Facile synthesis of hierarchical porous solid catalysts with acid-base bifunctional active sites for the conversion of cellulose to 5-hydroxymethylfurfural

Article abstract of DOI:10.1039/c8nj03812k

Herein, rod-shaped modified halloysite nanotubes (m-HNTs) with suitable wettability and basic active sites were utilized as stable particles to stabilize W/O Pickering HIPEs. Immediately following the thermal polymerization treatment and the subsequent sulfonation process, we have synthesized hierarchical porous solid catalysts (HPSCs) with acid-base bifunctional active sites. The as-prepared HPSCs have hierarchical meso-macroporous structures, strongly acidic surfaces and basic catalytic active sites, which can remarkably improve the catalytic activity for the conversion of cellulose to 5-hydroxymethylfurfural (HMF). This suggests that the stable particles with basic active sites embedded in the porous structure of HPSCs can significantly improve the catalytic activity. The results showed that a maximum yield of HMF (45.6%) can be obtained and the catalyst was recycled in four consecutive cycles without significant loss of catalytic activity. Moreover, this provides a new way for the development of multistage pore structure catalysts for catalytic conversion of carbohydrates.

Full text of DOI:10.1039/c8nj03812k

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Page 1 of 33
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Facile synthesis of hierarchical porous solid catalyst with acid-base bifunctional
active sites for the conversion of cellulose to 5-hydroxymethylfurfural
a
a
a
b
a
a
Kai Wang , Yunlei Zhang , Jiaojiao Zhao , Changhao Yan , Yanan Wei , Minjia Meng ,
a
a*
a*
Xiaohui Dai , Chunxiang Li and Yongsheng Yan
a
Institute of Green Chemistry and Chemical Technology, School of Chemistry and
Chemical Engineering, Jiangsu University, Zhenjiang, 212013, P. R. China
b
Research Center of Fluid Machinery Engineering and Technology, Jiangsu University,
Zhenjiang, 212013, P. R. China
*
Corresponding author: Professor Yongsheng Yan, Chunxiang Li
E-mail: yys@ujs.edu.cn, lcx@mail.ujs.edu.cn
Tel: +86 511 88791800 Fax: +86 511 88791800

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Abstract
Herein, the rod-shaped modified halloysite nanotubes (m-HNTs) with suitable
wettability and basic active sites were utilized as stable particles to stabilize W/O
Pickering HIPEs. Immediately following the thermal polymerization treatment and
subsequent sulfonation process, we have synthesized hierarchical porous solid catalyst
(HPSC) with acid-base bifunctional active sites. The as-prepared catalyst HPSC has
hierarchically meso-macroporous structure, strongly acidic surface and basic catalytic
active sites, which can remarkably improve the catalytic activity for the conversion of
cellulose to 5-hydroxymethylfurfural (HMF). It suggests that the stable particles with
basic active sites embedded in the porous structure of HPSC can significantly improve
the catalytic activity. The results showed that the maximum yield of HMF (45.6%)
can be obtained and the catalyst was recycled in four consecutive cycles without
significant loss of catalytic activity. Moreover, it provides a new way for the
development of multistage pore structure catalyst for catalytic conversion of
carbohydrates.
Keywords: Pickering HIPEs; Hierarchical porous solid catalyst; Acid-base
bifunctional active sites; Cellulose; 5-Hydroxymethylfurfural

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1
. Introduction
With the reduction and over-exploitation of non-renewable fossil fuel causing the
serious energy and environment crisis, new clean renewable energy sources are
1
attracting more and more attention . Biomass is considered as an ideal alternative
energy because of its advantages of renewability, universality and low pollution.
Carbohydrates, as the most abundant component of biomass energy, can be used to
2
, 3
synthesize useful high value-added biofuels by biological or chemical conversion .
The 5-hydroxymethylfurfural (HMF) is now known to be useful high value-added
energy by the conversion of carbohydrates (e.g. cellulose, glucose, fructose, etc.),
which play a crucial role in mitigating the energy crisis. Furthermore, it is a key
4
-6
intermediate for the connection of biochemistry and petrochemical industry .
Due to the low cost and abundant resources of cellulose, it has become a current
7
-9
research hotspot for bio-energy conversion . The conversion of cellulose to HMF
mainly involves three reaction processes, hydrolysis of cellulose to glucose, glucose
1
0, 11
isomerization to fructose, and dehydration of fructose to HMF
. The detailed
process and mechanism description is shown in Scheme 1. Among diverse routes for
8, 12
the preparation of HMF from cellulose, chemical catalysis is a preferential method
.
Up to now, many reports showed the production of HMF by acid catalysts, such as
1
3
14
15
16
mineral acids , solid acids , metal ions and acidic ionic liquids . Among them,
solid acid catalysts are more popular in the degradation of cellulose to HMF because
of their higher catalytic activity, better recyclability and lower corrosion to
1
7, 18
equipment
. Joo et al. reported a preparation method of sulfated ZrO hollow
2

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high-activity catalysts with good dispersity, well-developed mesoporosity and a
1
9
strongly acidic surface for dehydration of fructose to HMF . In addition, Peng et al.
first synthesized large-pored mesoporous silica nanoparticles (LPMSN) for the sake
of applying to the conversion of cellulose to HMF, which -SO H or -NH groups were
3
2
2
0
grafted onto LPMSN to produce basic active sites . By comparing their work, we
conclude that acidic active sites, basic active sites and pore structure play a crucial
role in the yield of HMF. There is therefore still a strong demand for the development
of acid-base bifunctional catalysts with adjustable pore structure for the efficient
conversion of cellulose to HMF.
As is well known, porous polymer materials are widely used in catalysis,
adsorption, medicine and many other fields based on their high porosity, low density
2
1-23
and large specific surface area
. There are common several synthesis methods,
2
4
25
including direct templating , interfacial polymerization , and block copolymer
2
6
self-assembly . Among the above strategies, high internal phase emulsions (HIPEs),
2
7
wherein the volume fraction of the internal phase exceed 74% , is an alternative
approach for fabricating porous materials, which is attributed to its easy to control the
2
8, 29
pore size and easy to operate by one step
. This versatile method is simply handled
and time-saving. Besides, physicochemical property of polymers can be easily
tailored by varying parameters in the HIPEs templates. A porous solid catalyst
templated by the HIPEs obtains conventional pores, in addition to a large number of
interconnected pores to embed structure, and the feature further increases the
material’s permeability and mass transfer efficiency. However, its mechanical

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properties are poor, and the overall structure is prone to collapse.
Recently, Pickering HIPEs template method with micro/nano particles as stabilizers
3
0, 31
is receiving more and more attention
. The method utilizes the irreversible
adsorption of solid particles on the two-phase interface to realize the oil-water mixing,
so as to obtain a stable emulsion. To date, porous solid acid catalysts prepared by the
Pickering HIPEs template have been developed for the production of HMF. Our group
has reported the synthesis of macroporous polymerized solid acid catalyst
(PDVB-SS-X-SO3H) with open cell structure and strong acidity through Pickering
32
HIPEs template, and its yield of HMF was 29.6% under optimal conditions .
Although the presence of macropores significantly increased mass transfer efficiency,
its specific surface area has certain limitations. In this regard, hierarchically
meso-macroporous materials can overcome these obstructions because of the
3
3, 34
interconnected porous structures
. To our knowledge, we have found that stabilized
particles often do not possess catalytic activity. During the process of emulsion
polymerization, they embedded in the pore surface and reduced the number of active
sites, which weakened the catalytic activity of the catalyst. Thus, it remains
challenging to utilize particles with catalytically active sites as stabilizer for
fabricating porous catalysts by Pickering HIPEs template method.
In this work, the modified halloysite nanotubes (m-HNTs) with basic active sites
were utilized as stabilizer to form water-in-oil (W/O) Pickering HIPEs. It played a
vital role in the production of hierarchical porous solid catalyst (HPSC). As a natural
multi-walled nanotube silicate clay mineral, halloysite nanotubes (HNTs) have been

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the indispensable material of this article owing to its high specific surface area,
excellent chemical and thermal stability, low cost and easy to modify. The m-HNTs
were obtained by grafting vinyl modification and further precipitation polymerization.
Notably, the previously synthesized rod-shaped m-HNTs with basic active sites were
first used to stabilize W/O Pickering HIPEs. Eventually, through thermal
polymerization treatment and subsequent sulfonation process, the HPSC with
hierarchical porous structure and acid-base bifunctional active sites was generated. Its
hierarchical porous structure increases the permeability and mass transfer efficiency,
acid-base bifunctional active sites significantly enhance catalytic activity. The
different amounts of m-HNTs were added in the synthesis of HPSC for exploring the
influence of catalytic activity.
Scheme 1. The catalytic mechanism for conversing cellulose into HMF.
2
2
. Experimental section
.1. Chemicals and reagents
All the regents were of analytical grade and were used without further purification.
Styrene
(St),
divinylbenzene
(DVB),
azodiisobutyronitrile
(AIBN),
3
-methacryloxypropyltrimethoxysilane (KH-570), were purchased from Aladdin
Reagent Co., Ltd. Acrylamide (AM), sulfuric acid, toluene, acetone, acetonitrile, high
performance liquid chromatography (HPLC)-grade methanol, and chloroform were

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obtained from Sinopharm Chemical Reagent Co., Ltd. HNTs was collected from
Zhengzhou Jinyangguang Chinaware Co. Ltd., Henan, China. Hypermer 2296 were
kindly supplied by Foshan Excel Chemical Co., Ltd.
2
.2. Preparation of modified halloysite nanotubes (m-HNTs)
The schematic diagram of the preparation of m-HNT was shown in Scheme 2.
Firstly, the procedure for grafting vinyl groups on the surface of the HNTs (v-HNTs)
was as follows: 150 mL of toluene and 1.0 g of HNTs were placed in a 250 mL
three-necked flask and ultrasonically dispersed, followed by nitrogen for 30 minutes.
Then, 6.0 mL of KH-570 was added and the mixture was stirred and refluxed at 90 ºC
for 24 h. Then, the reaction products were washed with toluene several times, and
dried at 60 ºC for 12 h in a drying oven.
Secondly, a layer of alkaline polymer was coated on the surface of the v-HNTs by
35
precipitation polymerization . A typical procedure used was detailed as follows:
v-HNTs (0.2 g), AIBN (0.16 g), and acetonitrile (60 mL) was uniformly mixed by
ultrasonic dispersion. Then, add DVB (0.42 mL) and AM (1:2 M ratio), vibrated at 50
ºC for 6 h in water bath oscillator. Afterwards, the temperature was adjusted to 60 ºC
for 24 h. The resulting products were washed with acetonitrile several times by
centrifugation, and dried at 60 ºC until a constant weight was reached. Besides, the
molar ratios of DVB and AM were respectively adjusted to 1:4, 1:6 and 1:8 in order to
adjust the wettability of the modified halloysite nanotubes (m-HNTs). Eventually,
different proportions of the m-HNTs were obtained.

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2
.3. The synthesis of HPSC
The HPSC was synthesized by the following procedures as shown in the Scheme 2.
Firstly, 1:6 m-HNTs (1.5 wt% relative to the mass of oil phase), AIBN (0.08 g) and
Hypermer 2296 (0.27 g) were dissolved in the mixture of DVB (3.0 mL), St (1.0 mL),
and toluene (2.0 mL) in a round bottom flask with gentle stirring to form continuous
external phase. Secondly, the internal water phase (34 mL deionized water) was
gradually added dropwise into the oleic phase under continuous stirring, a stable W/O
HIPEs was obtained. Then, the Pickering HIPEs was transferred to a sealed glass
mould and polymerized in a 65 ºC water bath for 12 h. Subsequently, the polyHIPEs
(PHs) were washed extensively with acetone by soxhlet extraction for 48 h to remove
any impurities. Finally, the resultant polymer foams were dried to constant weight in a
drying oven at 60 ºC .
For the fabrication of HPSC, the as-prepared polymer foams were sulfonated by 98%
H SO to produce Brønsted acidic sites. To be specific, the polymer foams (1.0 g)
2
4
were dispersed into 30 mL of 98% H SO solution and continuously stirred at 70 ºC
2
4
for 12 h. After that, the reaction products were repeatedly washed with deionized
water until the filtrate was neutral, and then dried at 60 ºC for 12 h. Hereto, catalyst
HPSC-1 was successfully prepared. HPSC-2, HPSC-3 and HPSC-4 were synthesized
similarly except that the proportion of m-HNTs (1:6) was changed to 2.0 wt%, 2.5 wt%
and 3.0 wt% respectively. At the same time, we added a set of HPSC-0 prepared
without stabilizer m-HNTs (1:6) as a blank control.

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2
.4. The conversion of cellulose to HMF
The specific catalytic step of converting cellulose into HMF was as follows: Firstly,
in order to destroy the firm structure of cellulose, 0.05 g of cellulose and 1.0 g of
-butyl-3-methyl-imidazolium chloride ([BMIM]-Cl) ionic liquids were separately
1
added into a 25 mL round-bottomed flask, heated and stirred at a certain temperature
until homogeneous solution formed. Then, a certain amount of catalyst was added to
the mixed solution. After continuing the reaction for a definite period of time, the
reaction product was immediately diluted with deionized water to obtain a constant
volume. Finally, the product was analyzed by 1200 Agilent HPLC equipped with
Agilent TC-C18(2) Column (4.6 × 250 mm, 5.0 μm) and the temperature of the
column maintained at 25 ºC. The volume ratio of water to methanol in the mobile
-1
phase was 30:70 at a flow rate of 0.7 mL min . The wavelength of the UV detector
was set at 283 nm. At the same time, according to the standard curve method can be
directly measured HMF content.
The yield of HMF was defined as the ratio of the total number of moles (n , mol) of
1
carbon atoms in the HMF to the total number of moles (n , mol) of carbon atoms in
0
the reactor in which the initial cellulose was charged. The specific equation was as
follows.
YHMF (%) =100n /n0
1
Besides, the HMF selectivity and substrate conversion were calculated as
푚표푙푒푠 표푓 푟푒푎푐푡푒푑 푠푢푏푠푡푟푎푡푒
Substrate conversion (%) =
×100
푚표푙푒푠 표푓 푖푛푖푡푖푎푙 푠푢푏푠푡푟푎푡푒
푚표푙푒푠 표푓 퐻푀퐹 표푏푡푎푖푛푒푑
HMF selectivity (%) =
×100
푚표푙푒푠 표푓 푟푒푎푐푡푒푑 푠푢푏푠푡푟푎푡푒

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2
.5. Characterization of catalysts
The water contact angle (CA) of m-HNTs was characterized by the optical contact
angle measuring device. The morphology of catalysts was observed by field emission
scanning electron microscopy (SEM, JSM-7100F) which equipped with the energy
dispersive spectroscopy (EDS). Infrared spectra were recorded on a Nicolet
NEXUS-470 FTIR apparatus. The elemental compositions of samples were measured
by elemental analyzer. The BET specific surface areas of the catalysts were
determined by N adsorption and desorption isotherms using a Micromeritics TriStar
2
II 3020 instrument. X-ray photoelectron spectroscopy (XPS) of samples was analyzed
via the Vario EL III elemental analyzer (Elementar, Hanau, Germany) which was
conducted on a Thermo ESCALAB 250. The acidic and alkaline features of catalysts
were detected according to NH temperature-programmed desorption (NH -TPD) and
3
3
CO temperature-programmed desorption (CO -TPD) curves, which were recorded
2
2
using a thermal conductivity detector (TCD, DAS-7000SERIES). Thermogravimet-
ric analysis (TGA) was performed with a Netzsch simultaneous thermal analyzer STA
4
49C (crucibles: aluminum from Netzsch) under a nitrogen atmosphere and heating
rate of 5 ºC/min from 20 ºC to 800 ºC.

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Scheme 2. The preparation of m-HNTs and the synthesis of catalyst HPSC.
3
3
. Results and discussion
.1. Optimization of Material
In order to make the m-HNTs as stabilizer to stabilize the emulsion perfectly, we
adjusted the wettability of m-HNTs by adjusting the ratio of DVB to AM as described
in the experimental section. Fig. 1 showed the water contact angle of m-HNTs (1:2)
(Fig. 1a), m-HNTs (1:4) (Fig. 1b), m-HNTs (1:6) (Fig. 1c) and m-HNTs (1:8) (Fig. 1d).
Obviously, m-HNTs (1:2) and m-HNTs (1:4) display the contact angles for the water
as large as 148.3° and 135.7°, which indicates their excellent hydrophobicity. With the
increase of the amount of acrylamide, m-HNTs (1:6) and m-HNTs (1:8) showed water
contact angles of 115.2° and 84.8°, respectively, exhibiting hydrophobicity and
relatively hydrophilic. Several reviews have shown that stable particles are too
3
6-38
hydrophobic or too hydrophilic to stabilize W/O emulsion
. Therefore, the
m-HNTs (1:6) were very suitable for the preparation of HIPEs.

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In this work, polyHIPEs (PHs) with adjustable pore size and connectivity were
prepared by adjusting the internal volume fraction from 75% to 90%. Fig. 2a-d shows
the SEM images of PHs1, PHs2, PHs3 and PHs4. From the figure we can see that,
there are open cells and highly interconnecting pore throat structures exist in all
samples. It is noteworthy that there are many virgulate solid particles embedded in the
hierarchical pore structure. Statistical analysis of SEM micrographs showed that the
average cavity diameter (D) and interconnecting pore sizes (d), as listed in Table 1.
With the increase of internal volume fraction, the average cavity diameter and pore
throat size of macropores decrease significantly while the degree of interconnection
has been greatly enhanced. The presence of macropores and interconnected pores
provides the material high permeability and stability with significant effect on the
3
3, 39
catalytic performance
. In addition, when the internal volume fraction is 80%, the
effects of 1.5-3.0 wt% m-HNTs on PHs were investigated. Fig. 3a-d showed the SEM
images of PHs5-8 when the m-HNTs (1:6) loading was vary from 0.081 g to 0.162 g.
Clearly, all the samples also showed open-cell pore structure interconnected with the
cavity through the spherical pore throat. It can be seen that the average cavity
diameter increases gradually with the increase of the content of particles and the
interconnection pore size decreases gradually. Comparing with Fig. 3, we can see
from the SEM image (Fig. 4d) of the catalyst HPSC that sulfonation does not destroy
its original pore structure. Besides, the attachment of solid particles to the inner wall
of the pore demonstrated the successful preparation of the catalyst.

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Fig. 1. Contact angles for water of 1:2 m-HNTs (a), 1:4 m-HNTs (b), 1:6 m-HNTs (c)
and 1:8 m-HNTs (d).
Fig. 2. SEM images of PHs1 (a), PHs2 (b), PHs3 (c) and PHs4 (d).

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Fig. 3. SEM images of PHs5 (a), PHs6 (b), PHs7 (c) and PHs8 (d).
Fig. 4. TEM images of HNTs (a) and m-HNTs (b), SEM images of m-HNTs (c) and
HPSC (d).

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Table 1. The average cavity diameter (D) and average interconnecting pore diameter
(d) of PHs1-4.
a
b
c
Samples
Vol.fract (%)
D (μm)
d (μm)
PHs1
PHs2
PHs3
PHs4
75
80
85
90
125
117.9
92.2
68.7
10.9
9.8
8.3
3.1
a The volume fraction of the HIPEs. b Average void diameter of the PHs estimated
from the SEM images. c Average interconnecting pore diameter that directly
measured from the SEM images.
3
.2. Physical and chemical properties of catalyst
The corresponding FT-IR spectra in Fig. 5 show the relevant groups of m-HNTs
-1
and HPSC-1. From Fig. 5a we can see, the double peaks at about 3696 and 3624 cm
4
0
were assigned to Al-OH stretching vibrations which exists in the surface of HNTs .
Notably, there are four characteristic peaks in the m-HNTs spectrum. The band at
-1
2
926.72 cm was associated with the C-H stretching vibration of methylene groups,
which can be confirmed from the side that the vinylation of the surface of the HNTs
-1
were successfully modified. Then the peaks at 3451.51 and 1603.3 cm were ascribed
to the N-H stretching vibration and bending vibration in -NH groups, the peak at
2
-1
1
447.2 cm was assigned to the C-N bond, indicating that the basic polymer has been
successfully encapsulated in the surface of v-HNTs. Moreover, it also can be
confirmed from the TEM images (Fig. 4a-b) and the SEM image (Fig. 4c) of m-HNTs

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that there is polymer coating on the surface of HNTs. Additionally, the peak at 1685.9
-1
cm was belong to the asymmetric stretching vibration of -C=O group, which was
derived from the monomer acrylamide. Fig. 5b shows the FT-IR spectra of PHs and
HPSC-1. Compared with PHs, there are two obvious differences in the spectra of
-1
HPSC-1. The peak at 1033.58 cm is assigned to the C-S bond, and the band at
-1
1
168.46 cm belongs to -SO H groups, indicating that the sulfonic group were
3
successfully introduced into HPSC-1. Table 2 lists the elemental composition of each
sample, which also confirms the above viewpoint from the side.
Fig. 5. FT-IR spectra of HNTs, m-HNTs (a), PHs and HPSC-1 (b).
Fig. 6a-d shows the XPS analysis of raw HNTs, m-HNTs, PHs and HPSC. It can be
observed that the signals of O1s, Si2s, Si2p, Al2s and Al2p appear in these four
samples, which imply that HNTs as the basic material has been throughout this study.
Specifically, comparing with the spectrum of HNTs, a new peak at 398.64 eV of N1s
was observed for m-HNTs (Fig. 6c), which was associated with -NH groups. It means
2
that -NH groups were successfully introduced onto the surface of HNTs by
2
precipitation polymerization, which confirmed the result of FT-IR analysis. Moreover,
the peaks of Si and Al are significantly weakened or even masked, indicating that the

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alkaline polymer was successfully coated on the surface of HNTs. The XPS spectrum
of the PHs and the m-HNTs showed no significant differences, indicating that
m-HNTs as stabilizer have been successfully attached to the surface of PHs. As we
can see from Fig. 6b and d, the test result of HPSC is obviously different with other
samples. To be exact, the signal of C1s at around 284.7 eV is attributed to the C-S
bond of HPSC. The emerging peak of S2p at 168.33 eV demonstrates the existence of
-
SO H which consistent with the observation from FT-IR test. Furthermore,
3
energy-dispersive X-ray spectroscopy (EDS) analysis also verified the presence and
distribution of Si, Al, N and S (Fig. 7). Taking all these results into consideration, the
catalyst HPSC with acid-base bifunctional active sites has been successfully prepared.
Fig. 6. XPS measurements of HNTs, m-HNTs (a) and PHs, HPSC-1 (b), N1s spectra
(c) of m-HNTs, S2p spectra (d) of HPSC-1.

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Fig. 7. Elemental mapping images of HPSC.
Table 2. Elemental composition of the samples from elemental analysis.
Samples
C(%)
H(%)
N(%)
S(%)
HNTs
m-HNTs
PHs
0.23
41.5
0.57
7.51
5.58
6.57
0
0
0
3.06
2.37
2.5
61.59
60.13
0
HPSC-1
7.4

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The BET specific surface area and the pore size distribution of the catalysts were
investigated by nitrogen adsorption-desorption experiments. Fig. 8 shows the nitrogen
adsorption-desorption isotherms and the pore size distribution curves of HPSC-1,
HPSC-2, HPSC-3 and HPSC-4. Notably, the capillary condensation at a relative
pressure of 0.1 to 1.0 exhibited the typical type-IV isotherms, which demonstrates the
4
1
presence of mesopores in these samples . The pore structure properties of HPSC-1,
HPSC-2, HPSC-3 and HPSC-4 were listed in Table 3. The BET surface area (182.4
2
3
m /g) and pore volume (0.267 cm /g) of HPSC-3 were distinctly higher than HPSC-1
2
3
2
3
(
139.8 m /g, 0.222 cm /g), HPSC-2 (173.4 m /g, 0.250 cm /g) and HPSC-4 (170.3
2
3
m /g, 0.263 cm /g). Meanwhile, the pore size distribution of HPSC-3 calculated by
the BJH method shows that the HPSC-3 catalyst has an average pore size of about 5.6
nm. Notably, the results indicate that the synthesized HPSC catalyst has hierarchical
meso-macroporous structure which is in accordance with the observation from SEM
image. This feature is conducive to the conversion of cellulose to HMF, in which
interconnected macropores can enhance the mass transfer efficiency, the mesopores
inside the macropores can effectively increase the specific surface area of the catalyst
and greatly improve the catalytic activity.

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Table 3. Porosity properties of the catalyst HPSC-1, HPSC-2, HPSC-3 and HPSC-4.
a
2
b
c
3
Samples
HPSC-1
HPSC-2
HPSC-3
HPSC-4
SBET (m /g)
DBJH (nm)
V (cm /g)
t
139.8
5.4
5.3
5.6
5.8
0.222
0.250
0.267
0.263
173.4
182.4
170.3
a
b
SBET is the BET specific surface area. DBJH is BJH mean pore diameter calculated by
c
the desorption branches of the nitrogen sorption isotherms. V is the total pore volume
t
determined at the relative pressure of 0.99.
Fig. 8. Nitrogen adsorption-desorption isotherms of HPSC-1 (a), HPSC-2 (b),
HPSC-3 (c) and HPSC-4 (d).

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The acidity and basic strength of the catalyst HPSC-1 to HPSC-4 are evaluated by
NH -TPD and CO -TPD method, and the measured results are shown in Fig. 9. It is
3
2
well-known that the areas of the peaks represented the acidity and basic strength,
different kinds of acidic or basic sites are based on adsorptive temperatures of NH or
3
42
CO . Among the desorbed NH or CO , those occurring at temperatures below 150
2
3
2
ºC are assigned to physical absorption, while the peaks at higher temperatures
4
3
correspond to molecules released from acidic or basic sites . As revealed in Fig. 9a-d,
the integration of each desorption peak is the corresponding acidity or alkalinity. From
the NH -TPD curves of catalysts (Fig. 9a-b), it can be seen that the trend of the curve
3
is roughly equal. There are three peaks appear around 125 ºC , 457 ºC and 746 ºC .
Among them, the peak intensity appearing at 746 ºC is the strongest and belongs to
the sulfonic acid group. The CO -TPD curves of catalysts (Fig. 9c-d) also exhibit this
2
peculiarity except the location and strength of peaks. The acidic and basic properties
of the catalyst are exhibited in detail in Table 4. Obviously, the catalyst HPSC-3 has a
total acidity of 4.11 mmol/g, which is higher than the others. However, the catalyst
HPSC-4 has a highest total alkalinity (1.51 mmol/g) because of the highest amount of
m-HNTs used during its preparation process. The alkalinity of the catalyst is mainly
reflected in the amino group of m-HNTs, which plays an important role in the process
of catalyzing cellulose. The acid and basic sites of the catalyst are conducive to the
conversion of cellulose to HMF, their catalytic effect will be further demonstrated in
the following catalytic experiments.

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Table 4. The acidic and basic properties of catalysts.
Samples
HPSC-1
HPSC-2
HPSC-3
HPSC-4
Total acidity (mmol/g)
Total alkalinity (mmol/g)
3.40
2.97
4.10
3.78
1.27
1.35
1.39
1.51
Fig. 9. NH -TPD curves of HPSC-1, HPSC-2 (a) and HPSC-3, HPSC-4 (b), CO -TPD
3
2
curves of HPSC-1, HPSC-2 (c) and HPSC-3, HPSC-4 (d).
The chemical stability and chemical composition of HPSC-1, HPSC-2, HPSC-3
and HPSC-4 can be estimated by TG analysis as shown in Fig. 10. It can be seen from
the figure that the weight loss of the catalyst goes through two processes. The first

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slight weight loss is caused by the evaporation of water molecules in the sample,
which are HPSC-1 (9.6%), HPSC-2 (12.1%), HPSC-3 (11.9%) and HPSC-4 (14.8%)
respectively. The second mass loss of HPSC-1 (50.1%), HPSC-2 (49.9%), HPSC-3
(49.5%) and HPSC-4 (51.6%) occurred between 200 ºC and 700 ºC . Clearly, all of the
catalysts lost most of their weight due to decomposition of organic polymer templates
at high temperatures. Simultaneously, the surface of catalysts was covered with a large
number of sulfonic acid groups which will be decomposed at high temperatures. Thus,
all of the catalysts suffered heavy weight loss under higher temperatures. Since the
reaction temperature for conversion of cellulose to HMF was carried out below
2
00 ºC , the catalyst will not decompose in the reaction system. In a word, HPSC-1,
HPSC-2, HPSC-3 and HPSC-4 were found to have sufficiently high thermal stability
for moderate conversion.
Fig. 10. TGA curves of HPSC-1, HPSC-2, HPSC-3 and HPSC-4.
3
.3. Catalytic performance of catalysts
Because of insoluble property in conventional solvents, cellulose was pre-treated
with ILs of 1-butyl-3-methyl-imidazolium chloride ([BMIM]-Cl) to destroy its rigid
framework. Hence, all of the catalytic experiments in this paper are based on ionic

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liquids [BMIM]-Cl as solvents. In order to investigate the effect of the amount of
stable particles on the catalytic activity of the catalysts, we conducted five catalytic
experiments. We carried out catalytic experiments on five different catalysts, HPSC-0,
HPSC-1, HPSC-2, HPSC-3 and HPSC-4 respectively. To explore the optimum
conditions for the conversion of cellulose to HMF, the amount of ILs and substrate
was set as a fixed value and the control variable method was used to study the
influence of different factors. First of all, the effect of the amount of catalyst on HMF
yield was investigated. A mixture of cellulose (0.05 g) and [BMIM]-Cl (1.0 g) was
stirred and heated at 120 ºC for 30 min, and then 30 mg HPSC-1 was added into the
reactor and reacted under the same conditions for 60 min. After that, HPSC-1 was
replaced by HPSC-0, HPSC-2, HPSC-3 and HPSC-4, respectively, the catalytic effect
shown in Fig. 11a. It can be seen from the figure that the catalytic effect of HPSC-0
prepared without stabilizers is significantly lower than that of catalysts prepared with
stabilizers. This indicates that the stabilizer m-HNTs with basic active sites play an
important role in promoting the catalytic reaction. In addition, compared with the
other four groups, the maximum yield of HMF is 39.6% when 30 mg HPSC-3
involved in the catalytic reaction. Besides, it also can be seen from Fig. 11b that the
highest conversion of cellulose and the selectivity of HMF were obtained by using
HPSC-3 as a catalyst under the same catalytic conditions. Therefore, we selected
HPSC-3 as a catalyst to discuss its optimum catalytic conditions.
Then, the catalyst loading was discussed from 10 mg to 50 mg, while keeping the
other operating conditions constant. Fig. 11c showed that the maximum yield of HMF

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was 42.1%. Specifically, the yield of HMF increased from 26.3% to 42.1% when the
amount of catalyst HPSC-3 increased from 10 mg to 40 mg. However, the yield of
HMF was significantly reduced from 42.1% to 35.5% if the catalyst loading was
further increased from 40 mg to 50 mg. It is quite obvious that the amount of catalyst
exceed optimal amount will actually lower the yield of HMF due to the formation of
4
4, 45
by-products and the decomposition of HMF
. Thus, subsequent catalysis
experiments were carried out under the catalyst of 40 mg. In the control experiment,
the yield of HMF produced by the catalyst-free ILs-based reaction system was only
1
.5%, indicating that the as-prepared catalyst HPSC-3 has high catalytic performance.
Finally, the effect of reaction temperature and time on the conversion of cellulose to
HMF was evaluated under optimized catalyst loading and the results were shown in
Fig. 11d. Clearly, the yield of HMF increases gradually with increasing temperature
from 110 ºC to 140 ºC in the first 60 min. However, when the reaction time was
further extended to 150 min at each temperature, the yield of HMF decreased.
Considering the effects of reaction temperature and time, it is clear that the HMF yield
reaches a maximum of 45.6% at 120 ºC for 90 min. In summary, the optimized
conditions for HPSC-3 were 40 mg of catalyst, 90 min of reaction time and 120 ºC of
reaction temperature. The higher catalytic performance is mainly attributed to the fact
that the catalyst not only has acid-base bifunctional sites but also possess
macroporous-mesoporous multistage pore structure.

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Fig. 11. (a) Effects of the HPSC-0 to HPSC-4 catalysts with different physicochemical
properties on the HMF yield from cellulose. (50 mg cellulose, 1.0 g [BMIM]-Cl, 30
mg HPSC, T=120 ºC, t=60 min) (b) Effects of catalysts on the catalytic performances
in cellulose-to-HMF. (c) Influence of the loading amount of HPSC-3 catalyst on the
yield from cellulose in the ILs and the other reaction parameters were constant. (50
mg cellulose, T=120 ºC, t=60 min) (d) Effects of reaction temperature and time on the
conversion of cellulose to HMF.
3
.4. Recyclability of the catalyst
In order to evaluate the recyclability of the catalyst, the conversion of cellulose to
HMF was catalyzed over four cycles by using HPSC-3 as a catalyst. It is worth
mentioning that the entire catalytic experiments were carried out under the optimal
reaction conditions (40 mg HPSC-3, 120 ºC and 90 min). Fig. 12a showed the HMF

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yield of each run and the catalyst was recycled in four consecutive cycles with a total
loss of only 4.8%. This means that the activity of the catalyst is not excessively lost
during the recycling reaction. The slight loss may be attributed to by-products, which
prevent new reactant molecules from contacting the active sites on the surface of the
catalyst. Scanning electron micrograph (Fig. 12b) of HPSC-3 recovered after four
cycles showed that the multistage pore structure of the catalyst remained intact during
the catalysis. Moreover, it can be seen clearly from Fig. 12c that the content of S
element in HPSC-3 recovered after four cycles has only slightly decreased compared
to the corresponding value of fresh HPSC-3. And, peaks of Al2p, Si2p, N1s and S2p
were still detected in the XPS spectra of the recovered HPSC-3 after four cycles (Fig.
1
2d), which was consistent with the observation from elemental analysis. The
recyclability of HPSC-3 is superior due to the higher thermal stability of its acidic
sites and polymer structure, which is an important property in application. The above
results show that, to some degree, heterogeneous catalysts have great potential for
efficient separation and recovery of dehydration reactions.

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Fig. 12. (a) Reusability of HPSC-3 in the conversion of cellulose into HMF under the
optimized conditions. (50 mg cellulose, 1.0 g [BMIM]-Cl, 40 mg HPSC-3, T=120 ºC,
t=90 min) (b) SEM image of HPSC-3 recovered after the fourth cycle. (c) The
element contents of HPSC-3 and HPSC-3 recovered after the fourth cycle. (d) XPS
measurement of HPSC-3 recovered after the fourth cycle.
4
. Conclusions
In this study, the stable W/O Pickering HIPEs was obtained by using rod-shaped
m-HNTs with basic active sites and employing the Pickering HIPEs template
successfully synthesized hierarchical porous solid catalyst with acid-base bifunctional
active sites. The effect of the physicochemical properties of the synthesized catalysts
on the catalytic performance was evaluated in the one-pot conversion of cellulose into

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HMF. The results showed that the catalyst HPSC-3 performs outstandingly in catalytic
reaction and achieved the highest HMF yield of 45.6%. The remarkable catalytic
activity was attributed to the fact that the designed hierarchical pore materials with
both macropores and mesopores made their permeability and specific surface area
significantly enhanced. Meanwhile, the synergistic effect between Brønsted acidic and
basic sites has greatly improved the catalytic activity. Catalyst regeneration
experiments showed that the catalyst was recycled in four consecutive cycles with a
total loss of only 4.8% of the activity in HMF yield. In summary, this newly
developed catalyst is of great value for future widespread use in biomass conversion
and bioenergy applications.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of
China (No. 21606100), Natural Science Foundation of Jiangsu Province
(No.BK20160498), China Postdoctoral Science Foundation (No. 2016M601723).
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