PS-SO3H@phenylenesilica with yolk-double-shell nanostructures as Efficient and stable solid acid catalysts

Article abstract of DOI:10.1016/j.jcat.2014.09.018

Efficient and stable solid acids have been successfully synthesized by sulfonation of polystyrene (PS) in the hollow interiors of silica-based hollow nanostructures. It was found that larger and smaller inner void spaces result in the formation of PS-SO³H@phenylenesilica respectively with double-shell (DSNs) and yolk-double-shell nanostructure (YDSNs). PS-SO₃H@phenylenesilica with DSNs and YDSNs nanostructure shows comparable activity and is more active than Amberlyst-15 in the esterification reaction. PS-SO₃H@phenylenesilica with YDSNs nanostructure affords higher activity than that with DSNs nanostructure in the Friedel-Crafts alkylation of toluene with 1-hexene, which is mainly attributed to the fact that the unique YDSNs nanostructure could slow down the swelling rate of PS-SO₃H during the catalytic process. More importantly, PS-SO₃H@phenylenesilica with YDSNs nanostructure showed much higher recycle stability than Amberlyst-15 in the Friedel-Crafts alkylation of toluene with 1-hexene, probably due to the high thermal stability of the sulfonic acid group and the unique YDSNs nanostructure.

Full text of DOI:10.1016/j.jcat.2014.09.018

Journal of Catalysis 320 (2014) 180–188
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
PS-SO3H@phenylenesilica with yolk–double-shell nanostructures
as efficient and stable solid acid catalysts
Xiaomin Zhang a,b, Yaopeng Zhao a, 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 University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Efficient and stable solid acids have been successfully synthesized by sulfonation of polystyrene (PS) in
the hollow interiors of silica-based hollow nanostructures. It was found that larger and smaller inner void
spaces result in the formation of PS-SO3H@phenylenesilica respectively with double-shell (DSNs) and
yolk–double-shell nanostructure (YDSNs). PS-SO3H@phenylenesilica with DSNs and YDSNs nanostruc-
ture shows comparable activity and is more active than Amberlyst-15 in the esterification reaction.
PS-SO3H@phenylenesilica with YDSNs nanostructure affords higher activity than that with DSNs nano-
structure in the Friedel–Crafts alkylation of toluene with 1-hexene, which is mainly attributed to the fact
that the unique YDSNs nanostructure could slow down the swelling rate of PS-SO3H during the catalytic
process. More importantly, PS-SO3H@phenylenesilica with YDSNs nanostructure showed much higher
recycle stability than Amberlyst-15 in the Friedel–Crafts alkylation of toluene with 1-hexene, probably
due to the high thermal stability of the sulfonic acid group and the unique YDSNs nanostructure.
Ó 2014 Elsevier Inc. All rights reserved.
Received 17 July 2014
Revised 23 September 2014
Accepted 25 September 2014
Keywords:
Solid acid catalyst
Sulfonation
Polystyrene
Esterification
Friedel–Crafts alkylation
1. Introduction
These hybrid solid acids with larger surface areas show higher
activity than the parent ion-exchange resins. However, they are
Acid-catalyzed reactions have been widely used in the produc-
tion of chemicals. The homogenous mineral liquid acids, such as
HCl, H2SO4, and HF, are efficient acid catalysts due to their uniform
acid sites and strong acid strength. However, the employment of
liquid acids in industry often faces problems of wastewater gener-
ation, equipment corrosion, and recycling difficulty [1–9]. The
development of solid acids as replacements for liquid acids has
received growing research attention recently due to strict environ-
mental legislation and the requirement of green processes for
chemicals production in industry.
Ion-exchange resins, such as Amberlyst-15, are very attractive
solid acid catalysts because they are convenient to use and invari-
ably exhibit high concentrations of acid sites [10–15]. However,
they generally have low degrees of exposure of acid sites due to
low surface area and low thermal stability. Silica/ion-exchange
resins have been prepared for improving both the accessibility of
acid sites and the thermal stability of ion-exchange resins
[16–18]. In addition to silica/ion-exchange resins, sulfonic acid
group-functionalized mesoporous silicas have been widely studied
and successfully applied to many acid-catalyzed reactions [19–23].
not active enough for catalyzing reactions requiring high acid
strength.
The solid acids generally show lower acid strength than their
homogeneous counterparts [24–26]. One reason is the low concen-
tration of acid sites on the solid surface, which could not generate
the cooperative effects among acid sites that homogeneous acids
do. In our previous report, we found that the acid strength of sul-
fonated polystyrene could be greatly enhanced by compressing it
in a confined nanospace, showing that high acid concentration
could increase the acid strength of solid acids [27]. Compared with
traditional methods for the preparation of hybrid composites of sil-
ica/ion-exchange resins by mixing silica and ion-exchange resins
together either chemically or physically, the encapsulation method
provides a new approach to crowding sulfonated polystyrene
within a confined nanospace. This may generate a solid acid with
both high acid concentration and high stability. Moreover, the acid
concentrations of solid catalysts can be adjusted via tuning the
volume of the nanospace, which may have a big influence on the
catalytic performance of solid acids.
Here, we report the preparation of solid acids with sulfonated
polystyrene encapsulated within mesoporous benzene–silica hol-
low nanospheres. By adjusting the volume of hollow nanospace,
sulfonated polystyrene hollow nanospheres and nanospheres
⇑
Corresponding author. Fax: +86 411 4694447.
E-mail address: yangqh@dicp.ac.cn (Q. Yang).
http://dx.doi.org/10.1016/j.jcat.2014.09.018
0021-9517/Ó 2014 Elsevier Inc. All rights reserved.

X. Zhang et al. / Journal of Catalysis 320 (2014) 180–188
181
could be formed within the confined nanospace. The catalytic
performance of solid acids was tested in a series of important
acid-catalyzed reactions such as esterification of fatty acids and
Friedel–Crafts alkylation of toluene and compared with the
performance of sulfonated benzene–silica hollow nanospheres,
Amberlyst-15, and liquid acid. The relation of activity to the acid
strength of solid acids was also investigated.
sulfonated in the same way as described above for the generation
of SO3H-phenylenesilica HNs.
2.3. Characterization
The nitrogen sorption experiments were performed at 77 K
using a Micromeritics ASAP 2020. The BET surface area was calcu-
lated 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 desorption branch using the Barret–Joyner–Halenda (BJH)
and Horvath–Kawazone (HK) method. Pore volume was estimated
at a relative pressure P/P0 of 0.99. Transmission electron micros-
copy (TEM) was performed on a HITACHI 7700 at an acceleration
voltage of 100 kV. Before the measurement, the sample was dis-
persed in ethanol and deposited on a holey carbon film on a Cu
grid. High-resolution transmission electron microscopy (HR-TEM)
was performed on an FEI Tecnai G2 F30 S-Twin at an acceleration
voltage of 300 kV. Before the measurement, the sample was dis-
persed in ethanol and deposited on a holey carbon film on a Cu
grid. High-resolution scanning electron microscopy (HR-SEM)
was undertaken on a HITACHI S-4800 operating at an accelerating
voltage of 1–20 kV. FT-IR spectra were collected with a Nicolet
Nexus 470 IR spectrometer (KBr pellets were prepared) in the
range of 400–4000 cmÀ1. Elemental analyses were determined by
means of an Elementary Vario EL III analyzer. The thermogravimet-
ric analysis (TGA) was performed using a NETZSCH STA 449F3 ana-
lyzer from 30 to 900 °C with a heating rate of 10 °C/min under air
atmosphere. 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 and 29Si signals were referenced to
tetramethylsilane (TMS). The experimental parameters are as fol-
lows: 8 kHz spin rate, 3 s pulse delay, 4 min contact time, and
1000 scans.
2. Experimental
2.1. Chemicals and reagents
All reagents were of analytical grade and used as purchased
without further purification. 1,4-Bis-(trimethoxysilyl)benzene
(BTEB), cetyltrimethylammonium bromide (CTAB), and triethyl-
phosphine oxide were purchased from Sigma–Aldrich Company
Ltd. (USA). Fluorocarbon surfactant, FC-4, was bought from YickVic
Chemicals (Hong Kong). Tetraethoxysilane (TEOS) was obtained
from Nanjing Shuguang Chemical Group (China). Other reagents
were purchased from Shanghai Chemical Reagent, Inc., of the
Chinese Medicine Group.
2.2. Catalyst preparation
2.2.1. Preparation of PS-SO3H@phenylenesilica yolk–double-shell
nanospheres (YDSNs)
In the first step, PS@phenylenesilica yolk–shell nanospheres
(YSNs) were prepared. In a typical run, an aqueous solution
(60 mL) and ethanol (20 mL) with CTAB (0.2 g), PS template
spheres (0.2 g), and NH3ÁH2O (0.7 mL, 25 wt%) was stirred at
50 °C for 0.5 h. Then TEOS (0.4 g) was added and the mixture was
stirred for 2 h, followed by the addition of an aqueous solution
(3 g) containing FC4 (0.04 g), CTAB (0.08 g), and NH3ÁH2O (0.2 mL,
25 wt%). Then BTEB (0.50 mL) in ethanol solution (2 mL) was added
under vigorous stirring to the above synthesis medium. The tem-
perature was raised to 80 °C and maintained for 4 h under stirring.
After that, the mixture was transferred into a Teflon-lined auto-
clave and aged at 100 °C under static conditions for 36 h. The white
powder was collected by filtration and dried at room temperature.
To remove the surfactant, the as-synthesized materials (1 g) were
dispersed in 200 mL of ethanol containing 1.5 g of concentrated
HCl aqueous solution and the mixture was heated at 70 °C for
12 h. After filtration and drying at 60 °C overnight, PS@phenylene-
silica YSNs was obtained.
The acid strength of the solid acid catalysts was monitored by
reference to the 31P NMR chemical shift of triethylphosphine oxide
(TEPO) chemically adsorbed on acid sites. 31P NMR spectra were
obtained on a Bruker Avance III 600 spectrometer operating at a
frequency of 242.9 MHz using a 4 mm MAS probe. 31P MAS NMR
spectra were recorded using high-power proton decoupling with
a spinning rate of 12 kHz. One hundred scans were accumulated
with a
p/4 pulse width of 2.25 ls and a 30 s recycle delay. The
chemical shifts spectra were referenced to 85 wt% phosphoric acid
external standard.
The acid exchange capacity of solid acids was determined by
acid–base titration with NaOH solution.
PS-SO3H@phenylenesilica with yolk–double-shell nanostruc-
ture (YDSNs) was generated by the following sulfonation reaction
of PS@phenylenesilica YSNs. The sulfonation of PS@phenylenesilica
(1.0 g) was similar to our previously reported method using chloro-
sulfonic acid (10 mL) as sulfonation reagent and dichloromethane
as solvent (50 mL) at 0 °C for 12 h under argon. The obtained nan-
ospheres were denoted as PS-SO3H@phenylenesilica YDSNs.
2.4. Catalytic reactions
2.4.1. Esterification of lauric acid with ethanol
Typically, the desired amount of solid acids (0.05 mmol H+) was
added into a two-necked round flask equipped with a reflux con-
denser and a magnetic stirrer. Then 10 mmol of ethanol and
2 mmol of lauric acid were added to the flask and the mixture
was stirred at 80 °C for 6 h. The activity in esterification was eval-
uated by yield of ethyl laurate. Reaction products were analyzed
using a gas chromatograph (GC Agilent-6890A) equipped with an
FID as well as a PEG capillary column using tetradecane as internal
standard.
2.2.2. Preparation of PS@phenylenesilica double shell nanospheres
(DSNs)
The preparation route was almost the same as that for PS-SO3-
H@phenylenesilica YDSNs, except that 0.65 g TEOS was used
instead of 0.4 g for the formation of PS@phenylenesilica-L YSNs
(L was used for differentiating with PS@phenylenesilica prepared
using 0.4 g of TEOS).
2.4.2. Friedel–Crafts alkylation of 1-hexene with toluene
The following sulfonation of PS@phenylenesilica-L YSNs results
in the formation of PS-SO3H@phenylenesilica DSNs.
A mixture of 1-hexene (2 mmol), toluene (24 mmol), dodecane
(0.10 g, internal standard), and the solid acids (0.1 mmol H+) was
added in a round-bottomed flask (10 mL) equipped with a reflux
condenser and a magnetic stirrer. The reaction mixture was heated
to 120 °C. The activity in Friedel–Crafts alkylation was evaluated by
conversion of 1-hexene. Samples were taken out at desired
2.2.3. Preparation of SO3H-phenylenesilica hollow nanospheres (HNs)
PS@phenylenesilica YSNs was calcined in air at 350 °C for 2 h to
remove PS cores. The resultant phenylenesilica HNs were

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X. Zhang et al. / Journal of Catalysis 320 (2014) 180–188
intervals and analyzed using a gas chromatograph (GC Agilent-
7890A) equipped with an FID as well as a PEG capillary column
using dodecane as internal standard.
nanostructures (DSNs), respectively. Both PS-SO3H@phenylenesili-
ca YDSNs and PS-SO3H@phenylenesilica DSNs have uniformly dis-
persed spherical morphology with particle size 367 and 387 nm
respectively, as evidenced by the HR-SEM characterizations
(Fig. S1 in the Supplementary Material). The fact that sulfonated
samples have particle size and morphology similar to those of par-
ent samples suggests that PS@phenylenesilica YSNs has enough
mechanical stability to survive the sulfonation process. Notably,
PS-SO3H@phenylenesilica YDSNs have double shells surrounding
the core. The inner and outer shell thickness of PS-SO3H@phenyl-
enesilica YDSNs was ꢀ30 nm and the particle size of the inner core
was about 160 nm. PS-SO3H@phenylenesilica DSNs have particle
size 387 nm with inner and outer shell thickness 55 and 25 nm,
respectively.
2.4.3. Recycling the solid acids
Friedel–Crafts alkylation of 1-hexene with toluene was used as
a model reaction to test the stability of solid acids. After reaction,
the solid acids were separated from the reaction system by
filtration, thoroughly washed with ethanol, and dried overnight
under vacuum at 60 °C. Then the recovered solid acids were used
directly in the next run.
3. Results and discussion
HR-TEM element mapping analysis was used to illustrate the
location of sulfur and silica in PS-SO3H@phenylenesilica YDSNs
and PS-SO3H@phenylenesilica DSNs (Fig. 2). The element mapping
image of the sulfur shows that the PS-SO3H is located mostly in
the core and inner shell for PS-SO3H@phenylenesilica YDSNs and
mainly in the inner shell for PS-SO3H@phenylenesilica DSNs. To fur-
ther clarify the location of PS-SO3H, PS-SO3H@phenylenesilica
YDSNs and PS-SO3H@phenylenesilica DSNs were calcined in air at
350 °C to remove PS-SO3H. According to the TEM images shown in
Fig. 1, both samples have similar double-shell hollow nanostruc-
tures after calcination with particle size identical to that of the sam-
3.1. Preparation and characterization of PS-SO3H@phenylenesilica
YDSNs and PS-SO3H@phenylenesilica DSNs
The overall process for the synthesis of PS-SO3H@phenylenesil-
ica YDSNs and PS-SO3H@phenylenesilica DSNs is illustrated in
Scheme 1. Initially, PS@SiO2 core–shell nanospheres with different
thicknesses of the SiO2 layer surrounding PS spheres were used as
starting materials for the synthesis of PS@phenylenesilica with
yolk–shell nanostructures according to an organosilane-assisted
etching method [28] using 1,4-bis-(trimethoxysilyl)benzene
(BTEB) as organosilane precursor. As shown in Fig. 1A, the particle
size of PS@phenylenesilica YSNs was about 367 nm. By adjusting
the amount of sacrificed SiO2 layer, the particle size of yolk–shell
nanostructures could be enlarged to 387 nm for PS@phenylenesil-
ica-L YSNs. Notably, the inner void space between PS and the ben-
zene–silica shell is not completely empty and some cotton like
particles exist, which are probably the silica residues. This is quite
different from the PS@ethane-silica yolk–shell nanostructures with
empty inner void space using 1,2-bis-(trimethoxysilyl)ethane as
organosilane precursor [27]. This may be related to different
hydrolysis and condensation rates of the organosilane precursor.
The sulfonation of PS@phenylenesilica YSNs and PS@phenylenesil-
ica-L YSNs could directly introduce sulfonic acid groups onto PS.
ples before calcination. The formation of
a
double-shell
nanostructure after calcination suggests that a thin layer of silica
fused with PS-SO3H for both PS-SO3H@phenylenesilica YDSNs and
PS-SO3H@phenylenesilica DSNs, as demonstrated in Scheme 1,
which is probably formed by the condensation of silica residue dur-
ing the sulfonation process. When the samples before and after cal-
cination are compared, it can be figured out that PS-SO3H mainly
locates in the inner shell and core, which is consistent with the
results of HR-TEM.
N2 adsorption–desorption isotherms of PS-SO3H@phenylenesil-
ica YDSNs and PS-SO3H@phenylenesilica DSNs are type IV, with a
sharp capillary condensation step and an H3 hysteresis loop start-
ing from a relative pressure P/P0 of 0.40 (Fig. 3A), showing that
both samples have mesoporous structures. The H3 hysteresis loop
is from the inner void space. PS-SO3H@phenylenesilica YDSNs and
PS-SO3H@phenylenesilica DSNs have high BET surface area (406.1
and 422.0 m2/g), large pore volume (0.35 and 0.41 cm3/g), and
coexistence of mesopores and micropores (Fig. 3B and Table 1).
The surface area of the solid acids is much larger than that of com-
mercial Amberlyst-15, which will benefit the diffusion of guest
molecules during the catalytic process.
3.2. Morphology and structure characterizations
Base on the TEM image shown in Fig. 1, the sulfonation of
PS@phenylenesilica YSNs and PS@phenylenesilica-L YSNs results
in the formation of PS-SO3H@phenylenesilica yolk–double-shell
nanostructures (YDSNs) and PS-SO3H@phenylenesilica double-shell
3.3. Characterizations of chemical composition, thermal stability, and
acidity
Sulfonation
Sulfonation
BTEB
BTEB
YDSNs
The chemical composition of solid acids with different nano-
structures was carefully characterized by FT-IR and solid NMR
technique (Fig. 4). The FT-IR spectra of PS-SO3H@phenylenesilica
YDSNs and PS-SO3H@phenylenesilica DSNs exhibit a broad intense
peak around 1300–1000 cmÀ1 corresponding to the Si–O–Si stretch
vibration characteristic of silica-based materials. The peaks attrib-
uted to sulfonic acid groups at 581 and 1415 cmÀ1 appeared in the
FT-IR spectra of PS-SO3H@phenylenesilica YDSNs and PS-SO3-
H@phenylenesilica DSNs. The 13C CP-MAS NMR spectra of PS-SO3-
H@phenylenesilica YDSNs and PS-SO3H@phenylenesilica DSNs
clearly display the signal at around 142 ppm assigned to the aro-
matic carbon coordinated with sulfonic acid groups. These results
confirm the successful formation of PS-SO3H via sulfonation.
Two sets of chemical shifts in the range of À35 to À90 ppm and
À85 to À120 ppm, respectively, attributed to T and Q sites of
silicon species could be clearly observed in 29Si MAS NMR spectra
PS@SiO2
PS@phenylenesilica YSNs
DSNs
CH CH2
n
-O-Si-C6H4-Si-O-
PS:
n
Si
SiO2
SO3
Si
H
SO3H
SO3H
Scheme 1. General procedure for the synthesis of PS-SO3H@phenylenesilica YDSNs
and DSNs.

X. Zhang et al. / Journal of Catalysis 320 (2014) 180–188
183
A
D
B
C
E
F
Fig. 1. TEM images of PS-SO3H@phenylenesilica and the corresponding parent materials: PS@phenylenesilica YSNs (A) and PS-SO3H@phenylenesilica YDSNs before (B) and
after calcination (C); PS@phenylenesilica-L YSNs (D) and PS-SO3H@phenylenesilica DSNs before (E) and after calcination (F). Scale bar, 200 nm.
Si
S
A
B
Si
S
1
Fig. 2. HR-TEM images of PS-SO3H@phenylenesilica YDSNs (A) and PS-SO3H@phenylenesilica DSNs (B) and corresponding element mapping image analysis of silica and
sulfur along the area 1 shown in (A) and (B).
of PS-SO3H@phenylenesilica YDSNs and PS-SO3H@phenylenesilica
DSNs, showing that both materials are hybrids of benzene–silicas
(mainly distributed in the outer shell) and pure silicas (both in
the outer shell and in the inner shell). T/(T + Q) for PS-SO3H@phen-
ylenesilica YDSNs and PS-SO3H@phenylenesilica DSNs is respec-
tively 0.61 and 0.68, calculated by the integration of the peak
area. These results show that the hybrid solid acids were composed
of sulfonic acid groups, silicas, and phenylenesilica.
Based on the acid–base titration method, PS-SO3H@phenylene-
silica YDSNs and PS-SO3H@phenylenesilica DSN have acid
exchange capacity respectively of 1.0 and 1.2 mmol/g, much lower
than the S content from elemental analysis results (Table 1). This

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X. Zhang et al. / Journal of Catalysis 320 (2014) 180–188
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0.28
A
B
350
300
250
200
150
100
50
0.24
0.20
0.16
0.12
0.08
0.04
0.00
a
a
a
b
0
10 20 30 40 50 60 70
b
c
Micropore pore
diameter (nm)
b
c
0
0.0
0.2
0.4
0.6
0.8
1.0
0
50
100
150
Pore diameter (nm)
Relative pressure (p/p0)
Fig. 3. N2 adsorption–desorption isotherms (A) and the corresponding pore size distributions (B) of PS-SO3H@phenylenesilica YDSNs (a), PS-SO3H@phenylenesilica DSNs (b),
and PS-SO3H@phenylenesilica YDSNs after being used for the 9th cycle in the Friedel–Crafts alkylation reaction (c).
Table 1
Textural parameters, acidity, and sulfur content of solid acid catalysts.
SBET (m2/g)
Vtb (cm3/g)
DBJH (nm)
DHK (nm)
Acid exchange capacity (mmol/g)d
S content (mmol/g)e
a
c
c
Sample
Amberlyst-15
41.8 0.15
0.31
40
–
4.7 0.06
(3.6 0.04)f
1.0 0.03
4.9 0.03
(3.9 0.06)
2.3 0.05
(2.1 0.05)
2.1 0.03
0.5 0.03
PS-SO3H@phenylenesilica YDSNs
406.1 3.4
(235.6 3.2)
422.0 1.3
438.0 3.4
0.35
(0.16)
0.41
2.1 (2.0)
1.3
(2.1 0.06)f
1.2 0.03
PS-SO3H@phenylenesilica DSNs
SO3H-phenylenesilica HNs
2.1
2.1
1.2
–
0.49
0.49 0.02
a
b
c
d
e
f
SBET is the BET specific surface area.
Vt is the total pore volume determined at relative pressure 0.99.
DBJH and DHK are BJH mesopore diameter and HK micropore diameter calculated by the desorption branches of the nitrogen sorption isotherms.
based on acid–base titration.
based on elemental analysis.
Data in parentheses for Amberlyst-15 and PS-SO3H@phenylenesilica YDSNs were obtained respectively after four and nine cycles in the Friedel–Crafts alkylation reaction.
shows that some of the acid sites in the solid acids could not be
accessed during the titration process [27]. The control sample,
SO3H-phenylenesilica HNs, has comparable BET surface area and
pore diameter but much lower acidity than PS-SO3H@phenylene-
silica YDSNs and DSNs (TEM; see Fig. S2 in the Supplemental
Material). This shows that SO3H groups could be incorporated into
benzene–silica during the sulfonation process, similarly to the pre-
vious report [29]. This result also suggests that SO3H groups also
exist in the shells of PS-SO3H@phenylenesilica YDSNs and DSNs.
The thermal stability of solid acids was investigated by TG
analysis (Fig. 4). PS-SO3H@phenylenesilica YDSNs and PS-SO3H@
phenylenesilica DSNs exhibit similar TG curves with two step
weight losses at 300–450 and 450–700 °C due to the destruction
of sulfonated polystyrene frameworks and benzene–silica,
respectively [30,31]. PS-SO3H@phenylenesilica YDSNs and
PS-SO3H@phenylenesilica DSNs show higher thermal stability than
Amberlyst-15, with decomposition temperature around 200 °C,
probably due to the protective effect of the outer silica shells of
the unique hollow nanostructures.
The acid strength of solid acids was investigated using TEPO
(triethylphosphine oxide) as a base probe molecule in combination
with the 31P MAS NMR technique [32–36] (Fig. 5). Amberlyst-15
exhibits the chemical shift at 89.4 ppm that was assigned to TEPO
adsorbed on sulfonic acid group sites [27]. PS-SO3H@phenylenesil-
ica YDSNs with chemical shift at 85.0 ppm have lower acid
strength than PS-SO3H@phenylenesilica DSNs with chemical shift
at 87.0 ppm. Our previous studies show that the acid strength of
PS-SO3H could be enhanced or weakened by controlling its
aggregation and swelling state in a confined nanospace. Based on
the TEM images, the volume of PS after sulfonation for PS-SO3H@
phenylenesilica YDSNs and PS-SO3H@phenylenesilica DSNs is
10.8 Â 10À21 m3 and 9.6 Â 10À21 m3, respectively. This could
explain the lower acid strength of PS-SO3H@phenylenesilica
YDSNs. PS-SO3H@phenylenesilica YDSNs and PS-SO3H@ phenyl-
enesilica DSNs both have lower acid strength than Amberlyst-15.
As a comparison, the acid strength of SO3H-phenylenesilica HNs
(Fig. S2) was also characterized using the same method. SO3H-
phenylenesilica HNs with acid exchange capacity of 0.47 mmol/g
exhibit chemical shift at 61.9 ppm. The low acid strength of
SO3H-phenylenesilica HNs is probably due to the low acid concen-
tration [26].
3.4. Catalytic results
The catalytic performance of solid acids with different nano-
structures and acid strength was tested in acid-catalyzed liquid
phase reactions and compared with that of concentrated H2SO4
and Amberlyst-15 (Table 2). Two types of reactions, esterification
of fatty acid with ethanol and liquid-phase Friedel–Crafts alkyl-
ation, were chosen to clarify the influence of the nanostructure,
acid strength, and thermal stability of solid acids on their catalytic
performance.
3.4.1. Esterification of fatty acid with ethanol
All solid acids tested could catalyze the esterification of lauric
acid and ethanol with ethyl laurate as the only organic product.
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.
Liquid acid, H2SO4, affords high activity with TOF of 40.7 hÀ1
.
Under the same reaction conditions, PS-SO3H@phenylenesilica
YDSNs and PS-SO3H@phenylenesilica DSNs with TOF of
ꢀ24.1 hÀ1 exhibit comparable activity, probably due to the similar
composition and nanostructure of the two materials. Notably,

X. Zhang et al. / Journal of Catalysis 320 (2014) 180–188
185
A
a
B
1415cm-1
581cm-1
142 ppm
a
b
b
180 160 140 120 100
80
60
40
20
4000
3000
1000
Wavenumbers (cm-1)
Chemical shift (ppm)
120
100
80
D
C
T
Q
60
a
a
40
b
20
b
c
0
-20
0
200
400
600
800
1000
0
-50
-100
-150
Chemical shift (ppm)
Temperature ( C)
Fig. 4. FT-IR spectra (A), 13C CP-MAS NMR spectra (B), solid-state 29Si NMR spectra (C), and TG curves (D) of PS-SO3H@phenylenesilica YDSNs (a), PS-SO3H@phenylenesilica
DSNs (b), and Amberlyst-15 (c).
0.16
80 oC
50 oC
A
B
85.1
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
80
a
b
c
87
61.9
60
d
140
120
100
80
40
20
0
0.02
0.04
0.06
0.08
0.10
fw (g-1)
Chemical shift (ppm)
Fig. 5. 31P MAS NMR spectra (A) of PS-SO3H@phenylenesilica YDSNs (a), PS-SO3H@phenylenesilica YDSNs after being used for the 9th cycle in the Friedel–Crafts alkylation
reaction (b), PS-SO3H@phenylenesilica DSNs (c), and SO3H-phenylenesilica HNs (d); (B) KN criterion plot of fw (gÀ1) versus r (mol hÀ1 gÀ1) for the esterification of lauric acid
and ethanol at 50 and 80 °C.
SO3H-phenylenesilica HNs with TOF of 37.7 hÀ1 are more active
than PS-SO3H@phenylenesilica YDSNs and PS-SO3H@phenylenesil-
ica DSNs. Differently from PS-SO3H@phenylenesilica YDSNs and
PS-SO3H@phenylenesilica DSNs, SO3H-phenylenesilica HNs pre-
pared by sulfonation of phenylenesilica HNs have a uniform
distribution of acid sites in the shells. Thus, the high activity of
SO3H-phenylenesilica HNs is mainly attributed to a high exposure
of acid sites and a short diffusion length of the reactants and prod-
ucts. The hybrid solid catalysts afford a higher yield of ethyl laurate
than H2SO4. This is probably related to the fact that benzene–silica
provides a hydrophobic microenvironment, which may help to
slow the reaction rate of the reverse hydrolysis reaction and pre-
vent the poison of sulfonic acid groups by water. The fact that
the activity of solid acids has no direct relation to their acid
strength suggests that the exposure of acid sites is more important
than the acid strength in the esterification reaction.
3.4.2. Koros–Nowak (KN) criterion test
In order to establish that the measured catalytic activity is inde-
pendent of transport phenomena, the Koros–Nowak (KN) criterion

186
X. Zhang et al. / Journal of Catalysis 320 (2014) 180–188
Table 2
The catalytic performance of acid catalysts in the esterification reaction and the Friedel–Crafts alkylation reaction:
O
O
H2O
O
OH
OH
+
+
Catalysts
Esterificationa
Friedel–Crafts alkylationb
c
c
TOF (hÀ1
)
Yield (%)
TOF (hÀ1
)
Conv. (%)
H2SO4
40.7 0.3
3.3 0.1
24.1 0.1
20.5 0.2
37.7 0.6
80.4 0.3
41.2 0.9
85.6 0.8
83.8 0.4
86.3 0.8
4.0 0.1
23.7 0.2
11.5 0.2
7.3 0.1
–
10.0 0.8
98.3 0.5
94.9 0.5
31.3 1.0
5.6 0.2
Amberlyst-15d
PS-SO3H@phenylenesilica YDSNs
PS-SO3H@phenylenesilica DSNs
SO3H-phenylenesilica HNs
a
b
c
The esterification reaction: 80 °C, 6 h, 0.04 mmol of acid sites, 2 mmol of lauric acid and 10 mmol ethanol.
The Friedel–Crafts alkylation reaction: 120 °C, 6 h, 5 mmol % of acid sites, 2 mmol of 1-hexene, and 24 mmol of toluene.
For the calculation of TOFs, the acid sites (H+) were measured by the acid–base titration method.
The conversion of 1-hexene for Amberlyst-15 was obtained at 2 h.
d
test modified by Madon-Boudart has been employed [37,38].
PS-SO3H@phenylenesilica YDSNs was used as a model sample for
the esterification of lauric acid and ethanol. The reaction rates in
the kinetic regime should be proportional to the concentration of
active sites (H+ in the present case). Hence a series of batch exper-
iments were performed following identical reaction conditions at
50 and 80 °C. The KN criterion has been explained by plotting the
reaction rate (r) in mol hÀ1 gÀ1 of catalyst versus the weight of
H+ (fw) in gÀ1, as shown in Fig. 5B. The value of the slope calculated
from the graph in Fig. 5B is 0.97 and 1.06 at 50 and 80 °C, respec-
tively. Because the calculated values of the slope are close to unity,
it can be concluded that the rate of transport has no influence on
the reaction rates.
strength of their acid sites [27]. Based on these results,
Amberlyst-15 indeed has higher acid strength than PS-SO3H@
phenylenesilica YDSNs and PS-SO3H@phenylenesilica DSNs. So it
is reasonable that Amberlyst-15 shows higher activity. However,
PS-SO3H@phenylenesilica YDSNs with lower acid strength than
PS-SO3H@phenylenesilica DSNs show higher activity.
To clarify the reason for the low activity of PS-SO3H@phenylene-
silica DSNs, the catalytic reaction was stopped after 4 h and solid
catalysts were taken out and analyzed with TEM. The TEM image
shows that the double-shell nanostructure changed completely to
a hollow nanostructure (Fig. 6A) and the volume of sulfonated poly-
styrene calculated based on the TEM image was increased from the
original 9.6 Â 10À21 m3 to 17.4 Â 10À21 m3. As we have reported,
the swelling of PS-SO3H could decrease the acid strength of solid
acids. So the low activity of PS-SO3H@phenylenesilica DSNs is pos-
sibly due to the decreased acid strength caused by the swelling of
PS-SO3H during the catalytic process. PS-SO3H@phenylenesilica
YDSNs exhibit high stability and the yolk–double-shell nanostruc-
ture could be retained during the catalytic process, which we will
discuss later.
3.4.3. Friedel–Crafts alkylation
The catalytic performance of solid acids was also tested in
liquid-phase Friedel–Crafts alkylation of toluene with 1-hexene
at S/C of ꢀ20. Friedel–Crafts alkylations are industrially important
reactions used to produce numerous chemical compounds. Classi-
cal Friedel–Crafts chemistry often relies on liquid acid catalysts,
such as AlCl3 and HF, because high acid strength is needed to cat-
alyze this reaction. Due to the severe drawbacks of liquid acids, the
development of solid acids is an urgent need. To avoiding oligo-
merization of alkenes, a molar ratio of toluene/1-hexene of 12
was used. Only 10% 1-hexene conversion was found on H2SO4,
showing that H2SO4 has poor activity at low concentrations.
Amberlyst-15 affords the highest activity among all the catalysts
tested. Within 2 h, 98.3% of 1-hexene was converted to several iso-
mers of Me-C6H4–C6H13. A small amount of dialkylated product
(dihexylbenzene isomers) was observed as well.
Under similar reaction conditions, PS-SO3H@phenylenesilica
YDSNs and PS-SO3H@phenylenesilica DSNs with TOF respectively
of 11.5 and 7.3 hÀ1 are less active than Amberlyst-15 with TOF of
23.7 hÀ1. With reaction time prolonged to 6 h, 94.9% conversion
of 1-hexene could be obtained on PS-SO3H@phenylenesilica
YDSNs. PS-SO3H@phenylenesilica DSNs only give 31.3% conversion
of 1-hexene. Under similar reaction conditions, SO3H-phenylene-
silica HNs show very low activity, with 1-hexene conversion of
only 5.6%. Thus, the distinct difference in catalytic performance
of the solid acid catalysts is not related to the exposure degree of
acid sites. It has been shown in previous work that higher acid
strength was needed for Friedel–Crafts alkylation. So the catalytic
activity of solid acids in alkylations could be correlated to the
3.4.4. Stability of solid acid catalysts
One of the important advantages for solid acids is recyclability.
In view of their good performance, PS-SO3H@phenylenesilica
YDSNs were chosen for a stability test in Friedel–Crafts alkylation
of toluene with 1-hexene. As a comparison, the recyclability of
Amberlyst-15 was investigated under the same conditions. As
shown in Fig. 7, Amberlyst-15 exhibited a very high deactivation
rate during four successive cycles. The conversion of 1-hexene
decreased from 97.8% in the first cycle to 45.0% for the fourth cycle.
For PS-SO3H@phenylenesilica YDSNs, the conversion of 1-hexene
was quite steady in the first seven cycles. An obvious decrease in
conversion was observed for the 8th run; however, 70% conversion
of 1-hexene could still be obtained for the ninth run. The result
suggests that PS-SO3H@phenylenesilica YDSNs are more stable
than Amberlyst-15.
To understand the reason for the deactivation, the sulfur con-
tent of both PS-SO3H@phenylenesilica YDSNs and Amberlyst-15
was analyzed after the recycle reaction (Table 1). The sulfur con-
tent of Amberlyst-15 decreased from 4.9 to 3.9 mmol/g after four
reaction cycles, indicating that about 20% of sulfonic acid groups
were leached during the recycle process. So leaching of sulfonic
acid groups may be the main reason for the deactivation of

X. Zhang et al. / Journal of Catalysis 320 (2014) 180–188
187
A
B
C
Fig. 6. TEM image of PS-SO3H@phenylenesilica DSNs taken out at 4 h during the Friedel–Crafts alkylation (A) and PS-SO3H@phenylenesilica YDSNs after first (B) and ninth
cycles (C). Scale bar, 200 nm.
DSNs with PS-SO3H in a more aggregated state show higher acid
strength than PS-SO3H@phenylenesilica YDSNs with PS-SO3H in a
less aggregated state. PS-SO3H@phenylenesilica DSNs and PS-SO3-
100
H@phenylenesilica YDSNs show comparable activity in esterifica-
tion reactions due to the similar nanostructure and compositions.
However, PS-SO3H@phenylenesilica YDSNs are more active than
PS-SO3H@phenylenesilica DSNs in the Friedel–Crafts alkylation of
toluene with 1-hexene. This is due to the fact that the unique YDSNs
nanostructure affords superior resistance to the swelling of PS-SO3H
during the catalytic process. Meanwhile, PS-SO3H@phenylenesilica
YDSNs show much higher stability than Amberlyst-15 in the
Friedel–Crafts alkylation of toluene with 1-hexene. Our findings
suggest that increasing antiswelling ability of PS-SO3H in confined
nanospaces may result in a highly efficient and stable solid acid
catalyst.
80
60
40
20
0
1
2
3
4
5
6
7
8
9
Recycling times
Acknowledgment
Fig. 7. Recycling ability of PS-SO3H@phenylenesilica YDSNs (gray) and Amberlsyt-
This work was supported by the National Natural Science
Foundation of China (21232008 and 21273226).
15 (black) in F-C alkylation of toluene with 1-hexene.
Amberlyst-15. For PS-SO3H@phenylenesilica YDSNs, the sulfur
content remained almost unchanged even after nine cycles, sug-
gesting much higher stability of the sulfonic acid groups. However,
the acid exchange capacity of PS-SO3H@phenylenesilica YDSNs
increases from 1.0 to 2.1 mmol/g after nine cycles, suggesting that
the nanostructure of PS-SO3H@phenylenesilica YDSNs might
change during the continuous recycling process.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2014.09.018.
References
As shown in Fig. 6, the nanostructure of PS-SO3H@phenylenesil-
ica YDSNs did not change after the first run. However, swelling of
the core was clearly observed after nine cycles. This could explain
the increase in acid exchange capacity after reuse. 31P NMR charac-
terization (Fig. 5) also shows an upfield of chemical shift from 85.1
to 80.0 ppm after nine cycles, showing a decrease in acid strength
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combined results show that the decrease in acid strength and BET
surface area caused by gradual swelling of PS-SO3H is the main
reason for the deactivation of PS-SO3H@phenylenesilica YDSNs.
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