Isomerization of glucose at hydrothermal condition with TiO2, ZrO2, CaO-doped ZrO2 or TiO2-doped ZrO2

Article abstract of DOI:10.1016/j.cattod.2016.01.049

Catalytic activity of TiO₂, ZrO₂ and ZrO₂ solid solutions (with CaO or TiO₂) for the isomerization of glucose into fructose under hydrothermal conditions (120-180 °C for 5-15 min reaction time) was evaluated by experimental kinetic studies and surface acidity-basicity measurements. Kinetic studies were conducted with batch reactors heated by microwave. Under hydrothermal conditions regardless of the temperature, fructose yield was always below 10% and fructose selectivity was rapidly decreased with increasing glucose conversion. By adding 10 mM NaOH, fructose yield reached 20% at 160 °C for 5 min. In the presence of TiO₂, fructose yield and selectivity were similar (160 °C, 47% of glucose conversion and 14% fructose yield). ZrO₂ showed higher catalytic activity (160 °C, 63% glucose conversion and 21% fructose yield) compared with TiO₂. The catalytic activity of TiO₂ doped ZrO₂ was between TiO₂ and ZrO₂. To increase the basicity of ZrO₂, CaO was doped into the ZrO₂ matrix. For experiments with the 24 wt% CaO doped ZrO₂, fructose selectivity was higher than 70% even at 30% glucose conversion at 160 °C for 15 min. For systematical understanding catalytic activity of the metal oxides used, acidity and basicity of the catalysts were measured by temperature programmed desorption (TPD) using either CO₂ or NH₃. It was found that basicity on the surface increased with increasing the amount of CaO in the ZrO₂ solid solution while acidity increased with amounts of TiO₂. For a simple network model of glucose reactions, rate constants were fitted to the data assuming a simple network model and they were completed with the acid-base properties of TiO₂, ZrO₂ and CaO doped and TiO₂ doped ZrO₂. As a result, the reactivity was found to be correlated with the ratio of the base to acid sites on the surface and fructose formation was linearly proportional to the base/acid mole ratio on the surface of the catalysts.

Full text of DOI:10.1016/j.cattod.2016.01.049

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Contents lists available at ScienceDirect
Catalysis Today
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Isomerization of glucose at hydrothermal condition with TiO2, ZrO2,
CaO-doped ZrO2 or TiO2-doped ZrO2
Haruyuki Kitajimaa, Yoshimasa Higashinoa, Shiho Matsudaa, Heng Zhongb,
Masaru Watanabea,b,∗, Taku M. Aidaa, Richard Lee Smith Jr.a,b
a Graduate School of Environmental Studies, Tohoku University, 6-6-11 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
b Research Center of Supercritical Fluid Technology, Tohoku University, 6-6 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Catalytic activity of TiO2, ZrO2 and ZrO2 solid solutions (with CaO or TiO2) for the isomerization of glucose
into fructose under hydrothermal conditions (120–180 ◦C for 5–15 min reaction time) was evaluated by
experimental kinetic studies and surface acidity–basicity measurements. Kinetic studies were conducted
with batch reactors heated by microwave. Under hydrothermal conditions regardless of the temperature,
fructose yield was always below 10% and fructose selectivity was rapidly decreased with increasing
glucose conversion. By adding 10 mM NaOH, fructose yield reached 20% at 160 ◦C for 5 min. In the presence
of TiO2, fructose yield and selectivity were similar (160 ◦C, 47% of glucose conversion and 14% fructose
yield). ZrO2 showed higher catalytic activity (160 ◦C, 63% glucose conversion and 21% fructose yield)
compared with TiO2. The catalytic activity of TiO2 doped ZrO2 was between TiO2 and ZrO2. To increase
the basicity of ZrO2, CaO was doped into the ZrO2 matrix. For experiments with the 24 wt% CaO doped
ZrO2, fructose selectivity was higher than 70% even at 30% glucose conversion at 160 ◦C for 15 min. For
systematical understanding catalytic activity of the metal oxides used, acidity and basicity of the catalysts
were measured by temperature programmed desorption (TPD) using either CO2 or NH3. It was found
that basicity on the surface increased with increasing the amount of CaO in the ZrO2 solid solution while
acidity increased with amounts of TiO2. For a simple network model of glucose reactions, rate constants
were fitted to the data assuming a simple network model and they were completed with the acid-base
properties of TiO2, ZrO2 and CaO doped and TiO2 doped ZrO2. As a result, the reactivity was found to
be correlated with the ratio of the base to acid sites on the surface and fructose formation was linearly
proportional to the base/acid mole ratio on the surface of the catalysts.
Received 27 October 2015
Received in revised form 5 January 2016
Accepted 19 January 2016
Available online xxx
Keywords:
Glucose
Fructose
Hydrothermal
Isomerization
ZrO2 solid solution
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
63%). However, homogeneous catalysts have disadvantages in their
reuse and recycle due to the difficulty of their separation from the
Glucose is widely found in natural resources such as carbohy-
drates, saccharides and cellulosic materials and it is an important
chemical for food and biomass chemistry. Transformation of glu-
cose into its isomer, namely fructose, is important as a primary step
for modifying of sugars for dietary purposes or for as chemical use.
It is well known that the isomerization of glucose into fructose is
catalyzed by alkali (or solid base) catalysts through proton transfer
or Lewis acid catalysts through hydride transfer. Liu et al. [1] found
high catalytic activity of organic amines on glucose isomerization
in hot water at 100 ◦C for 30 min reaction time, where glucose con-
version was 51% and fructose yield was 32% (fructose selectivity:
reaction products. To overcome the difficulty of reuse and recycle
homogenous catalysts, solid catalysts are preferable.
Tin coated zeolite, which is a solid Lewis acid catalyst, has been
demonstrated to have high catalytic activity for glucose isomeriza-
tion in water at 110–140 ◦C, giving around 30% of fructose yield
at 50% of glucose conversion [2]. Tin-coated zeolite, on the other
hand, requires a complicated and long-period production process
(hydrothermal treatment at 180 ◦C for up to 40 d). Alkali-metal
ion exchange zeolite has also been demonstrated to have activ-
ity for glucose isomerization [3–5] but metal cations tend to leach
from the surface of the zeolites under hydrothermal conditions
(90–95 ◦C). Talcites have been tested for glucose isomerization in
hot water [3,5,6] and leaching of magnesium ion in the talcite was
observed [6].
∗
Corresponding author.
E-mail address: meijin@scf.che.tohoku.ac.jp (M. Watanabe).
http://dx.doi.org/10.1016/j.cattod.2016.01.049
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: H. Kitajima, et al., Isomerization of glucose at hydrothermal condition with TiO2, ZrO2, CaO-doped
ZrO2 or TiO2-doped ZrO2, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.01.049

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As a water-tolerate solid base catalyst, metallosilicates [7],
zirconia promoted with Cs [8], zironosilicate [9] and zirconium car-
bonate [10] have been tested to isomerize glucose into fructose
under hydrothermal conditions at 80–150 ◦C. Zirconiumsilicate and
zirconium carbonate have good stability under hydrothermal con-
ditions and their catalytic activity is also high (for zirconiumsilicate,
glucose conversion was 46% and fructose selectivity was 52% at
80 ◦C for 1 h; for ziroconium carbonate, glucose conversion was
45% and fructose selectivity was 76% at 120 ◦C for 20 min). Sur-
face basicity of these ziroconium-based catalysts was evaluated by
CO2-TPD [9] or titration method [10] and these basic properties
were considered to be related to the catalytic activity, but the cor-
relation between surface properties and catalytic activity was not
clarified. Previously, anatase-TiO2 and tetragonal ZrO2, which are
stable under hydrothermal conditions at 200 ◦C, were used to study
the of isomerization glucose [11,12] and it was found that zirco-
nium could promote the reaction. Surface basicity and acidity of
the zirconium was evaluated by CO2- and NH3-TPD and it was con-
sidered that higher basicity of the zirconia was closely related to
its higher catalytic activity than anatase-TiO2, of which its basicity
was weaker [12]. For the design of an appropriate solid base catalyst
for glucose isomerization having water-tolerance, zirconium-based
catalyst seems to be preferable.
According to Moreau et al. [5], the rate of glucose isomeriza-
tion depends on pH (NaOH amount) of water media in hot water
(90 ◦C). Lee et al. [13] clearly showed that the acidity–basicity of zir-
conium can be controlled by doped-metal cation and their amount.
In this work, catalytic activity of CaO- or TiO2-solid solutions with
zirconium for glucose isomerization in hydrothermal water at
120–180 ◦C was evaluated with experimental reaction studies and
correlation of the rates with the catalyst surface acidity–basicity.
For comparison, anatase-TiO2 and tetragonal-ZrO2 were tested as
catalysts for glucose isomerization along with glucose reaction with
and without NaOH. To determine the acidity–basicity of the metal
oxides used in this study, temperature programmed desorption
method (TPD: NH3-TPD for acidity and CO2-TPD for basicity) was
used. Based on the kinetic data, a simple network kinetic model was
developed and the rate constant of each reaction step was deter-
mined by fitting a model to the experimental data. The correlation
between the rate constant and acidity-basicity of the metal oxides
was elucidated.
sites. The amount of base sites on CaO-doped ZrO2 was higher than
that of acid sites and the basicity increased with increasing the
doped amount of CaO. On the TiO2-doped ZrO2 and the anatase-
TiO2, the amount of acid sites was higher than that of base sites.
Glucose and fructose were purchased from Wako-Pure Chemi-
cals, Co., Ltd. (Osaka, Japan) and used without further purification.
Pure water was obtained by ion exchange and distillation equip-
ment (Yamato).
2.2. Apparatus and procedures
A microwave irradiation apparatus containing high-pressure
glass reactor was employed to perform the glucose isomerization
experiments. The detail of the apparatus is given in previous reports
[14,15]. Briefly, multi-mode microwave oven (m-reactor, Shikoku
Keisoku, Co., Ltd.) was used to heat a high-pressure glass reactor
(inner volume: 10 mL, maximum pressure: 10 MPa, Hiper glasstor,
Takatsu Techne Co., Ltd.) The glass reactor was covered with poly-
carbonate outer tube and PEEK caps. A vacuum was used between
the outer cover and the glass reactor to reduce heat losses by con-
vection. Water was fed into the evacuated space to assist in the
termination of the reaction after the reaction completed.
An aqueous solution (5 g) of glucose (2 wt%) and catalyst (0.1 g)
were loaded into the glass reactor. After sealing the glass reactor,
1.3 MPa of nitrogen gas was introduced to inhibit solution boiling.
For comparison, glucose reaction experiments without catalyst and
with 10 mM of NaOH instead of pure water were conducted. The
loaded reactor was placed in the microwave oven set at 120–180 ◦C
and temperature was maintained at the settings for 3–30 min. After
the reaction, the product solution consisting of the reactants, prod-
ucts, water and catalyst was recovered with rinsing the reactor by
water after cooling and depressurization. Solid catalysts and water
soluble products were separated by vacuum filtration.
2.3. Analysis and definition
Liquid products were analyzed by HPLC (column: Shodex
SH1011, detector: RI, effluent: 1 mL/min of 5 mM H2SO4 sulfu-
ric acid aqueous solution, column oven: 60 ◦C) and yields were
defined on a molar basis. The detected components were glu-
cose, fructose, 5-hydroxymethyl-furfural (5-HMF), glyceraldehyde,
erythrose, glycoaldehyde, lactic acid, formic acid, acetic acid, and
levulinic acid. In this study, the focus of the research was on glucose
isomerization thus glucose yield, fructose yield and fructose selec-
tivity (fructose yield/glucose conversion) were mainly evaluated
(Fig. 1).
2. Experimental
2.1. Materials
Anatase TiO2 was purchased from Wako Co., Ltd., and used as
it is. Tetragonal ZrO2 was prepared from Zr(OH)4 obtained from
Nakarai Tesque (Kyoto, Japan) by calcination at 400 ◦C. The crystal
structure of the calcined ZrO2 was found to be mixture of mon-
oclinic and tetragonal from determination by XRD analysis [12].
ZrO2 solid solutions (CaO or TiO2 were doped into the ZrO2 matrix)
was provided by Daiichi Kigenso Co., Ltd. (Osaka, Japan). The doped
amounts of CaO were 4.3, 14, and 24 mol% denoted as Ca-4.3, Ca-
14, and Ca-24, respectively and those of TiO2 were 7.5 and 15 mol%
denoted as Ti-7.5 and Ti-15, respectively. These CaO- and TiO2-
doped ZrO2 were calcined at 700 ◦C for 1 h. The structure of all the
doped ZrO2 was cubic (from XRD analysis, not shown here) regard-
less of the kind and amount of doped metal oxide. Table 1 shows
some physical properties of the metal oxides that are used in this
study. The amounts of acid and base sites were measured by a tem-
perature programmed desorption (TPD) method. Probe substrate
for acid and base sites were NH3 and CO2 respectively. The pro-
cedures and the conditions of TPD measurement were described
below. In summary, ZrO2 has the similar amount of acid and base
3. Results and discussion
3.1. Experimental data
Time profiles of glucose reactions are shown in Fig. S1 (Sup-
plementary material). Glucose was relatively stable at 140–180 ◦C
for 30 min and the maximum conversion was 23% at 180 ◦C for
30 min and then fructose yield was 7%. Fig. S2 shows the glucose
reaction with 10 mM NaOH at 140 ◦C and 160 ◦C for 3–15 min. The
maximum conversion was 30% and then fructose yield was about
20%. For both cases (without catalyst and with NaOH), glucose
conversion and fructose yield reached a plateau that resembled
chemical equilibrium. Fig. 2 shows the fructose selectivity against
glucose conversion with and without NaOH. Without catalyst, fruc-
tose selectivity drastically dropped and the formation into the other
products was formed with progressing glucose reaction. In the
presence of NaOH, selectivity of glucose isomerization into fruc-
tose was enhanced, but the selectivity was still low with fructose
Please cite this article in press as: H. Kitajima, et al., Isomerization of glucose at hydrothermal condition with TiO2, ZrO2, CaO-doped
ZrO2 or TiO2-doped ZrO2, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.01.049

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Table 1
Physical properties of TiO2, ZrO2 and doped ZrO2 solid solutions used in this study.
Catalyst
Crystal structure
BET (m2/g)
Amount of acid site
(␮ mol/g)
Amount of base site
(␮ mol/g)
Density of acid site
Density of base site
(␮ mol/m2)
(␮ mol/m2)
ZrO2
TiO2
Monoclinic/tetragonal
Anatase
Monoclinic
Monoclinic
Monoclinic
110
4.7
670
79
550
42
6.1
16.8
8.8
9.0
6.1
5.0
8.9
11.3
10.4
6.4
Ca-4.3(700)
Ca-14(700)
Ca-24(700)
Ti-7.5(700)
Ti-15(700)
46.8
54.7
79.2
55.4
197.1
410
490
480
560
400
530
570
510
240
230
Monoclinic/tetragonal
Monoclinic/tetragonal
10.1
2.0
4.3
1.2
All calcination times were 1 h. Hyphenated values refer to the amount of dopant in weight percent doped into ZrO2.
Fig. 1. Microwave irradiation apparatus with high-pressure glass reactor. (1):
microwave oven, (2): polycarbonate outer tube, (3): thick-walled glass reactor, (4):
thermocouple, (5): PEEK cap, (6): Ar gas, (7): valve, (8): valve for gas replacement,
(9): cooling water, (10): vacuum pump, (11): pressure indicater.
Fig. 3. Fructose selectivity against glucose conversion: (a) with tetrago-
nal/monoclinic ZrO2 catalyst, (b) with anatase TiO2 catalyst. Symbols ꢀ: 120 ◦C, ᮀ:
140 ◦C, ꢁ: 160 ◦C.
120–160 ◦C under hydrothermal conditions for 3–30 min reaction
time. Figs. S3 and S4, 5-HMF yields are also shown because 4–7%
yields of 5-HMF were formed while 5-HMF formation was negligi-
ble with and without NaOH. The formation of 5-HMF is evidence
that some amounts of acid sites exist in ZrO2 and TiO2 catalysts
and that the acidity of TiO2 was most likely higher than that of
ZrO2. Catalytic activity of glucose isomerization of ZrO2 was higher
(maximum fructose yield and glucose conversion were 21 and 63%,
respectively) than that of TiO2 (maximum fructose yield and glu-
cose conversion were 14% and 47%, respectively). Fig. 3 shows the
fructose selectivity against glucose conversion in the presence of
ZrO2 and TiO2 at 120–160 ◦C. Except for the reaction at 120 ◦C in
the presence of ZrO2, fructose selectivity decreased with increas-
ing glucose conversion. The maximum selectivity of fructose was
71%, which was achieved at the reaction with TiO2, but the fruc-
tose selectivity drastically decreased with the increase of glucose
conversion. The reaction behavior at 120 ◦C in the presence of ZrO2
implies that an intermediate of glucose conversion into fructose
surely exists such as maltose, as reported in a previous study [16].
Fig. 2. Fructose selectivity against glucose conversion with and without 10 mM
NaOH. Symbols: ꢀ: no catalyst 140 ◦C, ᮀ: no catalyst 160 ◦C,ꢁ: no catalyst 180 ◦C,
᭿: NaOH 140 ◦C, ꢀ: NaOH 160 ◦C.
selectivity being about 70% at 30% of glucose conversion, 160 ◦C for
30 min reaction time.
Figs. S3 and S4 show the fructose yield versus reaction time for
the glucose isomerization reaction with ZrO2 or TiO2 catalysts at
Please cite this article in press as: H. Kitajima, et al., Isomerization of glucose at hydrothermal condition with TiO2, ZrO2, CaO-doped
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Fig. 4. Fructose selectivity against glucose conversion: (a): with Ti-7.5, (b): with
Ti-15. Symbols ꢀ: 120 ◦C, ᮀ: 140 ◦C, ꢁ: 160 ◦C.
Figs. 4 and 5 show the relationship between fructose selectiv-
ity and glucose conversion with TiO2-doped ZrO2 and CaO-doped
ZrO2, respectively. Figs. S5–S9 show the time profile of the reac-
tions. For the case of TiO2-doped ZrO2 (Ti-7.5 and Ti-15), fructose
selectivity was always lower than that for ZrO2 and higher than that
for TiO2 at all glucose conversions for a given reaction temperature.
By doping CaO in ZrO2 matrix, fructose selectivity was enhanced. In
the presence of Ca-4.3, the maximum fructose selectivity was 86%
at 25% glucose conversion at 140 ◦C for 15 min reaction time.
Fig. 6 shows a comparison between data in this work (Ca-14
and Ca-24) and that of the literature [9,10]. Yue et al. [9] conducted
glucose isomerization in the presence of zirconosilicate in a glass
reactor at temperatures from 80 to 160 ◦C for 1 h reaction time.
Son et al. [10] studied the catalytic effect of zirconium carbonate
and zirconium phosphate on glucose isomerization at conditions
of 120 ◦C for 20 min reaction time. Except for one datum obtained
by Son et al. [10] that had a 76% of fructose selectivity for 45% of
glucose conversion all the data were lower than this work. From
results in this work, it could be confirmed that zirconium doped
alkaline-earth metals are effective and are tolerate to water for the
isomerization of glucose. It can be expected that the catalytic activ-
ity of zirconium solid solutions can be improved if the relationship
between surface property and catalytic activity can be clarified.
Fig. 5. Fructose selectivity against glucose conversion: a): with Ca-4.3, b): with Ca-
14, C): with Ca-24. Symbols: ꢀ: 120 ◦C, ᮀ: 140 ◦C, ꢁ: 160 ◦C.
3.2. Reaction model and kinetic analysis
To analyze the catalytic activity, correlation between physi-
cal properties of catalysts and reactivity (fructose selectivity at a
glucose conversion) was attempted by assuming a simple reac-
tion network model (Fig. 7). As shown in Fig. 7, based on the
experimental results given in the previous section, there are three
reaction routes: (i) reversible isomerization between glucose and
Fig. 6. Fructose selectivity against glucose conversion for Ca doped ZrO2 for this
work and values reported by other researches. Symbols: ᭹: Yue et al. [9], ꢁ: Son
et al. [10], ꢁ: Ca-24, 160 ◦C, ꢀ: Ca-24, 140 ◦C, ᮀ: Ca-24, 120 ◦C, ᭿: Ca-14, 120 ◦C, ꢀ:
Ca-14, 140 ◦C.
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5
kgf
kfg
glucose
fructose
kgp
kpf
kpg
kfp
other products
Fig. 7. Simple reaction network model for glucose isomerization. Reaction rate con-
stants k defined as first order.
Here subscripts indicate the component; g is corresponded to glucose, f is fructose,
and p is other products; and combination of two characters in subscripts represents
direction of reaction; gf is glucose into fructose, fg is fructose into glucose and so on.
fructose, (ii) reversible conversion of glucose into other products,
(iii) reversible conversion of fructose into other products. In Fig. 7,
the rate constant of each pathway is described by kgf, kfg, kgp, kpg
,
kpf, and kfp and the units of these rate constants are all s−1 because
all pathways are assumed to be first order. Here subscripts indicate
the component; g is corresponded to glucose, f is fructose, and p
is other products; and combination of two characters in subscripts
represents direction of reaction; gf is glucose into fructose, fg is
fructose into glucose and so on. Based on the simple network model
(Fig. 7) and the kinetic constants, the rate of glucose (Glu) reaction
is expressed by Eq. (1), that of fructose (Fru) is by Eq. (2) and other
products (P) by Eq. (3).
ꢀ
ꢁ
D Glu
[
dt
]
= − kgf + kgf Glu + k Fru + k
P
(1)
(2)
(3)
[
]
[
]
pg [ ]
fg
ꢀ
ꢁ
d Fru
[
]
= kgf Glu − kfg + kfp fru + k
P
[
]
[
]
pf [ ]
dt
Fig. 8. Correlation between base/acid mole ratio and fructose forma-
tion/disappearance ratio at 160 ◦
C for ZrO2, TiO2, Ca-4.3, Ca-14, Ca-24, Ti-7.5
ꢀ
ꢁ
d P
[ ]
dt
= kgp Glu + k Fru − kpg + k
P
[ ]
[
]
[
]
fp
pf
and Ti-15catalysts. (a): relationship between rate constants and base/acid mole
ratio, Symbols: ꢀ: kfg, ᮀ: kgf, ꢁ: kpf, ꢁ: kgp, ᭹: kfp, (b) relationship between fructose
formation/disappearance ratio and base/acid ratio.
These three equations were solved by fourth order Runge–Kutta
method and the rate constants in the equations were determined by
fitting experimental results at all reaction conditions, to minimize
sum of squared residue (SSR) between experimental data (Expi)
and calculation (Calci) based on Eq. (4).
the reactants (glucose, fructose and an intermediate) was fast com-
pared with the reaction rates.
Fig.
8 shows the correlation of catalytic activity and
n
ꢂ
acidity–basicity. Fig. 8a shows the relationship between base/acid
ratio and the rate constants at 160 ◦C. Fig. 8b shows the relationship
between the fructose formation rate (kgf + kpf) and fructose disap-
pearance rate (kfg + kfp) at 160 ◦C as they depend on the base/acid
ratio on the catalysts. In both figures, the rate constants and fruc-
tose formation/disappearance ratio are linearly proportional to the
base/acid ratio of the catalyst even though there is some scatter in
the plots. As shown in Fig. 8, to enhance the catalytic activity for
glucose isomerization into fructose, the ratio of base site to acid site
should be increased.
Finally, the temperature dependence of the glucose isomeriza-
tion with Ca-doped ZrO2 is discussed. Fig. 9 shows the temperature
dependence of kgp, which is an important rate constant for glucose
isomerization, because inhibition of glucose conversion to other
products is main way to improve fructose selectivity especially
since the rate of fructose conversion into other products is negligi-
ble at the experimental conditions tested in this study. As shown
in Fig. 9, kgp becomes lower with decreasing temperature so that
the increase of kgp with increasing base/acid ratio is small at 120 ◦C.
Based on this result and previous discussion, an appropriate cata-
lyst for glucose isomerization should have a high base/acid ratio of
at least 1.0 or more, and an appropriate reaction condition such as
a reaction temperature of 120 ◦C or less.
SSR =
(Calci − Expi)2
(4)
i=1
Table 2 shows the rate constants at all reaction temperatures for
each experimental system (with and without catalysts) with their
SSR value. The fitting results are also shown in Figs. S1–S9. As shown
in Table 2, kpg was found to be equal to zero for all reactions and
kpf was determined to have a positive value. These results show
that the intermediate that exists in the pathway from other prod-
ucts (including intermediate) to glucose, kpg, can be neglected. In
Table 2, the rate constant of fructose formation (kgf + kpf) and that
of fructose decomposition (kfg + kfp) are also shown. As shown in
Table 2, the order of each rate constant typically followed:
kfg > kgf > kpf > kgp
(5)
This trend shows the rate of fructose into glucose was higher
than that of glucose into fructose at the experimental condition
that were investigated. However, for the case of some of the
CaO-doped ZrO2 catalysts at lower temperatures, fructose forma-
tion rate (kgf + kpf) was higher than fructose disappearance rate
(kfg + kfp). This indicates that glucose isomerization into fructose
is favored at lower temperatures for a high basicity catalyst.
For the further consideration of the usefulness and recyclable
of the ZrO2 catalysts, long duration test of the catalyst must be
performed and surface characteristics before and after the reactions
such as TPD, BET, XRD, SEM and so on, has to be conducted. These
will be studied in the next stage for practical realization.
3.3. Correlation of rate constants with acidity-basicity of catalysts
In this study, the reaction rate on the surface of catalysts
depended directly on the rate of chemical reaction because the
reaction temperature was below 160 ◦C and the mass transfer of
Please cite this article in press as: H. Kitajima, et al., Isomerization of glucose at hydrothermal condition with TiO2, ZrO2, CaO-doped
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G Model
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ARTICLE IN PRESS
6
H. Kitajima et al. / Catalysis Today xxx (2016) xxx–xxx
Table 2
Rate constants of network model.
Catalyst
Temp., ◦
C
k × 103 [s−1
]
SSR [–]
gf
gp
pg
pf
0.00
0.84
0.21
fp
fg
7.42
5.31
3.37
gf + pf
fg + fp
140
160
180
0.35
0.28
0.27
0.00
0.07
0.12
0.00
0.00
0.00
0.00
0.00
0.00
0.35
1.11
0.48
7.42
5.31
3.37
3.5
0.8
1.7
No catalyst
120
140
160
0.27
0.74
2.42
0.08
0.16
0.56
0.00
0.00
0.00
1.51
0.59
0.71
0.00
0.00
0.00
3.79
5.73
13.01
1.79
1.33
3.13
3.79
5.73
13.01
1.5
2.3
6.9
TiO2
120
140
160
0.22
1.10
2.30
0.10
0.00
0.35
0.00
0.00
0.00
1.05
3.61
0.89
0.00
4.70
1.92
2.16
4.59
7.07
1.27
4.71
3.19
2.16
9.30
8.98
0.6
4.0
17.0
Ti-15(700)
Ti-7.5(700)
ZrO2
120
140
160
0.30
0.88
1.92
0.06
0.18
0.63
0.00
0.00
0.00
0.00
0.73
0.69
0.10
0.11
0.00
2.09
4.39
6.81
0.30
1.61
2.61
2.19
4.51
6.81
7.0
7.0
11.3
120
140
160
0.31
1.62
2.35
0.76
0.43
1.34
0.00
0.00
0.00
11.60
1.05
1.17
3.56
0.00
0.00
5.18
5.89
7.07
11.92
2.67
3.53
8.73
5.89
7.07
9.6
8.3
15.6
120
140
160
0.63
1.86
2.71
0.17
0.34
0.62
0.00
0.00
0.00
1.43
1.42
1.20
0.00
0.00
0.00
3.23
7.44
7.16
2.06
3.28
3.91
3.23
7.44
7.16
3.9
7.3
25.5
Ca-4.3(700)
Ca-14(700)
Ca-24(700)
120
140
160
0.41
1.41
3.35
0.07
0.37
1.00
0.00
0.00
0.00
1.71
1.51
1.82
0.51
0.00
0.00
1.56
4.43
8.22
2.12
2.91
5.17
2.06
4.43
8.22
8.0
5.7
42.1
120
140
160
0.45
1.20
3.14
0.14
0.49
1.59
0.00
0.00
0.00
0.94
0.91
0.72
0.00
0.10
0.00
1.64
2.28
4.09
1.39
2.11
3.87
1.64
2.38
4.09
19.3
57.1
11.6
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.01.
049.
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The effect of solid base catalysts (TiO2, ZrO2 and CaO- or TiO2-
doped ZrO2) on the isomerization of glucose to fructose in water
was examined with microwave-assisted heating. For all catalysts
that were tested in this study, glucose conversion and fructose
selectivity was improved over non-catalytic system. The CaO-
doped ZrO2 selectively promoted fructose formation from glucose.
The correlation between the surface properties of metal oxides and
reactivity revealed that the base/acid mole ratio on the catalyst
is strongly related to the catalytic activity for glucose isomeriza-
tion. Solid catalysts with a base/acid ratio greater than 1.0 would
increase the fructose selectivity.
Please cite this article in press as: H. Kitajima, et al., Isomerization of glucose at hydrothermal condition with TiO2, ZrO2, CaO-doped
ZrO2 or TiO2-doped ZrO2, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.01.049