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Hydrothermal decomposition of xylan as a model substance
for plant biomass waste
Hydrothermolysis in
e
subcritical water
Hanna Pi
nkowska
*
, Pawe
Wolak, Adrianna Z
oci
nska
1
1
Department of the Chemical Technology, University of Economics in Wroc
ł
aw, ul. Komandorska 118/120, 53-345 Wroc
ł
aw, Poland
article info
abstract
Article history:
Received 26 April 2010
Received in revised form
12 May 2011
Accepted 2 June 2011
Available online 2 July 2011
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
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
Keywords:
Xylan
Hydrothermal reaction
Xylose
Decomposition
Carboxylic acid
Furfural
saccharide
e
decomposition
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.
e
1.
Introduction
hemicellulose inplant rawmaterial 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
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
7]
. 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
e
200. Because of its structure,
e
it
is amorphous and hydrophilic
[2,4]
. The presence of
e
11]
.
It can be run in sub- and
* Corresponding author. Tel./fax:
48 71 36 80 275.
E-mail address:
(H. Pinkowska).
þ
0961-9534/$
2011 Elsevier Ltd. All rights reserved.
doi:
see front matter
e
ª
3903
e
supercritical water. Critical parameters of water are:
T
cr
¼
of DP
132 and the empirical formula (C
5
H
8
O
4
)
n
. The
elementary composition of beech wood xylan contains carbon
e
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
374.2
C, P
cr
¼
22,05 MPa, g
cr
¼
41.2%, hydrogen
5.9% and
oxygen
52.8%
e
e
16]
.
Along with an increase in temperature and pressure, the
cleavage of hydrogen bonds occurs, and the dielectric
constant and ionic product change
[2,13
e
(C:H:O
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.
¼
17]
. 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
e
17]
. It is also used as reaction medium in waste substance
utilization processes
[16,18
e
20]
,
including conversion of
e
biomass into useful products
[2,21
24]
.
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
e
2.2. The reactor and the course of hydrothermal
decomposition xylan
(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 decompositionof rape straw in subcritical water
will be conducted based on xylan decomposition test results.
b
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 formof a U-pipe (a single loop), a heatingmantle
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
300
C for 0
30 min, at a pressure corresponding to the
vapor pressure curve at a given temperature or one that
slightly exceeded it. HPLC-grade water
e
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
e
25%
[2]
, a commonly used
concentration of biomass waste undergoing hydrothermal
decomposition
e
30]
. 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
about 10%
[28
e
e
e
15 min and kept at the same temperature to an accu-
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
racy of
2.
Experimental
15
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.
2.1.
Materials and reagents
e
For the tests, there was used beech wood xylan (Carl Roth
GmbH, Karlsruhe, Germany) with a degree of polymerization
3904
2.4. Analytical techniques and measurement
methodology
290
0 min
5 min
10 min
15 min
260
230
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
200
170
140
110
80
50
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
e
0
5
10
15
20
25
30
35
40
45
50
Time (min)
Fig. 1
An example of a program for heating up, retaining
at 260
C and cooling down of the reaction mixture during
hydrothermal decomposition of xylan.
e
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
0.2 cm
3
of phosphoric acid
m). 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:
m
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]
.
þ
e
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
Y
G
¼
100
Y
DM
Y
MS
Y
MN
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
¼
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.
3905
e
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
45
40
35
30
25
20
15
10
180
190
200
210
220
230
240
250
260
270
280
290
300
Temperature (°C)
Fig. 3
Effect of reaction temperature of hydrothermal
decomposition of xylan on the reducing sugars yield in the
WS fractions. Retention time: 0 min.
e
232.5
C, the reducing sugars yield
was falling and reached 33.9
e
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
36.7%, 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
250
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.
e
at 300
C the MN
Nevertheless, its amount was very small
e
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
e
xylo-oligosaccharides, but also of unreacted raw material
which might have been solubilized under the reaction
250
C, gasification was very
intensive. After the temperature of 250
C was exceeded, the
gas product fractions yield exceeded 50%. At temperatures
e
6
100
DM
MS
MN
G
90
5,5
80
70
5
60
4,5
50
40
4
30
20
3,5
10
3
0
180
190
200
210
220
230
240
250
260
270
280
290
300
180
190
200
210
220
230
240
250
260
270
280
290
300
Temperature (°C)
Temperature (°C)
Fig. 2
Changes of pH in the water fraction of the products
of hydrothermal decomposition of xylan depending on
reaction temperature. Retention time: 0 min.
Fig. 4
Effect of the reaction temperature of hydrothermal
decomposition of xylan on the DM, MS, MN and G fractions
yield. Retention time: 0 min.
e
e
 3906
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]
.
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 drymatter 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
3.2. The effect of reaction time on the course of
hydrothermal decomposition of xylan
30 min, the originally created water-
soluble products were then converted, by being gasified
gradually along with a growth in reaction time
[41,42]
.
e
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 theWS fraction obtained at 220
C was
3.76, and at 235
C
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
300
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
e
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
¼
the products of xylan hydrothermal
90
45
220°C
235°C
80
40
70
35
60
DM-200°C
G-200°C
DM-235°C
G-235°C
30
50
40
25
30
20
20
15
10
10
0
0
5
10
15
20
25
30
0
5
10
15
20
25
30
Time (min)
Time (min)
Fig. 5
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.
Fig. 6
The effect of the reaction time of hydrothermal
decomposition of xylan at 220
C and 235
C on the DM and
G fractions yield.
e
e
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