Reaction Mechanisms and Kinetics of Xylo-oligosaccharide Hydrolysis by Dicarboxylic Acids

Laboratory of Renewable Resources Engineering

Agricultural and Biological Engineering, Purdue University, West Lafayette, IN 47907

Introduction

Reaction Model

Abstract

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 xylo-

oligomers 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 )

Results

Conclusions

Acknowledgements

References

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 wood-

derived xylo-oligosaccharide by dicarboxylic

acids.

1. Lu Y, Mosier NS. Biomimetic catalysis for hemicellulose hydrolysis in corn

stover. Biotechnol. Prog. 2007;23:116-123.

2. Mosier NS, Ladisch CM, Ladisch MR. Characterization of acid catalytic

domains for cellulose hydrolysis and glucose degradation. Biotech.

Bioeng. 2002;79:610-618

3. 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.

4. 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.

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 xylo-

oligomer 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.

Youngmi Kim, Thomas Kreke, Michael Ladisch*

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.

Substrate: Mixed-hardwood derived, soluble xylo-

oligosaccharides from Mascoma Co. (initial pH 3.7)

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 (140-

180oC) for 1 min to 30 hr.

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

Table 1. Composition of the xylo-oligosaccharide

solution obtained from steam pretreated mixed

hardwood.

1 2k k

nX X F

3k

D

4k

Xn = xylo-oligomers;

X = xylose;

F = furfural;

D = degradation products (humins).

1

1 2 3

2 4

n

n

n

d Xk X

dt

d Xk X k X k X

dt

d Fk X k F

dt

2 3

1

max

2 3 1

1

2 3

ln

Selectivity Factor

k k

kt

k k k

k

k k

0

exp

[ ]

E

RT

m

k K

K k H

A monophasic model based on Saeman’s pseudo-homogeneous irreversible first-order reaction kinetics [4]

Based on the model, the following set of

differential equations was derived:

tmax, the time at which xylose

concentration reaches its maximum,

and selectivity of xylose formation

(selectivity factor), are given as:

The rate constant, k is correlated to temperature

and acid concentration by Arrhenius equation

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)

Xn, X, F, D = concentration of xylo-

oligosaccharides, xylose, furfural, and

degradation products (humic solids) (g/L);

t = time (hr);

k = rate constant (1/hr)

The severity factor, which gives a

numerical representation of the

combined temperature and

residence time and reaction severity,

is defined as:

0

100log log exp

14.75

TR t

Materials and Methods

Components As

received

After Acid

solution

added

gluco-oligomers (glucose

equivalent) 3.5 2.9

xylo-oligomers (xylose

equivalent) 64.2 53.5

glucose 0.4 0.3

xylose 9.9 8.3

lactic acid 0.5 0.4

glycerol 0.6 0.5

acetic acid (free) 6.1 5.1

acetyl (bound) 13.5 11.3

butanediol 0.1 0.1

hydroxymethylfurfural (HMF) nd nd

furfural nd nd

Figure 1. Representative Arrhenius plots showing (A) the temperature dependence of the kinetic rate constants, k1-4 for estimation of activation

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

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

-0.00124 -0.0012 -0.00116 -0.00112 -0.00108

ln (k

1) (1

/hr)

1/RT (mol/cal)

SA

MA

OA

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

-0.00124 -0.0012 -0.00116 -0.00112 -0.00108

ln (k

2) (1

/hr)

1/RT (mol/cal)

SA

MA

OA

-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

-0.00124 -0.0012 -0.00116 -0.00112 -0.00108

ln (k

3) (1

/hr)

1/RT (mol/cal)

SA

MA

OA

-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

-0.00124 -0.0012 -0.00116 -0.00112 -0.00108

ln (k

4) (1

/hr)

1/RT (mol/cal)

SA

MA

OA

ln k1 vs -1/RT ln k2 vs -1/RT

ln k3 vs -1/RT ln k4 vs -1/RT

A Ln k1 vs Ln[H+] Ln k2 vs Ln[H+]

Ln k3 vs Ln[H+] Ln k4 vs Ln[H+]

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

-6 -5 -4 -3 -2

ln (k

1)(

1/h

r)

ln [H+]

SA

MA

OA

-4.5

-4.3

-4.1

-3.9

-3.7

-3.5

-3.3

-3.1

-2.9

-2.7

-2.5

-6 -5 -4 -3 -2

ln (k

2)

(1/h

r)

ln [H+]

SA

MA

OA

-7

-6.5

-6

-5.5

-5

-4.5

-4

-3.5

-3

-2.5

-2

-6 -5 -4 -3 -2

ln (k

3)

(1/h

r)

ln [H+]

SA

MA

OA

-5.5

-5

-4.5

-4

-3.5

-3

-2.5

-2

-6 -5 -4 -3 -2

ln (k

4)

(1/h

r)

ln [H+]

SA

MA

OA

B

0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50

Sele

ctiv

ity

(k1/

(k2+

k 3))

[H+] (mM)

SA

MA

OA

140oC

0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50

Sele

ctiv

ity

(k1/

(k2+

k 3))

[H+] (mM)

SA

MA

OA

160oC

0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50

Sele

ctiv

ity

(k1/(

k 2+k

3))

[H+] (mM)

SA

MA

OA

180oC

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50

t m

ax (h

r)

[H+] (mM)

SA

MA

OA

140oC

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 10 20 30 40 50

t m

ax (h

r)

[H+] (mM)

SA

MA

OA

160oC

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50

t m

ax (h

r)

[H+] (mM)

SA

MA

OA

180oC

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 10 20 30 40 50

% M

ax. X

ylo

se Y

ield

[H+] (mM)

SA

MA

OA

140oC

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 10 20 30 40 50

% M

ax. X

ylo

se Y

ield

[H+] (mM)

SA

MA

OA

160oC

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 10 20 30 40 50

% M

ax. X

ylo

se Y

ield

[H+] (mM)

SA

MA

OA

180oC

0%

5%

10%

15%

20%

25%

30%

35%

40%

0 10 20 30 40 50

% F

urf

ura

l Yie

ld

[H+] (mM)

SA

MA

OA

140oC

0%

5%

10%

15%

20%

25%

30%

35%

40%

0 10 20 30 40 50

% F

urf

ura

l Yie

ld

[H+] (mM)

SA

MA

OA

160oC

0%

5%

10%

15%

20%

25%

30%

35%

40%

0 10 20 30 40 50

% F

urf

ura

l Yie

ld

[H+] (mM)

SA

MA

OA

180oC

(A) Selectivity factor

(B) tmax

(C) Max. Xylose Yield

(D) Furfural Formation

Figure 2. Experimental and calculated (A) selectivity factors; (B) tmax; (C) maximum xylose; and (D) furfural yields at tmax at different hydrogen

ion concentrations. Symbols represent experimental data. Lines represent calculated from model.

Negative correlation between diacids

and xylose degradation

ln k1 vs ln [H+] ln k2 vs ln [H+]

ln k3 vs ln [H+] ln k4 vs ln [H+]