Magnetically separable porous carbon nanospheres as solid acid catalysts

Article abstract of DOI:10.1039/c3ra43208d

A novel solid acid material, magnetically separable and SO ₃H-functionalized porous carbon nanosphere, was facilely synthesized by a simple activation route. The SiO₂ layer protected the magnetic core from dissolving during the process of activation with ZnCl₂, and retained the highly magnetic property. The obtained materials were characterized by N₂ adsorption-desorption technology and transmission electron microscopy, and the results indicated that this solid acid material possessed an excellent spherical morphology and a superior porosity with high surface area and large pore volume. The results of X-ray diffraction, fourier transform infrared spectra and energy dispersive X-ray spectra demonstrated the preservation of the magnetic core and the successful modification of -SO ₃H functional groups. The solid acid activated at a low temperature (400 °C) showed the highest acidity of 1.98 mmol H⁺ g ^-1, which was estimated by an indirect titration method. The high surface area, large pore volume and high acidity endued this solid acid material excellent catalytic performance for esterification and transesterification reaction. Besides, the solid acid catalyst possessed remarkable stability and recycling property. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra43208d

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Magnetically separable porous carbon nanospheres using as
solid acid catalysts
Binbin Chang, Yanlong Tian, Weiwei Shi, Jiyang Liu, Fengna Xi, Xiaoping Dong
*
A novel solid acid material, magnetically separable and
SO3H-functionalized porous carbon nanosphere, was facilely
synthesized by a simple activation route. The SiO2 layer protected the
magnetic core from dissolving during the process of activation with
ZnCl2, and retained the highly magnetic property. The obtained
materials were characterized by N2 adsorption–desorption technology
and transmission electron microscope, and the results indicated that this
solid acid material possessed an excellent spherical morphology and a
superior porosity with high surface area and large pore volume. The
results of X-ray diffraction, fourier transform infrared spectra and
energy dispersive X-ray spectra demonstrated the preservation of
magnetic core and the successful modification of –SO3H functional
groups. The solid acid activated at a low temperature (400 oC) showed
the highest acidity of 1.98 mmol H+g-1, which was estimated by an
indirect titration method. The high surface area, large pore volume and
high acidity endued this solid acid material excellent catalytic
performance for esterification and transesterification reaction. Besides,
the solid acid catalyst possessed remarkable stability and recycling
property.
1. Introduction
Acid catalysts are essential for various chemical reactions in industry, including the fields of oil
refining, petrochemicals and other hydrocarbon chemicals. Traditional homogeneous acid
catalysts such as sulfuric acid, perchloric acid and other liquid acids are used extensively in
industry for the production of industrially important chemicals. Although these liquid acid
catalysts are inexpensive and highly efficient, it is difficult to separate them from the reaction
mixture, resulting in abundant acid waste1–3
.
Solid acid catalysts have shown great potential to replace liquid acid as reusable and
environmentally-safe acid catalysts. Among them, carbon-base solid acids are considered as ideal
catalysts for many chemical reactions owing to their chemical inertness and superior thermal
stability. Particularly, sulfonated carbon materials are the most promising of solid acid developed
for the production of biodiesel4. However, these sulfonated amorphous carbon–base solid acids are
usually prepared by the sulfonation of incompletely carbonized biomass, such as glucose, sucrose,
starch and biochar5–7. Consequently, these solid acids in general possess low surface area (< 2
m2g-1), poor porosity, relatively low acid density ranging from 0.37 to 1.34 mmol H+g-1 and poor
dispersion in liquid phase reaction mixture8.
To overcome these disadvantages and further improve catalytic performance,
SO3H–functionalized porous carbons (PCs) are used as fine powders in catalytic reactions.
SO3H–modified PCs with excellent catalytic performance have been successfully synthesized by
different routes9-16. However, the hard-template prepared sulfonated PCs often suffer a
high-temperature carbonization, and thus the surface of these materials possess limited
oxygen-containing and hydrogen-containing functional groups, which seriously restricts the
modification of –SO3H groups, subsequently resulting in the low acidity and low activity11,14,16
.
Recently, we reported an approach to obtain PC solid acid with high acidity and impressive
10
catalytic activity via the pretreatment of hard-template prepared mesoporous carbon with H2O2
.
Another route to enhance the acidity of PC solid acids is to reduce the carbonation temperature
and therefore retain an incompletely carbonized framework. Such strategy has been realized by a
soft-template route, where the porous resin frameworks are self-organized with the assistance of
triblock copolymers, such as Pluronic P123 and F127, and can keep the porous structure under a
low temperature carbonization9,17. Despite these sulfonated PCs exhibit high catalytic performance,
the separation of the fine powders from the catalytic systems is a great challenge.

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Design and synthesis of magnetically functionalized composites offer an alternative route for
the separation of fine powders by an outer magnetic field. Magnetic nanoparticles with a wide
range of compositions could be readily introduced into PC matrices. The conventional preparation
of magnetic PCs is usually through either a direct synthesis approach or a post loading method18–21
Normally, the magnetic nanoparticles can be well dispersed in case of a low content, resulting in a
poor crystallinity and magnetic property. However, it also has some problems by improving the
loading level to obtain highly magnetic property, such as the collapse of porous structure in the
direct synthesis approach and the pore blocking in the post loading method. Therefore, it is
extremely challenging to obtain strongly magnetic PC composite without pore blocking.
Constructing a core–shell structure by covering carbon shell out onto the surface of magnetic core
provides an opportunity to endow PC composites with strong magnetism and good monodispersity,
as well as inter-connected pore structure. Consequently, these materials have attracted increasing
.
attention in many applications, such as adsorption, separation, energy storage and conversion22,23
.
Great progress in fabrication of magnetic core–shell PCs has been made by developing several
synthesis strategies24–26. But, the tedious synthetic steps and low yields limit their wide
applications. Moreover, little attention has been paid to simultaneously exploring and utilizing the
high porosities of carbon matrices and the magnetic core–shell structure as active solid acid for
industrial catalysis.
Herein, we reported a successful synthesis of magnetically separable and SO3H–functionalized
nanospherical PC solid acid catalyst, Fe3O4@SiO2@PCS, which obtained via a facilely chemical
activation of Fe3O4@SiO2@C with ZnCl2. The SiO2 layer between the magnetic core and carbon
shell plays a key role for keeping the magnetic core from decaying by the strong acidity of ZnCl2
during the process of activation. The magnetic core–shell PC solid acid catalyst did not only
possess high surface area and narrow pore size distribution, but also exhibited high acidity and
excellently catalytic performance as well as a superb recycle. Such excellently magnetic
separation ability and high acidity make this material as an efficient solid acid catalyst for the
practical application of biodiesel production.
2. Experimental
2.1 Preparation of materials
Synthesis of Fe3O4 nanoparticles. The dispersible Fe3O4 nanoparticles were synthesized
according to the method reported previously27. In a typical experiment, FeCl3·6H2O (3.25 g),
trisodium citrate (1.3 g) and sodium acetate (NaAc, 6.0 g) were dissolved in ethylene glycol (100
mL) with magnetic stirring. The obtained yellow solution was then transferred and sealed into a
o
Teflon-lined stainless-steel autoclave and the autoclave was heated at 200 C for 10 h. The black
products were washed repeatedly with deionized water and ethanol and oven–dried at 60 oC for
more than 6 h.
Synthesis of Fe3O4@SiO2 microspheres. The core–shell Fe3O4@SiO2 microspheres were
prepared according to the previously reported method28. Briefly, 0.10 g of Fe3O4 particles were
treated with 0.1 M HCl aqueous solution (50 mL) by ultrasonication. After the treatment for 10
min, the magnetite particles were separated and washed with deioned water, and then
homogeneously dispersed in the mixture of ethanol (80 mL), deioned water (20 mL) and
concentrated ammonia aqueous solution (1.0 mL), followed by the addition of
tetraethylorthosilicate (TEOS, 0.03 g). After stirring at room temperature for 6 h, the Fe3O4@SiO2
microspheres were separated and washed with ethanol and water, and then dried at 60 oC for 6 h.
Synthesis of Fe3O4@SiO2@PC microspheres. In a typical experiment, 4 g of glucose was
dissolved in 40 mL of water to form a clear solution, and then 0.2 g Fe3O4@SiO2 microspheres
were dispersed in glucose solution. After vigorous stirring for 30 min, the suspension was
o
transferred to a Teflon–sealed autoclave and maintained at 180 C for 10 h. The products were
o
separated and washed with deionized water and ethanol, and then dried at 60 C for 6 h. The
resulting materials were denoted as Fe3O4@SiO2@C. The dried Fe3O4@SiO2@C materials were
impregnated in ZnCl2 solution for 6 h, and dried at 110 oC for 12 h and then was activated in N2 at
different temperatures (400, 500 and 600 oC) for 1 h. After cooling down, the activated samples
were thoroughly washed with distilled water and HCl solution (0.5 M). Finally, the materials were
dried under vacuum at 80 oC for 10 h, which were denoted as Fe3O4@SiO2@PC–x–y (x = 4,
referring to the mass ratio of ZnCl2/Fe3O4@SiO2@C of 4:1; y = 4, 5 and 6, meaning the activation
temperature is 400, 500 and 600 oC, respectively).

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Subsequently, these Fe3O4@SiO2@PC materials were sulfonated using concentrated sulfuric
acid at 150 oC for 10 h in a Teflon–sealed autoclave. Then, these products were washed repeatedly
o
with hot distilled water (> 80 C) until the sulfate ions were no longer detected in the wash water
(BaCl2 precipitation test) and were dried at 80 oC for 10 h. The resulting materials were denoted as
Fe3O4@SiO2@PCS.
2.2 Characterizations
o
Nitrogen adsorption–desorption isotherms were carried out at –196 C using a micromeritics
o
ASAP 2020 analyzer. Before adsorption, the samples were out-gassed at 150 C for 10 h. The
specific surface area (SBET) was evaluated using the Brunauer–Emmett–Teller (BET) method, and
the mesopore volume and micropore volume were calculated according to the
Barrett–Joyner–Halenda (BJH) formula and t-plot method, respectively. The pore size
distributions were calculated according to the Density–Functional–Theory (DFT) method. Fourier
transform infrared (FTIR) spectra were recorded on a Nicolet Avatar 370 spectrometer using the
stanndard KBr disk method. The morphology and core-shell structure were observed from a JEOL
JEM–2100 transmission electron microscope (TEM) with an accelerating voltage of 200 KV. The
energy dispersive X-ray (EDX) spectra were obtained using a Hitachi S–4800 field emission
scanning electron microscope equipped with an energy dispersive X-ray spectrometer.
The number of acid sites was estimated by using an indirect titration method29,30, which
involves an aqueous ion-exchange step of the catalyst H+ ions with base of NaHCO3, followed by
titration of the resulting solution with HCl aqueous solution (0.1 M). In a typical experiment,
30 mg of the catalyst was dispersed in 50 mL of 5 × 10-3 mol L-1 NaHCO3 solution, which was
stirred for 24 h and separated by filteration. Then 5 mL of filtrate was taken out for titration with
0.1 M of HCl aqueous solution. Titration was performed three times and the average number was
reported. The amount of acid groups in the solid acid catalysts was estimated by the
NaHCO3 consumed.
2.3 Catalytic testing
Catalytic reactions were performed in a round bottomed flask equipped with a reflux condenser,
magnetic stirrer and an oil bath maintained at a specified temperature. In the esterification of
aliphatic acid (oleic acid, palmitic acid and stearic acid) and acetic acid with methanol or ethanol,
aliphatic acid (0.01 mol), acetic acid (0.05 mol), and catalyst (0.1 g) were mixed in flask and the
reaction was carried out at 70 oC for 5 h. For the transesterification of soybean oil with methanol,
soybean oil (0.01 mol), and catalyst (0.3 g) were used at 150 oC for 6 h. The products in catalytic
reactions were analyzed by gas chromatograghy (GC-2014C, shimadzu) with a flame ionization
detector (FID), and methyl undecanoate was used as an internal standard.
3. Results and discussion
3.1 Characteristics of materials
Scheme 1 outlines the synthetic procedure of Fe3O4@SiO2@PCS material, which begins with the
synthesis of Fe3O4 nanoparticles. First, uniform Fe3O4 nanoparticles are coated with a thin layer of
amorphous SiO2 via a sol-gel approach to obtain nonporous Fe3O4@SiO2. Second, glucose
monomers polymerize on the surface of Fe3O4@SiO2 material under hydrothermal conditions at
o
180 C, which is higher than the normal glycosidation temperature and results in aromatization
and carbonization. Third, ZnCl2 is used as a dehydrating agent to chemically activate the carbon
layer to obtain the porous structure, Fe3O4@SiO2@PC. Finally, Fe3O4@SiO2@PC materials are
sulfonated using concentrated sulfuric acid in Teflon autoclaves at a high temperature to form
Fe3O4@SiO2@PCS material.
To reveal the morphology and structure of materials, TEM images of Fe3O4@SiO2,
Fe3O4@SiO2@C, and Fe3O4@SiO2@PCS are shown in Fig. 1. As displayed in Fig. 1a, the
Fe3O4@SiO2 particles exhibit good dispersity and uniform spherical morphology. Particularly, the
nonporous SiO2 layer with a thickness of 40~60 nm forms a complete enveloping of Fe3O4
nanoparticles and it therefore plays a key role for keeping interior magnetic cores from decaying
in the process of activation with ZnCl2. Additionally, this SiO2 layer is indeed favorable to
nucleation and growth of carbon outer layer31. Fig. 1b and 1c show the TEM images of

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Fe3O4@SiO2@C nanospheres. On the whole, carbon shell layer reveals spherical shapes with
thickness of about 50 120 nm. After the activation of carbon shell with ZnCl2 and the following
~
modification of sulfonation with concentrated sulfuric acid, the nanospherical morphology is
retained and its initial core-shell structure can be directly observed in Fig. 1d, which indicates that
the processes of activation and sulfonation do not destroy the structure and morphology of
Fe3O4@SiO2@C material.
To confirm the crystal structure of as-synthesized materials, XRD patterns of all samples were
characterized (Fig. 2). The characteristic diffraction peaks of Fe3O4 are present in all samples,
which can be assigned to the (220), (311), (400), (511) and (440) planes of a pure cubic Fe3O4
phase32.In comparison with pure Fe3O4 sample, an additional broad diffraction peak at 2θ =
20~30o appears in Fe3O4@SiO2 sample. This weak diffraction of amorphous silica verifies the
successful coating of SiO2 layer onto the magnetic core. After the hydrothermal treatment of
Fe3O4@SiO2 sample with glucose, an amorphous carbon shell is formed, which can be proved by
the enhanced intensity of broad diffraction and the shift of center to low angle. No evident
difference of the diffraction of magnetic core is found between Fe3O4 and Fe3O4@SiO2@C,
suggesting the modification of SiO2 layer and the carbon shell do not influence the crystal
structure and grain size of Fe3O4 phase. However, after the activation and further carbonization,
the diffraction of Fe3O4 become much sharper in Fe3O4@SiO2@PC and Fe3O4@SiO2@PCS,
which imply that the process of activation improve the crystallization of Fe3O4. The grain size of
Fe3O4 in Fe3O4@SiO2@PC is estimated to be 20.6 nm, which is larger than 7.9 nm for the Fe3O4
sample. Fig. 3 reveals the influence of SiO2 layer on the protection of magnetic Fe3O4 core in the
ZnCl2 activation process. According to the activation mechanism, HCl would be released from the
reaction of ZnCl2 and the incompletely carbonized framework. This acidic gas could react with
Fe3O4 as lacking a compact and inert layer between Fe3O4 core and carbon shell, which is verified
by the XRD pattern of Fe3O4@PC where the Fe3O4 diffractions disappear. On the contrary, as
coating a SiO2 layer between Fe3O4 core and carbon shell, Fe3O4 cores can be well protected
during the ZnCl2 activation. Fig. 4 shows a direct comparison of the magnetism of Fe3O4@PC and
Fe3O4@SiO2@PC. Without an outer magnet, these two materials can be well dispersed in
methanol to form a dark and stable suspension. As put a magnet close to these suspensions, the
Fe3O4@SiO2@PC sample is immediately attracted by the magnet. However, the Fe3O4@PC
sample does not exhibit any magnetism because of the dissolution of Fe3O4 by ZnCl2.
FTIR spectra were used to further identify the successful coating of silica and carbon layers
onto magnetic particles, and the presence of functional groups after sulfonation (Fig. 5). All of
samples present the characteristic band of Fe–O appeared at 475 and 575 cm-133. Compared with
Fe3O4 sample, two additional adsorption bands at 950 and 1088 cm-1 are found in Fe3O4@SiO2
sample, which are ascribed to the characteristic stretching vibration of Si–O34, verifying the
formation of the SiO2 shell. In Fe3O4@SiO2@C material, the bands at 1710 and 1620 cm-1 should
be attributed to C=O and C=C stretching vibrations, respectively, and the results support the
concept of aromatization of glucose during hydrothermal treatment35. The bands centered at 1000
~
1300 cm-1 are related to the C–O stretching vibration and –OH bending vibration36. These
absorptions overlap with the Si–O absorption at 950 and 1088 cm-1, and thus the latter become
illegible. All these evidences identify that Fe3O4@SiO2 are successfully coated by a carbon shell.
After activation, the absorption peaks of C–O become illegible and even the characteristic
absorption band of C=O at 1710 cm-1 disappear, which should be due to the chemical activation
with ZnCl2 by the release of H2O molecule37. After sulfonation treatment, the new absorption
bands at 1108 and 1190 cm-1 in the spectra of Fe3O4@SiO2@PCS are assigned to the symmetric
stretching vibrations of S=O38, and another additional absorption band at 620 cm-1 is attributed to
the bending vibration of –OH groups hydrogen bonded to –SO3H groups 39,40. The EDX analysis
also shows a similar result (Fig. 6). Si and O elements are observed in Fe3O4@SiO2 sample,
demonstrating the successful coating of SiO2 layer. The finding of C element in Fe3O4@SiO2@C
sample also indicates that the Fe3O4@SiO2 nanospheres are successfully covered by a carbon shell.
Furthermore, the signal of Fe in Fe3O4@SiO2@PC sample suggests the protection of magnetic
core by SiO2 layer in the process of activation. In the meantime, the intensity of O signal
becoming weak in Fe3O4@SiO2@PC is corresponding to the FTIR results. Finally, the S signal in
Fe3O4@SiO2@PCS sample gives the evidence of –SO3H groups modified on the carbon shell
surface.

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N2 sorption technology is used to investigate the porous properties of materials, as well as the
effect of activation temperature and sulfonation on the pore structure (Fig. 7). With the
enhancement of activation temperature, the N2 adsorption–desorption isotherms transform
gradually to typical type IV curves with a clear hysteresis loop at relative pressure from 0.4 to 0.7,
which suggest the pore of Fe3O4@SiO2@PC evolve from micropore to mesopore. The specific
surface area and pore volume also undergo a great change (Table 1). When the activation
temperature is 400 oC, the pyrolysis reaction just commences, thereby producing some micropores
and low micropore volume. This result should be ascribed to the inadequacy of heat energy
produced at the low activation temperature for some substantial evolution of volatiles necessary
o
for pore development. As the temperature is increased to 500 C, the pyrolysis reaction is
completely carried out, and the developed pore is mainly mesoporous. Mesopore size, mesopore
surface area and total pore volume increase, however the BET surface area decreases because
o
ofthe disappearance of micropores. With further enhancing the activation temperature to 600 C,
there are no evident changes in pore size, indicating that the pyrolysis reaction has finished at 500
oC. The decrease of mesopore surface area and mesopore volume should be due to a sintering
effect at high temperature, where the shrinkage of framework and the realignment of carbon
structure result in the reducing of pore areas as well as volume41. After sulfonation, the surface
area, pore volume and pore size of these samples all decrease, and the result should be related to
the presence of a large number of –SO3H groups. The decrement in surface area of
Fe3O4@SiO2@PCS–4–4 is much more than those of Fe3O4@SiO2@PCS–4–5 and
Fe3O4@SiO2@PCS–4–6, and the similar results can be detected in pore volume and pore size
(Table 1), which suggest the much higher density of –SO3H groups modified on the surface of
Fe3O4@SiO2@PCS–4–4. The acidities of these materials listed in Table 1 are 1.98 mmolH+g-1 for
Fe3O4@SiO2@PCS–4–4, 1.32 mmolH+g-1 for Fe3O4@SiO2@PCS–4–5 and 1.14 mmolH+g-1 for
Fe3O4@SiO2@PCS–4–6, respectively. This phenomenon can be attributed to the different
activation temperature. At low activation temperature, the pyrolysis reaction is incomplete and the
surface of Fe3O4@SiO2@PC–4–4 still possesses numerous oxygen-containing and
hydrogen-containing functional groups, which are favorable for the immobilization of –SO3H
groups on carbon surface via forming covalent bonds or hydrogen bonds10.
3.2 Catalytic performances for biodiesel preparation
In past decades, heterogeneous catalysts that functioned as efficient and stable catalysts for
biodiesel production have attracted much attention. Among them, solid acid catalysts are
promising candidates for preparation of biodiesel, because solid acids could simultaneously
catalyze esterification and transesterification reactions.
In this paper, as-prepared Fe3O4@SiO2@PCS catalysts were successfully tested in the
esterfication of aliphatic acid (oleic acid, palmitic acid, stearic acid and acetic acid) with methanol
or ethanol at 70 oC for 5 h. For comparison, the conventional homogeneous acid catalyst, sulfuric
acid, was also examined under the identical conditions. From the results shown in Table 2, the
yield of esters using Fe3O4@SiO2@PCS–4–6 catalyst are relatively low, only ca. 60 %, 70 %,
70 % and 55 % for methyl oleate, methyl stearate, methyl palmitate and ethyl acetate respectively,
which should be ascribed to its low acidity. With the decrease of activation temperature, the
acidity of Fe3O4@SiO2@PCS is obviously enhanced. Consequently, their catalytic performances
for esterfication reaction are also improved. Fe3O4@SiO2@PCS–4–4 catalyst gives rise to a
highest catalytic activity and the yields of esters of methyl oleatemethyl stearate, methyl palmitate
and ethyl acetate reach ca. 81 %, 86 %, 82 % and 76 % within 5 h, respectively. In addition, the
Fe3O4@SiO2@PCS–4–4 catalyst also exhibits much higher catalytic performance for the
esterfication reaction than those of other conventional solid Brønsted acid catalysts, such as niobic
acid (Nb2O5·nH2O)42, Amberlyst-15 (sulfonated polystyrene)43 and Nafion NR50
(perfluorosulfonated ionomer)44, and achieves about 80
~85% catalytic efficient of surfuric acid
under the same reaction condition. This result is attributed to its higher acidity than those of other
solid acid catalysts. Although the reference acid catalyst, sulfuric acid, is catalytically effective to
these esterification reactions, but it is difficult to separating from the reaction solution and unable
to compete with many process advantages offered by the solid acid catalysts.
The reusability of catalyst is extremely vital to estimate the efficiency of solid acid catalysts,
which contributes to reduce the cost of practical applications process. In case of carbon–base solid
acid catalysts, the deactivation of catalytic activity is worth of notice45,46. In order to evaluate the
reusability of catalyst, after each catalytic reaction the solid acid catalyst was magnetically

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separated and washed repeatedly with ethanol and distilled water. The washed catalyst was dried
at 80 oC and used for the next experiment. From the results shown in Fig. 8, no evident decrease in
catalytic activity is observed even after five consecutive cycles, which indicates that
Fe3O4@SiO2@PCS–4–4 possesses a good stability and reusability. Furthermore, for checking the
stability of –SO3H groups, the resulting mixture after catalytic reaction was washed with water to
extract sulfate ions possibly leached from catalyst. The BaCl2 precipitation test demonstrates no
sulfate ions exist in the washed water, suggesting the stability of –SO3H groups on the surface of
catalyst.
The Fe3O4@SiO2@PCS catalysts also show a good catalytic performance for the
transesterfication of soybean oil, as summarized on Table 3. Because of its the highest acidity,
Fe3O4@SiO2@PCS–4–4 still shows a highest catalytic activity for the transesterification of
soybean oil, and a ca. 80 % yield of esters at 150 oC for 6 h is obtained, which is much higher than
ca. 60
%
and 40
%
yield of esters gained over Fe3O4@SiO2@PCS–4–5 and
Fe3O4@SiO2@PCS–4–6, respectively. Besides, its catalytic performance is also better than those
of the conventional solid acids, such as silica-supported Nafion47, Amberlyst-1548, and so on49,
and is about 85 % that of sulfuric acid under the same reaction condition. Fig. 9 depicts the results
of Fe3O4@SiO2@PCS–4–4 catalyst reuse experiment for the transesterification of soybean oil
with methanol, and the material also act as a stable catalyst for transesterification. Even though
after five consecutive runs, it still keep the original catalytic activity and the esters yield still
reaches ca. 80 %. Consequently, these results suggest that the material presents great potential to
be a stable and highly active solid acid catalyst for preparation of biodiesel.
4. Conclusion
In summary, a magnetically separable of SO3H–functionalized PC nanospheres solid acid material
has been successfully prepared by the simple activation and sulfonation treatment. Under the
protection of SiO2 layer, the Fe3O4@SiO2@PCS still keep the structure of magnetic core after
ZnCl2 activation, and this accelerates its magnetic separation in liquid reaction system. Meanwhile,
the solid acid catalyst exhibits a high surface area, large pore volume and stable and high catalytic
performance for esterification and transesterification reactions. Importantly, deactivation of the
solid acid catalyst in the reaction system is inhibited and the solid acid can retain its initial activity
after least five consecutive catalytic cycles. Consequently, this material has great potential in
industry applications used as the solid acid catalyst due to its stability in catalytic activity and
magnetic separation property.
Acknowledgments
The authors gratefully acknowledge the financial support from the National Natural Science
Foundation of China (21001093), the Qianjiang talent project of Zhejiang Province of China
(2011R10048), the Science Foundation of Zhejiang Sci-Tech University (0913848-Y) and the
project-sponsored by the Scientific Research Foundation (SRF) for the Returned Overseas
Chinese Scholars (ROCS), State Education Ministry (SEM).
Notes
Department of Chemistry, School of Sciences, Zhejiang SciꢀTech University, 928 Second Avenue, Xiasha Higher
Education Zone, Hangzhou 310018, China
*Corresponding author. Fax: +86 571 86843228; Tel: +86 571 86843228; Eꢀmail: xpdong@zstu.edu.cn

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Table 1 Textural parameters of Fe3O4@SiO2@PC and Fe3O4@SiO2@PCS materials
a
b
c
d
Sample
SBET
Smicro
Vmeso
Vmicro
De
(nm)
Acidity f
(m2g-1)
3297.4
2918.4
2426.4
(m2g-1) (cm3g-1)
(cm3g-1)
0.45
—
(mmol H+ g-1)
Fe3O4@SiO2@PC–4–4
Fe3O4@SiO2@PC–4–5
Fe3O4@SiO2@PC–4–6
1082.2
—
1.16
1.66
1.57
0.37
1.29
1.31
1.62/2.18
2.73
—
—
—
—
2.73
—
Fe3O4@SiO2@PCS–4–4 1118.3
Fe3O4@SiO2@PCS–4–5 2310.1
Fe3O4@SiO2@PCS–4–6 2015.6
788.5
—
0.26
—
1.57/1.86
2.70
1.98
1.32
1.14
—
—
2.70
a specific surface area estimated using BET method
b
micropore surface area calculated using the V-t plot method
c mesopore volume
d micropore volume
e pore size calculated using DFT method
f measured by acid-base titration

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Table 2. Comparisons of esterification of aliphatic acid with methanol or ethanol over different
acid catalysts
Yield of ester (%)
Catalyst
Methyl oleate Methyl stearate
Methyl palmitate
Acetic ester
Fe3O4@SiO2@PCS–4–4
Fe3O4@SiO2@PCS–4–5
Fe3O4@SiO2@PCS–4–6
H2SO4
81
65
60
95
86
74
70
98
82
76
70
96
76
60
55
96
Reaction conditions: catalyst 0.1 g; aliphatic acid 0.01 mol; acetic acid 0.05 mol; methanol 0.2 mol; ethanol 0.5
mol; reaction temperature 70 oC; reaction time 5 h; H2SO4 3wt% of acid

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Table 3. Comporisons of transesterification of soybean oil by different acid catalysts
Yield of esters (%)
Catalyst
2 h
52
38
21
65
6 h
78
62
43
89
Fe3O4@SiO2@PCS–4–4
Fe3O4@SiO2@PCS–4–5
Fe3O4@SiO2@PCS–4–6
H2SO4
Reaction conditions: catalyst 0.3 g; soybean oil 0.01 mol; methanol 0.2 mol; reaction temperature 150 oC; H2SO4
3wt% of soybean oil

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Scheme 1 Schematic illustrations of preparing Fe3O4@SiO2@PCS material

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Figure Captions
Fig. 1 TEM images of Fe3O4@SiO2 (a), Fe3O4@SiO2@C (b, c) and Fe3O4@SiO2@PCS (d), and
the insert is the single nanosphere of Fe3O4@SiO2@PCS.
Fig. 2 The XRD patterns of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2@C, Fe3O4@SiO2@PC and
Fe3O4@SiO2@PCS samples
Fig. 3 The XRD patterns of Fe3O4@PC and Fe3O4@SiO2@PC samples
Fig. 4 The photographs of dispersed Fe3O4@SiO2@PC (a) and Fe3O4@PC (b) in methanol and
after being attracted by an outer magnet
Fig. 5 FTIR spectra of all samples
Fig. 6 EDX spectra of the Fe3O4@SiO2, Fe3O4@SiO2@C, Fe3O4@SiO2@PC and
Fe3O4@SiO2@PCS nanospheres
Fig.
7
N2 adsorption–desorption isotherms (a) and pore size distributions (b) of
Fe3O4@SiO2@PC–x–y before and after sulfonation
Fig. 8 Recyclability of Fe3O4@SiO2@PCS–4–4 catalyst for esterification of aliphatic acid
(catalyst 0.1 g; aliphatic acid 0.01 mol; methanol 0.2 mol; 70 oC; 5 h)
Fig. 9 Recyclability of Fe3O4@SiO2@PCS–4–4 catalyst for transesterification of soybean oil
(catalyst 0.3 g; soybean oil 0.01 mol; methanol 0.2 mol; 150 oC; 6 h)

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Fig. 7

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Fig. 8

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Fig. 9