Sonication assisted rehydration of hydrotalcite catalyst for isomerization of glucose to fructose

Article abstract of DOI:10.1016/j.molcata.2014.06.019

A series of hydrotalcite catalysts, including mother hydrotalcite (HT-M), calcined hydrotalcite (HT-C), and rehydrated hydrotalcites (HT-RM and HT-RSX) were prepared for isomerization of glucose to fructose. Two types of rehydration methods such as mechanical stirring method and sonication assisted method were employed for rehydration of hydrotalcite. Structure of hydrotalcite was successfully recovered to a layered double hydroxide structure via two rehydration processes (Memory effect of hydrotalcite). All rehydrated hydrotalcites (HT-RSX) with a sonication assisted method exhibited a higher fructose yield than rehydrated hydrotalcite (HT-RM) with a vigorous mechanical stirring in this reaction. In consideration of catalytic activity and rehydration time, sonication rather than mechanical stirring played an efficient role to improve catalytic activity of hydrotalcite in this reaction. Characterization results clearly revealed that the enhanced catalytic activity of HT-RSX catalyst was due to its abundant surface weak base sites resulted from a facile vertical breaking and exfoliation of hydrotalcite layers during the sonication assisted rehydration process. Yield for fructose over rehydrated hydrotalcite catalysts showed a tendency to increase with decreasing the crystallite size of catalyst and with increasing the weak base sites of catalyst. Among the catalysts tested, HT-RS10 with the smallest crystallite size and the largest weak base sites exhibited the highest fructose yield in this reaction.

Full text of DOI:10.1016/j.molcata.2014.06.019

Journal of Molecular Catalysis A: Chemical 393 (2014) 289–295
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Sonication assisted rehydration of hydrotalcite catalyst for
isomerization of glucose to fructose
a
a
b
a,∗
Gihoon Lee , Yeojin Jeong , Atsushi Takagaki , Ji Chul Jung
a
Department of Chemical Engineering, Myongji University, Yongin 449-728, South Korea
Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of hydrotalcite catalysts, including mother hydrotalcite (HT M), calcined hydrotalcite (HT C), and
rehydrated hydrotalcites (HT RM and HT RSX) were prepared for isomerization of glucose to fructose.
Two types of rehydration methods such as mechanical stirring method and sonication assisted method
were employed for rehydration of hydrotalcite. Structure of hydrotalcite was successfully recovered to
a layered double hydroxide structure via two rehydration processes (Memory effect of hydrotalcite). All
rehydrated hydrotalcites (HT RSX) with a sonication assisted method exhibited a higher fructose yield
than rehydrated hydrotalcite (HT RM) with a vigorous mechanical stirring in this reaction. In consid-
eration of catalytic activity and rehydration time, sonication rather than mechanical stirring played an
efficient role to improve catalytic activity of hydrotalcite in this reaction. Characterization results clearly
revealed that the enhanced catalytic activity of HT RSX catalyst was due to its abundant surface weak
base sites resulted from a facile vertical breaking and exfoliation of hydrotalcite layers during the son-
ication assisted rehydration process. Yield for fructose over rehydrated hydrotalcite catalysts showed a
tendency to increase with decreasing the crystallite size of catalyst and with increasing the weak base
sites of catalyst. Among the catalysts tested, HT RS10 with the smallest crystallite size and the largest
weak base sites exhibited the highest fructose yield in this reaction.
Received 27 May 2014
Accepted 10 June 2014
Available online 26 June 2014
Keywords:
Hydrotalcite
Rehydration method
Sonication
Isomerization
Glucose
©
2014 Published by Elsevier B.V.
1
. Introduction
[12,13]. However, biological conversion of glucose using enzyme
has various drawbacks: (i) limiting operating temperature range,
(ii) high-purity reactant, (iii) short life time of enzyme compared
to inorganic catalyst, (iv) high operating cost for mass production
[14–16]. Therefore, many researchers have extensively investi-
gated glucose conversion reactions over inorganic catalysts with
an aim of synthesizing valuable chemicals from glucose such as
fructose, lactic acid, glyceric acid, and glyceraldehyde [17,18].
Fructose is commonly added to foods and drinks for palatability
and taste enhancement due to its low cost and high sweetness. In
addition, fructose can be used as a starting material for the synthesis
of value-added chemicals (e.g., 5-hydroxymethylfurfural, levulinic
acid, furandicarboxylic acid, dimethyl furan, and dihydroxymethyl-
furan) [19,20]. Therefore, isomerization of glucose to fructose is
one of the important processes in biomass converting technolo-
gies. Although biological isomerization of glucose to fructose using
xylose isomerase is industrially operated, as already mentioned, the
biological process has many disadvantages over the chemical pro-
cess. In this respect, we attempted to produce fructose form glucose
via isomerization reaction over inorganic catalyst in this study.
Base catalysts are well-known to be efficient for isomeriza-
tion of glucose to fructose [21]. Among various base catalysts,
The rapid depletion of fossil fuels such as coal and crude
oil has resulted in increased utilization of renewable energy
1–5]. Biomass represents one of the promising renewable energy
[
sources, because bioenergy can help in reducing the oil depend-
ency and fossil-based greenhouse gas emission [6–9]. Biomass is
all biologically-produced organic materials derived from recently
living organisms, including plants, animals, and their by-products.
There are two ways to use biomass as an energy source. One is a
direct combustion to produce heat, and the other is an indirect con-
verting biomass to various forms of biofuels or valuable chemicals
[
10,11].
In biomass converting technologies, glucose has been consid-
ered as a key material. Glucose is used as a main energy source in
most organisms (from bacteria to humans), so that it can be bio-
logically converted to various chemicals. In other words, glucose
is a platform chemical in producing biomass-derived chemicals
∗ Corresponding author. Tel.: +82 31 330 6390; fax: +82 31 337 1920.
E-mail addresses: jcjung@mju.ac.kr, jcjung98@gmail.com (J.C. Jung).
http://dx.doi.org/10.1016/j.molcata.2014.06.019
1
381-1169/© 2014 Published by Elsevier B.V.

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G. Lee et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 289–295
HT_RS60
HT_RS40
HT_RS20
HT_RS10
HT_RS5
HT_RM
HT_M
HT_C
HT_RM
10
20
30
40
50
60
70
2
Theta (degree)
Fig. 1. XRD patterns of mother hydrotalcite (HT M), calcined hydrotalcite (HT C),
and rehydrated hydrotalcite (HT RM) catalysts.
10
20
30
40
50
60
70
2
Theta (degree)
hydrotalcite was chosen as a base catalyst for isomerization
reaction of glucose in this study. Hydrotalcites are one of the
layered double hydroxide (LDH) clays with a general formular
Fig. 2. XRD patterns of rehydrated hydrotalcites (HT RSX) with different sonication
time (X).
II
III
n−
II
III
of [M 1 − xM x(OH )][A
·mH O]. Here, M and M are diva-
2
x/n
2
2. Experimental
n−
lent and trivalent metal cations, respectively. Also, A represents
compensating anion located in the interlayer space [22–25]. It is
noteworthy that hydrotalcite has a well-known memory effect.
Hydrotalcite can be changed into mixed metal oxide with a spinel
structure via thermal decomposition under air atmosphere (cal-
cination), and then, the mixed metal oxide can be reconstructed
to original hydrotalcite with a layered double hydroxide structure
by immersion in water (rehydration) [26–28]. A further important
consideration is that base properties of hydrotalcite can be different
through rehydration process. It was revealed in our previous work
that rehydrated hydrotalcite served an efficient catalyst for isomer-
ization of glucose to fructose [8]. The enhanced catalytic activity of
rehydrated hydrotalcite compared original hydrotalcite was due to
its abundant weak base sites formed by vertical breaking and exfo-
liation of layers in the hydrotalcite structure during the rehydration
process.
Sonication has been considered as a realizable technique for
particle size reduction of clay [29,30]. The sonication can give a
shock-wave impact on the surface and interlayer of clays, which
can result in particle size reduction of clays [31–33]. In this
work, therefore, sonication assisted rehydration of hydrotalcite
was conducted to facilitate vertical breaking and exfoliation of
hydrotalcite layers during the rehydration process. A series of rehy-
drated hydrotalcites with different sonication time were prepared
by a sonication assisted rehydration method. For a comparison,
rehydrated hydrotalcite by a mechanical stirring method was also
prepared. The rehydrated hydrotalcites were characterized by XRD
and FE-SEM analyses. Base properties of rehydrated hydrotalcites
were determined by titration experiments. Effect of sonication
assisted rehydration on the catalytic performance of hydrotalcite in
the isomerization of glucose to fructose was investigated. Finally,
correlation between catalytic activity of rehydrated hydrotalcite
and its crystallite size or base property was also studied.
2
.1. Catalyst preparation
Mg-Al hydrotalcite (HT M) with a ratio of Mg/Al = 3 was pur-
chased as a mother catalyst from Sigma–Aldrich. The provided
catalyst was calcined at 450 C during 10 h under an air stream to
obtain the calcined Mg-Al hydrotalcite (HT C). The calcined catalyst
was rehydrated in decarbonated water under nitrogen atmosphere
by two methods: (i) with a vigorous mechanical stirring at 60 C for
◦
◦
◦
2
6
4 h, (ii) with a sonication at 60 C for desired time (5, 10, 20, 40, and
0 min). The rehydrated samples were obtained by filtrations. The
◦
obtained solid product was finally dried overnight at 80 C to yield
the rehydrated hydrotalcite. The rehydrated hydrotalcite under
vigorous mechanical stirring without sonication was denoted as
HT RM. In addition, the hydrotalcites rehydrated using sonication
assisted rehydration were represented by HT RSX, where X repre-
sents sonication time in minutes.
2.2. Catalyst characterization
Crystal structures of the prepared catalysts (HT M, HT C, HT RM,
and HT RS) were analyzed by XRD measurements (Shimadzu, XRD-
7
4
the surface morphology of the catalysts. Surface area of the catalysts
was determined using a BET apparatus (BEL Japan, BELSORP-max).
Base properties of the catalysts were determined using the
irreversible adsorption of noninteracting organic acids, according
to the similar method in the literature [25]. Phenol (pKa = 9.9) was
used as a probe molecules to measure the strong basicity of the
000) using Cu-K␣ radiation (ꢀ = 1.54056 A˚ ) operated 40 kV and
0 mA. SEM analysis (Jeol, JSM-6700F) was carried out to examine
Table 2
Crystallite size of the catalysts calculated by Scherrer equation.
Catalyst
Crystallite size (nm)
0 3
Table 1
0
1 1 0
Catalytic performance of HT M, HT C, and HT RM catalysts in the isomerization of
glucose to fructose at 100 C after a 3 h-catalytic reaction.
◦
HT M
HT RM
44.5
18.7
7.6
30.5
22.7
13.6
16.8
19.7
20.5
22.2
Catalyst
Conversion of
glucose (%)
Selectivity for
fructose (%)
Yield for
fructose (%)
HT RS5
HT RS10
HT RS20
HT RS40
HT RS60
9.6
HT M
HT C
HT RM
9.3
20.1
39.1
57.9
68.1
78.6
5.4
13.7
30.8
11.3
13.2
13.6

G. Lee et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 289–295
291
Fig. 3. SEM images of (a) mother hydrotalcite (HT M), (b) calcined hydrotalcite (HT C), and rehydrated hydrotalcites ((c) HT RM and (d) HT RS10).
sample. A strong acid as acrylic acid (pKa = 4.3) was also employed
to probe the total basicity of the sample. The number of weak
base sites was calculated by subtracting strong basicity from total
basicity. Prior to the basicity measurement, all catalysts were pre-
phase and reaction products were separated using a Biorad Aminex
HPX87H column. Conversion of glucose and selectivity for fructose
were determined using the following equations. Yield for fructose
was calculated by multiplying conversion and selectivity.
◦
treated overnight at 60 C under static air condition to remove any
moles of glucose reacted
moles of glucose supplied
physisorbed organic molecules. 0.05 g of the pretreated sample was
charged into the stoppered bottle, and then, 10 ml of standard solu-
tion of organic acid (phenol/acrylic acid) in cyclohexane was also
added to the bottle. The resulting solution was thoroughly mixed
for 2 h at room temperature. In order to avoid the interacting of the
Conversion of glucose (%) =
Selectivity for fructose (%) =
× 100
× 100
moles of fructose formed
moles of glucose reacted
sample with atmospheric CO and water, the expose of the sample
2
to atmosphere was minimized. In all experiments, 2 h is found to
be enough to achieve equilibrium condition at room temperature.
Concentration of the non-adsorbed organic acid in solution was
determined by UV–vis spectrometer (Sinco, S-3100). Calibration
curves derived from standard solutions were employed for the cal-
culation. From these results, concentration of the adsorbed organic
acid, and subsequently, basicity of the sample were finally obtained.
3
. Results and discussion
.1. Memory effect of hydrotalcite catalyst
Memory effect of hydrotalcite catalyst was confirmed by XRD
3
measurements (Fig. 1). Purchased hydrotalcite (HT M), which used
as a mother catalyst in this work, showed the characteristic XRD
2.3. Isomerization of glucose to fructose
Table 3
Catalytic performance of rehydrated hydrotalcites (HT RM and HT RSX) in the isom-
◦
Isomerization of glucose to fructose was conducted in a batch-
erization of glucose to fructose at 100 C after a 3 h-catalytic reaction.
type glass reactor. 0.3 g of glucose, 0.1 g of catalyst, and 10 ml of
dimethylformamide were used for a typical reaction. The reaction
was performed at 100 C for 3 h. After the reaction was completed,
the reactor was cooled in an ice bath. The catalysts were filtered
off using syringe filter and the filtrate was analyzed by high per-
formance liquid chromatography (Young Lin instrument, YL 9100)
equipped with an ultraviolet–visible (UV–vis) detector and a refrac-
Catalyst
Conversion of
glucose (%)
Selectivity for
fructose (%)
Yield for
fructose (%)
◦
HT RM
HT RS5
HT RS10
HT RS20
HT RS40
HT RS60
39.1
43.8
41.5
40.2
39.6
39.8
78.6
78.7
88.0
88.8
86.4
85.4
30.8
34.5
36.6
35.7
34.3
34.0
tive index (RI) detector. 0.005 M H SO was used as a mobile
2
4

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G. Lee et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 289–295
38
36
34
32
30
HT_RM
10
20
30
40
50
60
Sonication time (min)
Fig. 4. Yield for fructose over HT RSX catalysts in the isomerization of glucose to
fructose at 100 C after a 3 h-catalytic reaction, plotted as a function of sonication
time (X).
◦
Fig. 5. Comprehensive correlations between yield for fructose, crystallite size, and
weak base sites.
enhanced catalytic activity of HT RM catalyst is thought to be a
more abundant base sites of HT RM formed by vertical breaking
and exfoliation of layered structure during the rehydration process.
peaks of hydrotalcite materials. Concretely, the peaks correspond-
◦
◦
ing to the basal planes ((0 0 3), (0 0 6), and (0 0 9) at 2ꢁ = 11.6 , 23.3 ,
◦
and 34.8 , respectively) and those for the nonbasal planes ((1 1 0)
◦
◦
and (1 1 3) at 2ꢁ = 60.7 and 62.0 , respectively) were observed in
the HT M catalyst. This means that HT M catalyst retained a layered
double hydroxides structure, which is the characteristic structure
of hydrotalcite [23]. Hydrotalcite calcined under an air stream
3.2. Sonication assisted rehydration of hydrotalcite
Sonication assisted rehydration of hydrotalcite was employed
(
HT C) showed a spinel structure of Mg-Al oxide, in good agreement
to improve the catalytic activity of rehydrated hydrotalcite in the
isomerization of glucose to fructose. A series of rehydrated hydro-
talcites (HT RSX) with different sonication time (X) were prepared.
Fig. 2 shows the XRD patterns of HT RSX catalysts. For compari-
son, the XRD pattern of rehydrated hydrotalcite (HT RM) with a
vigorous mechanical stirring was also represented in Fig. 2. Most
of rehydrated hydrotalcites (HT RSX) with a sonication assisted
rehydration retained layered double hydroxides structures of
hydrotalcite, indicating that HT RSX catalysts were successfully
rehydrated by a sonication assisted rehydration method. The XRD
peaks corresponding to spinel structure of Mg-Al oxide was found
in the HT RS5 catalyst. It is inferred that 5 min was not enough
to rehydrate hydrotalcite using a sonication assisted rehydration,
leading to a mixed phase of layered double hydroxides structure
and spinel structure in the HT RS5 catalyst.
In order to confirm the effect of sonication assisted rehydration,
the crystallite size of hydrotalcites was calculated by Scherrer equa-
tion using the XRD peaks for the (0 0 3) and (1 1 0) planes. Crystallite
size of rehydrated hydrotalcites was listed in Table 2. Rehydrated
hydrotalcites (HT RM and HT RSX) retained a smaller crystallite
size than their mother catalyst (HT M). Once again, we confirmed
that vertical breaking and exfoliation of layers structure in the
hydrotalcite were occurred during the rehydration process. Inter-
estingly, a smaller crystallite size was found in all HT RSX catalysts
rather than HT RM catalyst. This means that sonication facilitated
the vertical breaking and exfoliation of layers structure during the
rehydration of hydrotalcite, leading to the abundant surface base
sites of HT RSX. Therefore, it is expected that HT RSX catalyst may
show a better catalytic activity than HT RM in isomerization of
glucose to fructose.
with the previous reports [23,24]. It is noteworthy that rehydrated
hydrotalcite (HT RM) with a vigorous mechanical stirring exhib-
ited the characteristic XRD peaks of hydrotalcite, indicating that
the layered double hydroxides structure was recovered by the rehy-
dration process. XRD results clearly showed that hydrotalcite could
be successfully rehydrated in decarbonated water with a vigorous
mechanical stirring. As shown in Fig. 1, HT RM catalyst retained a
broader XRD peaks for (0 0 3) and (1 1 0) planes than HT M catalyst.
As reported in our previous work [8], rehydration of hydrotalcite
resulted in a smaller crystallite size formed by vertical breaking and
exfoliation of layered structure.
Catalytic performance of hydrotalcite catalysts (HT M, HT C,
and HT RM) in the isomerization of glucose to fructose was
summarized in Table 1. The catalytic activity was obtained at the
◦
reaction temperature of 100 C after a 3 h-catalytic reaction. As
expected, HT RM catalyst showed a much better catalytic activity
than HT M catalyst in this reaction. This means that rehydration is
an efficient method to improve catalytic activity of hydrotalcite in
the isomerization of glucose to fructose. In our previous work [8],
it was revealed that the weak base sites of hydrotalcite catalyst
were responsible for this reaction. Therefore, main reason for the
Table 4
Basicity of the catalysts (HT M, HT RM, and HT RSX).
Catalyst
Base sites
Total base site
(
Strong base site
Weak base site
(mmol/g)c
mmol/g)a
(␮mol/g)b
HT M
HT RM
HT RS10
HT RS20
HT RS40
HT RS60
0.28
0.55
0.69
0.58
0.54
0.55
1.058
4.976
4.668
4.881
4.144
4.530
0.279
0.545
0.685
0.575
0.536
0.545
Surface morphology of hydrotalcite catalysts was observed by
SEM analysis. Mother hydrotalcite (HT M) and rehydrated hydro-
talcites (HT RM and HT RS10) retained layered structures, while a
typical surface morphology for mixed metal oxide was observed in
HT C catalyst (Fig. 3). It is interesting to note that layered double
hydroxide structure was more developed in the rehydrate hydro-
talcites (HT RM and HT RS10) than mother hydrotalcite (HT M).
a
Acrylic acid was used as a probe molecular.
Phenol was used as a probe molecule.
Weak base site = Total base site − Strong base site.
b
c

G. Lee et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 289–295
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G. Lee et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 289–295
Especially, it was revealed that HT RS10 (Fig. 3(d)) retained a more
developed layered structure and a smaller crystallite size in com-
parison with HT RM (Fig. 3(c)). In addition, BET surface areas of
Conversion of glucose
Selectivity for fructose
Yield for fructose
100
HT M, HT C, HT-RM, and HT RS were found to be 2.6, 34.8, 30.6, and
80
2
3
7.9 m /g, respectively. These results were consistent with those
obtained from XRD analysis. Therefore, sonication assisted rehy-
dration is thought to be an efficient way to obtain more abundant
surface base sites of hydrotalcite by the developed layered structure
and the small crystallite size.
6
4
2
0
0
0
0
Catalytic activity of rehydrated hydrotalcites (HT RSX) with a
sonication assisted rehydration was investigated by conducting
◦
the isomerization of glucose to fructose at 100 C. The obtained
catalytic activity after a 3 h-catalytic reaction was summarized in
Table 3. Results of rehydrated hydrotalcites (HT RM) with a vigor-
ous mechanical stirring were also listed as a reference. Conversion
of glucose over the HT RSX catalysts was higher than that over
the HT RM catalyst. In respect to the HT RSX catalysts, conversion
decreased with increasing sonication time (X). It is thought that
conversion of glucose over rehydrated hydrotalcite catalysts may
be closely related to its crystallite size. It can be inferred from cat-
alytic activity (Table 3) and crystallite size (Table 2) that conversion
of glucose increased with decreasing crystallite size of rehydrated
hydrotalcite. HT RS5 with the smallest crystallite size showed the
highest conversion of glucose in this reaction. However, selectivity
for fructose over the HT RSX catalysts exhibited a different trend
compared to conversion of glucose. Except for the HT RS5, selec-
tivity for fructose was almost the same in all HT RSX catalyst. A
low selectivity was particularly observed in the HT RS5 catalyst.
HT RS5 retained a mixed phase of layered double hydroxides struc-
ture and spinel structure due to the insufficient sonication time
80
100
120
140
160
o
Reaction temperature ( C)
Fig. 7. Catalytic performance of HT RS10 in the isomerization of glucose to fructose
after a 3 h-catalytic reaction, plotted as a function of reaction temperature.
work [8], glucose isomerization reaction is facilitated by weak base
site of the catalyst at low temperature, while glucose degradation
reaction is accelerated by strong base site of the catalyst at high
temperature (Fig. 5). Therefore, we focused on the weak basicity of
rehydrated hydrotalcite catalyst in this work.
As listed in Table 4, weak basicity of the rehydrated hydro-
talcite was different depending on the rehydration method and
time. Rehydrated hydrotalcites (HT RM and HT RSX) retained more
weak base sites than their mother catalyst (HT M). This indicates
that rehydration was an efficient method to increase the weak
basicity of hydrotalcite catalyst. In particular, sonication assisted
rehydration rather than mechanical stirring rehydration was more
favorable for obtaining abundant weak base sites. Weak base
sites of hydrotalcite catalysts roughly increased with decreasing
their crystallite size (Tables 2 and 4). Among the catalysts tested,
HT RS10 with the smallest crystallite size retained the largest weak
base sites.
From the characterization and activity of the hydrotalcite cat-
alyst, it can be inferred that catalytic activity of hydrotalcite in
glucose isomerization reaction may be closely related to its crys-
tallite size and weak basicity. In order to confirm these results,
correlation plots between yield for fructose (from Table 3), crys-
tallite size calculated using the XRD peak for the (0 0 3) plane (from
Table 2), and weak base sites (from Table 4) were obtained. As
shown in Fig. 6, yield for fructose showed a tendency to increase
with decreasing the crystallite size of catalyst and with increasing
the weak base sites of catalyst. It was again confirmed that the crys-
tallite size and the weak basicity of catalyst played a crucial factor
determining the catalytic activity of hydrotalcite in this reaction.
HT RS10 with the smallest crystallite size and the largest weak base
sites exhibited the highest fructose yield in this reaction. Sonica-
tion assisted rehydration served an efficient method to improve
catalytic activity of rehydrated hydrotalcite due to its abundant
surface weak base sites formed by a facile vertical breaking and
exfoliation of hydrotalcite layers during the rehydration process.
(Fig. 2), leading to facile side reactions by the spinel structure of
Mg-Al oxide. Consequently, yield for fructose, calculated by mul-
tiplying conversion and selectivity, was the highest over HT RS10
catalyst. This means that appropriate sonication time is required
for high catalytic activity of rehydrated hydrotalcite in the glucose
isomerization reaction.
We plotted the yield for fructose over the HT RSX catalysts with
respect to sonication time (X), with an aim of confirming the effect
of sonication rehydration method on the catalytic performance of
hydrotalcite in this reaction. As shown in Fig. 4, all rehydrated
hydrotalcites (HT RSX) with a sonication assisted method exhibited
better catalytic activities than rehydrated hydrotalcite (HT RM)
with a vigorous mechanical stirring. In consideration of catalytic
activity and rehydration time, sonication was a good method for
rehydration of hydrotalcite to improving its catalytic activity in
isomerization of glucose to fructose. Yield for fructose over the
HT RSX catalysts showed a volcano-shaped curve with respect to
sonication time (X). As expected, the maximum yield was obtained
by the HT RS10 catalyst.
3.3. Effect of basicity on the catalytic activity of rehydrated
hydrotalcite
It is generally accepted that base property of the catalyst is
responsible for isomerization of glucose to fructose. In order to ver-
ify the effect of base property on the catalytic activity of rehydrated
hydrotalcite in this reaction, irreversible adsorption of noninter-
acting organic acid such as phenol and acrylic acid was conducted.
HT RS5 catalyst was excluded in this experiment due to its mixed
phase of layered structure and spinel structure. The determined
basicity of rehydrated hydrotalcites was summarized in Table 4.
Total basicity was examined by acrylic acid adsorption, while strong
basicity was measured by phenol adsorption. Weak basicity was
calculated by difference in strong basicity and total basicity. Many
researchers agree that weak base site of the catalyst plays a key role
in isomerization of glucose to fructose. According to our previous
3.4. Effect of reaction temperature
Effect of reaction temperature on the catalytic activity of rehy-
drated hydrotalcite was examined. For this purpose, HT RS10 was
chosen as a model catalyst, and then, the glucose isomeriza-
tion reaction was conducted in the reaction temperature range
of 80–160 C (Fig. 7). Glucose conversion increased with increas-
ing reaction temperature (kinetic effect), while fructose selectivity
decreased with increasing reaction temperature (thermodynamic
◦

G. Lee et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 289–295
295
effect). As shown in Fig.6, high reaction temperature is unfavor-
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compensation between thermodynamic effect and kinetic effect led
to a volcano-shaped curve of fructose yield with respect to reaction
temperature.
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4
. Conclusion
Hydrotalcite catalyst was successfully rehydrated using two
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[
[
[
methods (mechanical stirring method and sonication assisted
method). Memory effect of hydrotalcite through the calcina-
tion and rehydration process was well confirmed by XRD and
SEM analyses. Rehydrated hydrotalcites retained a layered dou-
ble hydroxide structure, which is a structure of original mother
hydrotalcite. Rehydrated hydrotalcites showed a better catalytic
activity than mother hydrotalcite in the isomerization of glucose
to fructose, indicating that rehydration is an efficient method to
improve catalytic activity of hydrotalcite in this reaction. Par-
ticularly, all rehydrated hydrotalcites (HT RSX) with a sonication
assisted method showed better catalytic activities than rehydrated
hydrotalcite (HT RM) with a vigorous mechanical stirring. When
considering catalytic activity and rehydration time, sonication was
a good way for rehydration of hydrotalcite to improving its catalytic
activity in this reaction. Yield for fructose over HT RSX catalysts
showed a volcano-shaped curve with respect to sonication time (X).
Among the catalysts tested, HT RS10 showed the highest yield for
fructose. The enhanced catalytic activity of HT RS10 was attributed
to its abundant surface weak base sites formed by a facile vertical
breaking and exfoliation of hydrotalcite layers during the sonica-
tion assisted rehydration process.
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This work was supported by the framework of international
cooperation program managed by the National Research Founda-
tion of Korea (NRF-2012K2A2A4019876) and the 2013 Research
Fund of Myongji University.