Functionalized ionic liquids supported on silica as mild and effective heterogeneous catalysts for dehydration of biomass to furan derivatives

Article abstract of DOI:10.1016/j.molcata.2015.09.020

A series of functional ionic liquid supported silica nanoparticles with different acidity (ILs/SiO₂) have been prepared by covalent bonds using novel, non-toxic ethanol as solvent. Compared with using toluene or acetonitrile as solvent in traditional prepared method, the process represents a green, environmental friendly route to synthesize supported functional ionic liquid catalysts (SFILCs). These SFILCs, with the benefits of ionic liquid and the advantages of immobilization, were first systematically utilized in biomass dehydration reactions and proved to be efficient heterogeneous catalysts for the preparation of furan derivatives from various carbohydrates. However, the supported dual acidic functional ionic liquid catalyst (3-sulfobutyl-1-(3-propyltriethoxysilane) imidazolium hydrogen sulfate) was found to be more active and selective than other ILs/SiO₂ and the conversion of fructose, glucose, xylose, sucrose, cellobiose was up to 99, 96, 94, 76 and 99%. Moreover, the reducing sugar (TRS) yield of microcrystalline cellulose dehydration was up to 77%.

Full text of DOI:10.1016/j.molcata.2015.09.020

Accepted Manuscript
Title: Functionalized ionic liquids supported on silica as mild
and effective heterogeneous catalysts for dehydration of
biomass to furan derivatives
Author: Haimei Xu Huahua Zhao Huanling Song Zhichao
Miao JianYang Jun Zhao Ning Liang Lingjun Chou
PII:
S1381-1169(15)30095-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.09.020
MOLCAA 9636
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
9-5-2015
21-9-2015
24-9-2015
Please cite this article as: Haimei Xu, Huahua Zhao, Huanling Song, Zhichao
Miao, JianYang, Jun Zhao, Ning Liang, Lingjun Chou, Functionalized ionic liquids
supported on silica as mild and effective heterogeneous catalysts for dehydration
of biomass to furan derivatives, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2015.09.020
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Functionalized ionic liquids supported on silica as mild and effective
heterogeneous catalysts for dehydration of biomass to furan derivatives
ab
a
ac
ab
Haimei Xu, huhm@licp.cas.cn Huahua Zhao, Huanling Song, Zhichao Miao,
a
a
d
ac
JianYang, Jun Zhao, Ning Liang and Lingjun Chou* ljchou@licp.cas.cn
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China.
University of Chinese Academy of Sciences, Beijing 100049, China
b
c
Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences,
Suzhou 215123, China
d
Gansu Institute of Business and Technology, Lanzhou 730000, China.

Graphical abstract

HIGHLIGHTS
●
●
A series of ILs covalently grafted silica have been prepared using ethanol solvent.
Superior catalytic activity was achieved with IL-SO
3
H-HSO
4
/SiO in biomass
2
conversion.
●
IL-SO
3
H-HSO
4
/SiO showed good catalytic stability.
2

Abstract
A series of functional ionic liquid supported silica nanoparticles with different
acidity (ILs/SiO ) have been prepared by covalent bonds using novel, non-toxic
2
ethanol as solvent. Compared with using toluene or acetonitrile as solvent in
traditional prepared method, the process represents a green, environmental friendly
route to synthesize supported functional ionic liquid catalysts (SFILCs). These
SFILCs, with the benefits of ionic liquid and the advantages of immobilization, were
first systematically utilized in biomass dehydration reactions and proved to be
efficient heterogeneous catalysts for the preparation of furan derivatives from various
carbohydrates. However, the supported dual acidic functional ionic liquid catalyst
(
3-sulfobutyl-1-(3-propyltriethoxysilane)imidazolium hydrogen sulfate) was found to
be more active and selective than other ILs/SiO and the conversion of fructose,
2
glucose, xylose, sucrose, cellobiose was up to 99%, 96%, 94%, 76% and 99%.
Moreover, the reducing sugar (TRS) yield of microcrystalline cellulose dehydration
was up to 77%.
Keywords: Carbohydrates; 5-HMF; Furfural; Ionic liquids; Immobilization.

1
Introduction
Since the global consumption of the liquid petroleum tripled in the ensuing years,
continued reliance on petroleum feedstock will lead to an unacceptable amount of
greenhouse gas emissions which has been the major concern.[1] Biofuel has potential
to be sustainable substitutes as its economic, efficient, and environmentally benign
properties.[2] Among current biofuel sources, furan derivatives, such as
5
-hydroxymethylfurfural (5-HMF) and furfural, obtained from renewable biomass
derived carbohydrates are versatile and key intermediates that are attracting much
attention in biofuels chemistry and the petroleum industry.[3] Last few years have
witnessed rapid progress in the conversion of biomass for the preparation of 5-HMF
and furfural as a result of their versatile applications.[4]
Ionic liquids (ILs), viewed by some as a sort of novel reaction medium, have
gained a variety of applications in catalysis as solvents, catalysts and catalyst carriers
due to their non-volatility, wide temperature range for the liquid state, excellent
solubility for organic and inorganic materials and structure tunability.[5] The acid
group functionalized ionic liquid is an important subgroup in the ionic liquid based
catalysts class, and has been used innumerous types of reactions over the past few
years.[6] Studies involving the utilization of the acid (both Lewis and Brønsted )
functionalized ionic liquids as catalysts for the dehydration reaction of biomass to
5
-HMF have also been reported.[7, 8]
Although, homogeneous acid catalyzed biomass dehydration processes using
acid functionalized ILs is quite effective but presents serious drawbacks in terms of
separation and recycling of costly ILs, equipment corrosion as well as ILs toxicity.[9]

An approach to overcome these limitations is the development of supported functional
ionic liquid catalysts (SFILCs) that require smaller amounts of ionic liquids and
simultaneously minimize limitations associated to their viscosity, separation,
corrosiveness and toxicity. SFILCs combine attractive features of homogeneous
catalysis such as the uniform nature of the catalytic centers, high specificity and
selectivity of the catalyst, with important features of heterogeneous catalysts, such as
high interfacial surface area, high system stability, reusability and potential use in
fixed-bed reactor.
Many work have focused on supported ILs on different materials, such as
silica,[10] polymers,[11] and magnetic materials.[12] Compared to polymers and
magnetic materials, silica is used as carriers for immobilizing ionic liquids widely for
the reasons of lower price and availability. Furthermore, it was found that it could
contain ionic liquids in comparatively high amounts, depending on the surface area of
the silica. [13] There are several reports on the preparation of ionic liquids covalently
grafted silica (ILs/SiO ) and the application used as catalysts in carbohydrates
2
conversion reactions.[14] However, unfortunately, the use of silica immobilizing acid
functionalized ionic liquids in dehydration of other saccharides, especially in
hydrolysis of cellulose is scarce. In addition, the traditional preparation method is
using toluene or acetonitrile as solvent, which is harmful to both human body and
environment.
In this study, a series of covalently-bound amorpous silica supporting acid
functionalized ionic liquids (ILs/SiO ) with different acidity and functional group
2
were synthesized using ethanol as the solvent, which is a way of synthesize SFILCs in
a green route under mild conditions and were utilized for a variety of sugars and
cellulose conversion under mild reaction conditions. As well as this, the reusability of
the immobilized acid functional ionic liquids was also investigated.

2
2
Materials and methods
.1 Materials
Silica of porous particles (100–200 mesh) used as the support was purchased from
the company of Qingdao Chemical Industry and microcrystalline cellulose (average
particle size 50m) from Sigma-Aldrich. Deionized water, acetonitrile (HPLC grade)
and methanol (HPLC grade) were degassed ultrasonically prior to use. Phenol and
furfural were redistilled before used. [BMIM]Cl was supplied by Center of Green
Chemistry and Catalysis, Lanzhou Institute of Chemical Physics (Lanzhou, China).
All other chemicals used were of high purity and were used directly without further
purification.
2
2
.2 Synthesis of catalysts
.2.1 Silica pretreatment and activation
Silica was activated by refluxing in acetone for 24 h at room temperature and then
o
washed thoroughly. The sample was then dried at 100 C overnight and was activated
o
4
2
9
h at 250 C.
.2.2 Synthesis of ILs
According to step 1 in Scheme 1, imidazole (2.72 g, 40 mmol) was heated at
5oC under a nitrogen atmosphere. After imidazole was melt, 3-chloropropyltriethoxy
-
silane (9.7 ml, 40 mmol) was added and the mixture was refluxed overnight under a
nitrogen atmosphere. After the reaction, the mixture was extracted five times with
anhydrous ether. The solvent was removed by rotatory evaporation under reduced
pressure, and the product (N-(3-propyltriethoxysilane) imidazole chloride) was

obtained as a transparent viscous liquid (IL) (71% yield).
Detailed presentation of physical data of IL:
1H NMR (300 MHz, CDCl
): 0.60 (m, 2H), 1.15 (t, 9H), 1.96 (t, 2H), 3.61 (q, 6H),
3
4
.22 (t, 2H), 7.48 (s, 1H), 7.51 (s, 1H), 8.74 (s, 1H);
13C NMR (75 MHz, CDCl
): 11.2, 19.6, 54.2, 54.4, 60.2, 121.7, 124.5, 137.2.
3
Then, 5 g IL (18.3 mmol) was dissolved in ethanol (15 g) in flask and 1,4-butane
o
sultone (1.87 mL, 18.3 mmol) was added dropwise into above solution under 15 C,
o
o
then the final mixture was stirred at 30 C for 5 h and heated at 45 C for another 24 h
under nitrogen flow (step 2 in Scheme 1). The mixture was evaporated under reduced
pressure and extracted five times with diethyl ether. The diethyl ether was removed at
a reduced pressure to get a viscous yellow liquid 3-sulfobutyl-1-(3-propyltriethoxy
-
silane) imidazolium chloride (IL-SO H) (64% yield).
3
Detailed presentation of physical data of IL-SO
3
H:
1H NMR (300 MHz, DMSO-d6): 0.50 (t, 2H), 1.11 (t, 9H), 1.74 (m, 4H), 2.50 (t, 2H),
3
.59 (q, 6H), 4.20 (t, 2H), 4.41 (t, 2H), 7.61 (s, 2H), 9.05 (s,1H);
13C NMR (75 MHz, DMSO-d6): 11.2, 19.5, 24.4, 33.3, 46.4, 49.8, 52.6, 59.0, 60.2,
1
22.0, 124.6, 136.2.
The IL-SO
3
H (1.0 g) was dissolved in absolute ethanol (30 mL) and conc.
H
2
SO (0.54 mL, 10 mmol) was added dropwise into the above solution as shown in
4
o
step 3 sheme 1. The final mixture was stirred at 85 C for 24 h under reflux conditions.
The solvent was removed by rotatory evaporation and the mixture was extracted five
times with anhydrous ether under reduced pressure. The resulting transparent viscous
liquid is 3-sulfobutyl-1-(3-propyltriethoxysilane) imidazolium hydrogen sulfate
(
IL-SO
1H NMR (300 MHz, DMSO-d6): 0.50 (t, 2H), 1.11 (t, 9H), 1.74 (m, 4H), 2.50 (t, 2H),
.59 (q, 6H), 4.20 (t, 2H), 4.41 (t, 2H), 7.61 (s, 2H), 9.05 (s,1H);
13C NMR (75 MHz, DMSO-d6): 11.2, 19.5, 24.4, 33.3, 46.4, 49.8, 52.6, 59.0, 60.2,
22.0, 124.6, 136.2.
3
H-HSO
4
) and used as the compared reaction.
3
1

2
.2.3 Grafting of ILs on silica gel
The preparation process was according to known process[15] and shown in step 4
in scheme 1. The IL-SO H (1.0 g) was dissolved in absolute ethanol (30 mL) and
3
treated with silica gel (2.0 g). After heating the slurry under reflux conditions for 24 h,
the solid was isolated by filtration and washed with ethanol (5 * 10 mL), then was
o
dried under 100 C. The resulting solid is denoted as IL-SO
3
H/SiO (powder, 84%
2
yield).
In the next step (see step 5 in Scheme 1), conc. H
2
SO (0.54 mL, 10 mmol) was
4
added dropwise into the suspension solution of the above silica supported IL in
ethanol (30 mL) according to previous method.[15] The final mixture was stirred at
8
5oC for 24 h and by filtration, washed and dried to get supported dual acidic ionic
liquid IL-SO H-HSO /SiO (57% yield).
3
4
2
The immobilization processes of N-(3-propyltriethoxysilane) imidazolium
chloride and N-(3-propyltriethoxysilane) imidazolium hydrogen sulfate on silica gel
were according to above process, shown in step 6, 7 in scheme 1 and the products
2
were donated as IL/SiO , IL-HSO
4
/SiO .
2
2
.3 Characterization of catalysts
Element analysis (C, H, N, S) was measured by an Elementar Analysensysteme
GmbH VarioEL cube V1.2.1) to determine the chemical composition of catalysts.
(
The thermogravimetric-differential scanning calorimetry (TG-DSC) curves were
obtained from a NETZSCH STA 449F3 thermogravimetric analyzer over the
o
o
temperature range of 20 to 1000 C at a heating rate of 10 C/min. N
adsorption–desorption measurements was carried out using an Autosorb-iQ analyzer
U.S.). Pore size distributions and the specific surface areas were calculated via the
2
(

BJH method and BET method respectively. Fourier transform infrared (FT-IR)
spectra were recorded on a Nicolet (iS10) IR spectrometer. Solid 13C Nuclear
Magnetic Resonance (NMR) experiments were performed using a Bruker Avance
1
13
Ⅱ
WB 400 MHz NMR spectrometer. H NMR and C NMR spectra were recorded
using a Bruker AX (300 MHz) spectrometer using CDCl or DMSO-d as solvent and
3
6
reference. 29Si MAS-NMR spectra were recorded on a Varian-Infinity Plus
Spectrometer at 400 MHz. Determination of acidic sites: The amounts of acidic site
after synthesis of ILs/SiO were measured by means of acid–base titration. In a typical
2
experiment 0.2 g of acidic ionic liquid modified silica catalyst was added to 5 g
aqueous solution. The resulting suspension was allowed to equilibrate for 6 h and then
−2
titrated with 5.05×10 M (aq.) NaOH using phenolphthalein as the indicator.[16]
2
2
.4 Catalytic activity
.4.1 Conversion of carbohydrates
All the dehydration reaction experiments were performed in a 5 mL round
bottom flask under magnetic stirring and equipped with a condenser. In a typical
procedure, specific amount of substrates and powdered catalyst (ILs/SiO ) were
2
charged into the DMSO (1.8g). The reactor was raised to a certain temperature and
held at this temperature for a given period of time. After reaction, the solid catalyst
was centrifuged and washed with ethanol; the supernatant liquid were diluted and then
analyzed by HPLC based on the external standard method. The amount of catalyst
was decided as 0.05 g and the substrates as 0.2 g after optimization in advance.

2.4. 2 Conversion of cellulose
For the conversion process for cellulose, it was the same as that for cellobiose,
glucose, xylose except exclusion of the pre-treatment step. For pre-treatment,
cellulose (0.2 g) was added into [BMIM]Cl (2 g), and the whole mixture was heated
o
at 160 C for 0.5 h with stirring for dissolution of cellulose.
2.5 Analysis of substrates and products
2.5.1 Analysis of sugars, 5-HMF and furfural.
After reaction, the mixture was centrifuged and the supernatant liquid was
analyzed by high-pressure liquid chromatography (HPLC). Substrates were analyzed
using an Agilent 1100 Series HPLC equipped with a PHENOMENEX REZEX
+
RHM-Monosaccharide H (8%) (300 mm ×7.8 mm, 5m) column and a refractive
o
index detector (RID). The column temperature was maintained at 70.0 C and water
was used as mobile phase (flow rate: 0.60 ml/min). 5-HMF was quantified using a
reversed-phase C18 column and a multi-wavelength detector at 280 nm connected to
HP1090 HPLC. The mobile phase was 50.0% (v/v) acetonitrile aqueous solution at a
flow rate of 0.5 ml/min. To ensure the reproducibility of the reaction, repeated
experiments were carried out under identical reaction conditions. The obtained results
including conversion and yield data were found to be reproducible in the range of ±
5.0% variation.
Conversion of substrates and the yield of products were calculated according to
the following equations:
The conversion of substrates (mol%) = the molar of substrates converted *100 /
the molar of initial substrates.
The yield of products (mol%) = the molar of products produced *100 / the molar

of initial substrates × n. (n is 1 for monosaccharide and n is 2 for disaccharide)
The selectivity of products (%) = the molar of products produced *100/the molar
of substrates converted × n. (n is 1 for monosaccharide and n is 2 for disaccharide)
In the cases using DMSO as a solvent, the products were extracted with diethyl
ether prior to gas chromatography–mass spectrometry (GC) analysis. Qualitative
analysis of 5-HMF was performed by Agilent 7890A equipped with HP-5 capillary
column and Triple-Axis detector.
To obtain the reliable data of the reaction, repeated experiments were carried out
under identical reaction conditions. The obtained results including conversion of
substrates and yield of products were found to be reproducible in the range of ± 5.0%
variation. The experimental error in injection was below 2%.
2
.5.2. Total reducing sugar (TRS) assay for cellulose dehydration
.50 mL of the clear solution from the centrifuge tube was transferred to a vial and
.50 mL of DNS reagent was added.[17] The resulting solution was heated at 100 °C
2
0
for 5.0 min to develop the red-orange color. Then the absorbance was measured at 484
nm using 1 cm quartz cells and Carey 8753 UV–Vis spectrophotometer. TRS
concentration in the solution was calculated by employing a standard curve prepared
using glucose.
The yield of TRS (wt%) = the weight of TRS produced *100 / the weight of
initial cellulose.

2
.6 Recyclability and stability study of catalyst
Reused experiments were carried out by triplicating reagent and catalyst quantities
(
to ensure enough catalyst was present for each reuse) under otherwise identical
conditions in typical reaction runs. Upon reaction completion, catalysts were filtered
o
off, washed thoroughly with ethanol and dried under 150 C for 2 h prior to their next
use with fresh reagents.
3
Results and discussion
3.1 Characteristics
3
.1.1 Nuclear Magnetic Resonance
In our experiments, the dual acidic ionic liquid modified silica gel was
2
9
13
29
characterized by Si MAS NMR as well as C NMR spectra (Fig. 1). The Si MAS
NMR spectrum signals center for silica after the immobilization of the ionic liquid
2
3
(
IL-SO
3
H-HSO
4
/SiO ) is at -92, -101 and -109 ppm, which are assigned to Q and Q
2
4
n
and Q groups [Q = Si(OSi)
n
(OH)4−n], respectively, indicating the presence of
residual silanol groups. These results were consistent with literature.[18]
A chemical shift (at δ = -59 and -67 ppm) indicates the formation of Si–O–Si
linkage of 3-chloropropyltriethoxysilane (CPTES) to the silicon atoms of the silica via
one and two siloxane bonds, i.e. Si–O–SiR–(OEt)
2
and(Si–O)
2
SiR–OEt, assigned to
1
2
m
m
T and T groups [T = R-Si(OSi)m(OR/OH)(3-m)], respectively. The absence of T
shifts in the silica anchored triethoxysilane derivatives of functional ionic liquid
spectrum indicates the successful incorporation of CPTES on silica via covalent
1
3
bonding. [19] In addition, the solid-state C CP-MAS NMR spectrum (Fig. 1) of
IL-SO H-HSO /SiO H-HSO
which is consistent with the 13C NMR of IL-SO
3
4
2
3
4
confirmed the presence of the ionic liquid on the silica surface and further supported
our above conclusion.

3
.1.2 Elemental analysis
The loading amount of ionic liquids on silica gel was determined by element
analysis.
The amount of attached molecules onto the silica surface (Lo) calculated by
elemental analysis, using the following expression: Lo= the percentage of nitrogen or
sulfur*1g / nitrogen or sulfur atomic weight.
Comparing the amount of attached molecules on the silica surface (Table 1), it can
be concluded that the degree of functionalization of different catalysts were different.
The nitrogen analysis of ILs/SiO
.1, 1.5 and 1.8 mmol/g of imidazoles were, respectively, grafted on the surface of
silica gel. However, as it can be observed, the N/S ratio was higher than expected
value ( (1/2N) : S was 0.5 for IL-SO H-HSO /SiO and 1 for other catalysts) except
IL-HSO /SiO due to the presence of imidazole which was not bonded with -SO H.
So we can conclude the anion displacement of HSO and Cl was sufficient but the
reaction of imidazole with 1, 4-butane sulfonic acid lactone was not completed.
2
(N, 3.6, 4.2, 3.0 and 3.6 mmol/g) indicated that 1.8,
2
3
4
2
4
2
3
-
-
4
3
.1.3 Fourier transforms infrared spectra
As can be seen from Fig. 2, there were the stretching vibration peaks of Si–O-Si
-1
-1
-1
(
v: 1100 cm ), O–H (1638 cm ) and the stretching vibration Si–OH (v: 3450 cm )
clearly observed on curve 1. However, the FTIR spectrum of ILs/SiO (Curve 2-5)
2
-1
exhibit two characteristic peaks at the positions of 1570 and 1460 cm , which were
attributed to the imidazole ring, indicating that imidazole was successfully grafted on
-1
the surface of silica gel both in ILs/SiO . Both a very weak band at 1208 cm totally
2
appeared on curves 4 and 5 assigned to the S=O stretching vibration in the silica,
which convinced us that the HSO -
4
was successfully grafted on imidazole.[20]

3
.1.4 N
2
adsorption-desorption
N
2
adsorption–desorption isotherms at 77 K of SiO
2
and ILs/SiO were depicted
2
in Fig. 3. And BJH pore-size distributions of the samples were shown in the inset of
Fig. 3.
The samples showed a type IV isotherm with H1 hysteresis, characteristic of highly
mesoporous materials. Immobilization of ionic liquid significantly affected the
surface area and pore size distribution of the modified samples. The SiO
2
sample
3
2
possessed the pore volume of 0.97 cm /g and the BET surface area of 361 m /g, as
shown in Table 1. Immobilization of ILs decreased the surface area and shifts the pore
-
to smaller volume. The both introduction of HSO
IL-SO H-HSO /SiO
4
and R-SO
3
H on the
2
3
4
2
further decreased surface area to only 155 m /g. Similar
reduction after immobilization of other ILs in the BET surface area, pore volumes and
pore diameters was also observed in Table 1.
3.1.5 Thermogravimetric Analysis
The thermogravimetric analysis (TG) curve showed (Fig. 4) an initial weight loss
under 100°C for both catalysts due to surface silanol groups and the adsorbed water in
silica.
Complete loss of all the covalently attached organic structure was seen in the
1
63-579°C temperature range, and the organic fraction corresponded to 28%, 33%,
1%, 43% weight of the acidic ionic liquid modified silica for IL/SiO
/SiO , IL-SO H/SiO , IL-SO H-HSO /SiO . However, for the acid functional
4
2
,
IL-HSO
4
2
3
2
3
4
2
IL, this decomposition of the organic structure occured in three steps. And the three
peaks (in the DTG curve at 163, 367 and 487°C) were related to 6.8, 19.6 and 15.9%
weight losses. From TG analysis data, the SO
3
H and HSO group loading of the
4
catalyst can be calculated as 0.4 and 1.5 mmol/g (total S loading amount is 1.9
mmol/g), which was agree with the result of element analysis (1.9 mmol/g).

3
.1.6 Acidity determination
Titration of acidic ionic liquid modified silica catalyst with 5.05×10− 2 M NaOH
+
(
aq), using phenolphthalein as the indicator gave H /g of ILs/SiO
2
and the results
were summarized in Table 1. The acidic site loading in IL-SO
3
H-HSO /SiO and
4
2
IL-HSO
4
/SiO
2
was found to be 2.0 and 1.5 mmol/g, which indicates 2.0 mmol/g and
1
.5 mmol/g sulfonic acid groups was respectivly grafted on to the surface of the silica
support. The agreement between the acid–base titration as well as with the organic
group loading determined by TGA (1.5, 1.9 mmol/g) and element analysis (1.5, 1.9
mmol/g), is a clear evidence that the sulfonic groups are principally located on the
porous surfaces of the silica, where they are accessible for adsorption and catalytic
reaction processes. On the other hand, the acidic site loading in SiO /IL is more than
2
the sulfur loading measured with element analysis, which is due to the presence of H+
on imidazole as shown in scheme 1. However, the fact that the acidity of
IL-SO
3
H/SiO
2
(1.0 mmol/g) is higher than the SO H group loading amount (0.4
3
mmol/g) proved again that the reaction of imidazole with 1, 4-butane sulfonic acid
lactone is not completed.
3
3
.2 Reactions
.2.1 Conversion of fructose into 5-HMF
In order to demonstrate the best performance of ILs/SiO
2
on the
fructose-to-HMF conversion, IL-SO
3
H-HSO
4
/SiO was selected as a representative
2
catalyst for discussing the effects of reaction time and temperature on fructose
dehydration (shown in Supplementary Fig. S1 and the optimized conditions were got
(
i.e., substrate concentration of 10 wt% (the mass of fructose of 0.2 g and the mass of
o
solvent of 1.8 g), the mass ratio of substrate to catalyst of 4:1, 110 C, 120 min)). And

parallel control reactions of fructose conversion employing ILs/SiO
2
together with
neat ionic liquids (IL-SO H-HSO ) and SiO as the catalysts were carried out at
3
4
2
optimized conditions (Table 2 and entries 1-6). All these four SFILCs showed
excellent fructose conversion, as well as 5-HMF yield. The yield of 5-HMF ranged
slightly (from 93.8% to 97.5%) depending on the sort of supported ionic liquids,
however when the reaction was carried out using silica as catalyst, only with 31.3%
product detected by HPLC analysis, which could be reasonably attributed to the
enough acid sites of all immobilized ionic liquids for fructose conversion. In our
experiments, it was found that the catalytic activity of immobilized ionic liquid
(
IL-SO
3
H-HSO
4
/SiO
2
) was similar with the neat ionic liquid (IL-SO
3
H-HSO ),
4
revealing that SFILCs, with important features of heterogeneous catalysts, (i.e. high
interfacial surface area, high structure stability, reusability), also have attractive
features of homogeneous catalyst such as high catalysis selectivity.
3
.2.2 Catalytic conversion of other carbohydrates into 5-HMF or furfural
Apart from fructose, other carbohydrates (glucose, xylose, sucrose, cellobiose)
were also tested. We have tabulated the results of the carbohydrate dehydration
reaction in Table 2 (entries 8-23).
Glucose is a monomer unit of cellulose, which has great potential and promising
application prospect; however, dehydration of glucose to 5-HMF is much more
challenging in comparison with fructose dehydration. The four FPILCs were thus
utilized to catalytic conversion of glucose for the production of 5-HMF, where the
reaction time and temperature required were significantly longer and higher
respectively, due to its relatively stable structure. Treatment of glucose with IL/SiO ,
2
o
IL-HSO
4
/SiO
2
and IL-SO
3
H/SiO
2
in DMSO at 160 C for 180 min gave relatively low

yield of 5-HMF (18.7%, 19.2% and 23.5% respectively); while moderated result on
glucose-to-HMF (i.e. 43.3% of 5-HMF yield and 96.1% conversion) were achieved,
using IL-SO
3
H-HSO
4
/SiO as the catalyst (Table 2 and entries 12-15). These data
2
indicated that the catalytic efficiency of supported ionic liquids in term of 5-HMF
Reaction conditions: the mole of catalyst is 1.1 mmol, the mass ratio of substrate
to catalyst is 4:1, the substrate concentration is 10 wt% (=the mass of substrate/(the
o
mass of substrate + the mass of solvent) ), the reaction temperature is 110 C, reaction
a
time is 120 min; The product is 5-HMF. IL-SO
3
H-HSO
4
/SiO was recycled (the fifth
2
o
b
cycle) by washing with ethanol and drying at 150 C. Reaction conditions: reaction
o
c
d
temperature is 160 C, reaction time is 180 min; Yield: furfural. The catalytic test
was performed in the presence of [BMIM]Cl. 5-HMF: 5-hydroxymethylfurfural, TRS:
Reducing sugar.
yield or selectivity is closely related to the acidity and the functional groups in the
catalysts. This is in agreement with the work of Ronen Weingarten and George W.
Huber,[21] where they find that higher 5-HMF selectivity was observed with
relatively high Brønsted acid sites.
Observed in an order as IL/SiO
2
< IL-SO
3
H/SiO
2
< IL-HSO
4
/SiO
2
<
IL-SO H-HSO /SiO , reaction activity expressed in terms of furfural yield (Table 2
3
4
2
and entries 16-19) increased considerably from 31.0 to 49.7% because the presence of
Brønsted acid sites is particularly important to the selective conversion of xylose into
furfural,[22] whereas the conversion of xylose was fluctuant between 93.8% and
9
5.6%. It should be also noted that IL/SiO
yield of 31.0% and xylose conversion of 94.2%, the reason was very possibly related
to the fact chlorine anion in SiO /IL positively affected xylose conversion, although
Brønsted acids could strongly favor furfural formation.[23]
2
with lowest acid sites afforded the furfural
2

Ionic liquids modified silica, similarly, were both highly effective catalysts for
sucrose (Table 2 and entries 8-11), consisting of one glucose molecule and one
fructose molecule, dehydration reaction in the solution of DMSO. It took
o
IL-SO
3
H-HSO
4
/SiO
2
only 120 min to finish the reaction at 110 C with 75.6% of
sucrose conversion, offering the value of 5-HMF selectivity as high as 82.4%. Under
the same reaction condition, IL-HSO /SiO was also very active catalysts, yield of
-HMF as 61.6% was achieved, which was 5% much more in comparison with that of
4
2
5
SiO
2
/IL and IL-SO
3
H/SiO
2
. Principally, this remarkable improvement could be
-
substituting Cl-,
attributed to the significantly high Brønsted acid sites after HSO
4
+
which lead to a greatly enhancement of the interact of substrate and H over the
surface of silica nanoparticles.[24] It should be noted that the conversion of sucrose to
5
-HMF is very attractive, but lower than that of fructose. The decrease in the 5-HMF
yield in the case of sucrose compared to the fructose is presumably the result of a
slower conversion of glucose compared with fructose.[25] In other words, sucrose
behaved similarly to the 1:1 mixture of fructose and glucose.
Cellobiose, a kind of disaccharide consisting of two glucose molecules, was
employed as substrates. As it is shown in Table 2 (entries 20-23), IL-HSO
4
/SiO
2
and
IL-SO H-HSO /SiO remarkably improved the 5-HMF yield. The activity order in
3
4
2
term of 5-HMF yield and substrate conversion coincides with the trend that was
observed for sucrose dehydration reaction, indicating that acidity of ionic liquids still
play a role in the process of sugars hydrolysis. The cellobiose-to-HMF conversion
contains three steps: hydrolysis of cellobiose-to-glucose, isomerization of
glucose-to-fructose and dehydration of fructose-to-HMF. Because acid catalysts can
facilitate the first step, we suggest that it is the reason why the highest yield of
glucose appeared in the case of IL-HSO
4
/SiO
2
and IL-SO
3
H-HSO
4
/SiO . In addition,
2

it can also be proposed that the cellobiose-to-glucose is the rate-determining step
among the three reactions. Therefore, catalysts with stronger acidity would favor the
production of glucose that was then converted to 5-HMF, resulting in high yields of
5-HMF.[26]
To sum up, there is a direct correlation between 5-HMF or furfural yields, as well
as carbohydrates conversions and the fraction of Brønsted sites. The strong acid sites
-
of SO
3
H groups together with HSO
4
in the IL-SO
3
H-HSO
4
/SiO must have fully
2
played the roles as active centers for the a variety of carbohydrates dehydration
reaction.
3.2.3 Conversion of cellulose
To gain insight into the catalyst activity for biomass conversion, we utilized all
these four catalysts for further study of dehydration of microcrystalline cellulose in
the presence of [BMIM]Cl (Table 2 and entries 24-27). On the basis of above
discussions, both solid dual acid functional ILs produced good 5-HMF yields in the
dehydration of fructose and glucose, herein, their effectiveness in the
cellulose-to-HMF conversion via a single step proceeding in [BMIM]Cl
o
(
1-butyl-3-methylimidazolium chloride) with DMSO as co-solvent at 160 C for 3 h
were also examined. Satisfactorily, IL-SO
one-pot cellulose-to-HMF conversion, achieving an glucose yield of 20.4% and TRS
yield of 77.1%. In comparison, the use of IL/SiO for the same catalytic process gave
3
H-HSO
4
/SiO remained effective for the
2
2
a lower TRS yield of 57.4% under otherwise identical conditions. These results well
-
indicated the synergistic effect of HSO
4
and -SO H group in the bifunctional catalysts
3
on the selective conversion of cellulose into glucose. In fact, several groups have also
-
synthesized -SO
3
H and HSO
4
bi-functionalized ionic liquids for effective cellulosic
conversion.[27]

3
.3 Recyclability and stability study of catalyst
The reusability of the ILs/SiO was also investigated for dehydration of fructose
with IL-SO H-HSO /SiO at the optimized conditions (Supplementary Fig. S2). In
2
3
4
2
this regard, after the first run of reaction, the catalyst was filtered and then washed
o
with ethanol and dried at 150 C for 2 h. The recycled catalyst was again used for the
fructose conversion with identical conditions. The results showed the conversions of
fructose maintained at around 99% (unshown here) till the fifth run. Additionally, no
significant loss of 5-HMF yield was observed till the fifth run (Table 2 and entries 7).
It meant that the immobilized functional groups (i.e., SO
3
H and HSO ) did not leach
4
largely during the complicated and repeated process. And the element analysis of the
used catalyst confirmed that less than 0.3% of total S leached into DMSO in five
cycle (shown in Table 1). Meanwhile, the FT-IR spectrum of the recovered
IL-SO
3
H-HSO
4
/SiO was well consistent with that of the fresh one (Supplementary
2
Fig. S3), revealing a durable structure of the catalyst.
4
. Conclusions
In conclusion, a series of new ILs/SiO
2
have been developed in a green route and
were systematically used in conversion of various carbohydrates into furan derivatives
for the first time, displaying good catalytic activity. Moreover, supported dual acid
functionalized ionic liquids (IL-SO
3
H-HSO
4
/SiO ) with higher acidity showed more
2
efficient catalytic activity in comparison with other SFILCs. The catalyst also showed
high recyclability with a minor loss of performance. SFILCS, with grant greater
variety and regulate the physicochemical properties, generated an appealing catalyst
which changed the limitations of ILs associated to the viscosity, separation, cost and
toxicity. The novel catalysts integrating the benefits of ionic liquid with the common
advantages of immobilizations open up a wide avenue of green biomass refinery
processes toward the production of 5-HMF.
Acknowledgements
The authors would like to acknowledge the financial support from the National
Natural Science Foundation of China (No. 21133011 and No. 21373244).

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[

Fig. 1 29Si CP-MAS NMR and 13C CP-MAS NMR spectrum of silica after
immobilization of the ionic liquid (IL-SO H-HSO /SiO sample)
3
4
2

Fig. 2 FT-IR spectra: (1) SiO
2
(2) IL/SiO
2
; (3) IL-SO
3
H/SiO ;
2
(
4) IL-HSO /SiO ; (5) IL-SO
4
2
3
H-HSO /SiO
4
2
Fig. 3 N
2
adsorption–desorption isotherms: (1) SiO
2
; (2) IL/SiO
2
; (3) IL-SO
3
H/SiO ;
2
(
4) IL-HSO
4
/SiO ; (5) IL-SO H-HSO /SiO
2
3
4
2

Fig. 4 TG curves of ILs/SiO
2

Scheme 1 Synthesis of ionic liquids and the silica supported ionic liquids catalysts

Table 1 The physical parameters obtained for SiO
2
and ILs/SiO .
2
Loading
amount
S
BET
Vp
Pore diameter
[
mmol/g]
Acidic sites
sample
[
mmol/g]
2
3
[
m /g]
[cm /g]
[nm]
9.6
1/2 N
0
S
0
SiO
2
361
220
0.97
-
IL/SiO
2
0.59
0.46
0.45
9.6
9.5
9.6
1.8
2.1
1.5
0
0.9
1.0
1.5
IL-SO
3
H/SiO
2
2
168
172
0.4
1.5
IL-HSO /SiO
4
IL-SO
3
H-HSO
4
4
/SiO
/SiO
2
2
155
-
0.38
-
7.8
-
1.8
1.8
1.9
1.6
2.0
-
IL-SO
ed
3
H-HSO
-us

Table 2 Results for conversion of various carbohydrates catalyzed by ILs/SiO2.
Yield (%)
Entry Substrate
Catalyst
Conv.(%)
TRS Glucose
5-HMF/ Frufrual
1
2
3
4
5
6
7a
8
9
fructose
fructose
fructose
fructose
fructose
fructose
fructose
Sucrose
Sucrose
Sucrose
Sucrose
glucose
glucose
glucose
glucose
SiO
IL/SiO
IL-SO
IL-HSO
2
66.7
98.0
> 99
> 99
> 99
> 99
> 99
72.6
76.6
76.3
75.6
89.1
91.9
92.8
96.1
94.2
93.8
95.6
94.0
> 99
> 99
> 99
> 99
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
31.3
97.1
96.5
93.8
97.5
> 99
92.7
54.3
57.6
61.6
62.3
18.7
19.2
23.5
43.3
31.0
34.4
44.5
49.7
14.7
15.0
28.6
31.4
2.8
2
-
3
H/SiO
2
2
-
4
/SiO
-
IL-SO
IL-SO
IL-SO
3
H-HSO
3
H-HSO
3
H-HSO
4
4
4
/SiO
/SiO
2
2
-
-
-
IL/SiO
IL-SO
IL-HSO
IL-SO
IL/SiO
IL-SO
IL-HSO
IL-SO
IL/SiO
IL-SO
IL-HSO
IL-SO
IL/SiO
IL-SO
IL-HSO
IL-SO
IL/SiO
IL-SO
IL-HSO
IL-SO
2
-
3
H/SiO
2
2
-
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
0
4
/SiO
-
1
3
H-HSO
4
4
4
4
4
/SiO
/SiO
/SiO
/SiO
/SiO
2
2
2
2
2
-
2b
3b
4b
5b
-
2
3
H/SiO
2
2
-
4
/SiO
-
3
H-HSO
-
6b, c xylose
7b, c xylose
8b, c xylose
9b, c xylose
-
2
3
H/SiO
2
2
-
4
/SiO
-
3
H-HSO
-
0b
1b
2b
3b
cellobiose
6.1
6.3
4.0
4.5
2
cellobiose
cellobiose
cellobiose
3
H/SiO
2
2
4
/SiO
3
H-HSO
4b, d cellulose
5b, d cellulose
6b, d cellulose
7b, d cellulose
57.4 11.4
39.6 9.9
62.0 22.4
77.1 20.4
2
3
H/SiO
2
2
-
5.1
4
/SiO
-
2.9
3
H-HSO
-
5.9