Designed synthesis of sulfonated polystyrene/mesoporous silica hollow nanospheres as efficient solid acid catalysts

Article abstract of DOI:10.1039/c4ta00241e

We report the successful synthesis of hybrid hollow nanospheres (HNs) with sulfonated polystyrene (PS-SO₃H) aligned uniformly in the mesoporous channel of a silica shell. The fabrication process involved the sulfonation of silica HNs with polystyrene highly dispersed in the mesoporous shell which was prepared by co-condensation of a mixture of tetraethoxysilane (TEOS) and alkoxysilyl-functionalized poly(methyl acrylate) (PMA) around PS nanospheres in a base medium using cetyltrimethylammonium bromide (CTAB) as structural directing agent followed by THF treatment. The surface properties of the hybrid HNs were adjusted by the amount of PMA incorporated in the silica shell during the synthesis process and also by modification with an octyl group through a grafting method. The hybrid HNs, with acid exchange capacity in the range 0.8 to 2.0 mmol g^-1, could efficiently catalyze the esterification reaction of lauric acidwith ethanol. All hybrid HNs show much higher activity than commercial Amberlyst-15 catalyst and the TOF of the optimized hybrid HNs is almost identical to that of concentrated sulfuric acid. The high activity of the hybrid HNs is mainly attributed to the uniform distribution of the PS-SO₃H group in the mesoporous silica shell, the penetrating mesopore, and surface hydrophobicity. It was found that the recycle stability of hybrid HNs could be enhanced greatly by octyl group modification, which may prevent the leaching of PS-SO₃H during the catalytic process. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ta00241e

Journal of
Materials Chemistry A
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PAPER
Designed synthesis of sulfonated polystyrene/
mesoporous silica hollow nanospheres as efficient
solid acid catalysts†
Cite this: J. Mater. Chem. A, 2014, 2,
7546
a
a
Xiaomin Zhang,ab Lei Zhang and Qihua Yang
*
We report the successful synthesis of hybrid hollow nanospheres (HNs) with sulfonated polystyrene (PS–
SO3H) aligned uniformly in the mesoporous channel of a silica shell. The fabrication process involved the
sulfonation of silica HNs with polystyrene highly dispersed in the mesoporous shell which was prepared
by co-condensation of a mixture of tetraethoxysilane (TEOS) and alkoxysilyl-functionalized poly(methyl
acrylate) (PMA) around PS nanospheres in a base medium using cetyltrimethylammonium bromide
(CTAB) as structural directing agent followed by THF treatment. The surface properties of the hybrid HNs
were adjusted by the amount of PMA incorporated in the silica shell during the synthesis process and
also by modification with an octyl group through a grafting method. The hybrid HNs, with acid exchange
capacity in the range 0.8 to 2.0 mmol gꢀ1, could efficiently catalyze the esterification reaction of lauric
acid with ethanol. All hybrid HNs show much higher activity than commercial Amberlyst®-15 catalyst
and the TOF of the optimized hybrid HNs is almost identical to that of concentrated sulfuric acid. The
high activity of the hybrid HNs is mainly attributed to the uniform distribution of the PS–SO3H group in
the mesoporous silica shell, the penetrating mesopore, and surface hydrophobicity. It was found that the
recycle stability of hybrid HNs could be enhanced greatly by octyl group modification, which may
prevent the leaching of PS–SO3H during the catalytic process.
Received 15th January 2014
Accepted 17th March 2014
DOI: 10.1039/c4ta00241e
www.rsc.org/MaterialsA
acylation reactions due to their high surface areas, abundant
mesoporosity, and high concentration of sulfonic acid groups.4,5
Introduction
Thus, increasing the surface area and pore volume is of extreme
importance for improving the catalytic performance of IERs.
Owing to their outstanding properties, such as high surface
areas, large pore volumes, tunable pores sizes, and variable
morphologies, mesoporous silicas have attracted much
research interest and activity since their discovery.19–27 Recently,
different kinds of polymer–mesoporous silicas have been
synthesized due to the combined advantages from both the
polymers with versatile functional groups and mesoporous
silicas with high thermal stability, ordered porous structure,
and high surface area.28–32 The general strategies to incorporate
polymer building blocks into mesoporous silicas involve in situ
polymerization in the channels of mesoporous silicas,32,33 a post
graing approach,31 construction of mesoporous silicas in the
presence of polymers,34 and so on. Usually, it is difficult to
fabricate polymer–mesoporous silicas with high polymer
content by these methods owing to the threat of mesostructure
damage or pore blocking, and because the lack of effective
interactions between polymer and silica prevent the leakage of
polymers. Furthermore, most polymer–silica hybrid materials
do not have a well dened morphology, therefore high acces-
sibility of the active sites and fast diffusion of the reactants and
the products cannot be guaranteed.
The replacement of conventional mineral liquid acids by solid
acids for green chemistry has been intensively investigated
because of environmental considerations and safety
concerns.1–6 In the past years, various types of solid acid cata-
lysts have been developed, including zeolites,7–9 sulfated metal
oxides,10–12 and polymer based resins13,14 etc. Among these solid
acid catalysts, commercially available sulfonated ion-exchange
resins (IERs), represented by Amberlyst®-15 with an acidic
exchange capacity as high as 4.7 mmol gꢀ1, are widely used in a
wide range of acid-catalyzed liquid phase reactions. However,
the IERs do not exhibit high activity because most of the acidic
sites buried within the polymer beads are inaccessible in many
reactions owing to their low surface areas and lack of
porosity.15–18 Recently, Xiao and co-workers have successfully
synthesized mesoporous polymer based solid acids which
showed superior catalytic activities in esterication and
aState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, 457 Zhongshan Road, Dalian 116023. E-mail: yangqh@dicp.
ac.cn; Web: http://www.hmm.dicp.ac.cn; Fax: +86-411-84694447; Tel: +86-411-
84379552
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta00241e
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Previous studies have shown that the morphology of solid CTAB as surfactant micellar templates, PS nanospheres as co-
catalysts greatly affects their catalytic performance because of templates of hollow nanospheres, and PMA–Si and TEOS as
the inuence of morphology on the diffusion length of the precursors. In a typical synthesis, 0.20 g of CTAB was dissolved
reactants and products, the exposure degree of active sites, the in a mixed solvent prepared from 60 mL of deionized water and
crystalline phase, and so on. Recently, solid catalysts with 20 mL of ethanol at 50 ꢁC then stirred for 30 minutes. Next, the
the morphology of hollow nanospheres (HNs) have attracted desired amount (0.1 to 0.20 g) of PS nanospheres, dispersed in
much attention because the shortened diffusion lengths can 2 mL of deionized water by sonication for 30 minutes, were
not only increase the reaction rates, but also can improve added to the above mixture and the mixture was stirred at 50 ꢁC
selectivity by decreasing the possibility for side reactions, for another 30 minutes, followed by the addition of 0.7 mL of
especially for the deep reactions of the unstable products.26,35–40 ammonium hydroxide solution (25%). TEOS, at a concentration
Recently, our group found that silica hollow nanospheres, with of 0.625 g in 2 mL of ethanol, was mixed with 0.50 g of PMA–Si
chiral functional groups and SO3H/Pd incorporated in the shells in 2 mL of THF. Ten minutes later, the mixture was added
of HNs, are efficient solid catalysts for asymmetric reactions and dropwise under constant stirring. Aer stirring for 2 hours at
one-pot synthesis of methyl isobutyl ketone from acetone, 50 ꢁC, the mixturꢁe was transferred into a Teon-lined autoclave
respectively.36,38 Zeng and co-workers reported that and aged at 100 C, under static conditions, for 36 hours. The
H4SiMo12O40 functionalized silica HNs exhibited excellent solid product was recovered by ltration, washed with water and
catalytic activity toward benzylation of toluene.40 However, the ethanol, and dried at 60 C overnight. Aer surfactant extrac-
use of hollow nanospheres as catalysts is still in its infancy. tion in ethanol–HCl (0.75 wt% of HCl), PS/nPMA–SiO2 core shell
ꢁ
Recently, we found that dissolved polymer could be retained in nanospheres (CSNs) were obtained. Then, the solid product
the mesoporous silica shell functionalized with polymer fragments (1 g) was dispersed in 200 mL of THF and the mixture was
ꢁ
during the synthesis of silica HNs using polymer nanospheres as heated at 60 C for 12 hours. Aer centrifugation and washing
hard templates.41 Based on this nding, we report the fabrication with ethanol, PS/nPMA–SiO2 HNs were obtained, where n (n ¼
of efficient solid acid catalysts, PS–SO3H/PMA–SiO2 HNs (PS: 2.5, 3.3, and 5) denotes the weight ratio of PMA–Si to PS used in
polystyrene; PAM: poly(methyl acrylate)) with high content of the synthesis process.
polymers (66.4 wt%) aligned uniformly in the mesoporous silica
In a controlled experiment, PS/SiO2 HNs were synthesized in
shell through sulfonation of PS/PMA–SiO2 HNs which were a similar method to PS/5PMA–SiO2 HNs except 0.65 g TEOS was
synthesized by diffusion of PS core into the mesoporous silica shell used as the silica precursor
functionalized with PMA fragments. The catalytic performance of
In a controlled experiment, SiO2 HNs were obtained via
hybrid solid acids was tested in the esterication of fatty acids with calcination of the sample PS/SiO2 HNs under air.
ethanol, therefore, the relation of polymer content and surface
modication with catalytic performance was investigated.
Modication of PS/2.5PMA–SiO2 HNs with octyl group. PS/
2.5PMA–C8–SiO2 CSNs was synthesized by dispersing PS/
2.5PMA–SiO2 CSNs in 100 mL n-hexane containing 8 mmol of
octyltriethoxysilane and reuxing the mixture at 70 ꢁC for 24
hours. Then, the solid products were isolated and dried at 60 ꢁC
overnight. The following ethanol–HCl extraction and THF
treatment process was almost identical to that of PS/2.5PMA–
SiO2 HNs. Finally, PS/2.5PMA–C8–SiO2 HNs was obtained.
Sulfonation of PS/nPMA–SiO2 HNs, PS/SiO2 HNs, PS/
2.5PMA–C8–SiO2 HNs and SiO2 HNs. 1 g of hybrid HNs was
outgassed at 120 ꢁC under vacuum in a two-necked round ask
for 3 hours. Then, 50 mL of CH2Cl2 containing 10 mL of
chlorosulfonic acid was added to the ask at 0 ꢁC. The solution
was stirred for 12 hours under an argon atmosphere. The
suspension was washed with copious amounts of water until the
ltrate was neutral. Aer centrifugation and washing with
ethanol, the powder products were dried at 60 ꢁC overnight. The
resulting powder products were denoted accordingly as PS–
SO3H/nPMA–SiO2 HNs, PS–SO3H/SiO2 HNs, and PS–SO3H/
2.5PMA–C8–SiO2 HNs by using PS/nPMA–SiO2 HNs, PS/SiO2
HNs, and PS/2.5PMA–C8–SiO2 HNs as precursor, respectively.
Experimental section
Chemicals and reagents
All materials were of analytical grade and used as received without
any further purication, with the exception of styrene. Styrene was
washed through an inhibitor remover column and distilled under
vacuum before use. Cetyltrimethylammonium bromide (CTAB)
and [3-(methacryloxy)propyl]-trimethoxysilane (3-MOP, 98%) were
purchased from Sigma-Aldrich Company Ltd. (USA). Tetraethox-
ysilane (TEOS) was obtained from Nanjing Shuguang Chemical
Group (China). Methyl methacrylate (MMA, 98%), potassium
persulfate (K2S2O8), azobisisbutyronitrile (AIBN), tetrahydrofuran
(THF), ethylether, and other reagents were purchased from
Shanghai Chemical Reagent, Inc. of the Chinese Medicine Group.
Poly(methacrylate)-organosilane (PMA–Si) was synthesized by free
radical polymerization of [3-(methacryloxy)propyl]-trimethox-
ysilane (3-MOP) and methacrylate monomer (MA), initiated by
AIBN, according to our previous report.41,42 The PS template
spheres were prepared by emulsion polymerization in a water
system according to a modied method.43
Esterication of fatty acid with ethanol
The esterication of lauric acid with ethanol was carried out in a
two-necked round ask equipped with a reux condenser and a
Synthetic procedures
Synthesis of PS/nPMA–SiO2 HNs and PS/SiO2 HNs. PS/ magnetic stirrer. The catalyst was pretreated at 120 ꢁC under
nPMA–SiO2 HNs were synthesized under basic conditions using vacuum for 3 hours. In a typical run, 5 mL of ethanol and
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2 mmol of lauric acid were added to the ask containing 0.05 g vapor adsorption experiments were both treated by a freeze–
of catalyst. The mixture was stirred at 80 ꢁC and the aliquot was pump–thaw technique for three cycles prior to the adsorption.
withdrawn by a syringe at regular intervals and analyzed using a
The acid exchange capacity of the catalysts was determined
precalibrated gas chromatograph (Agilent 6890) equipped with by acid-base titration with a standard NaOH solution. In a
an FID detector and PEG capillary column (30 m ꢂ 0.25 mm ꢂ typical procedure, 0.05 g of solid was degassed at 120 ꢁC for
0.25 um). Tetradecane was used as the internal standard.
3 hours followed by the sample being suspended in 25 mL of
To recycle the solid acid catalyst, the catalyst was ltered 2 M aqueous NaCl solution. The resulting suspension was
from the reaction system, thoroughly washed with ethanol, stirred at room temperature for 24 hours until equilibrium was
dried under vacuum at 60 ꢁC overnight, and directly used in the reached and, subsequently, titrated with standard NaOH
next cycle.
solution.
Characterization
Results and discussion
The nitrogen sorption experiments were performed at 77 K
using a Micromeritrics ASAP 2020. Samples were degassed at
120 ꢁC for 6 hours prior to the measurements. The BET surface
area was calculated from the adsorption data in the relative
pressure P/P0 range from 0.04 to 0.2. Pore size distributions
were determined from the adsorption branches using the Bar-
ret–Joyner–Halenda (BJH) method. Pore volume was estimated
at the 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 (HR-TEM) was performed on a FEI Tecnai
G2 F30 S-Twin at an acceleration voltage of 300 kV. Before
measurement, the sample was dispersed in ethanol and
deposited on a holey carbon lm on a Cu grid. SEM was
undertaken by using a FEI Quanta 200F scanning electron
microscope operating at an acceleration voltage of 1–30 kV.
High-resolution scanning electron microscopy (HR-SEM) was
undertaken on a HITACHI S-4800 scanning electron microscope
operating at an acceleration voltage of 1–20 kV. FT-IR spectra
were collected with a Nicolet Nexus 470 IR spectrometer (KBr
pellets were prepared) in the range 400–4000 cmꢀ1. S elemental
analyses were determined by means of an Elementary Vario EL
III analyzer. The MAS 13C NMR spectra were recorded on a
Bruker DRX-400. The experimental parameters were as follows:
8 kHz spin rate, 3 s pulse delay, 4 min contact time, and 1000
scans. The acidic property of the solid materials was monitored
with reference to the 31P NMR chemical shi of triethylphos-
phine oxide (TEPO) chemically adsorbed on the solid acid
catalysts. 31P NMR spectra were performed 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 ms and a 30 s recycle delay. The chemical shi
spectra were referenced to an 85 wt% phosphoric acid external
standard. X-ray photoelectron spectroscopy (XPS) measure-
ments were performed on a VG ESCALAB MK2 system, using an
aluminium Ka X-ray source at 250 W and 12.5 kV. The thermo-
gravimetric analysis (TGA) was performed using a NETZSCH
STA 449F3 analyzer from 30 ꢁC to 900 ꢁC at a heating rate of 10
ꢁC minꢀ1 under air atmosphere. The adsorption of water and
benzene vapors was measured at 273 K on a Hiden Isochema
Intelligent Gravimetric Analyzer aer the sample was degassed
Synthesis of PS/nPMA–SiO2 HNs and PS–SO3H/nPMA–SiO2
HNs
PS–SO3H/nPMA–SiO2 HNs were obtained by sulfonation of PS/
nPMA–SiO2 HNs with chlorosulfonic acid (Scheme 1). Polymer
nanospheres, such as polystyrene (PS), poly(methyl meth-
acrylat) (PMMA) etc., have been widely used as hard templates
for the synthesis of hollow nanospheres.43–47 Aer hard template
removal, either by calcination or an extraction method, the
hollow nanostructure was generated. In our previous report,41 it
was found that the dissolved PMMA chains could be kept in the
mesochannels of silica shells for the formation of hybrid silica
HNs with a high concentration of PMMA based on the “disso-
lution and entrapment” strategy, which was proposed for the
rst time. Specically, the functional group PMA poly-
(methacrylate), which is incorporated into the mesoporous SiO2
shell, can efficiently entrap the dissolved PMMA. Here, a similar
strategy was employed for the synthesis of PS/nPMA–SiO2 HNs
(Scheme 1). In the rst step, PS nanospheres were coated with a
mixture of TEOS and PMA–Si using CTAB as the mesostructure
48
¨
directing agent according to a modied Stober method.
During the sol–gel process, the hydrolysis and polymerization of
TEOS and PMA–Si formed a hybrid mesoporous shell on the PS
template nanospheres and, consequently, PS/nPMA–SiO2 core
shell nanospheres (CSNs) were obtained (Fig. 1 and S1†). Aer
THF treatment, PS/nPMA–SiO2 HNs were obtained. The
successful transformation from core shell to hollow nano-
spheres aer THF treatment is shown in Fig. 1, using PS/5PMA–
Scheme 1 General procedure for the synthesis of mesoporous silica
for 6 h at 393 K. The ultrapure water and benzene used in the HNs with uniform distribution of PMA and sulfonated PS.
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monodispersed hollow nanospheres, and the particle size
increases as the ratio of PMA–Si silane precursor to PS
increases. The particle size increased from about 340 nm to
400 nm as the n increase from 2.5 to 5. Accordingly, the thick-
ness of the hybrid silica shells also increased from 50 nm to 80
nm. This is reasonable because more silane precursors are used
as the n increases. Based on the TEM image, PS/nPMA–SiO2 HNs
and PS/SiO2 HNs all have the mesoporous channels aligning
perpendicular to the cores, which would benet the diffusion of
reactants and products during the catalytic process. In
comparison to PS/SiO2 HNs, PS/nPMA–SiO2 HNs have larger
mesopores and a less condensed shell. For surface modica-
tion, PS/2.5PMA–SiO2 HNs were graed with octyl for the
generation PS/2.5PMA–C8–SiO2 HNs. PS/2.5PMA–C8–SiO2 HNs
have the same morphology as the parent PS/2.5PMA–SiO2 HNs
based on the TEM characterization, showing that the graing
process does not damage the nanostructure and morphology of
the hybrid HNs (Fig. 2).
The textural properties of the obtained hybrid hollow
nanospheres were investigated by nitrogen adsorption–
desorption at 77 K. Fig. 3A and B show the N2 adsorption–
desorption isotherms and corresponding pore size distribution
of PS/SiO2 HNs and PS/nPMA–SiO2 HNs, respectively. The
adsorption–desorption isotherms of the samples display type-IV
isotherm patterns which is typical for mesoporous silica
synthesized with CTAB as a surfactant.40 PS/SiO2 HNs exhibit a
sharp capillary condensation step at P/P0 less than 0.3, while PS/
nPMA–SiO2 HNs show an H4-type hysteresis loop starting from
the relative pressure P/P0 at 0.45, indicating that PS/nPMA–SiO2
HNs have larger mesopores than PS/SiO2 HNs. This is also
veried by the pore size distribution curves and consistency
with the TEM results. The pore size distribution curves also
show that PS/SiO2 HNs have a more uniform distribution of
mesopores than PS/nPMA–SiO2 HNs, probably due to the
interruption of the PMA polymer chain on the mesostructure
formation. The textural parameters of PS/SiO2 HNs and PS/
nPMA–SiO2 HNs are summarized in Table 1. The BET surface
area of PS/SiO2 HNs and PS/nPMA–SiO2 HNs varies in the range
Fig. 1 TEM images of PS template nanospheres (A), PS/5PMA–SiO2
CSNs (B) and PS/5MA–SiO2 HNs (C).
SiO2 HNs as a representative. It should be mentioned that the
particle size of PS/5PMA–SiO2 HNs remains unchanged aer
THF treatment, thus exhibiting that the dissolved PS is mainly
distributed in the mesopore of the shell. The above result clearly
suggests that the dissolution of PS cores during the THF
extraction process is the driving force for the formation of the
hollowed structure. The loading of PS could be facilely tuned by
varying the ratio of PS to silica precursor during the silica
coating process.
As a control sample, PS/SiO2 HNs, without PMA–Si incorpo-
rated in the shell, were prepared in a similar way to PS/5PMA–
SiO2 HNs. The results of thermo gravimetric analysis (TG) show
that weight loss for PS/SiO2 HNs and PS/5PMA–SiO2 HNs before
and aer THF treatment was 10% and 3.65 wt%, respectively
(Fig. S2†). These results indicate that the PMA–Si incorporated
into the mesoporous shell can efficiently entrap the dissolved PS
in the mesoporous shell. The sulfonation of PS/5PMA–SiO2 HNs
and PS/SiO2 HNs results in the formation of hybrid solid acids.
Characterization of PS/nPMA–SiO2 HNs
The TEM images of PS/nPMA–SiO2 HNs and PS/SiO2 HNs are
shown in Fig. 2. The TEM images show that all samples are
Fig. 3 N2 adsorption–desorption isotherms (A) and the pore size
distribution curves of PS/SiO2 HNs (a), PS/5PMA–SiO2 HNs (b), PS/
Fig. 2 TEM images of PS/2.5PMA–SiO2 HNs (A), PS/3.3PMA–SiO2 HNs 3.3PMA–SiO2 HNs (c), PS/2.5PMA–SiO2 HNs (d) and PS/2.5PMA–C8–
(B), PS/SiO2 HNs (C) and PS/2.5PMA–C8–SiO2 HNs (D).
SiO2 HNs (e).
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Table 1 The textural parameters and polymer content of PS/nPMA–SiO2 HNs and PS/SiO2 HNs before and after sulfonationa
Samples
SBET (m2 gꢀ1
)
Dpb (nm)
Vp (cm3 gꢀ1
)
Polymer contentc (wt%)
PS/SiO2 HNs
297 (331)
180 (149)
153 (164)
89 (128)
88 (115)
(45)
2.2 (2.5)
5.5 (6.4)
4.4 (4.8)
5.4 (4.4)
5.8 (4.2)
(40)
0.19 (0.30)
0.26 (0.21)
0.21 (0.21)
0.14 (0.14)
0.12 (0.13)
(0.31)
32.5
64.2
66.4
68.2
72
PS/5PMA–SiO2
PS/3.3PMA–SiO2
PS/2.5PMA–SiO2
PS/2.5PMA–C8–SiO2
Amberlyst®-15
—
a
Data in parentheses are for the samples aer sulfonation. b Pore size distribution estimated from BJH model. c Polymer content estimated based
on TG results.
89 to 297 m2 gꢀ1 and pore volume varies in the range 0.14 to nanostructure with almost identical particle size and shell
0.26 cm3 gꢀ1. In comparison to PS/2.5PMA–SiO2 HNs, only a thickness to the parent hybrid HNs, thus indicating the stability
slight decrease in the BET surface area was observed for of the hybrid HNs in strongly acid media. Taking PS–SO3H/SiO2
PS/2.5PMA–C8–SiO2 HNs.
as an example, the SEM image (Fig. 4F) clearly shows that the
Based on the TG analysis, the polymer content was calcu- sample aer sulfonation has
a
similar nanospherical
ꢁ
lated by the weight loss in the range 200–700 C (Table 1 and morphology to that before sulfonation. The HR-SEM images,
Fig. S3†). The weight loss of PS/SiO2 HNs, in the temperature shown in Fig. S5,† further verify that all sulfonated hybrid HNs
range 200–700 ꢁC, is 32.5%. For the PS/nPMA–SiO2 HNs, much are composed of uniform and mono-dispersed nanospheres
higher polymer content was obtained because of the introduc- with porous rough surfaces.
tion of the PMA polymer. PS/5PMA–SiO2 HNs affords a weight
The successful sulfonation of PS in the hybrid nanospheres
loss of 64.2%. From PS/5PMA–SiO2 HNs to PS/2.5PMA–SiO2 was veried by FT-IR, XPS, and 13C NMR spectroscopy (Fig. 5, S6
HNs, only about a 5.3% increase in weight loss was observed, and S7†). In the FT-IR spectra (Fig. S6†), the appearance of a
indicating that the polymer–silica weight ratio has an upper characteristic vibration of PMA at 1740 cmꢀ1 arising from a C]
limit in the hybrid HNs. The above results show that PS/nPMA– O stretching vibration41,42 suggested the successful incorpora-
SiO2 HNs, with high PS and PMA contents, have been success- tion of the PMA polymer fragment into the mesoporous network
fully synthesized. The weight loss of PS/2.5PMA–C8–SiO2 HNs is of PS/nPMA–SiO2 HNs as well as PS–SO3H/nPMA–SiO2 HNs. The
about 3.8% higher than PS/2.5PMA–SiO2 HNs based on the TG new peaks, attributed to sulfonic acid groups at 581, 1011, and
results (Fig. S4†).
1415 cmꢀ1, appeared in the FT-IR spectra of PS–SO3H/SiO2 HNs
and PS–SO3H/nPMA–SiO2, while the intensity of C–H deforma-
tion vibration of monosubstituted phenyl groups at 700–
750 cmꢀ1 decreases. In the XPS spectra (Fig. 5A), the S 2p peak at
173.2 eV was observed, conrming the incorporation of sulfonic
acid groups. The 13C NMR spectrum of PS–SO3H/SiO2 HNs
clearly shows a signal at around 142 ppm, which could be
assigned to the aromatic carbon coordinated with sulfonic acid
groups. These results imply a successful sulfonation of PS with
the current sulfonation method (Fig. S7†).
Characterization of PS–SO3H/nPMA–SiO2 HNs
PS–SO3H/SiO2 HNs, PS–SO3H/nPMA–SiO2 HNs, and PS–SO3H/
2.5PMA–C8–SiO2 HNs were obtained by direct sulfonation ꢁof
the corresponding hybrid HNs with ClSO3H in CH2Cl2 at 0 C
for 12 hours. The TEM images of sulfonated hybrid HNs are
shown in Fig. 4. PS–SO3H/SiO2 HNs, PS–SO3H/nPMA–SiO2 HNs,
and PS–SO3H/2.5PMA–C8–SiO2 HNs have a uniform hollow
The acid strengths of Amberlyst®-15, PS–SO3H/SiO2 HNs and
PS–SO3H/5PMA–SiO2 HNs were investigated using TEPO (trie-
thylphosphine oxide) as a probe molecule in combination with
the 31P MAS NMR technique (Fig. 5 B). The 31P chemical shi of
Fig. 4 TEM images of PS–SO3H/SiO2 HNs (A), PS–SO3H/2.5PMA–
SiO2 HNs (B), PS–SO3H/3.3PMA–SiO2 HNs (C), PS–SO3H/5PMA–SiO2 Fig. 5 XPS spectra (A) of PS–SO3H/SiO2 HNs (a) and PS–SO3H/5PMA–
31
HNs (D), PS–SO3H/5PMA–C8–SiO2 HNs (E) and SEM image of PS– SiO2 HNs (b); (B) P MAS NMR spectra of Amberlyst®-15 (a),49 PS–
SO3H/SiO2 HNs (F).
SO3H/SiO2 HNs (b) and PS–SO3H/5PMA–SiO2 HNs (c).
7550 | J. Mater. Chem. A, 2014, 2, 7546–7554
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Fig.
6 TEM images of PS–SO3H/5PMA–SiO2 HNs (A) and EDS
mapping analysis of Si element (B) and S element (C) according to TEM
image shown in A.
TEPO, chemisorbed on the acid site, is sensitive to acid
strength, and stronger acids will usually lead to larger 31P
chemical shis of TEPO. The 31P NMR spectrum of Amberlyst®-
15 exhibits a signal at 89.4 ppm which was assigned to TEPO
adsorbed on SO3H acid sites.49 PS–SO3H/SiO2 HNs and PS–
SO3H/5PMA–SiO2 HNs exhibit a signal at 84.5 ppm and 86 ppm,
respectively. The lower chemical shi indicates PS–SO3H/SiO2
HNs and PS–SO3H/5PMA–SiO2 HNs have a weaker acid strength
than Amberlyst®-15. However, PS–SO3H/5PMA–SiO2 HNs
exhibit a larger chemical shi than PS–SO3H/SiO2 HNs, thus
indicating that the incorporation of PMA polymer could help to
enhance the acid strength.
For further illustration of the distribution of sulfonic acid
groups in the mesoporous shells of PS–SO3H/nPMA–SiO2 HNs,
HR-TEM element mapping analysis was used as direct evidence
for the location of sulfur (Fig. 6). The HR-TEM element mapping
images clearly show that the Si and S element was uniformly
distributed in the outer shells of the hollow nanospheres. The
sulfonic acid groups, located uniformly in the outer shells, may
favor the access of reactants to active sites during the catalytic
process.
PS/SiO2 HNs display a sharp diffraction peak at low diffrac-
tion angle, showing the sample has an ordered mesoporous
structure. A broad diffraction peak was observed in the XRD
pattern of PS/5PMA–SiO2 HNs. This suggests that the incorpo-
ration of PMA polymer causes the deterioration of the meso-
structure. PS–SO3H/5PMA–SiO2 HNs have a similar XRD pattern
to that of PS/5PMA–SiO2 HNs, conrming that the sulfonation
process does not cause the destruction of the mesoporous
structure (Fig. S8†).
Fig. 7 N2 adsorption–desorption isotherms (A) and the corresponding
pore size distribution of PS–SO3H/SiO2 HNs (a), PS–SO3H/5PMA–SiO2
HNs (b), PS–SO3H/3.3PMA–SiO2 HNs (c), PS–SO3H/2.5PMA–SiO2
HNs (d) and PS–SO3H/2.5PMA–C8–SiO2 HNs (e).
acid exchange capacity and sulfur content, while higher content
of PS in the hybrid HNs benets a high content of acid sites. The
acid exchange capacity of the sulfonated hybrid HNs varies from
0.90 to 2.0 mmol gꢀ1 while the sulfur content varies from 0.99 to
2.2 mmol gꢀ1. For the PS–SO3H/nPMA–SiO2 HNs samples, the
acid exchange capacity and the sulfur content were very similar.
However, for PS–SO3H/SiO2 HNs, the sulfur content was higher
than the acid exchange capacity. The H-bond interactions may
exist among sulfonic acid groups and hydroxyl groups in the
mesoporous shells for PS–SO3H/SiO2 HNs, which may reduce
the accessibility of acid sites during the titration process. For
the PS–SO3H/nPMA–SiO2 HNs samples, the incorporation of the
polymer PMA may help to weaken the interactions among
sulfonic acid groups and silicon hydroxyl groups. Thus, the acid
exchange capacity is similar to the sulfur content. Based on the
results, we can see that the acid exchange capacity and the
sulfur element content were consistent, and this consistency
between H+ and sulfur suggests that all of the acid sites were
available during the titration process.
Catalytic performance of hybrid mesoporous silica hollow
nanospheres
The sulfonated hybrid HNs exhibit type-IV isotherm patterns Esterication of fatty acids with ethanol was chosen as the
similar to the parent materials (Fig. 7). It should be mentioned model reaction for the investigation of the catalytic perfor-
that an obvious H4 hysteresis loop appears in the N2 isotherm of mance of the sulfonated hybrid HNs (Table 2). The esterica-
PS–SO3H/SiO2 HNs, which is probably due to the existence of tion of fatty acids with alcohol to produce esters represents an
defects caused during the sulfonation process. The textural important pretreatment step in the production of biodiesel
parameters of the sulfonated hybrid HNs are summarized in from high free fatty acid feed stocks. Initially, a controlled blank
Table 1. Only a slight decrease in BET surface area was observed experiment was carried out for the esterication of lauric acid
aer sulfonation. The BET surface area of the sulfonated hybrid with ethanol and only 1.5% yield of laurate ethyl was obtained.
varies in the range 331 to 115 m2 gꢀ1, which was much higher For the sulfonated SiO2 HNs samples only 4.9% yield of laurate
than that of Amberlyst®-15.
ethyl was obtained, thus indicating that the amount of sulfonic
In order to quantitatively analyze the number of active sites acid groups graed to surface silanol groups using the same
in the sulfonated hybrid HNs, the sulfur content and acid method could be negligible. Under similar reaction conditions,
exchange capacity were obtained by element analysis and acid- all sulfonated hybrid HNs could efficiently catalyze the reaction,
base titration methods, respectively (Table 2). Increasing the thus showing that the sulfonic acid groups are the active sites
amount of PMA in the hybrid HNs results in a decrease in the for this reaction. The catalytic performance of concentrated
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Table 2 The sulfur content, acid exchange capacity and surface properties of sulfonated hybrid HNs and their catalytic performance in the
esterification of lauric acid with ethanol
S contenta
(mmol gꢀ1
H+ contentb
Adsorbed benzene–water
molar ratio
Sample
)
(mmol gꢀ1
)
TOF (hꢀ1
)
Yield (%)
H2SO4
Amberlyst®-15
—
—
—
—
—
29.3 ꢃ 0.1
2.4 ꢃ 0.1
13.6 ꢃ 0.3
27.3 ꢃ 0.2
21.1 ꢃ 0.2
16.3 ꢃ 0.1
19.1 ꢃ 0.1
—
93.7 ꢃ 1.7
65.3 ꢃ 0.8
87.8 ꢃ 0.1
88.9 ꢃ 0.6
91.6 ꢃ 0.2
91.6 ꢃ 0.6
94.7 ꢃ 0.1
4.9
4.7
2.0
0.9
1.5
1.9
1.8
—
PS–SO3H/SiO2 HNs
PS–SO3H/5PMA–SiO2
PS–SO3H/3.3PMA–SiO2
PS–SO3H/2.5PMA–SiO2
PS–SO3H/2.5PMA–C8–SiO2
SiO2 HNs
2.29
0.99
1.43
1.80
1.79
—
0.08
0.11
0.12
0.24
0.27
—
a
Measured by element analysis. b Measured by acid-base titration method.
sulfuric acid and commercially available Amberlyst®-15 was catalysts could increase catalytic activity due to the protective
also investigated. Among all samples, concentrated sulfuric effect of acid sites from water attack, the increased diffusion
acid affords the highest reaction rate (TOF of 29.3 ꢃ 0.1 hꢀ1) as rate of the hydrophobic reactants, and high enrichment effect
expected and Amberlyst®-15 gives the lowest reaction rate with for the reactants.5,52,53 For PS–SO3H/nPMA–SiO2 HNS, the TOF
a TOF of 2.4 ꢃ 0.1 hꢀ1. The low activity of Amberlyst®-15 is decreases monotonically with the increase in surface hydro-
mainly due to its low surface area. The TOF of the sulfonated phobicity. This is due to a decrease in the BET surface area
hybrid HNs varies in the range 27.3 ꢃ 0.3 to 16.3 ꢃ 0.1 hꢀ1. The because the textural properties also affect the catalytic activity of
relatively high activity of PS–SO3H/5PMA–SiO2 HNs and PS– the solid materials, in addition to the surface properties.
SO3H/SiO2 HNs, in comparison with Amberlyst®-15, suggests
The catalytic stability of the sulfonated hybrid HNs was
that PS–SO3H is uniformly aligned in the mesoporous silica tested using PS–SO3H/2.5PMA–SiO2 HNs and PS–SO3H/
shell and may benet the increase in accessibility of reactants to 2.5PMA–C8–SiO2 HNs as model catalysts in the esterication of
the sulfonic acid groups and the mesoporous shell would lauric acid with ethanol (Fig. 8). For the second cycle, the yield
facilitate the fast diffusion of reactants and products during the of ethyl lauric over PS–SO3H/2.5PMA–SiO2 HNs decreases
catalytic process. Actually, most solid acid catalysts that were sharply from 96.3% to 71.1%. For the next several cycles, the
reported exhibit much lower activity than H2SO4. It should be conversion only slightly decreases. Aer ten cycles, the conver-
mentioned that PS–SO3H/5PMA–SiO2 HNs with a TOF of 27.3 ꢃ sion is about half that of the rst cycle. The acid exchange
0.3 hꢀ1 is almost as active as H2SO4.
capacity of PS–SO3H/2.5PMA–SiO2 HNs aer 10 cycles was
Though PS–SO3H/SiO2 HNs have a higher BET surface area 1.0 mmol gꢀ1, only half of the fresh catalyst. This shows that the
and acid exchange capacity than PS–SO3H/nPMA–SiO2 HNs, PS– leaching of active sites is the main reason for the deactivation of
SO3H/nPMA–SiO2 HNs are more active than PS–SO3H/SiO2. It the hybrid HNs. However, for PS–SO3H/2.5PMA–C8–SiO2 HNs
has been reported that PMA is a hydrophobic polymer and its aer 10 cycles, the conversion only decreases from 96.3 to
incorporation may modify the surface properties of the hybrid 79.6%, and the activity of the catalyst could still retain 82% of
HNs.42 Therefore, water/oil adsorption experiments were per-
formed to investigate the surface hydrophobicity. As repor-
ted,38,50,51 the water and benzene adsorption capacities at the
low relative pressure range (P/P0 # 0.30) were used for the
calculation (Table 2). The results show that the adsorbed
benzene/water molar ratio for PS–SO3H/nPMA–SiO2 HNs is
higher than that for PS–SO3H/SiO2 HNs, showing that PS–SO3H/
nPMA–SiO2 HNs have more hydrophobic surface properties
than PS–SO3H/SiO2 HNs. Therefore, the low activity of PS–SO3H/
SiO2 nanocomposites is attributed to their more hydrophilic
surface properties and smaller pore size. The octyl modication
could further increase the surface hydrophobicity.52 PS–SO3H/
2.5PMA–C8–SiO2 HNs, with octyl group incorporation, show a
higher activity than PS–SO3H/2.5PMA–SiO2 HNs (19.1 ꢃ 0.1
versus 16.3 ꢃ 0.1 hꢀ1) (BET surface area and pore volume were
almost the same for the two samples). In combination with the
catalytic results, it could be suggested that a hydrophobic
surface is more favorable for obtaining higher catalytic activity.
Fig. 8 Catalytic recycle stability of PS–SO3H/2.5PMA–SiO2 HNs (a)
and PS–SO3H/2.5PMA–C8–SiO2 HNs (b) in the esterification of lauric
Previous studies suggest that hydrophobicity of the solid
acid with ethanol.
7552 | J. Mater. Chem. A, 2014, 2, 7546–7554
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Journal of Materials Chemistry A
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In summary, we have demonstrated an efficient strategy for the
synthesis of hybrid hollow nanospheres with a high content of
sulfonic groups uniformly distributed in mesoporous silica
channels. The hybrid HNs have a BET surface area from 115 to
331 m2 gꢀ1, and pore size from 2.5 nm to 6.4 nm, depending on
the polymer content incorporated in the silica shell. The hybrid
HNs could be used as efficient solid acid catalysts for acid-
catalyzed liquid reactions, such as esterication of lauric acid
with ethanol. The studies show that the incorporation of PMA
and surface modication by octyl groups could increase the
surface hydrophobicity, which causes an increase in the cata-
lytic activity. Under similar conditions, the hybrid HNs, with
optimized surface properties and sulfonic acid content, show
comparable activity to concentrated sulfuric acid. The incor-
porated octyl group could prevent the leaching of PS–SO3H
during the catalytic process, thus, the hybrid HNs could be
recycled in a stable manner. Our results provide a new approach
for the synthesis of solid acid catalysts by efficiently combining
PS–SO3H and silica and a hollow nanostructure together.
Acknowledgements
31 T. Okayasu, K. Saito, H. Nishide and M. T. W. Hearn, Green
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32 A. Martin, G. Morales, F. Martinez, R. van Grieken, L. Cao
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This work was nancially supported by the National Natural
Science Foundation of China (no. 21325313, 21232008).
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