Efficient and stable PS-SO3H/SiO2 hollow nanospheres with tunable surface properties for acid catalyzed reactions

Article abstract of DOI:10.1016/j.apcata.2016.02.008

Facile synthesis of hybrid solid acids with finely engineered surface properties was successfully realized via sulfonation of polystyrene (PS) dispersed in nanopores of silica hollow nanosphere modified with different organic group. It was found that octyl or perfluorinated octyl could efficiently increase the surface hydrophobicity of the hybrid solid acids. Benefited from the facile adjustability of the surface properties, the catalytic performance of the solid acids could be readily improved in both the esterification of lauric acid with ethanol and the transterification between tripalmitin and methanol. The solid acid modified with perfluorinated octyl shows even higher activity than liquid sulfuric acid in the transesterification reaction. The octyl group incorporated in the silica shell could also prevent the leaching of PS-SO₃H during the catalytic process.

Full text of DOI:10.1016/j.apcata.2016.02.008

Accepted Manuscript
Title: Efficient and stable PS-SO3H/SiO2 hollow nanospheres
with tunable surface properties for acid catalyzed reactions
Author: Jingjing Chen Jian Chen Xiaomin Zhang Jinsuo Gao
Qihua Yang
PII:
S0926-860X(16)30063-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.02.008
APCATA 15767
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
30-10-2015
2-2-2016
5-2-2016
Please cite this article as: Jingjing Chen, Jian Chen, Xiaomin Zhang, Jinsuo
Gao, Qihua Yang, Efficient and stable PS-SO3H/SiO2 hollow nanospheres with
tunable surface properties for acid catalyzed reactions, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2016.02.008
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Efficient and stable PS-SO3H/SiO2 hollow nanospheres with tunable
surface properties for acid catalyzed reactions
Jingjing Chen[ab], Jian Chen[ac], Xiaomin Zhang[a], Jinsuo Gao*[b], Qihua Yang*[a]
[a] State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China.
[b] Key Laboratory of Industrial Ecology and Environmental Engineering (MOE),
School of Environmental Science and Technology, Dalian University of Technology,
Linggong Road 2, Dalian 116024, China.
[c] Graduate School of the Chinese Academy of Sciences, Beijing 100049, China.
* To whom correspondence should be addressed. E-mail: yangqh@dicp.ac.cn;
jsgao@dlut.edu.cn

Graphical abstract
O
Si
O
O
O
*
*
n
SO3H
SO3H
Silica hollow nanospheres with sulfonated polystyrene and octyl groups dispersed in
nanopores act as efficient solid catalysts for esterification and transesterification
reactions.

Highlights

PS-SO3H/SiO2 hybrids hollow nanospheres with high content of octyl group in
the shell were synthesized by one-pot co-condensation method using
chlorosilanes as precursor.



The surface hydrophilicity/hydrophobicity of PS-SO3H/SiO2 hybrids hollow
nanospheres could be facilely adjusted.
The solid acid modified with perfluorinated octyl shows even higher activity than
liquid sulfuric acid in the transesterification reaction.
The octyl group incorporated in the silica shell could also prevent the leaching of
PS-SO3H during the catalytic process.
Abstract
Facile synthesis of hybrid solid acids with finely engineered surface properties was
successfully realized via sulfonation of polystyrene (PS) dispersed in nanopores of
silica hollow nanosphere modified with different organic group. It was found that
octyl or perfluorinated octyl could efficiently increase the surface hydrophobicity of
the hybrid solid acids. Benefited from the facile adjustability of the surface properties,
the catalytic performance of the solid acids could be readily improved in both the
esterification of lauric acid with ethanol and the transterification between tripalmitin
and methanol. The solid acid modified with perfluorinated octyl shows even higher
activity than liquid sulfuric acid in the transesterification reaction. The octyl group
incorporated in the silica shell could also prevent the leaching of PS-SO3H during the
catalytic process.
Keywords: solid acid; octyl; hydrophobicity; esterification;transesterification.

1. Introduction
The development of sustainable and green energy is of quite importance due to
energy crisis and environment pollution. Similar to solar-, hydro-, and wind-energy,
biofuel including bioethanol and biodiesel have received considerable attentions as
sustainable energy sources.[1-7] Compared with traditional fossil energy, biodiesel
has low sulfur and aromatic content, high flash point, high cetane number, good
lubricity. [1, 8, 9] Compared with base catalysts, acid catalysts can simultaneously
catalyze the esterification and transesterification reactions to achieve biodiesel
production without soap formation.[8] However, liquid acids meet many problems in
industrial application, such as waste water production, equipment corrosion and so
on.[10-14] Therefore, the usage of solid acids for transesterification reactions is highly
desirable.[5, 12-14]
Polystyrene sulfonic acid resins (PS-SO3H) represent one of the most efficient
organic solid acids and have widely been used in industry.[15-17] However, this kind
of solid acid generally has poor thermal stability and very low surface area. For
solving the above problems, mesoporous polystyrene sulfonic acid [5, 18] and
polystyrene sulfonic acid/inorganic oxide composites have been synthesized.[19, 20]
There are two major factors that influence the catalytic performance of solid acid
catalysts. Firstly, the textural parameters and the morphology of the solid acids could
greatly affect the diffusion rates of reactants and products during the catalytic
process.[8, 21-26] Secondly, the surface wettability (i.e. the hydrophobicity and
hydrophilicity) may have big influence on the adsorption and desorption of the
reactants and products and also the surface transfers between catalysts and
molecules.[27-30] The high performance solid acid catalysts should have suitable
surface wettability for fast adsorption of reactants on the active sites and also the fast

desorption of products from the active sites. [31] For achieving the above goal, the
surface wettability, nanostructure and the morphology of the solid acid catalysts
should be carefully engineered. Recently, we have reported the synthesis of
PS-SO3H/MS hybrids hollow nanospheres with poly(methyl acrylate) (PMA)
modified shell for acid catalyzed reactions.[33] The hollow nanostructure with shell
thickness of  50 nm could facilitate the fast diffusion of reactants and products due to
its shortened diffusion pathway. Also, it was found that further modification of silica
shell with octyl group via grafting method could increase both the activity and
stability of the solid acid catalysts. However, the amount of octyl group incorporated
in the silica hollow nanospheres is limited and also the sharp decrease in BET surface
area was obseved using grafting method. Our recent studies suggest that higher
content of organic functional groups could be incorporated into mesoporous silicas
via one-pot co-condensation method using chlorosilanes.[34] Thus, it is possible for
the synthesis of PS-SO3H/SiO2 hybrids hollow nanospheres with high content of octyl
group in the shell by one-pot co-condensation method using chlorosilanes as
precursor to further investigate the influence of octyl groups on the activity and
stability of the hybrid solid acids.
Herein, we report the synthesis of sufonated polystyrene functionalized hollow
nanospheres with high acid concentration and finely engineered surface properties.
PS-SO3H/SiO2 hollow nano-spheres were facilely fabricated via one-pot condensation
of tetraethoxysilane (TEOS) with chlorosilanes (Cl3SiC8H4F13 or Cl3SiC8H17), using
polystyrene (PS) nanospheres and cetyltrimethylammonium bromide (CTAB) micellar
as template followed by the structure transformation from core-shell to hollow and
sulfonation. Benefited from the facile adjustability of the surface properties, the
catalytic activity of the solid acids was readily improved. In the esterification between

lauric acid with ethanol and the transterification between tripalmitin and methanol, the
solid acids show much higher activity than Amberlyst-15 and liquid acid, H2SO4.
2. Experimental section
2.1. Chemicals and reagents
All materials were of analytical grade. Cetyl-trimethylammonium bromide (CTAB,
99%) was purchased from Sigma-Aldrich Company Ltd. (USA). Tetraethoxysilane
(TEOS, 99.9%) was obtained from Nanjing Shuguang Chemical Group (China), and
1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane (C8H4F13SiCl3, 97%) and
n-octyltrichlorosilane (C8H17SiCl3, 98%) were obtained from Alfa Aesar (UK). Other
reagents such as anhydrous ethanol (99.5%) and tetrahydrofuran (THF, 99%). were
purchased from Shanghai Chemical Reagent, Inc. of the Chinese Medicine Group.
Styrene (99%) was washed through an inhibitor remover column for removing the
inhibitor and then distilled under vacuum before use. The other materials were used as
received without any further purification. PS nanospheres with particle size of about
240 nm were prepared according to previous report. [32]
2.2. Characterization
FT-IR spectra were collected with a Nicolet Nexus 470 IR spectrometer (KBr
pellets were prepared) in the range of 400-4000 cm-1. N2 sorption isotherms were
carried out on a Micromeritics ASAP2020 volumetric adsorption analyzer. Before the
sorption measurements, samples were out-gassed at 120 oC for 6 h. The BET surface
area was calculated from the adsorption data at a relative pressure P/P0 in the range of
0.04-0.2. Pore size distributions were determined from the adsorption branch using
nonlocal density functional theory (NLDFT) method. Pore volume was estimated at a
relative pressure P/P0 of 0.99. Transmission electron microscopy (TEM) was

performed on a HITACHI 7700 at an acceleration voltage of 100 kV. High resolution
transmission electron microscopy (HRTEM) was performed on a FEI TecnaiG2 F30
S-Twin at an acceleration voltage of 300 kV. Before measurement, the sample was
dispersed in ethanol and deposited on a holey carbon film on a Cu grid. SEM was
undertaken by using a FEI Quanta 200F scanning electro-microscope operating at an
acceleration voltage of 1–30 kV.
Solid-state NMR spectra were obtained with a Bruker DRX 400 spectrometer
equipped with a magic-angle spin probe using a 4-mm ZrO2 rotor.13C signals were
referenced to tetramethylsilane (TMS). The experimental parameters are as follows: 8
kHz spin rate, 3 s pulse delay, 4 min contact time, and1000 scans.
Water contact angle measurements were performed on KRÜSS DSA100, and the
sample films were prepared via pressing the material powder at a pressure of 8 MPa
on tablet press.
The acid exchange capacity of solid acids was determined by acid-base titration
with NaOH solution. In a typical experiment, solid samples (40~50 mg) was degassed
at 120 oC for 3 h and suspended in NaCl solution (2M, 25 mL). After stirring at room
temperature for 24 h to reach the equilibrium, the resulting suspension was titrated
with a standard NaOH solution (8.38 x10-3 M).[32,33,35,36]
2.3. Synthesis of PS@SiO2 core-shell nanospheres (CSNs) with different surface
properties
PS@SiO2 CSNs with different surface properties were prepared by coating PS
nanospheres with TEOS and chlorosilane under basic conditions using CTAB as
surfactant. In a typical synthesis, the solution containing 0.20 g of CTAB, 60 mL of

o
deionized water and 20 mL of ethanol was stirred at 50 C for 30 minutes, followed
by the addition of PS nanospheres (0.20 g) dispersed in 2 mL of deionized water. The
o
above mixture was stirred at 50 C for another 0.5 h, followed by the addition of
ammonium hydroxide solution (0.7 mL, 25%). Then, TEOS (0.6175 or 0.585 or
0.585g) in 2 mL ethanol was added into above mixture under vigorous stirring. After
5 min, chlorosilanes (36 or 72 or 116 μL) was added slowly. After stirring at 50 oC for
o
2 h, the mixture was transferred into a Teflon-lined autoclave and aged at 100 C
under static conditions for 36 h. The solid product was recovered by filtration, washed
with water and ethanol, and dried at 60 oC overnight. The surfactant was extracted by
dispersing the solid product (1g) in ethanol-HCl (200 mL, 0.75 wt % of HCl) and the
o
mixture was refluxed at 70 C for 8 h. The samples were denoted as PS@C8-SiO2-x
(x=0.05 or 0.10) CSNs and PS@C8F-SiO2-0.10 CSNs respectively prepared with
n-octyltrichlorosilane and 1H,1H,2H,2H-perfluorooctyltrichlorosilane as chlorosilane
precursors, where x is the mol ratio of chlorosilane/(chlorosilane + TEOS).
In a controlled experiment, PS@SiO2 CSNs were synthesized in a similar method
to PS@C8-SiO2-x CSNs except only TEOS (0.65 g) was used as the silica precursor.
2.4. Synthesis of PS/SiO2 hollow nanospheres (HNs) with different surface
properties
PS/SiO2 HNs was prepared by dispersing PS@SiO2 CSNs (1 g) in 200 mL of
o
THF and the mixture was heated at 60 C for 4 h. After centrifugation and washing
with ethanol, PS/SiO2 HNs were obtained. The samples were denoted as
PS/C8-SiO2-x (x=0.05 or 0.10) HNs, PS/C8F-SiO2-0.10 HNs and PS/SiO2 HNs using
PS@C8-SiO2-x (x=0.05 or 0.10) CSNs, PS@C8F-SiO2-0.10 CSNs and PS@SiO2
CSNs as precursors, respectively.
2.5. Synthesis of PS-SO3H/SiO2 hollow nanospheres (HNs) with different surface

properties
o
PS/SiO2 HNs (1g) was outgassed at 120 C under vacuum in a two-necked round
flask for 3 h. Then, 50 mL of CH2Cl2 containing 10 mL of chlorosulfonic acid was
added to the flask at 0 oC. The mixture was stirred for 12 h under an argon atmosphere.
The suspension was washed with copious amounts of water until the filtrate was
neutral. After centrifugation and washing with ethanol, the powder products were
o
dried at 60 C overnight. The resulting powder products were denoted as
PS-SO3H/C8-SiO2-x (x=0.05 or 0.10) HNs, PS-SO3H/C8F-SiO2-0.10 HNs and
PS-SO3H/SiO2 HNs using PS/C8-SiO2-x (x=0.05 or 0.10) HNs, PS/C8F-SiO2-0.10
HNs and PS/SiO2 HNs as precursors, respectively.
2.6. General procedure for acid-catalyzed reactions
The catalysts were pretreated at 120 °C under vacuum for 3 h before
acid-catalyzed reactions.
Esterification of lauric acid with ethanol was carried out in a two-necked round
flask equipped with a reflux condenser and a magnetic stirrer. In a typical experiment,
10 mmol of ethanol and 2 mmol of lauric acid were added to the flask charged with
PS-SO3H/SiO2 catalysts (0.04 mmol of H+). The mixture was stirred at 80 °C for 6 h.
The products were collected by a syringe at regular intervals and analyzed using a
precalibrated gas chromatograph (Agilent 7890) equipped with a flame ion detector
(FID) and PEG capillary column (30 m × 0.25 mm × 0.25 mm). Tetradecane was used
as an internal standard.
Transesterification of tripalmitin and methanol was carried out in a two-necked
round flask equipped with a reflux condenser and a magnetic stirrer. In a typical
experiment, 0.72 mmol of tripalmitin and 2 mL of methanol were added to the flask
charged with PS-SO3H/SiO2 catalysts (0.04 mmol of H+). The mixture was stirred at

80 °C for 21 h. The products were analyzed by Agilent 7890 gas chromatography with
a flame ionization detector (FID) and HP-INNOWax capillary column (30 m × 0.25
mm × 0.25 mm). Dodecane was used as an internal standard.
3. Results and discussion
The general procedure for the synthesis of PS-SO3H/SiO2 HNS was outlined in
Fig. 1A. In the first step, PS@SiO2 CSNs with different surface properties were
synthesized via sol-gel coating of PS by one-pot condensation of TEOS and
chlorosilanes (Cl3SiC8H4F13 or Cl3SiC8H17) using CTAB as template. TEM images
clearly reveal the core shell nanostructure with dark core (PS) and gray shell layer
(SiO2) (Fig. 1B). The surface properties of the PS@SiO2 CSNs could be facilely
adjusted by the type and amount of chlorosilane precursor. After THF treatment, PS
core was dissolved and diffused into the mesopore of the silica shell for the formation
of hollow nanospheres as evidenced by the TEM image in Fig. 1C. Then, sulfonation
of PS/SiO2 HNs results in the formation of PS-SO3H/SiO2 HNs with different surface
properties. The TEM images clearly verify that all PS-SO3H/SiO2 HNs functionalized
with different type and different amount of organic groups are composed of hollow
nanospheres (Fig. 2). SEM image shows the sample is composed of mono-dispersed
nanospheres with porous rough surfaces and uniform partcile size distribution in the
range of 370 to 350 nm, taking PS-SO3H/C8-SiO2-0.10 HNs as an example. Based on
the TEM image, the average shell thick of PS-SO3H/SiO2 HNs is around 60 to 70 nm.
It should be mentioned that the location of the sulfonic acid site is very important
for catalysis. The HRTEM element mapping was used to reveal the location of
sulfonic acid sites, taking PS-SO3H/C8-SiO2-0.10 HNs as an example (Fig. 2C). The
result clearly shows that Si and S are mainly distributed in the shell of hollow
nanospheres. This further confirms that PS diffuses into the mesopore of silica shell

during THF treatment of core-shell structured nanospheres.
The N2 adsorption-desorption isotherms of
PS-SO3H/SiO2
HNs,
PS-SO3H/C8-SiO2-x (x=0.05, 0.10) HNs, and PS-SO3H/C8F-SiO2-0.10 HNs are
showed in Fig. 3. Clearly, PS-SO3H/SiO2 HNs display typical type-IV isotherms with
a steep capillary condensation in the relative pressure range between 0.20<P/P0<0.40,
showing that this sample has mesoporous structure in the shell. The hysteresis loop at
0.45<P/P0<0.95 is probably from the hollow interior of hollow nanospheres.
Compared with PS-SO3H/SiO2 HNs, the capillary condensation step is less defined
for PS-SO3H/C8-SiO2-x (x=0.05, 0.10) HNs, and PS-SO3H/C8F-SiO2-0.10 HNs,
suggesting that the incorporation of octyl and perfluorinated octyl group deteriorate
the mesostructural order in the shell [37]. PS-SO3H/SiO2 HNs show higher BET
surface area and larger pore volume and pore diameter than PS-SO3H/C8-SiO2-x
(x=0.05, 0.10) HNs, and PS-SO3H/C8F-SiO2-0.10 HNs (Table 1).
PS-SO3H/C8-SiO2-0.05 with less octyl group has larger BET surface ares and pore
diameter than PS-SO3H/C8-SiO2-0.10 with higher octyl group. The BET surface area
and
pore
diameter
decreases
in
the
order
of
PS-SO3H/SiO2
PS-SO3H/C8F-SiO2-0.10 PS-SO3H/C8-SiO2-0.05 PS-SO3H/C8-SiO2-0.10.
As shown in Fig. 4A, the FT-IR spectra of all PS-SO3H/SiO2 HNs clearly displays
vibration peaks at 1020 cm-1 and 573 cm-1, respectively assigned to of S=O [6, 38]
symmetric stretching vibration and S-C stretching vibration [39], which confirms the
successful incorporation of sulfonic acid group onto aromatic ring during sulfonation
process. The vibration peaks in the range of 900 cm-1 to 650 cm-1 are from the C–H
bending vibration and C=C stretching vibration of phenyl groups. Compared with PS-
SO3H/SiO2 HNs, the intensified vibration peaks at 2930cm-1 (CH2,νas) [34] for
PS-SO3H/C8-SiO2-x (x=0.05, 0.10) HNs and the new peak at 1240 cm-1 (CF2,CF3,νas)

[6, 7] for PS-SO3H/C8F-SiO2-0.10 clearly show the existence of octyl and
1H,1H,2H,2H-perfluorinated octyl groups as the surface modified samples (Fig.
4A).
The solid-state 13C CP/MAS NMR spectra of PS-SO3H/SiO2 HNs and
PS-SO3H/C8-SiO2-0.10 HNs are presented in Fig. 4B. The strong signals at 40 ppm
and 43 ppm are attributed to the aliphatic carbon chain on aromatic ring. The chemical
shifts at 127 ppm, 141 ppm (incorporated with SO3H) [5, 33] and 152 ppm
(incorporated with vinyl) could be assigned to aromatic rings. In addition, the new
13
chemical shifts at 13.1 ppm, 21.6 ppm, 28.5 ppm and 31.5 ppm in the C CP/MAS
NMR spectrum of PS-SO3H/C8-SiO2-0.10 HNs is from carbon species of octyl groups.
The combined results of FT-IR and 13C CP/MAS NMR confirm the successful
formation of organo-modified hybrid solid acids.
The sulfur content and acid exchange capacity was measured respectively by
element analysis and acid-base titration methods (Table 1). The sulfur contents of
samples vary in the range of 3.24 mmol/g to 2.85 mmol/g. The acid exchange
capacity of samples in the range of 1.25 mmol/g to 0.97 mmol/g is much lower than
sulfur content. The inconsistence between acid exchange capacity and sulfur content
suggests that some sulfonic acid sites are buried in the silica shell and cannot be
accessed during the acid-base titration process. The acid exchange capacity of the
samples decreases with the content of octyl group increasing, probably due to the
decrease in both the surface area and pore diameter. PS-SO3H/C8F-SiO2-0.10 gives
the lowest sulfur content and acid exchange capacity among all the samples.
The organic groups incorporated into the mesoporous silica shell of
PS-SO3H/C8-SiO2-x (x=0.05, 0.10) and PS-SO3H/C8F-SiO2-0.10 could adjust
hydrophobic and hydrophilic properties of the hybrid solid acids. All hollow

nanospheres before sulfonation, except PS/SiO2 HNs, located on the interface
between water and n-hexane, however, all samples uniformly dispersed in the water
after sulfonation (Fig. 5). The distribution phenomenon of samples before sulfonation
confirms that the incorporation of organic groups indeed could modify the surface
properties of HNs. After sulfonation, the incorporation of sulfonic acid groups onto
the PS distributed in the mesopore of silica shell could increase the surface
hydrophilicity of the samples. This could be confirmed by the decrease in water
contact angle of the samples before and after sulfonation. Thus, the samples could be
well dispersed in water phase. The water contact angle of the hybrid solid acids
increases in the order of PS-SO3H/C8-SiO2-0.10 > PS-SO3H/C8F-SiO2-0.10 
PS-SO3H/C8-SiO2-0.05  PS-SO3H/SiO2 (Table 1). This shows that octyl and
perfluorinated octyl groups could increase the surface hydrophobicity of the hybrid
solid acids.
The catalytic performance of hybrid solid acids with different surface properties
was tested in both the esterification of lauric acid and ethanol and the
transesterification of tripalmitin and methanol (Table 2). The esterification of lauric
acid and ethanol produce ethyl laurate as the only organic product with water as
byproduct. Water usually acts as a byproduct in various acid-catalyzed reactions,
which can easily poison the acid sites and result in opposite reactions, so the surface
properties of solid acid catalysts are quite important for achieving excellent catalytic
performance.
In the esterification of lauric acid and ethanol, Amberlyst-15 shows the lowest
activity due to the low BET surface area (the BET surface area of Amberlyst-15 is
only 41.8 m2/g) that causes most of the acid sites buried in the bulk polymer beads,
while liquid acid (H2SO4) shows the highest activity with turnover frequency (TOF)

of 36.7 h-1. Under similar reaction conditions, all PS-SO3H/SiO2 HNs catalysts are
more active than Amberlyst-15, which is mainly attributed to the high surface area of
hybrid solid acids benefiting higher exposure degree of acid sites. As the surface
hydrophobicity increases, the TOF of the hybrid solid acids increases from 15.2 h-1 to
27.5 h-1, showing the surface hydrophobicity could increase the catalytic activity.
PS-SO3H/C8F-SiO2-0.10HNs affords the highest TOF (27.5 h-1) among all the solid
hybrid solid acids, indicating the unique properties of fluorine in catalytic reactions.
[5-7]
The kinetic curves for the esterification and transterification reaction catalyzed by
different types of catalysts are displayed in Fig. 6. Though H2SO4 affords the highest
initial rate among all catalysts tested, the reaction rate decreases sharply after 1 h,
suggesting the deactivation of the liquid acid catalysts probably due to H2O formed
during the catalytic process. As a result, the final product yield is less than 85%.
PS-SO3H/SiO2 without organic groups in the shell affords 90% yield. As the surface
hydrophobicity increases, the product yield increases, which suggests that solid acids
with hydrophobic organic groups in the shell could protect the sulfonic acid sites from
deactivation by water. Among all solid acid catalysts, PS-SO3H/C8-SiO2-0.10 HNs
takes the shortest reaction time to achieve 96% yield though its TOF is not the highest.
This is probably related with its surface hydrophobicity and relatively larger BET
surface area. Compared with PS-SO3H/2.5PMA-C8-SiO2 with poly(methyl acrylate)
(PMA) and octyl modified shell [33], the solid catalysts, PS-SO3H/C8-SiO2-0.10 and
PS-SO3H/C8F-SiO2-0.10, reported in this work show much higher TOF and slightly
higher product yield. This is mainly attributed to the strong surface hydrophobicity
and larger BET surface area of the solid acids prepared using co-condensation
method.

The transesterification of tripalmitin with methanol has been widely investigated
for the production of biodiesel. [40] Liquid acid, H2SO4, could smoothly catalyze the
transesterification reaction to afford 52.3% yield of biodisel. However, Amberlyst-15
gives very low biodiesel yield of 17.3%. Compared with lauric acid, tripalmitin with
larger molecular size is more difficult to diffuse through Amberlyst-15 with low BET
surface area. As a result, Amberlyst-15 only shows low activity in the
transesterification reaction. On the contrary, all hybrid solid acids could efficiently
catalyze the transesterification reaction to afford much higher yield than H2SO4,
showing the mesopore in the silica shell allows the free diffusion of tripalmitin to
access the acid sites. It is also observed that the hybrid solid acids with stronger
surface hydrophobicity afford higher biodiesel yield and TOF. It should be noted that
PS-SO3H/C8-SiO2-0.10 HNs and PS-SO3H/C8F-SiO2-0.10 HNs are even more active
than H2SO4. The high activity of hybrid solid acids is probably due to its enrichment
effect for hydrophobic tripalmitin.
Based on the kinetic curves for the transesterification reaction (Fig. 6B), H2SO4
with higher initial reaction rate also affords much slower reaction rate after 2.5 h,
similar to those observed in the esterification reaction, showing the deactivation of
liquid acids during the transesterification reaction. The initial reaction rate of
PS-SO3H/SiO2 HNs is also identical to that of PS-SO3H/C8-SiO2-0.05 HNs, but the
reaction rate of the later sample is faster than the former sample after 7.5 h. This
further confirms the incorporation of organic groups in the silica shell could protect
the sulfonic acid sites from deactivation.
The above catalytic results suggest that hybrid solid acids with surface
hydrophobicity show high activity in both the esterification of lauric acid and ethanol
and the transesterification of tripalmitin and methanol. This is probably related to the

anti-poisoning effect of acid sites by water and enrichment effects for hydrophobic
substrates derived from the increased surface hydrophobicity.
The reusability of hybrid solid acids was tested with PS-SO3H/C8-SiO2-0.10 HNs
as model catalysts and esterification of lauric acid and ethanol as model reaction. As
shown in Fig. 7, the catalytic activity of PS-SO3H/C8-SiO2-0.10 HNs gradually
decreases in the course of repeating use, but the decrease is not so obvious. After 10
cycles, the conversion only decreases from 97.5 to 86.2 %, and the activity of the
catalyst could still retain 88 % of the fresh one. It should be noted that the reaction
time is extended from 6 h to 8 h after the 8th cycle. Also, the product yield was
measured for 20 min run for evaluating the stability of PS-SO3H/C8-SiO2-0.10 (Fig. 7).
The data for the 20 min run show the obvious decrease in product yield for the first
two cycles. The product yield remains constant for the next three cycles and an
obvious decrease in product yield was observed for the sixth run. The above results
show the gradual deactivation of the catalyst during the recycle process. For
understanding the decrease in activity of the hybrid solid acids during recycle process,
the used PS-SO3H/C8-SiO2-0.10 HNs was characterized by acid titration, elemental
analysis, TEM technique (Fig. 7B) and N2 sorption analysis (Table 1). The sulphur
content and acid exchange capacity of PS-SO3H/C8-SiO2-0.10 were measured after 10
cycles. It was found that sulphur content and acid exchange capacity of used
PS-SO3H/C8-SiO2-0.10 were 3.08 and 2.35 mmol/g, respectively. The sulfur content is
almost the same as that of fresh catalyst, suggesting negligible leaching of PS-SO3H
during the recycle process. It is noteworthy to mention that acid exchange capacity of
the used PS-SO3H/C8-SiO2-0.10 is much higher than that of fresh one (2.35 versus
1.13 mmol/g). The TEM image (Fig. 7B) show that used PS-SO3H/C8-SiO2-0.10 still
keeps the original hollow nanostructure with no significant changes in shell thickness

and particle size. However, the BET surface area decreases sharply from 262 m2/g to
130 m2/g after 7 cycles and to 27 m2/g after 10 cycles (Table 1). The above results
suggest that the swelling of PS-SO3H in the silica shell occurs during the recycle
process. The swelling of PS-SO3H increases the exposure degree of acid sites, as a
result, the activity of the catalyst only decreases slightly during recycle process
though the BET surface area decreases sharply. The fact that no obvious leaching of
PS-SO3H was observed during the recycle process shows that the octyl group
incorporated in the silica shell could efficiently prevent the leaching of PS-SO3H. [33]
Based on the elemental analysis, acid exchange capacity and TEM characterization,
the gradual deactivation of PS-SO3H/C8-SiO2-0.10 is probably due to the combined
effect of PS-SO3H swelling and decrease in BET surface area.
4. Conclusion
In summary, we report the successful synthesis of highly efficient solid acid
catalyst with controllable surface wettability and unique hollow nanostructure via
one-pot co-condensation method. The surface properties and the catalytic
performance of the solid acids could be facilely tuned by incorporation of octyl or
1H,1H,2H,2H-perfluorooctyl in the silica shell. Compared with post-synthesis
grafting method, the co-condensation method could result in the formation of silica
hollow nanospheres with both high content of organic groups and open mesoporous
structure. The primary results suggest that stronger surface hydrophobicity could
increase the catalytic activity of the solid acid catalysts and also could prevent the
acid sites from deactivation by water formed during the catalytic process.
PS-SO3H/C8-SiO2-0.10 HNs with optimized surface hydrophobicity and mesoporous
structure show much higher activity than H2SO4 or Amberlyst-15 in the esterification

and transesterification reactions and is among the most efficient solid acids reported
in the literature. The octyl groups in the silica shell could prevent the leaching of
PS-SO3H during the catalytic process, as a result, the solid acids could be recycled
though gradually deactivation was observed.
Acknowledgements
We gratefully acknowledge financial support for National Natural Science Foundation
of China (21325313, 21321002) and supported by the key research program of the
Chinese Academy of Sciences (Grant No. KGZD-EW-T05).
References
[1] D.Y.C. Leung, X. Wu, M.K.H. Leung, Applied Energy, 87(2010) 1083-1095.
[2] A.F. Lee, K. Wilson, Catal. Today, 242 (2015) 3-18.
[3] J.M. Marchetti, V.U. Miguel, A.F. Errazu, Renewable & Sustainable Energy Reviews, 11 (2007)
1300-1311.
[4] F. Liu, W. Li, Q. Sun, L. Zhu, X. Meng, Y. H. Guo, F. S. Xiao, Chemsuschem. 4 (2011)
1059-1062.
[5] F. Liu, X. Meng, Y. Zhang, L. Ren, F. Nawaz, F. S. Xiao, J. Catal. 271(2010) 52-58.
[6] F. Liu, A. Zheng, I. Noshadi, F. S. Xiao, Appl. Catal. B-Environ. 136 (2013) 193-201.
[7] F. Liu, L. Wang, Q. Sun, L. Zhu, X. Meng, F. S. Xiao, J. Am. Chem.
Soc. 134(2012) 16948-16950.
[8] F. Su, Y. Guo, Green Chem. 16 (2014) 2934-2957.
[9] M. Sasidharan, A. Bhaumik, ACS Appl. Mater. Interfaces. 5 (2013) 2618-2625.
[10] A. Corma, Chem. Rev. 95 (1995) 559-614.
[11] A. Corma, Curr. Opin. Solid State Mater. Sci. 2 (1997) 63-75.
[12] A. Corma, H. Garcia, Chem. Rev. 103 (2003) 4307-4365.
[13] J. H. Clark, Acc. Chem. Res. 35 (2002) 791-797.
[14] M. J. Climent, A. Corma, S. Iborra, Chem. Rev. 111(2011) 1072-1133.
[15] M. Hart, G. Fuller, D. R. Brown, J. A. Dale, S. Plant, J. Mol. Catal. A:
Chem. 182 (2002) 439-445.
[16] E. Andrijanto, E. A. Dawson, D. R. Brown, Appl. Catal. B-Environ. 115 (2012) 261-268.
[17] P. Barbaro, F. Liguori, Chem. Rev. 109 (2009) 515-529.
[18] L. Wang, F.-S. Xiao, Green Chem. 17 (2015) 24-39.
[19] Z. Fan, Y. Zhao, F. Preda, J. M. Clacens, H. Shi, L. Wang, X. Feng,
F. D.Campo, Green Chem. 17 (2015) 882-892.
[20] W. Long, C. W. Jones, Acs Catal. 1 (2011) 674-681.
[21] J. Liu, C. Li, Q. Yang, J. Yang, C. Li, Langmuir 23(2007) 7255-7262.

[22] M. Sasidharan, H. Zenibana, M. Nandi, A. Bhaumik, K. Nakashima, Dalton Trans. 42 (2013)
13381-13389.
[23] C. S. Oh, K. W. Koo, C. S. Han, J. W. Kim,
H. C. Kim, Y. S. Lee, Y. T. Choi, Y. H.
Kim, Journal of the Korean Institute of Metals and Materials, 47(2009) 516-521.
[24] P. Wang, S. Bai, J. Zhao, P. Su, Q. Yang, C. Li, Chemsuschem. 5 (2012) 2390-2396.
[25] J. Gao, J. Chen, X. Li, M. Wang, X. Zhang, F. Tan, S. Xu, J. Liu, J.
Colloid Interface Sci. 444(2015) 38-41.
[26] J. Gao, X. Zhang, Y. Lu, S. Liu, J. Liu, Chem. Eur. J. 21(2015) 7403-7407.
[27] W. J. Zhou, L. Fang, Z. Fan, B. Albela, L. Bonneviot, F. De Campo,
M. Pera.Titus, J. M. Clacens, J. Am. Chem. Soc. 136(2014) 4869-4872.
[28] K. Inumaru, T. Ishihara, Y. Kamiya, T. Okuhara, S. Yamanaka, Angew.
Chem. Int. Ed. 46 (2007) 7625-7628.
[29] L. Wang, F. S. Xiao, Chemcatchem. 6(2014) 3048-3052.
[30] B. Karimi, M. Khorasani, Acs Catal. 3(2013) 1657-1664.
[31] J. P. Dacquin, H. E. Cross, D. R. Brown, T. Duren, J. J. Williams, A.
F. Lee, K. Wilson, Green Chem. 12(2010) 1383-1391.
[32] X. Zhang, Y. Zhao, S. Xu, Y. Yang, J. Liu, Y. Wei, Q. Yang, Nat. Commun. 2014, 5.
[33] X. Zhang, L. Zhang, Q. Yang, J. Mater. Chem. A. 2(2014) 7546-7554.
[34] J. Peng, Y. Yao, X. Zhang, C. Li, Q. Yang, Chem. Commun. 50(2014) 10830-10833.
[35] X. Zhang, Y. Zhao, Q. Yang, J.Catal. 320(2014) 180-188.
[36] M. Zhang, A. Sun, Y. Meng, L. Wang, H. Jiang, G. Li, Microporous
Mesoporous Mater. 204(2015) 210-217.
[37] M. P. Kapoor, Q. Yang, A. S. Inagaki, Chem. Mater. 16(2004) 1209-1213.
[38] R. Xing, N. Liu, Y. Liu, H. Wu, Y. Jiang, L. Chen, M. He, P. Wu,
Adv. Funct. Mater. 17(2007) 2455-2461.
[39] M. Sasidharan, S. Fujita, M. Ohashi, Y. Goto, K. Nakashima, S. Inagaki,
Chem. Commun. 47(2011) 10422-10424.
[40] G. W. Huber, S. Iborra, A. Corma, Chem. Rev. 106(2006) 4044-4098.

Sulfonation
Coating
THF
A
O
Si
O
=CTAB
=TEOS
O
*
*
n
Cl
Si
Cl
Cl
SO3H
SO3H
Fig. 1. (A) Schematic illustration of the synthetic route of PS-SO3H/SiO2 HNs with
PS-SO3H/C8-SiO2-0.10 as a model and the corresponding TEM images of (B) PS@C8-SiO2-0.10
CSNs and (C) PS/C8-SiO2-0.10 HNs.
Fig. 2. (A) TEM images of (a) PS-SO3H/SiO2 HNs, (b) PS-SO3H/C8-SiO2-0.05 HNs, (c)
PS-SO3H/C8-SiO2-0.10 HNs and (d) PS-SO3H/C8F-SiO2-0.10 HNs, Scale bar: 200 nm, (B) SEM
images and (C) HRTEM images and corresponding element mapping image analysis of
PS-SO3H/C8-SiO2-0.10 HNs (silica and sulfur along the area 1 shown in (C)).

500
450
400
350
300
250
200
150
100
50
a+200
b+150
c+70
d
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Fig. 3. N2 adsorption–desorption isotherms of (a) PS-SO3H/SiO2 HNs, (b) PS-SO3H/C8-SiO2-0.05
HNs, (c) PS-SO3H/C8-SiO2-0.10 HNs and (d) PS-SO3H/C8F-SiO2-0.10 HNs.
9
*
10
B
A
11
12
12
12
12
a
13
12
12
9
SO3H
10
b
13
13
a
c
11
11
c
d
9
10
1-8
(n-octyl)
573
2930
Ar‐H
1020
1240
180 160 140 120 100 80 60 40 20
0
3000 2900
1000
500
Wavenumber (cm-1)
Chemical shift (ppm)
Fig. 4. (A) FT-IR spectra of (a) PS-SO3H/SiO2 HNs, (b) PS-SO3H/C8-SiO2-0.05 HNs, (c)
PS-SO3H/C8-SiO2-0.10 HNs and (d) PS-SO3H/C8F-SiO2-0.10 HNs, (B) 13 C NMR of (a) PS-SO3H/
SiO2 HNs and (c) PS-SO3H/C8-SiO2-0.10 HNs.

a
b
f
c
d
e
g
h
Fig. 5. Pictures of samples dispersed in water and n-hexane: (a) PS/SiO2 HNs, (b)
PS/C8-SiO2-0.05 HNs, (c) PS/C8-SiO2-0.10 HNs, (d) PS/C8F-SiO2-0.10 HNs, (e) PS-SO3H/SiO2
HNs, (f) PS-SO3H/C8-SiO2-0.05 HNs, (g) PS-SO3H/C8-SiO2-0.10 HNs, and (h)
PS-SO3H/C8F-SiO2-0.10 HNs.
c
100
80
60
40
20
0
100
80
60
40
20
0
d
b
a
A
B
c
e
d
b
a
f
e
f
0
1
2
3
4
5
6
7
0
5
10
15
20
25
Time (h)
Time (h)
Fig. 6. The kinetic curves for the esterification (A) and transterification (B) reaction catalyzed by
PS-SO3H/SiO2 HNs, PS-SO3H/C8-SiO2-0.05 HNs, PS-SO3H/C8-SiO2-0.10 HNs,
PS-SO3H/C8F-SiO2-0.10 HNs, H2SO4 and Amberlyst-15.

B
A
7th
the yield obtained at 6h
100
80
60
40
20
0
the yield obtained at 20min
10th
1
2
3
4
5
6
7
8
9
10
Running times
Fig. 7. (A) The yield obtained at different reaction times on PS-SO3H/C8-SiO2-0.10 HNs in the
esterification of lauric acid with ethanol during recycle experiments (the reaction time is
prolonged from 6h to 8 h after the 8th cycle) and (B) TEM images of PS-SO3H/C8-SiO2-0.10 HNs
after the (a) 7th and (b) 10th recycle.

Table 1. Physical parameters of PS-SO3H/SiO2 HNs, PS-SO3H/C8-SiO2-x HNs and PS-SO3H/C8F-SiO2-0.10 HNs.
Acid exchange
SBET
Vt
DDFT
Water contact
angle [d]
S content
Sample
capacity
(mmol/g) [e]
1.25±0.04
1.18±0.05
1.13±0.02
0.97±0.03
-
(m2/g)[a]
(cm3/g)[b]
(nm) [c]
(mmol/g) [f]
PS-SO3H/SiO2
318
210
222
262
130
27
0.39
0.13
0.21
0.19
0.10
0.02
3.2
2.2
1.4
2.7
--
37.5 (31.5)
52.3 (60.3)
70.8 (92.3)
62.8 (103.7)
-
3.24±0.015
3.21±0.009
3.20±0.002
2.85±0.016
-
PS-SO3H/C8-SiO2-0.05
PS-SO3H/C8-SiO2-0.10
PS-SO3H/C8F-SiO2-0.10
PS-SO3H/C8-SiO2-0.10[g]
PS-SO3H/C8-SiO2-0.10[h]
1.6
-
2.35
3.08
[a] SBET is the BET specific surface area. [b] Vt is the total pore volume determined at relative pressure 0.99. [c]
DDFT is pore diameter determined from the adsorption branch calculated based on the NLDFT. [d] data in
parentheses refers to the sample before sulfonation. [e] based on acid–base titration, the error bar is calculated
based on three parallel experiments. [f] based on elemental analysis, the error bar is calculated based on three
parallel experiments. [g] after recycled for seven times in esterification reaction. [h] after recycled for ten times in
esterification reaction

Table 2. Catalytic activity of PS-SO3H/SiO2-x HNs in the esterification of lauric acid and ethanol
and in the transesterification of tripalmitin and methanol.
Esterification [a]
Transesterification [b]
Sample
Yield (%)
TOF (h-1) [c]
36.7±0.6
Yield (%)
52.3±1.4
TOF (h1) [c]
H2SO4
84.4±2.2
3.21±0.44
Amberlyst-15
53.3±1.2
91.5±3.8
93.2±5.7
96.7±2.9
97.2±2.0
94.7±0.1
6.9±0.6
15.2±0.6
16.7±0.2
23.7±1.0
27.5±0.5
19.1±0.1
17.3±1.8
80.9±4.2
86.8±2.8
95.9±3.7
96.2±1.2
---
0.24±0.17
2.82±0.52
3.55±0.47
4.69±0.20
4.76±0.08
---
PS-SO3H/SiO2
PS-SO3H/C8-SiO2-0.05
PS-SO3H/C8-SiO2-0.10
PS-SO3H/C8F-SiO2-0.10
PS-SO3H/2.5PMA-C8-SiO2
[d]
[a] Reaction condition: 2 mmol of lauric acid, 10 mmol of ethanol, 0.04 mmol of H+, 80 C, 6 h;
The selectivity to ethyl laurate is 99.9%. The error bar is calculated based on three parallel
experiments. [b] Reaction condition: 0.52 mmol of tripalmitin, 2 mL of methanol, 0.04mmol of H+,
80 oC, 21 h; The selectivity to methyl palmitate is nearly 100%. The error bar is calculated based
on three parallel experiments. [c] The TOF was defined as mmol product obtained per acid site per
hour with yield about 15%. [d] Data taken from ref. [33].