Effect of cerous phosphates with different crystal structures on their acidity and catalytic activity for the dehydration of glucose into 5-(hydroxymethyl)furfural

Article abstract of DOI:10.1039/c6ra02602h

A series of cerous phosphate (CP) catalysts with different crystal structures were synthesized by a hydrothermal method at different temperatures (120, 140, 160, 180 and 200 °C) and their performances for the dehydration of glucose into 5-(hydroxymethyl)furfural (HMF) were also thoroughly investigated. These catalysts were characterized by XRD, N₂ adsorption-desorption, SEM, in situ DRIFT, NH₃-TPD and XPS. The results indicate that changing the temperature of synthesis will lead to a transformation of the crystal phase and morphology from 120 °C (nanoparticles, hexagonal structure) to 200 °C (nanorods, monoclinic structure); also, the different crystal phases possess different surface Ce⁴⁺ amounts and acidities. A good linear correlation is found between the Lewis acid content and the surface Ce⁴⁺ amount among these CP catalysts, and very good linearity is also displayed between the Lewis acid amount and the conversion of glucose or the selectivity of HMF, which indicates that Lewis acidity plays an important role in the dehydration of glucose to HMF. CP120, which has a hexagonal crystal structure, exhibits the best catalytic activity (97% conversion of glucose and 61% yield of HMF) because it has the highest amounts of Lewis acid and total acid.

Full text of DOI:10.1039/c6ra02602h

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Page 1 of 40
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Effect of Cerous Phosphates with Different Crystal Structures on Their
Acidity and Catalytic Activity for Dehydration of Glucose into
5ꢀ(hydroxymethyl)furfural
Lina Wang, Fulong Yuan, Xiaoyu Niu, Chuanhong Kang, Pengying Li, Zhibin Li*, Yujun
Zhu*
Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of
Education, School of Chemistry and Materials, Heilongjiang University, Harbin, 150080 P. R. China
*
Corresponding Author: Yujun Zhu, Email: yujunzhu@hlju.edu.cn, Zhibin Li, Email:
lizb@hlju.edu.cn, Tel: +86ꢀ451ꢀ86609650; Fax: +86ꢀ451ꢀ86609650
Abstract
A series of Cerous phosphates (CP) catalysts with different crystal structures were
synthesized by hydrothermal method at different temperature (120, 140, 160, 180 and 200 oC)
and their performance for the dehydration of glucose into 5ꢀ(hydroxymethyl)furfural (HMF)
were also well investigated. These catalysts were characterized by XRD, N2
adsorptionꢀdesorption, SEM, Inꢀsitu DRIFT, NH3ꢀTPD and XPS. The results indicate the
change of the synthesized temperature will lead to the transformation of crystal phase and
morphology from 120 oC (nanoparticles, hexagonal structure) to 200 oC (nanorods,
monoclinic structure), and different crystal phases possess different surface Ce4+ amount and
acidity. A well linear correlation is found between Lewis acid contents and the surface Ce4+
amount among these CP catalysts, and a very well linearity is also displayed between Lewis
acid amount and conversion of glucose or selectivity of HMF, which indicates Lewis acid
plays an important role in the dehydration of glucose to HMF. CP120 with hexagonal crystal
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structure exhibits the best catalytic activity (conversion 97% of glucose and yield 61% of
HMF) due to the maximum amount of Lewis acid and total acid.
Keywords: Cerous Phosphates; Ce4+ Lewis acid; hexagonal structure; dehydration of glucose;
5ꢀ(hydroxymethyl)fulural
Introduction
As the decline of fossil resource and the concern of CO2 emission, the utilization of
renewable sources such as solar, nuclear, wind energy and biomass resource, have attached
extensive interests.1ꢀ4
Biomass consist of cellulose, lignose, starch, glucose and fructose have
been wildly studied due to the advantage of wide distribution, huge production, renewable,
nonꢀpollution5,6 and prospective application values.7,8 5ꢀ(hydroxymethyl)furfural (HMF), as a
sort of green platform molecule derived from biomass, is recognized as a feedstock for the
production of advanced chemicals and has been applied in numerous industrial processes. For
instance, 2,5ꢀDimethylfuran from catalytic hydrogenation of 5ꢀHMF, can be used as the
additives of automotive liquid biofuel, 2,5ꢀFurandicarboxylic acid from oxidation of 5ꢀHMF
was also considered an important valuable chemical as the substitution of terephthalic acid.9
Presently, HMF was mainly produced from the catalytic dehydration of fructose and
glucose.1ꢀ5,10,11 High HMF yield transformation of fructose has been reported under aqueous
phase, organic solvents phase and water/organic mixture phase.2,10 Unfortunately, the
transformation of fructose was not suitable for large industrial application due to its high cost
even with superior yield of HMF.12,13 Glucose offered another alternation as its low cost and
easy obtaining, in which glucose firstly was converted to fructose through a isomerization
process,11,14 then followed conversion to HMF by the dehydration process (Scheme 1).15
2

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Many attempts have been applied to find better catalysts for this reaction,16 for example
11
H2SO4 , HCl17, metallic oxide14 and chromium chloride18 were employed, however,
inorganic liquid acid catalysts (H2SO4 and HCl) leaded to the increase of humins formed from
direct dehydration of glucose and lower selectivity to HMF.11,17 In recent years, solid acid
catalysts have been applied in catalytic transformation of glucose to HMF. In a study by
Zhang and coꢀworkers, amorphous catalysts of Cr2O3, SnO2, SrO and grapheme oxideꢀferric
oxide were used to convert glucose into HMF, however, the metal oxide catalysts fully lost the
catalytic activities after calcinations.14 Zhang et al. also reported the conversion of glucose to
HMF over hydroxyapatite supported chromium chloride, but the yield was below 40%.18
Another study by Jadhav et al. focused on application of zeolite in ionic liquid for producing
HMF from Dꢀglucose, but the ionic liquids they used were harmful and expensive.19
Therefore, designing a high yield and low cost catalyst was considered as a much more
important aspect in the conversion of glucose to HMF. Reported work by Ronen et al. studied
the formation of fructose via isomerization mainly from Lewis sites and HMF from both
Brønsted and Lewis sites.20 Alam and coꢀworkers reported titanium hydrogenphosphate (TiHP)
material consisting of both Brønsted and Lewis acid sites, they proposed Brønsted acid sites
could accelerate the produce of furan ring opening product when the temperature turned much
higher.21 Therefore, a catalyst with combination of both Lewis acid (for isomerization) and the
subsequent Br nsted acid (for dehydration configuration) may be more prospectively for
Ø
glucose dehydration to HMF.21ꢀ24
The newly metal phosphates20,25ꢀ27 were proposed in this reaction, in which metal could
provide Lewis acid sites and Brønsted acid sites could obtain from geminal P(OH) groups
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possibly.28 A series of metal Zr/Sn(IV) phosphate catalysts with modified surface acid
distribution were synthesized, which exhibited improved catalytic performance due to higher
amounts of polyphosphate species.20 Another cerium(IV) phosphates in this reaction were also
reported but showed a lower yield (23%–24%) and high stable performance even after several
cycles’ reaction.27 As the perspective of cerium phosphate species in the glucose to HMF,
much more work still need to do to improve their application.
In the present work, a series of CePO4 catalysts with different crystalline types have been
synthesized by a convenient hydrothermal method at different temperatures and their activity
have been carried out for dehydration of glucose to HMF. The relationships about total acid
amount, Br nsted and Lewis acids to yield and conversion have also been studied. Meanwhile,
Ø
various reaction conditions, such as reaction temperature, time, loading of catalyst, amount of
reaction medium and reusability have also been evaluated to find an optimal reaction
conditions for the HMF production.
Experimental
Catalysts preparation
CePO4 (CP) catalysts were prepared by a hydrothermal method. In a typical experiment,
Ce(NO3)3 (716 mg) and NaH2PO4 (869 mg) with mole ratio of 1:1 were dissolved in 20 ml
diluted water, respectively, and mixed these two solutions and stirred for 1.5 h, and then
transferred into a Teflon reactor with stainless autoclave. The above stainless autoclave was
o
heated at different temperatures (120, 140, 160, 180 and 200 C) for 8 h in the oven, then
cooled to room temperature. The resulting solid was filtered and washed with diluted water
o
for several times, following dried at 100 C for 12 h. The obtained catalysts were denoted as
4

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CP120, CP140, CP160, CP180 and CP200 according to the hydrothermal temperature.
Characterization of catalysts
Xꢀray diffraction (XRD) patterns of catalysts were evaluated using a D/maxꢀIIIB
diffractometer with a Cukα radiation (λ =1.5418 Å) combined with a Niꢀfilter. The
morphology of the CP catalysts was investigated by Scanning Electron Microscopy (SEM)
(Hitachi Sꢀ4800) with an electron gun working at an accelerating voltage of 5 kV. The textural
parameters were evaluated from the N2 physisorption using Micromeritics Tristar II analyzer
o
at ꢀ196 C. Xꢀray photoelectron spectra (XPS) were performed on a KratosꢀAXIS ULTRA
DLD with an Al Kα radiation source. Binding energies (BE) were referenced to the C (1s)
binding energy of carbon taken to be 284.7 eV.
Temperatureꢀprogrammed desorption of ammonia (NH3ꢀTPD) was carried out in a full
automatic adsorption instrument (XQ TPꢀ5080, China) with a thermal conductibility detector
(TCD) and performed in the following procedure. A fixed catalyst bed with 100 mg sample
was enclosed in a quartz tube and fixed by two balls of quartzꢀwool. Firstly, the sample was
o
heated from room temperature to 100 C with a heating rate of 10 oC/min under He flow (30
ml/min), and then maintained at this temperature for 60 min. the sample was cooled to room
temperature, switched to NH3 flow (30 ml/min) for 1 h. Finally, the sample was heated to 700
oC with a rate of 10 oC/min under He flow (30 ml/min).
In situ DRIFTS experiments were performed on a Nicolet 6700 spectrometer with in situ
diffuse reflectance pool and high sensitivity MCT detector which cooled by liquid N2. The
catalyst was loaded in the Harrick IR cell and heated to 100 oC under N2 flow (40 ml/min) for
60 min to remove adsorbed impurities, and then cooled to room temperature. The background
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spectrum was collected under a N2 atmosphere and was subtracted from the sample spectra.
When the adsorption of NH3 on the sample was investigated, the spectra were recorded with
N2 background. The DRIFTS spectra were recorded by accumulating 32 scans with a
resolution of 4 cmꢀ1. The total flow rate of the feeding gas was kept 40 mL/min (20 ml/min N2,
20 ml/min NH3).
Catalytic activity measurement
The dehydration reaction of glucose was performed with 50 mg catalyst, 50ꢀ250 mg glucose
and 5ꢀ25 ml dimethylsulfoxide (DMSO) in a 100 ml threeꢀnecked flask coated with oil bath at
130ꢀ170 oC for 15ꢀ150 min under N2 atmosphere and stirring condition. After the reaction, the
mixture was cooled and centrifuged. The obtained liquid was analyzed by high performance
liquid chromatography (HPLC). The quantitative analysis of glucose was monitored using
HPLC (Shimadzu 20A) equipped with a refractive index detector and a BioꢀRad Aminex
HPXꢀ87H column. 5ꢀHMF was analyzed using HPLC (DIONEX Ultimate 3000) with an
InterSustain C18 column and a 284 nm UV detector.
Results and discussion
XRD characterization
Fig. 1 showed the XRD patterns of the CP catalysts synthesized at different hydrothermal
temperatures. For CP120, the diffraction peaks at 2θ of 14.3, 20.0, 25.4, 29.1, 31.4, 37.5, 41.9,
o
48.2, 53.3 were ascribed to the hexagonal crystalline structure of CePO4 in Fig. 1A, which
match well with the standard data of JCPDS 34ꢀ1380. However, with the increase in synthesis
temperature, the intensity of peaks at 14.3 and 20.0 o indexed to (100) and (101) crystal plane
of CePO4 with hexagonal structure29, respectively, gradually reduced and the former one
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diminished in CP160, then these two peaks generally disappeared in CP180 and entirely
vanished in CP200. Meanwhile, the new peaks at 17.1, 18.9, 21.3 27.0, 34.4 and 41.1o were
observed in CP160 (Fig. 1C), which were attributed to (101) (011) (111) (200) (202) and (103)
planes of CePO4 with monoclinic crystalline phase (JCPDS 32ꢀ0199), respectively. This
crystalline phase dominated fully in CP200 sample.30 This means the crystal phase has been
transformed to an utterly monoclinic crystal phase for CP200 sample. It was interesting to
note that CP120 exhibited a hexagonal structure and CP200 presented the monoclinic phase,
while CP160, CP180 and CP200 demonstrated a mixture structure of these two crystal phase.
It reveals the change of hydrothermal temperature can lead to transformation of different
crystal phases.
N2 adsorption-desorption measurement
The textural properties of the CP catalysts derived from the nitrogen physisorption are
described in Fig. 2a and summarized in Table 1, the pore distributions are also displayed in
Fig. 2b. It can be seen that the CP catalysts exhibited type IV isotherms with a H3 hysteresis
loop at 0.4<p/p0<0.9 according to the IUPAC classification,31 which mean the formation of
intercrystal mesopores associated with aggregates of particles in the CP samples.32,33 It was
also noteworthy that pore volumes decreased from 0.39 cm3/g to 0.26 cm3/g (Table 1, Fig. 2).
From Fig. 2b, the pore diameter of CP120 was about 25 nm (Fig. 2bꢀA), pore diameter turned
gradually wider with the increase in hydrothermal temperature, in which the value of that was
more than 46 nm, along with a new pore diameter of 101 nm in the CP200. In addition, the
BET surface area was 108, 88, 82, 65 and 54 m2 /g for CP120, CP140, CP160, CP180 and
CP200, respectively (Table 1), which reduces with the increase in the synthesis temperature. It
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results from the change of crystal phase from hexagonal structure to monoclinic phase (Fig.
1).
SEM images
The morphology of the CP catalysts was characterized by SEM and shown in Fig. 3. It can
be seen easily that CP120 displayed a moderate aggregation and partial accumulation of
mesosphere structure with particles dimension at 30ꢀ60 nm. CP200 denoted obvious nanorod
morphology with the length of 100ꢀ300 nm and diameter of 50 nm, while the nanorods
formed more cumulated pore structure than that of CP120. For CP160, mixture morphology
of relative irregular nanosphere and nanorod was observed. As the variation of synthesis
temperature, the morphology of the CP catalysts gradually converted from nanosphere
(CP120) to nanorod shape (CP200), and others exhibited the irregular morphology due to
their mixture of hexagonal and monoclinic crystalline phases.
NH3-TPD measurement
The acidity of the CP catalysts was measured through NH3ꢀTPD as the result shown in Fig.
o
4. CP120 and CP140 catalysts exhibited a serials of desorption peaks in range of 50ꢀ350 C,
which were attributed to the weak NH3 adsorption.23 These peaks demonstrate that there are
various surface weak acid sites in CP120 and CP140. It was noteworthy that a small NH3
o
desorption peak around 450 C can be observed for CP140, which was assigned to the strong
o
acid sites.23 While the desorption peaks around 50ꢀ350 C became very weak for CP160,
CP180 and CP200, to the contrary, the peak at about 500 oC attributed to the strong acid sites
strengthened. The total amount of the acid sites were calculated by NH3ꢀTPD which was 1.67,
1.39, 1.29, 1.21 and 0.79 mmol/g for CP120, CP140, CP160, CP180 and CP200, respectively.
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It demonstrates that the total acid amount decreases with increasing the hydrothermal
temperature, and CP120 possesses the most total acid amount assigned to weak acidity sites
among all these catalysts. These results can be attributed to the full transformation from
hexagonal to monoclinic phases. Consequently, it is suggested that CP catalyst with
monoclinic crystalline phase may provide strong acid site, while in another aspect, weak acid
is mainly formed in hexagonal crystalline structure.
NH3-DRIFT measurement
As we know, Br
measurement. Thus, DRIFT spectra of NH3 absorbed on the surface acid site of the CP
catalysts were performed in order to obtain the amount of Br nsted and Lewis acid. Fig. 5
Ø
nsted and Lewis acid sites can not be distinguished clearly by NH3ꢀTPD
Ø
shows the IR spectra of the CP catalysts after NH3 adsorption. The strong band at 1690 cmꢀ1
+
was observed for the CP catalysts, which was assigned to the NH4 bending vibration on
Br
nsted acid site,34,35 elsewhere, two weaker ones at 1221 and 1578 cmꢀ1 were detected
Ø
which were attributed to the asymmetric and symmetric bending vibrations of coordinated
NH3 on Lewis acid site.36ꢀ38 Combined the total acid amount form NH3ꢀTPD (Table 1), the
amount of Br
Ønsted and Lewis acid can be calculated according to the peak area ratio of
BrØ
nsted acid site (1690 cmꢀ1) to total Lewis acid sites (1221 and 1578 cmꢀ1). The results
illustrate the amount of Lewis acid is 0.93, 0.69, 0.47, 0.31 and 0.28 mmol/g for CP120,
CP140, CP160, CP180 and CP200, respectively, which exhibits a declining tendency from
CP120 to CP200 with the increase in synthesized temperature. This can be ascribed to the
hexagonal and monoclinic crystalline structure in CP120 and CP200, respectively. However,
the amount of Br
Ø
nsted acid for these catalysts do not exhibit obvious regularity, there is a
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decrease of Br nsted acid amount in CP140ꢀCP180 catalysts (0.70ꢀ0.90 mmol/g). As we have
Ø
known from XRD results, CP catalysts with different hydrothermal synthesis temperatures
exhibit different crystallite types. CP140, CP160 and CP180 exhibit a mixture of two
crystalline structures, which causes an unstable Br nsted acid site due to different surface
Ø
properties. Moreover, IR spectra of CP120ꢀCP140 and CP160ꢀCP200 demonstrated different
surface acid strength, which is in accordance with the result from NH3ꢀTPD (Fig. 4).
XPS measurement
The CP catalysts were further analyzed by XPS to identify the surface nature and the results
were shown in Fig. 6. The complex spectra of Ce3d were decomposed into eight components
by GaussianꢀLorentz fitting procedure (Fig. 6). The subꢀbands labeled v1 and u1 represented
the 3d104f1 initial electronic state of Ce3+, and the subꢀbands labeled u0, u2, u3, v0, v2 and v3
represent the 3d104f0 state of Ce4+.39,40 The relative abundance of the Ce4+ (%) species of each
sample was estimated by considering the deconvolution peak area of Ce3d binding energies.
In Table 1, corresponding to the Ce4+ amount of these catalysts from CP120 to CP200 were
78%, 65%, 51%, 37% and 33%, respectively. As the synthesis temperature increased, the
content of Ce4+ gradually reduced along from CP120 to CP200. As it has been reported Ce4+
provides Lewis acid site,36 which means the decrease in the Ce4+ amount can lead to the
decrease in the amount of Lewis acid site. Thus, CP120 shows the maximum Lewis acid
amount that also certifies the DRIFT results. Fig. 7 gives a plot of Lewis acid amount (Table 1)
versus the Ce4+ amount (Table 1) for the CP catalysts. It is interesting that a striking linearity
is displayed between the Lewis acid amount and the Ce4+ amount of the CP catalysts (Fig.
7A). It was found that Lewis acid amounts decreased with the BET surface area (Table 1).
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Moreover, a near linearity relation was observed between Lewis acid amounts and BET
surface area for CP catalysts (Fig. S1). Considering the effect of surface area, Lewis acid
amounts on the basis of surface area were calculated for CP catalysts and shown in Table 1.
Plot of surface Ce4+ amount versus Lewis acid amount on the basis of surface area (Table 1)
for the CP catalysts were given (Fig. 7B). It was also found a linearity relation between
surface Ce4+ amount and Lewis acid amount on the basis of surface area, which suggests that
surface area plays a positive influence on the Lewis acid amount. Thus, the result proves that
Lewis acid is related to the surface Ce4+ of the CP catalysts, and its amount is proportional to
the content of surface Ce4+.
Catalytic performance
The catalytic performances of the CP catalysts were investigated for the dehydration of
glucose to HMF displayed in Fig. 8. It was observed a lower than 1% conversion of glucose
in this reaction without catalyst addition, which indicated a superior stability of glucose at this
condition.41 From the Fig. 8, the conversion of glucose was 97%, 90%, 84%, 80% and 78%
for CP120, CP140, CP160, CP180 and CP200, respectively, corresponding yields of HMF for
these CP catalysts were 61%, 52%, 45%, 41% and 40%, respectively. It is easy to see that the
yield of HMF and conversion of glucose decrease with the increase in synthesis temperature
from CP120 to CP200. Among these CP catalysts, the CP120 catalyst has a preferable
catalytic performance of conversion to glucose (97%) and yield to HMF (61%), which is
2ꢀ
better than most catalysts reported in literatures,12,20,23,42 such as SO4 /ZrO2,
2ꢀ
SO4 /ZrO2ꢀAl2O3, niobia/carbon, aluminium doped MCMꢀ41 silica and solid metal (IV)
phosphate. JiménezꢀLópez and coꢀworkers reported the glucose conversion of 69 % and HMF
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yield of 23 % were achieved at 175 ºC over mesoporous tantalum oxide as catalyst.12
Dibenedetto et al. used the same elements (Ce, P, O) to prepare different material of complex
structures. These materials were used to catalyze fructose to HMF, the best yield to HMF
(52%) was obtained, and the deactivation phenomenon was also observed.27
As we have shown in Fig. 1, the structure of the CP catalysts changed from hexagonal
(CP120) to monoclinic (CP200) with increase in synthesis temperature, along with the
different catalytic performances. It indicates that the crystalline structure plays a noticeable
influence on catalytic performance of the CP catalysts, which can be ascribed to the change of
surface acidity with the modification of catalyst structure from hexagonal to monoclinic phase.
Generally, the dehydration of glucose to HMF contains two stages: firstly, isomerization of
glucose to fructose under Lewis acid sites, and then fructose dehydration to HMF over
Br
nsted acid.23,24 In order to study the stage, fructose dehydration to HMF was carried out
Ø
over CP120 for 5, 10 and 15 min (Fig. S1). When the reaction time was 5 min, the conversion
to fructose and selectivity to HMF could get about 93% and 49%, respectively. As the time
increased to 10 min, the fructose almost converted completely, the conversion to fructose was
97%, corresponding 55% of selectivity to HMF. From the results, it can be found that the
stage of fructose dehydration to HMF was very fast. In our previous report, it also confirmed
that the dehydration of fructose to HMF was very fast over the carbonꢀbased solid acid
catalyst.43 Moreover, compared it with glucose dehydration to HMF, the conversion to glucose
and selectivity to HMF for the reaction time of 15 min under the same reaction conditions
were only 66% and 40% (Fig. S2), respectively, which suggest that the reaction rate of
glucose dehydration to HMF is slower than that of fructose dehydration to HMF. So, the
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glucose isomerization to fructose is considered as the key rate determining step in the glucose
dehydration to HMF. While, the isomerization of glucose to fructose is sensitive to Lewis acid
in the initial stage of the glucose dehydration to HMF, so the modification of catalyst acidity
will be very important for the reaction.
As it was mentioned, the structures of the CP catalysts changed from hexagonal (CP120) to
monoclinic (CP200) phase, simultaneously, the crystal morphologies were converted from
nanosphere (CP120) to nanorod (CP200) shape in accordance with the decrease of BET
surface area. The CP catalysts exhibited specific surface acid sites of Lewis and Br nsted
Ø
(Table 1). The Lewis acid of CP120 took up the highest content among all CP catalysts from
Fig. 5 and Table 1, which showed somehow the best activity. From CP140 to CP200, Lewis
acid amounts gradually reduced, they also displayed lower and lower activity for the
dehydration of glucose with decrease in the amount of Lewis acid, even they had a large
amount of Br nsted acid site. It is interesting that a striking linearity relation can be found
Ø
between the conversion of glucose and the amount of Lewis acid for the CP catalysts (Fig.
9A). The relationship of selectivity to HMF versus the amount of Lewis acid also
demonstrates the same situation as above described results (Fig. 9A). Considering the effect
of surface area, plots of conversion to glucose and selectivity to 5ꢀHMF versus Lewis acid
amount on the basis of surface area (Table 1) for the CP catalysts were given, respectively
(Fig. 9B). It was also found a very well linearity relation between conversion to glucose
(selectivity to 5ꢀHMF) and Lewis acid amount on the basis of surface area. These results can
prove that the surface Lewis acid plays an important role in the dehydration of glucose to
HMF.11,14 It also confirms that the isomerization of glucose to fructose is key stage in this
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reaction. In addition, Fig. 9B results also reveal positive influence of surface area on the
catalytic activity of CP catalysts. Thus, the highest conversion (97%) and selectivity (63%) in
CP120 was achieved, which should be accounted for the most amounts of Lewis acid sites
and the highest surface area. But glucose cannot convert completely to HMF, which should be
ascribed to the secondary processes of HMF rehydration process and condensation with
sugars into humins.44,45
More details of CP120 based on its excellent catalytic activity have been investigated in
order to understand the effects of various experimental parameters to the reaction including
reaction temperature, time, DMSO amount and ratio of glucose to catalyst.
Effect of temperature: Fig. 10 shows the effect of different reaction temperatures on
catalytic activity. As illustrated in Fig. 10, a low HMF yield of 10% with the glucose
conversion of 32% and HMF selectivity of 32% was obtained at reaction temperature of 130
oC. The conversion to glucose increased quickly from 32% to 95% with rising of reaction
o
temperature from 130 C to 150 oC. 97% conversion to glucose and 61% yield to HMF were
o
obtained at 160 C. It indicates that rising temperature is beneficial to glucose dehydration to
o
HMF. However, when the temperature was increased further to 170 C, the yield and
selectivity of HMF gradually dropped to 60% accompanying slight increase in glucose
conversion (near 100%), meanwhile the reaction solution gradually converted into a more
dark color, which means formation of more humins41 that can cover on the active sites of
catalyst surface. Furthermore, the selectivity of HMF showed similar trends as its yield, a
o
o
better HMF selectivity of 63% was obtained at 160 C. So 160 C is much more proper than
other temperature for this reaction.
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Effect of catalytic time: The impact of reaction time on activity has been explored as is
shown in Fig. 11. The trend of activities has not shown a linearly increment by the extension
of reaction time. When the reaction time was 15 min, the conversion (66%) was lower, that is
maybe time is short and glucose cannot convert completely on the surface of catalyst. In the
range of 30ꢀ60 min, conversion (95%ꢀ97%) and selectivity (57%ꢀ63%) displayed a moderate
increase. When the reaction time increased to 90 and 120 min, tiny difference was obtained.
Obviously to see the reduction of catalytic performance growth rate happens with the
extension of reaction time. Apparently when the reaction time was longer than 60 min, it
showed less effect on the property compared with that of shorter one (30ꢀ60 min). This can be
attributed to the presence of products like humins which hinder the ongoing reaction process
by covering the catalyst surface active site,41 meanwhile, the longer time reaction shows
lower catalytic performance benefits. Consequently, 60 min is optimal time for this reaction.
Effect of the ratio of catalyst to glucose: The effect of the ratio of catalyst to glucose (g/g)
on activity is shown in Fig. 12. The amount of CP120 was 50 mg and dosage of glucose was
increased from 50 mg to 250 mg to change the ratio of catalyst to glucose from 1:1 to 1:5. It is
notable to see that the conversion was almost unchanged with the increase in the amount of
glucose, however, HMF selectivity showed obviously a different tendency. When the ratio of
catalyst to glucose decreased from 1:1 to 1:4, HMF selectivity gradually increased from 41%
to 62%, however, when the ratio was 1:5, the selectivity of HMF appeared to decrease rapidly
to 43%, which in turn also made HMF yield down to 41%. When the amount of catalyst is too
much, it will make an interaction between glucose and new generated HMF on catalyst
surface, which leads to lower HMF selectivity due to formation of the formic, levulinic acids
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and humins.41 But the less amount of catalyst cannot completely catalyze the reaction. From
the above result, the conclusion can be made like that optimum ratio of catalyst to glucose in
this reaction is 1:4.
Effect of DMSO amount: It was reported that DMSO had an effective influence on
improvement of HMF yield by their extractability to suppress undesired byꢀproduct and
sequentially increase the selectivity of 5ꢀHMF.46ꢀ49 Here, the effect of DMSO amount was
also investigated in this reaction as the results shown in Fig. 13. From the results, it is easy to
see that 5 ml solvent showed a negative effect toward glucose conversion which displayed a
minimum HMF yield (40%). When increasing the solvent amount to 10 ml, a obvious
increment of HMF yield (from 40% to 61%) was observed, however, further increasing the
amount of solvent did not improve reaction activity even cost the loss of HMF selectivity
from 62% to 46%, product yield decreased from 61% to 41%, all these results indicate that
much more solvent amount (the optimum is 10 ml as shown above) will reduce the contact
with the substrate and decrease the reactant concentration and in turn decreased the reaction
activity. Therefore, 10 ml DMSO has been taken for dehydration of glucose to HMF in this
study.
As the results are presented in Fig. 10, 11, 12 and 13, which suggest that CP120 catalyst
shows an excellent catalytic performance for glucose dehydration to 5ꢀhydroxymethylfurfural
o
in the condition of 50 mg catalyst, 200 mg glucose, 160 C , 60 min and 10 ml DMSO as
solvent.
Reusability: The stability of the CP120 catalyst was also tested based on its excellent
catalytic activity for glucose dehydration to 5ꢀHMF. After the reaction was completed and
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filtered, used CP120 was washed with methanol and DMSO, and then dried at 100 oC for 12 h,
labeled as CP120ꢀusedꢀ1 and CP120ꢀusedꢀ2 for first and second reused cycle, respectively.
The reusability test results are showed in Fig. 14. It can be seen that the yield of fresh CP120
was about 54% and the glucose conversion was 95%. After first and second cycle, the activity
reduced obviously. The yield of HMF decreased from 54% to 33%, indicating that the catalyst
partially deactivated during the recycling process. In this reusability reaction, the activity of
CP120ꢀusedꢀ1 declined much more than that of CP120ꢀusedꢀ2. After first reused cycle,
CP120ꢀusedꢀ1 demonstrated 14% lower yield of HMF than the fresh CP120. The glucose
conversion decreased from 85% to 75% was founded in the second used cycle, in which the
selectivity of HMF only showed a tiny decrease from 47% to 45%, which indicated that the
deactivation was not obvious after the first used cycle. XRD pattern of CP120ꢀusedꢀ1 (Fig. S3)
showed 2θ sites of the diffraction peaks were not obvious alteration and only displayed a
decrease in crystalline compared with CP120, indicating the hexagonal structure of
CP120ꢀusedꢀ1 was still maintained. Moreover, a reaction solution was obtained by hot
filtration, and glucose was added into the above solution to assess if any leaching of active
catalytic species into the solution. The result indicates that there is no activity in this reaction
solution at the same conditions. So the active catalytic species are not leach into the reaction
solution. In addition, it was noticed that the used CPꢀ120 color changed into dark due to
formation of byꢀproduct humans that can cover up of surface acid site leading to catalyst
deactivation according to literatures.41,44,45
Conclusions
A series of CePO4 catalysts with hexagonal and/or monoclinic CePO4 crystal types have
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been successfully synthesized by hydrothermal method. The increase in hydrothermal
temperature exchange catalyst crystal phase gradually from hexagonal to monoclinic, which
lead to change of BET surface area and acidity. Catalytic performance combined with
characterization results suggest that surface acid strength and the distribution of different acid
site type influence the catalyst activity significantly. The amount of Lewis acid based on
either catalyst mass or surface area increases with the content of surface Ce4+ among these CP
catalysts, which shows a striking linearity between them. A very well linearity is also
displayed between Lewis acid amount based on either catalyst mass or surface area and
conversion of glucose and selectivity of HMF. Therefore, we put forward to Lewis acid plays
a key role in dehydration of glucose to HMF, which is derived from the surface Ce4+ of the CP
catalysts. The best catalytic performance is obtained over CP120 nanoparticles with 30ꢀ60 nm
in the dehydration of glucose to HMF, which exhibits a HMF yield of 61% at the glucose
conversion of 97% using 10 ml DMSO as solvent, 50 mg catalyst and 200 mg glucose at 160
oC for 60 min. It is attributed to the most amount of Lewis acid and the highest BET surface
area among these CP catalysts.
Acknowledgements
This work was supported by Natural Sciences Fund of Heilongjiang Province (B2015009),
Postdoctoral Scienceꢀresearch Developmental Foundation of Heilongjiang Province of China
(LBHꢀQ12022), Program for Innovative Research Team in University (IRTꢀ1237), the
Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education
Ministry (2013ꢀ1792) and Ministry of Human Resources and Social Security (2013ꢀ277),
Innovative Research Project of Key Laboratory of Functional Inorganic Material Chemistry,
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Ministry of Education.
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Table 1 physicochemical properties of the CP catalysts
Lewis acid
amount
(ꢁmol/m2) c
NH3 adsorption (mmol/g) b
Ce4+/(Ce3++Ce4+) d
Total Acidity
(mmol/g) a
Catalyst SA(m2/g) PV(cm3/g)
(%)
Br
Ø
nsted (B)
0.74
Lewis (L)
0.93
CP120
CP140
CP160
CP180
CP200
108
88
82
65
54
0.39
0.39
0.36
0.30
0.26
1.67
1.39
1.29
1.21
0.79
8.6
7.8
5.7
4.8
5.2
78
65
51
37
33
0.70
0.69
0.82
0.47
0.90
0.31
0.51
0.28
a. Calculated from the NH3ꢀTPD
b. Calculated from the NH3ꢀTPD and DRIFT based on catalyst mass
c. Lewis acid amount based on special surface area
d. Calculated from the XPS
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Scheme 1 reaction pathway for glucose to HMF
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Figures Captions
Fig. 1 XRD patterns of the CP catalysts (A: CP120, B: CP140, C: CP160, D: CP180, E: CP200)
Fig. 2 N2 adsorptionꢀdesorption isothermals (a) and pore distribution (b) of the CP catalysts (A: CP120, B:
CP140, C: CP160, D: CP180, E: CP200)
Fig. 3 SEM images of A: CP120, B: CP160, C: CP200
Fig. 5 DRIFT spectra of the CP catalysts (A: CP120, B: CP140, C: CP160, D: CP180, E: CP200)
Fig. 6 XPS of the CP catalysts (A: CP120, B: CP140, C: CP160, D: CP180, E: CP200)
Fig. 7 plots of A: Lewis acid amount based on catalyst mass (mmol/g) and B: Lewis acid amount based on
surface area (ꢁmol/m2) versus Ce4+ amount for the CP catalysts
Fig. 8 activity of the dehydration of glucose over the CP catalysts (
HMF yield,
glucose
conversion,
HMF selectivity; Reaction conditions: Glucose: 200 mg, catalyst: 50 mg, solvent: 10 ml
DMSO, temperature: 160 oC)
Fig. 9 plots of conversion to glucose and selectivity to 5ꢀHMF versus A: Lewis acid amount based on
catalyst mass (mmol/g) and B: Lewis acid amount base on surface area (ꢁmol/m2) for the CP catalysts
Fig. 10 effect of temperature on the dehydration of glucose over CP120 (
conversion,
HMF yield,
glucose
HMF selectivity; Reaction conditions: Glucose: 200 mg, catalyst: 50 mg, solvent: 10 ml
DMSO, time: 30 min)
Fig. 11 effect of reaction time on the dehydration of glucose over CP120 (
HMF yield,
glucose
conversion,
HMF selectivity; Reaction conditions: Glucose: 200 mg, catalyst: 50 mg, solvent: 10 ml
DMSO, temperature: 160 oC)
Fig. 12 effect of the ratio of catalyst to glucose (g/g) on the dehydration of glucose over CP120 (
HMF
yield,
glucose conversion,
HMF selectivity; Reaction conditions: Glucose: 200 mg, solvent: 10
ml DMSO, temperature: 160 oC)
Fig. 13 effect of the solvent amount on the dehydration of glucose over CP120 (
HMF yield,
glucose conversion,
HMF selectivity; Reaction conditions: Glucose: 200 mg, catalyst: 50 mg,
DMSO, temperature: 160 oC)
Fig. 14 reusability on the dehydration of glucose over CP120 (
HMF yield,
glucose conversion,
HMF selectivity; Reaction conditions: Glucose: 200 mg, catalyst: 50 mg, solvent: 10 ml DMSO, time:
30 min temperature: 160 oC)
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Fig. 1 XRD patterns of the CP catalysts (A: CP120, B: CP140, C: CP160, D: CP180, E: CP200)
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Fig. 2 N2 adsorptionꢀdesorption isothermals (a) and pore distribution (b) of the CP catalysts (A: CP120, B:
CP140, C: CP160, D: CP180, E: CP200)
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C
A
B
500 nm
500 nm
500 nm
Fig. 3 SEM images of A: CP120, B: CP160, C: CP200
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Fig. 4 NH3ꢀTPD profiles of the CP catalysts (A: CP120, B: CP140, C: CP160, D: CP180, E: CP200)
29