Beta zeolite: a universally applicable catalyst for the conversion of various types of saccharides into furfurals

Article abstract of DOI:10.1039/c5cy00719d

Carbohydrates, which consist of various types of saccharides, have been regarded as the potential alternative to fossil resources. However, the conversion of carbohydrates to chemical products generally requires energy- and time-consuming multiple processes. The present study shows that beta zeolites with both Lewis and Bronsted acid sites, prepared by facile calcination of the ammonium form of zeolite, promoted the direct conversion of various types of saccharides into furfurals. Beta zeolites with a large number of Lewis acid sites effectively promoted the direct conversion of glucose, other hexoses and even cellulose into 5-hydroxymethylfurfural (HMF). In the direct conversion of cellulose, the developed reaction system gave 42% yield of HMF; this value is significantly high for a heterogeneous reaction system. On the other hand, a beta zeolite with a small number of Lewis acid sites showed a better performance in the conversion of xylose into furfural than the one with a large number of Lewis acid sites. Suitable acid properties for the conversion of glucose and xylose are discussed in terms of reactivity of each substrate toward isomerization.

Full text of DOI:10.1039/c5cy00719d

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Beta zeolite: a universally applicable catalyst for
the conversion of various types of saccharides
into furfurals†
Cite this: DOI: 10.1039/c5cy00719d
R. Otomo, T. Tatsumi and T. Yokoi*
Carbohydrates, which consist of various types of saccharides, have been regarded as the potential
alternative to fossil resources. However, the conversion of carbohydrates to chemical products generally
requires energy- and time-consuming multiple processes. The present study shows that beta zeolites with
both Lewis and Brønsted acid sites, prepared by facile calcination of the ammonium form of zeolite, pro-
moted the direct conversion of various types of saccharides into furfurals. Beta zeolites with a large num-
ber of Lewis acid sites effectively promoted the direct conversion of glucose, other hexoses and even cel-
lulose into 5-hydroxymethylfurfural (HMF). In the direct conversion of cellulose, the developed reaction
system gave 42% yield of HMF; this value is significantly high for a heterogeneous reaction system. On the
other hand, a beta zeolite with a small number of Lewis acid sites showed a better performance in the
conversion of xylose into furfural than the one with a large number of Lewis acid sites. Suitable acid prop-
erties for the conversion of glucose and xylose are discussed in terms of reactivity of each substrate toward
isomerization.
Received 19th May 2015,
Accepted 25th June 2015
DOI: 10.1039/c5cy00719d
www.rsc.org/catalysis
reaction route that consists of the isomerization of glucose to
fructose and the subsequent dehydration of fructose to HMF
Introduction
Biomass resources including carbohydrates are potential
alternatives to fossil resources for the production of
has been proposed. Each step requires different types of
active species; for example, the isomerization step requires a
base, a Lewis acid, and isomerase. The dehydration step gen-
1–4
chemicals and fuels.
Carbohydrates mainly exist in nature
11–13
in the form of polysaccharides such as cellulose and hemicel-
lulose, from which multi-step reactions are currently required
for the production of useful materials. Complex mixtures pro-
duced through the hydrolytic decomposition of carbohydrates
consist of oligo-, di-, and monosaccharides including e.g.,
cello-oligosaccharides, cellobiose, glucose, galactose, man-
erally requires a Brønsted acid.
Recently, we found that the number of Lewis acid sites on
the aluminosilicate beta zeolite was increased by calcination
or steam treatment at high temperatures and thus
dealuminated beta zeolites were found to be effective bifunc-
tional catalysts for the conversion of glucose into HMF; Lewis
acid sites in the beta zeolites promoted the isomerization of
glucose, which is the rate-determining step, through a
hydride transfer mechanism and Brønsted acid sites
5
,6
nose, xylose, and arabinose. A multi-step process is less fea-
sible from a practical viewpoint and it is favorable to convert
all these saccharides together without purification.
1
4
In the past decade, considerable efforts have been devoted
in the conversion of carbohydrates into furfurals such as fur-
fural and 5-hydroxymethylfurfural (HMF), which are the
promptly promoted the dehydration of fructose to HMF.
Herein, we report that dealuminated beta zeolites are univer-
sally applicable catalysts for the conversion of various types
of saccharides into their corresponding furfurals.
7
–10
potential platform chemicals in biomass refinery.
Furfural
and HMF can be synthesized typically from xylose and glu-
cose, respectively, by the elimination of three water mole- Experimental
cules. For the conversion of glucose into HMF, a two-step
Materials
Except for cellulose and xylan, the saccharide substrates were
Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuta-cho
purchased from Kanto chemical. Cellulose powder was pur-
chased from Aldrich. These compounds were used without
any further purification. Xylan, made from beech wood and
purchased from Aldrich, was stirred at ambient temperature
4259, Midori-ku, Yokohama 226-8503, Japan. E-mail: yokoi@cat.res.titech.ac.jp
†
Electronic supplementary information (ESI) available: Data on structural
characterization of catalysts, changes in the cellulose particle before and after
reaction and discussion on improved reactivity of cellulose. See DOI: 10.1039/
c5cy00719d
−
1
in pure water (25 ml gxylan ) prior to use in the pretreatment
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step. Xylan was recovered by centrifugation, dried at 100 °C
and used as substrate.
obtained by collecting the reaction results at different time
points. Reaction runs were repeated several times, achieving
similar results with differences of ±3ꢀ. The reaction mixture
was filtered prior to the quantitative analysis to remove the
solid catalyst. The homogeneous solution after the filtration
was separated into the aqueous and organic phases by
adding an excess amount (~3 g) of NaCl as a salting-out
agent. The sugar substrates and water-soluble products in the
aqueous phase were analyzed by an HPLC apparatus
Preparation and characterization of catalysts
Aluminosilicate beta zeolite was synthesized by the fluoride
1
4,15
method.
Aluminum isopropoxide (Kanto chemical) was
added to a tetraethylammonium hydroxide (TEAOH) solution
(
(
Alfa Aesar) with stirring. Then, tetraethyl orthosilicate
TEOS, from TCI) was added and the mixture was stirred
(
Shimadzu, LC-20A series) equipped with an REZEX RHM-
until the evaporation of ethanol was complete. Finally, a
hydrofluoric acid solution (Wako chemical) was added. The
molar composition of the gel was 1.0 TEOS : 0.5 TEAOH : 0.067
Monosaccharide column (Phenomenex) in 5 mM H SO solu-
2
4
tion as eluent with RI and UV (280 nm) detectors. Since man-
nose, an isomer of glucose, cannot be separated from fruc-
tose under these analysis conditions, representative reaction
mixtures were analyzed using another REZEX RCM-
Monosaccharide (Phenomenex) column in pure water as elu-
ent, finding that the yields of mannose were much lower
than those of fructose. Therefore, the total yield of these two
isomers is shown as the yield of fructose. For the same rea-
son, the total yield of xylulose and lyxose, isomers of xylose,
was shown as the yield of xylulose. Furfural, HMF and other
products in the organic phase were analyzed by another
HPLC apparatus (Shimadzu LC-10A) equipped with a Gemini
C18 column (Phenomenex) with a UV detector (260 nm) in a
i
AlIJO Pr) : 8 H O : 0.5 HF. The as-obtained thick gel was trans-
3
2
ferred to a Teflon-lined stainless steel autoclave and crystal-
lized at 140 °C for 5 days with tumbling at 40 rpm. The solid
product was recovered by filtration, washed with distilled
water and dried overnight at 100 °C, followed by calcination
at 580 °C. The calcined sample was stirred in 1.0 M ammo-
nium nitrate solution at 80 °C for ion-exchange. The
ammonium-form sample was calcined at 500 and 750 °C in
a muffle furnace, which are designated as Beta-Cal500 and
Cal700, respectively.
Powder X-ray diffraction (XRD) patterns of the prepared
samples were collected on a Rigaku Ultima III diffractometer
using a Cu Kα radiation (40 kV, 20 mA). Elemental analyses
were performed on an inductively coupled plasma-atomic
emission spectrometer (ICP-AES, Shimadzu ICPE-9000). Mor-
phology of the cellulose particle was observed by scanning
electron microscopy using a Hitachi S-5200 microscope. The
8
0/20 H O : MeOH solution as eluent. Calibration curves of
2
saccharide substrates and products were used for their quan-
tification. When cellulose, starch and xylan were used as sub-
strate, the conversion was calculated by differences in weight
of solids before and after reaction runs. For cellulose, the
amount dissolved and/or consumed at the zero time point at
2
7
solid-state Al MAS NMR spectra of hydrated samples were
recorded on a JEOL ECA-600 spectrometer at a resonance fre-
quency of 156.4 MHz using a 4 mm sample rotor with a spin-
ning rate of 15.0 kHz. The chemical shifts were referenced
relative to an aluminum nitrate aqueous solution.
200 °C was below 2ꢀ and was neglected.
Results and discussion
Synthesis and characterization of catalysts
The number and type of acid sites on the catalysts were
measured by the IR technique using pyridine as the probe
molecule. The IR spectra were obtained by a Jasco FT-IR 4100
spectrometer equipped with an MCT detector. Sample pow-
der (50 mg) was pelletized to a self-supporting disk 1 cm in
During the calcination of the ammonium form of zeolite,
dealumination, which is the partial hydrolysis of Si–O–Al
bonds, occurred to form hexa-coordinated Al species, as
shown in the NMR peaks at 0 ppm (Fig. S1†). Beta-Cal750
went through more severe dealumination than Beta-Cal500,
as is seen in the further decrease in the peak of the tetrahe-
dral Al species at 57 ppm. Although the local structure of the
Al atoms were changed by the calcination, the framework
structures were retained (Fig S2†). The number of Brønsted
and Lewis acid sites on Beta-Cal500 were 0.25 and 0.13
diameter, which was held in
a glass cell. After the
pretreatment at 450 °C, the sample was cooled to 150 °C and
an excess amount of pyridine (~1 kPa) was introduced into
the cell. The gas phase was evacuated at 250 °C for 10 min
and a spectrum was recorded at 150 °C. For quantitative cal-
culation of adsorbed pyridine, the reported molar extinction
−
1
mmol g , respectively (Table 1). For Beta-Cal750, the more
−
1
coefficients of the absorption bands at 1455 and 1545 cm
for Lewis and Brønsted acid sites were used, respectively.
1
6
Table 1 Textural and acid properties of beta zeolite catalysts further
Catalytic tests
Acid sites
(mmol g )
Catalytic reactions were performed in mixed solvents (water/
dimethyl sulfoxide/tetrahydrofuran) by using a Teflon-lined
stainless steel autoclave (50 ml). Zero time was taken at the
point when the temperature of the mixture reached the set
value. At a time point, the reaction was quenched by cooling
the autoclave in an ice bath. Time course profiles were
−1
a
b
S
BET
V
micro
2
−1
−1
Catalyst
Si/Al
(m g
)
(ml g
)
Brønsted
Lewis
Beta-Cal500
Beta-Cal750
15
15
632
618
0.24
0.23
0.25
0.17
0.13
0.18
a
b
BET specific surface area. Micropore volume.
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severe dealumination led to a smaller number of Brønsted
acid sites and a larger number of Lewis acid sites (0.17 and
The developed beta zeolites were also capable of promot-
ing the conversion of the hemicellulose fraction. Fig. 1 shows
the reaction profiles in the conversion of xylose into furfural
over Beta-Cal500 and Cal750 at 180 °C. The reaction
proceeded through a successive reaction mechanism in
which xylose is isomerized to xylulose, followed by the dehy-
dration of xylulose to furfural, being analogous to the mecha-
nism in the conversion of glucose into HMF. Beta-Cal750
showed a higher conversion of xylose than Beta-Cal500 due
to its larger number of Lewis acid sites. Beta-Cal750 showed
57ꢀ yield of furfural at 3 h, and it was not significantly
changed until the conversion reached ~100ꢀ for 6 h, while
Beta-Cal500 showed a constant yield of 72ꢀ after 4 h. The
conversion of xylose into furfural has been extensively stud-
ied using various types of heterogeneous catalysts including
−
1
0
.18 mmol g , respectively).
Conversion of mono- and di-saccharides
14
As we have reported, Beta-Cal750 showed superior catalytic
performance than Beta-Cal500 in the conversion of glucose
because Beta-Cal750 had a larger number of Lewis acid sites
enough to promote the isomerization of glucose, which was
the rate-determining step (Table 2, entries 1 and 2). Hence,
studies on the conversion of other hexoses were conducted
using Beta-Cal750 under the same reaction conditions as
those for the conversion of glucose. Beta-Cal750 showed 80ꢀ
conversion with 52 and 55ꢀ selectivities to HMF in the con-
version of galactose and mannose, respectively (Table 2,
entries 3 and 4), indicating that the isomerization reaction
over Lewis acid sites on the beta zeolite is not specific to the
glucose molecule, but universally applicable for various hex-
oses. Fructose was dehydrated to HMF in a higher selectivity
2
1–34
35
36,37
zeolites,
oxides,
metal halide, metal oxides,
sulfated metal
and mesoporous
3
8–40
41,42
supported heteropolyacids,
4
3
silica containing sulfo groups in the batch and continuous-
flow reactor systems. Among the reports on the batch reactor
system, our developed beta zeolites showed a fairly high fur-
fural yield.
(
66ꢀ) than that in the conversion of glucose, as is often
1
1,17–19
observed in other studies (Table 2, entry 5).
This is
The different selectivities between the two types of beta
zeolites were probably due to the different numbers of Lewis
probably because glucose was partially consumed by side
reactions including the etherification to anhydroglucose (4ꢀ
selectivity). Furfural was found in 4–7ꢀ selectivity in all the
reaction runs above, which would be formed through a side
2
0
reaction of fructose with loss of the C-1 atom.
When cellobiose was used with the number of glucose
unit constant, cellobiose was totally consumed after 3 h of
the reaction and the apparent conversion of glucose was 77ꢀ
with 52ꢀ selectivity to HMF (Table 2, entry 6). Notably, simi-
lar results were achieved in the conversion of glucose and cel-
lobiose, implying that the hydrolysis of cellobiose to glucose
proceeded in a high selectivity and is faster than the isomeri-
zation of glucose. Sucrose, which consists of glucose and
fructose, was more efficiently converted into HMF. This is
because sucrose readily hydrolyzed to glucose and fructose,
and the reaction of sucrose can be regarded as a reaction of the
mixture of glucose and fructose. After 3 h, the apparent conver-
sions of the glucose and fructose units were 76 and 94ꢀ,
respectively, with 62ꢀ selectivity to HMF (Table 2, entry 7).
Table 2 Conversion of hexose mono- and disaccharides into HMF over
a
beta zeolites
b
c
Entry Catalyst
Substrate
Time (h) Conv. (ꢀ) Selec. (ꢀ)
1
2
3
4
5
6
7
Beta-Cal500 Glucose
Beta-Cal750 Glucose
Galactose
3
3
3
3
1
3
3
57
78
80
80
97
77
76, 94
36
55
52
55
66
52
62
Mannose
Fructose
Cellobiose
Sucrose
d
a
Fig. 1 Conversion of xylose into furfural over (a) Beta-Cal500 and (b)
Beta-Cal750. (●) Conversion of xylose, (■) xylulose yield, (▲) furfural
yield, and (◆) unknown products yield. Reaction conditions: catalyst,
0.1 g; xylose, 0.67 mmol; water, 4.5 ml; DMSO, 0.5 ml; THF, 15 ml;
temperature, 180 °C.
Reaction conditions: catalyst, 0.1 g; substrate, 0.67 mmol based on
monomer unit; water, 4.5 ml; DMSO, 0.5 ml; THF, 15 ml;
temperature, 180 °C. Conversion of substrate. For disaccharides,
b
c
conversion is based on the monomer unit. Selectivity to HMF.
d
Apparent conversion of glucose and fructose units, respectively.
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acid sites. In the previous work, we found that Brønsted acid
sites promoted the dehydration of fructose to HMF, but Lewis
acid sites inevitably caused side reactions of fructose into
change in the color of the reaction mixture; the reaction mix-
ture was originally colored white but turned clear yellow or
pale brown after the reaction, indicating that the starch
immiscible under ambient conditions were completely
decomposed into soluble products. Glucose and HMF were
found in 19 and 39ꢀ yields, respectively. Upon increasing
the reaction temperature to 200 °C, a similar result was
achieved for 1 h of the reaction. At 2 h of the reaction at 200
°C, the HMF yield was increased to 44ꢀ, which was slightly
lower than those obtained from glucose and cellobiose (ca.
50 and 49ꢀ respectively). Obviously, the selectivity in the
conversion of glucose into HMF determined the overall yield
of HMF starting from starch.
1
4
undetectable products. Similarly, a large number of Lewis
acid sites in Beta-Cal750 could promote side reactions of
xylulose, as is seen in the higher selectivity to unknown prod-
2
3,31,34
ucts including polymerized products (Fig. 1b).
In this
way, Beta-Cal500 with the small number of Lewis acid sites
showed a high yield of furfural. Similar behavior has been
reported by Huber et al. in the combination of the Lewis acid
4
4
metal triflate and hydrochloric acid. However, the opposite
tendency was seen in the conversion of glucose into HMF
(Table 2, entries 1 and 2), implying that the suitable acid
properties vary depending on the reactant. Xylose is much
more reactive than glucose and a relatively small number of
Lewis acid sites is enough to promote the isomerization of
xylose. Rather, an excess number of Lewis acid sites would
lead to a decrease in the selectivity to furfural. On the other
hand, glucose, which is less reactive, requires a large number
of Lewis acid sites; otherwise, the isomerization would not
proceed and consequently HMF would not be obtained.
It was also found that Beta-Cal500 can promote the con-
version of arabinose into furfural, although the conversion
was slightly lower than that of xylose (Table 3). Xylobiose, a
dimer of xylose, was successfully converted into furfural in
Cellulose is a crystalline glucan composed of 1,4-β-glyco-
sidic bonds, whose rigid structure makes it difficult to hydro-
lyze. Hence, cellulose has been converted into chemical prod-
ucts via multi-step reactions (Scheme 1). Prior to the
hydrolysis, cellulose is transformed to amorphous glucan by
4
5–47
some types of pretreatment such as ball-milling.
Then,
amorphous glucan is hydrolyzed into cello-oligosaccharides
and glucose in the presence of acid catalysts. Finally, glucose
is converted into the desired compounds through various
types of chemical reactions. Since such a multi-step reaction
is energy- and time-consuming, the direct conversion of cellu-
lose into chemical products has attracted much attention.
Hence, we have tackled the direct conversion of cellulose
using the zeolite catalyst.
Prior to the catalytic reactions, as a control, the reactivity
of cellulose against hydrolysis was examined in the absence
of a catalyst (Fig. 2). For 5 h of the reaction at 180 °C, glucose
was obtained in 16ꢀ yield at 44ꢀ conversion of cellulose. A
trace amount of cellobiose was also detected (cello-oligosac-
charides larger than cellobiose are not detected in our analy-
sis). Obviously, cellulose is less reactive than starch due to its
robust crystalline structure. However, the reactivity was
remarkably improved by increasing the reaction temperature
to 200 °C but the yield of HMF was still quite low (ca. 2ꢀ);
the conversion of cellulose was increased to 65ꢀ at only 1 h
with the glucose yield of 27ꢀ.
6
4ꢀ yield. Thus, Beta-Cal500 has the capability of effectively
promoting the conversion of pentose carbohydrates into
furfural.
Conversion of polysaccharides
It was found that various types of hexoses and disaccharides
were directly converted into HMF with almost similar perfor-
mance, stimulating us to study the conversion of polysaccha-
rides (Fig. 2). Cornstarch, which is a representative amor-
phous glucan, was employed as a substrate. After 5 h of the
reaction at 180 °C, the conversion of the starch was almost
1
00ꢀ. This complete conversion was also supported by the
When Beta-Cal750 was added to the reaction mixture at
00 °C, the yield of HMF was increased from 2 to 17ꢀ with
Table 3 Conversion of pentose mono- and disaccharides into furfural
over Beta-Cal500a
2
the decrease in the yield of glucose from 27 to 20ꢀ at 1 h of
the reaction. Although the conversion of cellulose was not
significantly changed by the addition of the catalyst, the yield
of HMF was remarkably increased. This behavior clearly dem-
onstrated that the hydrolysis of cellulose was successfully
combined with the subsequent conversion into HMF over
Beta-Cal750. In the absence of the catalyst, the total yield of
glucose, fructose, and HMF was lower than that in the pres-
ence of the catalyst, indicating that glucose and fructose were
consumed by side reactions without a catalyst added. When
the reaction time was extended to 5 h, the yield of HMF was
increased up to 42ꢀ, which was similar to the yield (ca. 44ꢀ)
obtained in the conversion of the starch.
b
c
Entry
Substrate
Xylose
Conditions
Conv. (ꢀ)
Selec. (ꢀ)
1
2
3
4
5
6
180 °C, 3 h
180 °C, 5 h
180 °C, 3 h
180 °C, 5 h
180 °C, 3 h
200 °C, 1 h
200 °C, 2 h
200 °C, 2 h
88
97
77
89
88
92
71
74
70
66
73
73
4
Arabinose
Xylobiose
Xylose
Xylan
d
e
7
8
88
f
e
90
68
a
Reaction conditions: catalyst, 0.1 g; substrate, 0.67 mmol based on
monomer unit; water, 4.5 ml; DMSO, 0.5 ml; THF, 15 ml;
b
temperature, 180 °C.
Conversionc of substrate. For xylobiose,
d
conversion is based on xylose unit. Selectivity to furfural. Xylan
e
without pretreatment. Conversion of xylan was determined by
the weight of recovered solids. Xylan pretreated (see the
f
Note that microcrystalline cellulose can be hydrolyzed into
smaller saccharides by neither any particular pretreatment
Experimental section).
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Fig. 2 Conversion of polysaccharides into HMF under different reaction conditions. Reaction conditions: Beta-Cal750, 0.1 g; substrate, 0.67 mmol
based on monomer unit; water, 4.5 ml; DMSO, 0.5 ml; THF, 15 ml.
4
9
nor any catalyst added. The morphology of the crystalline cel-
lulose before and after 5 h of the reaction at 180 °C was
observed by the FE-SEM technique. Interestingly, it was
found that the mean particle size of cellulose is somehow
containing LiCl showed 33ꢀ yield of HMF. Ionic liquids
have been extensively used as solvent for the conversion of
polysaccharides including cellulose and excellent results have
been reported; for example, Chen et al. reported 89ꢀ yield of
1
3
decreased after the reaction (Fig. 3a and b). XRD and C CP/
MAS NMR analysis revealed that the recovered cellulose was
as highly crystalline as that before the reaction (Fig. S3 and
S4†). We tentatively assume that the reactivity of cellulose
would be significantly improved by the reduction in the parti-
cle size (the details are described in the ESI†). The SEM
image for the mixture of the cellulose and the catalyst parti-
cles recovered after the reaction showed that the beta catalyst
particles with truncated bipyramidal shape ~1 μm in size
were on the large cellulose particle (Fig. 3c), supporting the
assumption that the decomposition of cellulose into soluble
products was independent of zeolite catalysts.
2
HMF from cellulose with CrCl catalyst in the imidazolium
5
0
ionic liquid at 120 °C for 6 h. Although ionic liquids
achieved excellent results, they are very expensive, unstable
under hydrothermal conditions, and difficult to reuse. There-
fore, heterogeneous catalysts were also employed but ordinar-
ily exhibited poor performance compared to the homoge-
showed 13ꢀ yield of HMF in water at 250
°C. In ionic liquid, the polymer-based sulfonic acid catalyst
achieved ~30ꢀ yield of HMF. Bokade et al. achieved 46ꢀ
yield of HMF directly from cellulose by using H-ZSM-5 modi-
neous ones; TiO
2
5
1
5
2
5
3
fied by post-synthetic desilication. Beta-Cal750 gave 42ꢀ
yield of HMF; this value is significantly high for a
To date, various types of catalysts have been applied to the
direct conversion of cellulose into HMF. Antal et al. reported
the hydrolysis of cellulose with sulfuric acid which yielded
4
8
5
ꢀ of HMF. Raines et al. reported that the combined cata-
lysts of CrCl3 and HCl in dimethylacetamide solvent
Fig.
3 SEM images of the cellulose (a) before the reaction, (b)
Scheme 1 Multi-step and direct pathways for conversion of cellulose
into HMF.
recovered after the reaction without a catalyst and (c) recovered after
the reaction with Beta-Cal750.
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heterogeneous reaction system and is as high as the result of Notes and references
Bokade et al. Because the MFI-type structure has only 10-
membered ring pores in which a glucose molecule cannot
enter, ZSM-5 requires post-synthetic modification for making
mesopores on catalyst particles. Beta zeolite has 12-
membered ring pores that can accommodate a glucose mole-
cule and so special modifications are not necessary. Thus,
our developed beta zeolite has advantages over the conven-
tional catalysts in the points of its preparation and catalytic
performance.
Xylan, which is a hemicellulose mainly composed of a
xylose unit, was also applied in the reaction system at 200 °C.
Unfortunately, a very poor yield of furfural was obtained
using xylan without any pretreatment, although a high yield
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(Table 3, entry 8). The hemicellulose structure of xylan could
not be decomposed under such mild conditions. It is known
that hemicellulose usually contains alkali cations such as
sodium and potassium because plants themselves accumu-
late alkali cations for their growth and because hemicellulose
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