Direct conversion of carbohydrates to 5-hydroxymethylfurfural using Sn-Mont catalyst

Article abstract of DOI:10.1039/c2gc35699f

5-Hydroxymethylfurfural (HMF) is a very important intermediate in the fine chemical industry. This study aims to investigate the direct conversion of glucose or glucose-based carbohydrates, such as sucrose, cellobiose, inulin, starch and cellulose, to HMF by using a Sn-Mont catalyst. With the use of this catalyst, a HMF yield of 53.5% was achieved from glucose in a mono-phase medium of tetrahydrofuran (THF)/dimethylsulfoxide (DMSO) at 160 °C for 3 h. The success of one-step conversion of glucose to HMF is attributed to the Sn-Mont catalyst containing two types of acid sites, Lewis acid and Bronsted acid sites. The former one plays a crucial role in the isomerization of glucose to fructose and the latter one is active in the dehydration of generated fructose to HMF. Furthermore, Sn-Mont catalyst also demonstrated excellent activity in the conversion of disaccharides and polysaccharides and as high as 39.1% HMF was directly obtained from cellulose in a THF/H₂O-NaCl bi-phasic system.

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PAPER
Direct conversion of carbohydrates to 5-hydroxymethylfurfural using
Sn-Mont catalyst
Jianjian Wang, Jiawen Ren,* Xiaohui Liu, Jinxu Xi, Qineng Xia, Yanhong Zu, Guanzhong Lu and
Yanqin Wang*
Received 8th May 2012, Accepted 14th June 2012
DOI: 10.1039/c2gc35699f
5-Hydroxymethylfurfural (HMF) is a very important intermediate in the fine chemical industry. This study
aims to investigate the direct conversion of glucose or glucose-based carbohydrates, such as sucrose,
cellobiose, inulin, starch and cellulose, to HMF by using a Sn-Mont catalyst. With the use of this catalyst,
a HMF yield of 53.5% was achieved from glucose in a mono-phase medium of tetrahydrofuran (THF)/
dimethylsulfoxide (DMSO) at 160 °C for 3 h. The success of one-step conversion of glucose to HMF is
attributed to the Sn-Mont catalyst containing two types of acid sites, Lewis acid and Brønsted acid sites.
The former one plays a crucial role in the isomerization of glucose to fructose and the latter one is active
in the dehydration of generated fructose to HMF. Furthermore, Sn-Mont catalyst also demonstrated
excellent activity in the conversion of disaccharides and polysaccharides and as high as 39.1% HMF was
directly obtained from cellulose in a THF/H2O–NaCl bi-phasic system.
1. Introduction
The conversion of sustainable biomass resources is currently
regarded as the most interesting processes for the production of
fuels and chemicals to satisfy the anxious need of our industrial
society. 5-Hydroxymethylfurfural (HMF), which is one of the
Scheme 1 Schematic representation of the glucose isomerization to
fructose using enzyme, Lewis acid or base catalysts and the fructose
dehydration to HMF with acid catalysts.
most versatile and important building blocks obtainable from
biomass, is now considered as a promising candidate for the pro-
duction of various non-petroleum valuable chemicals and fuels
through specific catalytic transformation.1–5
Traditionally, HMF is produced by acid-catalyzed dehydration
of mono-saccharides, such as fructose or glucose. Since the ring
structure of glucose is more stable than that of fructose,1b much
more satisfying results are obtained from fructose as the reactant
catalyzed by homogeneous or heterogeneous acid catalysts.
Dumesic et al. has reported that about 70% yield of HMF was
obtained from fructose in a biphasic system catalyzed by hydro-
chloric acid.3a As we know, homogenous acids are inexpensive
and act as high activity for the acid-catalyzed dehydration reac-
tions, but they have the drawbacks in terms of separation and
recycling in addition to the equipment corrosion. Shimizu et al.
reported that 100% HMF yield can be obtained in DMSO with
improved removal of generated water catalyzed by Amberlyst-
15.4b But, unfortunately, these acidic resins had low thermal
stability due to their organic frameworks and were normally used
below 130 °C, which partially limited their applications. Very
recently, as high as 91.2% HMF yield was obtained by using a
carbon-based catalyst in our group and this material also showed
good stability and reusability.6 However, the most important
industrial method for fructose production is by the enzymatic
isomerization of glucose syrup, a process yielding only about
50% fructose.7,8 So if glucose or glucose-based carbohydrate
was directly used as the raw reactant, the industrial process for
the production of HMF would be much more economical and
beneficial.
The challenge of using glucose as a raw material is the stab-
ility of the glucose ring, which makes the dehydration processing
much more difficult. It has been reported that the conversion of
glucose into HMF could proceed in two steps: the first isomeri-
zation of glucose to fructose in the presence of enzyme, Lewis
acid or base catalysts (Scheme 1),9–12 followed by the dehy-
dration of fructose to HMF. Enzymatic process is highly selec-
tive, but it has several disadvantages including the use of
buffering solutions to maintain pH, narrow operating tempera-
tures, strict feed purification requirements, and periodic replace-
ment of the enzyme due to irreversible deactivation.8,12a,13 On
the other hand, combination of solid acid and base in a single
high-boiling-point solvent, realizing the goal of glucose dehy-
dration to HMF,10,11 would make this process much more
Key Lab for Advanced Materials, Research Institute of Industrial
Catalysis, East China University of Science and Technology, Shanghai,
200237, P. R. China. E-mail: wangyanqin@ecust.edu.cn,
cutecube@ecust.edu.cn; Fax: +86-21-64253824
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complicated and greatly increase the cost of the separation of
HMF from the solvent.3,14
clay was washed with water until the washing water was neutral
(pH = 7). Finally, Sn-Mont was dried at 100 °C overnight, fol-
lowed by being ground to powder in a mortar with a pestle and
stored in a glass bottle.
The common Lewis acid, CrCl2/CrCl3 or SnCl4 has shown
good activity for the conversion of glucose into HMF in ionic
liquid solvent and the yield of HMF was in the range of 60% to
70%.15a,b But the chromium ion is toxic and the process for the
recycling of SnCl4 is still a problem. Alternatively, a hetero-
geneous catalyst containing Sn4+ ions would be a good choice to
convert glucose-based carbohydrates to HMF. Davis et al. found
that Sn-beta could efficiently catalyze the isomerization of
glucose to fructose and then convert the generated fructose to
HMF rapidly with hydrochloric acid at low pH.8,15c Sn4+ ions as
Lewis acids were active for the glucose isomerization to fructose
and the partially hydrolyzed Sn–OH groups were acted as
Brønsted acid for the generated fructose dehydration, and this
analysis was also in accordance with Corma’s reports in the
MPV reaction.16,17 Recent NMR studies have showed that Sn-
beta isomerizes glucose to fructose via 1,2-intra-hydride transfer
with an intermediate of 3-deoxy-glucosone.12b However, for the
synthesis of Sn-beta, a certain amount of hydrofluoric acid (HF)
must be used.8,14,15c HF is highly corrosive, noxious, easy to
volatilize and is very harmful to humans and the environment,
and so makes this synthetic process less eco-friendly. Besides,
single Sn-beta catalyst was only active for the isomerization of
glucose to fructose and an external acid catalyst, such as HCl or
Amberlyst-15, should be added to the reaction system to convert
the generated fructose to HMF.
In recent years, an abundant and cheap clay material
exchanged with Sn4+ ions was widely used in many organic
reactions, such as cyanosilylation of carbonyl compounds, ring-
opening polymerization of lactones, Baeyer–Villiger oxidation
reaction, and much satisfied results was obtained.18,19 However,
there were almost no reports about tin-ion-exchanged clay used
in the dehydration of carbohydrates. Herein, an Sn-Mont cata-
lyst, which was easily prepared via the ion-exchange of natural
calcium montmorillonite (Ca-Mont) with an aqueous tin tetra-
chloride solution, was used to investigate its activity in the con-
version of glucose, disaccharides, even polysaccharides to HMF.
2.3 Characterizations of the catalyst
XRD, EDX, SEM and BET characterization. X-ray powder
diffraction (XRD) patterns were recorded on a Bruker diffracto-
meter with Cu Kα radiation (λ = 1.5406 Å). The energy disper-
sive X-ray analysis (EDX) was employed for the quantitative
elemental analysis and measured on a Hitachi S-3400N SEM
coupled parallel to an energy dispersive X-ray spectrophotometer
(Bruker Quantax 200). The nitrogen adsorption experiment was
measured at 77 K on a NOVA 4200e sorption analyzer. Before
measurements, the samples were outgassed at 200 °C for 6 h.
Pyridine-adsorbed FT-IR characterization. The IR spectrum
of the pyridine-adsorbed Sn-Mont was obtained in the
transmission mode using a Nicolet Model 710 spectrometer.
Firstly, the catalyst (50 mg) was grounded into fine powders and
pressed into a very thin self-supporting wafer. The disc was
mounted in a quartz IR cell equipped with a CaF2 window and a
vacuum system. Prior to adsorption, the sample disc was pre-
treated in situ at 200 °C for about 2 h under evacuation, then
cooled to room temperature when pyridine vapor was introduced
into the cell for 0.5 h. The physically adsorbed pyridine was
removed by evacuating for 1 h and a spectrum was subsequently
recorded.
NH3-TPD characterization. Temperature-programmed de-
sorption of ammonia (NH3-TPD) was performed on an apparatus
PX200 (Tianjin Golden Eagle Technology Limited Corporation).
The sample (50 mg) was pretreated at 200 °C for 2 h and then
cooled to room temperature under a N2 flow. Pure NH3 was
injected until adsorption saturation was reached, followed by a
flow of N2 for 1 h. Then the temperature was raised from 90 to
500 °C with a heating rate of 10 °C min−1 and the amount of
desorbed ammonia was detected by using thermal conductivity
detector (TCD) at 110 °C. Besides, a blank measurement was
conducted without adsorption of ammonia and after deducted
the blank measurement, the NH3-TPD curve of Sn-Mont was
obtained.
2. Experimental
2.1 Materials
5-Hydroxymethylfurfural (99.5%) and levulinic acid were pur-
chased from Alfa Aesar Chemical Reagent Company. Amber-
lyst-15, inulin and cellulose were purchased from Aladdin
Reagent Limited Company. Calcium montmorillonite (Ca-Mont)
was purchased from Zhejiang San-Ding Technological Reagent
Limited Company (China). SnCl4·5H2O, SnO2, NaCl, fructose,
glucose, sucrose, cellobiose and starch was purchased from Sino-
pharm Chemical Reagent Co., Ltd (Shanghai, China). Tetra-
hydrofuran (THF), dimethyl formamide (DMF), dimethyl
sulfoxide (DMSO), dimethyl acetamide (DMA), N-methyl pyr-
rolidone (NMP) were chemical pure solvents and purchased
from shanghai chemicals company.
2.4 Dehydration of carbohydrates to HMF
The reactions were performed in a stainless steel autoclave
(50 mL) heated in a temperature-controlled oil bath. The mixture
of carbohydrates, solvent (monophase: 4.2 mL THF with 1.8 mL
DMSO; biphase: 5 mL organic phase with 1 mL H2O) and cata-
lyst (0.2 g) were put in the sealed autoclave equipped with a
magnetic stirring bar. N2 gas was used for purging air inside the
reactor and keeping a certain pressure to prevent boiling. After
reaction, the reactor was cooled to room temperature by loading
cooling water. The solid catalyst was removed by filtration, and
the filtrate was taken for analysis. The recovered Sn-Mont cata-
lyst was washed with water and ethanol, then dried at 100 °C for
the next run.
2.2 Catalyst preparation
In a typical synthesis, Ca-Mont was ion-exchanged with an
aqueous solution of SnCl4·5H2O at 50 °C for 24 h. The collected
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2.5 Product analysis
The analysis of the reaction products were carried out by means
of an HPLC apparatus (Agilent 1200 Series) equipped with an
XDB-C18 column (Eclipse USA) operated at 35 °C based on the
external standard. An auto-sampler (Agilent G1329A) was used
to enhance the reproducibility. HMF was quantified with an
ultraviolet detector (Agilent G1314B) at 282 nm. The eluent
with flow rate of 0.6 mL min−1 was a mixture of methanol to
water volumetric ratio of 20 : 80. The carbohydrates concen-
tration was analyzed with a refractive index detector (Agilent
G1362A). The eluent was pure water with a flow rate of
1.0 mL min−1. Conversion of glucose or fructose and yield of
HMF were defined as follows:
Fig. 1 NH3-TPD profile of the Sn-Mont catalyst.
Glucose or fructose conversion ¼ moles of glucose or
fructose reacted=moles of starting glucose or fructose
HMF yield ¼ moles of HMF produced=moles of
starting glucose or fructose
For disaccharides and polysaccharides, HMF yields were
defined as follows:
HMF yield ¼ moles of HMF produced=moles of
hexose units in poly ꢀ saccharides
3. Results and discussion
3.1 Characterization of catalyst
XRD, EDX, SEM and BET analysis. The X-ray powder
diffraction (XRD) patterns (not shown) confirmed that the struc-
ture of montmorillonite was maintained and no peaks of tin com-
pounds appeared, which means the ion-exchanging process had
no influence on the structure of then montmorillonite material.
Furthermore, the EDX analysis showed that no calcium atoms
were detected in the final Sn-Mont material, indicating that all of
the calcium ions were fully exchanged. The SEM image showed
that the prepared sample did not display well shaped crystalline
Fig. 2 Pyridine-adsorbed FT-IR spectra (pure power pressed discs) on
Sn-Mont catalyst.
Lewis and Brønsted acid sites.19a The results showed that
Sn-Mont exhibited both Brønsted acidity and Lewis acidity.
3.2 Determination of the suitable conditions for glucose
conversion in mixed organic solvent
particles and the BET surface area was 149.1 m2 g−1
.
Effect of solvent. The activity of Sn-Mont for the glucose
conversion in various solvents was investigated and the results
are presented in Fig. 3. From the results, we can see that the
Sn-Mont catalyst exhibited low activity in water, with the
glucose conversion of 40.2% and HMF yield of 6.5% at 160 °C
for 3 h. This is because water is not favorable for the dehydration
reaction and can promote the rehydration of HMF to by-
products, such as levulinic acid. Except water, good HMF yields
were obtained in organic solvents, such as in DMF, DMA, NMP
and DMSO. Especially in DMSO, the higher yield of HMF
(43.6%) was obtained because in which the furanoid form is pre-
ferred and the side reactions are remarkably suppressed.6,20
However, these solvents have high boiling-points (>150 °C) and
much energy should be inputted to separate HMF from the reac-
tion medium. On the other hand, a poor yield of HMF was
obtained from low the boiling-point solvent as well, due to the
poor solubility of glucose in pure THF. To solve this problem,
NH3-TPD analysis. The temperature programmed desorption
of ammonia was measured to characterise the acidity of the Sn-
Mont catalyst (Fig. 1). There are two desorption peaks, the low
temperature desorption peak indicated weak adsorption acidic
sites, and the peak at high temperature is attributed to the inter-
action of NH3 with moderate acid sites.
Pyridine-adsorbed FT-IR analysis. The type of acid on
Sn-Mont catalyst was determined by pyridine adsorption infrared
spectroscopy and the spectrum in 1390–1580 cm−1 range is
shown in Fig. 2. There are three bands in this region. The one at
about 1444 cm−1, arising from the C–C stretching vibration of a
coordinatively bonded pyridine complex, indicated the presence
of Lewis acid sites. The one at 1540 cm−1 is attributed to the
C–C stretching vibration of pyridium ion and has been used for
the identification of the Brønsted acid sites. Another peak at
about 1490 cm−1 was characteristic of pyridine adsorbed on both
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Fig. 3 Glucose conversion and HMF yield in diverse solvents by
Sn-Mont catalyst. (Reaction conditions: 6 mL reaction mixture, 5 wt%
glucose, 0.2 g Sn-Mont catalyst, 160 °C, 3 h.)
we tried to use a THF–DMSO mixed organic solvent system and
found the yield of HMF was increased with the increase of THF
percentage in DMSO. At the volume ratio of 70 : 30, the yield of
HMF reached 53.5% at 160 °C for 3 h, so this novel Sn-Mont
catalyst was very efficient to directly catalyze glucose dehydra-
tion to HMF in this THF–DMSO system without adding any
other Brønsted acid. Further increase the percentage of THF,
the solubility of glucose decreases rapidly. Therefore,
a
THF–DMSO mixture of 70 : 30 (v/v) was employed as reaction
media hereafter.
Fig. 4 Influence of the reaction temperature on HMF yield (a) and
glucose conversion (b). (Reaction conditions: 6 mL of reaction solution,
THF–DMSO = 70 : 30 v/v, 5 wt% glucose, 0.2 g Sn-Mont catalyst.)
Effect of reaction temperature with reaction time. Sub-
sequently, the influence of reaction temperature on HMF yield
and glucose conversion was investigated and the results are
shown in Fig. 4. At lower temperatures (100 °C and 120 °C), the
yield of HMF was very low and the conversion of glucose was
below 60%. Further increasing the reaction temperature to
140 °C, the activity of the Sn-Mont catalyst increased rapidly
and a higher HMF yield and glucose conversion were obtained.
As high as 53.5% yield of HMF was gained with 98.4% glucose
conversion at 160 °C for 3 h. In this study, reaction at higher
temperatures was not applied since higher temperatures will
cause high energy-consumption and difficulties in control.
Therefore, the reaction temperature of 160 °C was employed as
the reaction conditions hereafter.
Table 1 Summary of the glucose conversion and HMF yield catalyzed
by various heterogeneous catalystsa
Entry
Catalyst
Conversion (%)
Yield (%)
1
Sn-Mont
Sn-Mont
Sn-Mont
Sn-Mont
Amberlyst-15
Ca-Mont
No catalyst
SnO2
98.4
98.6
98.7
>99%
78.7
48.0
5.2
25.0
31.4
65.7
53.5
48.2
42.4
78.1
7.0
0
0
0
0
8.3
2b
3c
4d
5
6
7
8
9
SnCl4·5H2O
10
Sn-Monte
Effect of glucose concentration. Generally, the substrate con-
centration has a significant influence on the HMF yield for
heterogeneous catalytic reactions. Thus the effect of initial
glucose concentrations including 5 wt%, 7wt% and 10 wt% on
the dehydration of glucose to HMF was investigated (Table 1,
entries 1, 2 and 3). It can be seen that the glucose conversion
and HMF yield were partly affected by the initial concentration
of glucose. The highest HMF yield was 53.5% at 5 wt% of
initial glucose concentration. In addition, when the glucose con-
centration was increased to 10 wt%, the yield of HMF was still
high (42.4%), indicating the high efficiency of Sn-Mont catalyst.
The partial loss of HMF at high glucose concentration might be
attributed to the higher glucose concentration leading to higher
a Reaction conditions: 10 mL of reaction solution, THF–DMSO =
70 : 30 (v/v), 160 °C, 3 h. b ,c The initial concentration of glucose was
7 wt% and 10 wt%, respectively, and other default initial concentration
was 5 wt%. d Fructose was used as the reactant for 1 h. e Sn-Mont was
calcined at 600 °C for 4 h.
rates of side-reactions with the production of some undesired
by-products.3,21
Effect of the dose of Sn-Mont catalyst. Although the mass
ratio of Sn-Mont catalyst to glucose is 2 : 3 (m/m), the reaction
can proceed at lower catalyst loading and the results were shown
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Table 2 Glucose conversion and HMF yield with Sn-Mont as the
and then the generated fructose being dehydrated to HMF
by Brønsted acid. Tin ions used as Lewis acid sites to
isomerize aldose (xylose or glucose) to ketose (xylulose and
fructose, respectively) has been widely reported.8,14,15c,22
When Amberlyst-15, a typically strong Brønsted acid catalyst
was used, a poor HMF yield of 7% with 78.5% glucose
conversion at 160 °C for 3 h was obtained (Table 1, entry 5).
This indicated that this novel Sn-Mont catalyst containing
both types of acidic sites led to the direct conversion of
glucose to HMF in a one-pot process. Besides, no significant
amount of fructose was detected in the reaction solution and
the conversion of fructose was nearly up to 100% with 78.1%
HMF yield at 160 °C in 1 h if fructose was used as the reactant
(Table 1, entry 4), which meant that the dehydration of fructose
to HMF is very fast and the Brønsted acid on the Sn-Mont was
active enough for the generated fructose to be rapidly dehydrated
to HMF.
catalysta
Entry
Sn-Mont (g)
Conversion (%)
Yield (%)
1
2
3
4
5
0.4
0.2
0.1
0.04
0.02
98.2
98
91.2
89.9
89.5
33.3
53.5
48.5
44.6
39.8
a Reaction conditions: 5 wt% glucose, 160 °C, 3 h, 6 mL of reaction
solution, THF–DMSO = 70 : 30, v/v.
The pyridine-adsorbed FT-IR spectrum confirmed the exist-
ence of two types of acid sites on the Sn-Mont catalyst, which
was also in accordance with the experimental results. However,
the pristine Ca-Mont catalyst, with two types of acid sites as
well, only resulted in the formation of certain amounts of un-
determined by-products without HMF formation (Table 1, entry 6),
which indicated that tin ions were the main Lewis active sites to
catalyze the glucose isomerization to fructose. In blank solution
(without adding any catalyst), no HMF was detected, indicating
the auto-catalysis was negligible (Table 1, entry 7). Davis et al.
reported that the active sites for the isomerization reaction were
Sn4+ ions exchanged into the framework of the beta zeolites.8,12a
In our experiments, we also found a similar phenomenon that
either SnO2 or SnCl4·5H2O showed low isomerization activity
and the conversion of glucose was below 32% (Table 1, entries 8
and 9). After being exchanged with montmorillonite, the
glucose conversion was almost complete. However, after calci-
nation at 600 °C for 4 h to condense Sn–OH groups, Sn-Mont
showed much lower activity for glucose conversion, only 8.3%
yield of HMF was obtained with 65.7% glucose conversion
(Table 1, entry 10), which meant that Sn–OH were the Brønsted
acid sites to catalyze the generated fructose dehydration to HMF
as schemed on Fig. 6.
Fig. 5 Glucose conversions and HMF yields of Sn-Mont in cycle
usage test. (Reaction conditions: 0.2 g Sn-Mont, 5 wt% glucose, 6 mL
of reaction solution, THF–DMSO = 70 : 30, v/v. 160 °C, 3 h.)
in Table 2. As the catalyst amount is decreased to 0.02 g,
glucose conversion and HMF yield were still high, about 89.5%
and 39.8%, respectively. With the aim to obtain high yields of
HMF, the ratio of catalyst to glucose of 2 : 3 (m/m) was chosen
hereafter.
3.3 Catalyst recyclability
The recyclability is of great importance for applying a catalyst in
industrial processes. The recyclability of the Sn-Mont catalyst
was evaluated through six repeated reactions. The results in
Fig. 5 indicated that the Sn-Mont catalyst exhibited excellent
stability for glucose conversion into HMF. After being used six
times at 160 °C for 3 h, the glucose conversions and HMF
yields were always about 98% and 53%, respectively. The SEM
and N2 sorption showed that the morphology of the used
Sn-Mont did not change and the BET surface area of it remained
about 149 m2 g−1. Moreover, the NH3-TPD profile of the used
Sn-Mont was similar to that of the fresh one. Therefore, it can be
concluded that the catalyst was stable in this system, at least
being used six times.
3.5 Conversion of di/poly-saccharides on Sn-Mont catalyst
The robustness of Sn-Mont catalyst in synthesizing HMF
from glucose opens up the possibility of using more complex
carbohydrates, such as disaccharides (sucrose and cellobiose)
and polysaccharides (inulin, starch and cellulose) as renewable
raw material. Synthesis of HMF from cellobiose, inulin,
starch, and cellulose, involves the hydrolysis, isomerization
and dehydration steps with an acid or enzyme and makes the
process very complicated.2,23–26 Here, sucrose, cellobiose and
inulin could be converted into HMF with our catalyst and the
yield of HMF was above 40% without coupling with other
acid or enzyme catalysts (Fig. 7). However, the HMF yield
from starch and cellulose was quite poor under the same con-
ditions. This is because the hydrolysis of starch/cellulose to
glucose is difficult in the organic phase, affording a HMF yield
below 2%.
3.4 Comparison of catalyst activity and effect of Sn-Mont
catalyst on reaction pathways
Generally, the conversion of glucose to HMF would undergo
two successive steps: isomerization to fructose by Lewis acid
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To enhance the HMF yield, NaCl was added to solvents as indi-
cated in the previous report.27 As expected, we found that almost
all of HMF was transferred into the organic phase (0.0249.1 g mL−1,
almost no HMF in aqueous phase) and the HMF yield
increased to 59.3%, higher than that obtained in the monophase
as well, which indicated that the addition of NaCl not only
increased the partitioning coefficient between THF and water,
but also inhibited the generated HMF from further dehydration
to levulinic acid. Moreover, the THF/H2O–NaCl system was also
effective to convert fructose and di-/poly-saccharides. HMF
yields from fructose, sucrose, cellobiose and inulin were 78.8%,
43.6%, 40.1% and 42.6%, respectively (Fig. 7). These values are
a little higher than that obtained in single phase systems, indica-
ting that this biphase system could inhibit the side reactions. The
stability of the Sn-Mont catalyst in this THF/H2O–NaCl biphasic
system was also confirmed. After being used 4 times at 160 °C
for 3 h, the yield of HMF was always about 58%. The leaching
of tin in the aqueous phase was negligible (<0.005% detected by
ICP-AES), which was also proved the stability of the GTS cata-
lyst. More importantly, HMF yields obtained from starch and
cellulose were greatly exceeded in the monophase system
without the addition of other mineral acids and the yields
reached 44.4% and 39.1%, respectively. These results are very
promising since they show that Sn-Mont can be used directly for
the conversion of complex carbohydrates to HMF without coup-
ling with other acid catalysts.
Fig. 6 Plausible reaction mechanism for glucose isomerisation to fruc-
tose following dehydration to HMF on Sn-Mont catalyst.
4. Conclusions
In summary, a Sn-Mont catalyst was synthesized easily via an
eco-friendly ion-exchanged method. This Sn-Mont catalyst was
used for the direct conversion of glucose or glucose-based carbo-
hydrates into HMF and displayed high activity and good
stability. For glucose, the yield of HMF can mount to 53.5% in
THF–DMSO mixed solvent and 59.3% in the biphase.
Especially, for raw biomass starch and cellulose, the HMF yields
can reach 44.4% and 39.1%, respectively, in THF/H2O–NaCl
biphasic system. It is confirmed that the key to successfully
achieving the direct conversion of the glucose-unit carbohydrate
to HMF is that Sn-Mont contains both Brønsted acid and Lewis
acid sites and combines the isomerization process with the
dehydration step in an one-pot reaction system without the
addition of any other acid catalysts. Hence, the approach we
described with the Sn-Mont catalyst in mono-/biphase system,
due to its green preparation, high activity, and high reusability,
has excellent potential for the conversion of biomass into bio-
fuels and platform chemicals.
Fig. 7 HMF yields from various carbohydrates in THF/DMSO mono-
phase (a) and biphase THF/H2O–NaCl solvent with adding NaCl (b).
(Reaction conditions: (a) 0.2 g Sn-Mont, 5 wt% carbohydrates, 6 mL of
reaction solution, THF–DMSO = 70 : 30, v/v, 160 °C, 3 h; (b) 0.2 g Sn-
Mont, 5 wt% carbohydrates, 6 mL of reaction solution, 0.37 g NaCl,
1 mL H2O, 5 mLTHF, 160 °C, 3 h.)
3.6 Catalyst activity in biphasic solvent for carbohydrates
conversion
In order to further lower the boiling point of the reaction system
and make the separation of HMF easy and effective, water was
utilized to replace DMSO to generate a biphasic reaction system
for glucose conversion. After reaction at 160 °C for 3 h, the
HMF concentration in the organic phase (0.01675 g mL−1) was
4.4 times higher than that in aqueous phase (0.00381 g mL−1),
and the total HMF yield was about 41.7%, a little lower than in
the monophase solvent, due to partial HMF dehydration to levu-
linic acid in the aqueous phase which is catalyzed by Sn-Mont.
Acknowledgements
This project was supported financially by the National Basic
Research Program of China (no. 2010CB732306), the National
Natural Science Foundation of China (no. 20973058), the Com-
mission of Science and Technology of Shanghai Municipality
(10XD1401400) and the Fundamental Research Funds for the
Central Universities.
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