Facile synthesis of hierarchical pore foam catalysts with Br?nsted-Lewis acid sites for the one-pot conversion of cellulose to 5-hydroxymethylfurfural

Article abstract of DOI:10.1039/c6ra14615e

Herein, hierarchical pore foam catalysts (HPFCs) containing Br?nsted-Lewis acidic sites and with hierarchical porous properties were synthesized, and their catalytic performance was applied to convert cellulose into HMF through a one-pot reaction. Sulfated ZrO₂ nanostructures (SZs) were synthesized through grafting ZrO₂ onto the surface of mesoporous silica nanoparticles, and superacid was further introduced onto the surface of the ZrO₂ shells through sulfuric acid treatment and calcination. The representative SZs firstly acted as stabilizers in the synthesis of oil-in-water (O/W) Pickering high internal phase emulsions (Pickering HIPEs). After a subsequent ion-exchange process, Br?nsted acidic sites were located on the surface of HPFCs. The resulting HPFCs exhibit advantageous properties for catalysis, such as favorable meso-macro porosity, strongly acidic surfaces, and structural stability. Compared with other catalysts (e.g., one without superacid and mesopores (HPFCs-1) and another (HPFCs-2) without mesopores), HPFCs-3 manifested more efficiently (42%) the production of 5-hydroxymethylfurfural (HMF) from cellulose. This study develops a promising method for the fabrication of specific functional solid catalysts, which can be applied to the value-added conversion of abundant cellulose.

Full text of DOI:10.1039/c6ra14615e

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Facile synthesis of hierarchical pore foam catalysts with Brønsted-Lewis acid sites for
conversion of one-pot cellulose to 5-hydroxymethylfurfural
Yating Shena, b, Yunlei Zhanga, b, Enwei Zhua, b, Yao Chenc, Pei Jina, Meng Liua, Yongsheng Yana, b*
Chunxiang Li a, b*
,
a Institute of Green Chemistry and Chemical Technology
b School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, China
c
School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013,
China
*Corresponding author: Professor Yongsheng Yan
Eꢀmail: yysxszyz@126.com Tel.: +86 511 88791800 Fax: +86 511 88791800
*Corresponding author: Professor Chunxiang Li
Eꢀmail: lcx@ujs.edu.cn Tel.: +86 511 88791800 Fax: +86 511 88791800
Abstract
Herein, the hierarchical pore foam catalysts (HPFCs) containing BrønstedꢀLewis acidic sites
and hierarchical porous properties, were synthesized and their catalytic performance was applied
to convert cellulose into HMF by oneꢀpot reaction. Sulfated ZrO2 nanostructures (SZs) were
synthesized by grafting ZrO2 onto the surface of mesoporous silica nanoparticles, and superacid
was further introduced onto the surface of ZrO2 shells by sulfuric acid treatment and calcination.
The representative SZs were firstly acted as stabilizers in the synthesis of oilꢀinꢀwater (O/W)
Pickering high internal phase emulsions (Pickering HIPEs). After subsequent ionꢀexchanging
process, Brønsted acidic sites were located on the surface of HPFCs. The resulting HPFCs exhibit
advantageous properties for the catalysis, such as favorable mesoꢀmacro porosity, a strongly acidic
surface and structural stability. Compared with the catalysts (e.g. the one without superacid and
mesoporous (HPFCsꢀ1) and another (HPFCsꢀ2) without mesoporous), HPFCsꢀ3 was manifested
more efficiently (42%) in the production of 5ꢀhydroxymethylfurfural (HMF) from cellulose. This
study develops a promising way for fabrication of specific functional solid catalysts applied in
valueꢀadded conversion of the abundant cellulose.
Keywords: Pickering high internal phase emulsions; Superacid; BrønstedꢀLewis acidic sites;
Cellulose; 5ꢀHydroxymethylfurfural

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1. Introduction
With growing concerns on global warming and declining reserves of easily accessible, the
production of renewable chemicals and fuels from renewable biomass has attracted worldwide
interest for nurturing sustainable societies and the next chapter of human civilization.1ꢀ5 Cellulose,
a polymer composed of glucose units, is the most abundant and nonfood biomass resource, and the
versatile precursor to biodegradable plastics and fuels.6ꢀ7 Among these chemicals,
5ꢀhydroxymethylfurfural (HMF) plays a leading role in bioꢀbased energy,8 because its derivatives
hold vast potential. And it could eventually match the array of compounds derived from
petroleum.9 Therefore, efficient utilization of cellulose for the production of key intermediate
HMF is critical for biomass studies, from both fundamental science and industrial applications.
Among these routes of converting cellulose into HMF, transformations via catalysis have been
reported to be one of the most attractive methods in chemical processes. In consideration of
multiple steps involved in the direct transformation of cellulose into HMF which was shown in the
Scheme 1, namely, hydrolysis, isomerization, and finally dehydration to HMF,10 catalyst plays a
vital role in the catalytic conversion of cellulose to HMF, as reaction rates, product yields and
selectivity are also highly dependent on the catalyst characteristics11ꢀ15. Over the past few decades,
many catalysts have been witnessed in production of HMF, such as mineral acids, amino acids,
enzymes, boric, boronic acids and metal ions16ꢀ20. Qi et al. reported a twoꢀstep process for
converting cellulose into HMF21. Specifically, the cellulose was hydrolyzed into glucose catalyzed
by a strong acidic cation exchange resin in the first step, then CrCl3 was used to catalyze the
hydrolysis products into 5ꢀHMF after the resin was separated from the system. Although these
homogeneous catalysts possess excellent performance, the separation of catalysts and product
purification is a major challenge. Recently, our team combined heterogeneous catalysts of
HIPEsꢀCr(III) and HIPEsꢀSO3H for conversions of cellulose, glucose and fructose into HMF22.
This result verified the effect of Brønsted acidic sites in celluloseꢀtoꢀglucose hydrolysis and
fructoseꢀtoꢀHMF dehydration, and Lewis acidic sites optimized for glucoseꢀfructose conversion.
Therefore, compared with multiple reactions, oneꢀpot synthesis was an attractive methodology
with respect to energy conservation and timeꢀsaving23ꢀ27. To this end, intense research activity
should focus on exploring efficient catalytic system for oneꢀpot synthesis of HMF directly from
renewable cellulose.
One strategy for integrating the multiꢀsteps conversion into a oneꢀpot manner is employing the
catalysts possessing all the desired characters in each step, which will fulfill the sequential
conversions involved in the celluloseꢀtoꢀHMF transformation. Correspondingly, solid superacid
catalysts containing Brønsted acidic sites and Lewis acidic sites, have attracted extensive attention

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in the field of catalysis of conversion biomass into HMF. Since Hino et al. firstly authenticated the
attendance of superacidic properties in sulfateꢀmodified ZrO2 surfaces28, the synthesis of this type
of materials has been intensely studied. For example, mesoporous catalysts were synthesized by
grafting and sulfonating zirconia conformal monolayers onto SBAꢀ15 by Osatiashtiani and
coꢀworkers, which exhibited both Brønsted and Lewis acidity in conversion of glucose to HMF29.
Joo et al. also demonstrated the tailored synthesis of sulfated ZrO2 hollow nanostructures with
wellꢀdefined characteristics of wellꢀmaintained hollow morphologies, an active tetragonal
crystalline phase, good dispersity in liquid media, wellꢀdeveloped mesoporosity and a strongly
acidic surface, and enhanced catalytic activities in the dehydration of fructose to HMF30. However,
it was hard for large size cellulose to diffuse deep inside the mesoporous solid through the small
pore diameters that hence limited the accessibility of active sites and lost the merits of high
surface area31ꢀ33. In addition, it was difficult to maintain the small particle morphology in the
reaction process, and the separation of catalysts from the reaction mixture was also inconvenient.
These shortcomings strongly hinder the practical application of mesoporous materials in the field
of cellulose transformation.
Fortunately, hierarchically bimodal mesoꢀmacroporous materials can overcome these obstacles
specific for HMF production from cellulose due to the interconnected porous structures with
different length scales34,35. Namely, the presence of macropores permit the mass and bulk transfer
of the substances, while the smaller pores create a large internal surface area that greatly enhance
the catalytic36. Li and coꢀworkers successfully synthesized a series of mesoꢀmacroporous
polymeric catalysts, which contained macropores and interconnecting throats, while the
interfacially adsorbed microgel stabilizers drove the formation of smaller pores throughout the
macroporous walls after drying and sintering37. Currently, emulsion template as a method for
preparing such hierarchical porous polymeric catalysts are popular because it offers a route to
synthesizing polymer foams with controllable pores morphology and physical properties38ꢀ40. In
our previous work, the catalysts with a hierarchical porous structure and biꢀfunctional sites were
synthetized by waterꢀinꢀoil (W/O) high internal phase emulsions (HIPEs) template and
bifunctionalized mesoporous silica nanoparticles, then served for the highly efficient synthesis of
top valueꢀadded HMF from renewable cellulose41. Notably, innovation of neoteric methods for
fabrication of catalysts with hierarchically macroꢀmesoporous structures will be beneficial for
increasing HMF yield from cellulose by oneꢀpot reaction. Therefore, multiꢀcatalysts with
hierarchical porous structure and BrønstedꢀLewis acids can not fail to enhance significantly
reaction rates, product yields, and selectivity. Nevertheless, there are few reports about such
multifunctional polymeric solid catalysts for now, and composition of this catalysts remains a

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severe, tough and challenging study.
Herein, sulfated ZrO2 nanostructures (SZs) were synthesized and played a crucial role in the
manufacture of hierarchical pore foam catalysts (HPFCs) which catalytic application in the
conversion of cellulose to HMF. ZrO2@SiO2 coreꢀshells were synthesized by smudging zirconium
propoxide (ZrPr) layer on the changed surface of silicon sphere templates using the surface
hydroxyl. After introducing sulfate functional groups onto the surface of ZrO2@SiO2 by
immersing in sufficient sulfuric acid solution and finally calcination, the SZs was generated. More
specifically, asꢀprepared SZs were first applied to stabilize O/W Pickering HIPEs. Definitively,
through polymeric treatment and subsequent ionꢀexchanging process in a sulfuric acid solution,
the catalysts of HPFCs were produced. The different kinds of silicon spheres were used in these
courses for exploring the influence of mesoporous in the conversion of cellulose into HMF.
2. Experimental section
2.1. Materials and reagents
Tetraethyl orthosilicate (TEOS), cetyl trimethyl ammonium bromide (C16TBA), ammonium
fluoride (NH4F), ammonium hydroxide (NH3·H2O), carboxy methylated cellulose (CMC),
zirconium propoxide (ZrPr), ethanol, pure HMF, 1ꢀbutylꢀ3ꢀmethylꢀimidazolium chloride
([BMIM]ꢀCl, >99%), cellulose, Tween85, sulfuric acid (98%), hydrochloric acid (HCl), liquid
paraffin, sodium pꢀstyrene sulfonate (SS), acrylamide (AM), N,Nꢀmethyleneꢀbisꢀacrylamide
(MBA), potassium persulfate (K2S2O8), acetone, EDTA and xylenol orange were purchased from
Aladdin reagent CO, LTD (Shanghai, China). HPLCꢀgrade methanol was purchased from
Sinopharm Chemical Reagent Co, Ltd. (Shanghai, China). All other chemicals were purchased
from local suppliers and used without further purification.
2.2. Preparation of SZs
SZs was synthesized via three steps using a slightly modified method, as shown in the Scheme
2. Firstly, C16TAB (0.455 g) and NH4F (0.75g) were dissolved in 125 mL deionized water in a 250
mL threeꢀneck, roundꢀbottom fask sufficiently after ultrasound for 15 minutes. Soon afterwards,
the homogeneous solution was heated in water bath kettle to reach the desired temperature (80 ºC)
and accompanied with mechanical stirring. Then, 2.25 mL TEOS was dropped to the aforesaid
solution with continuous stirring for 2 h. The mesoporous silica nanoparticles (mesoꢀSiO2) were
obtained by dispersion and differential centrifugation, washed by ethanol and 37.5% HCl for three
times and dried in vacuum drying oven for 12 h.
Secondly, the nonporous silicon nanoparticles (nonꢀSiO2) were synthesized by the following

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method. 6mL TEOS was dissolved in the homogeneous solution of 90 mL ethanol and 10mL
deionized water, then 3.14 mL 25 wt% NH3·H2O was added into the uniform mixture with
continuous magnetic stirring for 1 h. The products were collected, washed several times by
ethanol solely and dried in vacuum drying oven for 12 h.
Eventually, 0.1 g silicon material (mesoꢀSiO2 or nonꢀSiO2) was soaked and dissolved in 5 mL
ethanol, and mixed with the solution composed of 10 mg CMC, 20 mL ethanol and 0.1 mL
deionized water. After stirring for 30 min, a desired amount of ZrPr (1.0 mL) in ethanol (4.5 mL)
was injected by a syringe at a rate of 0.25 mL minꢀ1, the mixture was stirred at 600 rpm for 20 h at
room temperature. The precipitate was isolated by centrifugation and washed with ethanol.
Afterwards, the asꢀprepared ZrO2@SiO2 coreꢀshell particles (typically 0.2 g) were dispersed in an
aqueous solution of H2SO4 (0.1 mol Lꢀ1, 5 mL) and stirred for 1 h. The resulting samples were
isolated by centrifugation, washed with deionized water and ethanol and dried under vacuum. The
dried ZrO2@SiO2 samples were calcined at the desired temperature for 2 h in air to obtain the
sulfated zirconia samples (mesoꢀSZs or nonꢀSZs).
2.3 Fabrication of HPFCs
The HPFCs were synthesized by the following procedures as shown in the Scheme 3. Firstly,
the asꢀprepared SZs(0.3 g), SS (1.2 g), AM (0.41 g), K2S2O8 (0.043 g), MBA (0.31 g), Tween 85
(0.08 g) were dissolved in distilled water (5.0 mL) in a 100 mL threeꢀneck flask and ultrasound for
15 min for homogenization. Secondly, 22.67 mL of the internal oleic phase (liquid petrolin) was
gradually dripped into the aqueous phase under continuous stirring. Then, the obtained O/W
Pickering HIPEs was dumped into a vitreous ampoule and polymerized in a water bath at 60 ºC.
After 12 h, the polymerized Pickering HIPEs were washed extensively with acetone by Soxhlet
extractor, then dried at 80 ºC for 12 h in vacuum oven.
In order to obtain Brønsted acidic sites, the poly(Pickering HIPEs) was further ionꢀexchanged in
a sulfuric acid solution. The poly(Pickering HIPEs) (0.5 g) was dispersed into 50 mL of 1.0 mol
Lꢀ1 sulfuric acid solution. After stirring for 24 h at room temperature, the sample was washed with
large amount of deionized water until the filtrate was neutral, and dried at 80 ºC for 24 h under
vacuum drying oven.
2.4 Catalytic test
Due to the rigid chemical structure, insoluble in many conventional solvents and difficulties
depolymerize of cellulose42, ionic liquids of [BMIM]ꢀCl was employed as the most ideal green
solvents for transforming cellulose owing to the character of nonvolatile, nonflammable, highly

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polar, and easy to separate from reactants and products43. Firstly, cellulose (0.1 g) and [BMIM]ꢀCl
(2.0 g) were mixed in 25 mL graduated Pyrex glass tube and preheated at designed temperature for
30 min under digital magnetic stirring in an oil bath for a homogeneous system. In the reaction
step, catalyst was added into the mixture while keep heating and stirring at 1200 r minꢀ1 under the
optimized conditions. All the reaction steps were repeated three times and the average yields of
HMF were calculated.
Next, the reaction products were diluted and metered volume with deionized water in the 100
mL volumetric lucifugal flask. Finally, liquid samples were diluted 50 times with deionized water,
and analyzed by 1200 Agilent HPLC equipped with Agilent TCC18(2) Column (4.6×250 mm, 5.0
ꢁm) and UV detector at 283 nm. The mobile phase was mixed water and methanol with a volume
ratio of 3:7, meanwhile the flow rate and column oven were set to 0.7 mL minꢀ1 and 25 ºC,
respectively. The percentage of HMF was obtained according to the standard curve method and
the standard curve was shown in Figure S1.
The yield of HMF (mol%) was evaluated on a carbon basis calculated by Eq. (1):
nHMF
YHMF (%)=
×100%
(1)
n
cellulose
nHMF: mole number of HMF.
cellulose: mole number of glucose unit in initial cellulose.
n
2.5 Characterization of catalysts
The morphology of SZs and HPFCs was analyzed via field emission scanning electron
microscopy (SEM, JSMꢀ7100F), which was carried out with an electron microscope equipped
with a field emission electron gun and energy dispersive spectrometer (EDS). Infrared spectra
were recorded on a Nicolet NEXUSꢀ470 FTIR apparatus (U.S.A.), which were obtained using the
KBr (1:100) pressed pellet method with a Bruker VERTEX 70 Fourier transform infrared
spectrometer. Xꢀray photoelectron spectroscopy (XPS) spectra was recorded by the vario ELIII
elemental analyzer (Elementar, Hanau, Germany), which was performed on a Thermo ESCALAB
250 with Al Kα radiation at y=901 for the Xꢀray sources, the binding energies were calibrated
using the C 1s peak at 284.9 eV. The acidic features of samples were measured according to NH3
temperatureꢀprogrammed desorption (NH3ꢀTPD) curves, which were recorded using a thermal
conductivity detector (TCD, DASꢀ7000SERIES, Beijing, China) and the heating range was
40ꢀ800 ºC under flowing helium at the rate of 10 ºC/min for NH3 desorption. Xꢀray diffraction
(XRD) tests were conducted on a Bruker D8 diffractometer, using a wavelength of 1.5418 Å (Cu
Kα, λ=1.5418 Å) and an angle of 5ꢀ80 ºC with 7 ºC/min. Thermogravimetric analysis (TGA) was

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performed with a Netzsch simultaneous thermal analyzer STA 449C (crucibles: aluminum from
Netzsch) under a nitrogen atmosphere and heating rate of 10 ºC/min from 30 ºC to 800 ºC. The
BrunauerꢀEmmettꢀTeller (BET) specific surface areas, total pore volumes, and the average pore
widths of the catalysts were determined from the adsorption and desorption isotherms of N2 using
a Micromeritics TriStar II 3020 instrument. Prior to characterization, the samples were degassed
under a N2 stream for 12.0 h. The specific surface area of the samples was calculated by applying
the BET theory to the nitrogenꢀadsorption data within the P/P0 range. Poreꢀsize distributions were
calculated from the desorption branch of the corresponding nitrogen isotherms, by
BarrettꢀJoynerꢀHalenda (BJH) method.
3. Results and discussion
3.1 Morphology of samples
With the introduction of sulfate ions onto the surface of ZrO2@SiO2, sulfated nanostructures
containing strongly acidic surface can be prepared. And, corresponding TEM images are exhibited
in Fig. 1. Clearly, the average diameter of nonꢀZrO2@SiO2 (Fig. 1b) is 133±5 nm, which is thicker
than nonꢀSiO2 (128±5 nm) showed in Fig. 1a. This result was mainly ascribed to the successful
coverage of ZrPr by solꢀgel method with CMC. Likewise, the size of mesoꢀSiO2 (125±5 nm) was
increased to 137±5 nm after grafting ZrO2. However, there was just a little increase for
nonꢀ/mesoꢀZrO2@SiO2 after sulfonating and calcinating processes, as presented in Fig. 1c and Fig.
1f. This is due to some organics, which came from the raw materials and attached to silica surface,
were oxidized during the highꢀtemperature calcination in air. Noteworthy, mesoporous of silica
were not blocked during the complex processes (Fig. 1e and Fig. 1f), and the mesopores ensure
that the catalysts possess with hierarchical pores.
Nowadays, the Pickering HIPEs was stabilized by solid colloidal particles that irreversibly
adsorbed at the interface, and the synthetic particles including modified silica particles44ꢀ46, titania
particles47 and zinc(II) oxide nanoparticles48. In this study, the catalysts with controllable physical
and chemical properties were synthesised by polymerizing O/W Pickering HIPEs, which were
stabilized by low polymeric surfactant (2.0 wt%, Tween 85) and moderate SZs. Wherein, the
catalysts were synthesised with freeꢀparticles, nonꢀSZs and mesoꢀSZs, and named HPFCsꢀ1,
HPFCsꢀ2 and HPFCsꢀ3, respectively. The SEM images of HPFCsꢀ1 (A, a), HPFCsꢀ2 (B, b) and
HPFCsꢀ3 (C, c) were presented in Fig. 2, respectively. Obviously, all the catalysts armed
macrospores with wellꢀdefined, openꢀcell and interconnecting pore throat structures but different
sizes. For HPFCsꢀ1, the shells ranged from 9 µm to 21 µm and the size of interconnecting pores
was 2ꢀ5 µm. Dramatically, there are larger shells and smaller interconnecting pores in HPFCsꢀ2

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(0.5ꢀ3 µm, 12ꢀ27 µm) and HPFCsꢀ3 (0.5ꢀ3 µm, 13ꢀ26 µm). These results are attributed to the
bigger droplet size of the emulsions stabilized by particles and surfactant than that by surfactant
solely, and stabilized emulsions mechanically against coalescence power of the particles
surrounding the droplets during polymerization49. Moreover, the SZs with high energy absorbed at
the waterꢀoil interface and hindered the formation of interconnecting pores during polymerization
or superfluous particles buried the accomplished connection holes50. Clearly, the existence of a
mass of SZs in the edge of interconnecting pores, as exhibited in the Fig. 2b and Fig. 2c, further
prove the above theory. Notably, a large number of silica spheres in HPFCsꢀ2 and HPFCsꢀ3 also
confirmed effectively the successful introduction of SZs.
Insert Fig. 1 TEM images of non-SiO2 (a), non-ZrO2@SiO2 (b), non-SZs (c), meso-SiO2 (d),
meso-ZrO2@SiO2 (e), and meso-SZs (f).
Insert Fig. 2 SEM images of HPFCs-1 (A, a), HPFCs-2 (B, b) and HPFCs-3 (C, c).
The nitrogen adsorptionꢀdesorption isotherms and the size distribution curves of mesoꢀSiO2
(a), mesoꢀZrO2@SiO2 (b), mesoꢀSZs (c), HPFCsꢀ1 (d), and HPFCsꢀ2 (e) and HPFCsꢀ3 (f) are
shown in Fig. 3. Typical typeꢀIV isotherms with step capillary condensation in the relative
pressure range of 0.1ꢀ1.0 were obtained for all samples. The porous parameters of mesoꢀSiO2,
mesoꢀZrO2@SiO2, mesoꢀSZs, HPFCsꢀ1, HPFCsꢀ2 and HPFCsꢀ3 were listed in Table 1. The BET
surface area (192.7.6 m2 gꢀ1), the BJH pore radius (3.9 nm) and pore volume (0.23 cm3 gꢀ1) of
mesoꢀSiO2 were obviously higher than those of mesoꢀZrO2@SiO2 (143.5 m2 gꢀ1, 2.3nm, 0.17 cm3
gꢀ1) and mesoꢀSZs (144.3 m2 gꢀ1, 2.8nm, 0.15 cm3 gꢀ1). These results were mainly attributed to the
introduction of ZrO2 that blocked the mesopores of mesoꢀSiO2, thereby resulting in smaller pores
in mesoꢀZrO2@SiO2 and mesoꢀSZs. Meanwhile, the BET surface area (109.7 m2 gꢀ1) and pore
volume (0.06 cm3 gꢀ1) of HPFCsꢀ3 were obviously higher than HPFCsꢀ2 (97.6 m2 gꢀ1, 0.05 cm3 gꢀ1)
and HPFCsꢀ1 (45.5 m2 gꢀ1, 0.03 cm3 gꢀ1). These are attributed to the introduction of SZs which
stacked in the edge of macropore of HPFCsꢀ3 and HPFCsꢀ2 resulting in the smaller pores and high
BET surface area. And, the BJH pore radius was only detected in HPFCsꢀ3(19.8 nm). In short, the
typical hierarchical porous structure of the polymer foams was confirmed from SEM and nitrogen
adsorptionꢀdesorption isotherm analyses of HPFCsꢀ3. Thus, the asꢀprepared HPFCsꢀ3 catalysts
that possess a hierarchically macroꢀmesoporous structure was suitable for conversion of cellulose
to HMF, in which the interconnected macropores would allow ready mass transfer, and the
mesopores along the macroporous walls might have a specific large surface area and greatly
enhance the catalytic activities.
Insert Table 1 Porosity properties of the catalysts meso-SiO2, meso-ZrO2@SiO2, meso-SZs,
HPFCs-1, HPFCs-2 and HPFCs-3.

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Insert Fig. 3 Nitrogen adsorption-desorption isotherms of meso-SiO2 (a) (inset: pore
distribution), meso-ZrO2@SiO2 (b) (inset: pore distribution), meso-SZs (c) (inset: pore
distribution), HPFCs-1 (d) (inset:pore distribution), HPFCs-2 (e) (inset: pore distribution)
and HPFCs-3 (f) (inset: pore distribution).
3.2 Physical and chemical properties of catalysts
The elemental compositions of catalysts were measured by elemental analyzer and presented in
Fig. 4d. The element N of HPFCsꢀ1 (4.8%), HPFCsꢀ2 (4.7%) and HPFCsꢀ3 (5.0%) came from the
monomer A.M and crosslinker MBA. The slight difference may be caused by the failure of
consistent distribution in the catalysts. For HPFCsꢀ1, the containing S (6.1%) was only
represented ꢀSO3H groups on the surface, which was originated from the other monomer SS. And,
the HPFCsꢀ2 and HPFCsꢀ3 displayed more percentages of S of 7.3% and 7.7%, respectively. This
result is mainly caused by the inserted stable particles SZs which contained a mass of elemental
sulphur. The comprehensive qualitative detection of elemental compositions of HPFCs and SZs
were taken by the EDS, and the results were shown in Fig. 4 and Fig. S2. Clearly, the existence of
element Zr and S (Fig. S2) proved definitely the successful graft of sulfated ZrO2 onto the surface
of silica spheres. By reading EDS spectra of HPFCsꢀ1 (a), HPFCsꢀ2 (b) and HPFCsꢀ3 (c), the
elements of C, O, Na, and S were discovered, which all came from the monomers of the O/W
Pickering HIPEs. Moreover, the element Zr and Si were only detected in HPFCsꢀ2 and HPFCsꢀ3,
suggesting that SZs have been successfully incorporated onto the structure of catalysts.
Insert Fig. 4 EDS images of HPFCs-1 (a), HPFCs-2 (b) and HPFCs-3 (c) and the element
contents of the catalysts (d).
The relevant groups of HPFCs (a) and SZs (b) were detected by FTꢀIR spectrometer and the
corresponding FTꢀIR spectra were listed in Fig. 5. Clearly, the SZs display absorption peaks in the
range of 400ꢀ500 cmꢀ1, which assigned to ZrꢀO stretching52. And the characteristic peaks at 797.98
2ꢀ
cmꢀ1 for mesoꢀSZs and 818.06 cmꢀ1 for nonꢀSZs assigned to bidentate SO4 ions coordinated to
ZrO2, indicating a strongly superacidic surface. The bands at about 1103 cmꢀ1 of mesoꢀSZs and
1120 cmꢀ1 of nonꢀSZs also appear, which may be attributed to the formation of SiꢀO stretching.
Although the SZs was the production with high temperature resistance, the representative peaks
still can be observed, reflecting the stability of the surface sulfate groups. The above peaks all
appeared in the spectra of HPFCsꢀ2 and HPFCsꢀ3 but HPFCsꢀ1, indicating the successful grafting
of SZs. The band at about 3200 cmꢀ1 assigned to OꢀH stretching vibration in ꢀCOOH and phenolic
ꢀOH groups generated in the process of sulfonation. Additionally, the peak around 1700 cmꢀ1 of
catalysts is asymmetric stretching vibration of radical group ꢀC=O, which was derived from the
polymerized monomer acrylamide and crossꢀlinking agent methyleneꢀbisacrylamide. And, the

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peaks around 1030 cmꢀ1 attribute to CꢀS bond, peaks around 1160 cmꢀ1 belong to ꢀSO3H group, the
peek at about 2900 cmꢀ1 is assigned to CꢀH bond, and the peek at about 1415 cmꢀ1 is the
characteristic of CꢀN bond, which were all observed in HPFCs.
Insert Fig. 5 FT-IR spectra of HPFC (a) and SZs (b).
Fig. 6 shows the XRD datum of HPFCs and SZs. Clearly, the peaks were observed for SZs (Fig.
5b) of the tetragonal phase at 2θ = 30.2, 50.3, and 60.5 and 2θ = 30.1, 50.3, and 60.1, which can
be attributed to the (111), (210), and (220) planes, respectively. The results further demonstrated
the successful grafting of sulfated ZrO2 onto the surface of silicic materials by the pretreatment
with ZrPr and CMC, following with calcination under high temperature. Any peeks could not be
found in the spectrum of HPFCsꢀ1 (Fig. 5a) with reasonable ground of no stable particles in the
process of preparation. However, the peaks observed in spectrum of HPFCsꢀ2 (Fig. 5a) for the
etragonal phase at 2θ = 30.0 and 50.5, slightly different with SZs, indicating a few changes of
relevant parameters of ZrO2 during the process of synthesis emulsion and the following
polymerization. Moreover, these peaks also discovered in the Fig. 5a for HPFCsꢀ3 and HPFCsꢀ3
of the etragonal phase at 2θ = 30.0 and 50.3.
Insert Fig. 6 XRD patterns of HPFCs (a) and SZs (b).
Fig. 7 shows the XPS datum for HPFCs. The signals of S2s, S2p, C1s, and O1s can be observed
in HPFCsꢀ1, HPFCsꢀ2 and HPFCsꢀ3. Correspondingly, the high resolved peeks of C1s showed at
284.8 eV for HPFCsꢀ1, 284.7 eV for HPFCsꢀ2 and 284.9 eV for HPFCsꢀ3, associating with the
existence of CꢀS bond in all the preparative catalysts. The signals of S2p at around 167.5 eV, 167.3
eV and 167.9 eV for HPFCsꢀ1, HPFCsꢀ2 and HPFCsꢀ3, respectively, confirm the attendance of
group ꢀSO3H consistent with the above FTIR tests and the existence of Brønsted acidity in these
catalysts. Moreover, the peaks of Si2p and Zr3d appeared in HPFCsꢀ2 and HPFCsꢀ3 but HPFCsꢀ1,
indicating SZs were grafted into the polymer frameworks. As shown in Fig. 7, two peaks located
at 175.9 eV for HPFCsꢀ2 and 175.7 eV for HPFCsꢀ3 of Zr3d, corresponding to OꢀZrꢀO and
ZrꢀOꢀZr species, indicating the successful grafting of Lewis acid onto the catalysts. It is worth
mentioning that the peak Zr3d of HPFCsꢀ3, shown in Fig. 7d, is stronger than the same kind of
peak of HPFCsꢀ2. The anticipated difference possibly because the more ZrO2 was introduced to
the mesoꢀSiO2 when compared with nonꢀSiO2. Undoubtedly, a large number of mesoporous are
not only favorable for loading more efficient ZrO2, but also stimulative for an increase in specific
surface of the catalysts. Overall, these results confirm that carbonaceous catalysts contain
Brønsted acid and Lewis acid, which will be beneficial for enhancing of catalytic performance in
the production of HMF from cellulose.
Insert Fig. 7 XPS measurements (a) of HPFCs, S2p spectra (b) of HPFCs, Si2p spectra (c) of

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HPFCs-2 and HPFCs-3 and Zr3d spectra (d) of HPFCs-2 and HPFCs-3.
The more immediate characterization and quantification of the total acid density of HPFCsꢀ1,
HPFCsꢀ2 and HPFCsꢀ3 were evaluated by NH3ꢀTPD method, and the results provide reliable
datum were shown in Fig. 8. Moreover, the areas of the peaks represented the amount of acidic
sites, which were further calculated and summarized in Table 2, respectively. It is worth
mentioning that the different kinds of acidic sites are based on adsorptive temperatures of NH3,
and 150ꢀ250 ºC, 250ꢀ500 ºC and > 500 ºC were designated as the weak, medium, strong acid,
respectively32. Meanwhile, the amounts of acid were the rough calculation of integral area of
peeks. Obviously, the total amounts of acid was closely related to the contents of 1.726 mmol gꢀ1
(the superposition of 0.362 mmol gꢀ1 at 110 ºC, 0.503 mmol gꢀ1 at 317 ºC, 0.227 mmol gꢀ1 at 449
ºC and 0.634 mmol gꢀ1 at 700 ºC) for HPFCsꢀ1, 2.405 mmol gꢀ1 (the superposition of 0.662 mmol
gꢀ1 at 168ºC, 0.765 mmol gꢀ1 at 301 ºC and 0.878 mmol gꢀ1 at 713 ºC) for HPFCsꢀ2 and 2.677
mmol gꢀ1 (the superposition of 0.887 mmol gꢀ1 at 165 ºC, 0.893 mmol gꢀ1 at 301 ºC and 0.897
mmol gꢀ1 at 715 ºC) for HPFCsꢀ3, respectively. Undoubtedly, the acid strength have a tremendous
increase after employing the SZs particles as the stabilizer for O/W Pickering HIPEs. The increase
of acid amount has an important effect on the conversion cellulose into HMF. Furthermore, the
peeks of strong acid of HPFCsꢀ2 and HPFCsꢀ3 appeared at the higher temperature than HPFCsꢀ1,
which is probably because stronger acidic sites from the stable particle of SZs partly. Although the
same weight of SZs were employed in HPFCsꢀ3 and HPFCsꢀ2, the slight augment of acidic
amounts was emerged due to the more surface of mesoꢀSiO2 than nonꢀSiO2. Thus, the strong acid
properties of the catalysts can be easily obtained by controlling the stabilizer, thereby offering a
simple method for adjusting the strong acid strength of polymeric catalysts.
Insert Fig. 8 NH3-TPD curves of HPFCs-1 (a), HPFCs-2 (b), HPFCs-3 (c).
Insert Table 2 Acidic properties of the HPFCs-1, HPFCs-2 and HPFCs-3.
Finally, the TG analysis of HPFCsꢀ1 (a), HPFCsꢀ2 (b) and HPFCsꢀ3 (c) was tested and the
datum were shown in the Fig. 9. As we all know, the TG spectra showed mass loss steps at
different temperature, and the following DSC curves indicated endothermic or exothermic
reactions which was revealed by temperature of the peaks. The headmost mass loss of HPFCsꢀ1
(22.85%), HPFCsꢀ2 (15.07%) and HPFCsꢀ3 (14.59%) happened in the region of 25 ºC to 250 ºC,
associating with the loss of absorbed water. Simultaneously, the endothermic peeks of HPFCsꢀ1
(71.3 ºC) showed higher than HPFCsꢀ2 (66.4 ºC) and HPFCsꢀ3 (64.7 ºC) in the respective DSC
curves because of the more water captured by HPFCsꢀ1, indicating the successful introduction of
SZs for HPFCsꢀ2 and HPFCsꢀ3. With the temperature rising to 800 ºC, the major mass loss of
HPFCsꢀ1 (42.69%), HPFCsꢀ2 (39.67%) and HPFCsꢀ3 (42.84%) emerged between 450 ºC and 500

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ºC. And, the DSC curves revealed the endothermic peaks of HPFCsꢀ1 (332.7 ºC), HPFCsꢀ2 (353.0
ºC) and HPFCsꢀ3 (352.7 ºC, 442.1 ºC). The significant weight loss and large endothermic peaks
were resulted from pyrolysis of organic matter and decomposition of ꢀSO3H groups. Notably, the
remaining mass of HPFCsꢀ2 (45.57%) and HPFCsꢀ3 (42.57%) were more than HPFCsꢀ1 (34.46%)
at 800 ºC, may be residual the heat processed stable SZs besides of carbon generated by
calcination of the polymers. Overall, it was found that the HPFCsꢀ1, HPFCsꢀ2 and HPFCsꢀ3
possessing high enough thermal stability for the moderate conversion.
Insert Fig. 9 TG-DSC curves of HPFCs-1 (a), HPFCs-2 (b), HPFCs-3 (c).
3.3 Catalytic performance
In order to investigate the effectiveness of the catalysts integrated different pore structure and
acidic strength roundly on the production of HMF from cellulose, HPFCsꢀ1, HPFCsꢀ2 and
HPFCsꢀ3 acted as catalysts respectively. The influence of catalysts loading on HMF yield was first
studied. 30 to 80 mg of catalysts were added to the reactive system with the same condition as
summarized in Fig. 10a. The maximum yield of 22.6%, 39.1% and 42.0% were obtained at
loading of 70 mg HPFCsꢀ1, 70 mg HPFCsꢀ2 and 60 mg HPFCsꢀ3. Obviously, the increasing
catalyst loading induced a gradual increase of HMF yield before the optimum dosage. Then, the
superabundant amount of catalysts aroused reduction of HMF yield, which is facilitated the
decomposition side reaction or polymerization of HMF15. Among these catalysts, HPFCsꢀ3
revealed the best catalytic performance, followed by the HPFCsꢀ2 and HPFCsꢀ1. And, HPFCsꢀ2
displayed much more effectively than HPFCsꢀ1 in the significant conversion. These results may
be attributed to the existence of SZs and Lewis acidic sites in HPFCsꢀ2, and the additive
mesopores in HPFCsꢀ332,33
.
Then, under the respective optimized catalyst loading, the effect of reaction temperature and
time were evaluated on conversion of cellulose into HMF, and the results were shown in the Fig.
10b, Fig. 10c, and Fig. 10d. It is not difficult to observe these common rules for the catalysts of
HPFCsꢀ1 and HPFCsꢀ2 and HPFCsꢀ3 applied in the conversion. Firstly, the HMF yield increases
rapidly along with the accessorial reaction time during the initial period, then decreases with the
followꢀup progression of the reaction time after the optimum one. It is possible because the
movement of solvent closer to the substrate molecule becomes an energetically spontaneous
process by lengthening the reaction time51. Then, the major sideꢀreaction, produced by
acidꢀcatalyzed hydration of HMF to levulinic acid and formic acid, has an augment with duration
of catalysis. Secondly, HMF yield shows positive correlation with the reaction temperature at the
beginning, then the negative correlation was appeared because of the excessively high temperature.
This phenomena can ascribe to the reduced viscosity of the ionic liquids by enhancing the reaction

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temperature, thereby increasing catalyst activity53,54. However, this function not only accelerate
degradation of cellulose to HMF, but also simultaneously increase the formation of byꢀproducts,
including unidentified soluble polymers and insoluble compounds that covered the active sites of
the catalysts.
Moreover, each temperature has a best reaction time for the maximum yields. For HPFCsꢀ1 (Fig.
10b), the most effective conversions (20.9%, 23.2%, 22.0%, 20.1% and 17.2%) of various
temperatures (110 ºC, 120 ºC, 130 ºC, 140 ºC and 150 ºC) were emerged at 90 min, 70 min, 50
min, 40 min and 40 min. For HPFCsꢀ2 (Fig. 10c), the optimal combinations of reaction
temperature and time were 90 min at 110 ºC, 70 min at 120 ºC, 70 min at 130 ºC, 50 min at 140 ºC
and 50 min at 150 ºC for the yields of 30.1%, 40.0%, 33.0%, 27.3% and 23.8%. Similarly, the
highest yields (31.8%, 42.0%, 37.5%, 30.4% and 25.3%) of HPFCsꢀ3 (Fig. 9d) were obtained at
110 ºC for 90 min, 120 ºC for 50 min, 130 ºC for 50 min, 140 ºC for 40 min and 150 ºC for 40 min.
Obviously, 120 ºC is the optimal temperature for dissolution and degradation of macromolecular
cellulose in [BMIM]ꢀCl. Then, with the gradually reducing reaction temperature, the reaction time
must be lengthened to get the ideal HMF yield, as shown in the Fig. 10b, Fig. 10c, and Fig. 10d.
This phenomenon is possibly attributed to the interaction of time and temperature in the catalytic
process.
Furthermore, the maximum yield (42.0%) of HPFCsꢀ3 appeared at 120 ºC for 50 min, which is
the highest efficiency comparing with the catalysts of HPFCsꢀ2 (40.0% at 120 ºC for 70 min) and
HPFCsꢀ1 (23.2% at 120 ºC for 70 min). The less catalysts with higher yield also verified the best
catalytic activity of HPFCsꢀ3. The result is ascribed to simultaneous existence of sulfated zirconia
and mesopores in the HPFCsꢀ355. And the yield discrepancy of HPFCsꢀ1 with HPFCsꢀ2 is greater
than discrepancy of HPFCsꢀ2 with HPFCsꢀ3, indicating the more effective of Lewis acidic sites
from SZs than the incremental specific surface area from the mesopores. In a word, the idea of
preparation the innovative catalysts with the treated mesoporous silica nanoparticles by sulfated
zirconia is a feasibility, effective and progressive design.
Insert Fig. 10 (a) Influence of the loading amount of HPFCs-1, HPFCs-2, HPFCs-3 on the
yield from cellulose in the ionic liquid [BMIM]-Cl, and the other reaction parameters were
constant: [cellulose] = 100 mg, T = 120 ºC, t = 50 min. (b) Effects of reaction temperature
and time on the conversion of cellulose to HMF with reaction conditions: 70 mg HPFCs-1,
100 mg cellulose and 2.0 g [BMIM]-Cl. (c) Effects of reaction temperature and time on the
conversion of cellulose to HMF with reaction conditions: 70 mg HPFCs-2, 100 mg cellulose
and 2.0 g [BMIM]-Cl. (d) Effects of reaction temperature and time on the conversion of
cellulose to HMF with reaction conditions: 60 mg HPFCs-3, 100 mg cellulose and 2.0 g
[BMIM]-Cl.

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3.4 Repetitiveness of the catalysts
On account of green and sustainable chemistry, the regeneration of catalyst was very necessary
to the practical production of HMF, and the results was presented in the Fig. 11a. The reactions
were carried out under the optimized reaction conditions (60 mg catalysts, 120 ºC and 5 h) in the
presence of HPFCsꢀ3. It is worth mentioning the solid acidic catalyst was reclaimed by simply
method of vacuum filter the liquid admixture after the previous reaction and washed by deionized
water, then dried at 30 ºC for 12 h in a vacuum oven for the next reaction. The catalyst was
recycled in four consecutive cycles with a total loss of only 5.2 % activity. The slight reduction
may be from the byꢀproduct, which prevented the new reactant molecules from contacting to the
acidic sites on the surface of catalysts. Besides, we conducted a hot filtration test of catalytic
system to further confirm the catalyst stability by quantitative determining the acid and zirconia of
filtrate. Notably, the average pH of filtrates is 6.16, indicating the few loss of acid during catalysis.
And, the filtrate not detected containing zirconia by EDTA complex titration method with xylenol
orange indicator, demonstrating the stability of ZrꢀO bond in the catalysis. The catalyst HPFCsꢀ3
recovered after the fourth cycles was again characterized by elemental analyzer, SEM and EDS.
The rigid porous structure also could be observed from the SEM images (Fig. 11b), indicating that
the reclaimed HPFCsꢀ3 still possess superior recyclability and reusability with high mechanical
stability of the polymer network. Moreover, Zr and Si peaks were still detected in the EDS spectra
of HPFCsꢀ3 recovered after the fourth cycles (Fig. 11c). And, the elemental compositions of S
(6.7%) declined only slightly comparing with the corresponding values for asꢀprepared HPFCsꢀ3
(Fig. 11d). Therefore, the excellent recyclability of HPFCsꢀ3, indicating the stability of acidic sites
and polymer network, was very important for the widely applications in coming industrialized
production of HMF from abandoned biomass.
Insert Fig. 11 (a) Reusability of HPFCs-3 in the conversion of cellulose into HMF in
[BMIM]-Cl under the optimized conditions: 120 ºC, 40 mg of catalyst based on 100mg of
cellulose, 50 min, 2.0 g [BMIM]-Cl. (b) SEM images of HPFCs-3 recovered after the fourth
cycle. (c) EDS images of HPFCs-3 recovered after the fourth cycle. (d) The element contents
of HPFCs-3 recovered after the fourth cycle.
4. Conclusion
In this study, hierarchical porous foam catalysts with wellꢀdefined characteristics were explored
for the oneꢀpot cellulose to HMF production. The synthetic method involves the formation of a
ZrO2 layer on the surface of a SiO2 templates, sulfuric acid treatment to introduce sulfate ions, and
finally calcination to produce sulfated ZrO2 shells. And the Pickering HIPEs template method was

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adopted to structure the BrønstedꢀLewis acidic and hierarchical porous catalysts. The influence of
the sulfated ZrO2 and the additive mesoporous on catalytic effectiveness was demonstrated by
comparison of the prepared three catalysts of the HMF yield. The highest HMF yield (42%) was
obtained in the oneꢀpot conversion of cellulose into HMF with HPFCsꢀ3, confirming the important
roles of BrønstedꢀLewis acidic sites (sulfated ZrO2) and the hierarchical pores. In conclusion, this
newly developed catalysts could be valuable for future study toward the wide application in
biomass conversion and bioenergy.
ACKNOWLEDGMENTS
This work was financially supported by the National Natural Science Foundation of China (No.
21107037, No. 21176107, No. 21306013), Natural Science Foundation of Jiangsu Province (No.
BK20160498, No. BK2011514), National Postdoctoral Science Foundation (No. 2013M530240),
Postdoctoral Science Foundation funded Project of Jiangsu Province (No. 1202002B, No.
1501024A).
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51 S. W. Zou, Z. J. Wei, Y. Hua, Y. H. Deng, Z. Tong, C. Y. Wang, Polym. Chem., 2014, 5,
4227ꢀ4234.
52 D. Liu, P. Yuan, H. M. Liu, J. G. Cai, D. Y. Tan, H. P. He, J. X. Zhu and T. H. Chen, Appl.Clay
Sci., 2013, 80, 407ꢀ412.
53 M. E. Zakrzewska, E. BogelꢀLukasik and R. BogelꢀLukasik, Chem. Rev., 2011, 111, 397ꢀ417.
54 A. Dutta, D. Gupta, A. K. Patra, B. Saha, A. Bhaumik, ChemSusChem, 2014, 7, 925ꢀ933.
55 W. H. Peng, Y. Y. Lee, C. Wu and K. C. W. Wu, J. Mater. Chem., 2012, 22, 23181ꢀ23185.
Fig. 1. TEM images of nonꢀSiO2 (a), nonꢀZrO2@SiO2 (b), nonꢀSZs (c), mesoꢀSiO2 (d), mesoꢀ
ZrO2@SiO2 (e), and mesoꢀSZs (f).
Fig. 2. SEM images of HPFCsꢀ1 (A, a), HPFCsꢀ2 (B, b) and HPFCsꢀ3 (C, c).

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200
175
150
125
100
75
180
b
a
0.11
0.10
0.09
0.08
0.07
0.06
0.05
0.04
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
160
140
120
100
80
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
Pore Radius (nm)
Pore Radius (nm)
60
40
50
Adsorption
Desorption
Adsorption
Desorption
20
0
25
0.0
0.2
0.4
0.6
0.8
1.0
0.2
0.6
0.8
1.0
Relativ0e.4Pressure(P/P0)
Relative Pressure(P/P0)
50
40
30
20
10
0
160
140
120
100
80
c
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0.000
d
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0
10 20 30 40 50 60 70
Pore Radius (nm)
1
2
3
4
5
6
7
8
Pore Radius (nm)
60
40
Adsorption
Desorption
20
Adsorption
Desorption
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure(P/P0)
Relative Pressure(P/P0)
120
100
80
60
40
20
0
100
e
f
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0.000
0.006
0.005
0.004
0.003
0.002
0.001
0.000
80
60
40
20
0
0
10 20 30 40 50 60 70
Pore Radius (nm)
0
10 20 30 40 50 60 70
Pore Radius (nm)
Adsorption
Desorption
Adsorption
Desorption
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure(P/P0)
Relative Pressure(P/P0)

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Fig. 3 (a) Nitrogen adsorption–desorption isotherms of mesoꢀSiO2 (inset: pore distribution), (b)
mesoꢀZrO2@SiO2 (inset: pore distribution), (c) mesoꢀSZs (inset: pore distribution), (d) HPFCsꢀ1
(inset:pore distribution), (e) HPFCsꢀ2 (inset: pore distribution) and (f) and HPFCsꢀ 3 (inset: pore
distribution).
a
b
C
C
S
S
O
Na
O
Zr
Si
Na
Zr
0.5
2.0
2.5
3.0
1.0KeV1.5
0.5
1.0
2.0
2.5
KeV 1.5
50
45
40
35
30
C
N
c
C
d
S
S
O
Na
8
6
4
2
0
Zr
Si
Zr
1.0 KeV 1.5
0.5
2.0
2.5
HPFCs-1
HPFCs-2
HPFCs-3
Fig. 4. EDS images of HPFCsꢀ1 (a), HPFCsꢀ2 (b) and HPFCsꢀ3 (c) and the element contents of
the catalysts (d).
non-SZs
meso-SZs
HPFCs-1
HPFCs-2
HPFCs-3
a
b
818.06
1120.22
449.67
500
1376.95
1668.22
2924.64
2854.52
443.68
500
3500 3000 2500 2000
1500-11000
3500 3000 2500 2000 1500
-11000
Wave number (cm )
Wave number (cm )

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Fig. 5. FTꢀIR spectra of HPFC (a) and SZs (b).
HPFCs-1
HPFCs-2
HPFCs-3
non-SZs
meso-SZs
a
b
20
30
40
50
60
70
20
30
40
50
60
2θ/°
2θ/°
Fig. 6. XRD patterns of HPFCs (a) and SZs (b).
HPFCs-1
HPFCs-2
HPFCs-3
HPFCs-1
HPFCs-2
HPFCs-3
a
b
1000
400
0
150
160
170
180
190
800Bind6in00g energy (eV)200
Binding energy (eV)
HPFCs-2
HPFCs-3
HPFCs-2
HPFCs-3
c
d
90
95
100
105
110
115 155
160
165
170
175
Binding energy (eV)
Binding energy (eV)

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Fig. 7. XPS measurements (a) of HPFCs, S2p spectra (b) of HPFCs, Si2p spectra (c) of HPFCsꢀ2
and HPFCsꢀ3 and Zr3d spectra (d) of HPFCsꢀ2 and HPFCsꢀ3.
a
b
100 200 300 400 500 600 700 800
0
100 200 300 400 500 600 700 800
Temperature (°C)
Temperature (°C)
c
0
100 200 300 400 500 600 700 800
Temperature (°C)
Fig. 8. NH3ꢀTPD curves of HPFCsꢀ1 (a), HPFCsꢀ2 (b), HPFCsꢀ3 (c).

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100
80
71.3oC
100
1.5
a
b
1.5
22.85%
351.7oC
15.07%
353.0oC
66.4oC
80
1.0
1.0
39.67%
42.69%
60
0.5
60
0.5
40
0.0
TG
DSC
TG
DSC
40
0.0
-0.5
100 200 300 400 500 600 700 800
100 200 300
600 700 800
T(
°
400C)500
T(°C)
100
80
2.0
c
14.59%
1.5
1.0
0.5
0.0
64.7oC
42.84%
60
352.7oC
442.1oC
40
TG
DSC
100 200 300 400 500 600 700 800
T(°C)
Fig. 9. TGꢀDSC curves of HPFCsꢀ1 (a), HPFCsꢀ2 (b), HPFCsꢀ3 (c).

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110
120
130
140
150
°
°
°
°
°
C
C
C
C
C
a
24
21
18
15
12
9
b
40
30
20
10
0
HPFCs-1
HPFCs-2
HPFCs-3
20
40
60
120 140
160
t 8m0 in 100
( )
30
40
50
60
70
80
Catalyst loading (mg)
45
40
35
30
25
20
15
110
120
130
140
150
°C
110
120
130
140
150
°C
d
c
40
35
30
25
20
15
°
C
C
C
C
°
C
C
C
C
°
°
°
°
°
°
20
40
60
120 140 160
t ( )
80 mi1n00
20
40
60
120 140 160
t
80(mi1n0)0
Fig. 10. (a) Influence of the loading amount of HPFCsꢀ1, HPFCsꢀ2, HPFCsꢀ3 on the yield from
cellulose in the ionic liquid [BMIM]ꢀCl, and the other reaction parameters were constant:
[cellulose] = 100 mg, T = 120 ºC, t = 50 min. (b) Effects of reaction temperature and time on the
conversion of cellulose to HMF with reaction conditions: 70 mg HPFCsꢀ1, 100 mg cellulose and
2.0 g [BMIM]ꢀCl. (c) Effects of reaction temperature and time on the conversion of cellulose to

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HMF with reaction conditions: 70 mg HPFCsꢀ2, 100 mg cellulose and 2.0 g [BMIM]ꢀCl. (d)
Effects of reaction temperature and time on the conversion of cellulose to HMF with reaction
conditions: 60 mg HPFCsꢀ3, 100 mg cellulose and 2.0 g [BMIM]ꢀCl.
50
a
40
30
20
10
0
C0
C1
C2
C3
C4
45
40
c
C
d
40.2
S
35
10
O
Na
6.7
S
4.9
N
Zr
Si
5
0
0.5
1.0
2.0
2.5
3.0
K1e.V5
C
Fig. 11. (a) Reusability of HPFCsꢀ3 in the conversion of cellulose into HMF in [BMIM]ꢀCl under
the optimized conditions: 120 ºC, 40 mg of catalyst based on 100mg of cellulose, 50 min, 2.0 g
[BMIM]ꢀCl. (b) SEM images of HPFCsꢀ3 recovered after the fourth cycle. (c) EDS images of
HPFCsꢀ3 recovered after the fourth cycle. (d) The element contents of HPFCsꢀ3 recovered after
the fourth cycle.

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Hydrolysis
Isomerization
OH
OH
OH
OH
O
O
O
O
OH
HO
O
CH2OH
OH
HOCH2
OH
O
HO
HO
HO
O
OH
HO
HO
n-2
OH
OH
HO
OH
OH
Glucose
Cellulose
Dehydration
Fructose
H
O
O
HO
HMF
Scheme 1. The catalytic mechanism for conversing cellulose into HMF.
Sulphonating
ZrO2
calcining
packaging
H+
O
Brønsted acid site
H
O
O
O
meso-SZ
meso-ZrO2@SiO2
meso-SiO2
Zr
O
O
O
S
Zr
O
O
O
O
ZrO2
Sulphonating
calcining
packaging
Zr+
Lewis acid site
non-SZ
non-SiO2
non-ZrO2@SiO2
Scheme 2. Synthesis of SZs.
Emulsification
Polymerization
Sulfonation
Oil-water interface
HPFCs
Pickering HIPEs
SZs
Tween85
Oil phase
Aqueous phase

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Scheme 3. Synthesis of HPFCs and conversion of cellulose into HMF.
Table 1 Porosity properties of the catalysts mesoꢀSiO2, mesoꢀZrO2@SiO2, mesoꢀSZs, HPFCsꢀ1,
HPFCsꢀ2 and HPFCsꢀ3
Samples
mesoꢀSiO2
mesoꢀZrO2@SiO2
mesoꢀSZs
SBETa (m2 gꢀ1)
RBJHb (nm)
Vtc (cm3 gꢀ1)
192.7
3.9
2.3
2.8
ꢀ
0.23
143.5
0.17
144.3
0.13
HPFCsꢀ1
45.5
0.03
HPFCsꢀ2
97.6
ꢀ
0.05
HPFCsꢀ3
109.7
19.8
0.06
a
b
SBET is the BET specific surface area. RBJH is BJH mean pore radius calculated by the
desorption branches of the nitrogen sorption isotherms. c Vt is the total pore volume determined at
the relative pressure of 0.99.

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Samples
HPFCsꢀ1
HPFCsꢀ2
HPFCsꢀ3
Acid strength (ºC)
Acid amounts (mmol gꢀ1)
Total acidity (mmol gꢀ1)
110
317
449
700
168
301
713
165
301
715
0.362
0.503
0.227
0.634
0.602
0.765
0.878
0.887
0.893
0.897
1.726
2.405
2.677
Table 2 Acidic properties of the HPFCsꢀ1, HPFCsꢀ2 and HPFCsꢀ3

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211x136mm (96 x 96 DPI)