Synthesis and evaluation of macroporous polymerized solid acid derived from Pickering HIPEs for catalyzing cellulose into 5-hydroxymethylfurfural in an ionic liquid

Article abstract of DOI:10.1039/c4ra06870j

Stable water-in-oil (W/O) Pickering high internal phase emulsions (HIPEs) with an internal phase volume fraction of 84.8% were first stabilized by both the hydrophobic silica nanoparticles and span 80. Then, based on the obtained Pickering HIPEs, several macroporous polymerized solid acid (PDVB-SS-X-SO₃H, X = 0, 0.2, 0.6) were prepared by polymerizing divinyl benzene (DVB) and sodium p-styrenesulfonate (SS), and were subsequently combined with sulfonation in H₂SO₄. All the as-prepared PDVB-SS-X-SO₃H possessed open-cell structure, interconnecting pores and strong acidity, and were adopted as catalysts to convert cellulose into 5-hydroxymethylfurfural (HMF) in the presence of [Emim]Cl-mediated solvents. The results showed a maximum yield of 29.6% for PDVB-SS-0.2-SO₃H, 12.9% for PDVB-SS-0-SO₃H and 15.5% for PDVB-SS-0.6-SO₃H under the same condition within 2.0 h at 120 °C, suggesting that the pore sizes and strong acidic sites played a key role in cellulose conversion. PDVB-SS-X-SO₃H can be very easily recycled at least four times without a significant loss of activity. This research opens up a new method to synthesize porous solid acid catalyst materials through Pickering HIPEs templates. This journal is

Full text of DOI:10.1039/c4ra06870j

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Synthesis and evaluation of macroporous
polymerized solid acid derived from Pickering
HIPEs for catalyzing cellulose into 5-
Cite this: RSC Adv., 2014, 4, 43029
hydroxymethylfurfural in an ionic liquid
a
a
b
a
b
Heping Gao, Yinxian Peng,* Jianming Pan,* Jun Zeng, Changhua Song,
b
b
b
Yunlei Zhang, Yongsheng Yan and Weidong Shi
Stable water-in-oil (W/O) Pickering high internal phase emulsions (HIPEs) with an internal phase volume
fraction of 84.8% were first stabilized by both the hydrophobic silica nanoparticles and span 80. Then,
based on the obtained Pickering HIPEs, several macroporous polymerized solid acid (PDVB-SS-X-SO
X ¼ 0, 0.2, 0.6) were prepared by polymerizing divinyl benzene (DVB) and sodium p-styrenesulfonate
3
SS), and were subsequently combined with sulfonation in H SO . All the as-prepared PDVB-SS-X-SO H
3
H,
(
2
4
possessed open-cell structure, interconnecting pores and strong acidity, and were adopted as catalysts
to convert cellulose into 5-hydroxymethylfurfural (HMF) in the presence of [Emim]Cl-mediated solvents.
The results showed a maximum yield of 29.6% for PDVB-SS-0.2-SO
5.5% for PDVB-SS-0.6-SO
3
3
H, 12.9% for PDVB-SS-0-SO H and
Received 9th July 2014
Accepted 26th August 2014
ꢀ
1
3
H under the same condition within 2.0 h at 120 C, suggesting that the pore
3
sizes and strong acidic sites played a key role in cellulose conversion. PDVB-SS-X-SO H can be very
DOI: 10.1039/c4ra06870j
easily recycled at least four times without a significant loss of activity. This research opens up a new
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method to synthesize porous solid acid catalyst materials through Pickering HIPEs templates.
corrosive nature and high corrosion of equipment has limited
the further application of homogeneous acid catalysts in
1
Introduction
With the decrease in fossil fuel reserves, cellulose, an abundant industry. With the enhancement in environmental protection
and renewable resource, is attracting increasing attention as a awareness, heterogeneous acid catalysts are increasingly being
1
replacement for fossil fuels. For using cellulose to produce fuel, used in the degradation of cellulose to HMF, due to their
producing 5-hydroxymethylfurfural (HMF) from cellulose in one environmentally friendly nature, better recyclability, green
2
step is a very important process. In the literature, it has been chemical processes, ease of separation of the catalyst from the
shown that HMF can be simply converted into furan bio-fuels, solution system, and reduced corrosion to equipment. More-
which can replace fossil fuels and chemicals by condensation over, in recent years, solid acid catalysts, such as solid immo-
9
7
8
and hydrogenolysis reactions. For example, HMF can be bilized liquid acid, zeolites, sulde, natural clay minerals,
1
0
11
12
hydrogenolysised into 2,5-dimethylfuran (2,5-DMF), which is a ion-exchanged resins, heteropoly acids (HPA), oxides, metal
3
13
14
potential fuel having higher energy density than ethanol. In salts, sulfonated carbon-based solid acid and polymer-based
addition, 2,5-bis(hydromethyl)furan (2,5-BHF) and 2,5-difor- solid acid, are being widely used in various acid-catalyzed
15
mylfuran (2,5-DFF), which are widely used in the synthesis of reactions.
4
polymers, can also be produced from HMF. In recent years, the
Recently, more and more researchers have found that porous
catalytic conversion of cellulose into HMF has been investigated solid acids have extraordinary catalytic activities due to excel-
by various acid catalysts. Homogeneous acid catalysts, such as lent thermal stability, adjustable pore, high specic surface
5
6
HCl and organic acids, have been used and high yields of HMF area, lower density, high catalytic activity and environmentally
16
have been obtained from the degradation of cellulose. However, friendliness. For example, Fujian Liu et al. successfully
the disadvantages of the difficult separation of the catalyst from prepared porous polydivinylbenzene (PDVB) based solid strong
the liquid reaction medium, environmental unfriendliness, acid (PDVB-SO
drawing group of SO
solid acid of PDVB-SO
H-SO
CF
CF
H, which could be synthesized by the
) by graing the strong electron with-
onto the network of preformed porous
3
2
3
2
3
3
a
School of biology and Chemical Engineering, Jiangsu University of science and sulfonation of superhydrophobic porous PDVB or the copoly-
technology, Zhenjiang 212013, China
merization of DVB with sodium p-styrene sulfonate. Caio
Tagusagawa et al. prepared several porous Ta–W mixed oxide
b
17
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang
2
12013, China. E-mail: zhenjiangpjm@126.com
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5 6
solid acids from TaCl and WCl in the presence of poly block
copolymer surfactant pluronic P-123. Although the catalytic
application effect was obvious, the catalysts were prepared in
2
Experimental
2.1 Chemical and reagents
harsh reaction conditions and it was hard to control the All the regents were of analytical grade and were used without
morphology of the product. To overcome these problems, our further purication. Tetraethoxysilane (TEOS), DVB, formic
1
8,19
groups
chromic chloride bi-functionalized catalysts, i.e. ATP–SO
and HNTs–SO H–Cr(III), were successfully synthesized by graing trile (AIBN), 3-methacryloxypropyltrimethoxysilane (KH-570),
the –SO H and Cr(III) onto the surface of treated attapulgite (ATP) cellulose, toluene, ethanol, methanol, span 80, K SO , potas-
and halloysite nanotubes (HNTs), respectively. On the other hand, sium persulfate (KPS), NH $H O (25 wt%), sodium p-styrene
have worked on two aspects. On one hand, two acid- acid, levulinic acid, 5-HMF (>99%), 1-ethyl-3-methyl-
3
H–Cr(III) imidazolium chloride ([Emim]Cl, >99%), azobisisobutyroni-
3
3
2
4
3
2
based on HNTs by precipitation polymerization and Pickering sulfonate (SS), and acetone were purchased from Aladdin
emulsion polymerization, two polymeric solid acid catalysts, i.e. reagent CO., LTD (Shanghai, China).
HNTs-polystyrene (PSt) – polydivinylbenzene (PDVB)-SO
HNTs-PSt-PDVB-SO H(II), were successfully prepared aer
sulfonation by 98% H SO . Therefore, the emulsion templates
method was found to be easier and more convenient for
synthesizing the porous solid acid material.
3
H(I) and
3
2
.2 Instruments
2
4
ꢁ
1
Infrared spectra (4000–400 cm ) were recorded on a Nicolet
NEXUS-470 FTIR apparatus (U.S.A.). The morphology of PDVB-
SS-X-SO H was observed by eld emission scanning electron
3
microscopy (SEM, JSM-7100F). The image of the silica particles
was obtained by transmission electron microscopy (TEM, JEM-
20
The high internal phase emulsions (HIPEs) with the
controlled volume fraction of droplet higher than 74% were
adopted to synthesize porous solid acid catalyst materials and
subsequent polymerization of the continuous phase,
2100). The optical micrographs of Pickering HIPEs were
collected by a DMM-330C optical microscope equipped with a
high performance digital camera (CAIKON, China). X-ray
photoelectron spectroscopy (XPS) of PDVB-SS-X-SO
recorded by the Vario EL III elemental analyzer (Elementar,
Hanau, Germany). The acidity of PDVB-SS-X-SO was
measured by NH temperature-programmed desorption (NH -
TPD, nesorb3010) instrument. The water contact angle of
silica nanoparticles were measured through the optical contact
angle measuring device (KSV CM200). The detailed test proce-
dure is described as follows. First, silica nanoparticles sample
plates were prepared by spreading a thin layer of silica nano-
particles with double-sided adhesive tape on the glass. Then, 2.0
mL of deionized water was injected on the sample surface
through the syringe pump and the images of the water droplet
were obtained using the camera in measuring device aer the
water droplet was formed on the sample surface (30s). Finally,
these images were analyzed using the supplied soware to
determine the contact angle of the sample.
2
1
commonly known as polyHIPEs. Moreover, the porous
structure is adjustable by changing the volume ratio of the oil
and water phase. However, the traditional polyHIPEs synthe-
sized from surfactants, such as span 80, with between 5 and
3
H was
3
H
5
0% (ref. 22) stabilized water-in-oil (W/O) HIPEs have poor
2
3
3
3
mechanical properties. However, the poor mechanical prop-
erties of polyHIPEs can be improved by increasing the
continuous organic phase volume using particle reinforce-
2
4
ments or by changing the composition of the monomer.
Among them, increasing small amounts of non-ionic poly-
meric surfactant to W/O particle-stabilized HIPE templates
(
Pickering-HIPEs) can lead to the formation of poly-Pickering-
HIPEs with an open porous structure and good mechanical
2
5
properties. To the best of our knowledge, the use of Pickering
HIPEs to produce the porous polymer solid acid catalyst is
rarely reported.
3
Inspired by the discussion above, PDVB-SS-X-SO H (X is
the volume of span 80 and X ¼ 0, 0.2, 0.6) with open-cell
structure, interconnecting pores and strong acidity were
designed and synthesized by applying to Pickering HIPEs
template method. First, in the St ¨o ber's method, modied
2.3 Synthesis and hydrophobic modication of silica
nanoparticles
2
6
hydrophobic silica particles and span 80 were both used to Synthesis of silica nanoparticles by hydrolysis and condensa-
stabilize W/O Pickering HIPEs with 84.8% in the internal tion reaction was carried out according to previous literature
26
2 4
phase. The internal (water) phase contained K SO and with a slight modication. In a typical run, 6.0 mL of TEOS was
sodium p-styrene sulfonate (SS) and the external (oil) phase added into a solution containing 90 mL alcohol and 10 mL H O.
2
0
consisted of divinylbenzene (DVB) and a,a -azoisobutyroni- Then, 3.14 mL of NH $H O (25 wt%) was slowly injected into the
3
2
ꢀ
trile (AIBN), which were emulsied to form stable W/O mixture at 30 C, and the reaction was allowed to proceed for 1.0
Pickering HIPEs. Then, poly-Pickering-HIPEs were h under mechanical stirring. Next, the reaction products were
sulfonated by 98% H SO to introduce the group of –SO H washed with alcohol several times by centrifugation, and
2
4
3
ꢀ
and PDVB-SS-X-SO H was obtained. The morphology and subsequently dried at 80 C for 3.0 h in a drying oven. Finally,
3
various properties of PDVB-SS-X-SO H were characterized the uniformly sized silica nanoparticles were obtained.
3
and its catalytic activity was studied in detail by optimizing
The silica nanoparticles were further modied to attain
the reaction time, temperature and catalysts loading hydrophobicity. A typical procedure used is detailed as follows:
amounts in catalyzing cellulose to HMF (Scheme 1). It could rst, a mixture of silica nanoparticles (1.0 g), toluene (150 mL),
ꢀ
be seen that PDVB-SS-X-SO
3
H showed good recyclability and
H
2
O (15 mL), and KH-570 (3.5 mL) was stirred for 24 h at 40 C.
chemical stability.
Then, the reaction products were washed with alcohol several
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Scheme 1 PDVB-SS-X-SO H catalyzed conversion of cellulose into HMF in [Emim]Cl under atmospheric pressure.
ꢀ
times by centrifugation, and dried at 80 C for 3.0 h in a drying In a typical run, the mixtures of 1.0 g of PDVB-SS-X particle and
ꢀ
oven. Finally, hydrophobic silica nanoparticles were obtained.
30 mL of 98% H SO were continuously stirred at 70 C for 12 h.
2
4
The reaction products were ltered, repeatedly washed with
deionized water to remove the excess H
vacuum at 80 C for 3.0 h. Finally, the PDVB-SS-X-SO
obtained.
2
SO
4
, and then dried in
2
.4 Synthesis of PDVB-SS-X-SO
3
H
ꢀ
3
H were
For the fabrication of stable W/O Pickering HIPEs, the recipes of
the aqueous phase and the organic phase used are listed in
Table 1. Two phases were mixed to achieve homogeneous W/O
Pickering HIPEs. First, the aqueous phase was added drop by
2.5 Conversion of cellulose to HMF
drop to the organic phase with fast and continuous stirring. The conversion of cellulose to HMF was carried out as described
27
Second, the HIPEs were transferred into a plastic centrifuge in previous literature with a slight modication. In a typical
ꢀ
tube and polymerized in a circulating air oven at 60 C for 12 h. run, the pretreatment of degraded cellulose was performed by
ꢀ
Then, the obtained poly-Pickering-HIPEs were dried at 30 C for adding 0.1 g of D-cellulose powder and 2.0 g of [Emim]Cl into a
ꢀ
2
4 h in a vacuum oven until a constant weight was achieved. 15 mL of round-bottom ask and stirring the mixture at 120 C
Small molecules in poly-Pickering-HIPEs were removed by for 30 min, which resulted in the breaking of the hydrogen
ꢀ
Soxhlet extraction in deionized water at 80 C for 24 h, and then bonds of cellulose molecules, and the reduction of the degree of
ꢀ
in acetone for an additional at 80 C for 24 h. Finally, the poly- crystallization and the regularity of the cellulose molecules.
Pickering-HIPEs, named as PDVB-SS-X (X standing for the Then, the 40 mg of sulfonated PDVB-SS-X-SO H was added into
3
ꢀ
volume of span 80 and X ¼ 0, 0.2, 0.6) were dried at 80 C for the mixed solution, and the cellulosic reaction was further
ꢀ
12 h.
carried out at 120 C for 2.0 h. Finally, the reaction products
For the preparation of sulfonated solid acid catalysts, PDVB- were diluted 5000 times in deionized water, and the concen-
SS-X were sulfonated by aromatic substitutions in 98% H
2
SO
4
.
tration of HMF was analyzed by a 1200 Agilent high
Table 1 Recipes of HIPEs
External, organic phase
Internal, aqueous phase
a
Sample
DVB (mL)
AIBN (g)
Silica (g)
Span 80 (mL)
Water (g)
K
2
SO
4
(g)
KPS (g)
SS (g)
PDVB-SS-0-SO
PDVB-SS-0.2-SO
PDVB-SS-0.6-SO
3
H
3.0
3.0
3.0
0.04
0.04
0.04
1.0
0.3
0.3
0
0.2
0.6
16.8
16.8
16.8
0.08
0.08
0.08
0.04
0.04
0.04
0.2
0.2
0.2
3
H
H
3
a
PDVB-SS-X-SO
3
H, X stand for the volume of span 80.
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performance liquid chromatography (HPLC) equipped with an nanoparticles are shown in Fig. 1c and d. The values of silica
ꢀ
Agilent TC-C18 Column (4.6 ꢂ 250 mm, 5.0 mm) and a UV nanoparticles of 34.7 and hydrophobic silica nanoparticles of
ꢀ
detector. During the analysis procedure, 20 mL of HMF samples 123.2 further illustrated the success of modication with vinyl
were injected manually, separated through the Agilent TC-C18 groups. Moreover, the hydrophobic silica nanoparticles prop-
column and detected by a UV-vis spectrophotometer at 283 erly stabilized W/O HIPEs, which was conrmed by Binks's work
ꢀ
nm. The column temperature was set at 25 C and the ow rate regarding using the particles with the water contact angle of
ꢀ
28
of optimized mobile phase consisting of water and methanol 123 to stabilize W/O HIPEs.
ꢁ1
with a volume ratio of 30 : 70 was set at 0.7 mL min . Then,
based on the standard curve obtained with standard
substances, the concentration of HMF was obtained, and the
HMF yield (Y, mol%) was calculated using the following
equation:
3
.2 Formation of Pickering W/O HIPEs
In this work, the process of preparing Pickering W/O HIPEs and
PDVB-SS-0.2-SO H is shown in Fig. 2. The internal (water) phase
contained 80 mg of K SO , 40 mg of KPS and 200 mg of SS in
6.8 mL H O, and the external (oil) phase consisted of 3.0 mL of
DVB, 40 mg of AIBN, 200 mg of hydrophobic silica nano-
(mol) are the moles of HMF and the moles particles, and 0.2 mL of span 80. When the internal and external
of glucose units of initial cellulose, respectively.
phases were mixed, the phase-separated system was formed
Fig. 2a). Then, a stable Pickering W/O HIPE with an internal
3
2
4
n
1
1
2
Y% ¼ ꢂ 100%
n
0
1 0
where n (mol) and n
(
phase volume fraction of 84.8% was prepared aer emulsica-
tion (Fig. 2b). Digital photographs of the drops of Pickering
HIPEs dispersing in water and toluene are shown in Fig. 2c. The
drops of the emulsions were diffused in toluene and main-
tained their original shape, indicating the formation of W/O
Pickering HIPEs. In addition, the micrograph of the Pickering
W/O HIPEs is shown in Fig. 2d, and the uniform and compact
liquids revealed the typical type of HIPEs. The morphology of
3
Results and discussion
3.1 Fabrication of hydrophobic silica nanoparticles
The morphologies of silica nanoparticles and modied silica
nanoparticles were observed by TEM. As shown in Fig. 1a and b,
silica nanoparticles and hydrophobic silica nanoparticle
samples exhibited a sphere-shaped morphology with similar
particle sizes (around 100 ꢃ 20 nm). It was clearly seen that the
surfaces of silica nanoparticles were smooth. However, the
surface of the hydrophobic silica nanoparticles had a pale
shadow on their rough surface, and some of those were linked
together, suggesting that the vinyl groups may be graed on the
surface of silica nanoparticles. The image of the water contact
angle of silica nanoparticles and hydrophobic silica
29
HIPEs was also observed by Xiaodong Li et al. As shown in
Fig. 2e, the digital photographs of PDVB-SS-0.2-SO H monolith
soaking in water revealed that a lot of water permeated into the
PDVB-SS-0.2-SO H monolith by pressing into the water via an
3
3
external force. This indicates that a large number of inter-
connected pores exist inside the PDVB-SS-0.2-SO H monolith.
3
3
.3 Characterizations of PDVB-SS-X-SO
Fig. 3 shows the SEM images of PDVB-SS-0 (a), PDVB-SS-0-HF (b)
i.e. poly-Pickering-HIPEs removed silica nanoparticles by HF),
the triangular domain of PDVB-SS-0-HF (c), PDVB-SS-0-HF-SO
d), PDVB-SS-0.2-SO H (e), and PDVB-SS-0.6-SO H (f). Clearly,
3
H
(
3
H
(
3
3
SEM image (Fig. 3a) shows that PDVB-SS-0 has a closed-cell pore
structure (pore size of 100 ꢃ 8 mm) but no pore throats are
observed, indicating the impermeable nature of PDVB-SS-0. We
learned preliminarily that the PDVB-SS-0 would exhibit the
worst catalytic activity because the reactants and active sites of
3
PDVB-SS-0-SO H may be isolated by closed-cell pores. An
obvious double shell structure composed of poly-SS (PSS)
internal layer and PDVB external layer is shown in Fig. 2b, and
the formative reason may be that the SS monomers in water
phase were moved to the interface of oil–water through a phase
inversion process and form a PSS layer when the polymerization
of PDVB-SS-0 emulsion occurs. In addition, upon enlarging the
image of the triangular domain of PDVB-SS-0-HF (Fig. 2c), many
clear 100 nm pores, which were formed by corroding the
stabilized silica nanoparticles in HF are seen on the surface.
The fact that the stabilized silica particles distribute in the
triangle region to strengthen the stability of HIPEs was
Fig. 1 TEM images of (a) silica nanoparticles, and (c) hydrophobic silica
nanoparticles; contact angles of water images of (b) silica nano-
particles, and (d) hydrophobic silica nanoparticles.
30
conrmed. Moreover, Silverstein et al. also obtained the same
result by TEM. The double shell and small pores structures of
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Fig. 2 Exposition of prepared Pickering W/O HIPEs and PDVB-SS-X-SO H using the experimental progress of PDVB-SS-0.2-SO H (a) phase-
separated system, (b) Pickering W/O HIPEs with an 84.8 vol% internal phase fraction, (c) Pickering W/O HIPEs dispersed in water and toluene, (d)
micrograph of the Pickering W/O HIPEs, (e) PDVB-SS-0.2-SO H monolith soaked in water.
3
3
structure of PDVB-SS-0-HF-SO H (Fig. 3d) was similar to that of
PDVB-SS-0-HF (Fig. 3b), which revealed that the sulfonation
process had no obvious effect on material morphology. More-
over, this also suggested that PDVB-SS-X-SO H had excellent
3
chemical and mechanical stability. With the increase in the
volume of span 80, the different pore sizes of PDVB-SS-0-HF-
SO
SO
3
H (Fig. 3d), PDVB-SS-0.2-SO
H (Fig. 3f) were obtained and were about 100 mm, 10 mm and
3
H (Fig. 3e) and PDVB-SS-0.6-
3
50 mm, respectively. The differences in the results of our
observation may be attributed to three reasons, rst, the
adsorption of surfactant on particles promoted their attach-
ment to the drop interface, leading to a decrease in the droplet
size. Second, increasing the surfactant concentration lowered
the oil–water interfacial tension, facilitating drop break up
31
during emulsication. Thirdly, the droplets of W/O Pickering
HIPEs are instability and easy to combined each other to form
bigger droplets during polymerization process leading to form
bigger pore sizes. Similar results have been also reported by
32
Wong et al.
The results of energy dispersive spectrometer (EDS) analysis
equipped with SEM for PDVB-SS-0, PDVB-SS-0-HF, PDVB-SS-0-
SO
3
H, PDVB-SS-0-HF-SO
3 3
H, PDVB-SS-0.2-SO H, and PDVB-SS-
0.6-SO
3
H are listed in Fig. 4a–f, respectively. When comparing
Fig. 4a and b, the peak of the element Si of PDVB-SS-0-HF dis-
appeared, and the same phenomenon was observed in PDVB-
Fig. 3 SEM images of (a) PDVB-SS-0 (b) PDVB-SS-0-HF, (c) the
triangular domain of PDVB-SS-0-HF, (d) PDVB-SS-0-HF-SO
PDVB-SS-0.2-SO H, and (f) PDVB-SS-0.6-SO H.
3
H, (e) SS-0-HF-SO H when comparing Fig. 4c and d, which suggested
3
3
3
that the silica nanoparticles were successfully removed. Inter-
estingly, as shown by Fig. 4a–d, the peaks of element S of PDVB-
PDVB-SS-0-HF increased the odds of contact with reactants in SS-0-SO H and PDVB-SS-0-HF-SO H were stronger than those of
3
3
the catalytic reaction. Meanwhile, we further sulfonated the PDVB-SS-0 and PDVB-SS-0-HF, respectively, which indicated
PDVB-SS-0-HF, and discovered by SEM that the morphology that the internal phase SS monomers were successfully
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Fig. 4 Energy dispersive spectrometer (EDS) and elemental analysis of (a) PDVB-SS-0 (b) PDVB-SS-0-HF, (c) PDVB-SS-0-SO
3
H, (d) PDVB-SS-0-
3 3 3
HF-SO H, (e) PDVB-SS-0.2-SO H, and (f) PDVB-SS-0.6-SO H.
polymerized into PSS, and –SO H group was also graed on (2.62%) were increased than those of PDVB-SS-0 (0.06%),
3
PDVB via the sulfonation process. In addition, this fact could be PDVB-SS-0-HF (0.09%) and PDVB-SS-0.2-SO H (1.09%), respec-
3
further conrmed by SEM results of the formation of PSS layer. tively, because –SO
The peaks of elements S and O from PDVB-SS-0.6-SO H (Fig. 4f) PDVB-SS-X-SO H. This was also conrmed by the results of the
were both evidently stronger than those of PDVB-SS-0.2-SO EDS analysis.
Fig. 4e). This result can be attributed to the formation of larger Fig. 5 shows the FT-IR spectra of PDVB-SS-0-HF-SO
cell structure by Pickering HIPEs templates in the presence of PDVB-SS-0-SO
3
H groups were successfully graed onto
3
3
3
H
(
3
H (a) and
ꢁ
1
ꢁ1
3
H (b). The peaks of 1039 cm , 1170 cm and
ꢁ
1
3
more span 80, and that the larger pore was easier to be graed 1487 cm , which are associated with the C–S bond, –SO H
by –SO H groups. The results of element analysis for PDVB-SS-0, group and benzene ring, respectively, could be clearly found in
3
PDVB-SS-0-HF, PDVB-SS-0-SO H, PDVB-SS-0-HF-SO H, PDVB- the samples of PDVB-SS-0-HF-SO H and PDVB-SS-0-SO H, sug-
3
3
3
3
SS-0.2-SO
3
3
H, and PDVB-SS-0.6-SO H are also shown in the cor- gesting the presence of sulfonic group and benzene ring in
33
ꢁ1
responding gures. The weights of S in PDVB-SS-0-SO
0.65%), PDVB-SS-0-HF-SO H (0.98%) and PDVB-SS-0.6-SO
3
H
H
these samples. The band around 1107 cm associated with
Si–O–Si antisymmetric stretching vibration was only found in
(
3
3
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3 3
the network of PDVB-SS-X-SO H and that the –SO H group
played a key role in increasing the acid strength of PDVB-SS-X-
SO H. The signal of S2p of PDVB-SS-0-HF-SO H and PDVB-SS-0-
3
3
SO H were found in the same position with a binding energy of
3
1
69 eV, suggesting no obvious effect of sulfonation for PDVB-SS-
X-SO H aer removing silica nanoparticles by HF. The results
also supported the fact that PDVB-SS-X-SO H possessed rigid
3
3
network structure and good chemical stability, which were also
proven based on the morphology structure analysis of PDVB-SS-
0
-HF and PDVB-SS-0-HF-SO H performed earlier.
3
The acidic features of PDVB-SS-0-SO H, PDVB-SS-0-HF-SO H,
3
3
PDVB-SS-0.2-SO H, and PDVB-SS-0.6-SO H solid catalysts were
3
3
determined by NH
hydrogen-bounded NH
3
-TPD. The physically adsorbed and
3
Fig. 5 FT-IR spectra of (a) PDVB-SS-0-HF-SO H, and (b) PDVB-SS-0-
ꢀ
34
3
SO H.
3
can desorbe at #150 C, and the acid
at higher tempera-
at desorption tempera-
site bound resulted in other desorbed NH
3
35
tures. In addition, the desorbed NH
3
ꢀ
ꢀ
ꢀ
ꢀ
tures of 150–250 C, 250–350 C, 350–500 C and >500 C were
the sample of PDVB-SS-0-SO
nanoparticles were completely washed out by HF in the samples
of PDVB-SS-0-HF-SO H. This result was also conrmed by the
3
H, suggesting that the silica
measured as weak, medium, strong and very strong of the acid
36
sites, respectively. As shown in Fig. 7, it can be clearly seen that
3
medium, strong and very strong acid sites exist in PDVB-SS-X-
results of element and EDS analysis.
SO
acidic sites, which are calculated and summarized in Table 2.
The total acidic amounts of PDVB-SS-0-SO H, PDVB-SS-0-HF-
SO H, PDVB-SS-0.2-SO H, and PDVB-SS-0.6-SO
3
H, and the areas of the peaks represented the amount of
Fig. 6 shows the results of the X-ray photoelectron spec-
troscopy (XPS) measurements of PDVB-SS-0-HF-SO H and
3
PDVB-SS-0-SO H. PDVB-SS-0-HF-SO H and PDVB-SS-0-SO H
3
3
3
3
H
were
exhibited the signals of S, C and O, possibly indicating the
presence of –SO H group in these samples. Correspondingly,
3
3
3
ꢁ
1
ꢁ1
1
6
4
70 mmol g (i.e. respective acidic amounts of 25 mmol g
,
3
ꢁ
1
ꢁ1
ꢀ
9 mmol g , and 76 mmol g for the acidic strengths of 300 C,
the high resolution XPS spectrum of C1s showed that the
signals at around 284.7 and 275.4 eV, associated with C–C and
C–S bonds, respectively, could be found in these samples,
ꢀ
ꢀ
1
ꢁ1
70 C and 578 C), 207 mmol g (i.e. respective acidic amounts
ꢁ
ꢁ1
ꢁ1
of 48 mmol g , 76 mmol g , and 83 mmol g for the acidic
ꢀ
ꢀ
ꢀ
ꢁ1
strengths of 289 C, 420 C and 578 C), 233 mmol g (i.e.
3
further suggesting the successfully introduction of –SO H into
3 3
Fig. 6 X-ray photoelectron spectroscopy measurements of (a) survey, (b) C1s, (c) S2p of PDVB-SS-0-HF-SO H and PDVB-SS-0-SO H.
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determined by experience; the specic optimization reaction
progress is listed as follows: rst, cellulose (100 mg) was pre-
ꢀ
treated in [Emim]Cl (2.0 g) at 120 C for 0.5 h. Then, the catalyst
(40 mg) was added into the reactor and kept for another 3.0 h in
the same condition. The effect of the different quantities of
catalyst for the yield of HMF from cellulose under the same
condition was studied, and the results are shown in Fig. 8a. It
could be observed that only 9.8% yield of HMF was obtained
ꢀ
when 20 mg of PDVB-SS-0.2-SO
3
H was used at 120 C for 2.0 h
but the yield of HMF unexpectedly reached to 29.6% as the
amount of PDVB-SS-0.2-SO H increased to 40 mg, and the
3
amount of catalyst was signicantly lower than that of Wang
38
3
Pan. This fact indicated that the addition of PDVB-SS-0.2-SO H
showed excellent catalytic performance in degrading cellulose
to HMF. It may be because the increasing catalytic sites with the
increased amount of catalyst boosts the dehydration reaction.
However, when the amount of catalyst was increased from 40
mg to 60 mg, there was a smooth decrease in HMF yield. The
same tendency of the HMF yield with the increasing amount of
Fig. 7 NH
3
-TPD curves of (a) PDVB-SS-0.2-SO
3
H, (b) PDVB-SS-0.6-
H.
SO H, (c) PDVB-SS-0-HF-SO
3
3
H, and (d) PDVB-SS-0-SO
3
Table 2 Properties of the catalyst and catalytic efficiency
Acid
Total acid
39
catalyst was also reported by Hu and co-workers. This
phenomenon implied that the side reaction of cellulose-
degradation also accelerated due to the over-use of catalyst,
which probably facilitated the decomposition of HMF or the
Acid
amounts
amounts
(mmol g
HMF
(%)
a
ꢀ
ꢁ1
ꢁ1
Sample
strength ( C) (mmol g
)
)
PDVB-SS-0-SO
3
H 300
25
69
76
48
76
83
46
92
95
34
69
141
170
12.9
20.6
29.6
15.5
4
5
70
78
40
polymerization of HMF. Moreover, the addition of excess
catalyst limited the mass transfer and decreased the amount of
PDVB-SS-0-HF- 289
SO 420
78
207
233
244
41
cellulose to join in the reaction at the same time.
3
H
5
275
379
Meanwhile, we also studied the effects of reaction tempera-
ture and time on the yield of HMF from cellulose, and the
results are shown in Fig. 8b. Under the same reaction time (i.e.
PDVB-SS-0.2-
SO
3
H
548
ꢀ
2
.0 h and 40 mg of catalyst), 29.6% HMF yield at 120 C was
PDVB-SS-0.6-
SO H
3
284
359
ꢀ
signicantly higher than the 15.3% HMF yield at 110 C, 21.6%
ꢀ
ꢀ
508
HMF yield at 130 C, and 25.7% HMF yield at 140 C. This
indicated that reaction temperature has a signicant inuence
on the HMF yield. Proper high reaction temperature can
accelerate the degradation of cellulose, and the decreasing
trend was more obvious with higher reaction temperature,
which suggested that higher reaction temperature can improve
the rehydration and polycondensation of HMF to undesired
byproducts such as soluble polymers and insoluble humins.
Moreover, the same results were also obtained by Zhou and co-
a
3
PDVB-SS-X-SO H, X stand for the volume of span 80.
ꢁ1
ꢁ1
respective acidic amounts of 46 mmol g , 92 mmol g , and 95
ꢁ
1
ꢀ
ꢀ
ꢀ
mmol g for the acidic strengths of 275 C, 379 C and 548 C),
ꢁ1
ꢁ1
and 244 mmol g (i.e. respective acidic amounts of 34 mmol g
,
ꢁ1
ꢁ1
6
9 mmol g , and 141 mmol g for the acidic strengths of 284
ꢀ
ꢀ
ꢀ
C, 359 C and 508 C), respectively, which is in agreement with
the results of the content of S of 0.65%, 0.98%, 1.09%, and
42
workers. There are two possible reasons. First, increasing the
reaction temperature reduces the viscosity of ionic liquids and
increases the contact area between the catalyst and the reac-
tants, which was benecial for the accelerated degradation of
cellulose. Second, the by-product of HMF covered the acidic
sites of catalysts, and the reaction of HMF to other substances
was enhanced with the further increase in reaction tempera-
ture. Furthermore, the by-products, including formic acid and
levulinic acid, were examined by HLPC; Zakrzewska and co-
workers also reported that HMF was decomposed to some
2.62% by elemental analysis, respectively. The acidic amount
provides the number of acidic sites which can help in improving
the efficiency of the conversion of cellulose to HMF. In brief,
PDVB-S S-X-SO H had much stronger acidic sites and the exis-
tence of this feature, i.e. 145 mmol g at 470 C and 578 C for
PDVB-SS-0-SO
SS-0-HF-SO H, 187 mmol g at 379 C and 548 C for PDVB-SS-
37
3
ꢁ1
ꢀ
ꢀ
ꢁ1
ꢀ ꢀ
3
H, 159 mmol g at 420 C and 578 C for PDVB-
ꢁ
1
ꢀ
ꢀ
ꢀ
3
ꢁ1
ꢀ
0
0
.2-SO
.6-SO
3
3
H, and 210 mmol g at 359 C and 508 C for PDVB-SS-
H, could be effective to catalyze the reaction of cellulose
43
unidentied products.
to HMF.
To discuss the possible inuence to the conversation of
cellulose to HMF, the HMF yield of PDVB-SS-0-SO
PDVB-SS-0-HF-SO H (20.6%), PDVB-SS-0.2-SO H (29.6%) and
PDVB-SS-0.6-SO
3
H (12.9%),
3
.4 Conversion of cellulose to HMF using PDVB-SS-X-SO
3
H
3
3
as the catalyst
3
H (15.5%) under the same conditions (40 mg of
ꢀ
The optimal reaction conditions for the conversion of cellulose catalyst, 2.0 h of reaction time, and 120 C of reaction temper-
to HMF using PDVB-SS-0.2-SO
3
H
as the catalyst were ature) were obtained and are listed in Table 2. It is clear that
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Fig. 8 Optimization of reaction conditions to cellulose to HMF using PDVB-SS-0.2-SO
3
H as the catalyst. Effects of (a) the amount of
3
PDVB-SS-0.2-SO H, and (b) reaction time and temperature under 40 mg of catalyst and 2.0 g of [Emim]Cl.
PDVB-SS-0.2-SO
according to the performance of PDVB-SS-X-SO
mentioned discussion, we observed that PDVB-SS-0.2-SO
smaller pore sizes (about 10 mm) and moderate amount of strong of PDVB-SS-X-SO
acid (187 mmol g ). Thus, we concluded that the results may be cellulose to HMF. This work is being continued in an effort to
attributed to two aspects: rst, the smaller pore sizes helped in prepare macroporous polymerized solid acids, and the
increasing contact frequency between reaction substrates and results of this work have provided useful experience for
catalytic acid sites. Second, moderate amounts of strong future research toward the development of new energy,
acid was more conducive for the improvement of the catalytic especially in the area of bioenergy.
effect.
3
H obtained higher HMF yield. Moreover, PDVB-SS-0.6-SO
H in the above- system with 20.6%, and PDVB-SS-0-SO
H had It could be concluded that the pore sizes and acidic features
H were important for the conversion of
3
H system with 15.5%, PDVB-SS-0-HF-SO
3
H
3
3
H system with 12.9%.
3
3
ꢁ
1
Acknowledgements
3.5 Recycling of catalysis system
This work was nancially supported by the National Natural
Science Foundation of China (no. 21107037, no. 21176107, no.
The recyclability of the catalyst is very important for the
practical production of HMF and for the demand of green and
sustainable chemistry. Therefore, the recycling activity of the
catalyst was examined using a set of experiments. The solid
2
1306013), Natural Science Foundation of Jiangsu Province (no.
BK20131223), National Postdoctoral Science Foundation (no.
013M530240), Special National Postdoctoral Science Founda-
2
acid catalyst PDVB-SS-0.2-SO H was recovered by ltering the
3
tion (no. 2014T70480), Postdoctoral Science Foundation funded
Project of Jiangsu Province (no. 1202002B) and Programs of
Senior Talent Foundation of Jiangsu University (no. 12JDG090).
mixture aer the end of the rst reaction, washing 10 times
with 5.0 mL deionized water, and drying at 30 C for 24 h in a
vacuum oven. Subsequently, the solid acid catalysts were re-
used for the conversion of cellulose to HMF up to four times
under the same conditions, and the yields of HMF were 23.4%,
ꢀ
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