Acid-catalyzed conversion of xylose in methanol-rich medium as part of biorefinery

Article abstract of DOI:10.1002/cssc.201100745

Acid treatments of xylose have been performed in a methanol/water mixture to investigate the reaction pathways of xylose during bio-oil esterification. Xylose was mainly converted into methyl xylosides with negligible humins formed below 130°C. However, humins formation became significant with the dehydration of xylose to furfural and 2-(dimethoxymethyl)furan (DOF) at elevated temperatures. The conversion of xylose to methyl xylosides protected the C1 hydroxyl group of xylose, which stabilized xylose and suppressed the formation of sugar oligomers and polymerization reactions. In comparison, the conversion of furfural to DOF protected the carbonyl group of furfural. However, the protection did not remarkably suppress the polymerization of furfural at high temperatures because of the shift of the reaction equilibrium from DOF to furfural with a prolonged residence time. In addition, the acid treatment of furfural produced methyl levulinate in methanol and levulinic acid in water, which was catalyzed by formic acid. Copyright

Full text of DOI:10.1002/cssc.201100745

DOI: 10.1002/cssc.201100745
Acid-Catalyzed Conversion of Xylose in Methanol-Rich
Medium as Part of Biorefinery
Xun Hu, Caroline Lievens, and Chun-Zhu Li*[a]
Acid treatments of xylose have been performed in a methanol/
water mixture to investigate the reaction pathways of xylose
during bio-oil esterification. Xylose was mainly converted into
methyl xylosides with negligible humins formed below 1308C.
However, humins formation became significant with the dehy-
dration of xylose to furfural and 2-(dimethoxymethyl)furan
(DOF) at elevated temperatures. The conversion of xylose to
methyl xylosides protected the C1 hydroxyl group of xylose,
which stabilized xylose and suppressed the formation of sugar
oligomers and polymerization reactions. In comparison, the
conversion of furfural to DOF protected the carbonyl group of
furfural. However, the protection did not remarkably suppress
the polymerization of furfural at high temperatures because of
the shift of the reaction equilibrium from DOF to furfural with
a prolonged residence time. In addition, the acid treatment of
furfural produced methyl levulinate in methanol and levulinic
acid in water, which was catalyzed by formic acid.
Introduction
Flash pyrolysis of biomass produces a bio-oil that has been rec-
ognized as a potential fuel and chemical feedstock.[1–3] Howev-
er, the raw bio-oil cannot be directly used because of its intrin-
sic deleterious properties such as corrosiveness and thermal
instability.[4,5] Esterification can overcome the undesirable prop-
erties by removing the organic acids in bio-oil with alcohols.[6]
In addition to organic acids, carbohydrates are another main
component of bio-oil.[7,8] The presence of carbohydrates in-
creases not only the viscosity but also the instability of bio-oil
as carbohydrates have a high tendency to polymerize.[9–12]
Thus, the reaction pathways of carbohydrates in the esterifica-
tion of bio-oil are of concern.
furfural? Moreover, does a methanol-rich medium change the
degradation pathways of xylose and furfural? These are the
key questions that we have tried to answer in this study.
Xylose was used as the model compound of C5 carbohydrates.
A methanol/water mixture was used as the reaction medium
as water is invariably present in bio-oil. Amberlyst 70, which is
a commercial solid acid catalyst, was used to catalyze the con-
version of xylose in the methanol/water mixture.
Results and Discussion
The simple carbohydrates in bio-oil mainly refer to the C6
and C5 carbohydrates, which originate from the degradation
of cellulose and hemicelluloses in biomass. Acid treatments of
the C6 or C5 carbohydrates in water have been intensively in-
vestigated,[13–15] and the conversion of the C6 carbohydrates in
methanol has also been investigated recently.[16–18] However,
few studies focused on the conversion of the C5 carbohydrates
in methanol-rich media, the understanding of which helps us
to elucidate the catalytic behavior of the C5 carbohydrates in
bio-oil esterification and to select appropriate conditions to
avoid side reactions.
Effects of reaction temperature on the conversion of xylose
in methanol/water medium
Xylose conversion, product distribution, and humins formation
at the temperatures from 90 to 1708C are depicted in Figure 1.
The required reaction temperature was reached at 0 min on
the x-axes of the graphs in Figure 1. Prolonged reaction time
and increased temperature promote the conversion of xylose,
as shown in Figure 1a. Methanolysis of xylose dominated
below 1308C, producing methyl-a-d-xylopyranoside (MAXP,
Figure 1b) and methyl-b-d-xylopyranoside (MBXP, Figure 1c) as
the main products and methyl-a-d-xylofuranoside (MAXF) and
methyl-b-d-xylofuranoside (MBXF) as minor products. MAXF
and MBXF are not presented, as their concentrations were low
and became negligible with the progress of the reaction. The
In water, it is known that the acid treatment of C5 carbohy-
drates such as xylose, a typical C5 carbohydrate from the deg-
radation of hemicellulose, produces furfural.[19] In methanol
and in the presence of acids, it is known that xylose can be
converted to methyl xylosides;[20] however, further conversion
of methyl xylosides in methanol is not understood. In addition,
both xylose and furfural have a high tendency to polymerize in
acidic aqueous media.[21,22] Polymerization is undesirable as it
diminishes the utilization efficiency of xylose and may lead to
catalyst deactivation. Hence, a question is raised. Does a metha-
nol-rich medium suppress the polymerization of xylose and
[a] Dr. X. Hu, Dr. C. Lievens, Prof. C.-Z. Li
Fuels and Energy Technology Institute
Curtin University of Technology
GPO Box U1987, Perth, WA 6845 (Australia)
Fax: (+61)8-9266-1138
E-mail: chun-zhu.li@curtin.edu.au
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100745.
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Figure 1. Effect of reaction temperature [90 ( ), 110 ( ), 130 ( ), 150 ( ),
and 1708C ( )] on a) xylose conversion, yields of b) MAXP, c) MBXP, d) DOF,
*
and e) furfural as well as the weight ratio of humins to converted xylose (f).
Methanol/water mass ratio: 4.5; catalyst dosage: 5 wt%; xylose loading:
5.58 wt%; reaction time: 180 min; stirring rate: 600 rpm.
production of different methyl xylosides is probably because
of the different steric configurations of xylose in the reaction
medium as shown in Scheme 1.
Scheme 1. The main reaction pathways of xylose in methanol-rich medium.
At 1308C with prolonged residence time, the methyl xylo-
sides degraded slightly, producing small amounts of 2-(dime-
thoxymethyl)furan (DOF, Figure 1d) and furfural (Figure 1e).
Further increasing the temperature to 1508C resulted in the re-
markable degradation of the methyl xylosides and correspond-
ingly produced more DOF and furfural. DOF and furfural were
formed almost simultaneously, but the furfural concentration
was higher than that of DOF and soon reached a maximum,
whereas the DOF concentration increased monotonously.
It seems that furfural was the primary product from the degra-
dation of methyl xylosides and DOF was the secondary prod-
uct from the acetalization of furfural with methanol. However,
it is difficult to draw a conclusion because of the equilibrium
between furfural and DOF. At 1708C, significant amounts of
furfural and DOF were formed initially, but they were soon
converted to something that cannot be detected by GC–MS,
which is probably the polymeric material, humins.
Figure 1 f also shows that humins formation was negligible
below 130, slight at 150, and significant at 1708C. Humins for-
mation was closely related to the product distribution. The
methanolysis of xylose to methyl xylosides below 1308C does
not lead to remarkable humins formation, whereas the produc-
tions of furfural and DOF above 1508C was accompanied by
a significant amount of humins formation. Furfural has a high
tendency to polymerize in water.[21] Apparently, at 1708C, the
methanol-rich medium used did not prevent the polymeri-
zation of furfural. Although some furfural was converted to its
acetal (DOF), the formation of humins consumed furfural,
which would shift the equilibrium between furfural and DOF
towards furfural and eventually consume DOF. These results
also indicated that, in the esterification of bio-oil under similar
conditions, the xylose in bio-oil could be converted into useful
chemical feedstocks such as methyl xylosides, furfural, and
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Acid-Catalyzed Conversion of Xylose
DOF below 1508C, whereas it was mainly converted to humins
above this temperature.
As humins formation was found to be closely related to the
product distribution, which was further affected by the resi-
dence time, the change of humins formation with residence
time was investigated at 1708C. The analysis of humins formed
on heating to 1708C over one, two, and three hours at 1708C
is collected and presented in Figure 2. During heating up, the
Figure 2. Effect of the residence time on humins formation (as weight ratio
of humins to converted xylose) in the acid-treatment of xylose at 1708C.
Methanol/water mass ratio: 4.5; catalyst dosage: 5 wt%; xylose loading:
5.58 wt%; stirring rate: 600 rpm.
humins formation was slight and the main reaction was the
methanolysis of xylose to methyl xylosides (Figures 1b and
1c), which further indicates that the methanolysis of xylose to
methyl xylosides did not result in remarkable humins forma-
tion. In the first hour at 1708C, the degradation of methyl xylo-
sides produced a large amount of furfural and DOF (Figures 1d
and 1e) and humins formation was slight. In the second hour,
the furfural and DOF concentrations decreased significantly
and humins formation was substantial. In the third hour, the
concentrations of furfural and DOF were low and humins for-
mation reached a plateau. These results indicated that the
degradation of methyl xylosides to furfural or DOF did not
lead to a significant amount of humins formation, but the fur-
ther degradation of furfural and DOF mainly went to humins.
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Figure 3. Effect of methanol/water mass ratios [CH3OH/H2O=10 ( ), 4.5 ( ),
~
1 ( ), and 0.22 ( )] on a) xylose conversion, yields of b) MAXP, c) MBXP,
*
d) DOF, and e) furfural as well as the weight ratio of humins to converted
xylose (f). T=1508C; catalyst dosage: 5 wt%; xylose loading: 5.58 wt%; reac-
tion time: 180 min; stirring rate: 600 rpm.
Figure 3 f shows that humins formation was favored in the
water-rich medium and suppressed in the methanol-rich
medium. In the water-rich medium, high concentrations of un-
converted xylose and furfural were present. Both xylose and
furfural have a high tendency to polymerize to form homo-
polymers or a copolymer,[23,24] as shown in Scheme 2. In the
methanol-rich medium, the C1 hydroxyl group of xylose and
the carbonyl group of furfural are protected with methanol by
etherification and acetalization reactions, which are responsible
for the suppression of humins formation in the methanol-rich
medium. In the water medium, the C1 hydroxyl group of
xylose is easily protonated, forming a carbocation,[25] as shown
in Scheme 3. The carbocation is very reactive in the acidic envi-
ronment and can react with xylose to form various disaccha-
rides or even sugar oligomers.[25] These reactions are termed
“reversion reactions” and are recognized as important side re-
actions, which lower the utilization efficiency of sugars.[25]
To measure the formation of sugar oligomers from xylose,
the acid treatment of xylose at 1108C in water was performed.
As the possible polymerized sugar products cannot be detect-
ed by GC–MS, the products were processed by the derivatiza-
tion method before analysis by GC–MS. d-Xylononitrile, 4-O-
(2,3,4-tri-O-acetyl-b-xylopyranosyl)-2,3,5-triacetate (XAXT, Fig-
Effects of methanol/water mass ratios on xylose conversion
The acid treatment of xylose at 1708C mainly converted xylose
into humins, as presented above. Thus, the effects of the
methanol/water mass ratio on the xylose conversion were in-
vestigated at 1508C (Figure 3). High methanol/water ratios
(methanol-rich medium) favored the methanolysis of xylose, re-
sulting in faster conversion of xylose (Figure 3a) and higher
concentrations of methyl xylosides (Figures 3b and 3c) and
DOF (Figure 3d). In comparison, low methanol/water ratios
(water-rich medium) resulted in a much slower conversion of
xylose and more furfural produced (Figure 3e). The furfural
production decreased with prolonged reaction time in both,
methanol- and water-rich media, as it was probably converted
to DOF or humins.
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Figure 4. Acid-treatment of xylose in water ( ) and in methanol-rich
*
medium ( ): Effect of reaction time on a) xylose conversion and b) XAXT
abundance. In water: T=110 8C; catalyst dosage: 5 wt%; xylose loading:
5.58 wt%; stirring rate: 600 rpm. In methanol-rich medium: T=110 8C; cata-
lyst dosage: 5 wt%; xylose loading: 5.58 wt%; methanol/water mass ratio:
4.5; stirring rate: 600 rpm.
Scheme 2. The polymerization of xylose in water and in methanol.
ure 4b), which is the derivatization product of d-xylobiose, was
detected. The structure and mass spectrum of XAXT are shown
in Scheme S1 and Figure S1 in the Supporting Information. The
conversion of xylose (Figure 4a) was much lower in water than
that in the methanol-rich medium as methanol was very reac-
tive to form methyl xylosides with xylose. The production of
d-xylobiose reached a maximum with prolonged reaction time.
d-Xylobiose contains a C1 hydroxyl group, and it was possible
that d-xylobiose continued to react with xylose or other reac-
tive intermediates to form some larger sugar oligomers. No
disaccharides were detected at 1108C in the methanol-rich
medium. Apparently, the methanolysis of xylose dominated,
and the formation of the disaccharides was suppressed in the
presence of excess methanol.
xylose to furfural and the subsequent polymerization of furfu-
ral and/or the sugar oligomers dominated, which led to the
formation of humins as the main product. The weight ratio of
the humins to the xylose converted in the experiment with
a xylose loading of 23 wt% (65%) was higher than that with
a xylose loading of 15 wt% (61%). The high initial xylose load-
ing produced more polymeric intermediates and favored poly-
merization reactions.
In the methanol-rich medium, methanol will react with
xylose to form methyl xylosides because of the high reactivity
and the small steric effect of methanol. The methyl xylosides
were relatively stable and would not easily be converted back
in the methanol-rich medium to the reactive xylose carbocat-
ion (Scheme 3). Consequently, the formation of disaccharides
or sugar oligomers in the methanol-rich medium was sup-
pressed. Furthermore, the copolymer formed from the conden-
sation of xylose and furfural may also be suppressed because
of the stabilization of xylose as methyl xylosides.
To confirm whether the sugar oligomers acted as intermedi-
ates in the polymerization or not, the acid-treatment of xylose
at 1708C in water was performed. High xylose loadings (15
and 23 wt%) and correspondingly high catalyst loadings were
used as the high concentration of xylose favored the formation
of sugar oligomers. The results presented in Figure S2 showed
that d-xylobiose was also formed, reached a maximum, and
then decreased significantly. A similar case was that of furfural
(Figure S2); at 1708C in the acidic medium, the dehydration of
In the methanol-rich medium with prolonged residence
time, instead of conversion back to xylose, the methyl xylo-
sides continued to degrade to some intermediates and eventu-
ally to DOF and furfural (Scheme 2). It is believed that the
methanol also helped to stabilize the intermediates from the
Scheme 3. The formation of disaccharides in water and methyl xylosides in methanol.
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Acid-Catalyzed Conversion of Xylose
degradation of methyl xylosides, which might also contribute
to the suppression of humins formation. Furfural is another
precursor of humins. In the methanol-rich medium, some of
the carbonyl groups in furfural were protected with methanol
by acetalization, which may affect the tendency of furfural to
polymerize. To further investigate the effects of this protection,
the reactions of furfural with pure methanol and water were
performed, respectively.
Reaction pathways of furfural in aqueous and methanolic
media
The reactions of furfural with water and methanol were per-
formed at 1708C as the humins formation from furfural is ex-
pected to be significant at this temperature. Furfural conver-
sion and the product distribution are presented in Figure 5.
Most furfural was converted at the end (reaction time:
180 min) in both water and methanol (Figure 5a). One differ-
ence is that DOF was produced as the main intermediate in
methanol (Figure 5b). Although some other products were
formed, their concentrations were small or at trace levels. The
main product was humins, the weight of which with respect to
the loaded furfural was 55.67% in water and 50.55% in metha-
nol. The slight difference in humins formation indicates that
the conversion of furfural to DOF (and protection of the car-
bonyl group) did not effectively suppress polymerization at
1708C. There is a reaction equilibrium between furfural and
DOF in methanol, and the polymerization of furfural and the
acetalization of furfural to DOF occurred in parallel. Although
most furfural was initially converted to DOF in methanol, the
polymerization of furfural consumed furfural, which would shift
the reaction equilibrium from DOF to furfural with prolonged
residence time and eventually consume most of the DOF
(Scheme 4). Thus, the long residence time in acidic medium
and the high reaction temperature still led to humins forma-
tion from furfural. With a short residence time, the conversion
of furfural into DOF probably helps to suppress humins forma-
tion from furfural as the formation of DOF decreases the con-
centration of furfural in the reaction medium.
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Figure 5. Acid treatment of furfural in water ( ) and in methanol ( ). Effect
of reaction time on a) furfural conversion and yields of b) DOF, c) formic
acid, d) levulinic acid, e) methyl levulinic acid, and f) dimethyl succinate. Fur-
fural initial concentration: 5.58 wt%; T=1708C; catalyst dosage: 5 wt%; stir-
ring rate: 600 rpm.
ulinic acid (Figure 5d), a platform molecule from the hydrolysis
of C6 carbohydrates,[27] from furfural was unexpected. The hy-
drogenation of furfural to 2-furylmethanol followed by hydroly-
sis or methanolysis produces levulinic acid or its ester.[28–30]
However, 2-furylmethanol was not detected, which was proba-
bly because formic acid, the potential hydrogen donor,[31–33]
reduced furfural to some intermediates and then to levulinic
acid in the acidic environment. In methanol, instead of levulin-
ic acid, methyl levulinate was produced (Figure 5e), which was
the product from levulinic acid and methanol. In addition,
In addition to humins formation, some interesting minor
products were also detected. In water, formic acid was detect-
ed (Figure 5c), which is known as a product in the degradation
or polymerization of furfural.[26] However, the formation of lev-
Scheme 4. The reaction pathways of furfural in water and in methanol.
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methyl levulinate was also detected in the conversion of
xylose at 1708C in the methanol-rich medium. In addition to
methyl levulinate, methyl formate was also detected in trace
amounts (not shown).
To measure the effect of formic acid on the formation of lev-
ulinic acid from furfural, 4.5 wt% of formic acid was added to
the water medium to catalyze the conversion of furfural (the
other conditions were the same). The yield of levulinic acid is
presented in Figure 5d. The formic acid added was not mark-
edly consumed, but its presence facilitated the production of
levulinic acid in low concentration. The polymerization of furfu-
ral and the formation of levulinic acid occurred in parallel. The
domination of the polymerization of furfural suppressed the
conversion of furfural to levulinic acid. Therefore, a highly
active hydrogenation catalyst may be required to facilitate the
decomposition of formic acid to hydrogen to promote the pro-
duction of levulinic acid from furfural. In addition to these
products, dimethyl succinate was also present in the conver-
sion of furfural in methanol and in the conversion of xylose in
methanol-rich medium at 1708C (Figure 5 f). Dimethyl succi-
nate should be the product of the reaction between methanol
and succinic acid, but succinic acid was not detected in the
conversion of furfural in water. Similarly, the high tendency of
furfural to polymerize suppressed the formation of dimethyl
succinate.
Effects of catalyst dosage on product distribution and
humins formation
The amount of catalyst may influence the reaction rates of hy-
drolysis, methanolysis, and polymerization. Hence, the catalyst
dosage was expected to significantly affect the product distri-
butions and the humins formation. The effect of catalyst
dosage on xylose conversion in the methanol-rich medium
was investigated at 1508C with catalyst dosages ranging from
2 to 8 wt% (Figure 6). Higher catalyst dosages promoted the
conversion of xylose, but the effects were relatively small (Fig-
ure 6a). However, the degradation of methyl xylosides was
greatly affected (Figures 5b and 6c). The degradation of
methyl xylosides involved the protonation of three hydroxyl
groups and their subsequent dehydration. Hence, methyl xylo-
side degradation was sensitive to catalyst dosage and favored
by high catalyst dosages. Consequently, more DOF and furfural
were produced with the high catalyst dosage of 8 wt% (Fig-
ures 6d and 6e). The humins formation presented in Figure 6 f
showed that the higher the catalyst dosage used, the more
humins were formed. With a low catalyst dosage (2 wt%), the
degradation of methyl xylosides was much slower. Conse-
quently, the concentrations of methyl xylosides were high and
those of furfural and DOF were low, which were the main rea-
sons for the reduced humins formation with this low catalyst
loading. Of course, prolonged residence time might also in-
crease humins formation with a catalyst dosage of 2 wt%.
However, the above results support that the conversion of
xylose to methyl xylosides lowered the reactivity of xylose to
polymerization and consequently suppressed humins
formation.
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Figure 6. Effect of catalyst dosage [2 ( ), 5 ( ), and 8 wt% ( )] on a) xylose
*
conversion, yields of b) MAXP, c) MBXP, d) DOF, and e) furfural as well as the
weight ratio of humins to converted xylose (f). T=1508C; methanol/water
ratio: 4.5; xylose loading: 5.58 wt%; stirring rate: 600 rpm.
Characterizations of humins with FTIR and UV fluorescence
spectroscopy
The functional groups of the humins were characterized by
using FTIR and UV fluorescence spectroscopy. The FTIR spec-
troscopic results (Figure S3 and Table 1) showed that the
humins contain a wide range of functional groups including
hydroxyl groups, carbonyl groups, carbon–carbon double
bonds, aliphatic CꢀH bonds, and aromatic rings. UV fluores-
cence spectroscopy showed a strong excitation from l=330–
500 nm (Figure S4), which indicated that the humins contain
large conjugated p-bonded systems.[34] Thus, humins is a poly-
meric material with various functional groups.
Conclusions
Humins formation in the acid treatment of xylose was investi-
gated by variation of the reaction parameters. A high reaction
temperature, long residence time, low methanol/water mass
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Acid-Catalyzed Conversion of Xylose
tants was typically approximately 390 mL. The initial concentration
of xylose was 5.58 wt% (ca. 18.98 g) for most of the experiments,
which is specified in the figure legends. The autoclave was purged
with nitrogen three times after the introduction of reactants and
then heated to the desired temperature at 68Cminꢀ1 with a stirring
rate of 600 rpm. The selection of the stirring rate of 600 rpm was
based on the observation that no mass transfer limitations were
found with stirring rates above 300 rpm in preliminary experi-
ments. A sample was taken immediately after reaching the reaction
temperature, and further samples were taken at 20 min intervals.
The holding time at the reaction temperature was 180 min for all
experiments. The initial pressure in autoclave was approximately
1 bar before heating, and the final pressure depended on the reac-
tion temperature and the reaction medium. The humins formed as
isolated particles, deposited on the catalyst, or adhered to the re-
actor wall were collected after the reactor cooled and dried in
a vacuum oven at 1008C for 4 h to constant weight to determine
the amount of the humins formed.
Table 1. Assignments of the FTIR peaks of the humins.
n˜ [cmꢀ1
]
Assignment
3660–3590
3040–3000
2990–2800
2700–2500
1820–1650
1650–1500
1420–1410
1250–1000
900–690
OꢀH stretch: alcohols, phenols
C=CꢀH stretch: aromatics, unsaturated bonds
CꢀH stretch: aliphatics
OH stretch
C=O stretch: carbonyls
C=C stretch: substituted aromatics
CH2 deformation: unsaturated bonds
CꢀO stretch, OꢀH deformation: alcohols, ethers
CꢀH out of plane deformation: substituted aromatics
ratio, and high catalyst dosage were favorable for humins for-
mation. Both xylose and furfural contributed to polymerization
in water. In the methanol-rich medium, xylose was converted
to methyl xylosides, which stabilized xylose and suppressed
the formation of sugar oligomers and polymerization reactions.
Although furfural can be converted into 2-(dimethoxymethyl)-
furan (DOF) in the methanol-rich medium, this did not remark-
ably suppress polymerization at 1708C because of the shift of
the reaction equilibrium from furfural to DOF with prolonged
residence time. These results are helpful to understand the re-
action pathways of xylose in the esterification of bio-oil and to
select the appropriate reaction conditions to produce platform
molecules and avoid the occurrence of side reactions. The acid
treatment of furfural also produced methyl levulinate in meth-
anol and levulinic acid in water, which was found to be cata-
lyzed by the degradation product of furfural, formic acid. More
attention may need to be paid to this reaction pathway as
methyl levulinate and levulinic acid are platform molecules for
diverse chemicals.
Analytical methods
Samples were analyzed by using
a Hewlett–Packard GC–MS
(HP6890 series GC with an HP5973 MS detector) with a capillary
column (HP-INNOWax, length=30 m, internal diameter=0.25 mm,
film thickness=0.25 mm). Standard solutions covering the concen-
tration range of the samples were used to obtain calibration
curves to calculate the concentrations of the compounds of inter-
est. The sample (1 mL) was injected into the injection port set at
2508C with a split ratio of 50:1. The column was operated in a con-
stant flow mode using 3.0 mLminꢀ1 of helium as the carrier gas.
The column temperature was initially maintained at 408C for 3 min
before increasing to 2608C at a heating rate of 158C minꢀ1. The
identification of each compound was achieved by matching its
mass spectrum with that in the spectral library and was confirmed
by injecting the standard where available. A derivatization method
was used to determine xylose, broadly following the procedure in
the literature.[35] A typical chromatograph after derivatization of the
products is shown in Figure S5. FTIR spectra of the humins were
recorded by using a Perkin–Elmer Spectrum GX FTIR/Raman Spec-
trometer with a spectral resolution of 4 cmꢀ1 at room temperature.
The spectrum represents the average of at least six scans. The
weight ratio of the humin-type polymer to KBr was 0.5 wt%. The
UV fluorescence spectra of the humins were recorded by using
a Perkin–Elmer LS50B spectrometer. The synchronous spectra were
recorded with a constant energy difference of ꢀ2800 cmꢀ1. The slit
Experimental Section
Materials
All chemicals used in this study were of analytical grade and used
without further purification. 2-(Dimethoxymethyl)furan (DOF) was
purchased from LC Scientific Inc. (Canada). Methyl-a-d-xylopyrano-
side (MAXP) and methyl-b-d-xylopyranoside (MBXP) were pur-
chased from Carbosynth Limited (UK). Xylose, furfural, methyl levu-
linate, levulinic acid, formic acid, and dimethyl succinate were pur-
chased from Sigma Aldrich. Methanol was obtained from Merck
Australia. Amberlyst 70 (Rohm & Haas), a commercial solid acid cat-
alyst with a maximum operating temperature of 1908C and con-
centration of acidic sites of ꢁ2.55 eqkgꢀ1, was used without fur-
ther pretreatment. The stability of Amberlyst 70 and the leaching
of the ꢀSO3H group were tested, and the results showed that the
decomposition of the catalyst and the leaching of the ꢀSO3H
group were insignificant under the reaction conditions used in this
study.
widths were 2.5 nm and the scan speed was 200 nmminꢀ1
.
The definitions of xylose conversion and product yields were as fol-
lows:
1ꢀmol of xylose in product
ð1Þ
ð2Þ
Conv: ðmol %Þ ¼
Yield ðmol %Þ ¼
ꢂ 100 %
ꢂ 100 %
mol of xylose loaded in reactor
mol of product produced
mol of xylose loaded in reactor
Acknowledgements
Experimental procedures
We gratefully acknowledge the financial support of the Austra-
lian Government through the Second Generation Biofuels Re-
search and Development Grant Program and the International
Science Linkages (ISL) Program.
The experiments were performed in a stainless steel, high-pressure
batch reactor (Parr 4572, Parr Instrument Co.). In each experiment,
given amounts of xylose, methanol, water, and Amberlyst 70 were
mixed and introduced into the reactor. The volume of the reac-
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Received: November 19, 2011
Published online on && &&, 0000
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ChemSusChem 0000, 00, 1 – 9
ÝÝ
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FULL PAPERS
Methanolysis makes the difference:
In a methanol/water medium, the con-
version of xylose to methyl xylosides
protects the C1 hydroxyl group of
xylose, which stabilizes xylose and sup-
presses the formation of sugar oligo-
mers and polymerization reactions.
X. Hu, C. Lievens, C.-Z. Li*
&& – &&
Acid-Catalyzed Conversion of Xylose
in Methanol-Rich Medium as Part of
Biorefinery
ChemSusChem 0000, 00, 1 – 9
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
&
9
&
ÞÞ
These are not the final page numbers!