An Effective Heterogeneous Catalyst of [BMIM]3PMo12O40 for Selective Sugar Epimerization

Article abstract of DOI:10.1002/cplu.201800154

The development of heterogeneous catalysts for the epimerization of sugars has received much less attention than that for the isomerization of sugars. To date, molybdates are the most effective catalysts for the epimerization of sugars, although they lack stability toward hydrolysis of their active sites in water. To solve the issue of the formation of a highly water-soluble heteropolyblue (PMo_red) for phosphomolybdates (PMos) in aqueous reaction systems, herein, a 1-butyl-3-methylimidazolium phosphomolybdate ([BMIM]₃PMo₁₂O₄₀) was synthesized through an ion-exchange method. This catalyst was effective and selective for the C2-epimerization of sugars under mild reaction conditions (<100 °C; 1–2 h) with good water-tolerant properties. The reaction was confirmed to occur in a heterogeneous manner and no leaching of PMo_red was detected by means of UV/Vis spectroscopy. Moreover, the catalyst can be simply separated by filtration and reused for at least eight cycles without a drop in catalytic activity. XRD, FTIR, and X-ray photoelectron spectroscopy measurements indicate that the catalyst is stable under the reaction conditions. In a comparison of the catalytic activity and surface wettability with those of other PMo salts, that is, 1-ethyl-3-methylimidazolium phosphomolybdate ([EMIM]₃PMo₁₂O₄₀), 1-hexyl-3-methylimidazolium ([HexMIM]₃PMo₁₂O₄₀), [choline]₃PMo₁₂O₄₀, and cetyltrimethylammonium phosphomolybdate ([CTA]₃PMo₁₂O₄₀), it is found that [BMIM]₃PMo₁₂O₄₀ has more appropriate hydrophobic–hydrophilic balance, which should be responsible for better catalytic activity and stability.

Full text of DOI:10.1002/cplu.201800154

Accepted Article
Title: An effective heterogeneous catalyst of [BMIM]3PMo12O40 for
selective sugar epimerization
Authors: Asep Bayu, Akihiro Yoshida, Surachai Karnjanakom, Zhijun
Zuo, Xiaogang Hao, Abuliti Abudula, and Guoqing Guan
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An effective heterogeneous catalyst of [BMIM]3PMo12O40 for
selective sugar epimerization
Asep Bayu,[b] Akihiro Yoshida,[a],[b] Surachai Karnjanakom,[b] Zhijun Zuo,[c] Xiaogang Hao,[c] Abuliti
Abudula[b] and Guoqing Guan*[a],[b]
Abstract: Development of heterogeneous catalysts for epimerization
of sugars has received much less attention than that for
isomerization of sugars. To date, molybdates are the most effective
catalysts for epimerization of sugars although they lack the stability
toward hydrolysis of their active sites in water. To solve the issue on
the formation of a highly water-soluble heteropolyblue (PMored) for
phosphomolybdates (PMo) in aqueous reaction systems, in this
reactions involve the isomerization of sugars in their
transforming routes, the current catalyst developments are
mainly concentrated on the design of catalysts for isomerisation
of sugars. However, the increasing market demand of
epimerized sugars and their derivatives in nutraceuticals
industry, e.g. mannose and sugar alcohols as a dietary
supplement,[4] has attracted much interest on development of
chemo-catalytic processes for epimerization of sugars recently.
Importantly, epimerization has been considered as an effective
route to synthesize “rare sugars”, a family of monosaccharides
existed with only small amounts as natural products,[5] from
widely available and cheap sugars.[6] Various kinds of
applications on rare sugars have been reported especially in the
fields of the pharmaceutical and nutraceutical industries.[5] For
instances, D-talose was reported to have important biological
study,
a
1-butyl-3-methylimidazolium phosphomolybdate
([BMIM]3PMo12O40) was synthesized via an ion-exchange
method. It is found that this catalyst was effective and selective
for the C2-epimerization of sugars at mild reaction conditions
(<100oC; 1-2 h) with a good water-tolerant property. The
reaction was confirmed to occur as a heterogeneous manner
and no leaching of PMored was detected by using UV-Vis
spectroscopy. Moreover, the catalyst can be simply separated
by filtration and reused for at least 8 cycles without catalytic
activity dropping. XRD, FT-IR and XPS measurements indicate
that the catalyst was stable in the reaction conditions. In the
comparison of its catalytic activity and surface wettability with
other PMo salts, i.e. [EMIM]3PMo12O40, [HexMIM]3PMo12O40,
[Choline]3PMo12O40 and [CTA]3PMo12O40, it is found that
[BMIM]3PMo12O40 had more appropriate hydrophobic-hydrophilic
balance, which should be responsible for its better catalytic
activity and stability than the former PMo-based catalysts.
activities such as anti-tumor, antimicrobial and marker of O-
7]
antigents;[6a,
D-lyxose can be used as a precursor for
synthesizing antitumor compound of agelapshins;[8] and D-
tagatose is becoming as a promising low-calorie sugar[9] and a
candidate drug to treat type 2 diabetes which has entered the
phase III clinical trial.[10] Therefore, the artificial production of
rare sugars is also required to satisfy their growing market
demands. In this context, developing the catalyst for chemo-
catalytic epimerization of sugars is one of attractive options
besides enzymatic-based processes.[11] In addition, this strategy
should be also useful for producing novel bio-based chemicals in
the future carbohydrate refinery besides the current sugar
valorisation via the isomerization pathways.[12]
Introduction
Generally, chemo-catalytic processes of carbohydrates
refinery are focused on valorisation of sugars into valuable
oxygenated chemicals such as furfural/5-hydroxymetylfurfural
(HMF),[1] levulinates[2] and lactates[3] via dehydration, rehydration
and [3+3] retro-aldol reaction, respectively. Since those
CH2OH
CH2OH
O
OH
O
OH
OH
CH2OH
O
OH
OH
OH
OH
OH
OH
CH2OH
OH
OH
(2)
(3)
(1)
Scheme 1. Representative scheme of glucose (1), fructose (2) and
mannose (3)
[a]
Prof. Dr. G. Guan, Prof. Dr. A. Yoshida
Department of Renewable Energy, Institute of Regional
Innovation, Hirosaki University
The epimerization of sugars was firstly observed on
glucose under basic conditions, which is known as the
Lobry de Bruyn-Alberda van Ekenstein reaction.[13] However,
it occurred together with aldose-ketose interconversion
2-1-3, Matsubara, Aomori 030-0813, Japan
E-mail: guan@hirosaki-u.ac.jp
Tel.: +81-17-762-7756; Fax: +81-17-735-5411
A. Bayu, S. Karnjanakom, Prof. Dr. G. Guan, Prof. Dr. A. Yoshida
Graduate School of Science and Technology
Hirosaki University
1-Bunkyocho, Hirosaki 036-8560, Japan
Prof. Dr. Z. Zuo, Prof. Dr. X. Hao
Department of Chemical Engineering
Taiyuan University of Technology
[b]
[c]
(Scheme 1) and gave a mixture of (1), (3) and (2) in the
product with the ratio of 4:1:4.[14] In 1972, Bilik reported that
a molybdic acid solution prepared by dissolving MoO3 in hot
water can effectively and selectively catalyze the glucose-
mannose epimerization with a final glucose:mannose molar
ratio of 3:1 at 90oC.[15] In this case, a dimolybdate ion, i.e.
the active species for the Bilik reaction,[16] formed a
tricentric intermediate complex with sugar, leading the
Taiyuan 030024, China
Supporting information for this article is given via a link at the end
of the document.
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10.1002/cplu.201800154
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with a good water-tolerant property. The reaction proceeded with
a heterogeneous manner and the spent catalyst was easily
separated and reused at least eight cycles without significant
dropping of its activity. The material showed the similar activity
for the glucose epimerization as the homogeneous H3PMo12O40
catalyst. Moreover, the appropriate surface wetting property of
BMIM-PMo was found to play an important role in its excellent
catalytic activity.
occurrence of 1,2-C skeletal rearrangement related to its
aldehyde/ketone functional group.[16b,
Other molybdate
17]
catalysts such as (NH4)6Mo7O24 and H3PMo12O40 were also
found to be effective and selective for 1,2-C rearrangement
in sugar epimerization.[18] Unfortunately, all of these
molybdates are soluble in hot water and in fact work as
homogeneous catalysts. Thusly, they are difficult to be
separated from the solution and reused.
To solve this problem, several attempts have been
conducted to develop molybdate-based heterogeneous solid
catalysts instead of these homogeneous ones. For instances,
Results and Discussion
6-
2-
immobilizations of Mo7O24 and MoO4 on anion-exchange
Catalyst Characterization
19]
resin,[17a,
layered double hydroxide of zinc-alumina[19b] and
As shown in Fig. 1, X-ray diffraction pattern (XRD) of the
synthesized BMIM-PMo is corresponding well to that of the
simulated one based on the reported crystallographic
parameters of [BMIM]3PMo12O40 (CCDC 650261).[21] Three
BMIM cations combined with each PMo anion in its structure
was confirmed by the weight loss in TGA profile (Fig. S1). The
anhydrous salt of BMIM-PMo was indicated from no significant
SBA-15[19b] were reported to have comparable activity as their
homogeneous ones. However, only Mo-exchanged SBA-15
showed good stability in a 3-cycle test, and the activities of
others especially the Mo-exchanged resin were significantly
reduced to 30-85% due to the leaching of Mo.[19] Meanwhile, it is
reported that Keggin-type phosphomolybdate (PMo) solidified
with Ag+ and Sn4+ countercations (i.e., Ag3PMo12O40 and
Sn0.75PMo12O40) exhibited identical activity and selectivity to the
homogenous acid; however, they were also soluble in hot
water.[18b] Even at mild conditions (60oC, 0.5 h), Ag3PMo12O40
and Cs3PMo12O40 were suffered from the significant leaching of
Mo species (13-21%).[4b] In these systems, the formation of
water-soluble hetero-polymolybdate-blue (PMored) is the main
drawback on the application of PMo-based catalysts in the
aqueous sugar epimerization reactions. Recently, the truly
heterogeneous solid Mo-based catalysts, i.e., the layered
bimetallic oxides such as LiNbMoO6 and HNbMoO6 were found
to have high hydrothermal stability and good reusability.[20]
However, rigorous reaction conditions (i.e. 100-120oC, 1.5 h)
were necessary for achieving the equilibrium state of the
epimerization reaction in water. The similar reaction conditions
(100oC, 1.5-3 h, 3 MPa of N2) were also required for a novel
solid catalyst, i.e., Sn-isolated on organic framework, to catalyze
the epimerization of sugars in ethanol.[12]
o
weight lost in the range of 130-300 C[21, 23] as well as no broad
peak of IR absorbance of water at 3400-3600 cm-1.[21] The
typical IR spectra of BMIM cation were identified at 619 cm-1
(imidazole C2-N1-C5 bending); 1165 cm-1 (imidazole H-C-C & H-
C-N bending); 1567 cm-1 (imidazole ring - ); 2869, 2933, 2959
cm-1 (aliphatic  C-H); and 3093, 3113 and 3148 cm-1 (imidazole
ring  C-H) (Fig. 2). Furthermore, the fingerprint of IR absorption
3-
associated with the Keggin anion of PMo12O40 was observed
from the sharp peaks at 791 (Mo-Oc); 870 (Mo-Ob); 961 (Mo=Od)
and 1064 cm-1 (P-Oa). It is simultaneously supported with the
appearances of three weak peaks at 218, 260 and 300 nm on a
DRUV spectrum, which were also attributed to the charge
transfer transitions of the Keggin unit (Fig. 3). In addition, a
hexagonal prism-shaped particle was observed in the SEM
image (inset of Fig. S2), which is consistent with the
orthorhombic crystal structure of BMIM-PMo reported by Rao et
al.[21] Thus, it can be concluded that the BMIM-PMo catalyst was
successfully synthesized, which maintained the structures of
3-
both BMIM and Keggin-type PMo12O40
.
Figure 1. X-ray diffraction patterns of the synthesized [BMIM]3PMo12O40 (A)
compared with its standard (B)[21]
In this work, a PMo salt of 1-butyl-3-methylimidazolium
(BMIM-PMo), which is commonly known as a catalyst for
selective oxidation reactions,[22] was found to be effective for the
sugar epimerization at mild reaction conditions (<100oC, 1-2 h)
Figure 2. FT-IR spectra of [HexMIM]3PMo12O40 (a), [BMIM]3PMo12O40 (b),
[EMIM]3PMo12O40 (c), [Choline]3PMo12O40 (d), and [CTA]3PMo12O40 (e)
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ordered structure. Meanwhile, from the SEM image (Fig. S2),
one can see that [CTA]3PMo12O40 had the micellar-like structure,
which should be corresponded to the lateral packing of the
bounded alkyl chain of CTA on Keggin anion. This is supported
by the broad peak with a crystal size of about 0.41 nm in the
WAX pattern of [CTA]3PMo12O40 at 2 of 21.4o (Fig. S3B), which
was reported to correspond to the lateral packing of the alkyl
chains in some surfactant-polyxometalate salts.[24] This
characteristic usually leads the materials to posses the specific
wetting surface properties with polar/non polar solvents.[24c] As
expected, the measured water contact angle shows that
[CTA]3PMo12O40 was highly hydrophobic (Fig. S4A). Since three
CTA cations are bounded onto the surface of Keggin anion in
[CTA]3PMo12O40, these observation indicates that its external
surface composed of hydrophobic alkyl chains of CTA and a
micellar-like structure was formed as shown by the SEM image
(Figs. S2 and S5). These properties should be also responsible
to its catalytic performance that will be discussed in more details
in the next section.
Figure 3. Diffuse reflectance UV-Visible (DRUV) spectra of the synthesized
catalysts
For comparison, other phosphomolybdate salts with
different
organic
countercations
such
as
1-ethyl-
1-hexyl-3-
choline
methylimidazolium
methylimidazolium
([Choline]3PMo12O40)
([EMIM]3PMo12O40),
([HexMIM]3
and
Table 1. Glucose epimerization over PMo-based catalystsa
PMo12O40),
cetyltrimethyl-ammonium
Selectivityc (%)
Conversion
Entry
Catalystb
([CTA]3PMo12O40) were also synthesized. The formations of
these salts of PMo were confirmed by FTIR measurement. As
shown in Fig. 2, the imidazole ring of imidazolium (1165-1567
cm-1, 3090-3159 cm-1), the broad band of the hydroxyl group of
choline (3500 cm-1) and the long alkyl group of CTA (2850-2930
cm-1) were observed. TGA measurements also confirmed the
compositions of three organic cations for each PMo anion (Fig.
S1). The wide-angle X-ray diffraction (WAX) measurements also
indicated the cation exchange on PMo12O403- (Fig. S3A). Herein,
the typical XRD patterns of H3PMo12O40, i.e. the sharp peaks at
2 of 10.6o, 11.1o, 19.0o, 21.4o, 24.8o, 25.7o and 29.0o,[23] were
changed for all the synthesized materials. The introduction of
organic cation should be responsible for changing the
arrangement of cations and PMo anion, resulting the different
XRD patterns of those PMo salts when compared with
H3PMo12O40. Moreover, it was supported by the blue shift of UV
absorption at 426 nm as that for BMIM-PMo (Fig. 3). Here, one
can see that the blue shift was aligned with the number of
carbon of the countercations in PMo salts. It was reported that
the UV absorptions in the wavelengths of 350-426 nm on UV
(%)
(2)
(3)
H2O
1
2
3
4
5
6
7
-
3
-
-
-
-
-
-
-
-
H3PMo12O40
30
25
28
29
10
4
90
89
83
94
93
84
BMIM-PMo
[EMIM]3PMo12O40
[Choline]3PMo12O40
[HexMIM]3PMo12O40
[CTA]3PMo12O40
EtOH/H2O (9:1)
< 1
8
9
-
-
-
-
-
-
H3PMo12O40
[BMIM]3PMo12O40
[CTA]3PMo12O40
21
27
26
90
96
96
10
11
a55.6 mM of glucose, 78oC for 1 h; bmolar ratio of sugar and
PMo12O403- was 50:1; cbased on C- yield of the product.
Catalytic Epimerization of Sugars
Table
1 summarizes the catalytic activities of the
3-
spectra of Keggin anion of PMo12O40 were resulted from the
synthesized PMo salts with organic cations on the epimerization
of glucose to mannose. Particularly, the investigations were
focused on the catalytic reaction in water because the reported
PMo-based catalysts were usually highly soluble in aqueous
solvent. It is considered that the epimerization reaction occurred
ligand to metal charge transfer (O2-  Mo6+) in the Keggin
units.[21] As such, exchanging of proton in H3PMo12O40 with those
organic cations in PMo salts should affect the metal charge
transfer on Keggin anion because of their different
intermolecular interactions.
3-
owing to the activity of PMo12O40 catalyst (Table 1, entry 1). In
Based on the WAX measurements, the typical diffractions
for crystalline materials were also observed for all the
synthesized PMo salts except [CTA]3PMo12O40, which had a
non-crystalline phase as indicated by its broad XRD pattern (Fig.
S3A). No sharp difractions in the WAX region of [CTA]3PMo12O40
should be resulted from the poor arrangement of PMo anions
due to the bulky alkyl chain of CTA. In contrast, the relatively
smaller sizes of choline or planar structures of imidazolium
countercations allowed them to form a three-dimensionaly
consistent with the previous report,[18b] the homogeneous
H3PMo12O40 was active and selective for this reaction in the
present conditions (Table 1, entry 2). The generation of PMored
was indicated by the changing of the product solution color into
green-blue (the inset image in Fig. 4) and the UV-Vis
absorbance at 310-335 nm (Figs. S6). This peak was reported to
be attributed to the charge transfer change of oxygen-Mo,[25]
which is suspected to be formed after epimerization reaction.[18b]
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Also, it was supported by the UV-Vis measurement of the
reaction product solution, in which, a new absorbance peak at
the region of 500-900 nm was found and its intensity was
aligned with the intensity of the blue color of the solution.
the removal of catalyst, indicating that this catalyst truly worked
as heterogeneously. Moreover, as shown in Fig. 5A, this catalyst
exhibited comparable activity to its homogenous analogue of
H3PMo12O40, which is relatively faster than that is expected from
its relatively poor BET surface area (i.e. 8-11 m2/g). Herein, the
surface property of BMIM-PMo is proposed to have influence on
attracting the polar molecular of sugars, leading the reaction to
be comparable with its homogenous acid. This could be seen
from the responsibility of this surface property on the catalytic
3-
performances of BMIM-PMo and other Keggin PMo12O40 salts
as discussed later. In this context, the equilibrium state was
achieved after 2 h of the reaction. The reusability of BMIM-PMo
was also examined in order to gain further insight for its
application potency. As shown in Fig. 6, the BMIM-PMo
recovered by a simple filtration exhibited no significant activity
loss. This good stability was also supported from no significant
changes of its structure after the reaction by XRD and FTIR
measurements (Figs. S7 and S8). Furthermore, XPS analysis
also indicated that the catalyst was stable after several reaction
cycles (Figs. S9-S12).
The catalytic activity of BMIM-PMo was compared with
other PMo salts for the glucose epimerization. [EMIM]3PMo12O40
and [Choline]3PMo12O40 exhibited the comparable activity as
BMIM-PMo (Table 1, entries 4 and 5). However, they suffered
from a significant leaching of PMored as indicated by the
appearance of blue color and the UV absorption of PMored in
their product solutions (Fig. 4). Meanwhile, [HexMIM]3PMo12O40
and [CTA]3PMo12O40 showed obviously lower catalytic activities
when compared with other imidazolium-based ones (Table 1,
entries 6 and 7). It should be noted that [CTA]3PMo12O40 was
observed to float on the surface of the solution during the
reaction which should be resulted from its strong hydrophobicity
as discussed above.
Figure 4. UV-Vis spectra of the product solution after glucose epimerization in
water at 78oC for 1h, which were obtained from H3PMo12O40 (b),
[Choline]3PMo12O40 (c), [EMIM]3PMo12O40 (d), [BMIM]3PMo12O40 (e) and
[CTA]3PMo12O40 (f) compared with that from the solution of H3PMo12O40 before
reaction (a). The inset figure shows the visual color of their product solutions.
The solution was diluted 187.5 times prior the measurement.
Figure 5. Glucose (o) epimerization to mannose () over H3PMo12O40 (green
colour) and [BMIM]3PMo12O40 (black colour) in water (55.6 mM of glucose in
water, 50:1 molar ratio of glucose and catalyst, 78oC) (A). The equilibrium
state is marked by the dashed line. The heterogeneity test was carried out by
performing filtration after 15 min of reaction (B).
When BMIM-PMo was employed as the catalyst for the
glucose epimerization, the similar selectivity as that using
homogeneous acid was achieved. Interestingly, in contrast with
its homogenous acid, no color change was observed in the
product solution (inset of Fig. 4). This is also supported by the
result that no significant UV absorption related to PMored at the
region of 310-335 nm was observed for the reaction solution (Fig.
4). The catalyst remained undissolved during the reaction and
was easily collected by simple filtration. The heterogeneous
state of the reaction was confirmed as follows: the catalyst was
removed from the reaction solution by filtration in 15 min after
starting the reaction, and the filtrate was remained heating until
the end of the reaction (Fig. 5B). No reactions proceeded after
Figure 6. The reusability of [BMIM]3PMo12O40 in aqueous glucose
epimerization at 78oC for 1 h. The catalytic activity of each runs was compared
with the activity of the fresh catalyst.[26]
The lowest catalytic activity of [CTA]3PMo12O40 in water
should be resulted from its strong hydrophobic surface which
limited the contact of the soluble reactants in water to the active
sites on the catalyst. Similarly, the lower catalytic activity of
[HexMIM]3PMo12O40 than other imidazolium-based PMo salts
should be also related to its relatively high hydrophobic
properties due to the long alkyl chain of the hexyl group (Fig. S4).
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In contrast, [CTA]3PMo12O40 showed activity in the less polar
solvent, i.e. a mixture of ethanol/water (Table 1, entry 11). Thus,
an appropriate surface wetting property of PMo salt should play
an important role in its catalytic activity and the surface
hydrophilicity of solid catalyst affects the contact of the reactants
with the active sites in aqueous reaction system. Furthermore, it
should have great influence on the stability of the solid catalyst.
As shown in Fig.4, the appearance of PMored was found in the
product solutions of [EMIM]3PMo12O40 and [Choline]3PMo12O40 in
water, indicating that they were more hydrophilic than BMIM-
PMo. Herein, the less hydrophobic counter cations such as
EMIM and choline could not completely prevent the dissolution
of surface PMo species in water. In fact, when these catalysts
were solely added to hot water for a while and thereafter their
filtrates were used for the glucose epimerization, the mannose
was obviously detected after the reactions (Fig. S13). The
similar phenomena were also observed for [CTA]3PMo12O40 and
BMIM-PMo in ethanol solution, i.e. about 18% and 11% of
glucose were converted, respectively, which implied the lack
stability of these organic cations in this less polar solvent.
Although BMIM-PMo did not suffer from significant leaching in
water, the peak shift in the XRD pattern of the spent catalyst
indicated that its structure was slightly expanded (Fig. S7).
Therefore, the appropriate hydrophilic-hydrophobic balance of
BMIM cation should be play an important role for the excellent
catalytic activity as well as the stable recyclability of BMIM-PMo
toward the epimerization of sugars in water reaction system.
This conclusion is consistent to the recent reports that the
suitable hydrophobicity/hydrophilicity of the solid catalysts
generally had great impact on their catalytic activity and
reusability in most of aqueous reactions.[12, 27]
reactions with the same manner as H3PMo12O40, by which the
reactions is known to occur via the 1,2-C shift mechanism[18b]
.
Meanwhile, no reaction occurred when cellobiose was used as
the substrate (Table 2, entry 5). Herein, the lack of Brønsted
acid (proton) sites on BMIM-PMo for cleaving the -(1,4)-
glycosidic linkage in cellobiose should be the main reason
because it is necessary to produce monomeric sugars prior to
the epimerization. This is also aligned with the poor performance
of the unprotonated form of the Nb-Mo mixed oxide catalyst for
the cellobiose epimerization.[20]
Figure 7. Catalytic activity of [BMIM]3PMo12O40 at varying reaction
temperatures
The catalytic and water-tolerant properties of BMIM-
PMo in water were also depending on the reaction temperature.
Although the reaction rate of the epimerization was increased at
a higher reaction temperature, the catalyst suffered from the
o
significant leaching after reaction at above 100 C as shown by
the intense blue color of the product solution. At this state, the
peaks corresponding to smaller molecules such as erythrose,
glycolaldehyde, glycolic acid and formic acid were found in the
HPLC chromatogram, indicating that some unwanted reactions
also occurred in parallel with the epimerization reaction. The
generation of erythrose and glycolaldehyde implies that BMIM-
PMo was active to catalyze the [2+4] retro-aldol reaction at this
mild reaction condition. This is a very interesting feature of this
catalyst since much higher reaction temperature (>150 oC) is
usually required for the retro-aldol reaction of hexoses especially
in water.[28] Furthermore, glycolic and formic acid were detected
in the reaction solution. This fact indicates that the oxidation
reaction should also occur at the higher reaction temperature,
although the reaction was carried out in the inert atmosphere.
The main reason for the formation of these oxidized products is
probably because PMo worked as an oxidizer in the harsh
condition. Meanwhile, BMIM-PMo was also active and selective
for sugar epimerization in the ethanol based reaction system
(Table 1, Fig. 7) although the lack stability of BMIM cation
toward alcoholysis led PMo species to leach into the ethanol
solution as described above.
Table 2. Epimerization of various sugars over [BMIM]3PMo12O40
in watera
Conversionc
Selectivity
(%)
Sugarb
(wt%)
Entry
Isomer
Epimer
1
2
3
4
5
Glucose
Mannose
Xylose
Galactosed
Cellobiosee
25
39
35
8
-
-
-
-
-
89
84
83
84
-
-
aunless otherwise stated, the reaction was conducted at 78 oC for
1 h;b50:1 molar ratio of sugar/catalyst, cthe theoretical equilibrium
conversion of glucose, mannose, xylose and galactose are 30,
70, 33 and 20 %, respectively;[16a, 18b, 20] d2 h; e25:1 molar ratio of
sugar/catalyst, 98oC, 4 h.
BMIM-PMo was also applicable for the epimerization of
other kinds of sugars. As shown in Table 2, xylose and
galactose were easily converted into lyxose and talose,
respectively, with almost the same selectivity observed in the
conversion of glucose to mannose (Table 2, entries 1-4). Since
mannose, talose and lyxose are the C-2 epimers of the
corresponding starting sugars, BMIM-PMo should catalyze the
Conclusions
In summary, it is found that a phosphomlybdate-ionic liquid
cation hybrid material of BMIM-PMo was an effective and stable
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10.1002/cplu.201800154
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solid catalyst for the aqueous sugar epimerization at mild
reaction conditions (<100oC, 1-2 h). The catalyst was easily
synthesized from their commercially available precursors, i.e.
BMIMCl and H3PMo12O40, via ion-exchange at room temperature.
This hybrid catalyst showed a comparable activity with the
homogenous H3PMo12O40 without suffering from leaching of
PMored. The catalyst can be simply separated from the solution
by filtration and remained active and selective for at least 8
cycles without a significant loss of its activity. The stability of
catalyst was confirmed by XRD, FTIR and XPS which showed
no significant changes of their structures after used several
times. Herein, the more appropriate hydrophobic-hydrophilic
balance of BMIM-PMo was considered to play an important role
on its good catalytic activity as well as its long-term stability.
The sample solution was placed in a rectangle quartz cell with the optical
length of 10 mm. Surface morphology of the catalysts was observed by
using a Hitachi SU8600 scanning electron microscope (SEM) with an
accelerating voltage of 15 kV. The water contact angle was measured by
using a contact angle meter (DME 201, Kyowa, Japan) at ambient
temperature. X-ray photoelectron spectroscopy (XPS) analysis of the
catalyst before and after reaction was acquired with a VG Scientific
ESCALab250i-XL unit (UK) and was calibrated with the C 1s peak at
284.6 eV.[29] The spesific BET surface area was measured on a
Quantachrome instrument (NOVA 4200e, U.S.A.).
Catalytic Reaction: The reaction was conducted in a screw caped
tube reactor coupled with a magnetic stirrer and a temperature controller
(Personal Organic Synthesizer Zodiac CCX, Eyela, Japan). Briefly,
appropriate amounts of a sugar solution and a catalyst were introduced
into the reactor and reacted in several operation conditions. After the
reaction, the reactor was cooled down in an ice bath and the product
solution was filtrated using a cellulose acetate membrane filter (0.2 m,
ADVANTEC). Afterwards, an aliquot of the liquid product was diluted with
distilled water and analyzed using a Shimadzu Prominence HPLC
equipped with a RID-10A detector and a UV detector operated at 210 nm
(Shimadzu) and an oven CTO-20A that operated at 40°C. A Supelcogel
C610-H column (30cm x 7.8mm) (Sigma) and 0.05 wt% of H3PO4 were
used as the column and the eluent, respectively. The amounts of
products were determined by the external standard method. The
conversion, yield and selectivity of products were calculated based on
the following equations 1, 2 and 3.
Experimental Section
Chemicals: All chemicals were commercially purchased as a
reagent grade without any further pretreatment. The hydrate form of
phosphomolybdic acid was calculated based on the weight loss of its
TGA profile. Distilled water was used in all experiments.
Synthesis of [BMIM]3PMo12O40. A 10 mL aqueous solution of 0.25
M H3PMo12O40 was added dropwise into a 10 mL aqueous solution of
0.625 M BMIMCl with stirring. At this state, a yellow precipitate was
immediately formed. Afterwards, the suspension was stirred for 12 h at
room temperature in dark. Then, the precipitate was separated by
filtration (0.2 m of Cellulose acetate membrane filter, ADVANTEC) and
washed with water until no dissolution of PMo12O403- was detected by the
AgNO3 test. The obtained yellow precipitate was dried in an oven at 50oC
for 12 h to give about 90-92% yield of [BMIM]3PMo12O40. The similar
procedure was used to synthesize other hybrid salts of PMo by replacing
BMIMCl with other cation sources (i.e. [EMIM] acetate, [HexMIM] chloride,
Choline chloride and CTA chloride).
푀표푙푎푟 표푓 푟푒푎푐푡푎푛푡
conversion mol% = (1 − 푀표푙푎푟 표푓 푟푒푎푐푡푎푛푡푡=푡) × 100
(1)
(2)
(3)
(
)
푡=0
푀표푙푎푟 표푓 푝푟표푑푢푐푡
푡=푡
(
)
yield mol% =
× 100
푀표푙푎푟 표푓 푟푒푎푐푡푎푛푡
푡=0
푦푖푒푙푑
(
)
selectivity % =
× 100
푐표푛푣푒푟푠푖표푛
Catalyst Characterization. Thermogravimetric analysis (TGA)
measurement was carried out on a thermogravimetric analyzer (TGA,
DTG-60H, Shimadzu, Japan) in a temperature range of 25-800oC under
a nitrogen atmosphere (50 mL min-1, 10oC min-1 of a heating rate). A
Rigaku SmartLab X-ray Diffractometer was used to record the X-ray
diffraction (XRD) patterns of the catalysts. In this context, a powder
sample was scanned in a 2 range from 10 to 90o with a 10o min-1 scan
rate using a Cu-K radiation source. Fourier transform infrared (FTIR)
spectra were measured by using a FT-IR spectrophotometer (FT-IR 4200,
Jasco, Japan) with the wavelength range of 500-4000 cm-1. A UV-Vis
spectrophotometer (V650, Jasco, Japan) was used to analyze the UV
absorbances of the solutions as well as the diffuse reflectance UV
(DRUV) spectra of the catalysts with a wavelength range of 210-900 cm-1.
Acknowledgements
This work is supported by Aomori City Government and
President Fund of Hirosaki University, Japan. A. Bayu and S.
Karnjanakom gratefully acknowledge the scholarship from
Ministry of Education, Culture, Sport, Science, and Technology
(MEXT) of Japan
Keywords: Sugars epimerization • Rare sugars •
Phosphomolybdates • Keggin catalyst • Hybrid catalyst
[2]
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562-582; dH. Li, Z. Fang, S. Yang, ChemPlusChem 2016, 81, 135-
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[BMIM]3PMo12O40 was found to be
an effective and stable water-
tolerant solid catalyst for
Asep Bayu, Akihiro Yoshida,
Surachai Karnjanakom, Zhijun Zuo,
Xiaogang Hao, Abuliti Abudula and
Guoqing Guan*
selective sugars epimerization.
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An effective heterogeneous
catalyst of [BMIM]3PMo12O40 for
selective sugar epimerization
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Author(s), Corresponding Author(s)*
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Title
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