to LA and FA (mol L-1 min-1)
R2
S
:
Reaction rate of HMF to humins (mol L-1 min-1)
:
Rate selectivity parameter (−)
t
:
Time (min)
T
:
Reaction temperature (°C)
Ti
:
Temperature of reaction mixture at t = 0 (°C)
Toven
:
Temperature of oven (°C)
TR
:
Reference temperature (°C)
U
:
Overall heat transfer coefficient (W m-2 K-1)
XHMF
:
Conversion of HMF (mol %)
YLA
:
Yield of LA (mol %)
Special symbols
Ĉ i
:
Estimated value of matrix Ci (i = HMF, LA)
Ci
:
Average value of matrix Ci (i = HMF, LA)
norm(C)
:
Norm of matrix C
%FITi
:
Fit percentage of the ith compound (i = HMF, LA)
60
A Kinetic Study on the Decomposition of 5-Hydroxymethylfurfural into Levulinic Acid
3.7 References
[1]
Klass, D. L., Biomass for Renewable Energy, Fuels, and Chemicals. Academic Press: New York,
1998.
[2]
Fitzpatrick, S. W. Production of levulinic acid from carbohydrate-containing materials. US
patent 5,608,105, 1997.
[3]
Ghorpade, V.; Hanna, M. A. Method and apparatus for production of levulinic acid via
reactive extrusion. US patent 5,859,263, 1999.
[4]
Farone, W. A.; Cuzens, J. E. Method for production of levulinic acid and its derivatives. US
patent 6,054,611, 2000.
[5]
Cha, J. Y.; Hanna, M. A., Levulinic acid production based on extrusion and pressurized batch
reaction. Ind.Crop.Prod. 2002, 16, 109-118.
[6]
Fang, Q.; Hanna, M. A., Experimental studies for levulinic acid production from whole kernel
grain sorghum. Bioresour.Technol. 2002, 81, 187-192.
[7]
Bozell, J. J.; Moens, L.; Elliott, D. C.; Wang, Y.; Neuenscwander, G. G.; Fitzpatrick, S. W.; Bilski,
R. J.; Jarnefeld, J. L., Production of levulinic acid and use as a platform chemical for derived
products. Resour.Conserv.Recy. 2000, 28, 227-239.
[8]
Leonard, R. H., Levulinic Acid As A Basic Chemical Raw Material. Ind.Eng.Chem. 1956, 48,
1331-1341.
[9]
Kitano, M.; Tanimoto, F.; Okabayashi, M., Levulinic Acid, a New Chemical Raw Material; Its
Chemistry and Use. Chem.Econ.Eng.Rev. 1975, 7, 25-29.
[10] Thomas, J. J.; Barile, G. R. In Conversion of cellulose hydrolysis products to fuels and chemical
feedstocks., 8th Symposium on Energy from Biomass and Waste, Lake Buena Fista, FL, 1984; pp
1461-1494.
[11] Ghorpade, V.; Hanna, M. A., Industrial Applications for Levulinic Acid. In Cereal Novel Uses
and Processes, Campbell, G. M.; Webb, C.; McKee, S. L., Eds. Plenum Press: New York, 1997;
pp 49-55.
[12] Timokhin, B. V.; Baransky, V. A.; Eliseeva, G. D., Levulinic acid in organic synthesis.
Russ.Chem.Rev. 1999, 68, 80-93.
[13] Grethlein, H. E., Chemical Breakdown of Cellulosic Materials. J.Appl.Chem.Biotechnol. 1978, 28,
296-308.
[14] Teunissen, H. P., Velocity measurements on the opening of the furane ring in hydroxymethylfurfuraldehyde. Recl.Trav.Chim.Pay-B. 1930, 49, 784-826.
[15] Heimlich, K. R.; Martin, A. N., A Kinetic Study of Glucose Degradation in Acid Solution.
J.Am.Pharm.Assoc. 1960, 49, 592-597.
[16] Mckibbins, S.; Harris, J. F.; Saeman, J. F.; Neill, W. K., Kinetics of the Acid Catalyzed
Conversion of Glucose to 5-Hydroxymethyl-2-furaldehyde and Levulinic Acid. Forest Prod.J.
1962, 12, 17-23.
[17] Kuster, B. F. M.; van der Baan, H. S., Dehydration of D-Fructose (Formation of 5Hydroxymethyl-2-Furaldehyde and Levulinic Acid) .2. Influence of Initial and Catalyst
Concentrations on Dehydration of D-Fructose. Carbohydr.Res. 1977, 54, 165-176.
[18] Baugh, K. D.; McCarty, P. L., Thermochemical Pretreatment of Lignocellulose to Enhance
Methane Fermentation: I. Monosaccharide and Furfurals Hydrothermal Decomposition and
Product Formation Rates. Biotechnol.Bioeng. 1988, 31, 50-61.
[19] Bard, Y., Nonlinear Parameter Estimation. Academic Press: New York (USA), 1974; p 61-71.
61
Chapter 3
[20] Knightes, C. D.; Peters, C. A., Statistical analysis of nonlinear parameter estimation for Monod
biodegradation kinetics using bivariate data. Biotechnol.Bioeng. 2000, 69, 160-170.
[21] Westerterp, K. R.; van Swaaij, W. P. M.; Beenackers, A. A. C. M., Chemical Reactor Design and
Operation. 2nd ed.; John Wiley & Sons Ltd.: 1984; p 84-87.
[22] Blanksma, J. J.; Egmond, G., Humin from Hydroxymethylfurfuraldehyde. Recl.Trav.Chim.PayB. 1946, 65, 309-310.
[23] Fogler, H. S., Elements of Chemical Reaction Engineering. 3rd ed.; Prentice Hall PTR: New Jersey
(USA), 1999; p 285.
62
Chapter 4
A Kinetic Study on the Conversion of
Glucose to Levulinic Acid*
Abstract
Levulinic acid has been identified as a promising biomass-derived platform
chemical. A kinetic study on one of the key steps in the conversion of biomass to
levulinic acid, i.e., the acid-catalysed decomposition of glucose to levulinic acid
has been performed. The experiments were carried out in a broad temperature
window (140−200 °C), using sulphuric acid as the catalyst (0.05−1 M) and an initial
glucose concentration between 0.1 and 1 M. A kinetic model of the reaction
sequence was developed including the kinetics for the intermediate 5hydroxymethyl-2-furaldehyde (HMF) and humins by-products using a power-law
approach. The yield of levulinic acid is favoured in dilute glucose solution at high
acid concentration. On the basis of the kinetic results, continuous reactor
configurations with a high extent of back-mixing are preferred to achieve high
levulinic acid yields.
Keywords: biomass; green chemistry; levulinic acid; kinetic studies; reactor configurations.
Based on: Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., A kinetic study on the conversion of
glucose to levulinic acid. Chem. Eng. Res. Des. 2006, 84, 339–349.
*
Chapter 4
4.1 Introduction
A substantial amount of research activities is currently undertaken worldwide
to identify attractive chemical transformations to convert biomass into organic
(bulk) chemicals and to develop economically feasible processes for these
transformations on a commercial scale. Our research activities involve the acidcatalysed decomposition of lignocellulosic biomass into valuable chemicals. An
attractive option is the conversion of lignocellulosic biomass into levulinic acid (4oxopentanoic acid) by acid treatment at relatively mild conditions. Levulinic acid
contains a ketone group and a carboxylic acid group. These two functional groups
make levulinic acid a potentially versatile building block for the synthesis of
various organic (bulk) chemicals as shown in Figure 4.1 [1-5]. For instance, 2methyltetrahydrofuran and various levulinate esters may be used as gasoline and
biodiesel additives, respectively. δ-Aminolevulinate is a known herbicide, and the
bisphenol derivative may be an interesting substitute for bisphenol A [6,7].
O
O
O
CH3
γ-Valerolactone
O
O
CH3
OH
H2 C
O
2-Methyltetrahydrofuran
O
Acrylic acid
OH
H2 N
CH3
O
δ-Amino levulinic acid
O
α-Angelica lactone
OH
OH
H3 C
OH
Levulinic acid
OH
O
H3 C
OH
HO
1,4-Pentanediol
CH3
O
O
OH
O
R
H3 C
O
Levulinate esters
H3 C
O
4,4-Bis-(4-hydroxyphenyl)
valeric acid
O
β-Acetylacrylic acid
Figure 4.1 Potentially interesting derivatives of levulinic acid.
On a molecular level, the conversion of lignocellulosic biomass to levulinic acid
is known to follow a complicated reaction scheme involving several intermediates
and by-products (Figure 4.2) [8,9]. Hemicellulose and cellulose, two of the three
main constituents of biomass, are carbohydrate-based polymers that can be broken
down to low molecular weight sugars by hydrolysis using an acid catalyst. The
acid-catalysed decomposition of the C6-sugar fragments (e.g., glucose) leads to 5hydroxymethyl-2-furaldehyde as the intermediate product, which is subsequently
rehydrated to give levulinic and formic acids as the final products. Hydrolysis of
the C5-sugars of hemicellulose may lead to furfural. In addition, other constituents
in the hemicellulose matrix may produce side products like acetic acid and
galacturonic acid [9]. Lignin, the third main constituent of lignocellulosic biomass,
64
A Kinetic Study on the Conversion of Glucose to Levulinic Acid
is a resin-like polymer matrix with various substituted phenolics present. During
the acid hydrolysis, various acid soluble lignin-derived components may be
formed, increasing the product complexity. The simplified reaction scheme given
in Figure 4.2 does not explicitly show the reactions leading to the undesired
insoluble-polymeric materials known as humins.
BIOMASS
Cellulose
Hemicellulose
CHO
O
H3C
OH
Acetic Acid
HO
CHO
CHO
OH
HO
COOH
Galacturonic Acid
OH
OH
OH
Acid-soluble compounds
OH
OH
HO
HO
Lignin
OH
CH2OH
CH2OH
Xylose
Glucose
O
HO
OH
Glycolic Acid
CHO
O
Furfural
HOH2C
5-Hydroxymethyl-2-furaldehyde
O
O
H
CHO
O
OH
Formic Acid
OH
H3C
O
LEVULINIC ACID
Figure 4.2 Possible pathways and products of the acid-catalysed hydrolysis of a
typical lignocellulosic material.
As part of a larger project to develop efficient reactor configurations for the
conversion of biomass to levulinic acid, we have initiated a study to determine the
kinetics of all steps involved in the process. A stepwise approach was followed,
starting with the conversion of 5-hydroxymethyl-2-furaldehyde (HMF) to levulinic
acid [10]. We here report our results of the kinetic study of the acid-catalysed
decomposition of glucose in a broad range of process conditions, including the
kinetics of the reactions leading to humins.
The acid-catalysed decomposition of glucose has been studied by a number of
authors [11-20]. However, in all studies, only the rate of decomposition of glucose
has been taken into account, often represented by a simple first-order reaction
(Table 4.1). The development of a kinetic scheme for the conversion of glucose to
levulinic acid, including the kinetics of by-product formation and incorporation of
65
Chapter 4
HMF as an intermediate, has not been reported to date. In addition, the rate
equations provided in the literature are often valid for small temperature,
substrate and/or catalyst concentration windows, whereas our study was
performed in a large window for all variables.
Table 4.1 Overview of kinetic studies of glucose decomposition.
T (°C)
CGLC,0
Cacid
Order in substrate and acid
(
)
1.02
Reference
170−190
5 wt %
H2SO4 (0.4−1.6 wt %)
R ∝ wH 2 SO 4
100−150
0.056 M
HCl (0.35 M)
R ∝ C GLC
[12]
160−240
0.278−1.112 M
H2SO4 (0.025−0.8 N)
R ∝ CacidCGLC
180−244
0.4−6 wt %
H2SO4 (0.5−4 wt %)
100−144
4−12 wt %
H2SO4 (4−20 wt %)
(
)
R ∝ (C ) C
[13]
170−230
0.006−0.33 M
pH 1−4
R ∝ C H + C GLC
[17]
190−210
0.125 M
pH 1.5−2.2
R ∝ C H + C GLC
[20]
R ∝ wH 2 SO 4
C GLC
0.8955
C GLC
1.33
H+
GLC
[11]
[15]
[16]
The results of this kinetic study will be used as input to obtain a full kinetic
model for the acid-catalysed hydrolysis of the lignocellulosic biomass to levulinic
acid. In addition, it allows the selection and the development of an efficient
continuous reactor technology, in which the yield of levulinic acid is optimised
and the amount of undesired by-products is reduced.
4.2 Experimental
4.2.1
Experimental procedure
All chemicals (analytical grade) were purchased from Merck (Darmstadt,
Germany) and used without further purification. The reactions were carried out in
glass ampoules with an internal diameter of 3 mm, a wall thickness of 1.5 mm, and
a length of 15 cm. An ampoule was filled at room temperature with a solution of
glucose and sulphuric acid in the predetermined amounts (Vliquid= 0.5 cm3). The
ampoule was sealed with a torch. A series of ampoules was placed in a special
rack and subsequently positioned in a constant-temperature oven (± 0.1 °C), which
was pre-set at the desired reaction temperature. At different reaction times, an
ampoule was taken from the oven and directly quenched into an ice-water bath (4
°C) to stop the reaction. The ampoules were opened, the reaction mixture was
taken out and subsequently diluted with water to 10 ml. Insoluble humins, formed
during the decomposition reaction, were separated from the solution by filtration
over a 0.2 µm cellulose acetate filter (Schleicher & Schuell MicroScience, Dassel,
Germany). The particle-free solution was then analysed using high performance
liquid chromatography (HPLC).
66
A Kinetic Study on the Conversion of Glucose to Levulinic Acid
4.2.2
Analytical methods
The composition of the liquid phase was determined using an HPLC system
consisting of a Hewlett Packard 1050 pump, a Bio-Rad Organic Acid column
Aminex HPX-87H and a Waters 410 differential refractive index detector. The
mobile phase consisted of aqueous sulphuric acid (5 mM), which was set at a flow
rate of 0.55 cm3 min-1. The column was operated at 60 °C. The analysis for a
sample was complete in 40 minutes. A typical chromatogram is shown in Figure
4.3. The concentrations of each compound in the product mixture were
determined using calibration curves obtained by analysing standard solutions of
known concentrations.
Glucose
Levulinic Acid
Formic
Acid
0
5
10
15
HMF
20
25
30
35
40
Retention Time / min
Figure 4.3 Typical chromatogram of a product mixture obtained from the acidcatalysed decomposition of glucose.
Identification of side-products of glucose decomposition (i.e., the reversion
products) was performed by connecting the HPLC system to an API3000 triple
quadrupole LC/MS/MS mass spectrometer (Perkin-Elmer Sciex Instruments,
Boston, MA). The mass spectrometer was supplied with an atmospheric pressure
ionisation source at a temperature of 400 °C.
4.2.3
Heat transfer experiments
At the start-up of the reaction, the reaction takes place nonisothermally due to
heating-up of the contents of the ampoule from the room temperature to the oven
temperature. To gain insight into the time required for heating-up the reaction
mixture and to compensate for this effect in the kinetic modelling studies, the
temperature inside the ampoules as a function of the time during the heating-up
process was determined experimentally. For this purpose, an ampoule equipped
with a thermocouple was filled with a representative reaction mixture. The
ampoule was then closed tightly using a special bolt-and-screw system to prevent
evaporation of the liquid. The ampoule was subsequently placed in the oven at a
67
Chapter 4
specified temperature and the temperature of reaction mixture was followed in
time. Before and after an experiment, the amount of liquid inside the ampoule was
measured to ensure that evaporation of the liquid did not occur.
The experimental profiles at different temperatures were modelled using a heat
balance for the contents in an ampoule:
(
d MC pT
dt
)
= UA t (Toven − T )
(4.1)
When assuming that the heat capacity of the reaction mixture is constant and not a
function of temperature, rearrangement of equation (4.1) will give:
dT UAt
(Toven − T ) = h(Toven − T )
=
dt MC p
(4.2)
Solving the ordinary differential equation (4.2) with the initial value T = Ti at t = 0
leads to:
T = Toven − (Toven − Ti ) exp − ht
(4.3)
The value of h was determined by fitting all experimental data at different oven
temperatures (100−160 °C) using a non-linear regression method and was found to
be 0.596 min-1. Figure 4.4 shows an experimental and modelled temperature
profile performed at an oven temperature of 100 °C. Equation (4.3) was
incorporated in the kinetic model to describe the non-isothermal behaviour of the
system at the start-up of the reaction.
120
Experimental data
Model according to equation (4.3)
100
o
T/ C
80
60
40
20
0
2
4
6
8
10
12
14
16
t / min
Figure 4.4 Heating profile of the reaction mixture at Toven = 100 °C.
68
A Kinetic Study on the Conversion of Glucose to Levulinic Acid
4.2.4
Determination of the kinetic parameters
The concentrations of all compounds involved in the decomposition reaction of
glucose were obtained from HPLC analysis. All concentrations were normalized
with respect to the initial concentration of glucose as follows:
X GLC =
YHMF =
YLA =
(C
GLC,0
− C GLC )
(4.4)
C GLC,0
(C
(C
HMF
− C HMF,0 )
(4.5)
C GLC,0
LA
− C LA,0 )
(4.6)
C GLC,0
The kinetic parameters were determined using a maximum-likelihood
approach, which is based on minimization of errors between the experimental
data and the kinetic model. Details about this procedure can be found in the
literature [21,22]. Error minimization to determine the best estimate of the kinetic
parameters was performed using the MATLAB toolbox fminsearch, which is based
on the Nelder-Mead optimisation method.
4.3 Results and discussions
4.3.1
Effects of process variables on the decomposition reaction of glucose
A total of 22 experiments were performed in a temperature window of 140−200
°C, CH2SO4 ranging between 0.05 and 1 M and CGLC,0 between 0.1 and 1 M. A typical
concentration profile is given in Figure 4.5. HMF was observed as an intermediate
product in all experiments. The CHMF showed a maximum with respect to reaction
time, although its maximum value is generally very low and less than 5% of the
CGLC,0. This observation indicates that the conversion of HMF to levulinic acid is
much faster than the conversion of glucose to HMF.
0.12
CGLC
0.10
CLA
Concentration / M
CHMF
0.08
0.06
0.04
0.02
0.00
0
20
40
60
80
100
120
t / min
Figure 4.5 Typical concentration profile (CGLC,0 = 0.1 M, CH2SO4 = 1 M, T = 140 °C).
69
Chapter 4
The rate of glucose decomposition is a strong function of the temperature, and
the time to reach 99 mol % of glucose conversion ranged between 12 h at 140 °C
(CGLC,0 = 0.1 M, CH2SO4 = 0.1 M) and 6 min at 200 °C (CGLC,0 = 1 M, CH2SO4 = 0.5 M).
The reaction rate is also considerably higher at higher acid concentrations. At 200
°C, only dilute solutions of sulphuric acid (0.05−0.1 M) could be used as catalyst.
Due to the very fast reaction rates at these conditions, representative sampling and
analysis proved not possible.
In all reactions, the formation of substantial amounts of black insolubleproducts, also known as humins, was observed. The composition and the
formation pathways of these polymeric sugar-derived compounds are poorly
understood [23]. The yield of levulinic acid is a function of the reaction time,
temperature, CGLC,0 and CH2SO4. The highest yield was about 60 mol % at CGLC,0 =
0.1 M, CH2SO4 = 1 M and T = 140 °C.
The yield of levulinic acid as a function of the reaction time and CGLC,0 (0.1−1M)
is given in Figure 4.6. It is evident that more dilute solutions of glucose results in
higher yields of levulinic acid. The effect of temperature on the yield is given in
Figure 4.7. The maximum yield decreases when operating at the high end of the
temperature window. The concentration of sulphuric acid only has a small effect
on the yield of levulinic acid (Figure 4.8).
70
CGLC,0 = 0.1 M
60
CGLC,0 = 0.5 M
CGLC,0 = 1.0 M
YLA / mol %
50
40
30
20
10
0
0
20
40
60
80
100
120
t / min
Figure 4.6 Yield of levulinic acid versus time for different CGLC,0 (T = 140 °C, CH2SO4
= 1 M).
70
A Kinetic Study on the Conversion of Glucose to Levulinic Acid
70
o
T = 140 C
o
T = 170 C
o
T = 200 C
60
YLA / mol %
50
40
30
20
10
0
1
10
100
t / min
Figure 4.7 Yield of levulinic acid versus time at different temperatures (CGLC,0 =
0.1 M, CH2SO4 = 0.5 M).
70
CH SO = 0.1 M
2
60
2
4
CH SO = 1.0 M
50
YLA / mol %
4
CH SO = 0.5 M
2
4
40
30
20
10
0
1
10
100
t / min
Figure 4.8 Yield of levulinic acid versus time at different CH2SO4 (CGLC,0 = 0.5 M, T
= 170 °C).
4.3.2
Development of a kinetic model for glucose decomposition to levulinic
acid
The acid-catalysed decomposition of glucose (1) to levulinic acid (LA, 3) and
formic acid (FA, 4) is schematically given in Scheme 4.1. In line with literature data
and our experimental findings, HMF (2) is considered as an intermediate product.
71
Chapter 4
H
OH
O
HO
H
H
HO
OH
H
- 3 H 2O
O
O
HO
O
+
OH H
O
+ 2 H2 O
H+
H
OH
H3 C
H
OH
O
2
1
+
4
3
Scheme 4.1 Acid-catalysed decomposition of glucose to LA.
This simplified scheme does not take into account the formation of humins and
other possible by-products. Substantial amounts of insoluble humins are formed
in the course of the reaction. There are strong indications that the humins may be
formed from both glucose and HMF [13,17]. LA is not a source for humins. This
was checked independently by reacting LA with 1 M sulphuric acid at 150 °C for 6
hours. It was found out that the concentration of LA was constant during the
reaction. The rate of formation of humins from glucose and HMF was included in
the kinetic model.
Fructose (5) is a known intermediate in the acid-catalysed decomposition of
glucose [24-26]. It is likely formed from glucose (1) according to a reaction
mechanism given in Scheme 4.2 [27,28]. Here, 1,2-enediol (6) is proposed as the
common intermediate. However, fructose could not be detected in our reaction
mixtures. This is not surprising, as previous studies [29,30] have already shown
that the dehydration of fructose to HMF is much faster than that of glucose.
Therefore, any fructose formed from glucose is expected to be converted to HMF
rapidly.
CHO
CH2OH
OH
OH
O
OH
HO
HO
HO
OH
OH
OH
OH
OH
CH2OH
CH2OH
CH2OH
1
OH
5
6
- 3 H 2O
HO
CHO
O
2
+ 2 H2 O
O
OH
O
3
+
O
H
OH
4
Scheme 4.2 Reaction mechanisms for the acid-catalysed decomposition of glucose
to LA.
72
A Kinetic Study on the Conversion of Glucose to Levulinic Acid
Other possible by-products are so-called reversion products, like levoglucosan
or 1,6-anhydro-β-D-glucopyranose (9), 1,6-anhydro-β-D-glucofuranose (10), isomaltose (11) and gentiobiose (12), is shown in Scheme 4.3. In acidic solutions, the
acyclic form of D-glucose (1) exists in equilibrium with its anomeric forms, i.e., αD-glucopyranose (7) and β-D-glucopyranose (8). The anomeric forms may be
involved in a number of reactions leading to reversion products [27,31]. Intramolecular condensation reactions produce anhydro sugars, mainly levoglucosan
and 1,6-anhydro-β-D-glucofuranose. Inter-molecular condensation reactions
between two glucose units will give disaccharides such as isomaltose and
gentiobiose. Several investigators [32,33] have also found and isolated other type
of disaccharides, i.e., (1→2)-linked and (1→3)-linked dimers. Most studies [33,31]
revealed that the yields of anhdyro sugars were higher than the yields of
disaccharides, although other investigator [32] found opposite results.
HO
OH
HO
O
HO
O
- H 2O
O
HO
OH
OH
O
- H 2O
OH
O
OH
HO
OH
10
OH
O
OH
7
HO
HO
OH
11
O
HO
OH
HO
OH
OH
1
OH
HO
OH
HO
O
OH
O
O
- H 2O
HO
HO
OH
9
O
- H 2O
HO
OH
HO
O
HO
OH
HO
O
HO
8
OH
12
Scheme 4.3 Reversion reactions of glucose in acid solutions.
Some of the reversion products were detected in our experiments (Figure 4.9).
Gentiobiose and levoglucosan were identified in the product mixture from the
retention times of their pure compound, i.e., 8.4 min (gentiobiose) and 13.6 min
73
Chapter 4
(levoglucosan). Isomaltose (7.7 min) and 1,6-anhydro-β-D-glucofuranose (12.6
min) were identified using LC-MS.
Glucose
Gentiobiose
1,6-Anhydro-β-D-glucofuranose
Levoglucosan
Isomaltose
0
4
8
12
16
20
Retention time / min
Figure 4.9 Identification of reversion products (CGLC,0 = 1 M, CH2SO4 = 0.1 M, T =
170 °C, t = 10 min).
The reversion products were observed at the initial stage of the reactions. At
full glucose conversion, reversion products were absent. The maximum
concentrations of the reversion products in the course of the reaction were very
low which made it very difficult to determine the concentrations of every
component accurately. Therefore, we have not incorporated the reversion
products in the kinetic model.
On the basis of these considerations, the following kinetic model (Scheme 4.4)
was applied to model the acid-catalysed decomposition of glucose.
H
OH
H
HO
HO
O
H
OH
H
R2G
Humins
R1G
OH
O
R1H
HO
CHO
O
OH
H3 C
O
+
H
OH
O
R2H
Humins
Scheme 4.4 Reaction network for the acid-catalysed decomposition of HMF to LA.
The reaction rates were defined using a power-law approach:
74
R1G = k1G (C GLC )aG
(4.7)
R2G = k 2G (C GLC )bG
(4.8)
A Kinetic Study on the Conversion of Glucose to Levulinic Acid
R1H = k1H (C HMF )aH
(4.9)
R2H = k 2H (C HMF )bH
(4.10)
The temperature dependence of the kinetic constants was defined in terms of
modified Arrhenius equations:
k1G = (C H + ) k1RG exp
αG
k 2G = (C H + ) k 2RG exp
βG
⎡ E1G ⎛ T −TR
⎢
⎜⎜
⎢⎣ R ⎝ TR T
⎞⎤
⎟⎟ ⎥
⎠ ⎥⎦
⎡ E2G ⎛ T −TR
⎜⎜
⎢
⎣⎢ R ⎝ TRT
⎞⎤
⎟⎟ ⎥
⎠ ⎦⎥
⎡ E1H ⎛ T −TR
⎜⎜
⎢
⎣⎢ R ⎝ TR T
⎞⎤
⎟⎟ ⎥
⎠ ⎦⎥
⎡ E2H ⎛ T −TR
⎜⎜
⎢
⎣⎢ R ⎝ TRT
⎞⎤
⎟⎟ ⎥
⎠ ⎦⎥
k1H = (C H + ) k1RH exp
αH
k 2H = (C H + ) k 2RH exp
βH
(4.11)
(4.12)
(4.13)
(4.14)
where T is a function of time defined in equation (4.3), and TR is the reference
temperature (140 °C).
The catalytic effect of sulphuric acid is included in reaction rates in term of CH+,
which can be calculated as follow:
C H + = C H 2 SO 4 +
(
1
− K a,HSO − + K a,2 HSO − + 4C H 2 SO 4 K a,HSO −
4
4
4
2
)
(4.15)
The term Ka,HSO4− in equation (4.15) represents the dissociation constant of (HSO4)−,
which ranges between 10-4.5−10-3.6 in the temperature window of 140−200 °C [34].
The kinetic constants and the reaction orders for the decomposition to HMF to
LA and FA have been determined earlier in our previous study [10]. The results
are given in Table 4.2. These values were used as input for the kinetic model for
glucose decomposition.
Table 4.2 Kinetic parameter estimates for the HMF decomposition to LA.
Parameter
k1RH
(M1-aH-αH
Estimate
min-1) a
110.5 ± 0.7
E1H (kJ mol-1)
k2RH (M1-bH-βH
a
0.340 ± 0.010
min-1) a
0.117 ± 0.008
E2H (kJ mol-1)
111 ± 2.0
aH (−)
0.88 ± 0.01
bH (−)
1.23 ± 0.03
αH (−)
βH (−)
1.38 ± 0.02
1.07 ± 0.04
TR = 140 °C
75
Chapter 4
In a batch system, the concentrations of the compound involved in
decomposition reaction can be represented as follow:
dC GLC
= −(R1G + R2G )
dt
(4.16)
dC HMF
= R1G − (R1H + R2H )
dt
(4.17)
dC LA
= R1H
dt
(4.18)
4.3.3
Modelling results
The best estimates of the kinetic parameters, as determined by minimization of
the errors between all experimental data and the kinetic model, are shown in Table
4.3. The experimental data consisted of 660 data points (22 experiments, 10
samples per experiment, concentrations of LA, HMF and glucose for each sample).
Comparisons of the experimental data and the output of the kinetic model
demonstrate a good fit for a broad range of reaction condition (Figure 4.10). A
parity chart (Figure 4.11) shows the goodness-of-fit between the experimental and
model data.
Table 4.3 Kinetic parameter estimates for the glucose decomposition to LA.
Parameter
k1RG
(M1-aG-αG
Estimate
min-1) a
152.2 ± 0.7
E1G (kJ mol-1)
k2RG
(M1-bG-βG
E2G (kJ
a
mol-1)
0.013 ± 0.001
min-1) a
0.013 ± 0.001
164.7 ± 0.6
aG (−)
1.09 ± 0.01
bG (−)
1.30 ± 0.02
αG (−)
1.13 ± 0.01
βG (−)
1.12 ± 0.02
TR = 140 °C
76
0.6
0.6
0.10
0.5
0.5
0.08
0.4
0.4
0.06
0.3
0.3
C/M
0.12
C/M
C/M
A Kinetic Study on the Conversion of Glucose to Levulinic Acid
0.04
0.2
0.2
0.02
0.1
0.1
0.00
0
60
120
180
240
300
360
0.0
420
0
20
40
60
t / min
0.0
120
2
1.0
0.8
0.08
0.8
0.6
0.06
0.6
C/M
0.10
C/M
1.0
0.4
0.04
0.4
0.2
0.02
0.2
6
9
12
0.00
15
0
3
6
t / min
9
12
15
0.0
0
1
2
(e) T = 200 C, CH SO = 0.05 M, CGLC,0 = 0.1 M
2
4
10
12
4
5
6
7
t / min
o
o
8
4
3
t / min
(d) T = 171 C, CH SO = 0.5 M, CGLC,0 = 1.0 M
2
6
2
1.2
3
4
o
4
0.12
0
2
(c) T = 171 C, CH SO = 1.0 M, CGLC,0 = 0.5 M
(b) T = 140 C, CH SO = 1.0 M, CGLC,0 = 0.5 M
4
1.2
0.0
0
t / min
o
o
C/M
100
t / min
(a) T = 140 C, CH SO = 0.1 M, CGLC,0 = 0.1 M
2
80
4
o
(f) T = 200 C, CH SO = 0.5 M, CGLC,0 = 1.0 M
2
4
Figure 4.10 Comparison of experimental data (…: CGLC, {: CHMF, U: CLA) and
kinetic model (solid lines).
100
XGLC
YLA
Predicted value (mol %)
80
60
40
20
0
0
20
40
60
80
100
Experimental value (mol %)
Figure 4.11 Parity plot of all experimental data and model prediction.
77
Chapter 4
4.4 Application of the kinetic model
4.4.1
Batch simulation and optimisation
With the model available, it is possible to gain insight into the conversion,
selectivity and yield of LA as a function of the process conditions. A typical batch
time for 90 mol % glucose conversion as a function of the temperature is given in
Figure 4.12.
5
10
4
10
batch time / min
3
10
2
10
1
10
0
10
-1
10
100
120
140
160
180
200
o
T/ C
Figure 4.12 Reaction time to achieve 90 mol % of glucose conversion in isothermal
batch reactor as a function of temperatures (CGLC,0 = 0.1 M, CH2SO4 = 1 M).
The kinetic model also allows the determination of the optimum reaction
conditions to achieve the highest selectivity of LA. For this purpose equation (4.4)
is differentiated to give:
dX GLC = −
dC GLC
C GLC,0
(4.19)
Combination of equations (4.16)–(4.19) leads to the following expressions:
dC GLC
= −C GLC,0
dX GLC
(4.20)
dC HMF R1G − R1H − R2H
=
C GLC,0
dX GLC
R1G + R2G
(4.21)
R1H
dC LA
=
C GLC,0
dX GLC R1G + R2G
(4.22)
Equations (4.20)–(4.22) were solved using the numerical integration toolbox
ode45 in the MATLAB software package from 0 to 90 mol % glucose conversion.
The selectivity of LA (σ LA) is defined as the ratio of the amount of the desired
product (LA) formed and the key reactant (glucose) converted.
78
A Kinetic Study on the Conversion of Glucose to Levulinic Acid
σ LA =
C LA − C LA,0
Y
= LA
C GLC,0 − C GLC X GLC
(4.23)
Figure 4.13 shows the predicted σ LA as a function of the T and CGLC,0 at 90 mol
% glucose conversion and a CH2SO4 of 0.5 M. The experimental data points are also
given, demonstrating the good fit between experiments and model.
100
CGLC,0 = 0.1 M
CGLC,0 = 0.5 M
CGLC,0 = 1.0 M
60
40
σ
LA
/ mol %
80
20
0
100
120
140
160
180
200
T / °C
Figure 4.13. Temperature effect on σ LA at XGLC = 90 mol % and CH2SO4 = 0.5 M.
Symbols (…), ({) and (U) represent experimental values of σ LA at CGLC,0 = 0.1, 0.5
and 1.0 M respectively.
The σ LA is strongly temperature depending, with high temperatures leading to
lower selectivity. This is in line with the observed activation energies for the main
and side reactions. The activation energy for humins formation from glucose
(Table 4.3) is significantly higher (164.7 kJ mol–1) than all other activation energies,
implying that the kinetics of this reaction is the most sensitive to temperature. To
reduce humins formation, reactions at low temperature are favoured. It is also
evident that higher CGLC,0 will lead to lower σ LA. This may be rationalised when
looking at the orders in substrate for the various reactions involved. The order of
glucose for the desired reaction to HMF (1.09) is lower than that of the side
reaction to humins (1.30), hence a higher CGLC,0 will lead to reduced σ LA.
4.4.2
Optimisation of continuous reactor systems
The yield of LA in continuous reactors will be a function of typical process
parameters (T, CGLC,0 and CH2SO4) and the extent of mixing in the reactor. In Figure
4.14, the yield of LA as a function of the glucose conversion at different
temperatures (140 and 200 °C) is provided for the two extremes with respect to
mixing, i.e., a plug-flow reactor (PFR) and a continuous ideal stirred tank reactor
(CISTR).
79
Chapter 4
100
100
o
T = 140 C
80
80
60
60
ΨLA / mol %
ΨLA / mol %
PFR
CISTR
40
20
0
o
T = 200 C
PFR
CISTR
40
20
0
20
40
60
80
XGLC / mol %
100
0
0
20
40
60
80
100
XGLC / mol %
Figure 4.14 Comparison of LA yields in two ideal continuous reactors at different
temperatures (CGLC,0 = 0.1 M, CH+ = 0.5 M).
Here the yield of LA, Ψ LA, is defined as the ratio between the amounts of LA
formed during the reaction and of glucose fed into the reactor.
Ψ LA =
out
in
C LA
− C LA
in
C GLC
(4.24)
The graphs were constructed from the mass balance design equations for the
two model reactors in combination with the rate equations for the reactions. The
reactor design equations of the PFR are similar to the design equations for the
batch reactor (equations (4.19)–(4.22)). The general reactor design equation for a
CISTR reads:
τ CISTR =
C iout − C iin
Ri
(4.25)
The relationship between glucose conversion (XGLC) and τ CISTR is given by the
following equation:
τ CISTR
in
X GLCC GLC
=
R1G + R2G
(4.26)
Substitution of equation (4.26) into equation (4.25) and executing some
rearrangement gives:
80
⎛ R − R1H − R2H ⎞ in
out
⎟⎟C GLC X GLC
= ⎜⎜ 1G
C HMF
+
R
R
1G
2G
⎝
⎠
(4.27)
⎛ R1H ⎞ in
out
⎟⎟C GLC X GLC
= ⎜⎜
C LA
+
R
R
2G ⎠
⎝ 1G
(4.28)
A Kinetic Study on the Conversion of Glucose to Levulinic Acid
Based on the results shown in Figure 4.14 it is clear that the LA yields increases
with the glucose conversion and that the yields in a CISTR are higher than in a
PFR, particularly at high conversion levels. The yields at low temperature are
higher than the yields at high temperature for both reactor configurations.
To select the optimum operating conditions for the reactor, it is also necessary
to consider the full process configuration. If a high glucose conversion is desired,
e.g., when the separation of the LA from the glucose/humins/sulphuric acid
mixture is difficult, it might be advantageous to apply a reactor with a high extent
of backmixing (Figure 4.14). A number of options are available like a stirred tank
reactor equipped with an impeller or a recycle reactor with a high recycle ratio. An
important feature will be the scaling properties of the insoluble, humins byproducts. However, information on this topic is lacking and further research will
be required. In case separation of the product mixture is simple and cheap, it
might be advantageous to operate at relatively low conversions of glucose to
reduce reactor volume and associated costs. At low conversions, the yield is not a
strong function of the extent of back-mixing (Figure 4.14) and other reactor
configurations may be applied as well.
4.5 Conclusions
A kinetic model for the acid-catalysed decomposition of glucose in a broad
operating window (CH2SO4= 0.05–1 M, CGLC,0 = 0.1–1 M, T = 140–200 °C) has been
developed. Glucose decomposes in a consecutive reaction mode to give LA as the
final product through HMF as the intermediate. Glucose as well as HMF
decomposes in parallel reaction modes to give insoluble humins as the byproduct. The model implies that the highest yield of LA in continuous reactor
configurations may be achieved by applying dilute solution of glucose, a high
concentration of sulphuric acid as the catalyst and using a reactor configuration
with a high extent of back-mixing.
4.6 Nomenclature
aG
:
Reaction order of CGLC in the decomposition of glucose to HMF (−)
αG
:
Reaction order of CH+ in the decomposition of glucose to HMF (−)
aH
:
Reaction order of CHMF in the decomposition of HMF to LA and FA (−)
αH
:
Reaction order of CH+ in the decomposition of HMF to LA and FA (−)
At
:
Heat transfer area (m2)
bG
:
Reaction order of CGLC in the decomposition of glucose to humins (−)
βG
:
Reaction order of CH+ in the decomposition of glucose to humins (−)
bH
:
Reaction order of CHMF in the decomposition of HMF to humins (−)
βH
:
Reaction order of CH+ in the decomposition of HMF to humins (−)
CGLC
:
Concentration of glucose (M)
81
Chapter 4
CGLC,0
:
Initial concentration of glucose (M)
CH+
:
Concentration of H+ (M)
CH2SO4
:
Concentration of sulphuric acid (M)
CHMF
:
Concentration of HMF (M)
CHMF,0
:
Initial concentration of HMF (M)
C iin
:
Concentration of the ith compound at the inflow (M)
C iout
:
Concentration of the ith compound at the outflow (M)
CLA
:
Concentration of LA (M)
CLA,0
:
Initial concentration of LA (M)
Cp
:
Heat capacity of reaction mixture (J g-1 K-1)
E1G
:
Activation energy of k1G (kJ mol-1)
E1H
:
Activation energy of k1H (kJ mol-1)
E2G
:
Activation energy of k2G (kJ mol-1)
E2H
h
:
Activation energy of k2H (kJ mol-1)
:
Heat transfer coefficient from the oven to the reaction mixture (min-1)
k1G
:
Reaction rate constant of glucose decomposition to HMF (M1-aG min-1)
k1RG
:
Reaction rate constant k1G at reference temperature (M1-aG-αG min-1)
k1H
:
Reaction rate constant of HMF for the main reaction (M1-aH min-1)
k1RH
:
Reaction rate constant k1H at reference temperature (M1-aH-αH min-1)
k2G
:
Reaction rate constant of glucose decomposition to humins (M1-bG min-1)
k2RG
:
Reaction rate constant k2G at reference temperature (M1-bG-βG min-1)
k2H
:
Reaction rate constant of HMF for the side reaction to humins (M1-bH min-1)
k2RH
:
Reaction rate constant k2H at reference temperature (M1-bH-βH min-1)
Ka,HSO4−
:
Dissociation constant of (HSO4)- (−)
M
:
Mass of the reaction mixture (g)
R
:
Universal gas constant, 8.3144 J mol-1 K-1
R1G
:
Reaction rate of glucose decomposition to HMF (mol L-1 min-1)
R1H
:
Reaction rate of HMF decomposition to LA and FA (mol L-1 min-1)
R2G
:
Reaction rate of glucose decomposition to humins (mol L-1 min-1)
R2H
t
:
Reaction rate of HMF decomposition to humins (mol L-1 min-1)
:
Time (min)
T
:
Reaction temperature (°C)
Ti
:
Temperature of reaction mixture at t = 0 (°C)
Toven
:
Temperature of oven (°C)
TR
:
Reference temperature (°C)
U
:
Overall heat transfer coefficient (W m-2 K-1)
wH2SO4
:
Weight percentage of sulphuric acid (%)
XGLC
:
Conversion of glucose (mol %)
YHMF
:
Yield of HMF (mol %)
YLA
:
Yield of LA (mol %)
82
A Kinetic Study on the Conversion of Glucose to Levulinic Acid
Greek symbols
σ LA
τ CISTR
:
Selectivity of LA (mol %)
:
Residence time of CISTR (min)
Ψ LA
:
Yield of LA in continuous reactors (mol %)
4.7 References
[1]
Leonard, R. H., Levulinic Acid as a Basic Chemical Raw Material. Ind. Eng. Chem. 1956, 48 (8),
1330-1341.
[2]
Kitano, M.; Tanimoto, F.; Okabayashi, M., Levulinic acid, a new chemical raw material; its
chemistry and use. Chem. Econ. Eng. Rev. 1975, 7, 25-29.
[3]
Thomas, J. J.; Barile, G. R. In Conversion of cellulose hydrolysis products to fuels and chemical
feedstocks., 8th Symposium on Energy from Biomass and Waste, Lake Buena Fista, FL, 1984;
pp 1461-1494.
[4]
Ghorpade, V. M.; Hanna, M. A., Industrial applications for levulinic acid. In Cereal Novel Uses
and Processes., Campbell, G. M.; Webb, C.; McKee, S. L., Eds. Plenum Press: New York, 1997;
pp 49-55.
[5]
Timokhin, B. V.; Baransky, V. A.; Eliseeva, G. D., Levulinic Acid in Organic Synthesis. Russ.
Chem. Rev. 1999, 68 (1), 73-84.
[6]
Bozell, J. J.; Moens, L.; Elliott, D. C.; Wang, Y.; Neuenscwander, G. G.; Fitzpatrick, S. W.;
Bilski, R. J.; Jarnefeld, J. L., Production of levulinic acid and use as a platform chemical for
derived products. Resour. Conserv. Recycl. 2000, 28, 227-239.
[7]
Werpy, T.; Petersen, G. Top Value Added Chemicals from Biomass Volume I-Results of Screening
for Potential Candidates from Sugars and Synthesis Gas.; NREL/TP-510-35523; National
Renewable Energy Laboratory (NREL): 2004.
[8]
Grethlein, H. E., Chemical Breakdown of Cellulosic Materials. J. Appl. Chem. Biotechnol. 1978,
28, 296-308.
[9]
Danon, B.; Girisuta, B.; Heeres, H. J., A Kinetic Study of the Acidic Hydrolysis of Water
Hyacinth to Levulinic Acid. In University of Groningen: Groningen (Netherlands), 2005.
[10] Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., A Kinetic Study on the Decomposition of 5Hydroxymethylfurfural into Levulinic Acid. Green Chem. 2006, 8, 701-709.
[11] Saeman, J. F., Kinetics of Wood Saccharification - Hydrolysis of Cellulose and Decomposition
of Sugars in Dilute Acid at High Temperature. Ind. Eng. Chem. 1945, 37, 43-52.
[12] Heimlich, K. R.; Martin, A. N., A Kinetic Study of Glucose Degradation in Acid Solution. J.
Am. Pharm. Assoc. 1960, 49, 592-597.
[13] McKibbins, S.; Harris, J. F.; Saeman, J. F.; Neill, W. K., Kinetics of the Acid Catalyzed
Conversion of Glucose to 5-Hydroxymethyl-2-furaldehyde and Levulinic Acid. Forest Prod. J.
1962, 12, 17-23.
[14] Fagan, R. D.; Grethlein, H. E.; Converse, A. O.; Porteus, A., Kinetics od the Acid Hydrolysis
of Cellulose Found in Paper Refuse. Environ. Sci. Technol. 1971, 5, 545-547.
[15] Smith, P. C.; Grethlein, H. E.; Converse, A. O., Glucose Decomposition at High-Temperature,
Mild Acid, and Short Residence Times. Solar Energy 1982, 28, 41-48.
83
Chapter 4
[16] Bienkowski, P. R.; Ladisch, M. R.; Narayan, R.; Tsao, G. T.; Eckert, R., Correlation of Glucose
(Dextrose) Degradation at 90 to 190-Degrees-C in 0.4 to 20-Percent Acid. Chem. Eng. Comm.
1987, 51, 179-192.
[17] Baugh, K. D.; Mccarty, P. L., Thermochemical Pretreatment of Lignocellulose to Enhance
Methane Fermentation .1. Monosaccharide and Furfurals Hydrothermal Decomposition and
Product Formation Rates. Biotechnol. Bioeng. 1988, 31, 50-61.
[18] Bergeron, P.; Benham, C.; Werdene, P., Dilute Sulfuric-Acid Hydrolysis of Biomass for
Ethanol-Production. Appl. Biochem. Biotechnol. 1989, 20-1, 119-134.
[19] Mosier, N. S.; Ladisch, C. M.; Ladisch, M. R., Characterization of acid catalytic domains for
cellulose hydrolysis and glucose degradation. Biotechnol. Bioeng. 2002, 79, 610-618.
[20] Xiang, Q.; Lee, Y. Y.; Torget, R. W., Kinetics of glucose decomposition during dilute-acid
hydrolysis of lignocellulosic biomass. Appl. Biochem. Biotechnol.. 2004, 113-16, 1127-1138.
[21] Bard, Y., Nonlinear Parameter Estimation. Academic Press: New York, 1974; p 61-71.
[22] Knightes, C. D.; Peters, C. A., Statistical analysis of nonlinear parameter estimation for
Monod biodegradation kinetics using bivariate data. Biotechnol. Bioeng. 2000, 69, 160-170.
[23] Horvat, J.; Klaic, B.; Metelko, B.; Sunjic, V., Mechanism of levulinic acid formation.
Tetrahedron Lett. 1985, 26 (17), 2111-2114.
[24] Harris, D. W.; Feather, M. S., Evidence for A C-2-->C-1 Intramolecular Hydrogen Transfer
During Acid-Catalyzed Isomerization of D-Glucose to D-Fructose. Carbohydr. Res. 1973, 30,
359-365.
[25] Harris, D. W.; Feather, M. S., Intramolecular C-2-->C-1 Hydrogen Transfer-Reactions During
Conversion of Aldoses to 2-Furaldehydes. J. Org. Chem. 1974, 39, 724-725.
[26] Harris, D. W.; Feather, M. S., Studies on Mechanism of Interconversion of D-Glucose, DMannose, and D-Fructose in Acid Solution. J. Am. Chem. Soc. 1975, 97, 178-182.
[27] van Dam, H. E.; Kieboom, A. P. G.; van Bekkum, H., The Conversion of Fructose and Glucose
in Acidic Media - Formation of Hydroxymethylfurfural. Starch-Starke 1986, 38, 95-101.
[28] Moreau, C.; Durand, R.; Razigade, S.; Duhamet, J.; Faugeras, P.; Rivalier, P.; Ros, P.; Avignon,
G., Dehydration of fructose to 5-hydroxymethylfurfural over H-mordenites. Appl. Catal. A:
General 1996, 145, 211-224.
[29] Kuster, B. F. M.; van der Baan, H. S., Dehydration of D-Fructose (Formation of 5Hydroxymethyl-2-Furaldehyde and Levulinic Acid) .2. Influence of Initial and Catalyst
Concentrations on Dehydration of D-Fructose. Carbohydr. Res. 1977, 54, 165-176.
[30] Kuster, B. F. M., 5-Hydroxymethylfurfural (HMF) - A Review Focusing on Its Manufacture.
Starch-Starke 1990, 42, 314-321.
[31] Helm, R. F.; Young, R. A.; Conner, A. H., The Reversion Reactions of D-Glucose During the
Hydrolysis of Cellulose with Dilute Sulfuric-Acid. Carbohydr. Res. 1989, 185, 249-260.
[32] Thompson, A.; Anno, K.; Wolfrom, M. L.; Inatome, M., Acid Reversion Products from DGlucose. J. Am. Chem. Soc. 1954, 76, 1309-1311.
[33] Peat, S.; Whelan, W. J.; Edwards, T. E.; Owen, O., Quantitative Aspects of the Acid Reversion
of Glucose. J. Chem. Soc. 1958, 586-592.
[34] Dickson, A. G.; Wesolowski, D. J.; Palmer, D. A.; Mesmer, R. E., Dissociation-Constant of
Bisulfate Ion in Aqueous Sodium-Chloride Solutions to 250-Degrees-C. J. Phys. Chem. 1990,
94, 7978-7985.
84
Chapter 5
A Kinetic Study on the Acid-catalysed
Hydrolysis of Cellulose to
Levulinic Acid*
Abstract
A variety of interesting bulk chemicals is accessible by the acid-catalysed
hydrolysis of cellulose. An interesting example is levulinic acid, a versatile
precursor for fuel additives, polymers and resins. A detailed kinetic study on the
acid-catalysed hydrolysis of cellulose to levulinic acid is reported in this paper.
The kinetic experiments were performed in a temperature window of 150−200 °C,
sulphuric acid concentrations between 0.05 and 1 M, and initial cellulose intakes
between 1.7 and 14 wt %. The highest yield of levulinic was 60 mol %, obtained at
a temperature of 150 °C, an initial cellulose intake of 1.7 wt %, and a sulphuric acid
concentration of 1 M. A full kinetic model covering a broad range of reaction
conditions was developed using the power-law approach. Agreement between the
experimental data and kinetic model is good. The kinetic expressions were used to
gain insights into the optimum process condition for the conversion of cellulose to
levulinic acid in continuous-reactor configurations. The model predicts that the
highest obtainable levulinic acid yield in continuous reactor configurations is
about 76 mol %, which was obtained when using reactors with a large extent of
back-mixing.
Keyword: cellulose, acid-catalysed hydrolysis, levulinic acid, green-chemicals.
Based on: Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., Kinetic study on the acid-catalyzed
hydrolysis of cellulose to levulinic acid. Ind. Eng. Chem. Res. 2007, 46, 1696–1708.
*
Chapter 5
5.1 Introduction
Cellulose is a natural polymer consisting of glucose units. It is abundantly
available on earth, and its annual production is estimated at 2×109 tons [1].
Cellulose may be converted to interesting bulk chemicals by acid-catalysed
hydrolysis reaction. During hydrolysis, the β-(1→4)-glycosidic bonds of cellulose
are cleaved to give glucose, that can be converted further to various organic (bulk)
chemicals. One attractive option is the conversion of glucose to levulinic acid (4oxopentanoic acid) by acid treatment. Levulinic acid is a versatile building block
for fuel additives, polymer- and resin-precursors [2]. Several reviews have been
published describing the properties and potential industrial applications of
levulinic acid and its derivatives [3-6].
Two different approaches are commonly applied for the acid-catalysed
hydrolysis of cellulose. The first uses high concentrations of mineral acids (e.g.,
15−16 N HCl or 31−70 wt % H2SO4) as catalysts and low operating temperatures
(20−50 °C) [7,8]. The major drawbacks are the high operating cost of acid recovery
and the use of expensive construction material for both the hydrolyser and the
acid recovery system. The second-approach uses highly diluted acids at high
operating temperatures (170−240 °C). This method is favoured and research
studies applying this approach are abundant [9-12].
Various kinetic studies on the acid-catalysed hydrolysis using a range of
cellulosic materials have been reported in the literature. The first systematic
kinetic study on biomass hydrolysis to glucose was performed in 1945 by Saeman
[13], who studied the hydrolysis reaction of Douglas fir in batch reactors. In this
study, the hydrolysis reaction is modelled by the following two consecutive firstorder reactions:
1
2
Cellulose ⎯⎯→
Glucose ⎯⎯→
Decomposition products of glucose
k
k
(5.1)
The reaction rate constants are represented by modified Arrhenius equations,
including the effects of temperature (T) and acid concentration (A).
− Ei
ki = ki,o Ami exp RT
i = 1,2
(5.2)
Here, ki,o is the frequency factor, mi is the reaction order in acid, R is the ideal gas
constant and Ei is the activation energy.
Further investigations were conducted by Fagan and co-workers [14] on Kraft
paper slurries. A nonisothermal plug-flow reactor was used to determine the
kinetics of the hydrolysis reaction. Further studies were performed on Solka-Floc
[15] and filter paper [16] in an isothermal plug-flow reactor. Malester and coworkers [17,18] carried out the kinetic studies using municipal solid waste (MSW)
as the cellulose source. The experiments were carried out in a 2-L batch steel
reactor using sulphuric acid at low concentrations as the catalyst. All these kinetic
studies applied the kinetic model developed earlier by Saeman [13] to analyse the
kinetic data. An overview of kinetic studies including the range of process
conditions and intakes is given in Table 5.1. For cellulose decomposition to
86
A Kinetic Study on the Acid-Catalysed Hydrolysis of Cellulose to Levulinic Acid
glucose, the activation energy is between 172 and 189 kJ mol-1. However, large
variations are observed in the order of acid concentration (1.0−1.78). A similar
observation also holds for the decomposition of glucose to (non-identified)
products, where the order in acid concentration varies between 0.55 and 1.02.
Table 5.1 Literature overview of kinetic parameters for the acid-catalysed
hydrolysis of cellulose.
Cellulose hydrolysis
Glucose decomposition
Substrate
k1,o
(min-1)
m1
E1
(kJ mol-1)
k2,o
(min-1)
m2
E2
(kJ mol-1)
Reaction conditions
Douglas fir
[13]
1.73×1019
1.34
179.5
2.38×1014
1.02
137.5
T = 170−190 °C
Csubstrate = 10 wt %
A = 0.4−1.0 wt %
Kraft paper
slurries [14]
28×1019
1.78
188.7
4.9×1014
0.55
137.2
T = 180−240 °C
Csubstrate = 2.5 wt %
A = 0.2−1.0 wt %
Solca-Floc
[15]
1.22×1019
1.16
177.8
3.79×1014
0.69
136.8
T = 180−240 °C
Csubstrate = 10 wt %
A = 0.5−2.0 wt %
Filter paper
[16]
1.22×1019
1.16
178.9
3.79×1014
0.69
137.2
T = 200−240 °C
Csubstrate = 2 wt %
A = 0.4−1.5 wt %
MSW [18]
1.16×1019
1.0
171.7
4.13×1015
0.67
142.4
T = 200−240 °C
Csubstrate = 1 wt %
Acid = 1.3−4.4 wt %
The acid-catalysed hydrolysis of cellulose is a heterogeneous reaction where
mass transfer effects may play an important role and under some conditions may
even determine the overall reaction rate. As such, the dimensions of the cellulosic
materials and their properties (e.g., crystallinity of the cellulose fraction) may have
significant effects on the overall rate of the hydrolysis reaction. Mass transfer
effects on the overall rate of the hydrolysis reaction of cellulose were investigated
by Saeman [13] by conducting the reaction with various cellulose particle sizes.
The hydrolysis reaction rate was unaffected when using particle sizes in the range
of 20−200 mesh (74−840 µm). Similar results were obtained by Malester and coworkers [18]. These results imply that under these conditions, the hydrolysis
reaction of cellulose can be treated as a homogenous reaction when the particle
size of cellulose is < 20 mesh (840 µm). Sharples [19,20] proposed a kinetic model
including the effects of the degree of crystalline of the cellulose on the reaction
rate. The cellulose applied in this study was pretreated with 18 wt % of sodium
hydroxide solution for 48 h at room temperature. A kinetic model with an inverse
relation between the hydrolysis reaction rate constant and the mean length of the
crystalline domains of the cellulose was proposed. Later investigations [21,22]
have shown that the Sharples model is not valid for virgin, untreated cellulose.
87
Chapter 5
All previous kinetic studies mainly focused on the optimisation of glucose
production. Only a few kinetic reports [23-25] are available for the acid-catalysed
hydrolysis of cellulose to levulinic acid. A complete kinetic model describing the
acid-catalysed hydrolysis of cellulose to levulinic acid including by-product
formation and covering a broad range of reaction conditions and intakes is
lacking. In addition, the acid-catalysed decomposition reactions of glucose and
HMF produce insoluble-solid product known as humins. These humins are
expected to be formed as well as when reacting cellulose with acids in an aqueous
environment. However, humins formation has never been included in the kinetic
models reported to date. A systematic kinetic study on the acid-catalysed
hydrolysis of cellulose to levulinic acid using sulphuric acid as the catalyst was
reported here. The effects of temperature, acid concentration and initial intake of
cellulose on the yield of levulinic acid were assessed, and a kinetic model
including humins formation and covering a wide range of reaction conditions was
developed. The results were applied to optimise the production of levulinic acid in
various reactor configurations.
5.2 Materials and methods
5.2.1
Chemicals
All chemicals used in this study were of analytical grade and used without
purification. Microcrystalline cellulose [9004-34-6] with an average particle size of
20 µm was purchased from Sigma-Aldrich. Concentrated sulphuric acid 95−97 wt
% [7664-93-9], glucose [14431-43-7] and formic acid [64-18-6] were purchased from
Merck GmbH (Darmstadt, Germany); 5-hydroxymethylfurfural [67-47-0] and
levulinic acid 98 wt % [123-76-2] were obtained from Acros Organics (Geel,
Belgium). Deionised water was applied to prepare the various solutions.
5.2.2
5.2.2.1
Experimental procedures
Kinetic experiments
The reactions were carried out in two types of glass ampoules with a wall
thickness of 1.5 mm and a length of 15 cm, differing in internal diameter (3 and 5
mm). The ampoules were filled with the predetermined amount of cellulose.
Subsequently, the acid-catalyst solution (0.2−0.5 cm3) was added. The ampoules
were sealed with a torch. The sealed ampoules were placed in a constanttemperature oven (± 1 °C). At various reaction times, ampoules were taken from
the oven and quenched in an ice-water bath (4 °C) to stop the reaction. The
ampoule was opened, and the liquid was separated from the solids using a microcentrifuge (Omnilabo International BV) for approximately 15−20 minutes at 1200
rpm. A certain amount of the clear solution was taken (100−200 µL) and diluted
with water (2 cm3). The composition of the solution was determined using high
performance liquid chromatography (HPLC).
88
A Kinetic Study on the Acid-Catalysed Hydrolysis of Cellulose to Levulinic Acid
The composition of the gas phase after the reaction was determined using GCMS. Gas samples were obtained by placing an ampoule in an air-tight plastic bag.
The plastic bag was flushed with helium and placed under vacuum. Subsequently,
the glass ampoule was broken, and the released gas was mixed with 10 cm3 of
helium gas.
The solid products were washed with water several times and dried overnight
in the oven at a temperature of 60 °C. The elemental composition of the dried solid
products was determined using elemental analysis. The particle structure of the
solid products was analysed using scanning electron microscope (SEM).
5.2.2.2
Heat-transfer experiments
At the start-up of the reaction, the ampoules were placed in a constanttemperature oven and the contents were heated-up to the pre-determined oven
temperature. To determine the temperature profile at the start of the reaction and
to compensate for this non-isothermal behaviour in the kinetic modelling, the
temperature inside the ampoule during the heating-up phase was determined
experimentally. For this purpose, a special ampoule with a thermocouple was
developed. The ampoule was filled with a representative reaction mixture
(without catalyst) and closed tightly using a special bolt-and-screw system to
prevent evaporation of the liquid. The ampoule was subsequently placed in the
oven and the temperature of reaction mixture was followed in time. Before and
after each experiment, the amount of liquid inside the ampoule was measured to
determine the amount of evaporation. In all cases, the loss of water was less than 1
wt %, indicating that the results were not biased by water evaporation.
The experimental profiles at different temperatures were modelled using a heat
balance for the contents in an ampoule:
(
d MC pT
dt
)
= UAt (Toven − T )
(5.3)
When assuming that the heat capacity of reaction mixture is constant and not a
function of temperature, rearrangement of equation (5.3) will give:
dT UAt
(Toven − T ) = h(Toven − T )
=
dt MC p
(5.4)
Solving the ordinary differential equation (5.4) with the initial value T = Ti at t = 0
leads to:
T = Toven − (Toven − Ti ) exp − ht
(5.5)
The value of h was determined by fitting all experimental data at different oven
temperatures (100−160 °C) using a non-linear regression method and was found to
be 0.596 min-1 (for small ampoules) and 0.359 min-1 (for large ampoules). Figure
5.1 shows an experimental and modelled temperature profile performed at an
oven temperature of 140 °C using both types of ampoules. Equation (5.5) was
89
Chapter 5
incorporated in the kinetic model to describe the non-isothermal behaviour of the
system at the start-up of the reaction.
180
Experimental data (small ampoules)
Experimental data (large ampoules)
Model according to equation (5.5)
160
140
T/K
120
100
80
60
40
20
0
2
4
6
8
10
12
14
16
t / min
Figure 5.1 Heating profile of the reaction mixture at Toven = 140 °C for both types
of ampoules.
5.2.3
Method of analyses
The composition of the liquid phase was determined using an HPLC system
consisting of a Hewlett Packard 1050 pump, a Bio-Rad organic acids column
Aminex HPX-87H and a Waters 410 refractive index detector. The mobile phase
consisted of an aqueous solution of sulphuric acid (5 mM) at flow rate of 0.55 cm3
min-1. The column was operated at 60 °C. The analysis for a sample was complete
within 55 min. The concentration of each compound in the liquid phase was
determined using calibration curves obtained by analysing standard solutions
with known concentrations.
The gas composition was analysed with GC-MS, which consists of a HP 5890
Series II gas chromatography and a HP 6890 detector. The composition of the gas
phase was determined using a CP-Porabond-Q column (length = 25 m and I.D. =
0.25 mm). The oven temperature was set at 40 °C for 2 min and increased to 240 °C
with an increment of 30 °C min-1. Helium was used as the carrier gas with a flow
rate of 1.5 cm3 min-1. Elemental analyses were performed at the Analytical
Department of the University of Groningen using an automated Euro EA3000
CHNS analyser. Solid-products particles were analysed using field emission
scanning electron microscope (FESEM) on a JEOL 6320F.
5.2.4
Determination of the kinetic parameters
The kinetic parameters were estimated using a maximum-likelihood approach,
which is based on the minimization of errors between the experimental data and
the kinetic model. The minimization of errors was initiated by providing initial
90
A Kinetic Study on the Acid-Catalysed Hydrolysis of Cellulose to Levulinic Acid
guesses for each kinetic parameter. The best estimates were obtained using the
MATLAB toolbox fminsearch, which is based on the Nelder-Mead optimisation
method.
The calculation of errors was based on the concentration of glucose (CGLC) and
levulinic acid (CLA). To compensate for the large spread in concentrations, the
concentrations were scaled and transformed to the yields of glucose (YGLC) and
levulinic acid (YLA), respectively. By definition, these vary between 0 and 1 and are
expressed as:
YGLC =
YLA =
C GLC
C CEL,0
(5.6)
C LA
C CEL,0
(5.7)
Here, the CCEL,0 is defined as the initial concentration of cellulose, expressed as the
amount of glucose units present in cellulose, and is determined using the
following relation:
C CEL,0 =
mass of cellulose × wt % of glucose in cellulose
molecular weight of glucose × volume of reaction mixture
(5.8)
5.3 Results and discussions
5.3.1
Reaction products
The generally accepted reaction pathway for the acid-catalysed hydrolysis of
cellulose to levulinic acid is schematically given in Scheme 5.1.
OH
H
H
H
HO
HO
O
HO
O
1
H
H
1
4
H
OH
H
H
H
OH
H
O
H
4
HO
O
H
H
OH
O
O
1
H
HO
4
H
OH
H
OH
H
OH
H
OH
H
H
n
O
OH
1
H+ + n H2O
OH
H
H
HO
HO
H
- 3 H2O
O
OH
H
O
O
2
H+
+ 2 H2O
HO
O
3
CHO
H+
OH
H3C
O
+
OH
H
4
O
5
Scheme 5.1 Acid-catalysed hydrolysis reaction of cellulose to levulinic acid.
91
Chapter 5
In the first step, the polymer chains of cellulose (1) are broken down into low
molecular weight fragments and ultimately to glucose (2) by the action of an acid
catalyst. The glucose is decomposed to 5-hydroxymethylfurfural (HMF, 3), which
is further converted in a serial mode to levulinic acid (LA, 4) and formic acid (FA,
5). All anticipated products (2−5) were detected in this study and identified and
quantified using HPLC analysis. A typical example of an HPLC chromatogram is
given in Figure 5.2.
Glucose
Levulinic Acid
Formic
Acid
HMF
Furfural
0
10
20
30
40
50
t / min
Figure 5.2 Typical HPLC chromatogram for the acid-catalysed hydrolysis of
cellulose (xCEL,0 = 7.7 wt %, CH2SO4 = 0.05 M, T = 200 °C, t = 16 min).
Besides the anticipated products, small amounts of glucose-reversion products
(e.g., levoglucosan, isomaltose or gentiobiose) and furfural were detected in the
liquid phase. The formation of the reversion products was also observed in our
previous study [26] on the acid-catalysed decomposition of glucose. The
maximum amount of glucose-reversion products was very low (less than 0.1 wt
%). The presence of furfural in the reaction mixture is at first-sight surprising. It is
a known product of the acid-catalysed decomposition of C5-sugars and
particularly of xylose (6), as shown in Scheme 5.2 [27-29]. It is likely that the
cellulose applied in this study is contaminated with C5-sugars, producing furfural
(7). Based on the intake of cellulose and the maximum experimentally observed
concentration of furfural, the amount of C5-sugars in the cellulose applied in this
study is about 1 wt %.
O
- 3 H2O
HO
HO
OH
OH
6
H+
O
CHO
7
Scheme 5.2 Acid-catalysed decomposition of xylose to furfural.
92
A Kinetic Study on the Acid-Catalysed Hydrolysis of Cellulose to Levulinic Acid
During all experiments, black insoluble-substances known as humins were
formed. These are well known products of side-reactions of the acid-catalysed
decomposition of glucose and HMF. The presence of these humins was confirmed
by elemental analysis on the solid products present after the reaction. The
elemental composition (in wt %) for a typical product (C, 55.2; H, 4.9) suggests
that the solids are a mixture consisting mainly of humins (typical composition: C,
63.1; H, 4.2) [30] and some unreacted cellulose (C, 42.2; H, 6.1). Further evidence
for the formation of substantial amounts of humins was obtained from SEM.
Typical SEM images of the cellulose particles applied in this study and the solid
products after the reaction are given in Figure 5.3. Clearly visible in the reaction
products are the typical round shaped, agglomerated humins particles with
particle sizes in the range of 5−10 µm. Furthermore, some unreacted cellulose is
also present, in line with the elemental analyses data.
(a)
(b)
Figure 5.3 SEM image of cellulose particles used in this study (a) and the solid
products obtained after the reaction (b).
Other possible by-products of the acid-catalysed hydrolysis of cellulose are
gas-phase components from thermal degradation reactions of reactants and/or
products. To gain insights into the extent of these reactions, the gas phase after the
reaction was analysed using GC and GC-MS. Both CO and CO2 could be detected;
however, the amounts were less than 0.1 wt % of the cellulose intake. This implies
that the formation of gas-phase compounds is only a minor reaction pathway
under the reaction conditions applied in our experiments.
5.3.2
Effects of process variables on the yield of LA
A total of 26 experiments were performed covering a wide range of reaction
conditions. Three operating temperatures (150, 175 and 200 °C) were used. In all
cases, sulphuric acid was used as the catalyst with concentrations varying between
0.05 M and 1 M. The initial intake of cellulose (xCEL,0) was varied between 1.7 and
14 wt %.
93
Chapter 5
0.25
Concentration / M
0.20
CGLC
CFA
CHMF
CFUR
CLA
0.15
0.10
0.05
0.00
0
5
10
15
20
25
t / min
Figure 5.4 Typical concentration profile during acid-catalysed hydrolysis of
cellulose (xCEL,0 = 7.7 wt %, CH2SO4 = 0.05 M, T = 200 °C).
The composition of the reaction mixture was followed in time and a typical
concentration profile is given in Figure 5.4. As anticipated on the basis of Scheme
5.1, the concentrations of both glucose and 5-hydroxymethylfurfural (HMF)
displayed an optimum. The maximum CGLC was 0.15 M, which equals to a glucose
yield (YGLC) of 30 mol %. The maximum CHMF obtained in all experiments were
generally much lower than the maximum CGLC, which indicates that the
conversion of HMF to LA and FA is much faster than the conversion of glucose to
HMF. In line with the reaction stoichiometry given in Scheme 5.1, LA and FA were
always formed in a 1:1 molar ratio. This finding also implies that these compounds
are stable under the reaction conditions employed and do not decompose to other
products.
The yield of LA (YLA) is a clear function of the operating temperature, with
high temperatures leading to reduced yields. This is illustrated in Figure 5.5,
where the yields of LA are plotted as a function of reaction time at three different
temperatures.
The yield of LA was improved when applying higher acid concentrations, see
Figure 5.6 for details. This effect was substantial and the yields increased from 31
to 54 mol % when increasing the acid concentration from 0.1 to 1 M.
A number of experiments were carried out using various initial intakes of
cellulose (1.7–14 wt %) at T = 150 °C and a catalyst concentration of 1 M. The initial
intake of cellulose has a significant effect on the yields of LA (Figure 5.7). Lower
intakes of cellulose resulted in higher yields of LA. These findings are in line with
previous studies on acid-catalysed decomposition of HMF and glucose, where low
substrate’s concentrations favoured high LA yields. The highest LA yield (60 mol
%) at full cellulose conversion was obtained at a temperature of 150 °C, an initial
cellulose intake of 1.7 wt % and a sulphuric acid concentration of 1 M.
94
A Kinetic Study on the Acid-Catalysed Hydrolysis of Cellulose to Levulinic Acid
60
T = 150 °C
T = 175 °C
T = 200 °C
50
YLA / mol %
40
30
20
10
0
0.1
1
10
100
1000
t / min
Figure 5.5 Effect of temperature on YLA (xCEL,0 = 1.7 wt % and CH2SO4 = 0.5 M).
CH SO = 0.1 M
60
2
4
CH SO = 0.5 M
2
50
2
YLA / mol %
4
CH SO = 1 M
4
40
30
20
10
0
10
100
1000
t / min
Figure 5.6 Effect of sulphuric acid concentration on YLA (T = 150 °C and xCEL,0 = 1.7
wt %).
CCEL,0 = 1.7 wt %
70
CCEL,0 = 7.7 wt %
60
CCEL,0 = 14 wt %
YLA / mol %
50
40
30
20
10
0
0
20
40
60
80
100
120
t / min
Figure 5.7 Effect of initial intake of cellulose on YLA (T = 150 °C and CH2SO4 = 1 M).
95
Chapter 5
5.3.3
Kinetic modelling
5.3.3.1
H
Development of a kinetic model
OH
H
H
HO
HO
H
H
O
HO
O
4
1
OH
H
OH
H
H
OH
4
HO
O
H
H
O
H
H
H
1
HO
O
4
1
H
OH
H
OH
O
H
H
n
H
OH
H
OH
H
R2C
Decomposition products
O
OH
R1C
OH
H
O
H
HO
HO
R1G
O
H
R1H
HO
CHO
O
O
OH
H3C
O
+
H
OH
O
OH
H
R2H
R2G
Humins
Humins
Scheme 5.3 Proposed reaction network for the acid-catalysed hydrolysis reaction
of cellulose to LA.
We here propose a novel kinetic model for the acid-catalysed hydrolysis of
cellulose to LA including the main reactions and possible side reactions. It is based
on the following considerations:
1. The reaction rate equations are quantified using the power-law approach
instead of a pseudo-first-order approach.
2. The reaction rate constants are defined in term of modified Arrhenius
equations that combine both effects of temperature and acid catalyst
concentration.
3. The first step in the acid-catalysed hydrolysis of cellulose is cleavage of β(1→4)-glycosidic bond in cellulose to glucose by an acid catalyst (see Scheme
5.3). The rate of this reaction is expressed by the following equations:
R 1C = k 1C (C CEL )aC
k1C = (C H + ) k1RC exp
αC
(5.9)
⎡ E1C ⎛ T −TR
⎜⎜
⎢
⎣⎢ R ⎝ TRT
⎞⎤
⎟⎟ ⎥
⎠ ⎦⎥
(5.10)
In this study, the concentration of cellulose is defined in terms of the number of
available glucose units in the cellulose.
4. It is assumed that cellulose does not selectively react to glucose. We propose
that decomposition products are formed in a parallel-reaction mode [31,32]. All
96
A Kinetic Study on the Acid-Catalysed Hydrolysis of Cellulose to Levulinic Acid
side reactions are lumped in one overall reaction (Scheme 5.3) and modelled
using the following relations:
R2C = k 2C (C CEL )bC
k 2C = (C H + ) k 2RC exp
βC
(5.11)
⎡ E2C ⎛ T −TR
⎜⎜
⎢
⎣⎢ R ⎝ TRT
⎞⎤
⎟⎟ ⎥
⎠ ⎦⎥
(5.12)
5. The main reaction of the acid-catalysed decomposition of glucose is the
formation of HMF with a reaction rate defined as:
R 1G = k 1G (C GLC )aG
k1G = (C H + ) k1RG exp
αG
(5.13)
⎡ E1G ⎛ T −TR
⎜⎜
⎢
⎣⎢ R ⎝ TR T
⎞⎤
⎟⎟ ⎥
⎠ ⎦⎥
(5.14)
In parallel, undesired products (humins) are produced for which the reaction
rate is defined as:
R 2G = k 2G (C GLC )bG
k 2G = (C H + ) k 2RG exp
βG
(5.15)
⎡ E2G ⎛ T −TR
⎜⎜
⎢
⎣⎢ R ⎝ TRT
⎞⎤
⎟⎟ ⎥
⎠ ⎦⎥
(5.16)
The kinetic parameters for the acid-catalysed reaction of glucose to HMF and
humins have been determined independently and the values of the kinetic
constants are given in Table 5.2 [26].
6. LA and FA are formed when HMF is treated with sulphuric acid, and the rate
of this reaction is expressed as:
R 1H = k 1H (C HMF )aH
k1H = (C H + ) k1RH exp
αH
(5.17)
⎡ E1H ⎛ T −TR
⎜⎜
⎢
⎢⎣ R ⎝ TR T
⎞⎤
⎟⎟ ⎥
⎠ ⎥⎦
(5.18)
Table 5.2 Kinetic parameters for the acid-catalysed decomposition of glucose [26].
Parameter
Estimate
aG
1.09 ± 0.01
bG
1.30 ± 0.02
αG
1.13 ± 0.01
βG
a
1.13 ± 0.02
E1G (kJ
mol-1)
E2G (kJ
mol-1)
152.2 ± 0.7
164.7 ± 0.6
k1RG
(M1-aG-αG
min-1) a
0.013 ± 0.001
k2RG
(M1-bG-βG
min-1) a
0.013 ± 0.001
The values were determined at a reference temperature (TR) of 140 °C.
97
Chapter 5
Similar to glucose, the side reaction of the acid-catalysed decomposition of
HMF results in the formation of humins:
R 2H = k 2H (C HMF )bH
k 2H = (C H + ) k 2RH exp
βH
(5.19)
⎡ E2H ⎛ T −TR
⎜⎜
⎢
⎣⎢ R ⎝ TRT
⎞⎤
⎟⎟ ⎥
⎠ ⎦⎥
(5.20)
The kinetic parameters for both main and side reactions of acid-catalysed
decomposition of HMF have been determined previously and are given in
Table 5.3. Experimentally, it was observed that the acid-catalysed decomposition of HMF was generally fast compared to the reaction of glucose to HMF
(vide supra). Only at high temperatures (> 200 °C), significant amounts of HMF
were formed. In all other cases, the maximum HMF yield was < 2.5 mol %
based on cellulose intake. Therefore, the acid-catalysed decomposition of HMF
was included in the kinetic model only for the high temperature (200 °C)
experiments.
7. Previously, we have shown that both LA and FA are stable under the reaction
conditions employed and do not decompose to humins or other organic
compounds in the wide-range of reaction conditions applied [30]. Therefore the
kinetic model does not include decomposition reactions of LA and FA.
8. The amount of gaseous products formed during the acid-catalysed hydrolysis
of cellulose and the acid-catalysed decompositions of glucose and HMF are
negligible (vide infra). Therefore, these decomposition reactions were not
included in the kinetic model.
For a batch reactor set-up, the concentration of the individual species as a
function of time, using the proposed kinetic model given in Scheme 5.3, may be
represented by the following ordinary differential equations:
dC CEL
= − R 1C − R 2C
dt
(5.21)
dC GLC
= R 1C − R 1G − R 2G
dt
(5.22)
dC HMF
= R 1G − R 1H − R 2H
dt
(5.23)
dC LA
= R 1H
dt
(5.24)
The rate-expressions equations (5.9–5.20) in combination with the mass balances
equations (5.21–5.24) and the temperature profile in equation (5.5) were applied to
model the experimental batch data.
98
A Kinetic Study on the Acid-Catalysed Hydrolysis of Cellulose to Levulinic Acid
Table 5.3 Estimated kinetic parameters for the acid-catalysed decomposition of
HMF [30].
Parameter
Estimate
aH
0.88 ± 0.01
bH
1.23 ± 0.03
αH
βH
1.38 ± 0.02
E1H (kJ
E2H (kJ
mol-1)
110.5 ± 0.7
111.3 ± 2.0
(M1-aH-αH
min-1) a
0.340 ± 0.010
k2RH (M1-bH-βH
min-1) a
0.117 ± 0.008
k1RH
a
1.07 ± 0.04
mol-1)
The values were determined at a reference temperature (TR) of 140 °C
5.3.3.2
Modelling results
A total of 26 experiments gave 280 sets of experimental data, where each set
consists of the concentrations of glucose, HMF and LA at certain reaction times.
The best estimates of the kinetic parameters and their standard deviations were
determined using a MATLAB optimisation routine and the results are given in
Table 5.4.
Figure 5.8 shows a good fit between the experimental concentrations of
glucose, HMF and LA and the kinetic model for a broad-range of reaction
conditions. This is also confirmed by the parity plot as shown in Figure 5.9.
Table 5.4 Estimated kinetic parameters for the acid-catalysed hydrolysis of
cellulose.
Parameter
Estimate
aC
0.98 ± 0.02
bC
1.01 ± 0.08
αC
0.96 ± 0.02
βC
E1C (kJ
E2C (kJ
mol-1)
151.5 ± 1.2
174.7 ± 1.4
(M1-aC-αC
min-1) a
0.410 ± 0.018
k2RC (M1-bC-βC
min-1) a
0.065 ± 0.014
k1RC
a
0.94 ± 0.07
mol-1)
The values were determined at a reference temperature (TR) of 175 °C
99
Chapter 5
Concentration / M
Concentration / M
0.25
0.20
0.15
0.10
0.25
0.4
0.20
0.3
0.2
0.1
0.05
0.00
0.5
Concentration / M
0.30
0
30
60
90
0.0
120 150 180 210
0
20
40
0.04
0.04
0.03
0.02
0.01
3
6
9
0
5
10
12
15
2
4
0.02
0.01
0.25
0.20
0.15
0.10
0.05
0
5
10
15
20
25
30
0.00
0
3
6
9
2
12
15
18
t / min
(f) T = 200 °C, CH SO = 0.1 M, xCEL,0 = 14 wt %
(e) T = 200 °C, CH SO = 0.05 M, xCEL,0 = 1.7 wt %
4
25
0.30
t / min
(d) T = 175 °C, CH SO = 1 M, xCEL,0 = 1.7 wt %
20
(c) T = 175 °C, CH SO = 0.5 M, xCEL,0 = 7.7 wt %
4
0.03
0.00
18
15
t / min
0.35
t / min
2
0.00
120
Concentration / M
0.05
Concentration / M
Concentration / M
2
4
0.05
0
100
0.05
(b) T = 150 °C, CH SO = 1 M, xCEL,0 = 14 wt %
(a) T = 150 °C, CH SO = 0.5 M, xCEL,0 = 7.7 wt %
0.00
80
0.10
t / min
t / min
2
60
0.15
2
4
4
Figure 5.8 Comparison of experimental data (…: CGLC, U: CHMF, {: CLA) and
kinetic model (solid lines).
70
Predicted Value of Yield / mol %
YGLC
60
YLA
50
40
30
20
10
0
0
10
20
30
40
50
60
70
Experimental Value of Yield / mol %
Figure 5.9 Parity plot for the experimental and modelled yield of glucose and LA.
100
A Kinetic Study on the Acid-Catalysed Hydrolysis of Cellulose to Levulinic Acid
5.3.3.3
Evaluation of mass-transfer effects
The kinetic data for the acid-catalysed hydrolysis of cellulose were determined
with the assumption that the overall reaction rate is not affected by mass-transfer
effects. The existence of internal-particle mass-transfer limitations may be
evaluated using the Weisz modulus (MW), which represents the ratio of the
reaction rate and the diffusion rate:
MW =
rGLC (d cel 6 )2
C H + ,cel DH + ,cel
(5.25)
In equation (5.25), the term rGLC is the reaction rate of glucose (mol glucose per m3
cellulose particle per second), dcel is the typical diameter of a cellulose particle, and
CH+,cel and DH+,cel are the concentration and the diffusion coefficient of H+ in the
solid phase, respectively. Mass transfer effects on the overall reaction rate are
negligible when the MW value is < 0.15 [33].
To evaluate the value for MW, a typical reaction rate at the high end of the
temperature range (200 °C) was determined, as the effects of mass transfer on
overall reaction are generally more profound at high temperatures. About 6×10-3
mmol of glucose was obtained from 8.2 mg of cellulose after 3 min. In combination
with a bulk density of the microcrystalline cellulose of 500 kg m-3, this leads to an
rGLC of approximately 2 mol m-3cellulose s-1.
The concentration of H+ in the cellulose particle may be estimated by the
following equation:
m=
C H + , cel
C H + , water
(5.26)
The value for m was calculated using literature data [34] and was estimated to be
about 8. When applying a 0.5 M sulphuric acid concentration, the value of CH+,cel is
approximately 4 kmol m-3cellulose.
Microcrystalline cellulose has a high capacity to retain water, and it swells to
almost 100% of its initial particle size at 20 °C [35]. The diffusion coefficient of
sulphuric acid in the swollen cellulose can be estimated as the diffusion coefficient
in a porous medium [36]:
DH + ,cel = DH + ,w ε 1.5
(5.27)
The diffusion coefficient of sulphuric acid in water, DH+,w, is about 1.8×10-9 m2
[37], and the void fraction (ε) of 0.5 was obtained from the swelling properties
of cellulose in water. Therefore, the diffusion coefficient of H+ in cellulose was
estimated to be 6.4×10-10 m2 s-1. With these data and by using an average particle
size of cellulose particle of 20 µm, an MW value of 3.1×10-6 is calculated. This value
is much less than 0.15 and indicates that diffusion limitations are absent.
Experimental verification was obtained by performing experiments with larger
cellulose particles (dcel between 45 and 55 µm), obtained by sieving the cellulose
s-1
101
Chapter 5
particles through two successive sieves (55 µm and 45 µm). The experimental
results are shown in Figure 5.10. Evidently, similar results were obtained for both
particle sizes, a clear indication that mass transfer limitations are absent and do
not bias the kinetic data.
0.06
0.05
dcel = 20 µm
dcel = 20 µm
dcel = 50 µm
0.05
dcel = 50 µm
0.04
0.04
CLA / M
CGLC / M
0.03
0.03
0.02
0.02
0.01
0.01
0.00
0.00
0
5
10
15
20
25
0
5
t / min
10
15
20
25
t / min
(a)
(b)
Figure 5.10 The effects of cellulose particle size on the concentrations of glucose (a)
and LA (b).
H
OH
H
H
HO
HO
O
HO
1
H
OH
H
O
H
H
H
4
H
OH
H
1
OH
4
H
O
HO
O
HO
1
H
H
OH
H
H
O
OH
O
H
4
H
H
n
OH
H
OH
H
Decomposition products
bC = 1.01
βC = 0.94
E2C = 174.7 kJ mol-1
O
OH
aC = 0.98
αC = 0.96
E1C = 151.5 kJ mol-1
OH
O
H
H
HO
HO
O
O
OH
HO
H
O
OH
H
aG = 1.09
αG = 1.13
E2G = 152.2 kJ mol-1
bG = 1.30
βG = 1.13
E2G = 164.7 kJ mol-1
Humins
CHO
O
aH = 0.88
αH = 1.38
E2H = 110.5 kJ mol-1
H3C
+
H
OH
O
bH = 1.23
βH = 1.07
E2H = 111.3 kJ mol-1
Humins
Figure 5.11 Schematic overview of reaction network and estimated kinetic
parameters for cellulose hydrolysis to LA.
102
A Kinetic Study on the Acid-Catalysed Hydrolysis of Cellulose to Levulinic Acid
5.3.3.4
Model implications
The yields of LA (YLA) were considerably reduced at high temperatures, as has
been discussed in subsection 5.3.2. The effects of temperature on YLA may be
explained using the kinetic model (see Figure 5.11).
The hydrolysis reaction of cellulose to decomposition products has the largest
activation energy (174.7 kJ mol-1), which implies that this reaction is most sensitive
to the temperature. The second-largest activation energy (164.7 kJ mol-1) was
observed for the decomposition reaction of glucose to humins. Conducting the
reaction at high temperatures favours these side reactions, and as a result more
side products (i.e., the decomposition products and humins) are produced and the
formation of LA is suppressed.
Experimentally, it has been observed that the yield of LA is higher when
applying higher acid concentrations, see Figure 5.6 for details. These findings may
be explained by assessing the reactions orders in acid concentrations for the
various reactions. For the hydrolysis reaction of cellulose, the reaction orders in
acid are similar for both the main (αC = 0.96) and side (βC = 0.94) reaction and no
effects of the acid concentration are expected. Also the order in acid for glucose
decomposition to HMF (αG = 1.13) and humins (βG = 1.13) are similar. However,
the reaction order in acid for the hydration reaction of HMF to LA has a slightly
higher value (αH = 1.38) than the side reaction (βH = 1.07). Therefore, high acid
concentrations favour the hydration reaction of HMF to LA and lead to increase
YLA.
Experimentally, it was found that low cellulose intakes lead to higher YLA. The
reaction order in cellulose for the reactions involving cellulose (see Figure 5.11) are
similar (aC = 0.98 and bC = 1.01). Therefore, no loading effects are expected on the
selectivity of this reaction. However, low cellulose loadings and subsequently low
glucose concentrations favour the formation of LA and suppress humins
formation. This is evident from the reaction orders of glucose (1.09) and HMF
(0.88) for the main reactions, which are smaller than those of the undesired side
reactions (1.30 and 1.23 for glucose and HMF, respectively).
5.3.3.5
Comparisons with previous kinetic studies
A number of kinetic studies have been reported in the literatures for the
hydrolysis of cellulose to glucose (Table 5.1). However, the range of process
conditions and substrate intakes is generally rather limited, for instance the intake
of lignocellulosic material was not varied within each study. The kinetic study
reported here has been set up and validated for a much wider range of conditions.
To investigate the broad applicability of the kinetic model presented in this paper,
the modelled reaction rate constants (kCEL,model) for a set of experimental conditions
were compared with the various literature models. For this purpose, a set of
reaction conditions (T, CH2SO4 and xCEL,0) was selected within the validity range of
our model (150 °C < T < 200 °C, 0.05 M (0.5 wt %t) < CH2SO4 < 1 M (9.8 wt %), 1.7
wt % < xCEL,0 < 14 wt %). The reaction rates constants of cellulose hydrolysis at
103
Chapter 5
various reaction conditions were calculated using equations (5.10) and (5.12), by
taking into account that kCEL,model = k1C + k2C. Similarly, the reaction rate constants
of cellulose hydrolysis from the literature models (kCEL,lit) were calculated using
the data provided in Table 5.1. The literature studies were performed using
feedstock with a various cellulose contents. For a proper comparison, the actual
cellulose content was calculated and used to determine the kCEL,lit. The kCEL,lit were
compared with kCEL,model, and the results are given in Figure 5.12. A good fit
between the kCEL,lit and kCEL,model was observed, especially for the kinetic models
derived by Saeman (1945) [13] and Malester et al. (1992) [18], indicating the broad
applicability of our kinetic model.
1
10
Saeman (1945)
Fagan et al. (1971)
Thompson et al. (1979)
Franzidis et al. (1983)
Malester et al. (1992)
0
kCEL,model / min
-1
10
-1
10
-2
10
-3
10
-3
10
-2
10
-1
10
10
0
1
10
-1
kCEL,lit / min
Figure 5.12 Comparisons between literature models and the kinetic model
provided here.
5.4 Applications of the kinetic model for reactor optimisation
5.4.1
Optimisation of LA production in a batch reactor
With the model available, it is possible to optimise the yields of LA in a batch
reactor as function of reaction conditions. The yields of LA were evaluated at 99
mol % cellulose conversion (XCEL), which is defined as follow:
X CEL = 1 −
C CEL
C CEL,0
(5.28)
Combination of equations (5.21–5.24) with the differentiated form of equation
(5.28) leads to the following expressions:
104
dC CEL
= −C CEL,0
dX CEL
(5.29)
dC GLC R 1C − R 1G − R 2G
=
C CEL,0
dX CEL
R 1C + R 2C
(5.30)
A Kinetic Study on the Acid-Catalysed Hydrolysis of Cellulose to Levulinic Acid
dC HMF R 1G − R 1H − R 2H
=
C CEL,0
dX CEL
R 1C + R 2C
(5.31)
dC LA
R 1H
=
C CEL,0
dX CEL R 1C + R 2C
(5.32)
Equations (5.29–5.32) were solved simultaneously using the numerical integration
toolbox ode45 in MATLAB software package. The yields of LA were subsequently
calculated using equation (5.7).
By using the solutions of ordinary differential equations (5.29–5.32), it is
possible to optimise the reaction conditions to obtain the highest YLA. For example,
the modelled YLA obtained at various temperatures and acid concentrations at 99
mol % conversion of cellulose is given in Figure 5.13. A number of experimental
data points are also given, demonstrating the goodness-of-fit between
experimental data and model.
Figure 5.13 confirms the experimental trends (subsection 5.3.2) that low
operating temperatures lead to higher YLA. However, the volumetric production
rate of LA (mol m-3 h-1) will decrease dramatically when operating at low
temperature. Figure 5.14 shows the reaction time needed to reach 99 mol %
conversion of cellulose in a batch reactor as a function of the temperature and acid
concentration.
70
CH SO = 0.1 M
2
2
60
4
CH SO = 1 M
2
YLA / mol %
4
CH SO = 0.5 M
4
50
40
30
20
100
120
140
160
180
200
T / °C
Figure 5.13 Effects of temperature and acid concentration on YLA evaluated at XCEL
= 99 mol % and xCEL,0 = 1.7 wt %. (…), (U) and ({) represent the experimental YLA
at CH2SO4 = 0.1, 0.5 and 1.0 M, respectively.
105
Chapter 5
6
10
CH SO = 0.1 M
2
4
CH SO = 0.5 M
5
10
2
4
CH SO = 1 M
2
Batch time / min
4
4
10
3
10
2
10
1
10
0
10
100
120
140
160
180
200
T / °C
Figure 5.14 Modelled batch time for XCEL = 99 mol % as a function of temperature
and acid concentration (xCEL,0 = 1.7 wt %).
Figure 5.15 shows the modelled YLA as a function of xCEL,0 and CH+ evaluated at
150 °C and a cellulose conversion of 99 mol %. It is evident that the highest YLA is
obtained at high acid concentrations and low initial cellulose intakes, in line with
the experimental results (subsection 5.3.2).
5.4.2
Optimisation of LA production in continuous reactors
The yields of LA in continuous reactors will be a function of reaction
conditions (T, CH2SO4 and xCEL,0) and the extent of mixing in the reactor. The kinetic
model derived here was used to model the performance of two extremes with
respect to mixing, i.e., a plug-flow reactor (PFR) and a continuous ideal stirred
tank reactor (CISTR). The reactor design equations of a PFR are similar to the one
for a batch reactor (equations (5.29–5.32)) provided that the time t is replaced by
the residence time τ PFR. The reactor design equation for a CISTR reads:
τ CISTR =
C iout − C iin
Ri
(5.33)
Applying equation (5.33) for cellulose and combining with the definition of
cellulose conversion in equation (5.28), leads to:
τ CISTR =
out
in
C CEL
− C CEL
X C in
= CEL CEL
− R 1C − R 2C R 1C + R 2C
(5.34)
Substitution of equation (5.34) into the design equation (5.33) for glucose, HMF
and LA leads to:
out
C GLC
=
106
R 1C − R 1G − R 2G
in
X CEL C CEL
R 1C + R 2C
(5.35)
A Kinetic Study on the Acid-Catalysed Hydrolysis of Cellulose to Levulinic Acid
Figure 5.15 Modelled effects of xCEL,0 and CH+ on YLA (T = 150 °C and XCEL = 99
mol %).
out
C HMF
=
out
C LA
=
R 1G − R 1H − R 2H
in
X CEL C CEL
R 1C + R 2C
R 1H
in
X CEL C CEL
R 1C + R 2C
(5.36)
(5.37)
The system of algebraic equations (5.35–5.37) was solved numerically using a
MATLAB toolbox fsolve, which gives the concentrations of glucose, HMF and LA
in the outlet of the reactor. The yield of LA in the continuous reactors (YLA) is
calculated as follow:
Ψ LA =
out
C LA
in
C CEL
(5.38)
Figure 5.16 shows the yields of LA as a function of cellulose conversion at
different temperatures (150 and 200 °C) for both the PFR and CISTR. It is clear that
the yield of LA increases with the cellulose conversion, and that at any conversion
level, the LA yield in a CISTR is always higher than in a PFR. The highest
obtainable yield is about 76 mol % at T = 150 °C, xCEL,0 = 1.7 wt % and CH2SO4 = 1 M
using a CISTR. This implies that continuous reactor configurations with a high
extent of back-mixing are preferred with respect to the yield of LA. The yields of
LA at low temperature are higher than at high temperature for both reactor
configurations.
107
Chapter 5
100
100
PFR
CISTR
T = 150°C
80
80
60
60
Ψ LA / mol %
Ψ LA / mol %
PFR
CISTR
40
20
0
T = 200°C
40
20
0
20
40
60
80
100
0
20
XCEL / mol %
40
60
80
100
XCEL / mol %
(a)
(b)
Figure 5.16 Comparisons of ΨLA in two ideal continuous reactors (PFR and CISTR)
at 150 °C (a) and at 200 °C (b) (xCEL,0 = 1.7 wt % and CH2SO4 = 1 M).
5.5 Conclusions
This study describes an in-depth experimental and modelling study on the
acid-catalysed hydrolysis of cellulose into LA. A broad range of reaction
conditions were applied, including variations in temperature between 150 and 200
°C, sulphuric acid concentrations between 0.05 and 1 M, and initial cellulose
intakes between 1.7 and 14 wt %. A power law approach was used to develop a
novel kinetic model for the reaction, including side reactions to humins. A good fit
between experimental data and modelling results was obtained. The highest yield
of LA may be obtained at the low end of the temperature window, a low initial
cellulose concentration, and a high sulphuric acid concentration. Modelling of the
reactions in continuous-reactor systems revealed that reactor configurations with a
high extent of back-mixing (e.g., a CISTR) give better yields of LA. The results of
this study will be applicable for the rational design and operation of dedicated
reactors for the conversions of various types of biomass feedstock to LA.
5.6 Nomenclature
aC
: Reaction order of CCEL in the cellulose hydrolysis to glucose
αC
: Reaction order of CH+ in the cellulose hydrolysis to glucose
aG
: Reaction order of CGLC in the glucose decomposition to HMF
αG
: Reaction order of CH+ in the glucose decomposition to HMF
aH
: Reaction order of CHMF in the HMF decomposition to LA
αH
: Reaction order of CH+ in the HMF decomposition to LA
108
A Kinetic Study on the Acid-Catalysed Hydrolysis of Cellulose to Levulinic Acid
A
: Acid concentration in Saeman’s model, wt %
At
: Heat transfer area, m2
bC
: Reaction order of CCEL in the cellulose hydrolysis to the decomposition products
βC
: Reaction order of CH+ in the cellulose hydrolysis to the decomposition products
bG
: Reaction order of CGLC in the glucose decomposition to humins
βG
: Reaction order of CH+ in the glucose decomposition to humins
bH
: Reaction order of CHMF in the HMF decomposition to LA
βH
: Reaction order of CH+ in the HMF decomposition to LA
CCEL
: Cellulose concentration, M
CCEL,0
: Initial concentration of cellulose, M
CFUR
: Furfural concentration, M
CGLC
: Glucose concentration, M
CHMF
: HMF concentration, M
CH2SO4 : Concentration of H2SO4, M
CH+
: Concentration of proton, M
CH+,cel
: Concentration of proton in the solid phase, M
C iin
: Concentration of the ith compound at the inflow of continuous reactors, M
C iout
: Concentration of the ith compound at the outflow of continuous reactors, M
CLA
: LA concentration, M
Cp
: Heat capacity of reaction mixture, J g-1 K-1
dcel
: Diameter of cellulose particle, m
DH+,cel : Diffusion coefficient of proton in the solid phase, m2 s-1
DH+,w
: Diffusion coefficient of proton in aqueous phase, m2 s-1
E1
: Activation energy of k1 in Saeman’s model, kJ mol-1
E1C
: Activation energy of k1C, kJ mol-1
E1G
: Activation energy of k1G, kJ mol-1
E1H
: Activation energy of k1H, kJ mol-1
E2
: Activation energy of k2 in Saeman’s model, kJ mol-1
E2C
: Activation energy of k2C, kJ mol-1
E2G
: Activation energy of k2G, kJ mol-1
E2H
: Activation energy of k2H, kJ mol-1
h
: Heat transfer coefficient from the oven to the reaction mixture, min-1
k1
: Reaction rate constant of cellulose hydrolysis in Saeman’s model, min-1
k1,o
: Frequency factor of k1 in Saeman’s model, min-1
k1C
: Reaction rate constant of cellulose hydrolysis to glucose, M1-aC min-1
k1RC
: Reaction rate constant k1C at reference temperature, M1-aC-αC min-1
k1G
: Reaction rate constant of glucose decomposition into HMF, M1-aG min-1
k1RG
: Reaction rate constant k1G at reference temperature, M1-aG-αG min-1
k1H
: Reaction rate constant of HMF decomposition to LA, M1-aH min-1
k1RH
: Reaction rate constant k1H at reference temperature, M1-aH-αH min-1
k2
: Reaction rate constant of glucose decomposition in Saeman’s model, min-1
109
Chapter 5
k2,o
: Frequency factor of k2 in Saeman’s model, min-1
k2C
: Reaction rate constant of cellulose hydrolysis to decomposition products, M1-bC min-1
k2RC
: Reaction rate constant k2C at reference temperature M1-bC-βC min-1
k2G
: Reaction rate constant of glucose decomposition into humins, M1-bG min-1
k2RG
: Reaction rate constant k2G at reference temperature M1-bG-βG min-1
k2H
: Reaction rate constant of HMF decomposition into humins, M1-bH min-1
k2RH
: Reaction rate constant k2H at reference temperature, M1-bH-βH min-1
m
: Distribution coefficient of proton in bulk and solid phase
m1
: Reaction order of acid concentration of k1 in Saeman’s model
m2
M
: Reaction order of acid concentration of k2 in Saeman’s model
MW
: Weisz modulus
rGLC
R
: Production rate of glucose, mol m-3cellulose s-1
Ri
: Reaction rate of the ith compound in the continuous reactor configurations, M min-1
R1C
: Reaction rate of cellulose hydrolysis to glucose, M min-1
R1G
: Reaction rate of glucose decomposition to HMF, M min-1
R1H
: Reaction rate of HMF decomposition to LA, M min-1
R2C
: Reaction rate of cellulose hydrolysis to decomposition products, M min-1
R2G
: Reaction rate of glucose decomposition to humins, M min-1
R2H
t
: Reaction rate of HMF decomposition to humins, M min-1
T
: Temperature, K
Ti
: Temperature of reaction mixture at t = 0, K
Toven
: Oven temperature, K
TR
U
: Reference temperature, K
xCEL,0
: Initial intake of cellulose, wt %
XCEL
: Conversion of cellulose, mol %
YGLC
: Yield of glucose, mol %
YLA
: Yield of LA, mol %
: Mass of the reaction mixture, g
: Universal gas constant, 8.3144×10-3 kJ mol-1 K-1
: Time, min
: Overall heat transfer coefficient, W m-2 K-1
Greek symbols
ε
τ CISTR
τ PFR
Ψ LA
110
: Void fraction
: Residence time in a CISTR, min
: Residence time in a PFR, min
: Yield of LA in continuous reactors, mol %
A Kinetic Study on the Acid-Catalysed Hydrolysis of Cellulose to Levulinic Acid
5.7 References
[1]
Sasaki, M.; Adschiri, T.; Arai, K., Production of cellulose II from native cellulose by near- and
supercritical water solubilization. J.Agric.Food Chem. 2003, 51, 5376-5381.
[2]
Bozell, J. J.; Moens, L.; Elliott, D. C.; Wang, Y.; Neuenscwander, G. G.; Fitzpatrick, S. W.;
Bilski, R. J.; Jarnefeld, J. L., Production of levulinic acid and use as a platform chemical for
derived products. 2000, 28, 227-239.
[3]
Leonard, R. H., Levulinic Acid As A Basic Chemical Raw Material. 1956, 48, 1331-1341.
[4]
Kitano, M.; Tanimoto, F.; Okabayashi, M., Levulinic Acid, a New Chemical Raw Material; Its
Chemistry and Use. 1975, 7, 25-29.
[5]
Ghorpade, V.; Hanna, M. A., Industrial Applications for Levulinic Acid. In Cereal Novel Uses
and Processes, Campbell, G. M.; Webb, C.; McKee, S. L., Eds. Plenum Press: New York, 1997;
pp 49-55.
[6]
Timokhin, B. V.; Baransky, V. A.; Eliseeva, G. D., Levulinic acid in organic synthesis. 1999, 68,
80-93.
[7]
Goldstein, I. S.; Pereira, H.; Pittman, J. L.; Strouse, B. A.; Scaringelli, F. P., The Hydrolysis of
Cellulose with Superconcentrated Hydrochloric-Acid. Biotechnol.Bioeng. 1983, 17-25.
[8]
Camacho, F.; Gonzalez-Tello, P.; Jurado, E.; Robles, A., Microcrystalline-cellulose hydrolysis
with concentrated sulphuric acid. J.Chem.Technol.Biotechnol. 1996, 67, 350-356.
[9]
Grethlein, H. E., Chemical Breakdown of Cellulosic Materials. 1978, 28, 296-308.
[10] Torget, R. W.; Kim, J. S.; Lee, Y. Y., Fundamental aspects of dilute acid
hydrolysis/fractionation kinetics of hardwood carbohydrates. 1. Cellulose hydrolysis. 2000,
39, 2817-2825.
[11] Kim, J. S.; Lee, Y. Y.; Torget, R. W., Cellulose hydrolysis under extremely low sulfuric acid
and high-temperature conditions. Appl.Biochem.Biotechnol. 2001, 91-3, 331-340.
[12] Mosier, N. S.; Sarikaya, A.; Ladisch, C. M.; Ladisch, M. R., Characterization of dicarboxylic
acids for cellulose hydrolysis. 2001, 17, 474-480.
[13] Saeman, J. F., Kinetics of Wood Saccharification - Hydrolysis of Cellulose and Decomposition
of Sugars in Dilute Acid at High Temperature. 1945, 37, 43-52.
[14] Fagan, R. D.; Grethlein, H. E.; Converse, A. O.; Porteous, A., Kinetics of Acid Hydrolysis of
Cellulose Found in Paper Refuse. Environ.Sci.Technol. 1971, 5, 545-&.
[15] Thompson, D. R.; Grethlein, H. E., Design and Evaluation of A Plug Flow Reactor for AcidHydrolysis of Cellulose. 1979, 18, 166-169.
[16] Franzidis, J. P.; Porteous, A.; Anderson, J., The Acid-Hydrolysis of Cellulose in Refuse in A
Continuous Reactor. Conservation & Recycling 1982, 5, 215-225.
[17] Green, M.; Kimchie, S.; Malester, A. I.; Rugg, B.; Shelef, G., Utilization of Municipal SolidWastes (Msw) for Alcohol Production. 1988, 26, 285-295.
[18] Malester, I. A.; Green, M.; Shelef, G., Kinetics of Dilute Acid-Hydrolysis of Cellulose
Originating from Municipal Solid-Wastes. 1992, 31, 1998-2003.
[19] Sharples, A., The Hydrolysis of Cellulose and Its Relation to Structure. J.Chem.Soc., Faraday
Trans. 1957, 53, 1003-1013.
[20] Sharples, A., The Hydrolysis of Cellulose and Its Relation to Structure .2. J.Chem.Soc., Faraday
Trans. 1958, 54, 913-917.
111
Chapter 5
[21] Lin, C. H.; Conner, A. H.; Hill, C. G., The Heterogeneous Character of the Dilute AcidHydrolysis of Crystalline Cellulose .2. Hydrolysis in Sulfuric-Acid. J.Appl.Polym.Sci. 1991, 42,
417-426.
[22] Lin, C. H.; Conner, A. H.; Hill, C. G., The Heterogeneous Character of the Dilute AcidHydrolysis of Crystalline Cellulose .3. Kinetic and X-Ray Data. J.Appl.Polym.Sci. 1992, 45,
1811-1822.
[23] Frost, T. R.; Kurth, E. F., Levulinic Acid from Wood Cellulose. Tappi 1951, 34, 80-86.
[24] Efremov, A. A.; Pervyshina, G. G.; Kuznetsov, B. N., Preparation of levulinic acid from
woody sources in the presence of sulfuric acid and its salts. Khim.Prir.Soedin. 1998, 226-230.
[25] Fang, Q.; Hanna, M. A., Experimental studies for levulinic acid production from whole kernel
grain sorghum. 2002, 81, 187-192.
[26] Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., A kinetic study on the conversion of glucose to
levulinic acid. 2006, 84, 339-349.
[27] Dunlop, A. P., Furfural Formation and Behavior. 1948, 40, 204-209.
[28] Root, D. F.; Saeman, J. F.; Harris, J. F.; Neill, W. K., Kinetics of the acid-catalyzed conversion
of xylose to furfural. 1959, 9, 158-165.
[29] Dias, A. S.; Pillinger, M.; Valente, A. A., Dehydration of xylose into furfural over micromesoporous sulfonic acid catalysts. J.Catal. 2005, 229, 414-423.
[30] Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., A Kinetic Study on the Decomposition of 5Hydroxymethylfurfural into Levulinic Acid. 2006, 8, 701-709.
[31] Abatzoglou, N.; Bouchard, J.; Chornet, E.; Overend, R. P., Dilute Acid Depolymerization of
Cellulose in Aqueous Phase - Experimental-Evidence of the Significant Presence of Soluble
Oligomeric Intermediates. Can.J.Chem.Eng. 1986, 64, 781-786.
[32] Mok, W. S. L.; Antal, M. J.; Varhegyi, G., Productive and Parasitic Pathways in Dilute AcidCatalyzed Hydrolysis of Cellulose. 1992, 31, 94-100.
[33] Weisz, P. B.; Prater, C. D., Interpretation of Measurements in Experimental Catalysis.
Adv.Catal. 1954, 6, 143-196.
[34] Malm, C. J.; Barkey, K. T.; May, D. C.; Lefferts, E. B., Treatment of Cellulose Prior to
Acetylation. 1952, 44, 2904-2909.
[35] Sarymsakov, A. A.; Baltaeva, M.; Shoikulov, B. B.; Nabiev, D. S.; Rashidova, S. S., Reaction of
microcrystalline cellulose with water. 2002, 38, 87-89.
[36] Wesselingh, J. A.; Krishna, R., Mass transfer in multicomponent mixtures. Delft University Press:
Delft, Netherlands, 2000.
[37] Umino, S.; Newman, J., Temperature dependence of the diffusion coefficient of sulfuric acid
in water. J.Electrochem.Soc. 1997, 144, 1302-1307.
112
Chapter 6
Experimental and Kinetic Modelling
Studies on the Acid-Catalysed
Hydrolysis of the Water Hyacinth Plant
to Levulinic Acid
Abstract
Levulinic acid (LA) is considered as a versatile precursor for green fuel additives,
polymers and resins. A comprehensive experimental and modelling study on the
acid-catalysed hydrolysis of the water hyacinth plant to LA is reported (T = 150–
175 °C, sulphuric acid concentrations between 0.1 and 1 M, water hyacinth intake
between 1 and 5 wt %). At high catalyst concentrations (> 0.5 M), LA is the major
organic acid whereas at low catalyst concentration (< 0.1 M) and high initial
intakes of water hyacinth, the formation of propionic acid instead of LA is
favoured. The highest yield of LA was 53 mol % (35 wt %) based on the amount of
C6-sugars in the water hyacinth (175 °C, initial water hyacinth intake of 1 wt %,
sulphuric acid concentration of 1 M). The LA yield as a function of the reaction
conditions was modelled using a kinetic model originally developed for the acidcatalysed hydrolysis of cellulose and good agreement between the experimental
and modelled data was obtained.
Keyword: Water hyacinth (Eichhornia crassipes), levulinic acid, acid hydrolysis,
green chemicals.
Chapter 6
6.1 Introduction
With an annual production of up to 1.7–2.0×1011 tons, biomass has been
identified as an important source for alternative fuels and added-value chemicals
[1-3]. However, only 6×109 tons of biomass are currently used for food and nonfood applications [4]. Food applications are by far the most important (96.5–97%).
A substantial amount of research is currently carried out worldwide to identify
attractive chemical transformations to convert biomass into organic (bulk)
chemicals. Examples are the production of organic acids from biomass-based
sugars through fermentation or thermochemical processes. A well known example
is lactic acid, which is easily converted to polylactic acid, a green polymer with
very interesting applications.
A wide variety of biomass sources is available for further conversion and
utilisation. Selection of the biomass feedstock is of paramount importance from
both techno- and socio-economical points of view. For ethical reasons, the biomass
feedstock should not compete with the food chain. Waste streams with a low or
even negative value, such as agricultural waste are preferred. Furthermore, it is
also advantageous to select sources that are not prone to diseases, only require a
limited amount of fertiliser, have a high growth rate per ha per year and are
preferably available throughout the year. Based on these criteria, the water
hyacinth could be an excellent biomass feedstock for further conversions and
utilisation.
The water hyacinth plant (Eichhornia crassipes) is a free-floating aquatic plant
originating from the Amazon River basin in South America [5,6]. Owing to its
beautiful lavender flowers, the water hyacinth was introduced to various
countries as an ornamental plant and has spread to more than 50 countries on five
continents [7-9]. The plant tolerates extremes in water level fluctuations, seasonal
variations in flow velocity, nutrient availability, pH, temperature and toxic
substances [7]. It can even grow at salinity levels up to 0.24% as was shown in
Indonesia [10]. Extremely high growth rates of up to 100–140 ton dry material ha-1
year-1 [11,12] have been reported, depending on the location and time of year. This
enormous growth rate is among the highest reported for a wide range of biomass
sources [11].
The coverage of waterways by water hyacinth has created various problems.
Examples are the destruction of ecosystems (Victoria Lake in Africa), irrigation
problems and an increase in mosquito populations. These negative effects have
pinpointed the water hyacinth as one of the world's worst weeds [13] and
stimulated the search for control measures. Chemical control of the water hyacinth
using herbicides is very effective but the long-term effects of these chemical
substances on the environment are unknown. Furthermore, the sprayed plants are
left to rot in the water, leading to pollution and eutrophication. So far, control by
manual and mechanical harvesting has been practised widely in countries
suffering from water hyacinth. However, as removal of the weed by both means is
extremely costly, the interest in valorisation of harvested water hyacinth plants
114
Experimental and Kinetic Modelling Studies …
has grown rapidly. Commercial utilization of the water hyacinth as a whole or
partly is considered to be a suitable method to reduce the cost of the removal.
A well-known approach to convert lignocellulosic material like the water
hyacinth to bulk chemicals is treatment of the biomass with a mineral acid like
sulphuric acid at elevated temperatures (100–250 °C). Upon this treatment, the
various fractions of lignocellulosic materials (lignin, hemicellulose and cellulose)
are converted to soluble low molecular weight components.
Hemicellulose, a polymeric material consisting of C5- and C6-sugars, is
degraded to various organic chemicals, including oligomers of various sugars [1416], monosaccharides [17-22], furfural generated from pentose dehydration [2325], furfural-degradation products [26,27] and acetic acid [28].
The cellulose fraction is converted to a variety of interesting bulk chemicals [2932]. An interesting example is levulinic acid (LA), a versatile building block for the
synthesis of various organic compounds. The esters of LA can either be used in the
flavouring and fragrance industries or as blending component in biodiesel [33].
The reaction of LA with phenol is known to produce diphenolic acid [34,35] that
can serve as a replacement for Bisphenol A in the production of polycarbonates,
epoxy resins and other polymers [36,37,33]. Other products derived from LA are δaminolevulinic acid, a biodegradable herbicide [38], succinic acid [39] and
methyltetrahydrofuran [40], a gasoline oxygenate [36]. Details on the properties
and potential industrial applications of LA and its derivatives are provided in
several reviews [41-43].
The aim of this study was to identify whether the harvested water hyacinth
plant is a useful biomass source for LA manufacture. The chemical composition of
the water hyacinth plant was determined, followed by systematic studies to
optimise the LA yield by altering the process conditions (temperature, water
hyacinth intake and acid concentration). Subsequently, the LA yields were
modelled using a recently developed kinetic model for cellulose.
6.2 Materials and Methods
6.2.1
Water hyacinth
Fresh water hyacinth plants were obtained from Intratuin B.V. (Groningen,
Netherlands). The plants were washed with water to remove sand and dirt. The
leaves were separated from the stems and the roots and were reduced in size to
about 2–3 mm using a mini-chopper (TEFAL Rondo 500). These finely cut parts
were dried overnight in an oven at 55 °C. The dried leave parts were chopped
(TEFAL Rondo 500) and sieved through a 0.5 mm pore-size sieve before use. The
leave parts with a size < 0.5 mm were used for this study.
115
Chapter 6
6.2.2
Chemicals
All chemicals used in this study were of analytical grade and used without
purification. Concentrated sulphuric acid 95–97 wt % [7664-93-9], glucose [1443143-7] and formic acid [64-18-6] were purchased from Merck GmbH (Darmstadt,
Germany); xylose [58-86-6], arabinose [28697-53-2], furfural [98-01-1], 5hydroxymethylfurfural [67-47-0], propionic acid 99 wt % [79-09-4], acetic acid 96
wt % [64-19-7] and LA 98 wt % [123-76-2] were obtained from Acros Organics
(Geel, Belgium). Deionised water was applied to prepare the various solutions.
6.2.3
Experimental procedures
6.2.3.1 Water hyacinth characterisation
Thermal gravimetric analysis (TGA) was used to determine the chemical
composition (cellulose, hemicellulose, lignin and the inorganic ash content) of the
water hyacinth plants used in this study. The elemental composition was
determined by elemental analysis. Two-stage acid-catalysed hydrolysis was used
to determine the type and amount of sugars and acetyl groups [44]. In the firststage, the water hyacinth was hydrolyzed in a concentrated solution of sulphuric
acid (72 wt %) at 30 °C for 120 min. After completion, the reaction mixture was
diluted with water to obtain an acid concentration of 4 wt %, and was rehydrolysed in the second-stage at 121 °C for 60 min. The liquid phase was
separated from the solids using a micro-centrifuge (Omnilabo International B.V.)
for approximately 15–20 minutes at 1200 rpm. Afterward, the sample was
neutralised using Ba(OH)2 until a pH of 5–7 was obtained and subsequently
centrifuged to obtain a particle-free solution. The composition of the particle-free
solution was determined using high performance liquid chromatography (HPLC)
equipped with a BioRad sugars column Aminex HPX-87P.
6.2.3.2 Kinetic experiments
The reactions were carried out in glass ampoules with a length of 150 mm, an
internal diameter of 3 mm and a wall thickness of 1.5 mm. The ampoules were
filled with a predetermined amount of dried water hyacinth. Subsequently, an
aqueous solution (0.2–0.5 cm3) of the sulphuric acid catalyst at the desired
concentration was added. The ampoules were sealed with a torch. The sealed
ampoules were placed in a constant temperature oven (± 1 °C). At various reaction
times, ampoules were taken from the oven and quenched in an ice-water bath (4
°C) to stop the reaction. The ampoule was opened, and the liquid was separated
from the solids using a micro-centrifuge (Omnilabo International B.V.) for
approximately 15–20 minutes at 1200 rpm. A certain amount of the clear solution
was taken (100–200 µL) and diluted with water (2 cm3). The composition of the
solution was determined using HPLC equipped with a BioRad organic acids
column Aminex HPX-87H.
116
Experimental and Kinetic Modelling Studies …
The composition of the gas phase after the reaction was determined using GCMS. Gas samples were obtained by placing an ampoule in an airtight plastic bag.
The plastic bag was flushed with helium and placed under vacuum. Subsequently,
the glass ampoule was broken, and the released gas was mixed with about 10 cm3
of helium gas.
6.2.4
Analytical equipment
The composition of the liquid phase was determined using an HPLC system
consisting of a Hewlett Packard 1050 pump and a Waters 410 refractive index
detector. Two different columns were applied. The mobile phase for Aminex HPX87P Sugar column was HPLC-grade water at a flow rate of 0.55 cm3 min–1, and the
column was operated at 80 °C. An aqueous solution of sulphuric acid (5 mM) at a
flow rate of 0.55 cm3 min–1 was used as the mobile phase for the Aminex HPX-87H
Organic Acid column, which was operated at 60 °C. The concentration of each
compound in the liquid phase was determined using calibration curves obtained
by analysing standard solutions with known concentrations.
The gas composition was analysed with GC-MS, which consisted of a HP 5890
Series II gas chromatography with a HP 6890 detector. The composition of the gas
phase was determined using a CP-Porabond-Q column (length = 25 m and I.D. =
0.25 mm). The oven temperature was set at 40 °C for 2 min and increased to 240 °C
with an increment of 30 °C min–1. Helium was used as the carrier gas with a flow
rate of 1.5 cm3 min–1.
Elemental analyses were performed at the Analytical Department of the
University of Groningen using an automated Euro EA3000 CHNS analyser.
Thermal analysis of oven-dried water hyacinth was performed on a Mettler
Toledo TGA/SDTA 851e with a heating rate of 10 °C min–1 in an inert atmosphere.
6.2.5
Modelling techniques and software
The kinetic parameters for the proposed kinetic model were estimated using a
maximum-likelihood approach, which is based on minimization of errors between
the experimental data and the kinetic model. Minimization of errors was initiated
by providing initial guesses for each kinetic parameter. The best estimates were
obtained using the MATLAB toolbox fminsearch, which is based on the NelderMead optimization method.
The optimisation experiments were modelled using Design-Expert 7 software
(Stat-Ease). The yield of LA was modelled using a standard expression as given in
equation (6.1):
3
3
3
YLA = b0 + ∑ bi xi + ∑∑ bij xi x j
i =1
(6.1)
i =1 j =1
The operating variables (water hyacinth intake, temperature and acid catalyst
concentration) are represented by the indices 1–3. The regression coefficients were
obtained by statistical analyses of the data. Significance of factors was determined
117
Chapter 6
by their p-value in the ANOVA analyses. A factor was considered significant if the
p-value was lower than 0.05, meaning that the probability of noise causing the
correlation between a factor and the response is lower than 0.05. Insignificant
factors were eliminated using backward elimination, and the significant factors
were used to model the data.
6.2.6
Definitions of LA yield
The yield of LA on a molar base (YLA) is defined as the ratio between the LA
concentration in the reaction product (CLA) and the concentration of the available
C6-sugars in the water hyacinth (CC6,0):
YLA (mol %) =
C LA
× 100%
C C6,0
(6.2)
In equation (6.2), the CC6,0 is the sum of the available C6-sugar monomers (glucose
and galactose) in the cellulose and hemicellulose fraction.
It is also possible to define the yield of LA on a weight base (YLA,wt), which is
defined as the mass ratio between the LA and the available C6-sugars in the water
hyacinth:
YLA,wt (wt %) =
C LA × M LA
× 100%
C C6,0 × M C6 - sugars
(6.3)
In equation (6.3), the terms MLA and MC6-sugars represent the molecular weight of
LA (116 g mol–1) and C6-sugars (180 g mol–1), respectively.
The yield of LA can also be defined as the ratio between the mass of LA and the
total mass of the oven-dried water hyacinth:
YLA, total (wt %) =
C LA × M LA
× 100%
m WH
(6.4)
where mWH represent the mass of the oven-dried water hyacinth.
6.3 Results and discussion
6.3.1
Determination of the water hyacinth composition
Detailed knowledge of the water hyacinth composition is essential to gain
insights into the highest theoretically possible LA yield and to rationalise the
product composition after the acid-catalysed hydrolysis reaction. The results of a
thermo gravimetric analysis (TGA) of water hyacinth leaves are given in Figure
6.1. Three distinct stages of weight losses are visible. The first-stage between 40
and 100 °C is due to evaporation of residual water. Cellulose and hemicelluloses
are degraded between 200–350 °C. Finally lignin decomposes between 420 and 500
°C. These temperature ranges are in line with a previous study [45] on the
characterisation of lignocellulosic biomass using TGA analysis. The residue is non118
Experimental and Kinetic Modelling Studies …
combustible and is defined as the ash content of water hyacinth. Gopal [7] has
reported a typical composition of the ash fraction of the water hyacinth: K2O: 6.3–
34.1%; Na2O: 1.8–1.88%; CaO: 8.4–12.8%; Cl–: 3.9–21%; (PO4)3–: 2.8–8.2%,
suggesting that the ash is basic in nature. The results of our analysis for the
organic and inorganic content, including the elemental composition, are given in
Table 6.1.
0.000
H2O
-0.002
TG / wt %
80%
60%
-0.004
DTG
Cellulose +
Hemicelluloses
-0.006
-0.008
40%
Lignin
-0.010
20%
-1
TG
DTG / wt % min
100%
-0.012
Ash
0%
-0.014
0
100
200
300
400
500
600
700
800
T / °C
Figure 6.1 Thermo Gravimetric (TG) and Differential Thermo Gravimetric (DTG)
curves of the oven-dried water hyacinth.
Table 6.1 Chemical composition of oven-dried water hyacinth leaves.
wt %
Thermal gravimetric analysis
Cellulose + Hemicellulose
46.74
Lignin
27.69
Ash
18.20
Water
7.37
Elemental analysis (dry basis)
C
42.18
H
6.41
O
27.52
N
4.25
Ash
19.65
Two-stage acid hydrolysis
Glucose
19.8
Galactose
6.5
Xylose
11.5
Arabinose
9.0
Acetyl groups
1.1
119
Chapter 6
The type and amounts of sugar monomers in the water hyacinth were
determined using a two-stage acid hydrolysis procedure. The total amount of C6sugars was 26.3 wt %, being glucose (19.8 wt %) and galactose (6.5 wt %). Mannose
could not be detected. Details about the amount and type of C6-sugars in the
water hyacinth plant are scarce. Most studies only reported the amount of
cellulose, which varies between 17.8 and 19.5 wt % [46-48]. Our analysis reveals
the presence of two C5-sugars, xylose and arabinose, in the water hyacinth leaves
in amounts of 11.5 and 9.0 wt %, respectively. Nigam (2002) reported similar
amounts for xylose (12.4 wt %), whereas their arabinose levels (2.2 wt %) were
considerably lower [11].
Considerable amounts of acetic acid (1.1 wt %) were detected in the reaction
mixture after the two-stage hydrolysis process. The presence of this acid is likely
the result of hydrolysis of acetyl groups in the hemicellulose fraction [49,28].
6.3.2
Exploratory experiments
Exploratory experiments on the acid-catalysed hydrolysis of the water hyacinth
plant to gain insights into the type and amount of reaction products were carried
at T = 175 °C, using a water hyacinth intake of 5 wt % and two sulphuric acid
concentrations (1.0 and 0.1 M).
6.3.2.1 Results for 1.0 M sulphuric acid
Typical concentration profiles for the various water-soluble compounds when
hydrolysing the water hyacinth plant in a strong acidic medium (1.0 M sulphuric
acid) are given in Figure 6.2.
50
50
(1) Glucose
(2) HMF
(3) LA
40
40
30
C / mM
C / mM
30
20
10
0
(7) Arabinose
(8) Furfural
(9) Acetic acid
(14) Propionic acid
20
10
0
5
10
t / min
15
20
0
0
5
10
15
20
t / min
Figure 6.2 Concentration profiles of various compounds present when
hydrolysing water hyacinth leaves (CH2SO4 = 1.0 M, xWH,0 = 5 wt %, T = 175 °C).
120
Experimental and Kinetic Modelling Studies …
The amount of LA in the reaction mixture after 20 min was about constant and
reached a maximum level of 32 mM, corresponding with a yield of 40 mol % on
the available C6-sugars in the water hyacinth. Besides LA, considerable amounts
of other organic acids (formic acid, acetic acid and propionic acid) were present
after 20 min of reaction time, although the concentrations were significantly lower
than that of LA.
A number of intermediate products with clear maximum concentrations were
observed (Figure 6.2). These were identified as monomeric sugars (glucose and
arabinose) as well as furan derivatives (5-hydroxymethylfurfural and furfural).
The concentration profiles of galactose and xylose cannot be presented
individually because of overlapping peaks in the HPLC analysis using the Aminex
HPX-87H column, which hampered the quantitative analysis. However, the
combined peak area of both sugars also showed a clear optimum, indicative for
the existence of consecutive reaction pathways.
On the basis of the product composition and literature precedents for other
biomass sources [50,51], a simplified reaction pathway for the acid-catalysed
hydrolysis of the water hyacinth is proposed in Scheme 6.1.
OH
H
Cellulose
HO
H+
O
O
H
HO
- 3 H2O
H
+ 2 H2O
HO
H+
OH
OH
O
CHO
H+
OH
H3C
OH
H
O
H
2
1
O
+
3
4
OH
H
HO
H+
- 3 H2O
O
H
HO
H
H+
OH
OH
H
1
OH
OH
H
H
H+
HO
O
- 3 H2O
H
H+
OH
OH
H
5
Hemicellulose
O
HO
OH
HO
H+
OH
6
- 3 H2O
H+
H+
OH
- 3 H2O
O
HO
O
CHO
H+
Decomposition products
8
OH
H+
OH
7
O
Acetyl groups
H+
OH
H3C
9
Scheme 6.1 Simplified reaction scheme for the acid-catalysed hydrolysis reaction
of water hyacinth.
121
Chapter 6
The cellulose in the water hyacinth is broken down into low molecular weight
fragments and ultimately to glucose (GLC, 1) by the action of the acid catalyst [52].
Subsequently, the glucose is decomposed to 5-hydroxymethylfurfural (HMF, 2),
which is further converted in a serial mode to LA (3) and formic acid (4) [53,54].
The anticipated products of hemicellulose hydrolysis are glucose, galactose (5),
xylose (6) and arabinose (ARA, 7) and these were indeed detected in the reaction
mixtures. The C6-sugars from the hemicellulose fraction (glucose and galactose)
are expected to be converted to HMF and subsequently to LA [51,55]. Both C5sugars (xylose and arabinose) are known to be decomposed to furfural (FUR, 8)
[23-25]. The furfural concentration also shows an optimum with respect to reaction
time (Figure 6.2), indicating subsequent reactions under these conditions. Acetic
acid (AA, 9) is most likely formed from the hydrolysis of the acetyl groups present
in the hemicellulose [49,28].
Besides these products, small amounts of other organic acids, i.e. lactic acid (10)
and propionic acid (14, less than 0.2 wt %) were detected in the liquid phase.
Further analysis by ion exchange chromatography (IEC) also revealed the
presence of minor amounts (less than 0.1 wt %) of pyruvic acid (11) and glycolic
acid (12). The presence of these acids is in line with previous study on C5-sugar
decomposition [23], which proposed that 10, 11 and 12 are, among others, the
decomposition products of C5-sugars and particularly of xylose in neutral or low
acidic media. A reaction scheme for the catalysed degradation of xylose is given in
Scheme 6.2.
H+
O
HO
HO
Intermediates 1
H+
OH
H+
O
OH
CHO
Destruction Products
8
6
H+
Intermediates 2
O
OH
HO
CH3
HO
HO
10
OH
CH3
O
O
O
Condensation Products
11
12
Scheme 6.2 Products of the catalysed-degradation of xylose [23].
Previous studies [56,57,28] have reported that propionic acid (PA, 14) is a
product of consecutive reactions of lactic acid via acrylic acid (13) as the
intermediate (see Scheme 6.3). In the first step, the hydroxyl group of lactic acid is
dehydrated to give acrylic acid. The latter is supposed to be hydrogenated to
propionic acid.
122
Experimental and Kinetic Modelling Studies …
OH
HO
H+
HO
∆
CH3
CH2
HO
H2
O
O
O
CH3
13
10
14
Scheme 6.3 Proposed reaction pathway for the conversion of lactic acid to
propionic acid at elevated temperature [56,57,28].
In all experiments, dark-brown insoluble-products were formed. These are
likely humins-type by-products from the acid-catalysed decomposition of glucose
and HMF, as well as from the condensation reactions of C5-sugars and furfural
[23].
Other possible by-products of the acid-catalysed hydrolysis of water hyacinth
are gas-phase components from thermal degradation reactions of reactants and/or
products. To gain insights into the extent of these reactions, the gas phase after the
reaction was analysed using GC and GC-MS. Both CO and CO2 could be detected;
however, the amounts were less than 0.1 wt % of the water hyacinth intake. This
implies that the formation of gas-phase compounds is only a minor reaction
pathway under the reaction conditions applied in our experiments.
6.3.2.2 Results for 0.1 M sulphuric acid
To gain insights into the effects of the acid catalyst concentration on the
product profiles, a series of experiments was carried out at low sulphuric acid
concentrations (0.1 M). The results are shown in Figure 6.3.
50
50
(1) Glucose
(2) HMF
(3) LA
40
40
30
C / mM
C / mM
30
20
10
0
(7) Arabinose
(8) Furfural
(9) Acetic acid
(14) Propionic acid
20
10
0
10
20
30
t / min
40
50
60
0
0
10
20
30
40
50
60
t / min
Figure 6.3 Concentration profile of various compounds present when hydrolysing
water hyacinth leaves (CH2SO4 = 0.1 M, xWH,0 = 5 wt %, T = 175 °C).
123
Chapter 6
The amount of LA at the end of the reaction is considerably lower (1 mol %) at
0.1 M when compared to 1 M sulphuric acid (40 mol %), see also Figure 6.4. This
suggests that the amount of available monomeric C6-sugars in the reaction
mixture is reduced considerably at low acid catalyst concentrations. This is clearly
supported by Figure 6.3, which shows that the maximum glucose concentration (8
mM) is considerably lowered when compared with a hydrolysis experiment in the
presence of 1 M sulphuric acid (20 mM). Glucose originates from both the
cellulose and hemicellulose fraction of the water hyacinth leaves. It is well possible
that glucose formation from the cellulose fraction is suppressed at low acid
catalyst concentration due to a slower rate of break down of the glycosidic bonds
in crystalline cellulose. In addition, the reaction rates of the subsequent reactions
of glucose to HMF and LA are also reduced at lower acid concentrations.
Consequently, only small amounts of LA are produced at these conditions.
On the contrary, the formation and decomposition reactions involving the C5sugars were not affected by the amount of acid catalyst present in the reaction
mixture. Significant amounts of C5-sugars (xylose and arabinose) were formed as
well as furfural. These observations indicate that breakdown of the hemicellulose
fraction at low acid concentrations is still very facile. This is further supported by
the observation that the maximum concentration of arabinose, a main component
in the hemicellulose fraction, is about equal to value observed at high acid catalyst
concentrations. All observations suggest that hemicellulose breakdown is still
facile at low acid catalyst concentrations. This is in line with earlier investigations
on hemicellulose decomposition and is ascribed to the low crystallinity of the
hemicellulose fraction [16].
The concentration of acetic acid in the mixture at the end of the reaction was
essentially similar to that at higher acid catalyst concentration (cf. Figures 6.2 and
6.3). Apparently, the acetyl groups in the hemicellulose fraction are easily
hydrolysed to acetic acid, even when using a dilute mineral acid catalyst.
Propionic acid was formed in larger amounts than found at higher acid
concentrations (Figure 6.4). Oeffner et al. (1992) studied the degradation of xylose
in aqueous media at different pH values and found substantial amounts of
propionic acid [58]. The formation of propionic acid was highest in neutral and
alkaline solutions, whereas furfural was the major product in acid media. This
suggests that our experiments were actually carried out at very low acidic or even
neutral conditions. We assume that the acid catalyst is neutralised by the basic
components present in the (high) ash fraction of the water hyacinth (vide supra)
[59,60]. This explanation is also supported by the fact that the water hyacinth
intake also has a profound effect on the selectivity. When using a constant
sulphuric acid catalyst concentration of 0.1 M and varying the water hyacinth
intake, higher intakes led to larger amounts of propionic acid (vide infra).
On the basis of the experimental data at different acid concentrations, it may be
concluded that the product composition at the end of the reaction and the
maximum concentration of intermediate products are a strong function of the
124
Experimental and Kinetic Modelling Studies …
acidity of the reaction medium (see Figure 6.4). In strong acidic medium, the
formation of LA is favoured, whereas propionic acid is the major acid formed at
lower acidity.
CH SO = 0.1 M
2
4
CH SO = 1.0 M
2
4
Maximum concentration (mM)
40
30
20
10
0
FUR,opt
(3) ARAARA,opt
(7) FUR
(9) PA PA,opt
GLCGLC,opt
(1) HMFHMF,opt
(2) LA LA,opt
(8) AAAA,opt
(14)
Compound
Figure 6.4 Maximum concentration of selected products as a function of the acid
catalyst concentration (xWH,0 = 5 wt % and T = 175 °C).
6.3.3
Optimisation experiments
A total of 12 experiments were performed, differing in temperature, sulphuric
acid concentration and initial intake of water hyacinth (see Table 6.2). The effects
of the process conditions on the LA yield (YLA) will be discussed in the following
section.
Table 6.2 Experimental conditions and LA yield for the optimisation experiments.
T
CH2SO4
xWH,0
t
YLA a
(°C)
(M)
(wt %)
(min)
(mol %)
1
150
0.1
1
0 – 720
38
2
150
0.1
5
0 – 720
2
3
150
0.5
1
0 – 240
47
4
150
0.5
5
0 – 240
49
5
150
1.0
1
0 – 240
51
6
150
1.0
5
0 – 240
51
7
175
0.1
1
0 – 60
34
8
175
0.1
5
0 – 60
1
9
175
0.5
1
0 – 30
46
10
175
0.5
5
0 – 30
41
11
175
1.0
1
0 – 30
53
12
175
1.0
5
0 – 30
46
No.
a
The yields were evaluated at the final reaction time and YLA is defined in equation (6.2).
125
Chapter 6
The highest experimental YLA is 53 mol % (or 9 wt % based on the mass of
oven-dried water hyacinth), and was obtained at T = 175 °C, xWH,0 = 1 wt % and
CH2SO4 = 1 M. To quantify the effect of process conditions on the LA yield, the data
were analysed using the Design-Expert software. As anticipated on the basis of the
screening experiments, the experiments carried out at a low acid catalyst
concentration in combination with a high water hyacinth intake favoured the
formation of propionic acid. These experiments (2 and 8) were excluded from the
statistical analysis. The following model, including the quadratic and interaction
terms, fits the experimental data well (R2 = 0.9998):
YLA = 50.176 + (− 0.109 )T + (− 4.762 )C H 2 SO 4 + (11.347 )x WH,0
+ (0.260 )C H 2 SO 4 T + (− 0.069 )x WH,0T + (− 1.0 )C H 2 SO 4 x WH,0
(6.5)
+ (− 16.944 )C H2 2 SO 4
Analysis of variance of the model is given in Table 6.3. A good agreement between
the empirical model and the experimental data was observed as shown in the
parity plot provided in Figure 6.5.
The model predicts that particularly the acid concentration has a profound
effect on the YLA. This is clearly illustrates in Figure 6.6 and supported by the
experiments (Table 6.2). For example, an increase in the acid concentration from
0.1 to 1 M (xWH,0 = 1 wt % and T= 150 °C), the YLA increases from 38 to 51 mol %.
Within the limited temperature window (150–175 °C), the reaction temperature
has a small but significant effect on the LA yield (Figure 6.6). The YLA is generally
reduced when performing the reaction at higher temperatures. This is in line with
the experimental and modelling studies on the acid-catalysed hydrolysis of
cellulose to LA [52].
To increase the YLA, it seems preferable to conduct the hydrolysis reaction in a
dilute solution (Figure 6.6).
Table 6.3 Analysis of variance of the preferred model.
p-value
Sum of
Degree of
Mean
Squares
Freedom
Square
Model
340.34
7
48.62
1723.33
0.0006
T (A)
42.11
1
42.11
1492.62
0.0007
CH2SO4 (B)
118.10
1
118.10
4186.01
0.0002
xWH,0 (C)
20.73
1
20.73
734.82
0.0014
AB
11.19
1
11.19
396.75
0.0025
AC
25.87
1
25.87
917.08
0.0011
Source
F-value
Prob > F
BC
2.00
1
2.00
70.89
0.0138
B2
15.25
1
15.25
540.53
0.0018
Residual
0.056
2
0.028
126
Experimental and Kinetic Modelling Studies …
60
Predicted YLA / mol %
55
50
45
40
35
30
30
35
40
45
50
55
60
Experimental YLA / mol %
Figure 6.5 Parity plot between the experimental data and the predicted data from
the empirical model.
WH,0
X
= 1 wt %
WH,0
55
50
50
/ mol %
55
LA
45
40
= 5 wt %
45
40
Y
Y
LA
/ mol %
X
35
35
30
1
30
1
0.8
H2SO4
/M
160
0.2
155
150
T / oC
175
170
0.6
165
0.4
C
0.8
175
170
0.6
165
0.4
C
H2SO4
/M
160
0.2
155
150
T / oC
Figure 6.6 Optimisation studies using the empirical model on the effects of
temperature and acid concentration at two water hyacinth intakes (xWH,0 = 1 and 5
wt %). Symbols (•) represent the experimental data.
Table 6.4 shows a comparison between the LA yields from earlier studies on a
variety of biomass sources with the results reported here for the water hyacinth
leaves. Clearly, the yields of LA depend strongly on the type of biomass feedstock
and reaction conditions. High LA yields are usually obtained by hydrolysing
biomass feedstock with a high content of C6-sugars, such as starch or pulp slurry,
at high temperatures. The LA yield from the water hyacinth is relatively low and
comparable with that found for wood sawdust. This low LA yield is the
consequence of the relatively low amounts of C6-sugars in the water hyacinth
leaves compared to other feedstocks.
127
Chapter 6
Table 6.4 Various methods to prepare LA from biomass feedstock.
Feedstocks
a
T
(°C)
Biomass C6intake
sugars
(wt %)
(wt %)
Cacid
(wt %)
Acid
t
YLA,total
YLA,wt
(h)
( wt %)
(wt %)
Reference
Wood
sawdust
190
20
50
1.5
HCl
0.5
9
36
[61]
Cane sugar
100
10
100
16
HCl
24
15
15
[62]
Bagasse
25-195 10
40
1.3
H2SO4 2
18
45
[63]
Corn starch
162
29
90
6.5
HCl
1
26
37
[64]
Starch
200
31
90
1.7
HCl
0.5
35
49
[65]
Pulp slurry
160
10
60
6
HCl
1
41
68
[66]
Water
hyacinth
175
1
26.3
9.5
H2SO4 0.5
9
35 (53)a
This study
The value in parentheses is the yield of LA in defined on a molar base (YLA).
The water hyacinth plant has an enormously high growth rate and values up to
~100 dry ton ha–1 year–1 have been reported [11]. With the yield data reported in
Table 6.4, a hectare of water hyacinth has the potential to produce 9 ton LA year–1.
6.4 Development of a kinetic model for the acid-catalysed
hydrolysis of water Hyacinth to LA
We recently published a kinetic model for the acid-catalysed hydrolysis of
cellulose to LA [52]. The model has been validated in the temperature range of
150–200 °C, sulphuric acid concentrations between 0.05 and 1 M and initial
cellulose intakes between 1.7 and 14 wt %. This kinetic model is the basis for the
kinetic model presented here to predict the LA yields and the amounts of glucose
for the acid-catalysed hydrolysis of the water hyacinth at different reaction
conditions. Adjustments of the cellulose model are required to compensate for the
fact that the water hyacinth is by far a more complex matrix than pure cellulose
and consists of different sugar-polymers together with lignin. The acid-catalysed
hydrolysis of this complex material is evidently not the same as for pure cellulose.
The kinetic model for the water hyacinth leaves is based on the following
considerations and assumptions:
1. Among the sugars present in the water hyacinth, only the C6-sugars are
converted to the desired product LA. These C6-sugars are glucose monomers
from the cellulose and hemicellulose fraction and galactose monomers from the
hemicellulose fraction. Scheme 6.4 shows the reaction network of acidcatalysed hydrolysis of water hyacinth. Undesirable by-products are humins
and these are formed in each of the reactions within the network [53,54,52].
128
Experimental and Kinetic Modelling Studies …
Scheme 6.4 Proposed reaction network for the acid-catalysed hydrolysis reaction
of water hyacinth to LA.
2. The reaction rate equations are quantified using the power law approach, and
the reaction rate constants are defined in term of modified Arrhenius
equations. The kinetic parameters for the conversion of cellulose, glucose and
HMF to LA have been determined in previous studies in our laboratory and
are used as input in the kinetic model [53,54,52].
3. The first step in the acid-catalysed hydrolysis of water hyacinth is the
depolymerisation of the cellulose and hemicellulose fractions into their sugar
monomers, and the reaction rates are represented as R1WH, R2WH and R3WH
(Scheme 6.4). To compensate for the fact that the cellulose fraction in the water
hyacinth differs from that of pure cellulose (e.g., crystallinity, particle size), a
correction factor is applied:
R 1WH = c 1WH R CEL→GLC
(6.6)
where RCEL→GLC represents the reaction rate of glucose formation from pure
cellulose.
4. Hemicellulose fraction is more easily hydrolysed than cellulose. Unfortunately,
no data are available for the rate of hydrolysis of the C6-sugars in the
hemicellulose fraction. Therefore, the depolymerisation rates of glucose and
galactose present in the hemicellulose are taken as follows:
R 2WH = c 2WH R CEL→GLC
(6.7)
R 3WH = c 3WH R CEL→GLC
(6.8)
5. Simultaneously, part of the water hyacinth is also decomposed to humin
byproducts (R4WH):
R 4WH = c 4WH R CEL→HUM
(6.9)
In equation (6.9), RCEL→HUM represents the decomposition rate of pure cellulose
to the undesired humins.
6. It is assumed that galactose decomposes in a similar fashion as glucose and
forms HMF and subsequently LA and FA. The reaction rate of galactose
129
Chapter 6
decomposition are assumed to be the same as the reaction rate of glucose
decomposition:
R1GAL = R1GLC
(6.10)
R 2GAL = R 2GLC
(6.11)
Several additional experiments using pure galactose have been performed to
validate this assumption. The experiments were carried out at 140 °C, 1.0 M
sulphuric acid and variable galactose intakes (0.1–1.0 M). The experimental
data were compared with the kinetic model developed for the acid-catalysed
decomposition of glucose (Figure 6.7). Clearly, the rate of the acid-catalysed
galactose decomposition is similar to that of glucose, proving the validity of
this assumption.
1.0
CGAL = 0.1 M
CGAL = 0.5 M
CGAL = 1.0 M
0.8
Glucose kinetic model
CGAL / M
0.6
0.4
0.2
0.0
0
30
60
90
120
t / min
Figure 6.7 Comparisons of experimental data for galactose decomposition to LA
with a kinetic model developed for glucose (T = 140 °C and CH2SO4 = 1.0 M).
7. Considering that the decomposition rates of galactose and glucose are equal
(vide supra), the proposed reaction network (Scheme 6.4) can be simplified to:
Scheme 6.5 Simplified reaction scheme of water hyacinth hydrolysis.
130
Experimental and Kinetic Modelling Studies …
Here, the production rates of C6-sugars from the water hyacinth are lumped as
follows:
R 1 = R 1WH + R 2WH + R 3WH = (c 1WH + c 2WH + c 3WH )R CEL → GLC = c 1 k 1C (C CEL )aC (6.12)
Meanwhile, the production rate of humins from the water hyacinth is given by:
R 2 = R 4WH = c 4WH R CEL → HUM = c 2 k 2C (C CEL )bC
(6.13)
The values for k1C, k2C, aC and bC were determined earlier in our group [52].
8. The initial concentration of C6-sugars fraction in the water hyacinth (CC6,0) is
determined using the following equation:
C C6,0 =
mass of water hyacinth × wt % of C6 - sugars in water hyacinth
molecular weight of C6 - sugar × volume of reaction mixture
(6.14)
9. At the start-up of the reaction, the temperature in the reactor is not constant
and the reaction proceeds non-isothermal. Additional experiments according to
a published procedure [53,54] were carried out to obtain a model to
compensate for this effect. This model was subsequently incorporated into the
kinetic model for the water hyacinth to describe the non-isothermal behaviour
of the system at the start-up of the reaction.
For a batch reactor set-up with no density- and volume-changes, the
concentrations of the individual species as a function of time, using the proposed
kinetic model given in Scheme 6.5, may be represented by the following ordinary
differential equations:
dC WH
= −R1 − R 2
dt
(6.15)
dC C6
= R 1 − R 1G − R 2G
dt
(6.16)
dC HMF
= R 1G − R 1HMF − R 2HMF
dt
(6.17)
dC LA
= R 1HMF
dt
(6.18)
The rate expressions of water hyacinth hydrolysis in equations (6.12) and (6.13)
are combined with the mass balance equations (6.15)–(6.18) to model the
experimental data. A total of 10 experiments gave 96 sets of experimental data,
where each set consists of the concentrations of glucose and LA at a certain
reaction time. The best estimates of the correction factor c1 and c2 were determined
using a MATLAB optimisation routine, and the results are given in Table 6.5.
131
Chapter 6
Table 6.5 Estimated correction factors for the acid-catalysed hydrolysis of water
hyacinth.
Parameter
Estimate
c1 [−]
0.74 ± 0.04
c2 [−]
1.94 ± 0.18
The term c1 is the summation of three individual rate constants, see equation
(6.12) for details. The value of c1 is less than 1, meaning that the rate of
depolymerisation of C6-sugars from the water hyacinth matrix is slower than that
of pure cellulose. The rate of decomposition of the hemicellulose fraction to C6sugars is expected to be much faster than that of cellulose. This implies that the
rate of formation of glucose from the water hyacinth matrix is slower than that
from pure cellulose. Pure cellulose is usually produced from cotton and has gone
through several treatments to remove the hemicellulose and lignin fractions. The
presence of these fractions in the water hyacinth, especially lignin as the binder,
and the differences in the cellulose structure likely reduces the rate of
depolymerisation of the cellulose fraction in the water hyacinth to glucose
monomers.
Figure 6.8 shows a good fit between the experimental concentrations of glucose
and LA and the kinetic model for a broad-range of reaction conditions. This is also
confirmed by the parity plot as shown in Figure 6.9.
6.5 Conclusions
The acid-catalysed hydrolysis of the water hyacinth to LA was carried out in a
broad range of reaction conditions, including variations in temperature (150 and
175 °C), sulphuric acid concentrations (between 0.1 and 1 M) and initial water
hyacinth intakes (1 and 5 wt %). The product distribution depends strongly on the
reaction condition applied, and two distinct reaction pathways may be
discriminated. At high sulphuric acid concentration, LA is the major organic acid
formed, whereas propionic acid is preferentially formed at low acid
concentrations. The highest yield of LA observed in this study was 53 mol % based
on the available C6-sugars in the water hyacinth or 9 wt % based on dried water
hyacinth. This value is at the low end when compared to other biomass sources,
due to the relatively low amounts of C6-sugars in the water hyacinth. Based on
this maximum yield of LA, it can be estimated that a hectare of lake occupied by
water hyacinth has the potential to produce LA at a rate of ~ 9 ton year–1. Finally, a
kinetic model originally developed for the acid-catalysed cellulose hydrolysis was
adapted and applied to model the LA yield from the water hyacinth plant. A good
fit between the experimental data and the kinetic model was obtained.
132
60
6
50
5
4
3
2
0
120
240
360
40
30
20
480
600
0
720
0
60
120
180
6
4
2
0
240
0
60
120
180
240
t / min
t / min
t / min
(a) T = 150 °C, CH SO = 0.1 M, xWH,0 = 1 wt %
(b) T = 150 °C, CH SO = 0.5 M, xWH,0 = 5 wt %
(c) T = 150 °C, CH SO = 1 M, xWH,0 = 1 wt %
2
2
4
4
2
0
10
20
30
40
50
40
30
20
10
0
60
8
Concentration / mM
Concentration / mM
6
0
5
10
15
t / min
2
20
25
6
4
2
0
30
0
4
8
t / min
2
4
12
16
(f) T = 175 °C, CH SO = 1 M, xWH,0 = 1 wt %
2
4
4
Figure 6.8 Comparison of experimental data (…: CGLC; {: CLA) and kinetic model
(solid lines).
60
Predicted Value of Yield / mol %
YGLC
YLA
50
40
30
20
10
0
0
10
20
30
20
t / min
(e) T = 175 °C, CH SO = 0.5 M, xWH,0 = 5 wt %
(d) T = 175 °C, CH SO = 0.1 M, xWH,0 = 1 wt %
4
10
50
8
0
2
4
60
10
Concentration / mM
8
10
1
0
10
Concentration / mM
7
Concentration / mM
Concentration / mM
Experimental and Kinetic Modelling Studies …
40
50
60
Experimental Value of Yield / mol %
Figure 6.9 Parity plot for the experimental and modelled yield of glucose and LA.
133
Chapter 6
6.6 Nomenclature
aC
: Reaction order for pure cellulose hydrolysis to glucose, [−]
bC
: Reaction order for pure cellulose decomposition to humins, [−]
c1
: Correction factor for the hydrolysis of water hyacinth to C6-sugars, [−]
c1WH
: Correction factor for R1WH, [−]
c2
: Correction factor for the hydrolysis of water hyacinth to humins, [−]
c2WH
: Correction factor for R2WH, [−]
c3WH
: Correction factor for R3WH, [−]
c4WH
: Correction factor for R4WH, [−]
CAA
: Acetic acid concentration, M
CARA
: Arabinose concentration, M
CC6
: C6-sugars concentration, M
CCEL
: Cellulose concentration, M
CFUR
: Furfural concentration, M
CGLC
: Glucose concentration, M
CC6,0
: Initial concentration of C6-sugars in water hyacinth, M
CC6
: C6-sugars concentration, M
CHMF
: HMF concentration, M
CH2SO4
: Concentration of H2SO4, M
CLA
: LA concentration, M
CPA
: Propionic acid concentration, M
CWH
: Water hyacinth concentration, M
k1C
: Reaction rate constant for pure cellulose hydrolysis to glucose, min-1
k2C
: Reaction rate constant for pure cellulose decomposition to humins, min-1
MC6-sugars
: Molecular weight of C6-sugars, g mol-1
MLA
: Molecular weight of LA, g mol-1
mWH
: Mass of the oven-dried water hyacinth, g
R1
: Reaction rate of water hyacinth hydrolysis to C6-sugars, M min-1
RCEL→GLC
: Reaction rate of glucose formation from pure cellulose, M min-1
RCEL→HUM : Reaction rate of humins formation from pure cellulose, M min-1
R1G
: Reaction rate of C6-sugars decomposition to HMF, M min-1
R1GAL
: Reaction rate of galactose decomposition to HMF, M min-1
R1GLC
: Reaction rate of glucose decomposition to HMF, M min-1
R1HMF
: Reaction rate of HMF decomposition to LA, M min-1
R1WH
: Reaction rate of decomposition of cellulose fraction to glucose, M min-1
R2
: Reaction rate of water hyacinth hydrolysis to humins, M min-1
R2G
: Reaction rate of C6-sugars decomposition to humins, M min-1
R2GAL
: Reaction rate of galactose decomposition to humins, M min-1
R2GLC
: Reaction rate of glucose decomposition to humins, M min-1
R2HMF
: Reaction rate of HMF decomposition to humins, M min-1
R2WH
: Reaction rate of decomposition of hemicellulose fraction to glucose, M min-1
R3WH
: Reaction rate of decomposition of hemicellulose fraction to galactose, M min-1
134
Experimental and Kinetic Modelling Studies …
R4WH
t
: Reaction rate of decomposition of water hyacinth to humins, M min-1
T
: Temperature, K
xWH,0
: Initial intake of water hyacinth, wt %
YGLC
: Yield of glucose, mol %
YLA
: Yield of LA on a molar base, mol %
YLA,wt
: Yield of LA on a weight base, mol %
YLA,total
: Yield of LA based on the total mass of water hyacinth, mol %
: Time, min
6.7 References
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Chapter 6
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138
Summary
A substantial amount of research activities is currently carried out worldwide
to identify attractive chemical transformation for the conversion of biomass to biofuels and green added-value chemicals. This research specifically focuses on the
conversion of the C6-sugars in the water hyacinth plant (Eichhornia crassipes) to
levulinic acid (LA) through an acid-catalysed hydrolysis reaction. LA has been
identified as a platform chemical for the synthesis of various organic chemicals
(see Figure A) with applications in the polymer, fuel additive and organic solvent
industries.
OH
O
HO
O
H3 C
O
OH
CH3
R
OH
H2 N
O
O
Diphenolic acid
Levulinate esters
O
δ-Aminolevulinic acid
O
O
O
CH3
OH
H3 C
O
Methyltetrahydrofuran
HO
Levulinic acid
OH
HO
O
Succinic acid
OH
1,4-butanediol
O
O
O
Tetrahydrofuran
γ-Butyrolactone
Figure A Potentially interesting derivatives of LA.
The water hyacinth plant grows extremely fast and growth rates up to 100–140
ton dry material per ha per year have been reported. As such, the water hyacinth
plant is as an excellent biomass feedstock for further conversions, for example to
LA. To define optimum reaction conditions and to develop efficient reactor
configurations for the conversion of the C6-sugars in the water hyacinth plant to
LA, the kinetics of the various chemical reactions involved should be determined.
The water hyacinth plant, like all lignocellulosic biomass sources, is a complex
material consisting of cellulose and hemicellulose polymers that are bound
together by lignin. Cellulose and hemicellulose are both involved in the
Summary
conversion of the water hyacinth plant to LA according to a complex reaction
network (Figure B). A number of side reactions also produce by-products (e.g.
insoluble humin compounds), increasing the complexity of the reaction network.
Both the intermediates and by-products complicate the development of a kinetic
model for the conversion of the C6-sugars in the water hyacinth plant to LA
considerably. Therefore, a stepwise approach was applied by investigating the
kinetics of the individual reactions separately, starting with 5-hydroxymethylfurfural (HMF).
Cellulose
Glucose
5-Hydroxymethylfurfural
Levulinic acid
+ Formic acid
Glucose
C6-sugars
Galactose
Biomass
Hemicellulose
C5-sugars
Xylose
Furfural
Arabinose
Lignin
Figure B Simplified reaction scheme for the conversion of the water hyacinth plant
to LA.
The first step was to select the optimal acid catalyst with respect to activity and
selectivity towards LA. A catalyst-screening study on the conversions of 5hydroxymethylfurfural (HMF) and glucose to LA is reported in Chapter 2. The
catalytic activities of various types of acids (homogeneous and heterogeneous
Brønsted acids) were tested. In the case of the reaction of HMF to LA, sulphuric
acid (H2SO4), hydrochloric acid (HCl) and hydrobromic acid (HBr) showed the
highest catalytic activities and LA yields. The HMF conversion and the LA yield
are correlated with the concentration of H+ in solutions, indicating the absence of
anion effects. When using glucose, H2SO4 and HCl showed the highest catalytic
activities and LA yields. It was proven that the products LA and formic acid
neither auto-catalyse the decomposition reactions of glucose to LA nor that of
HMF to LA. Of the solid acid catalysts tested, ZSM−5 gave very promising result
for the conversion of HMF to LA. With ZSM−5, catalyst recycle may be more facile
than with sulphuric acid, leading to simplified catalyst recycling strategies.
However, further optimisation studies will be required to identify whether it can
be a replacement for sulphuric acid.
Chapter 3 describes an in-depth experimental and modelling study on the acidcatalysed decomposition of HMF to LA in a batch reactor. Earlier studies generally
focused only on the overall decomposition rate of HMF without discriminating
between the rates of the main reaction to LA and the side reaction to humins.
Therefore, a general kinetic expression for a broad range of temperatures, catalyst
140
Summary
concentration and initial concentration of HMF for the main reaction as well as the
side reaction was developed using the power law approach. The kinetic
experiments were performed in a temperature window of 98–181 °C, acid
concentrations between 0.05–1 M and initial HMF concentrations between 0.1 and
1 M. The highest LA yield is obtained (94 mol %) at high acid concentrations (1 M)
and low initial HMF concentrations (0.1 M) and is essentially independent of the
temperature. Agreement between experimental data and kinetic model is good.
The rate expressions were applied to gain insights in optimum process conditions
for batch processing. The highest LA yield at a short batch times is obtained at
high temperature, a low initial concentration of HMF and a high acid
concentration.
A kinetic study on one of the key steps in the conversion of biomass to LA, i.e.,
the acid-catalysed dehydration of glucose to LA, is reported in Chapter 4. Glucose
decomposes in a consecutive reaction mode to give LA as the final product with
HMF as the intermediate. Glucose as well as HMF decomposes in parallel reaction
modes to give insoluble humins as the by-products. The kinetic experiments were
carried out in a broad temperature window (140−200 °C), using sulphuric acid as
the catalyst (0.05−1 M) and an initial glucose concentration between 0.1 and 1 M.
The highest yield was about 60 mol % at an initial glucose concentration of 0.1 M,
a sulphuric acid concentration of 1 M and a temperature of 140 °C. A kinetic
model using the power law approach was developed. Comparison of the
experimental data and the output of the kinetic model showed a good fit for a
broad range of reaction conditions. Optimisation for a batch reactor revealed that
the yields of LA are highest when applying low temperature, high sulphuric acid
concentration and low initial concentrations of glucose. The kinetic model also
implies that the highest yield of LA in continuous reactor configurations may be
achieved by applying dilute solution of glucose, a high concentration of sulphuric
acid as the catalyst and using a reactor configuration with a high extent of backmixing (e.g., a continuously ideally stirred tank reactor, CISTR).
A systematic kinetic study on the acid-catalysed hydrolysis of cellulose to LA
using sulphuric acid as the catalyst is reported in Chapter 5. A broad range of
reaction conditions was applied, including variations in temperature between 150
and 200 °C, sulphuric acid concentrations between 0.05 and 1 M and initial
cellulose intakes between 1.7 and 14 wt %. The kinetic models of HMF and glucose
decomposition were used as input to develop a novel kinetic model for the
reaction, including the side reactions to humins. A good-fit between experimental
data and modelling results was obtained. The highest yield of LA (60 mol %) may
be obtained at the low end of the temperature window (150 °C), a low initial
cellulose concentration (1.7 wt %) and a high sulphuric acid concentration (1 M).
The kinetic expressions were also used to gain insights into the optimum process
condition for the conversion of cellulose to LA in continuous-reactor
configurations. The model predicts that the highest attainable LA yield in
continuous reactor configurations is about 76 mol %, which is obtained when
using reactors with a large extent of back-mixing (e.g., a CISTR). The acid-
141
Summary
catalysed hydrolysis of cellulose is a heterogeneous reaction where mass transfer
effects may play an important role and under some conditions may even
determine the overall reaction rate. However, at our conditions, mass transfer
limitations were shown to be absent by performing reactions with different
cellulose particle sizes.
Experimental and modelling studies on the conversion of the C6-sugars in the
water hyacinth plant to LA are described in Chapter 6. The chemical composition
of the water hyacinth plant was determined, followed by systematic studies to
optimise the LA yield by altering the reaction conditions (temperature, water
hyacinth intake and acid concentration). The product distribution shows a strong
dependency on the reaction conditions, and two distinct reaction pathways may
be discriminated. At high acid-catalyst concentrations (> 0.5 M), LA is the major
organic acid whereas at low catalyst concentration (< 0.1 M) and high initial
intakes of water hyacinth, the formation of propionic acid instead of LA is
favoured. The highest yield of LA was 53 mol % based on the available C6-sugars
in the water hyacinth or 9 wt % based on dried water hyacinth. This value is at the
low end when compared to other biomass sources, due to the relatively low
amounts of C6-sugars in the water hyacinth plant. Based on this maximum yield
of LA, it can be estimated that a hectare of lake occupied by the water hyacinth has
the potential to produce LA at a rate of ~ 9 ton per year. Finally, a kinetic model
originally developed for the acid-catalysed cellulose hydrolysis was adapted and
applied to model the LA yield from the water hyacinth plant. A good fit between
the experimental data and the kinetic model was obtained.
142
Samenvatting (Dutch Summary)
Er is wereldwijd grote belangstelling voor de inzet van biomassa voor het
opwekken van energie en het maken van groene chemicaliën. Het onderzoek
beschreven in dit proefschrift concentreert zich op de conversie van de C6-suikers
in de water hyacint plant (Eichornia crassipes) naar levuline zuur (LA) door een
zuur gekatalyseerde hydrolyse reactie. LA is geïdentificeerd als een zeer
aantrekkelijk platform chemicalie, mede omdat het omgezet kan worden in een
breed scala aan interessante derivaten (zie Figuur A) met toepassingen in de
polymeer, brandstof additieven en oplosmiddelen industrie.
OH
O
HO
O
H3 C
O
OH
CH3
R
OH
H2 N
O
O
Diphenolic acid
Levulinate esters
O
δ-Aminolevulinic acid
O
O
O
CH3
OH
H3 C
O
Methyltetrahydrofuran
HO
Levulinic acid
OH
HO
O
Succinic acid
OH
1,4-butanediol
O
O
O
Tetrahydrofuran
γ-Butyrolactone
Figuur A Potentieel interessante derivaten van levuline zuur.
De water hyacint plant heeft vanwege de enorme groeisnelheid wereldwijd
grote problemen gecreëerd op meren en waterwegen. Het blijkt dat de plant 100–
140 ton droog materiaal per hectare per jaar kan aanmaken. Mede daarom is de
water hyacint een potentieel zeer interessante lignocellulosische biomassa bron
voor verdere conversie naar chemicaliën als LA.
De water hyacint plant bestaat, net als vele andere vormen van
lignocellulosische biomassa, uit cellulose en hemicellulose in een lignine matrix.
De C6-suikers in deze matrix zijn de bron voor LA (Figuur B). Voor het bepalen
van de optimale reactie condities en voor de ontwikkeling van efficiënte reactor
Samenvatting
configuraties, moet de kinetiek van de chemische reacties als weergegeven in
Figuur B bekend zijn. Helaas is dit niet het geval. Het bepalen van de kinetiek van
de afzonderlijke stappen is dan ook een van de belangrijkste doelstellingen van
het onderzoek beschreven in dit proefschrift. Vanwege de complexiteit is een
stapsgewijze aanpak toegepast, startende met de conversie van 5hydroxymethylfurfural (HMF) naar LA.
Cellulose
Glucose
5-Hydroxymethylfurfural
Levulinic acid
+ Formic acid
Glucose
C6-sugars
Galactose
Biomass
Hemicellulose
C5-sugars
Xylose
Furfural
Arabinose
Lignin
Figuur B Gesimplifeerd reactie schema voor de zuur gekatalyseerde hydrolyse
van de water hyacint
In eerste instantie is onderzoek verricht naar de beste katalysator voor de
reacties. Een katalysator screening studie voor de conversie van 5hydroxymethylfurfural (HMF) en glucose naar LA wordt beschreven in Hoofdstuk
2. De katalytische activiteit van verschillende typen zuren (homogene en
heterogene Brønsted zuren) is bepaald. In het geval van de reactie van HMF naar
LA laten zwavelzuur (H2SO4), zoutzuur (HCl) en waterstof bromide (HBr) de
hoogste katalytische activiteit en LA opbrengst zien. De conversie van HMF en de
LA opbrengst zijn gerelateerd aan de concentratie H+ in oplossing, een indicatie
voor de afwezigheid van anion effecten. Voor glucose laten H2SO4 en HCl de
grootste katalytische activiteit en LA opbrengst zien. De reactieproducten, LA en
mierenzuur, blijken de reacties van glucose naar LA niet te katalyseren. Van een
serie vaste zure katalysatoren geeft ZSM-5 de meest belovende resultaten voor de
conversie van HMF naar LA. Bij het gebruik van vaste zuren als ZSM-5 is
katalysator recycling veel makkelijker dan met zwavelzuur.
Hoofdstuk 3 beschrijft een diepgaand experimenteel en modellering onderzoek
naar de zuur gekatalyseerde reactie van HMF naar LA in een batch reactor.
Eerdere studies concentreerden zich alleen op de ontledingssnelheid van HMF,
zonder verschil te maken tussen de snelheden van de hoofdreactie naar LA en de
zijreacties naar humines. In Hoofdstuk 3 is een algemene kinetische vergelijking
opgesteld met gebruik van de ‘power-law‘ benadering. De kinetische
experimenten zijn uitgevoerd in een temperatuursgebied van 98–181 °C, zuur
concentraties van 0.05–1 M en initiële HMF concentraties van 0.1 and 1 M. De
hoogste LA opbrengst (94 mol %) is behaald bij hoge zuur concentraties (1 M), lage
144
Samenvatting
initiële HMF concentraties (0.1 M) en is relatief onafhankelijk van de temperatuur.
De overeenkomst tussen de experimentele data en het kinetisch model is goed. De
snelheids vergelijkingen zijn toegepast om inzicht te krijgen in de optimale proces
condities voor een batch systeem. De hoogste LA opbrengst bij korte batch tijden
is behaald bij een hoge temperatuur, een lage initiële HMF concentratie en een
hoge zuur concentratie.
Hoofdstuk 4 beschrijft een kinetische studie naar de zuur gekatalyseerde
dehydratie van glucose naar LA. Deze reactie verloopt via het intermediaire HMF
(Figuur B). Zowel glucose als HMF kunnen ontleden naar onoplosbare humines.
De kinetische experimenten zijn uitgevoerd in een breed temperatuur gebied (140–
200 °C), met zwavelzuur als katalysator (0.05–1 M) en een initiële glucose
concentratie tussen 0.1 en 1 M. De hoogste LA opbrengst was rond 60 mol % bij
een initiële glucose concentratie van 0.1 M en een temperatuur van 140 °C.
Vergelijking van de experimentele data en het kinetische model laten een goede fit
zien voor een brede range aan temperaturen en reactie condities.
Modelberekeningen laten zien dat de LA opbrengst in een batch reactor een
maximum heeft bij lage temperaturen, hoge zwavelzuur concentratie en een lage
initiële glucose concentratie. In het geval van continue reactor configuraties laat
het kinetisch model zien dat de hoogste opbrengst kan worden behaald bij
toepassing van verdunde glucose oplossingen, een hoge concentratie van
zwavelzuur als katalysator en het gebruik van een reactor configuratie met een
hoge mate van menging (bv. een geroerde tank reactor, CSTR).
Een systematische kinetische studie van de zuur gekatalyseerde hydrolyse van
cellulose naar levuline zuur met zwavelzuur als katalysator wordt beschreven in
Hoofdstuk 5. Een brede range aan reactie condities is toegepast (variaties in
temperatuur tussen 150 en 200 °C, zwavelzuur concentraties tussen 0.05 en 1 M en
initiële glucose concentraties tussen de 1.7 en 14 wt %). Het kinetisch model voor
de reactie van glucose naar LA is gebruikt als uitgangspunt voor de ontwikkeling
van een nieuw model voor de reactie van cellulose naar LA. De hoogste opbrengst
aan levuline zuur (60 mol %) is behaald bij de 150 °C, een lage initiële cellulose
concentratie (1.7 wt %) en een hoge zwavelzuur concentratie (1 M). De kinetische
vergelijking is ook gebruikt om inzicht te krijgen in de optimale proces condities
voor de conversie van cellulose naar levuline zuur in continue reactor
configuraties. Het model voorspelt dat de hoogst haalbare opbrengst voor levuline
zuur ongeveer 76 mol % is bij het gebruik van een reactor met een hoge mate van
menging (bv. CSTR). De zuur gekatalyseerde hydrolyse van cellulose is een
heterogene reactie waar massa transport effecten een grote rol kunnen spelen en
voor sommige condities zelfs volledig de overall reactie snelheid kunnen bepalen.
Experimenten met cellulose van verschillende deeltjesgroottes laten geen
verschillen in de LA opbrengsten zien, een indicatie dat de reacties in het
kinetische regime zijn uitgevoerd en dat de intrinsieke kinetiek bepaald is.
Een experimentele studie naar de conversie van de C6-suikers in de
waterhyacinth plant naar LA wordt beschreven in Hoofdstuk 6 met als doel om na
te gaan of geoogste water hyacint planten een geschikte biomassa bron zijn voor
145
Samenvatting
de productie van LA. De chemische samenstelling van de water hyacint plant is
bepaald, gevold door een systematische experimentele studie waar het effect van
procescondities (temperatuur, water hyacint intake en zuur concentratie) op de
LA opbrengst bepaald is. De product distributie is sterk afhankelijk van de
katalysator concentratie en twee reactie wegen kunnen worden onderscheiden. Bij
hoge zuurconcentraties (>0.5 M) wordt voornamelijk LA gevormd, bij lage zuur
concentraties (<0.1 M) en hoge initiële intakes van de water hyacint plant is de
vorming van propionzuur dominant. De hoogste opbrengst van LA was 53 mol %
gebaseerd op de beschikbare C6-suikers in de water hyacint plant ofwel 9 wt %
gebaseerd op gedroogde water hyacint. Deze waarde is aan de lage kant
vergeleken met andere biomassa bronnen. Dit is voornamelijk het gevolg van de
relatief lage hoeveelheden aan C6-suikers in de water hyacint plant. Gebaseerd op
deze opbrengst kan er ~ 9 ton LA per hectare per jaar geproduceerd worden. De
experimentele resultaten zijn gemodelleerd op basis van een kinetisch model
ontwikkelt voor de zuur gekatalyseerde cellulose hydrolyse naar LA (Hoofdstuk 5).
Het model blijkt de experimentele data goed te voorspellen.
146
Acknowledgements
This thesis is an accumulation of successes and failures of my PhD-project,
which has been accomplished by countless support of many people.
First of all, I would like to thank my promotors, Prof. H.J. Heeres and Prof.
L.P.B.M. Janssen, whose constant support finally lead me to the end of this PhDproject. Erik and Leon, thank you very much for your guidance throughout the
stages of this project. With your critical comments and deep thoughts, you always
encouraged me to gain more insights into my research and filled-up the holes in
my experimental results. Again, thank you for guiding me and giving me
examples how to become a good researcher in the academicals world.
Special thanks to Dr. Ignacio Melián-Cabrera, for his introduction to the zeolitesworld and for the fruitful discussions on the application of solid acid catalysts in
this project. Prof. J.A. Wesselingh is acknowledged for his help in estimating the
diffusion coefficients of cellulose particle. I also thank Prof. B. Roffel for his help in
correcting my MATLAB programs.
I also would like to thank the members of my reading committee Prof. A.A.
Broekhuis, Prof. J.G. de Vries and Prof. G.J. Harmsen, who have spent their precious
time to read my manuscript and gave valuable comments and remarks to enhance
the quality of this thesis.
My deepest appreciation is given for those who have helped me during my
experimental works. To Rob Cornellisen, Henk Knol and Jonathan Funk, who
provided the ampoules and always ready to seal the ampoules for me. To Anne
Appeldorn, Laurens Bosgra, Marcel de Vries and Erwin Wilbers for the technical
supports you provided. To Jan Henk Marsman for the advice on the analytical
methods, and to Peter Evers, Arnold Dalmolen, Harry Nijland, Hans van der Velde and
Annie van Dam for the analytical assistances. To Bart Danon, my master student,
who has done the exploratory study on the hydrolysis of the water hyacinth plant.
Thank you all, I could not have done it without your support.
My PhD period in the Chemical Engineering Department is one of the best
times in my life, and I would like to thank all my colleagues who have shared it
with me: Poppy Sutanto, Asaf Kleopas Sugih and Henky Muljana for being my best
partners in the academicals discussions and also in the Ping-Pong matches; my
office-mates Yao Jie, Gerard Kraai, Fesia Lestari Laksmana, Boelo Schuur and Jelle
Wildschut (thanks for translating my English Summary to Dutch); Farchad Husein
Mahfud for being a good partner in the laboratory and also the best house-mate for
three-years; Marya van der Duin-de Jonge, Michel Boesten, Francesco Pichhioni, Jos
Winkelman, Jaap Bosma, Gerald Jonker, Inge Noordergraaf, Fransesca Gambardela,
Acknowledgements
Franseca Fallani, Jasper Huijsman, Niels van Vegten, Burhan Sharafbayani, Olga
Polushkin, Anna Nizniowska, Tomasz Oniszczuk, Sameer Nalawade, Anant Samdani,
Hans Heeres, Nidal Hammoud Hassan, Marcel Wiegman, Vincent Nieborg, Youchun
Zhang, Asal Harmaneh, Danielle Keijzer, Oscar Rojas, Diana Santangelo, Diana Jijris,
Anindita Widyadhana, Mochamad Chalid, Judy Retti-Witono, C.B. Rasrendra, Agnes
Ardiyanti, Laura Justinia, Louis Daniel, Erna Subroto, Teddy, Henk van de Bovenkamp,
Abdul Osman, Camiel Janssen, Petit Wiringgalih, Wahyu, Siti Maemunah, Muhammad
Iqbal, Nadia Gozali and Chunai Dai for the friendships and sharing the good times in
our department.
In this occasion, allow me to thank my colleagues at the Chemical Engineering
Department of UNPAR (Parahyangan Catholic University, Bandung, Indonesia),
especially to Prof. Ignatius Suharto and Dr. Budi Husodo Bisowarno for their
continuous supports.
Groningen is a small city with a large population of Indonesia students. Some
of them have shared their best moments with me: Bu Ida (thanks for being my
paranimph), Bima, Mahesa, Wisnu, Pandu, Yongki, Indra, Tiara, Yeny, Mbak Tita,
Patrick, Opi, Pak Harry, Mbak Mia, Marly, Lingkan and AW (thanks for your idea
about my thesis lay-out). Thanks a lot, those shared moments will never be
forgotten. To Henk Stegeman, thanks for your friendship. I am grateful to Tante
Smith and Tante Alma for being my family during my stay in Groningen.
Thank you Papa and Mama for your endless love. And to my wife, Rina, thank
you for sharing your life with me and colouring my days.
Thank you Lord for guiding me the way of life You would have me to go.
148
List of Publications
1. Schaap, A. P.; Girisuta, B.; Heeres, H. J., The water hyacinth as a renewable
feedstock for green (bulk) chemicals. NPT Procestechnologie 2004, 11, 10-11.
2. Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., Green chemicals: From cellulose
waste to levulinic acid. Proceeding of the 6th Netherlands’ Catalysis and Chemistry
Conference 2005, Noordwijkerhout (the Netherlands), 7-9 March 2005 (oral
presentation).
3. Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., Green bulk chemicals: kinetic
study on the decomposition of 5-hydroxymethylfurfural into levulinic acid.
Proceeding of the 7th World Congress of Chemical Engineering 2005, Glasgow
(Scotland), 10-14 July 2005 (oral presentation).
4. Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., Green chemicals: conversion of
biomass to levulinic acid. Proceeding of the 6th International Conference on Process
Intensification 2005, Delft (the Netherlands), 27-29 September 2005 (oral
presentation).
5. Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., Green chemicals: A kinetic study
on the conversion of glucose to levulinic acid. Chem. Eng. Res. Des. 2006, 84,
339-349.
6. Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., A kinetic study on the
decomposition of 5-hydroxymethylfurfural into levulinic acid. Green Chem.
2006, 8, 701-709.
7. Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., Dehydration of glucose to 5hydroxymethylfurfural using aluminium salts as novel Lewis acid catalyst.
Proceeding of the 4th Asia Pacific Congress on Catalysis 2006, Singapore, 6-8
December 2006 (oral presentation).
8. Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J., Kinetic study on the acidcatalyzed hydrolysis of cellulose to levulinic acid. Ind. Eng. Chem. Res. 2007, 46,
1969-1708.
9. Girisuta, B.; Danon, B.; Manurung, R.; Janssen, L. P. B. M.; Heeres, H. J.,
Experimental and kinetic modelling studies on the acid-catalysed hydrolysis of
the water hyacinth plant to levulinic acid. 2007 (submitted to Bioresource
Technology).