Article
pubs.acs.org/IECR
Aqueous Phase Conversion of Hexoses into 5‑Hydroxymethylfurfural
and Levulinic Acid in the Presence of Hydrochloric Acid: Mechanism
and Kinetics
Diego Garcés, Eva Díaz, and Salvador Ordóñez*
Department of Chemical and Environmental Engineering, University of Oviedo, Julián Clavería s/n, 33006 Oviedo, Spain
S Supporting Information
*
ABSTRACT: The kinetics of the acid-catalyzed dehydration
of two common monosaccharides (glucose and fructose) to 5hydroxymethyl-furfural (HMF) and levulinic acid is studied in
this work. Reaction studies were performed in a stirred batch
reactor using aqueous sugar solutions (150 g/L). The
influence of reaction temperature (363−403 K) and catalyst
concentration (0.05−0.25 mol/L) was also studied. The
formation of the main products, as well as other minor
products such as humins (in both cases) or glucose dimers and
anhydroglucose (in glucose processing), is modeled considering a serial-parallel scheme reaction. The proposed model
allows predicting the formation of both main and side
products. Obtained results suggest that it is possible to work at total conversions of both monosaccharides, and to tune the
selectivity to HMF-LA by changing the operation temperature and/or catalyst concentration. As a general trend, higher
selectivities are obtained (to HMF or to LA, depending on reaction conditions) when fructose is used as a reactant. By contrast,
glucose dehydration leads to a larger amount of side products such as anhydroglucose and glucose oligomers.
1.12−15 The common conversion of glucose to HMF takes place
by isomerization of glucose to fructose before the dehydration
step. Higher selectivities of HMF from glucose have been
reported using ionic liquid and organic solvents,4,16−18
particularly when a solid catalyst is used for the isomerization
of glucose to fructose.19 However, the separation of HMF from
the organic solvent or ionic liquid is difficult, making more
advantageous the use of water as a solvent.6
In the case of aqueous systems, hydrolysis of HMF to levulinic
acid and formic acid also takes place. This process can be avoided
in biphasic systems using an immiscible organic phase to extract
HMF from the aqueous phase.1,18,21 However, levulinic acid is a
valuable chemical which can be used for polymer production
such as acrylate polymers, and fuel additives such as γvalerolactone, 2-methyl-tetrahydrofuran, or condensation adducts.22,23 Thus, the manufacture of levulinic acid could be
desirable from an economic point of view, since current levulinic
acid costs around €5500 per ton, versus a HMF market price of
€2500 per ton.23 Hence, there is interest to explore acid catalytic
dehydration for HMF and levulinic production from glucose and
fructose in aqueous media, which, depending on the operation
conditions, allows producing both platform molecules.
1. INTRODUCTION
The declining of petroleum reserves and tightening environmental policies about greenhouse gas emissions have boosted the
research on the efficient exploitation of natural resources, such as
lignocellulosic biomass, as feedstocks for the production of
chemicals and liquid fuels.1,2 The drawbacks of the production of
first-generation biofuels, such as its contribution to the increase
in food prices and the effect on deforestation and biodiversity
losses,2 promote the development of techniques for obtaining
second-generation biofuels from lignocellulosic materials.
Among the different components of lignocellulose biomass,
cellulose and hemicellulose can be converted into monosaccharides including, for example, glucose (obtained mainly
from cellulose) and xylose (obtained from hemicellulose),
glucose being the most abundant sugar. Thus, the transformation
of glucose into platform chemicals, such as 5-hydroxymethylfurfural (HMF) and levulinic acid, has recently attracted much
attention.3−5
HMF is obtained by acid-catalyzed dehydration of glucose or
fructose in aqueous media, where fructose is an intermediate in
glucose dehydration. HMF is a platform molecule for obtaining
chemicals, biofuels, and polymer precursors from renewable
resources.6−8 High yields of HMF from fructose have been
obtained in biphasic systems,7 with selectivities up to 89%, and
organic systems.9−11 On the other hand, in the case of aqueousphase glucose dehydration, lower yields were obtained (>25%)
due to both the low reaction rate and the byproduct formation,
such as anhydroglucose and other oligomers as shown in Scheme
© 2017 American Chemical Society
Received:
Revised:
Accepted:
Published:
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March 7, 2017
April 10, 2017
April 24, 2017
April 24, 2017
DOI: 10.1021/acs.iecr.7b00952
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Scheme 1. Reaction Pathway for Glucose Dehydration to HMF, Levulinic Acid, and Other Side Reactions
2.3. Analysis. The reactants were quantified by HPLC (1200
Series, Agilent) equipped with a G1362A RI detector, G1316A
column oven, G1322A degasser, G1311A quaternary pump,
G1329A automatic liquid sampler, and a Hi-Plex H column (300
× 7.7 mm, Agilent). The column was operated at 333 K, and the
detector at 328 K. Next, 5 mM H2SO4 was used as a mobile phase
at a flow rate of 0.6 mL/min. The injection volume was 20 μL of a
filtered (45 μm) sample with a 1:33 dilution. All samples were
analyzed using the RI detector. Since humins are not detected by
HPLC, and with the aim of taking them into account in the
calculations, it was supposed that all mass not detected by HPLC
corresponds to humins. Once the concentration of each
compound was determined, selectivities were calculated as
follows:
Although most current studies of dehydration of sugars are
focused on heterogeneous catalysis, homogeneous catalysis is the
industrial method currently used in the production of levulinic
acid because of the low price of mineral acids.24 Therefore, the
development of accurate kinetic models is needed for the design
of these processes.
In this work, dehydration reactions of glucose and fructose in
an aqueous medium are studied using hydrochloric acid as a
homogeneous catalyst. The choice of this acid is based on the
wide availability of hydrochloric acid, even being a low cost waste
in many industrial processes, and in addition, most of the
previous works related to dehydration kinetics of sugar were
developed using H2SO4 as a catalyst, an acid stronger than HCl
and, for that matter, with different effects on the dehydration
process. The kinetics of dehydration of fructose and glucose are
discussed separately to estimate the influence of the isomerization step of glucose to fructose in the dehydration process. A
study about the variable influence of acid concentration on each
one of the reaction steps is carried out.
n
Si = ni moli/∑ nj mol j
j=1
(1)
ni and nj being the number of carbons in the components i and j,
respectively, and moli and molj, the moles of the components i
and j in the reaction mixture, respectively. The sum ∑n1 Cjmolj
includes the component i.
The solids obtained for both reactions were analyzed in a TGDSC instrument (Setaram, Sensys) using α-alumina as an inert
reference material. Samples (20 mg) were treated in a nitrogen
flow (20 mL/min) with a temperature program of 5 K/min from
298 to 973 K trying to identify the different compounds involved
in these solids.
The value of the pH was followed during the experiments, but
very small variations have been detected. The measured pH
values were lower than 1 in all the experiments, and without
significant variations during the reaction.
2.4. Kinetic Modeling. The kinetic model considers the
reactor as an ideally stirred batch reactor (BR). Therefore, mass
balance for a given molecule follows a differential eq (eq 2). By
combining the reactor model with the appropriate power-law
rate expressions (eq 3) and Arrhenius eqs (eq 4), the complete
model is obtained (a system of ordinary differential equations),
which was implemented and solved in Scientist Software (2006,
MicroMath, USA).
2. EXPERIMENTAL SECTION
2.1. Materials. D-(−)-Fructose (≥99%), D-(+)-glucose
(≥99.5%), and sodium hydrogen carbonate (≥99.7%) were
purchased from Panreac Applichem. 5-(Hydroxymethyl)furfural
(≥99%), formic acid (98%), and levulinic acid (98%) for HPLC
calibration were acquired from Sigma-Aldrich, and hydrochloric
acid (37%) was purchased from Fisher Chemical. Solutions were
prepared using distilled water.
2.2. Experimental Procedure. Reactions were carried out
in a 0.5 L stirred batch autoclave reactor (Autoclave Engineers
EZE seal) with a back pressure regulator and a PID temperature
controller. The reaction volume was 0.25 L of an aqueous
solution of 45 g of D-(−)-fructose, for the first set of experiments,
or D-(+)-glucose, for the second set. Five different reaction
temperatures were studied (363−403 K). Once the desired
temperature was reached, 50 mL of a solution of hydrochloric
acid were added, resulting in an acid concentration of the
reaction mixture of 0.25 M. Air was purged with N2, and
dehydration was carried out with 10 bar of N2 with a stirring of
600 rpm for 4 h in the case of experiments at 363−383 K and for
3 h in the case of experiments of 393 and 403 K.
In order to study the effect of reaction media acidity, additional
experiments were carried out with acid concentrations of 0.05
and 0.1 M at 393 K for both reactants, keeping constant the other
conditions.
Samples were taken from the sampling port, filtered using 0.45
μm Nylon syringe filters, and diluted in a 1:33 ratio.
M
dCi /dt =
∑ −αijrj
j=1
(2)
with Ci being the concentration of the i component (mol L−1);
αij, the stoichiometric coefficient of component i on the reaction
j; and rj, the reaction rate for reaction j (mol L−1 s−1).
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Table 1. Summary or the Results Obtained for Conversion of Fructose and Glucose to HMF, Levulinic Acid, Formic Acid, AHG
and G2a (CGlucose = 180 g L−1; CHCl = 0.25 M)
selectivity (%)
a
entry
feed
T (K)
time (h)
conv. (%)
HMF
levulinic acid
formic acid
AHG
G2
1
2
3
4
5
6
7
8
9
10
fructose
fructose
fructose
fructose
fructose
glucose
glucose
glucose
glucose
glucose
363
373
383
393
403
363
373
383
393
403
4
4
4
3
3
4
4
4
4
4
54
78
93
97
100
20
27
38
49
49
69
49
28
11
3
0
9
16
9
6
26
42
59
76
83
0
15
28
51
43
6
9
13
13
14
0
0
6
7
7
0
0
0
0
0
0
0
13
16
21
0
0
0
0
0
100
76
37
17
23
G2: glucose dimer.
Figure 1. Evolution of the HMF (a) and LA (b) yields with reaction time and temperature for fructose dehydration. Temperatures: 363 K (▲), 373 K
(●), 383 K (■), 393 K (⧫), and 403 K (×). Solid lines represent the evolution of the concentration of different products according to the theoretical
model proposed.
N
In the above equation, kj0 is the pre-exponential factor in min−1, A
is the acid concentration in mol L−1, mj is the reaction order of
acid concentration, Eaj is the activation energy in J mol−1, R is the
universal gas constant in J mol−1 K−1, and T is the temperature in
K. Different kj values were found for each experiment, and then,
an Arrhenius fit between kj and temperature (T) was carried out
to find the activation energy of each individual reaction keeping
the HCl concentration constant. The kinetic model was fitted to
n
rj = kj ∏ Ck jk
k=1
(3)
with kj being the rate constant of the j reaction; Ck, the
concentration of the k component (mol L−1); and nk, the reaction
order of the k component.
kj = kj0 Amj exp( −Eaj /RT )
(4)
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Figure 2. Comparison between experimental and theoretical data for the evolution of fructose concentration between 363 and 403 K. For symbol codes,
see Figure 1.
fructose mixtures in aqueous media in the presence of HCl and
reported that the fructose concentration decreased sharply from
the beginning of the reaction, whereas the concentration of
glucose was reduced much more slowly.15 Nikolla and coworkers used Sn-Beta zeolite as a catalyst necessary for the
isomerization of glucose to fructose, while HCl was used for the
dehydration of fructose to HMF.19 In this study, the presence of
Sn-Beta zeolite enhanced the conversion of glucose from 26% to
75% under the same reaction conditions. Therefore, the absence
of a catalyst with Lewis acid sites slows the isomerization of
glucose to fructose, making it the controlling step of the overall
reaction. As a consequence, a small amount of fructose is rapidly
converted into HMF and, later, levulinic and formic acid due to
the presence of HCl. This fact was confirmed by the absence of
fructose in the samples of the glucose dehydration reactions.
Other works argue that there is a direct method of dehydration of
glucose to HMF, although it is much less important than the
transformation via isomerization.15
Regarding the reaction products, Figure 1 shows the evolution
of the HMF and LA yields with reaction temperature for the
fructose dehydration reaction. The formation of HMF is highly
dependent on the reaction temperature. The maximum HMF
yield (57%) was achieved at 393 K in only 40 min. The use of a
higher temperature resulted in lower HMF yields since, at a high
temperature, HMF is rehydrated to LA and formic acid so that
the LA yield showed a continuous increase with increasing
reaction temperature. According to Figure 1, obtained results
suggest that it is possible to tune the selectivity in fructose
dehydration. Especially, it is possible to maximize the selectivity
to HMF by an appropriate selection of the reaction time (or
space time, in a continuous process), working in the interval
393−403 K. By contrast, decreasing reaction temperature is not
an effective way to increase HMF selectivities. Concerning LA
selectivities, they are maximized as both reaction temperature
and time increase, with almost total conversions to LA being
possible.
Figure 2 shows the evolution of fructose concentration with
the temperature. As expected, fructose decreases more quickly as
reaction temperature increases, this effect being more marked
between 383 and 393 K.
experimental data by applying the linear regression model using
the coefficient of determination (R2). After that, reactions at
different acid concentrations were carried at constant temperatures to evaluate the influence of acid concentration on different
kinetic reactions. Consequently, the kj vs A ratio was plotted in
order to find the reaction order of acid concentration for each
reaction step.
3. RESULTS AND DISCUSSION
The dehydration of hexoses to HMF in aqueous media using acid
catalysis depends on several parameters, such as acid
concentration, temperature, and initial sugar concentration. In
this work, initial sugar concentration was initially fixed at 150 g
L−1 (13 wt %) since, according to the National Renewable
Energy Laboratory of the U.S. Department of Energy, the typical
range of sugar concentration of a cellulosic hydrolysate is 10−15
wt %. Thus, the effect of both temperature and acid
concentration was studied.
Scheme 1 shows the main reaction pathway for glucose
conversion to 5-HMF including the formation of condensation
products (humins) from polymerization of fructose and glucose
molecules.8,15 In this mechanism, several side reactions are
involved. Among them, the rehydration of HMF to levulinic and
formic acid in aqueous media,6−8,11,12,20,25−29 the dehydration of
glucose to anhydroglucose (AHG),3,14 and the polymerization of
glucose should be highlighted.1
3.1. Effect of the Temperature on Product Distribution.
Table 1 summarizes the results of conversion for fructose and
glucose dehydration reactions from 363 to 403 K and the
selectivity data for different reaction products. The conversion
evolution is the most remarkable difference between fructose and
glucose dehydration. For example, at 383 K, the conversion of
fructose (93% at 4 h reaction time) was considerably higher than
the conversion of glucose (38% at the same reaction time and
operation conditions). Although the presence of HCl catalyzes
sugar dehydration, the efficient transformation of glucose to
fructose by aldose-ketose isomerization requires the presence of
a Lewis acid site catalyst such as hydrotalcite.3 Toftgaard
Pedersen and co-workers studied the dehydration of glucose−
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Figure 3. Comparison between experimental and theoretical data for the evolution of glucose (a), 5-HMF (b), levulinic acid (c), formic acid (d), a
glucose dimer (e), and anhydroglucose (f) concentration between 363 and 403 K. For symbol codes, see Figure 1.
reached at the lowest time as the acid concentration increases.
Furthermore, after the maximum of LA selectivity was reached
for the highest acid concentration, it did not decrease, indicating
the strong stability of LA in the reaction conditions.
Table 2 shows the values of conversion for dehydration
reactions of fructose and glucose carried out at different acid
concentrations. In the case of dehydration of fructose, all the
reactions reached a conversion of over 96%. On the other hand,
in the case of glucose dehydration, obtained conversions are
largely lower (max. 36%), although conversions increase at
increasing acid concentrations
Figure 4e shows the evolution of the selectivity of HMF with
the reaction time of the glucose dehydration at 393 K for
different acid concentrations. The evolution of the selectivity of
HMF indicates that it is not a primary product since it is formed
after the equilibrium between glucose and side products (AHG
and dimer glucose) have been established.15 Furthermore, Figure
4a shows the evolution of HMF concentration and theoretical
HMF concentration data for dehydration reactions of glucose
with different acid concentrations at 393 K. The shape of the
evolution of HMF concentration is similar to those found in
previous works. The concentrations reached a maximum during
the reaction time since the conversion of HMF to levulinic acid
and formic acid is much faster than the conversion of glucose to
HMF.26 As the dehydration reaction of sugars and the hydration
process of HMF are catalyzed by the presence of acid, the higher
the acid concentration, the higher the slope of increase and
decrease of HMF concentration, as found in other work,10 and
the lower the HMF selectivity as shown in Figure 4e.
On the other hand, as shown in Figure 4f, the selectivity for LA
with the reaction time does not depend on acid concentration.
These results are consistent with those obtained by Rivas and co-
Regarding carbon balance, it decreases to 53% in the case of
the fructose dehydration at 393 K in the liquid phase. This
behavior could more likely be caused by the formation of humins.
The behavior of the glucose at these reaction conditions is
completely different, with the formation of several byproducts
out of the main isomerization−dehydration route being
observed. Thus, Figure 3 shows the evolution of the experimental
concentration data and theoretical data predicted by the kinetic
model, proposed below, of glucose, HMF, levulinic and formic
acid, AHG, and a glucose dimer at different temperatures.
This evolution shows a remarkable initial decrease of the
concentration of glucose, especially at the highest considered
temperatures, in good agreement with the strong initial increase
of the concentration of side products, such as AHG and the
glucose dimer. It is concluded that glucose is primarily converted
into side products by equilibrium controlled reactions. After
equilibrium, once AHG and glucose dimer concentrations are
stabilized, glucose follows the expected degradation route
yielding HMF.15 Since the isomerization from glucose to
fructose is the slowest step of the overall mechanism, when
small amounts of HMF are formed, the process of rehydration to
levulinic and formic acids takes place to a large extent, leading to
low HMF selectivities, even at the highest studied temperatures.
3.2. Effect of Acidity (pH). Figure 4 shows the influence of
the acid concentration on the selectivity of HMF and levulinic
and formic acid during the dehydration reaction of fructose.
Regarding HMF, being a primary product, its selectivity
decreased with the reaction time in all cases, but in the case of
the lowest acid concentration, this decrease was slight because
the subsequent rehydration of HMF to levulinic and formic acid
was facilitated by high acid concentrations, as shown in Figure 4b
and c. On the other hand, the maximum selectivity of HMF is
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Figure 4. Influence of the concentration of hydrochloric acid on the selectivity of HMF (a), LA (b), and FA (c) during the dehydration of fructose at 393
K, and on the selectivity of HMF (d) and LA (e) during the dehydration of glucose at 393 K. 0.05 M (■), 0.1 M (●), and 0.25 M (▲).
dehydration product, its presence is favored by the presence of
acid.
3.3. Kinetic Model. The reaction network seen in Scheme 1
was proposed based on separate fructose and glucose
dehydration. We propose a novel kinetic model for the acidcatalyzed dehydration of fructose and glucose to levulinic acid,
with a particular focus on possible side reactions which had
deserved little attention in previous works. In addition, most
dehydration reactions have been studied using sulfuric acid as a
catalyst, whose effects on the kinetics of dehydration have been
sufficiently described in previous works.26,28,29 However, we have
chosen in this work to delve into the kinetics of dehydration
using hydrochloric acid as a catalyst. To determine the overall
reaction mechanism, the different reaction products were
classified according to selectivity−conversion plots. Figure S1
shows the evolution of HMF selectivity with conversion for the
reaction of fructose dehydration. In this figure, HMF selectivity
begins to fall when the value of fructose conversion reaches 25%.
This fact suggests that HMF is a primary product formed in
fructose dehydration. However, according to Figure S2, levulinic
acid, and therefore formic acid, are secondary products formed
from HMF rehydration since LA selectivity increases when the
fructose conversion exceeds 25%. Although several authors
Table 2. Conversion Values for Different Acid Concentration
Reactions from Fructose and Glucose
conversion (%)
feed
temperature
(K)
reaction time
(min)
0.25 M
0.1 M
0.05 M
fructose
glucose
393
393
180
240
96.77
36.56
98.58
17.49
98.71
10.91
workers.30 According to this study, the absence of a catalyst
which facilitates the isomerization step makes the kinetic
constant of HMF formation extremely low in spite of the high
acid concentration.30 This is tentatively explained considering
that the overall process is controlled by the isomerization
glucose−fructose step, independently of the acid concentration.
On the other hand, Figure 5a and b show the evolution of
concentration of glucose dimer and anhydroglucose for
experimental and theoretical model data. As mentioned above,
both side products are obtained through polymerization and
dehydration equilibria, respectively, making these concentrations
remain stable after a given reaction time, as predicted by the
proposed model. In the case of anhydroglucose, being a
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Figure 5. Comparison between experimental and theoretical data for the evolution of AHG (a) and glucose dimer (b) concentration for dehydration of
glucose at a different acid concentration at 393 K. For symbol codes, see Figure 4.
propose a direct reaction of glucose to obtain HMF,15,31,32 our
results suggest that HMF is only formed as a primary product
when fructose is used as a parent reactant.
Concerning the glucose dehydration, HMF selectivity shows a
pattern of an intermediate product (Figure S3), the selectivity
being very low at low conversion (suggesting the formation of
other primary products such as anhydroglucose and glucose
dimers), and also presenting a slight decrease at the highest
conversion. This fact is due to HMF being an intermediate
product formed after the equilibrium among glucose and
anhydroglucose and glucose dimers, but before the formation
of levulinic acid and formic acid. By contrast, LA presents the
typical pattern (Figure S4) of a secondary product not
undergoing further reactions. LA selectivity increases with
conversion, this conversion being higher with temperature.
All these reactions are acid-catalyzed, so HCl concentration
must be included in the reaction rate equations. Although some
previous works suggest a linear relation of the different apparent
kinetic constants with HCl concentration,15 reaction orders on
acid concentration for each reaction were preliminarily evaluated
by power law equations to check this ratio.
For that, this study suggests that glucose primarily leads to the
formation of glucose (R1), and subsequently, dehydration of
fructose to glucose takes place (R2), being the rate of these
reactions expressed by the following equations:
R1 = k1(CG)nG1
(5)
k1 = k10 Am1 exp( −Ea1/RT )
(6)
R 2 = k 2(C F)nF2
(7)
k 2 = k 20 Am2 exp( −Ea2 /RT)
(8)
As mentioned previously, whereas the dehydration of fructose
to HMF takes place through a single step irreversible reaction
(R2), the dehydration of glucose occurs via isomerization from
glucose to fructose (R1). In this study, it is considered that the
isomerization reaction of glucose to fructose, in the absence of a
Lewis acid catalyst, is the slow step of the overall process of the
dehydration of glucose. Therefore, from the point of view of the
proposed kinetic model, it is assumed that the process of
dehydration of glucose is a one-step irreversible reaction (R8)
whose activation energy would be similar to that corresponding
to the isomerization reaction of glucose to fructose because this is
the slowest step of the overall process. For that, the rate
expressions of R1 and R8 are the same.
In parallel, undesired products (humins) are produced from
glucose (R3) and fructose (R4).11,33,34 For these products, the
reaction rates are defined as follows:
R3 = k 3(CG)nG3
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Table 3. Summary of the Pre-exponential Factors, Activation Energies, and Reaction Order for the Elemental Steps Mentioned in
Scheme 1a
a
reaction (j)
reactant (k)
Ea (kJ mol−1)
log10(kjo) (M(1−njk−mj) min−1)
njk
mj
2
3
4
5
6
7
8
fructose
glucose
fructose
HMF
glucose
glucose/G2
glucose
88 ± 3.04
135 ± 5.42
136 ± 5.39
91 ± 2.21
115 ± 4.87
77b ± 3.27
114 ± 4.01
10.53 ± 0.53
25.13 ± 0.82
15.72 ± 0.64
22.09 ± 0.74
27.39 ± 0.81
16.11 ± 0.54c
25.37 ± 0.78
1.00
2.00
1.00
1.00
1.00
2.00/1.00
1.00
0.35 ± 0.02
1.47 ± 0.03
1.51 ± 0.04
0.49 ± 0.02
1.45 ± 0.05
1.47 ± 0.02/0.67 ± 0.01
0.33 ± 0.01
Model fitting was done considering both experimental data from glucose and fructose. bEa = Ea7b − Ea7f. clog10(k70) = log10(k7f0) − log10(k7b0).
k 3 = k 30Am3 exp( −Ea3/RT )
(10)
R 4 = k4(C F)nF4
(11)
k4 = k40 Am4 exp( −Ea4 /RT )
(12)
may be represented by the following ordinary differential
equations.
To confirm that the obtained solids after the reaction were
only humins, thermogravimetric analyses (TGA) of the two solid
samples were carried in an inert atmosphere (N2). Figure S7
shows the TGA thermograms of two humins from the
dehydration of fructose and glucose, respectively, at a heating
rate of 5 K/min. The thermal behavior of both solid samples is
very similar. The thermal decomposition takes place between
430 and 950 K, losing about 50% of the sample weight. The
highest decomposition rate for both solids was observed at 650
K. These data are consistent with those found in previous works
in which the thermal decomposition has the same trend and the
same shape.35
On the other hand, glucose can also undertake both intra- and
intermolecular dehydration reactions (also known as reversible
reactions) yielding anhydroglucose (R6) and glucose dimer (R7),
respectively.15,36
nG6
R 6 = k6(CG)
nG7f
R 7 = k 7f (CG)
− k 7b(CAHG)
(15)
k 7f = k 7f 0Am7f exp( −Ea7f /RT )
(16)
m7b
k 7b = k 7b 0 A
exp( −Ea7b/RT)
(17)
Finally, levulinic acid and formic acid are formed when HMF is
treated with hydrochloric acid (R5), and the rate of this reaction is
expressed as follows:
R 5 = k5(C HMF)nHMF5
(18)
k5 = k50Am5 exp( −Ea5/RT )
(19)
dC HMF
= R 2 − R5
dt
(21)
dC G
= −R3 − R 6 − R 7 − R 8
dt
(22)
dC HMF
= R 8 − R5
dt
(23)
dCG2
= −R 7
dt
(24)
Using Scientist Software and using the rate expressions (eqs
5−19) in combination with the mass balances (eqs 20−24),
different reaction orders were tested for each involved chemical
and theoretical model for the dehydration of fructose and glucose
has been defined, looking for the combination which involves the
lowest value of the sum of squared errors as it was used in
previous studies.15 The formation of both humins (R3) and
glucose dimers (R7) from glucose are better described by second
order dependence on glucose concentration.15 In the case of the
formation of glucose dimers, according to previous works, the
disaccharide formation from glucose is a relatively simple
bimolecular reactions.36 All the other reactions are well-defined
by first order dependences on each reactant as occurs in previous
works.39
Thus, the activation energies, reaction orders, and preexponential factors for the seven rate expressions proposed are
given in Table 3. In the case of glucose dehydration, only
experiments performed in the 383−403 K temperature interval
were considered because the low concentration or reaction
products were obtained at lower temperatures.
Regarding Figures 1 and 2, the theoretical model fits very well
to the experimental evolution of the concentration of fructose at
different temperatures and HMF and LA yields for the
dehydration of fructose. However, according to Figures 3 and
4, the fit provided by the proposed model is worse in the glucose
dehydration experiments. This fact can be explained by the
higher complexity of the reaction network, involving the
presence of reaction products at very different concentration
intervals. However, the model adequately fits the evolution of
concentration and selectivity of anhydroglucose and the glucose
(14)
nAHG7b
(20)
And the concentration of different compounds involved in the
dehydration reaction of glucose as a function of time may be
represented by the following differential equations.
(13)
k6 = k60Am6 exp( −Ea6 /RT )
dC F
= −R 2 − R 4
dt
Although some previous works argue that the formic and
levulinic acid are formed directly from glucose and fructose, in
these experiments, it has been confirmed that the appearance of
these acids does not take place immediately but occurs after the
formation of HMF.15
Previous works have shown both levulinic and formic acids are
stable under similar reaction conditions and do not decompose
to humins or other organic compounds in the wide range of
reaction conditions applied.26
For a batch reaction setup, the concentration of the different
species involved in the dehydration of fructose as a function of
time, using the proposed kinetic mechanism shown in Scheme 1,
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Ind. Eng. Chem. Res. 2017, 56, 5221−5230
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Industrial & Engineering Chemistry Research
lower rates due to the lack of a catalyst active for glucose to
fructose isomerization. Due to that, acid concentration only
slightly affects the levulinic acid selectivities in glucose
dehydration. The formation of glucose dimers and anhydroglucose is also significant when using glucose as a reactant.
A kinetic model was proposed based on an experimental
investigation where the temperature and acid concentration were
used as variables. The model describes perfectly the evolution of
the concentration of major compounds as such fructose, and
HMF and levulinic acid in the case of the dehydration of fructose.
In the case of the glucose reaction, the model is able to predict the
reactivity trends (including the formation of lateral minor
products), in spite of the higher complexity of the reaction
network.
dimer, primary products of the dehydration of glucose.
Therefore, according to Figures 3 and 4, the theoretical
selectivity of HMF and levulinic acid is independent of the
acid concentration, whereas the concentration of the glucose
dimer and anhydroglucose increases with the acid concentration.
This fact is due to the slowest step of the process being the
isomerization of glucose to fructose, which controls the
subsequent steps such as the dehydration of fructose to HMF
and the rehydration of HMF to LA, but the isomerization process
does not control the dehydration of glucose to AHG and the
polymerization of glucose to the glucose dimer.
Some previous works about kinetics of homogeneous acid
catalyzed fructose dehydration have been found, and the values
of activation energy and pre-exponential factors have been
compared with data obtained in this study. However, there are
few articles studying homogeneous acid glucose dehydration.
Swift and co-workers have reported an activation energy for the
dehydration of fructose to HMF of 126 kJ/mol.37,38 This is a
result higher than that obtained in this work (88 kJ/mol). In
water, the activation energy for acid catalyzed dehydration of
fructose is around 126 and 160 kJ/mol for pH between 1.0 and
1.8. So taking into account, in our case, the initial pH value being
0.56, HCl acts as a catalyst reducing the activation energy of the
reaction, and by increasing the acid concentration, the value of
the activation energy is reduced. In the case of rehydration of
HMF to levulinic and formic acid, the activation energy reaches a
value of 91 kJ/mol, very similar to values found in other works
(94−97 kJ/mol).24,36,39 In regard to the activation energy for the
formation of humins from glucose and fructose, the values
obtained are 165 and 136 kJ/mol, respectively, very similar to
those found in previous works (148−171 kJ/mol).40,41
Referring to the HCl reaction orders with respect to
dehydration from fructose to HMF, the formation of humins
from glucose, the rehydration of HMF to levulinic and formic
acid, and the dehydration of glucose to HMF, the values obtained
were 0.35 ± 0.02, 1.47 ± 0.03, 0.49 ± 0.02, and 0.33 ± 0.01,
respectively. These reaction orders are below those shown in
previous works.25 This fact may be due to, according to most of
the previous works, H2SO4 being used as a catalyst and H2SO4
being an acid stronger than HCl. This means the presence of
H2SO4 affects to a greater degree the dehydration process, and
for that, the reaction order with respect to the acid is higher. On
the other hand, the highest reaction order with respect to HCl
corresponds to the formation of humins. This fact means that the
variation of acid concentration affects, to a greater extent, the
process of formation of humins than the dehydration process.
The presence of acid also influences in a special way the
formation step of AHG and dimer glucose from glucose due to
the high values of HCl reaction order. In this case, due to the
absence of a Lewis acid catalyst, which facilitates the isomerization step from glucose to fructose, the activation energy of the
formation of HMF and AHG from glucose is very similar.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.iecr.7b00952.
(Figure S1) Evolution of the selectivity of HMF with
fructose conversion at different temperatures (363−403
K). (Figure S2) Evolution of the selectivity of LA with
fructose conversion at different temperatures (363−403
K). (Figure S3) Evolution of the selectivity of HMF with
glucose conversion at different temperatures (383−403
K). (Figure S4) Evolution of the selectivity of LA with
glucose conversion at different temperatures (383−403
K). (Figure S5) TGA analyses of humins generated during
the dehydration of fructose (a) and glucose (b) (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*Tel: +34 985 103 437. Fax: + 34 985 103 434. E-mail:
[email protected].
ORCID
Salvador Ordóñez: 0000-0002-6529-7066
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work has been financed by Research Projects of the
Regional Government of Asturias (project reference GRUPIN14-078) and Spanish Ministry for Economy and Competitiveness (CTQ2014-52956-C3-1-R). D.G. thanks the
Government of the Principality of Asturias for a Ph.D. fellowship
(Severo Ochoa Program).
■
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