Chinese J. Chem. Eng., 14(5) 708—712 (2006)
RESEARCH NOTES
Kinetics of Levulinic Acid Formation from Glucose Decomposition at
High Temperature*
CHANG Chun(常春)a,b, MA Xiaojian(马晓建)b and CEN Peilin(岑沛霖)a,**
a
b
College of Material Science and Chemical Engineering, Zhejiang University, Hangzhou 310027,China
College of Chemical Engineering, Zhengzhou University, Zhengzhou 450002,China
Abstract Levulinic acid is a kind of new green platform chemical with wide application. The kinetics of levulinic acid
formation from glucose decomposition at high temperature was investigated. Glucose containing 1%, 3% or 5% H2SO4
was treated at 170℃ or 190℃. For the various experimental conditions assayed, the time-courses of glucose and glucose
degradation products (including 5-hydroxymethylfurfural and levulinic acid) were established. These variables were correlated with the reaction time based on the equations derived from a pseudo-homogeneous, first-order kinetic model,
which provided a satisfactory interpretation of the experimental results. The set of kinetic parameters from regression of
experimental data provided useful information for understanding the levulinic acid formation mechanism.
Keywords levulinic acid, kinetics, decomposition, platform chemical
1
INTRODUCTION
With the plentiful consumption of non-renewable
resources and aggravation of energy crisis, using renewable resources to replace non-renewable ones is
arousing more and more attention around the world[1].
Especially, using abundant renewable biomass to replace petroleum as raw materials for the chemical industry has important strategic significance.
Levulinic acid (LA, H8C5O3), or gammaketovaleric acid, is a short chain fatty acid having a
ketone carbonyl group and an acidic carboxyl group.
It is a versatile new platform chemical with numerous
potential uses for making textile dye, antifreeze, animal feed, coating material, solvent, food flavoring
agent, pharmaceutical compounds and resin[2].
There are many methods for preparing LA, and
the most widely used approach is dehydration of
biomass or carbohydrate with acid[3―6]. Recently, a
new process developed by Biofine Corporation laid
the foundation for effective LA production[7]. The
technology can be successfully employed using diverse cellulose-containing waste materials such as
paper mill sludge, urban waste paper, agricultural
residues and cellulose fines from papermaking as
starting materials. Those materials were hydrolyzed
with acid at high temperature in a novel two reactor
system and the final yield of LA was more than 60%,
one of the highest reported in the literature.
Although LA has been known since the 1870’s, it
has never attained much commercial significance. One
of the reasons for its slow development is the low
yields of LA[8]. Till now, the mechanism and kinetics
of LA formation are not understood clearly, which
should be studied deeply. Some papers devoted to ki-
netics and mechanism of LA formation under low
temperature 80—98℃[9], but few papers about LA
formation under high temperature were reported. The
goal of the present work is to study the kinetics of LA
formation from glucose decomposition under high
temperature above 160℃.
2 EXPERIMENTAL
2.1 Equipment and procedure
The experiment on hydrolysis reactions were carried out in a series of cylindrical pressurized reactors.
Each reactor had inner diameter of 35mm and depth of
130mm, the wall thickness of the reactor was 7.5mm,
and the total volume was 125ml. The reactors were
made of stainless steel (316L) to resist corrosion. The
temperature of the reactor contents was monitored by
a thermocouple. The reactors were heated in a salt
bath, whose temperature was controlled by an adjustable electric cooker and monitored using a thermalcouple with digital readout. When the reaction was
stopped, the reactor was quenched by quickly immersing in a water bath.
For each experiment, 60ml of a solution was put
into each reactor. The solution was made up by dissolving 5g of glucose in 100ml of sulfuric acid solution. A set of data were collected over a range of reaction time at fixed temperature. Experimental time
stopped with the quenching of the reactor. After
quenching, the samples were filtered and analyzed.
2.2
Analysis
Samples were filtered, and then the concentration of
remaining glucose, 5-hydroxymethylfurfural (5-HMF)
and levulinic acid were analyzed, respectively. Glucose
Received 2005-09-28, accepted 2006-03-09.
* Supported by the Natural Science Foundation of Henan Educational Committee (No.200510459056).
** To whom correspondence should be addressed. E-mail: [email protected]
Kinetics of Levulinic Acid Formation from Glucose Decomposition at High Temperature
was analyzed by glucose oxidase kits[10]. 5-HMF was
analyzed by a spectrophotometric method. HMF in
samples firstly reacted with 2-tiobarbituric acid (TBA)
in acid medium, and then heated to 40℃ for 30min.
Absorption spectra were acquired between 500nm and
300nm. The concentration of 5-HMF was determined
by using the derivative spectrophometry method at
435nm[11]. The concentration of LA was determined
by gas chromatography with a flame ionization detector (FID). LA was separated on a FFAP capillary
column (30m×0.32mm×0.33μm) with a linear tem－
perature program of 15℃·min 1 (initial 90℃, final
210℃, injector 240℃, detector 250℃)[12]. The sample
injection volume was 1μl. The retention time of LA
was 7.6min, and the retention time of the butyric acid
internal standard was 1.7min.
2.3
Kinetic model
Glucose can decompose into a variety of soluble
products such as 5-HMF, levulinic acid, formic acid
etc. and insoluble products. Due to the difficulty to
find a rigorous mechanism of hydrolysis reaction, it is
usual to use simplified model to determine the kinetics
of glucose decomposition. A pseudo-homogeneous
irreversible first-order reaction model was proposed in
the literature. Neglecting the formation of intermediates, glucose can decompose into 5-HMF during
thermo chemical reaction, which is a precursor of
LA[13]. Then, 5-HMF decomposes into LA and formic
acid under acidic conditions. During the reaction the
formation of humic solids was monitored. Some of
these black insoluble products can be easily separated
from the solution by filtration, which was in agreement with Baugh’s report[14]. Therefore, LA resulting
from 5-HMF decomposition can be viewed as part of
products from the overall glucose decomposition. The
model proposed in the literature was as follows:
Based on the above model, the following set of
differential equation can be obtained:
d c glu
dt
= − kc glu
(1)
dc5-HMF
= k1cglu − k2 c5-HMF
dt
(2)
dcLA
= k 2 c5-HMF
dt
(3)
with k=k1＋k3.
The analytic expression of concentration of glucose, HMF and LA are
cglu = cglu0 e− kt
(4)
c5-HMF =
k1cglu0
k2 − k
(e − kt − e− k2t ) + e− k2t c5-HMF0
(5)
cLA =
k1k2 cglu0 ⎛ 1 − e − kt 1 − e − k3t
−
⎜
k2 − k ⎝ k
k2
709
⎞
⎟+
⎠
c5-HMF0 (1 − e − k3t ) + cLA0
(6)
The kinetic coefficients can be correlated with
temperature by applying Arrhenius equation[15]:
k1 = k10 e − E1 RT
(7)
k2 = k20 e − E2
RT
(8)
− E3 RT
k3 = k30 e
(9)
It is common to modify the Arrhenius equations
to model the effect acid concentration as follow[16]:
k1 = a1C n1 e − E1 RT
(10)
k2 = a2C n2 e − E2
RT
(11)
k3 = a3C n3 e − E3 RT
(12)
Equations (4)—(12) were used to fit the experimental data. The parameters in above equations are
evaluated using the method of non-linear least squares
regression analyses by MATLAB 6.5.
In the experiments, it was found part of humic
solids adhered to the inner wall of the reactor tightly,
and it was impossible to collect the whole solids precisely. So, the experimental data of humic solids were
not discussed in the article.
3
RESULTS AND DISCUSSION
The reactions were conducted at temperature of
170℃, 190℃ and 210℃, with mass concentration of
sulfuric acid at 1%, 3% and 5%. With the increase of
reaction temperature, it was found the rate of glucose
decomposition increased sharply. When the reaction
temperature was 210℃ and concentration of sulfuric
acid was more than 3% , glucose and 5-HMF can
wholly decompose in 1.5min, the data of concentration of glucose, 5-HMF and LA were inapplicable to
kinetic studies. Therefore, 170℃ and 190℃ were
chosen as the reaction temperature.
Figures 1 and 2 show the concentration of glucose, 5-HMF, LA during the glucose decomposition
performed with 1%, 3% and 5% H2SO4 at 170℃ and
190℃ respectively. It can be observed that the trends
of concentration are in agreement with consecutive reaction, and it is also seen that both temperature and acid
concentration have a large effects on the rates of glucose decomposition, and formation of 5-HMF and LA .
Tables 1 and 2 list the sets of kinetic parameters
obtained from experimental data, whereas Figs.1 and 2
show the agreement between experimental and predicted
values of concentration of glucose, 5-HMF and LA.
3.1
Glucose decomposition
The decomposition rate of glucose can be modeled as a pseudo-first-order reaction [Eq.(1)][17]. In
such a reaction, the rate is directly proportional to the
concentration of the glucose[18]. From Table 1, it can
Chinese J. Ch. E. 14(5) 708 (2006)
Chinese J. Ch. E. (Vol. 14, No.5)
710
Figure 1 Experimental and predicted concentrations of
the products generated during the hydrolysis of 1%, 3% or
5% H2SO4 at 170℃ and different time
◆ glucose; ■ 5-HMF; ▲ LA
be seen that the glucose decomposition rate constant k
increases as the acid concentration and temperature
increases, and the values of k1 with different acid concentration are smaller than the values of k, indicating
that only part of glucose can decomposes into 5-HMF
and the yield of LA can not reach its theoretical yield.
3.2
5-HMF formation
As the precursor of LA, the mechanism of
October, 2006
Figure 2 Experimental and predicted concentrations of
the products generated during the hydrolysis of 1%, 3% or
5% H2SO4 at 190℃ and different time
◆ glucose; ■ 5-HMF; ▲ LA
5-HMF formation from glucose is very complicated,
many unidentified intermediates can be formed during
glucose decomposition[19]. Neglecting the formation
of intermediates, the formation of 5-HMF can be
modeled as consecutive reactions, and Figs.1 and 2
exhibited the change of 5-HMF concentration with
time displayed typical behavior of a consecutive
first-order pathway with a maximum concentration.
Kinetics of Levulinic Acid Formation from Glucose Decomposition at High Temperature
Table 1
T,
℃
170
190
Kinetic parameters obtained from optimization of experimental data
Acid concentration,
k1,
k,
－1
k2,
－1
－1
k3,
k1/k
k2/k
0.0108
0.8092
1.7986
－1
%
min
min
min
min
1
0.0566
0.0458
0.1018
3
0.0845
0.0605
0.2382
0.024
0.7160
2.8189
5
0.0964
0.0837
0.3170
0.0127
0.8682
3.2884
1
0.2939
0.1089
0.1230
0.185
0.3705
0.4185
3
0.2924
0.1734
0.4382
0.119
0.5930
1.4986
5
0.4875
0.2537
1.0510
0.232
0.5204
2.155
E1
E2
E3
86.33
56.95
209.5
average activation energy,
kJ·mol－1
Table 2 Generalized models for predicting kinetic parameters as a function of sulphuric acid concentration
Models
r2
3 ⎞
⎛
k1 = 4.597 × 109 C 0.427 exp ⎜ −86.33 × 10 ⎟
RT
⎝
⎠
0.994
3 ⎞
⎛
k2 = 4.318 × 107 C 0.973 exp ⎜ −56.95 × 10 ⎟
RT
⎝
⎠
0.958
3 ⎞
⎛
k3 = 1.20 × 1023 C 0.119 exp ⎜ 209.5 × 10 ⎟
RT
⎝
⎠
0.940
Table 1 shows that with the increases of temperature and acid concentration, 5-HMF formation
rate constant k1 also increased. But the relative rate of
5-HMF formation with respect to glucose decomposition had not the same rules as rate constant k1. Increasing acid concentration from 1% to 5% did not
enhance the formation rate of 5-HMF relative to overall glucose decomposition obviously as indicated by
k1/k ratio. The values of k1/k at 190℃ are lower than
those at 170℃, suggesting that the overall glucose
decomposition rate k increases more quickly than the
5-HMF formation rate k1, and higher temperature will
promote glucose decomposition.
3.3
711
Levulinic acid formation
LA is the product of 5-HMF decomposition[20].
As shown in Figs.1 and 2, concentration of LA increased quickly at the initial stage, and then it reached
to a certain value. Based on the model and experimental data, the calculated rate constant of LA formation increased with the increases of temperature and
acid concentration. It can be observed that the formation rate of LA relative to overall glucose decomposition can be enhanced by increasing acid concentration
from 1% to 5%, as indicated by an increasing k2/k ratio from Table 1. It is also found that values of k2/k
with same acid concentration at 190℃ are smaller
than those of 170℃. The reason is that the average
value of activation energy for glucose decomposition
was greater than that of 5-HMF decomposition. Tarabanko et al. reported the kinetics of LA formation
from glucose at 98℃，revealing the rate of LA formation from glucose depends slightly on the acid
concentration[9]. But, different results were obtained at
higher temperature conditions. The rate of LA formation increases sharply with the increase of the acid
concentrations.
The acid-catalyzed conversion of glucose into LA
can also be carried out in concentrated solutions of
acids at lower temperature, thus high consumption of
the acid and longer reaction time were required[21].
Although high yield of LA can be attained at low
temperature, the disadvantages of long reaction time,
corrosion to equipment and the difficulty of acid recovery limit its further use in industry. Contrarily, the
consumption of the acid and reaction time can be decreased sharply under high temperature conditions,
which was helpful to decrease the side products[22].
High temperature hydrolysis with dilute acid appears
to be in the best position from economic viewpoint.
Especially, when using biomass as raw materials to
product LA, high temperature can promote the LA
production effectively[7]. But, too high temperature
(>250℃) may also cause further decomposition of LA,
and different materials have different optimal conditions. In this study, the highest LA yield can reach up
to 80.7% (molar percent) of theoretical yield under
170℃ and 5% acid conditions.
NOMENCLATURE
a
C
cglu
c5-HMF
cLA
E
k
k0
n
T
t
constant in model
sulfuric acid mass concentration, % (by mass)
glucose concentration, mol·L－1
5-HMF concentration, mol·L－1
LA concentration, mol·L－1
activation energy, kJ·mol－1
rate constant, min-1
pre-exponential factor, min-1
constant in model
temperature, K
time, min
Chinese J. Ch. E. 14(5) 708 (2006)
712
Chinese J. Ch. E. (Vol. 14, No.5)
Subscripts
0
1,2,3
initial
steps in model
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