Glucose reactions with acid and base catalysts in hot compressed water at 473 K

Article abstract of DOI:10.1016/j.carres.2005.06.017

The effects of the homogeneous catalysts (H₂SO₄ and NaOH) and heterogeneous catalysts (TiO₂ and ZrO₂) on glucose reactions were examined in hot compressed water (473 K) by a batch-type reactor. From the homogeneous catalyst studies, we confirmed that the acid catalyst promoted dehydration, while isomerization of glucose to fructose was catalyzed by alkali. Anatase TiO₂ was found to act as an acid catalyst to promote formation of 5-hydroxymethylfuraldehyde (HMF). Zirconia (ZrO₂) was a base catalyst to promote the isomerization of glucose. The effects of the additives were also confirmed through fructose reactions.

Full text of DOI:10.1016/j.carres.2005.06.017

Carbohydrate
RESEARCH
Carbohydrate Research 340 (2005) 1925–1930
Glucose reactions with acid and base catalysts in hot
compressed water at 473 K
Masaru Watanabe, Yuichi Aizawa, Toru Iida, Taku M. Aida, Caroline Levy,
Kiwamu Sue and Hiroshi Inomata*
Research Center of Supercritical Fluid Technology, Tohoku University, 6-6-11-403 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
Received 14 December 2004; accepted 10 June 2005
Available online 14 July 2005
Abstract—The effects of the homogeneous catalysts (H2SO4 and NaOH) and heterogeneous catalysts (TiO2 and ZrO2) on glucose
reactions were examined in hot compressed water (473 K) by a batch-type reactor. From the homogeneous catalyst studies, we con-
firmed that the acid catalyst promoted dehydration, while isomerization of glucose to fructose was catalyzed by alkali. Anatase TiO2
was found to act as an acid catalyst to promote formation of 5-hydroxymethylfuraldehyde (HMF). Zirconia (ZrO2) was a base cat-
alyst to promote the isomerization of glucose. The effects of the additives were also confirmed through fructose reactions.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Glucose; HMF; Fructose; TiO2; ZrO2; Acid; Alkali
1. Introduction
Some researchers have reported controlling reactions
of glucose in hot compressed water with or without a
catalyst.6–8 Goto et al.6 and Sasaki et al.7 studied glu-
cose reactions in SCW at various temperatures and pres-
sures without any catalyst using a flow apparatus. They
suggested that dehydration (C–O bond splitting) was
dominant at lower temperatures (523–623 K), while
the retro-aldol condensation (namely C–C bond break-
ing) mainly occurred at higher temperatures (673–
773 K). Above the critical temperature, dehydration of
glucose was promoted with increasing pressure (water
density). As summarized recently, at high temperature
and low water density, radical reactions are likely to
take place; on the other hand, ionic reactions mainly
occur at low temperature and high water density.9 One
way to control glucose reactions is to change the temper-
ature and pressure (water density). In particular, for
ionic reactions, water density is the most important
controlling factor in the absence of a catalyst. However,
water density of the saturated liquid phase cannot be
changed drastically with pressure below the critical
point compared to the supercritical region. Thus, in
sub-critical conditions of water, additives must be used
to control ionic reactions.
Glucose, which is the monomer unit of cellulose, will be
one of the most important starting chemicals in the com-
ing society that is highly dependent on biomass. To de-
velop a sustainable society using biomass resources, a
highly selective glucose conversion process is strongly
needed. Hot-compressed water, including hydrothermal
and supercritical water (SCW) conditions, is quite a
powerful reaction media for glucose conversion. Kabye-
mela et al.1–4 proposed a number of primary reactions of
glucose in SCW. Sinag et al.5 suggested a simple but use-
ful reaction pathway for glucose in SCW. According to
these reaction mechanisms,1–5 the main reaction path-
ways of glucose conversion in SCW were dehydration
(namely C–O bond breaking) and C–C bond-breaking
reactions (such as retro-aldol condensation). Unsatu-
rated compounds such as furans and phenols are pro-
duced through dehydration and low molecular acids,
aldehydes, and gaseous form via bond-breaking
reactions.
*
Corresponding author. Fax: +81 22 795 7282; e-mail:
inomata@scf.che.tohoku.ac.jp
0008-6215/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2005.06.017

1926
M. Watanabe et al. / Carbohydrate Research 340 (2005) 1925–1930
Among the dehydration products of glucose, 5-
and used without pretreatment. Zirconia (ZrO2) was pre-
pared by calcination of zirconium hydroxide, which was
obtained from Nakalai Tesque, at 673 K for 3 h. The
ZrO2 was confirmed as a tetragonal–monoclinic mixture
phase from XRD analysis. Pure water, which was distilled
after deionization, was obtained with a water distillation
apparatus (Yamato Co., model WG-220).
hydroxymethylfurfural (HMF) becomes remarkably
important because it can be used as a monomer for useful
polymers such as polyethylene terephthalate, or an inter-
mediate for a medicinal purpose, and so on.10 Though
there are reactions that produce HMF at high yield from
biomass, there are no processes for HMF production.
This is mainly because of the use of strong acids and or-
ganic solvents such as DMSO, which require neutraliza-
tion and additional separation during the process.11
Thus, HMF is called a Ôsleeping giantÕ.12 Bicker et al.12
studied the effect of acetone and H2SO4 on glucose con-
version into HMF. A HMF selectivity of 48% was
obtained by dehydration of glucose in a supercritical
acetone (90 wt %)–water (10 wt %) solvent mixture with
10 mM H2SO4 at 453 K and 20 MPa. The C–O bond
splitting of the C–OH group in alcohols, namely dehy-
dration, was reported to be enhanced by adding acid cat-
alyst in hot compressed water at a lower temperature.13
Thus, the elimination of hydroxy groups in glucose was
enhanced by an acid catalyst. However, from the view-
point of green chemistry, the addition of a homogeneous
acid catalyst and organic solvent is not consistent with
the ÔgreenÕ requirement of the process.
An alternate catalyst to an effective acid catalyst for
organic reactions in hot compressed water is a metal
oxide. Tomita et al.14 employed an MoO3/Al2O3 cata-
lyst for the hydration of propene (hydration is also
catalyzed by an acid catalyst) in sub- and supercritical
water. They confirmed that MoO3/Al2O3 can be used
as an acid catalyst instead of H2SO4. Watanabe et al.15
conducted formaldehyde reactions in SCW using metal
oxides and reported that TiO2 and MoO3 behaved as
acid catalysts for formaldehyde reactions. This same
conclusion was reported for the study done on the dehy-
dration of 2-propanol in supercritical water.16
The reactions were carried out in an SS 316 stainless
steel tube bomb reactor with an inner volume of 6 cm3.
The loaded amount of glucose was 0.1 g and that of water
was 1.0 g. When the effect of homogeneous acid or alkali
on the glucose reaction was examined, 1.0 g of the acid or
alkali solution (1 mM) was loaded instead of pure water.
The loaded amount of the metal oxides was always 0.1 g.
Ar was loaded at 2.5 MPa to make easier the recovery of
the gaseous product after the reaction. After the loading,
the reactor was submerged into the heating bath. Quench-
ing the reactor into water bath stopped the reaction.
Reaction temperature was 473 K and reaction time ran-
ged from 60 to 600 s. From preliminary studies, the heat-
ing period in the fluidized sand bath was 60 s.17 The gas
products were recovered and the composition was deter-
mined. However, in this study, the volumes of the gas
products were always quite small, and thus no further
analysis was performed. The reactor was opened and
washed with pure water to recover the liquid samples.
Char formation was sometimes observed; however, the
amount of char was small and not analyzed further.
The identification and quantification of the product
gas was conducted by GC–TCD (Shimadzu, model
GC-7A, and Hitachi, model GC163). To understand
the product distribution in the recovered solution,
HPLC analyses were conducted with a KS-802 column.
In this study, we focused on only isomerization and
dehydration among the primary reactions, and the prod-
ucts of isomerization (fructose) and dehydration (1,6-
anhydroglucose (AHG), HMF, and furfural) were quan-
tified. The total amount of carbon in the recovered
water solution was measured using the TOC (total or-
ganic carbon detector, Shimadzu, model TOC-5000 A).
Product yield (mol%) of carbon compound was eval-
uated from the carbon base as shown below:
In this study, the effects of some additives on glucose
conversion were examined in hot compressed water, by a
batch reactor. At first, homogeneous acid and alkali
were added to examine the effect of protons on the reac-
tion of glucose. Next, metal oxides (TiO2 and ZrO2)
were used in the glucose reaction to understand the con-
trollability of the glucose reaction by the metal oxides.
Also, in order to confirm the effect of additives (homo-
geneous and heterogeneous catalysts) on the reaction,
reactions starting from fructose were conducted.
Product yield ½mol% ꢀ
An amount of carbon atom in a product
¼
The amount of carbon atom in the loaded glucose
ꢁ100
ð1Þ
2. Experimental
Glucose and fructose were obtained from Wako Pure
Chemical and used as received. Sodium hydroxide
(NaOH, 1 M aq solution) and sulfuric acid (H2SO4,
1 M aq solution) were purchased from Wako Pure Chemi-
cal and used as received. Titanium oxides (anatase and
rutile TiO2,) were obtained from Wako Pure Chemical
3. Results and discussion
3.1. Without additive
The reaction time (including the heating period) was
from 60 to 300 s. Gas and char formations were not

M. Watanabe et al. / Carbohydrate Research 340 (2005) 1925–1930
1927
tially formed. HMF was obtained with disappearing
fructose and AHG formed through the dehydration of
glucose. At longer reaction time (after 300 s), furfural
was obtained in a low yield (below 1 mol %). Figure 3
shows a simple reaction network of glucose in hot com-
pressed water at 473 K. This pathway was based on the
model by Kabyemela et al.,1–4 Sinag et al.,5 and Srokol
et al.8 Namely glucose isomerizes into fructose. Both
glucose and fructose are dehydrated into AHG and
HMF, respectively. Furfural is formed by the decompo-
sition of ketose and C-5 monosaccharide, which are
intermediates. These compounds also converted into
acid/aldehyde/phenols/gas/char as suggested by Sinag
et al.5 and Watanabe et al.18
TOC
100
90
Glucose 0.1 g
Water 1.0 g
80
Glucose
70
60
0
200
400
600
800
Time, s
Figure 1. Yield of glucose and TOC without additive in hot
compressed water at 473 K.
HOH2C
CH2 OH
OH
O
O
HOH2C
CHO
HO
3.2. With homogeneous catalysts
HO
HMF
Fructose
10
8
Glucose 0.1 g
Water 1.0 g
The glucose conversion and the TOC values are shown
in Figure 4, and the yields of the liquid products (fruc-
tose, AHG, HMF, and furfural) are shown in Figure
5, with and without additive. The reaction time was
300 s. From all these reactions, the amounts of gaseous
products were only trace amounts, and the result of the
gaseous products are not shown here. From Figure 4,
the conversion was promoted by adding the additives
except for rutile TiO2. As shown in Figure 5, fructose
H2C
OH
O
6
O
HO
4
OH
AHG
O
2
CHO
Furfural
0
0
200
400
Time, s
600
800
Figure 2. Yield of the liquid products in hot compressed water at
473 K.
observed. Figures 1 and 2 show the glucose conversion
and the products that were identified and quantified
(fructose, AHG, HMF, and furfural), respectively.
The values of TOC were always almost 100%. Glucose
gradually converted (30% conversion at 600 s of reaction
time), and the liquid products gradually formed with
glucose conversion. As shown in Figure 2, fructose ini-
100
TOC
80
Glucose 0.1 g
Water 1.0 g
300 s
60
40
20
0
Conversion
Without
catalyst
H2SO4 NaOH TiO2
(rutile)
TiO2
(anatase)
ZrO2
Figure 4. Effect of the additives on the conversion of glucose and the
TOC value at 473 K and 300 s.
HOH C
2
HOH C
CH OH
2
O
2
O
OH
HO
OH
OH
HO
OH
HO
Glucose
Fructose
Furans
O
H C
2
O
O
O
HOH C
CHO
CHO
OH
2
HO
OH
HMF
Furfural
AHG
acids / aldehyde / phenols/ gas / char
Figure 3. Reaction pathways of glucose in hot compressed water at
Figure 5. Effect of the additives on the yield of the liquid products at
473 K.
473 K and 300 s.

1928
M. Watanabe et al. / Carbohydrate Research 340 (2005) 1925–1930
tions of glucose into fructose (Fig. 5) and fructose into
Fructose 0.1 g
Water 1.0 g
300 s
glucose (Fig. 7) are promoted in the alkali condition.19
The equilibrium of the isomerization is quite important
to develop the process of glucose conversion in hot com-
pressed water. Kabyemela et al.1 measured the kinetics
of the epimerization of glucose. According to this re-
port, the equilibrium favors fructose formation in
SCW because the rate of isomerization of fructose into
glucose is negligibly small compared to that of glucose
into fructose. Under acidic conditions, HMF formation
from fructose was slightly promoted (Fig. 7). A similar
result was also reported by Antal et al.,20 who found
that the fructose yield (45%) from fructose in the pres-
ence of 2 mM H2SO4 was roughly two times higher than
that without any catalyst (25%) in hot compressed water
at (523 K and 34.5 MPa). Direct dehydration products
(AHG from glucose and HMF from fructose) were
not greatly enhanced by adding the acid (Figs. 4 and
6). Thus, the high conversion was probably related to
the formation of intermolecular dehydration to form
polymeric compounds (namely condensation) because
the gaseous products were not enhanced by the addition
of acid. Condensation is strongly dependent on the con-
centration of condensable materials such as glucose and
fructose.21 In this study, 0.56 M glucose or fructose was
used as the starting materials. This concentration and
the addition of acid may have possibly led to the promo-
tion of condensation reactions. A lower concentration of
the starting material may be favored to promote mono-
molecular dehydration (AHG from glucose and HMF
from fructose), and this possibility will be examined in
the near future. The effects of homogeneous acid and al-
kali on glucose reactions are suggested as shown in Fig-
ure 8.
TOC
100
80
60
40
20
0
Conversion
NaOH
Without H2SO4
catalyst
TiO2
(anatase)
ZrO2
Figure 6. Effect of the additives on the conversion of fructose and the
TOC value at 473 K and 300 s.
formation was inhibited by H2SO4, while the yields of
HMF and AHG were not suppressed. In the case of
addition of alkali NaOH, the conversion and the TOC
value (Fig. 4) were almost the same as those without
any catalyst. The addition of NaOH promoted the yield
of fructose and inhibited the yield of AHG.
In order to understand the effect of the additives on
glucose reactions in more detail, fructose conversions
at 473 K and 300 s were conducted. Figure 6 shows
the fructose conversion and TOC value in the absence
and presence of the additives. Figure 7 shows the liquid
product (glucose, HMF, and furfural) yield from the
fructose conversion at 473 K and 300 s as a function
of the additive. By adding the homogeneous acid, fruc-
tose conversion was enhanced and HMF formation
was slightly promoted. On the other hand, in the pres-
ence of NaOH, fructose conversion was almost the same
as without catalyst, and HMF formation was inhibited,
and glucose was obtained.
Through these results, the effect of the homogeneous
catalysts on glucose reactions were as follows: (1) pro-
motion of isomerization between glucose and fructose
is catalyzed by alkali,19 and (2) promotion of dehydra-
tion of fructose is promoted by acid. Both the isomeriza-
3.3. With heterogeneous catalysts
As shown in Figures 4 and 5, rutile TiO2 has no signif-
icant effect on glucose reactions under these conditions.
HOH C
2
HOH C
O
CH OH
2
2
alkali
O
OH
HO
OH
OH
HO
OH
HO
Glucose
Fructose
acid
Furans
O
H C
2
O
O
O
HOH C
CHO
CHO
OH
2
HO
OH
HMF
Furfural
AHG
Figure 7. Effect of the additives on the yield of the liquid products at
Figure 8. Effects of the homogeneous catalysts on glucose primary
473 K and 300 s.
reactions at 473 K.

M. Watanabe et al. / Carbohydrate Research 340 (2005) 1925–1930
1929
ZrO2 catalyst apparently acts as a base catalyst as
100
80
60
40
20
0
shown in Figures 4–7. That is, isomerization of glucose
and fructose was catalyzed by NaOH and ZrO2. In the
presence of anatase TiO2, glucose conversion was signif-
icantly enhanced (Fig. 4), and the HMF yield was
remarkably high (Fig. 5). Anatase TiO2 enhanced fruc-
tose conversion (Fig. 6) and the HMF yield (Fig. 7).
These effects of anatase TiO2 on glucose and fructose
conversions show that anatase TiO2 can be considered
as an acid catalyst.
TOC
Glucose 0.1 g
Water 1.0 g
Glucose
0
200
400
600
800
Time, s
The acidic and basic characteristics of these catalysts
(anatase TiO2 and ZrO2) on glucose and fructose reac-
tions were also consistent with the study we preformed
previously on formaldehyde reactions.15 That is, anatase
TiO2 was an acid and ZrO2 was a base for formaldehyde
reactions. The results of this study and the formalde-
hyde reactions suggest that the function of the metal
oxide as an acid and base catalyst can be applied for var-
ious other organic reactions.
Figure 9. Yield of glucose and TOC with anatase TiO2 in hot
compressed water at 473 K.
20
HMF
16
12
Glucose 0.1 g
Water 1.0 g
In a previous paper,15 we reported that the acidity and
basicity of metal oxides such as TiO2 and ZrO2 can be
roughly estimated from the electronegativity of the me-
tal ion. However, in this study and in the formaldehyde
study,14 anatase and rutile TiO2 show different catalytic
activities. Namely, rutile TiO2 is always inactive. Thus,
the estimation of catalytic activity from the electronega-
tivity of the metal ion may not be universal. One exam-
ple is Al2O3. The most stable state of Al2O3, a-Al2O3, is
an inactive species for various organic reactions; how-
ever, the transition state of Al2O3 such as c-Al2O3 have
an acid on the surface that makes it active.22 Also,
monoclinic ZrO2 and tetragonal ZrO2 show a different
acidity and basicity.23
We observed a difference between the heterogeneous
acid catalyst (anatase TiO2) and the homogeneous acid
catalyst (H2SO4) we used. The isomerization of fructose
to glucose proceeded in the presence of anatase TiO2
while it did not in the presence of H2SO4, as shown in
Figure 7. On the other hand, this isomerization was
catalyzed by alkali NaOH (Figs. 5 and 7). This result
implies that the heterogeneous acid catalyst, anatase
TiO2, has also a basic site to enhance fructose isom-
erization to glucose. This bifunctionality is one
of the advantages of using of the metal oxide as a
catalyst.
Figures 9 and 10 show the time profiles of glucose
conversion and product distribution, respectively, with
anatase TiO2. Glucose conversion was significantly en-
hanced by adding anatase TiO2 as shown in Figure 9.
With the enhancement of glucose conversion, consump-
tion of fructose was promoted, as shown in Figure 10.
AHG continuously formed within the reaction time
without a catalyst as shown in Figure 2, while its yield
with anatase TiO2 gradually decreased in the reaction
time (Fig. 10). HMF and furfural formation were also
increased by adding anatase TiO2. The yield of furfural
8
Fructose
AHG
4
0
Furfural
0
200
400
600
800
Time, s
Figure 10. Yield of the liquid products with anatase TiO2 in hot
compressed water at 473 K.
was always below 1 mol %. Anatase TiO2 selectively en-
hanced HMF formation from glucose. According to the
concept shown in Figure 8, HMF formation is produced
from fructose and the intermediate products of the
isomerization between glucose and fructose. These re-
sults (Figs. 7 and 10) indicate the main reaction for
HMF formation in the presence of anatase TiO2 is not
a reaction that includes fructose. As shown in Figure
7, the isomerization of fructose into glucose was pro-
moted by anatase TiO2. Also, as shown in Figure 10,
fructose decreased rapidly within the reaction time. This
indicates that the isomerization of glucose into fructose
occurred in a shorter reaction time (before 60 s), and
fructose was rapidly consumed after 60 s. These results
suggest that the increase of HMF formation by anatase
TiO2 is related to the intermediate compounds during
isomerization. The intermediate products of the isomer-
ization between glucose and fructose are ring-opened
compounds that are not stable.2 The acidic sites on the
surface of anatase TiO2 might promote the dehydration
of the intermediate. Thus, acidic and basic sites on the
surface of anatase TiO2 promoted HMF formation from
glucose in hot compressed water. In conclusion, the ef-
fect of anatase TiO2 on glucose in hot compressed water
at 473 K is suggested as shown in Figure 11.
Now, we are studying the catalytic activity of anatase
TiO2 by the use of TPD. Also, exploration of more

1930
M. Watanabe et al. / Carbohydrate Research 340 (2005) 1925–1930
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This research was supported by Industrial Technology
Research Grant Program in 02A44001d from New En-
ergy and Industrial Technology Development Organiza-
tion (NEDO) of Japan.