Hydrothermal decomposition of xylan as a model substance
for plant biomass waste

e Hydrothermolysis in

subcritical water

Hanna Pi

nkowska

*

, Pawe

1 Wolak, Adrianna Z1ocinska

Department of the Chemical Technology, University of Economics in Wroc

ław, ul. Komandorska 118/120, 53-345 Wrocław, Poland

a r t i c l e i n f o

Article history:

Received 26 April 2010
Received in revised form
12 May 2011
Accepted 2 June 2011
Available online 2 July 2011

Keywords:

Xylan
Hydrothermal reaction
Xylose
Decomposition
Carboxylic acid
Furfural

a b s t r a c t

Beech wood xylan, as a model substance for hemicellulose contained in plant biomass
waste, was subjected to thermohydrolysis in subcritical water. The composition of the
product fractions obtained as a result of its hydrothermal decomposition was studied: the
water fraction, the oil fraction and the solid fraction of charred post-reaction residue. An
increase in temperature favors xylan thermohydrolysis, leading to the production of
saccharides

e the products of its hydrolytic depolymerization. The yield of the saccharides

contained in the water-soluble product fraction reaches it maximum value at 220

C and

235

C, with the retention time of 0 min. Both extending reaction time up to 30 min and

further increasing the temperature favor the occurring of secondary reactions

e saccharide

decomposition

e leading to the production, among others, of carboxylic acids, furfurals

and aldehydes, and their further carbonization and gasification.

ª 2011 Elsevier Ltd. All rights reserved.

1.

Introduction

Plant biomass waste is a cheap, commonly available and
renewable source of energy and useful bioproducts. Its most
important components are cellulose, lignin and hemi-
cellulose. In plant cell walls, hemicellulose, together with
lignin and cellulose, make up the lignocellulosic complex

[1]

.

Hemicellulose is a copolymer that contains repeat units

comprising pentoses (xylose, arabinose), hexoses (mannose,
glucose, galactose) and uronic acids

[2]

. It also contains

D

-

xylopyranose,

D

-glucopyranose,

D

-galactopyranose,

L

-arabino-

furanose,

D

-mannopyranose,

D

-glucuronic acid,

D

-galacturonic

acid

[3]

, and others. Hemicellulose is a branched polymer with

a degree of polymerization of 100

e200. Because of its structure,

it is amorphous and hydrophilic

[2,4]

. The presence of

hemicellulose in plant raw material has an advantageous effect
on the use of lignocellulosic biomass in the production of paper
(it increases its strength). However, when this biomass is used
in other chemical processes, e.g. in the production of bio-
ethanol, hemicellulose is an adverse ingredient. Therefore,
effective chemical conversion of lignocellulosic biomass waste
often requires an initial procedure to remove hemicellulose

[5

e7]

. There are numerous physical, thermal (pyrolysis),

physicochemical (steam explosion, ammonia fiber explosion,
CO

2

explosion) and chemical (ozonolysis, acid hydrolysis,

alkaline hydrolysis) methods for successful removal of hemi-
cellulose from plant biomass waste

[6,7]

.

A new proposal for the removal of hemicellulose contained

in lignocellulosic biomass is its controlled hydrothermal
decomposition

[5,6,8

e11]

. It can be run in sub- and

* Corresponding author. Tel./fax:

þ48 71 36 80 275.

E-mail address:

[e-mail]/

(H. Pi

nkowska).

A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

h t t p : / / w w w . e l s e v i e r . c o m / l o c a t e / b i o m b i o e

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 0 2

e3 9 1 2

0961-9534/$

e see front matter ª 2011 Elsevier Ltd. All rights reserved.

doi:

10.1016/j.biombioe.2011.06.015

supercritical water. Critical parameters of water are:
T

cr

¼ 374.2

C, P

cr

¼ 22,05 MPa, g

cr

¼ 0.32 g cm

3

[12]

. The

properties of sub- and supercritical water differ significantly
from the properties water has in normal conditions

[2,13

e16]

.

Along with an increase in temperature and pressure, the
cleavage of hydrogen bonds occurs, and the dielectric
constant and ionic product change

[2,13

e17]

. Due to its

properties, sub- and supercritical water can play the role of
solvent, catalyst and reagent, e.g. in chemical synthesis
reactions and organic compound decomposition processes

[14

e17]

. It is also used as reaction medium in waste substance

utilization processes

[16,18

e20]

, including conversion of

biomass into useful products

[2,21

e24]

.

Examination of the course and optimal parameters of

decomposition of hemicellulose contained in plant biomass
waste can be performed using the commercially available
xylan, which can play the role of a model substance for
hemicellulose. Its main ingredients are xylopyranose residues
forming a polymer chain and linked by

b(1-4)-glycosidic

bonds. Depending on the origin of xylan, its backbone chain
may contain such side groups as arabinofuranose groups,
acetyl groups, 4-0-methyl-glucuronic groups, and others

[1]

.

Hydrothermal decomposition of xylan is a type of conver-

sion in which near-critical water’s ability can be used to
dissolve the raw material and catalyze its hydrolysis with the
help of H

þ

ions

[25]

.

There are few reports describing the course of hydro-

thermolysis of xylan in the world literature. The course of this
reaction was presented by Miyazawa

[26]

. The decomposition

reaction was carried out within a batch process at the
temperature of 200

C for 15 min, in the presence of CO

2

. As

a result of hydrothermal decomposition of xylan, the mono-
saccharide (xylose) yield reached around 15% in the liquid
product fraction. In another paper, beech wood xylan was
subjected to hydrothermal decomposition in the batch reactor
at 380

C at the pressure of 100 MPa for 5 s. The resultant yield

in the liquid product fraction amounted to 93.7%, with the
fraction

containing,

among

others,

2-furfural

(2-FA),

5-(hydroxymethyl)furfural (5-HMF), glycolaldehyde, dihy-
droxyacetone and the carboxylic acids: formic, acetic, glycolic,
lactic and pyruvic acids (yield of 27.2%). The saccharide
content in the liquid product fraction was not analyzed

[27]

.

The purpose of this paper was to study the course of

hydrothermal decomposition of xylan in subcritical water in
a batch reactor and to determine the influence of reaction
parameters: reaction temperature and time, on the degree of
xylan conversion achieved. Xylan hydrolysis was performed
without a catalyst, with the use of water as solvent and cata-
lyst. In this study, xylan played the role of a model substance
for plant biomass waste, such as rape straw. A study of
hydrothermal decomposition of rape straw in subcritical water
will be conducted based on xylan decomposition test results.

2.

Experimental

2.1.

Materials and reagents

For the tests, there was used beech wood xylan (Carl Roth
GmbH, Karlsruhe, Germany) with a degree of polymerization

of DP

¼ 132 and the empirical formula (C

5

H

8

O

4

)

n

. The

elementary composition of beech wood xylan contains carbon
e 41.2%, hydrogen e 5.9% and oxygen e 52.8%
(C:H:O

¼ 5:8.6:4.8) and trace amounts of sulfur and nitrogen.

Saccharides (arabinose, galactose, xylose and mannose), ace-
tic acid and lactic acid were provided by the company Fluka,
water, glyceraldehyde, glycoaldehyde, pyruvaldehyde, dihy-
droxyacetone (DHA) and furfurals (2-FA and 5-HMF) were
provided by Aldrich, whereas formic acid and oxalic acid plus
the reagents that were required for the preparation of Luff-
Schoorl

reagent

(copper

sulfate,

citric

acid,

sodium

carbonate) and used in chromatographic determination were
bought from POCh (Poland). In this study, the reagents used
were analytically pure or HPLC pure, depending on the
requirements of the analytical method applied.

2.2.

The reactor and the course of hydrothermal

decomposition xylan

Hydrolysis of xylan dried at 103

C for 24 h was performed in

a high-temperature (maximum working temperature of
500

C) and high-pressure (maximum pressure of 34.5 MPa)

4576A-type batch reactor manufactured by Parr (Moline,
Illinois, USA). The reactor was equipped with a 250 cm

3

vessel made of T316 stainless steel, 70 mm in height, 65 mm
in

internal

diameter,

with

15-mm-thick

walls,

with

a 1700 rev/min magnetic mixer, a manometer, an internal
cooler in the form of a U-pipe (a single loop), a heating mantle
with a 2.0 kW electrical heater, a fixed head, a reagent dozing
valve, a sampling device, a thermoelement placed in the
reactor’s vessel, a 4857-type controller of the machine,
a 4875-type power controller, and CALGrafix software
controlling the work of the apparatus assembly.

The reaction of xylan hydrothermolysis was performed at

180

e300

C for 0

e30 min, at a pressure corresponding to the

vapor pressure curve at a given temperature or one that
slightly exceeded it. HPLC-grade water

e before it was used e

was degassed in an ultrasonic bath and blown through with
nitrogen. Reagents were applied at the water to xylan weight
ratio of 98:2.

Typical content of hemicellulose fraction in the agricul-

tural residues is about 15

e25%

[2]

, a commonly used

concentration of biomass waste undergoing hydrothermal
decomposition

e about 10%

[28

e30]

. In such conditions, in the

material feeding the hydrothermal reactor, the content of
hemicellulose is approximately 2% (w/w).

The 100 cm

3

of xylan suspension in water was introduced

into the reaction vessel that had been originally heated up to
around 80

C. After the reactor was closed, the reaction vessel,

together with its contents, was blown through a few times
with nitrogen at 2 MPa, heated up to the planned temperature
for 10

e15 min and kept at the same temperature to an accu-

racy of

1

C. The xylan suspension in water was heated up,

until the reaction was stopped having reached the planned
temperature (retention time: 0 min) or having retained it in
the reactor for 2, 5, 10, 15, 20 and 30 min. The reaction mixture
ingredients were heated at the speed of around 10

e15

C/min.

Fig. 1

shows a curve illustrating the course of the program for

heating up, retaining at a target temperature and cooling
down of the reaction vessel contents.

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 0 2

e3 9 1 2

3903

After the conclusion of the reaction, the reaction vessel

was cooled down to around 90

C for around 10 min and after

the system was expanded, it was emptied and rinsed with
water, to reach the final volume of the aqueous fraction of
250 cm

3

.

2.3.

Separation of xylan hydrothermal decomposition

products

Through hydrothermal decomposition of xylan, there was
obtained

raw

liquid

product

containing

water-soluble

elements (the WS fractions) and a post-reaction solid
residue (the WN fractions). The WS fractions were separated
from the WN fractions by filtration at a lower pressure
through a PTFE membrane filter manufactured by Sartorius
(SRP 15 0.45

mm). In the WS fractions, the dry matter content

(DM) was determined by evaporating water in a vacuum drier
at 65

C down to dry matter. The WN fractions were subjected

to a 30-min extraction in an ultrasonic bath with 100 cm

3

of

methanol. After the extraction, the resulting mixture was
filtered at a lowered pressure to obtain filtrate containing
elements that were water-insoluble but methanol-soluble
(MS)-oil,

and

residue

containing

methanol-insoluble

substances (MN)-unreacted xylan and a charred solid
residue. From the MS fractions, methanol was removed
through distillation at a lowered pressure, whereas the solid
product, similarly to the MN residue, was dried in the vacuum
drier at 65

C down to solid mass.

The yield of fractions WS, WN, MS, MN and individual

products present in the WS fractions (reducing sugars,
saccharides, carboxylic acids, furfurals, other aldehydes and
DHA) was determined in relation to the weight of xylan sub-
jected to hydrothermal decomposition. The gas product
created during xylan hydrothermolysis was not collected. The
gas fractions (Y

G

) yield was calculated using the equation:

Y

G

¼ 100 Y

DM

Y

MS

Y

MN

in which Y

DM

is dry matter yield in the WS fractions, Y

MS

e

methanol-soluble fractions yield, Y

MN

e methanol-insoluble

fractions yield.

2.4.

Analytical techniques and measurement

methodology

For xylan, the elementary composition was determined with
the use of the Vario EL III apparatus manufactured by Ele-
mentar Analysensysteme GmbH, Hanau. In the WS fractions
of xylan hydrothermal decomposition products pH measure-
ments were performed using Elmetron pH-meter CPI-501 with
a glass electrode Hydromet

[31]

, the dry matter content was

determined by the weight method, and the reducing sugars
content (hemiacetals, that reduce Tollens reagent to give
a silver mirror

e components in post-reaction WS fractions

which have free aldehyde or ketone groups and possess the
property of reducing many metallic salts such as copper in
alkaline conditions) was determined by the Luff-Schoorl’s
method

[32]

. Moreover, in WS fractions the monosaccharide,

carboxylic acids, furfurals, other aldehydes and DHA content
was determined. Determinations were made by the HPLC
method with the use of a Merck-Hitachi liquid chromato-
graph, equipped with Knauer’s SmartLine 1000 gradient
pump. The saccharide (arabinose, galactose, xylose and
mannose) content was determined at 85

C with the use of the

Biorad Aminex HPX-87P column equipped with a precolumn.
Water with the flow rate of 0.6 cm

3

min

1

was used as the

mobile phase. Saccharide detection was carried out with the
use of Knauer’s RI K-2300 refractometric detector

[33]

. The

carboxylic acids (formic, acetic, lactic, oxalic) content was
determined with the use of the Eurospher C18 column (Kna-
uer), with 25 mM KH

2

PO

4

(pH corrected to 2.5 with the help of

85% H

3

PO

4

) as the mobile phase, at the flow rate of

1.5 cm

3

min

1

, with the use of Merck-Hitachi’s DAD L7455

detector at the wavelength of 210 nm

[34]

. The furfurals (2-FA

and 5-HMF) content was determined at 35

C with the use of

the Eurospher C18 column. As the mobile phase we used
a solution composed of acetonitrile and an A solution (2 cm

3

of

acetic acid

þ 0.2 cm

3

of phosphoric acid

e complemented with

water up to 1 dm

3

) at the 18: 82 v/v ratio. The mobile phase

flow rate was 1.2 cm

3

min

1

, while the components were

determined with the use of a DAD detector at the wavelength
of 280 nm

[35]

. With the use of the Shodex KC-811 column, the

glyceraldehyde, pyruvaldehyde and glycoaldehyde content
and the DHA content was determined. The analytes were
separated and identified using a 5.0 mM solution of H

3

PO

4

as

the mobile phase, at the flow rate of 1 cm

3

min

1

, with the

help of a DAD detector (wavelength of 210 nm)

[36,37]

.

3.

Results and discussion

3.1.

Effect of temperature on the course of hydrothermal

decomposition of xylan

The effect of temperature on the course of hydrothermal
decomposition of xylan was determined by heating the reac-
tion mixture up to the planned temperature and stopping the
reaction after that temperature was reached. The initial pH
value of a 2% (w/w) xylan solution is pH

¼ 6.32. After hydro-

thermal decomposition of xylan, obtained post-reaction
aqueous fractions of products with the final volume of 250 cm

3

were allowed to cool to room temperature and the pH was

50

80

110

140

170

200

230

260

290

0

5

10

15

20

25

30

35

40

45

50

T

e

mp

e

ra

tu

re

(°C

)

Time (min)

0 min

5 min

10 min

15 min

Fig. 1

e An example of a program for heating up, retaining

at 260

C and cooling down of the reaction mixture during

hydrothermal decomposition of xylan.

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 0 2

e3 9 1 2

3904

measured using pH-meter with a glass electrode. Together
with an increase in hydrothermolysis temperature, in the WS
fractions the solution pH dropped. After the temperature of
around 250

C was exceeded, at which the minimum pH was

reached, together with an increase in conversion temperature
a small increase in pH was observed.

Fig. 2

shows changes of

pH in the water product fractions occurring together with an
increase in the temperature of hydrothermal decomposition
of xylan.

Fig. 3

shows the effect of reaction temperature on the

reducing sugars yield (oligosaccharides and others aldoses
and ketoses) determined by Luuf-Schoorl’s assay, present in
the WS fractions of the reaction products, calculated in rela-
tion to the amount of xylan introduced into the reactor. In the
WS fraction obtained at 180

C in the reaction time 0, the yield

of reducing sugars reached 14.1%. Initially, together with an
increase in temperature, there was an increase in the yield of
reducing sugars present in the WS fractions, until the first
maximum amounting to 39.6% was reached at 220

C. In the

temperature range of 222.5

e232.5

C, the reducing sugars yield

was falling and reached 33.9

e36.7%, after which it increased

again. At 235

C the reducing sugars yield of 42.4% was

obtained in the WS product fraction. A further increase in
xylan hydrothermolysis temperature caused a gradual
decrease in the reducing sugars content in the WS fractions.

Fig. 4

shows the effect of xylan hydrothermal decomposi-

tion temperature on the dry matter yield obtained in the
water-soluble

product

fractions,

the

methanol-soluble

product fractions, the methanol-insoluble product fractions
and the gas product fractions. Within the entire temperature
range, together with its increase there was a decrease in the
dry matter content in the WS fractions in relation to the
weight of xylan introduced into the reactor. At 180

C the dry

matter content in the WS fraction amounted to 91.5%,
whereas at 300

C it amounted to 36.5%. This large dry matter

yield obtained in the water product fractions, particularly at
lower temperatures (up to around 240

C), probably resulted

from the presence not only of water-soluble products of
hydrothermal decomposition of xylan and xylan oligomers

e

xylo-oligosaccharides, but also of unreacted raw material
which might have been solubilized under the reaction

conditions

[38]

. The amount of dry matter obtained in the WS

fractions dropped with an increase in reaction temperature
and the progressing depolymerization of xylan.

Like in the case of the dry matter yield, the yield of the MS

product fractions containing oil fraction products (bio-oil) and
the yield of the MN product fractions containing char (solid
char residue)

[39,40]

and probably some part of unreacted

xylan, decreased with a growth in reaction temperature. In the
temperature range of 225

e250

C, the MS and MN fractions

were absent among the reaction products, and after the
temperature of 260

C was exceeded, their content started to

growth only slightly. The increase in the MS fractions yield up
to 4.3% at 300

C was probably caused by progressing thermal

degradation of the products contained in the WS fraction and
a simultaneous creation of the solid residue

e the char.

Nevertheless, its amount was very small

e at 300

C the MN

fraction yield was a mere 0.5%.

In the whole temperature range in which hydrothermal

decomposition of xylan was performed, the gas product
fractions yield grew. At 210

e250

C, gasification was very

intensive. After the temperature of 250

C was exceeded, the

gas product fractions yield exceeded 50%. At temperatures

3

3,5

4

4,5

5

5,5

6

180

190

200

210

220

230

240

250

260

270

280

290

300

pH of

W

S

f

rac

tions

Temperature (°C)

Fig. 2

e Changes of pH in the water fraction of the products

of hydrothermal decomposition of xylan depending on
reaction temperature. Retention time: 0 min.

10

15

20

25

30

35

40

45

180

190

200

210

220

230

240

250

260

270

280

290

300

Reduc

ing s

ugar

s

y

ield (

%

)

Temperature (°C)

Fig. 3

e Effect of reaction temperature of hydrothermal

decomposition of xylan on the reducing sugars yield in the
WS fractions. Retention time: 0 min.

0

10

20

30

40

50

60

70

80

90

100

180

190

200

210

220

230

240

250

260

270

280

290

300

F

rac

tions

y

ield

(%)

Temperature (°C)

DM

MS

MN

G

Fig. 4

e Effect of the reaction temperature of hydrothermal

decomposition of xylan on the DM, MS, MN and G fractions
yield. Retention time: 0 min.

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 0 2

e3 9 1 2

3905

over 250

C the gasification of reaction mixture ingredients

was still occurring, but the increase in the gas product frac-
tions yield was slower. At 300

C the gas product fraction yield

was 59.0%.

The process of hydrothermal decomposition of xylan runs

in a number of stages. In the first phase of the conversion, at
lower temperature ranges (up to around 240

C), hydrolytic

depolymerization of xylan occurred. During the depolymer-
ization, the quantitatively dominant group of products con-
tained in the water fractions were reducing sugars. After the
maximum yield was reached, an increase in temperature was
accompanied by a drop in their content in the WS fractions,
and a drop in the dry matter yield, but also a growth in the
amount of bio-oil, a slight growth in the solid char residue,
and a very clear growth in the gas product content. Bio-oil,
char and gas were produced as a result of degradation of the
products contained in the WS fractions

[39,41,42]

.

3.2.

The effect of reaction time on the course of

hydrothermal decomposition of xylan

With the retention time of 0 min, the largest amount of
reducing sugars in the water product fractions obtained
through hydrothermal decomposition of xylan was reached at
220 and 235

C. The effect of the time of hydrothermolysis of

xylan on the type and quantity of the products was deter-
mined by running its decomposition for the retention time of
2, 5, 10, 15, 20 and 30 min.

With a growth in reaction time, a slight drop in pH was

noted in the water-soluble product fractions. After a 30-min-
long reaction, the pH of the WS fraction obtained at 220

C was

3.76, and at 235

C

¼ 3.71.

Fig. 5

shows the effect of the time of hydrothermal

decomposition of xylan on the yield of reducing sugars present
in the WS fractions determined by Luuf-Schoorl’s method. At
a constant temperature, an increase in reaction time caused
a gradual decrease in their yield. The reducing sugars content
in the WS fraction was 21.0% for hydrothermolysis run at
220

C for 30 min, and 11.4% for one run at 235

C.

With reaction time increasing and the temperature

remaining the same, together with a decrease in the reducing

sugars yield in the WS fractions, changes occurred in the dry
matter yield and the gas product fractions yield, as well
(

Fig. 6

). The dry matter yield obtained in the WS fractions of

the products of hydrothermal decomposition of xylan kept
falling while reaction time grew. As a result of a reaction run
at 220

C for a retention time of 2 min, the dry matter yield was

77.5%, whereas after a 30-min reaction it was 17.0%. In turn, as
a result of xylan decomposition run at 235

C for 2 min, the dry

matter yield was 52.5%, whereas after 30 min it was 21.2%.
Within the whole reaction time range, the MS and MN frac-
tions were absent among the xylan decomposition products.

At each of the temperatures applied and within the whole

hydrothermolysis time range, there occurred a gasification of
the products derived from xylan that were present in the WS
fractions. After 30 min, the gas product fraction yield was
83.5% at 220

C and 75.1% at 235

C.

Similarly to the value of the temperature applied, also

reaction time was a factor that had a significant effect on the
yield of reducing sugars and the selected variables Y

DM

, Y

MS

,

Y

MN

and Y

G

. When retaining the reaction mixture at the target

temperature for 2

e30 min, the originally created water-

soluble products were then converted, by being gasified
gradually along with a growth in reaction time

[41,42]

.

3.3.

The water-soluble product fractions

Table 1

shows the composition of the water product fractions

obtained through hydrothermal decomposition of xylan at
180

e300

C at a retention time of 0 min, determined by liquid

chromatography. In the WS fractions, the primary xylan
hydrolytic depolymerization products were identified: xylose,
arabinose, trace amounts of other saccharides (mannose and
galactose), and also acetic acid and secondary products: other
carboxylic acids (formic, lactic and oxalic), furfurals (2-FA and
5-HMF), as well as aldehydes (glyceraldehyde, glycoaldehyde,
pyruvaldehyde), and DHA. The chromatograms obtained also
showed peaks of unidentified substances, which could have
been xylo-oligosaccharides or products of the degradation of
saccharides, carboxylic acids, aldehydes or DHA

[27]

.

Like in the case of the reducing sugars content in the

water fractions of the products of xylan hydrothermal

10

15

20

25

30

35

40

45

0

5

10

15

20

25

30

Reduc

ing s

u

gar

s

y

ield (

%

)

Time (min)

220°C

235°C

Fig. 5

e The effect of the reaction time of hydrothermal

decomposition of xylan at 220

C and 235

C on the yield of

reducing sugars in the WS fractions.

0

10

20

30

40

50

60

70

80

90

0

5

10

15

20

25

30

F

ra

ct

io

n

s yi

e

ld

(%

)

Time (min)

DM-200°C

G-200°C

DM-235°C

G-235°C

Fig. 6

e The effect of the reaction time of hydrothermal

decomposition of xylan at 220

C and 235

C on the DM and

G fractions yield.

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 0 2

e3 9 1 2

3906

decomposition, along with an increase in temperature up to
235

C at the retention time of 0 min, there was an increase in

the saccharide yield. In the WS fractions, the quantitatively
dominant saccharide was xylose, and its share in the
saccharide fraction exceeded 75

e85% (w/w). Two maximum

values of the saccharide yield were obtained

e at 220

C and

235

C. In the most favorable case

e a reaction run at 235

C

e

in the water product fraction the concentration of xylose was
2.4 g dm

3

. A further increase in reaction temperature caused

the saccharide content in the WS fractions to fall.

An increase in temperature caused an increase in the yield

of carboxylic acids contained in the water product fractions,
but only one maximum value was reached

e at 235

C. Among

carboxylic acids, the quantitatively dominant components
were acetic acid and formic acid. Their share in the acid
fractions, depending on the reaction temperature applied,
was 87.1

e95.2% (w/w). In some water fractions, there was also

lactic acid and trace amounts of oxalic acid. During hydro-
thermal decomposition of xylan, acetyl groups contained in
the side groups of the polymer chain are cleaved, facilitating
xylan depolymerization

[43]

. At 235

C, in the WS fraction the

highest acetic acid yield was 18.4% (1.47 g dm

3

) in relation to

the amount of xylan subjected to hydrothermolysis. The
largest amount of formic acid was obtained in a reaction run
at 260

C (0.16 g dm

3

). Formic acid could have been produced

as a result of the degradation of furfurals

[43]

, whereas lactic

acid and oxalic acid might have resulted from the dehydration
of the remaining components in the WS fractions

[44]

.

The furfural yield in the WS fractions of the products of

xylan hydrothermal decomposition grew along with a growth
in reaction temperature. The dominant component of the
furfurals was 2-FA

e a product of the degradation of pentoses

e xylose and arabinose

[45]

. Also, trace amounts of 5-HMF

were identified, which was produced through the decompo-
sition of hexoses (mannose and galactose). The 2-FA content
in relation to the amount of xylan subjected to hydro-
thermolysis grew within the whole range of reaction
temperatures. At 300

C, the 2-FA yield in the WS fraction

was 4.4%.

Aldehydes were absent from the water fraction of the

products of hydrothermal decomposition of xylan at 180

C.

Within the range of 200

e260

C, together with an increase in

reaction temperature, their yield grew, too. Having exceeded

the temperature of 260

C, there was a drop in the aldehyde yield

in the water product fractions. The aldehyde fractions con-
tained variable quantitative proportions of aldehydes that did
not indicate any distinct effect of reaction temperature on their
production. As an example, in the aldehyde fraction contained
in the WS product fractions, obtained at 220

C and 250

C, the

quantitatively

dominant

aldehyde

was

pyruvaldehyde,

whereas at 230

C and 235

C it was glyceraldehyde.

The presence of DHA was detected in WS fractions of the

products of xylan hydrothermolysis obtained at 220

C. Its

yield increased gradually within the whole temperature range
studied.

We were unsuccessful at balancing the content of

elements contained in the water fraction of xylan hydro-
thermolysis products with the amount of dry matter deter-
mined by the weight method.

The presence of xylose, formic acid, acetic acid and 2-FA in

the WS product fractions confirms that the hydrothermolysis
run according to the mechanism in which the initial stage was
xylan depolymerization through hydrolysis and pyrolytic
cleavage of glycosidic bonds. In a consecutive reaction, xylose
was converted into 2-FA, which was then converted into for-
mic acid

[43]

.

Table 2

shows the results of an analysis of the content of

saccharides, carboxylic acids, furfurals and other products
carried out by liquid chromatography

e in relation to the

amount of xylan subjected to hydrothermal decomposition

e

present in the WS fractions obtained as a result of xylan
hydrothermolysis run at 220

C and 235

C for 2

e30 min. With

a growth of conversion time, the saccharide and carboxylic
acids yield kept falling. In the saccharide fractions, the
quantitatively dominant element was xylose. Regardless of
the temperature of hydrothermal decomposition of xylan and
conversion time, the xylose content in the saccharide fraction
ranged from 83.6% to 94.5% (w/w).

In the carboxylic acid fractions the quantitatively domi-

nant elements were formic acid and acetic acid. Their content,
in relation to the content of the remaining acids, ranged from
93.1% to 96.9% (w/w) and, like in the case of the xylose content
in the saccharide fractions, it fell as reaction time grew.

The furfurals yield in the WS fractions of the products of

xylan hydrothermolysis grew with a growth in reaction time.
This product group was quantitatively dominated by 2-FA. Its

Table 1

e The yield

a

of products present in the WS fractions obtained through hydrothermal decomposition of xylan.

Retention time: 0 min.

Component

Yield in the water product fractions (%)

180

C

200

C

210

C

220

C

230

C

235

C

240

C

250

C

260

C

280

C

300

C

Saccharides

b

4.5

6.6

16.5

30.3

22.6

37.9

17.6

12.8

11.7

2.4

1.3

Carboxylic acids

c

0.9

10.6

13.5

14.8

16.6

22.0

19.9

13.9

6.0

4.1

3.7

Furfurals

d

e

e

0.3

0.6

1.7

1.9

2.9

3.1

4.2

4.4

4.5

Aldehydes

e

e

0.4

2.1

3.0

3.8

3.9

5.2

6.8

7.9

4.1

3.5

DHA

e

e

e

1.2

1.4

1.5

2.1

2.9

3.5

4.6

5.1

a The yield was calculated in relation to the amount of xylan subjected to the hydrothermal decomposition.
b Arabinose, galactose, xylose and mannose.
c Formic, acetic, lactic, oxalic acid.
d 2-FA and 5-HMF.
e Glyceraldehyde, pyruvaldehyde and glycoaldehyde.

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 0 2

e3 9 1 2

3907

content in the furfural fractions ranged from 95.1% to 99.8%
(w/w).

Extending the reaction time did not favor the production of

other aldehydes

e their yield was becoming smaller and

smaller. The quantitatively dominant aldehyde was pyr-
uvaldehyde, which was the only aldehyde identified in the
fractions obtained in reactions run for 15

e30 min. The glyco-

aldehyde and glyceraldehyde content was small. Their share
in the aldehyde content in the WS fractions obtained in
reaction run for 2 and 5 min did not exceed 16.3%.

In the case of DHA, its content in the WS fractions was the

highest among the products obtained in reactions run for 2
and 5 min. Extending the reaction time up to 30 min caused
a gradual drop in the DHA yield.

Like it was in the case of determining the effect of reaction

temperature on the composition of the water fraction of the
products of xylan hydrothermal decomposition, also in the
case of analyzing the effect of reaction time we were unable to
balance the total content of the components of the WS frac-
tions with the amount of dry matter determined by the weight
method. The discrepancies might have resulted from the
presence of unreacted xylan (particularly with a shorter
reaction time and at a lower temperature), and an incomplete
chromatographic detection of all the analytes present in the
water product fractions.

3.4.

Material balance for the hydrothermal

decomposition of xylan at 180

e300

C

The material balances for the hydrothermal decomposition of
2.0 g of xylan are shown in

Table 3

. In the entire studied

reaction temperature range at the retention time of 0 there
occurred a solubilization and dissolution of xylan

[2]

(trace

quantity of products contained in the MN fractions). The
content of oil fraction products present in the MS fractions
was also very small. In the lower reaction temperature range,
the percentage yield of identified products of xylan hydro-
thermal decomposition had small values. At a significant
content of products of xylan hydrothermal decomposition
present in the WS fractions, this was probably caused by the
presence of xylo-oligosaccharides

e products of hydrolytic

depolymerization of xylan

[46]

in these fractions. Their

content had not been determined in this article. Together with
an increase in reaction temperature to 240

C, the xylo-

oligosaccharides have in the process of hydrolysis reaction
turned into corresponding monosaccharides

e primary

products of hydrothermal decomposition of xylan and their
contents in the WS fractions increased. As temperature
increased, so did the recovery of products in the WS fractions,
reaching 100% in temperatures 235 and 240

C, but after

exceeding 240

C it decreased, due to progressing thermal

degradation and the gasification of the components of the
reaction mixture.

3.5.

Determination of the reaction kinetic parameters

Kinetics experiments were carried out at temperature 220 and
235

C and for different reaction times 0, 2, 5, 10, 15, 20 and

30 min. Since reactions taking place are complex, a simple
decomposition process is proposed, in which xylan is depo-
lymerized to form xylose and xylose is decomposed to form
identified degradation products

e 2-FA, other aldehydes and

DHA and non-identified products (unknown compounds were
included). Therefore, the overall reaction kinetics were
analyzed based on sequence given by equation, as shown in

Fig. 7 [47]

. The rate constants k

1

, k

2

and k

3

were obtained,

using the method of non-linear least squares regression
analyses by MicroMath Scientists for Windows Version 2.0.

Fig. 8

shows the experimental data and the model predi-

cation. There could be observed the good agreement between
the values

e experimental (markers) and calculated (solid

lines), with the determination coefficient 0.994 and 0.996 at
220 and 235

C, respectively.

The obtained apparent rate constants k

1

and k

2

are shown

in

Table 4

. The ratio k

1

/k

2

is 1.73 at 220

C and 2.23 at 235

C

and shows that hydrothermal depolymerization of xylan was
faster than the decomposition rate of xylose.

The activation energy E

a

and pre-expotential factor A were

calculated from a plot of lnk versus 1/T, using the Arrhenius
equation.

Table 5

shows the kinetic parameters E

a

and A

obtained for hydrothermal decomposition of xylan. The acti-
vation energy of xylan hydrolysis was higher than activation

Table 2

e The yield

a

of products present in the WS fractions obtained through hydrothermal decomposition of xylan.

Retention time: 2

e30 min.

Components

Yield in the water product fraction (%) for the time of (min.)

Temperature 220

C

Temperature 235

C

2

5

10

15

20

30

2

5

10

15

20

30

Saccharides

b

28.6

27.7

26.4

22.6

18.6

11.7

28.0

24.6

20.0

17.5

13.8

9.8

Carboxylic acids

c

14.9

14.6

14.0

13.8

11.8

11.0

29.9

21.9

17.8

11.7

8.9

5.5

Furfurals

d

0.7

1.5

1.6

2.3

2.3

3.0

0.7

1.6

2.0

2.4

2.5

3.0

Aldehydes

e

2.9

2.9

2.5

2.1

1.5

1.1

3.9

3.4

3.3

2.9

2.1

1.2

DHA

1.1

1.1

1.0

1.0

0.8

0.2

1.5

1.5

1.1

0.5

0.5

0.1

a The yield was calculated in relation to the amount of xylan subjected to the hydrothermal decomposition.
b Arabinose, galactose, xylose and mannose.
c Formic, acetic, lactic, oxalic acid.
d 2-FA and 5-HMF.
e Glyceraldehyde, pyruvaldehyde and glycoaldehyde.

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 0 2

e3 9 1 2

3908

energy of decomposition of xylose. This indicates, that the
course of decomposition of xylose may be difficult to
controlled, but the kinetic data are useful in helping to
understand the hydrothermolysis process of xylan.

3.6.

Hydrothermal decomposition pathway of xylan

Fig. 9

shows the probable course of hydrothermal decompo-

sition of xylan in subcritical water. The process of xylan
hydrothermolysis (1)

[48]

started from its depolymerization

and the cleavage of acetyl groups

[43]

and led to the

Table 3

e Material balances for hydrothermal decomposition of 2.0 g xylan. Retention time: 0 min.

180

C 200

C 210

C 220

C 230

C 235

C 240

C 250

C 260

C 280

C 300

C

Material out (g):
WS fractions, including:

1.86

1.86

1.80

1.62

1.57

1.37

1.25

0.94

0.93

0.88

0.73

Reducing sugars

a

0.28

0.45

0.54

0.79

0.68

0.85

0.79

0.50

0.45

0.40

0.28

Losses in the WS fractions

b

1.58

1.41

1.26

0.83

0.89

0.52

0.46

0.42

0.48

0.48

0.45

% Recovery of products in the WS fractions

c

14.00

22.50

27.00

39.50

34.00

42.50

39.50

25.00

22.50

20.00

14.00

WS fractions, including:
Saccharides

0.09

0.13

0.33

0.61

0.45

0.76

0.35

0.26

0.23

0.05

0.03

Carboxylic acids

0.02

0.21

0.27

0.30

0.33

0.44

0.40

0.28

0.12

0.08

0.07

Furfurals

e

e

0.01

0.01

0.03

0.04

0.06

0.06

0.08

0.09

0.09

Aldehydes

e

0.01

0.04

0.06

0.08

0.08

0.10

0.14

0.16

0.08

0.07

DHA

e

e

e

0.02

0.03

0.03

0.04

0.06

0.07

0.09

0.10

Losses in the WS fractions

d

1.75

1.51

1.15

0.62

0.65

0.02

0.30

0.14

0.27

0.49

0.37

% Recovery of products in the WS fractions

e

5.91

18.82

36.11

61.73

58.60

100.00

100.00

85.11

70.97

44.32

49.32

MS fractions

0.06

0.06

0.02

0.01

e

e

e

e

0.02

0.04

0.08

MN fractions

0.04

0.03

0.02

e

e

e

e

e

0.01

0.01

0.01

Losses

f

þ G fractions

0.04

0.05

0.16

0.37

0.43

0.63

0.75

1.06

1.07

1.12

1.27

a Reducing sugars determined by Luuf-Schoorl’s assay.
b Losses (L) calculated according to the equation L

¼ mass of WS fractions minus mass of reducing sugars present in the WS fractions.

c % Recovery was calculated in proportion to content of reducing sugars in the WS fractions.
d Losses calculated according to the equation L

¼ mass of WS fraction minus mass of saccharides, carboxylic acids, furfurals, other aldehydes

and DHA present in the WS fractions.
e % Recovery calculated in proportion to products identified with the help of HPLC

e saccharides, carboxylic acids, furfurals, other aldehydes

and DHA present in the WS fractions.
f Losses in all received product fractions were calculated according to equation L

¼ 2 g minus mass of WS fraction minus mass of MS fraction

minus mass of MN fraction.

Xylan

Xylose

Xylose

Identified degradation products

Non-identified degradation products

k

1

k

2

k

3

Fig. 7

e Simple reaction pathway for xylose hydrothermal

decomposition.

0

0,5

1

1,5

2

2,5

3

0

200

400

600

800

1000

1200

1400

1600

1800

Conc

ent

rat

ion (

dm

-3

)

Time (s)

Xylose- 220°C

Identified degradation products- 220°C

Xylose- 235°C

Identified degradation products- 235°C

Fig. 8

e Experimental and predicted concentration of the

xylose and identified degradation products in the WS
fractions (values shown by lines were calculated).

Table 4

e Kinetic parameters for hydrothermal

decomposition of xylan.

Rate constant

220

C

235

C

k

1

(s

1

)

0.001265

0.001935

k

2

(s

1

)

0.000731

0.000866

Table 5

e The Arrhenius parameters e activation

energies and expotential factors for the
hydrothermolysis of xylan.

Parameters

Value

E

a1

(kJ mol

1

)

58.89

A

1

(s

1

)

2.22

10

3

E

a2

(kJ mol

1

)

23.49

A

2

(s

1

)

0.226

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 0 2

e3 9 1 2

3909

production of xylose (2), arabinose (3) and acetic acid (4)

[43,45]

. In the second stage of the reaction, the pentoses

obtained were dehydrated and subjected to retro-aldol
condensation. As a result of the dehydration of the pentoses,
furfurals 2-FA (5) i 5-HMF (6)

[2]

were produced, from which

e

in the consecutive reactions of hydration, dehydration, tau-
tomerization and hydration

e further intermediate products

were produced, from which formic acid (9) and probably lev-
ulinic acid

[36,37]

were produced. As a result of retro-aldol

condensation of xylose and arabinose, the liquid product
fraction contained glycolaldehyde (7) and glyceraldehyde (8)

[2]

. Glyceraldehyde was further converted

e in a tautomeri-

zation reaction

e to dihydroxyacetone (10), and e in a dehy-

dration reaction

e to pyruvaldehyde (11)

[2]

. In turn, the

regrouping and dehydration of pyruvaldehyde might have
resulted in the production of lactic acid (12)

[36,37]

.

4.

Conclusions

This paper presents results of research into the course of
hydrothermolysis of 2% aqueous suspension of xylan as
a model substance for hemicellulose contained in plant
biomass waste (for example rapeseed straw).

Hydrothermal decomposition of xylan was carried out in

subcritical water in a batch process. Under the conditions
applied (the temperature of 180

e300

C, the time of 0

e30 min),

water played the role of solvent and acid catalyst for the
conversions. At lower temperatures, xylan decomposition led
to the production of water-soluble product fractions, in which
the quantitatively dominant elements were xylose and acetic
acid

e the products of hydrolytic depolymerization of xylan.

Their highest yield, amounting to 36.3% for xylose and 18.4% for
acetic acid, was reached in a reaction run at the temperature of
235

C, with the retention time of 0 min. The products contained

in the WS fractions and obtained in these conditions (180

e240

)

owing to trace contents of aldehydes can be applied as a base
material in the fermentation process leading to bioethanol.

The longer the reaction mixture was heated up, the faster

the speed was of the degradation of xylose and other primary
liquid fraction components leading to the creation of the
secondary products of their consecutive conversions

e formic

acid, 2-FA, aldehydes and DHA. An increase in the tempera-
ture of xylan hydrothermolysis favored the production of an
ever larger amount of gas product fractions, with an insig-
nificant increase in the yield of the char-containing fraction.
Also, extending the time of xylan hydrothermolysis up to
30 min did not have an advantageous effect on the yield of
saccharides and carboxylic acids contained in the water
product fraction. In a reaction run both at 220

C and 235

C,

their content decreased with a growth in conversion time. The
secondary products, obtained as a result of hydrothermal
decomposition of xylan in the higher temperature range,
contained in the WS fractions can be directly applied as fuel
supplements. In industrial processes, concentrations of xylo-
oligosaccharide or hemicellulose is higher (even 100 g dm

1

).

It seems advisable to determine the impact of xylan concen-
tration subjected to hydrothermal decomposition also in the
wider range of the type and quantity of the created products
present in the WS fractions.

Acknowledgment

The authors are gratefully acknowledge the financial support
for this work provided by the Ministry of Science and Higher
Education of Poland in 2008

e2010 in the form of research

project No. N N523 494134.

The authors would like to express their deepest gratitude

to Prof. Edmund Cibis, University of Economics in Wroc

1aw,

Poland, for contribution and helpful suggestions to the
description of reaction kinetics model.

r e f e r e n c e s

[1] Tokarzewska-Zadora J, Rogalski J, Szczodrak J. Xylan-

degrading enzymes

e characterization and application in

biotechnology. Biotechnologia 2005;69:163

e82 [22.02.10],

http://www.pfb.info.pl/files/kwartalnik/2_2005/
Tokarzewska-Zadora.pdf

[in Polish].

[2] Yu Y, Lou X, Wu H. Some recent advances in hydrolysis of

biomass in hot-compressed water and its comparisons with
other hydrolysis methods. Energ Fuel 2008;22:46

e60.

[3] Hansen NML, Plackett D. Sustainable films and coatings from

hemicelluloses: a review. Biomacromolecules 2008;9:
1493

e505.

O

OH

OH

O

H

3

CO

OH

O

COO

-

O

O

HO

O

O

O

OH

O

O

O

O

HO

H

3

C

O

(

)n

O

O

H

3

CO

HO

(1)

depolymerization

CH

3

COOH

(2)

(4)

O

OH

OH

OH

OH

O

OH

OH

OH

OH

(3)

retro-aldol condensation

+

+

dehydration

O

CHO

(5)

CHO

O

HOH

2

C

(6)

HO

H

O

(7)

HO

H

OH

O

+

(8)

keto-enol
tautomerization

dehydration

HO

OH

O

(10)

H

O

O

(11)

rearrangment
dehydration

OH

OH

O

(12)

+

O

OH

OH

O

H

3

CO

OH

O

O

O

HO

O

O

O

OH

O

O

O

HO

H

3

C

O

((

O

O

3

CO

HO

(1)

depolymeriza

z

z tion

CH

3

COOH

(2)

(4)

O

OH

OH

OH

OH

O

OH

OH

OH

OH

(3)

retro

rr -aldol condensation

+

+

dehyd

y

y ration

CHO

O

HOH

2

C

(6)

HO

H

O

(7)

HO

H

OH

O

+

(8)

keto-enol

tautomeriza

z

z tion

n

dehyd

y

y ration

HO

OH

O

(10)

H

O

O

(11)

rearran

dehyd

y

y r

+

(9)

degradation

HCOOH

Fig. 9

e The course of hydrothermal decomposition of

xylan

[2,36,37,48]

.

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 0 2

e3 9 1 2

3910

[4] Bogoczek R, Kocio

1ek-Balawejder E. Chemical organic

technology. 1st ed. Wroc

1aw: Wydawnictwo AE we

Wroc

1awiu; 1992 [in Polish].

[5] Tunc MS, Van Heiningen ARP. Hemicellulose extraction of

mixed southern hardwood with water at 150

C: effect of

time. Ind Eng Chem Res 2008;47:7031

e7.

[6] Hendriks ATWM, Zeeman G. Pretreatments to enhance the

digestibility of lignocellulosic biomass. Bioresour Technol
2009;100:10

e8.

[7] Kumar P, Barrett DM, Delwiche MJ, Stroeve P. Methods for

pretreatment of lignocellulosic biomass for efficient
hydrolysis and biofuel production. Ind Eng Chem Res 2009;
48:3713

e29.

[8] Kabel MA, Carvalheiro F, Garrote G, Avgerinos E, Koukios E,

Parajo JC, et al. Hydrothermally treated xylan rich by-
products yield different classes of xylo-oligosaccharides.
Carbohydr Polym 2002;50:47

e56.

[9] Laser M, Schulman D, Allen SG, Lichwa J, Antal MJ, Lynd LR.

A comparison of liquid hot water and steam pretreatments
of sugar cane bagasse for bioconversion to ethanol. Bioresour
Technol 2002;81:33

e44.

[10] Boussarsar H, Roge B, Mathlouthi M. Optimization of

sugarcane bagasse conversion by hydrothermal treatment
for the recovery of xylose. Bioresour Technol 2009;100:
6537

e42.

[11] Makishima S, Mizuno M, Sato N, Shinji K, Suzuki M,

Nozaki N, et al. Development of continuous flow type
hydrothermal reactor for hemicellulose fraction
recovery from corncob. Bioresour Technol 2009;100:
2842

e8.

[12] Cansell F, Rey S, Beslin P. Thermodynamic aspects of

supercritical fluids processing: application to polymers and
wastes treatment. Revue De L’Institut Francais Du Petrole
1998;53:71

e98.

[13] Bro¨ll D, Kaul C, Kra¨mer A, Krammer P, Richter T, Jung M,

et al. Chemistry in supercritical water. Angew Chem Int Ed
1999;38:2998

e3014.

[14] Akiya N, Savage PE. Roles of water for chemical reactions in

high-temperature water. Chem Rev 2002;102:2725

e50.

[15] Kruse A, Dinjus E. Hot compressed water as reaction

medium and reactant. Properties and synthesis reactions. J
Supercrit Fluids 2007;39:362

e80.

[16] Burczyk B. Water

e a useful and environmentally friendly

reaction medium. Przem Chem 2007;86:184

e94 [in Polish].

[17] Burczyk B. Green chemistry. 1st ed. Wroc

1aw: Oficyna

Wydawnicza Politechniki Wroc

1awskiej; 2006 [in Polish].

[18] Yesodharan S. Supercritical water oxidation: an

environmentally safe method for the disposal of organic
wastes. Curr Sci 2002;82:1112

e22.

[19] Goto M, Obuchi R, Hirose T, Sakaki T, Shibata M.

Hydrothermal conversion of municipal organic waste into
resources. Bioresour Technol 2004;93:279

e84.

[20] Kruse A, Dinjus E. Hot compressed water as reaction

medium and reactant: 2. Degradation reactions. J Supercrit
Fluids 2007;41:361

e79.

[21] Pi

nkowska H. Low-temperature biomass gasification in

subcritical and supercritical water. Przem Chem 2007;86:
599

e606 [in Polish].

[22] Pi

nkowska H. High-temperature biomass gasification in

supercritical water. Przem Chem 2007;86:1044

e55 [in Polish].

[23] He W, Li G, Kong L, Wang H, Huang J, Xu J. Application of

hydrothermal reaction in resource recovery of organic
wastes. Resour Conserv Recy 2008;52:691

e9.

[24] Brunner G. Near critical and supercritical water. Part I.

hydrolytic and hydrothermal processes. J Supercrit Fluids
2009;47:373

e81.

[25] Resende FLP, Neff ME, Savage PE. Noncatalytic gasification of

cellulose in supercritical water. Energ Fuel 2007;21:3637

e43.

[26] Miyazawa T, Funazukuri T. Polysaccharide hydrolysis

accelerated by adding carbon dioxide under hydrothermal
conditions. Biotechnol Prog 2005;21:1782

e5.

[27] Yoshida K, Saka S. Organic acid production from Japanese

beech by supercritical water treatment. In: The 2nd Joint
International Conference on Sustainable Energy and
Environment. C-032, Thailand; 2006. p. 1

e6, 22.02.2010,

http://www.jgsee.kmutt.ac.th/see1/cd/file/C-032.pdf

.

[28] Jacobsen SE, Wyman CE. Xylose monomer and oligomer

yields for uncatalyzed hydrolysis of sugarcane bagasse
hemicelluloses at varying solids concentration. Ind Eng
Chem Res 2002;41:1454

e61.

[29] Yuan XZ, Li H, Zeng GM, Tong JY, Xie W. Sub- and

supercritical liquefaction of rice straw in the presence of
ethanol

ewater and 2-propanolewater mixture. Energy 2007;

32:2081

e8.

[30] Lu X, Zhang Y, Agelidaki I. Optimization of H

2

SO

4

-catalyzed

hydrothermal pretreatment of rapeseed straw for
bioconversion to ethanol: focusing on pretreatment at high
solids content. Bioresour Technol 2009;100:3048

e53.

[31] Weil J, Sarikaya A, Rau SL, Goetz J, Ladisch CM, Brewer M,

et al. Pretreatment of yellow poplar sawdust by
pressure cooking in water. Appl Biochem Biotechnol 1997;68:
21

e40.

[32] Journal of Laws dated 24/12/2004, Minister of Agriculture and

Rural Development Regulation dated 02/12/2004 [in Polish].

[33] Sluiter A, Hames B, Ruiz R, Scarlata C, Sluiter J, Templeton D.

Determination of sugars, byproducts, and degradation
products in liquid fraction process samples. Laboratory
Analytical Procedure, Issue Date: 12.08.2006, Technical
Report NREL/TP-510-42623,

http://www.nrel.gov/biomass/

pdfs/42623.pdf

[accessed 22.02.10].

[34] Prevail HPLC Columns,

http://www.apexchrom.com/DL/

Prevail.pdf

[accessed 22.02.10].

[35] Alcazar A, Jurado JM, Pablos F, Gonzalez AG, Martin MJ. HPLC

determination of 2-furaldehyde and 5-hydoxymethyl-2-
furaldehyde in alcoholic beverages. Microchem J 2006;82:
22

e8.

[36] Aida TM, Tajima K, Watanabe M, Saito Y, Kuroda K,

Nonaka T, et al. Reactions of D-fructose in water at
temperatures up to 400

C and pressures up to 100 MPa. J

Supercrit Fluids 2007;42:110

e9.

[37] Aida TM, Sato Y, Watanabe M, Tajima K, Nonaka T,

Hattori H, et al. J Supercrit Fluids 2007;40:381

e8.

[38] Liu C, Wyman CE. The effect of flow rate of compressed hot

water on xylan, lignin and total mass removal from corn
stover. Ind Eng Chem Res 2003;42:5406

e16.

[39] Minowa T, Fang Z, Ogi T, Varhegyi G. Decomposition of

cellulose and glucose in hot-compressed water under
catalyst-free conditions. J Chem Eng Jpn 1998;31:131

e4.

[40] Fang Z, Minowa T, Smith RL, Ogi T, Kozi

nski JA. Liquefaction

and gasification of cellulose with Na

2

CO

3

and Ni in

subcritical water at 350

C. Ind Eng Chem Res 2004;43:

2454

e63.

[41] Minowa T, Zhen F, Ogi T, Varhegyi G. Liquefaction of

cellulose in hot compressed water using sodium carbonate:
products distribution at different reaction temperatures. J
Chem Eng Jpn 1997;30:186

e90.

[42] Minowa T, Fang Z, Ogi T. Cellulose decomposition in hot-

compressed water with alkali and nickel catalyst. J Supercrit
Fluids 1998;13:253

e9.

[43] Carvalheiro F, Esteves MP, Parajo JC, Pereira H, Girio FM.

Production of oligosaccharides by autohydrolysis of
brewery’s spent grain. Bioresour Technol 2004;91:93

e100.

[44] Watanabe M, Aizawa Y, Iida T, Aida TM, Levy C, Sue K, et al.

Glucose reactions with acid and base catalysts in
hot compressed water at 473 K. Carbohydr Res 2005;340:
1925

e30.

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 0 2

e3 9 1 2

3911

[45] Parajo JC, Garrote G, Cruz JM, Dominguez H. Production of

xylooligosaccharides by autohydrolysis of
lignocellulosic materials. Trends Food Sci Tech 2004;15:
115

e20.

[46] Weil J, Brewer M, Hendrickson R, Sarikaya A, Ladisch MR.

Continuous pH monitoring during pretreatment of yellow
poplar wood sawdust by pressure cooking in water. Appl
Biochem Biotechnol 1998;70-72:99

e111.

[47] Ladisch MR. Hydrolysis. In: Kitani O, Hall CW, editors.

Biomass handbook. New York: Gordon and Breach Science;
1989. p. 434

e51.

[48] Li X, Chen X. Biodegradation of polysaccharide sourced from

virulence factor or plant and pathogenic cell wall constituent
and its application in management of phytopathogenic disease.
In: Wang BY, editor. Environmental biodegradation research
focus. New York: Nova Science Publishers Inc; 2007. p. 19.

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 0 2

e3 9 1 2

3912