Reaction Mechanisms and Kinetics of Xylo-oligosaccharide Hydrolysis by Dicarboxylic Acids
Youngmi Kim, Thomas Kreke, Michael Ladisch*
Laboratory of Renewable Resources Engineering
Agricultural and Biological Engineering, Purdue University, West Lafayette, IN 47907
Results
Abstract
Figure 1. Representative Arrhenius plots showing (A) the temperature dependence of the kinetic rate constants, k1-4 for estimation of activation
Hydrothermal pretreatment of lignocellulosic materials generates a liquid stream rich in pentose sugar-oligomers. Cost-effective hydrolysis and
utilization of these soluble sugar-oligomers is an integral component of biofuel production. We report integrated rate equations for hydrolysis of xylooligomers derived from pretreated hardwood by dicarboxylic maleic and oxalic acids. The highest xylose yield observed with dicarboxylic acids was
96%, and compared to sulfuric acid, was 5 to 15% higher with less xylose degradation. Dicarboxylic acids showed an inverse correlation between xylose
degradation rates and acid loadings unlike sulfuric acid for which less acid results in less xylose degradation to aldehydes and humic substances. A
combination of high acid and low temperature leads to xylose yield improvement. Hydrolysis time course data at 3 different acid concentrations and 3
temperatures between 140 and 180ºC were used to develop a reaction model for the hydrolysis of xylo-oligosaccharides to xylose by dicarboxylic acids.
(This study is recently accepted in AIChE Journal )
energy, E at pH 1.9; (B) the acid dependence of the kinetic rate constants, k1-4 for estimation of pre-exponential factor parameters, k0 and m at
140ºC. SA: sulfuric acid; MA: maleic acid; OA: oxalic acid
B
-1
2
-1.5
ln (k2) (1/hr)
1.5
1
SA
MA
OA
k1
Selectivity Factor 
k2  k3
The severity factor, which gives a
numerical representation of the
combined temperature and
residence time and reaction severity,
is defined as:

 T 100 
log R0  log t  exp 


14.75



Acids:
Sulfuric acid (19 to 102 mM, pH 1.3 to 2.2)
Maleic acid (50 to 172 mM, pH 1.7 to 2.2)
Oxalic acid (33 to132 mM, pH 1.5 to 2.2)
in the final mixture
-2.5
-3
-2
-2.5
-3
SA
MA
OA
-4
-4.5
-0.00112
-0.00108
-0.00124
SA
MA
OA
MA
-4.3
OA
-6
-5
-4
-3
-2
-3
-4
-4.5
-5
MA
-5
-4
-3
ln [H+]
-4
SA
MA
-5
OA
-6
-3.5
-4.5
SA
140
180
SA
MA
OA
160
140
120
100
80
60
160oC
140
120
100
80
60
120
100
80
60
40
40
40
20
20
20
0
0
0
0
10
20
30
40
0
50
10
30
40
50
0
10
30
40
50
1.0
SA
MA
OA
2.5
SA
MA
OA
0.8
t max (hr)
6
5
4
t max (hr)
2.0
1.5
0.6
0.4
1.0
2
140oC
1
0.5
0
0.2
160oC
0.0
0
10
20
30
40
50
180oC
0.0
0
10
20
30
40
50
0
100%
90%
90%
90%
80%
80%
80%
70%
60%
50%
140oC
40%
30%
SA
70%
60%
50%
40%
160oC
30%
20%
MA
20%
10%
OA
10%
SA
MA
OA
10
20
30
40
10
20
OA
-5.5
-2
-6
-5
-4
-3
-2
ln [H+]
SA
MA
OA
30%
30
40
60%
50%
40%
50
15%
160oC
25%
20%
15%
0%
0%
[H+] (mM)
20
30
[H+] (mM)
40
50
1.
180oC
15%
0%
10
References
20%
5%
0
50
2.
5%
50
40
SA
MA
OA
25%
5%
40
30
30%
10%
30
20
35%
10%
20
10
The material in this work was supported by Mascoma
Corporation and the US Department of Energy
(Contract #DE-F-08G018103). We thank Todd Lloyd,
David Hogsett, and Yulin Lu for providing xylooligomer containing sugar stream used in this work.
We also thank Nathan Mosier for his comments and
suggestions.
Michael R. Ladisch is CTO at Mascoma Corporation.
40%
10%
10
SA
MA
OA
0
SA
MA
OA
30%
20%
0
180oC
30%
[H+] (mM)
35%
140oC
25%
Acknowledgements
10%
40%
40%
50
Conclusions
Analysis of empirical data and model parameters
explicitly indicated that:
1) hydrolysis of soluble sugar oligomers in
washate stream of steam exploded mixed
hardwood follows first-order hydrolysis kinetics;
2) a combination of low temperature and high acid
loading leads to increased xylose yields and
minimal sugar loss to degradation;
3) dicarboxylic acids outperform sulfuric acid by
preventing xylose degradation;
4) xylose degradation rate by dicarboxylic acids is
inversely dependent on pH;
5) sulfuric acid requires a lower pH than
dicarboxylic acid to give an equivalent level of
optimal xylose yield.
6) A monophasic model based on Saeman’s
pseudo-homogeneous irreversible first-order
reaction kinetics and specific acid catalysis
successfully modeled hydrolysis of woodderived xylo-oligosaccharide by dicarboxylic
acids.
70%
[H+] (mM)
[H+] (mM)
(D) Furfural Formation
40
0%
0
50
30
20%
0%
0%
20
[H+] (mM)
100%
0
10
[H+] (mM)
100%
53.5
20
[H+] (mM)
3.0
SA
MA
OA
9
2.9
10
[H+] (mM)
[H+] (mM)
(B) tmax
20
180oC
SA
MA
OA
160
Selectivity (k1/(k2+k3))
160
140oC
3
0.3
8.3
0.4
0.5
5.1
11.3
0.1
nd
nd
-4.1
Negative correlation between diacids
-2
and xylose degradation
Ln k4 vs Ln[H+]
-2.5
ln k4 vs ln [H+]
-6.5
-0.00108
180
180
35%
0.4
9.9
0.5
0.6
6.1
13.5
0.1
nd
nd
-0.00112
SA
ln [H+]
ion concentrations. Symbols represent experimental data. Lines represent calculated from model.
(A) Selectivity factor
After Acid
As
solution
received
added
64.2
-0.00116
-3.9
Figure 2. Experimental and calculated (A) selectivity factors; (B) tmax; (C) maximum xylose; and (D) furfural yields at tmax at different hydrogen
7
3.5
-0.0012
-3.7
-2
ln
[H+]+]
Lnk3k3vsvslnLn[H
1/RT (mol/cal)
(C) Max. Xylose Yield
gluco-oligomers (glucose
equivalent)
xylo-oligomers (xylose
equivalent)
glucose
xylose
lactic acid
glycerol
acetic acid (free)
acetyl (bound)
butanediol
hydroxymethylfurfural (HMF)
furfural
-3
-7
[H+] (mM)
Components
-4
-6
-5
-0.00116
-5
-5.5
-3.5
SA
MA
OA
-3.5
-4.5
-2.5
ln (k3) (1/hr)
ln (k4) (1/hr)
ln (k3) (1/hr)
-2
1/RT (mol/cal)
Materials and Methods
Hydrolysis: 2.4 mL solution consisting of a mixture of
2 mL xylo-oligosaccharide and 0.4 mL acid solutions
with compositions as shown in Table 1 was
hydrolyzed in a stainless steel tube reactor (3/8 in. OD
x 3 in length, 4 mL internal vol.).
The hydrolysis was carried out by placing the tube in
a Tecam® SBL-1 fluidized sand bath (Cole-Parmer,
Vernon Hills, IL) set to a target temperature (140180oC) for 1 min to 30 hr.
ln k4 vs -1/RT
-3.5
-0.0012
OA
-3.3
ln [H+]
-1.5
-0.00124
MA
-6
-0.00108
% Max. Xylose Yield
tmax
 k2  k3 
ln 

k
  1 
k2  k3  k1
-0.00112
-1.5
8
Table 1. Composition of the xylo-oligosaccharide
solution obtained from steam pretreated mixed
hardwood.
-0.00116
SA
-2
-3
-5
 m
Substrate: Mixed-hardwood derived, soluble xylooligosaccharides from Mascoma Co. (initial pH 3.7)
-0.0012
% Furfural Yield
tmax, the time at which xylose
concentration reaches its maximum,
and selectivity of xylose formation
(selectivity factor), are given as:
Xn, X, F, D = concentration of xylooligosaccharides, xylose, furfural, and
degradation products (humic solids) (g/L);
t = time (hr);
k = rate constant (1/hr)
0
-1
% Max. Xylose Yield
Xn = xylo-oligomers;
X = xylose;
F = furfural;
D = degradation products (humins).
0.5
-1
-4.5
k  K  exp
K = pre-exponential factor;
[H+] = measured initial aqueous hydronium ion
concentration at room temperature (M);
E = activation energy (cal/mol);
R = ideal gas law constant (=1.98 cal/mol∙K);
T = temperature (K)
1
-2
-0.5
% Furfural Yield
D
ln k3 vs -1/RT
-4
The rate constant, k is correlated to temperature
and acid concentration by Arrhenius equation
E
RT
K  k0  [ H ]
-3.1
1/RT (mol/cal)
-3.5
t max (hr)
k4
-0.00124
-0.00108
1.5
-1.5
0
-0.5
% Max. Xylose Yield
k3
d  Xn 
 k1   X n 
dt
dX 
 k1   X n   k2   X   k3   X 
dt
d F 
 k2   X   k4   F 
dt
-0.00112
0
% Furfural Yield
X n 
 X 
F
Based on the model, the following set of
differential equations was derived:
-0.00116
Selectivity (k1/(k2+k3))
A monophasic model based on Saeman’s pseudo-homogeneous irreversible first-order reaction kinetics [4]
-0.0012
1/RT (mol/cal)
Selectivity (k1/(k2+k3))
Reaction Model
-4
-4.5
-0.00124
-2.9
-1
SA
MA
OA
-3.5
-1
2
-0.5
-3
0
Hemicellulose, which represents 15-35% of lignocellulosic biomass, is made of complex, heterogeneous polysaccharides, consisting primarily of
xylan and smaller amounts of arabinan, galactan, uronic and acetic acid. Hydrothermal pretreatments, such as stream explosion and liquid hot water,
efficiently fractionate hemicellulose away from lignocellulosic materials by dissolution. Hydronium ions generated from auto-ionization of water at an
elevated temperature cause acetic acid to be released from the hemicellulose as well as catalyze formation of oligomers by partial hydrolysis of
hemicellulose. Post-hydrolysis of these oligomers results in monosaccharides that may be converted to biofuels and other chemicals. A broad array
of hemicellulolytic enzymes is required to fully depolymerize the oligosaccharides through hydrolysis of xylan-backbone, and removal of acetyl,
arabinan, and uronic acid side substituents. Acid catalysis offers two main advantages over enzymatic hydrolysis: short reaction time and reduced
catalyst cost.
Dicarboxylic, organic acids (maleic acid, oxalic acid, fumaric acid) have been identified as suitable hydrolytic molecules. They are less corrosive,
more selective, and may be thermally decomposed into non-toxic molecules (CO2, formic, fumaric acids) at the end of their use cycle, unlike sulfuric
acid [1]. Maleic acid, which mimics the structure of the active site in cellulase enzymes, has selectivity superior to sulfuric acid hydrolysis of sugar
polymers due to lower sugar degradation [2]. Oxalic acid, another dicarboxylic acid, is secreted by brown-rot fungi that degrade fiber structures in
plant materials and has demonstrated potential for hydrolysis of lignocellulosic materials [3].
This work reports a mathematical kinetic model for acid-catalyzed hydrolysis of soluble xylo-oligomers by dicarboxylic acids (maleic and oxalic
acids) and compares these results to sulfuric acid. The xylo-oligosaccharide solution was obtained from a common feedstock, i.e., the liquid from
aqueous pretreatment of mixed hardwood. A classic homogeneous, pseudo first-order hydrolysis kinetics was found to accurately represent the
hydrolysis of xylo-oligomers by all three acids, with differences between sulfuric, oxalic, and maleic acids being captured through the kinetic
parameters. This model enabled us to identify, assess, and compare catalytic performance and determine optimal hydrolysis conditions, together with
a mechanistic explanation of how the selected model represents the hydrolysis mechanism.
k2
-2.5
0.5
-0.5
k1
-2
+]
Lnkk22vs
ln
vsln
Ln[H
[H+]
-2.7
ln (k4) (1/hr)
ln (k1) (1/hr)
2.5
+]
Ln
ln k1 vs ln
Ln[H
[H+]
2.5
ln k2 vs -1/RT
-0.5
-2.5
ln (k2) (1/hr)
ln k1 vs -1/RT
3
Introduction
3
0
3.5
ln (k1) (1/hr)
A
3.
4.
0
10
20
30
[H+] (mM)
40
50
Lu Y, Mosier NS. Biomimetic catalysis for hemicellulose hydrolysis in corn
stover. Biotechnol. Prog. 2007;23:116-123.
Mosier NS, Ladisch CM, Ladisch MR. Characterization of acid catalytic
domains for cellulose hydrolysis and glucose degradation. Biotech.
Bioeng. 2002;79:610-618
Kootstra AM, Mosier NS, Scott EL, Beeftink HH, Sanders JPM. Differential
effects of mineral and organic acids on the kinetics of arabinose
degradation under lignocellulose pretreatment conditions. Biochem. Eng.
J. 2009;43:92-97.
Saeman JF. Kinetics of wood saccharification-hydrolysis of cellulose and
decomposition of sugars in dilute acid at high temperature. Ind. Eng.
Chem. 1945;37:43-52.