Water-tolerant heteropolyacid on magnetic nanoparticles as efficient catalysts for esterification of free fatty acid

Article abstract of DOI:10.1039/c3ra40219c

Heteropolyacids (HPAs) supported on magnetic nanoparticles (MNPs) have been prepared by a simple acid-base interaction between functionalized magnetic nanoparticles and HPAs. The surface of the catalyst had been modified by organic groups to protect the catalytic sites from destruction by water. The hybrid SiO₂-MNP-HPW consists of a core of magnetic iron oxide MNPs, silica shell, amino-silica and organic groups on the surface, which catalyzed the esterification of palmitic acid with methanol to give 90.4% FFA conversion within 2 h and a high TOF of 4.95 × 10² h^-1 under mild reaction conditions. The catalytic performance compared favourably to other solid acid catalysts, demonstrating the advantage of the small size of catalyst particles in reducing mass transfer limitations and providing the substrates with better access to the catalyst sites on the surface of carriers. The catalysts are easily separable from the reaction mixture under a magnetic field and showed high stability and recyclability, with no significant loss of productivity after five cycles of esterification.

Full text of DOI:10.1039/c3ra40219c

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Graphical Abstract
magnetic catalysts
C15
H
31COOH+CH
3OH
C15
H
31COOCH
3
+H O
2
a
b
a: without, b: with the external magnetic field.

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Water-tolerant heteropolyacid on magnetic nanoparticles as efficient catalysts for esterification of free
fatty acid
Xixin Duana, c,*, Jing Zhaoa, Wei Shia, Xia Zhanga, Xiaohong Wanga,*, Shiwu Lib,*
a Key Lab of Polyoxometalate Science of Ministry of Education, Northeast Normal University,
Changchun 130024, P. R. China
b Traffic College, Jilin University, Changchun 130025, P. R. China
c Faculty of Chemistry and Biology, Beihua University, Jilin 132013, P. R. China
Fax: 0086ꢀ431ꢀ85099759; Eꢀmail: wangxh665@nenu.edu.cn
Abstract: Heteropolyacids (HPAs) supported on magnetic nanoparticles (MNPs) have been prepared
by a simple acidꢀbase interaction between functionalized magnetic nanoparticles and HPAs. The
surface of the catalyst had been modified by organic groups to protect the catalytic sites from destroy
by water. The hybrid SiO2ꢀMNPsꢀHPW consists of a core of magnetic iron oxide MNPs, silica shell,
aminoꢀsilica and organic groups on the surface, which catalyzed the esterification of palmitic acid
with methanol to give 90.4% FFA conversion within 2 h and high TOF of 4.95×102 hꢀ1 under mild
reaction conditions. The good catalytic performance was good among the other solid acid catalysts,
demonstrating the advantage of the small size of catalyst particles in reducing massꢀtransfer
limitation and providing better accessibility of the catalyst sites on the surface of carriers for the
substrates. The catalysts are easily separable from the reaction mixture under a magnetic field and
showed high stability and recyclebility, with no significant loss of productivity after five cycles of
esterification.
Keywords: Heteropolyacids; esterification; magnetic separation; waterꢀtolarence.
1. Introduction
1

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Biodiesel recognized as ‘a green fuel’ is one of the alternative renewable fuel composed of fatty
acid methyl ester (FAME), typically derived from transesterification of triglycerides (TG) in
1ꢀ3
vegetable oils or animal fats or through esterification of free fatty acids (FFAs).
The major
drawbacks of transesterification of crude oils are the FFAs in feedstocks leading to the low efficiency
for base catalysts. A simple and efficient approach to improve the process is to first esterify FFAs
with methanol to their alkyl esters using an acid catalyst, and then transesterify triglycerides into
FAME. 4, 5 Homogeneous acid catalysts such as sulfuric acid are known for the esterification of FFA
but suffer from many drawbacks such as corrosion, pollution, complex downstream neutralization,
difficult separation, and no reusability of the catalysts. 3 Solid acid catalysts have served as important
functional materials for green and recyclable catalysts. However, the reported heterogeneous acid
catalysts have been thus far unsatisfactory for the esterification of FFA, because of the low densities
and low accessibility of the acid sites, easy deactivation, low thermal stability, low catalytic activity
and high cost. Moreover, owing to the formation of water as byꢀproducts, many of the acid catalysts
generally become deactivated or decomposed during the esterification processes, leading to highly
acidic and corrosive wastes. Thus, development of solid acid catalysts with high accessibility to the
acid sites, waterꢀtolerance is crucial to obtain satisfactory product yields in esterification of FFAs. 6
As continuing efforts towards the investigation of waste cooking oil as a potential biodiesel
7
feedstock,
we are interested in developing novel heterogeneous acid catalysts based on
heteropolyacids (HPAs) with high activity, stability, and recyclability for the esterification of FFA
from waste cooking oils. By now, there are some reports on the esterification of FFA catalyzed by
solid HPA catalysts. For example, Prof. Guo’s group synthesized a series waterꢀtolerant HPAs
8
catalysts loading on mesoporous Ta2O5 or ZrO2 for esterification and transeterification reactions,
2

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while the hgihest conversion of FFA is about 99% at 24 h and 65 ºC with acid/ethanol of 1: 9. J. A.
Dias and coꢀworkers designed H3PW12O40 loading on ZrO2 as esterification catalyst with an
optimum reaction performance (88% of oleic acid conversion at 20 wt% loading, 100 ºC, 4 h
9
reaction and 1:6 of acid:ethanol molar ratio). A. Patel et al. reported H3PW12O40 supported by
SBAꢀ15(the loading of H3PW12O40 as high as 28.5%) as catalysts for esterificatio of FFA, by which
92% conversion of oleic acid was obtained at 40 ºC, 4 h reaction and 1:40 of acid: methanol molar
ratio. 10 Nevertheless some efforts had been paid, but the low activity of current solid acid catalysts is
the main problems, which is mainly caused by the diffusional limitation of porous materials, or the
low acid loading and mass transfer limitation of the large size carriers. 11 However, another problem
of HPA in catalysis is the high solubility in polar media, which made POMs physically absorbed on a
support leach into reaction solution. Therefore, the attempt to heterogenization of HPAs has been one
area of research that has seen increasing interest. There are several methods for synthesis of stable
materials for leaching of HPAs in polar medium, such as capsulation of HPAs in solid matrix and
application of a support modified with functional groups such as carboxylic groups, lactam, amide,
imide or aminoꢀgroups for the anchoring of POMs. 12ꢀ20 Aminoꢀmodified silica materials are widely
used for immobilization of POMs. In spite of the decreasing of acidity after chemically anchoring of
HPAs to the aminoꢀmodified support, these HPA/NH2ꢀsupport systems can be used as catalyst for
different acid type reactions, showing easy handling, waterꢀtolerant properties. But in these
materials, HPA molecules were embedded on the porous of silica resulting in low efficiency and
requirement of high ratio of alcohol to acid.
To overcome these, our group choose magnetic nanoparticles (MNPs) as the carriers to prepare
nanoꢀsize solid catalyst containing HPAs and hydrophobic group on the surface. Magnetic
3

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nanoparticles are available from cheap materials via simple synthesis and easily tunable by structural
surface modifications, and also allow for the convenient and efficient recovery of nanocatalysts under
21
a magnetic field, thus facilitating catalyst recycling. Three examples of MNPs supporting HPA
catalysts are known: MNPs supported HPAs via nonꢀcovalent interaction as reusable acid catalysts
22
for Friedel–Crafts reaction with HPA loading 0.55 mmol/g;
HPA/chitosan/Fe3O4 shellꢀcore
23
materials with acid content of 0.13 mmol/g for acid catalytic reaction; HPAs supported on silica
24
functionalized Fe3O4 through sol/gel method.
These catalysts are efficient for other catalytic
reaction except for esterification reaction with moderate or low HPA loadings. We have focused on
the engineering of cheap and stable HPA acid catalyst for an esterification reaction. Herein, we report
novel core–shell structured iron oxide MNPs with SiO2 as shell, HPA and organic groups as core,
which acts as a highly active, waterꢀtolerant, stable, and easily recyclable nanoꢀsize solid acid
catalyst for the highꢀyielding esterification of FFAs with methanol.
2. Experimental
2.1. Preparation of catalysts
H3PW12O40 (HPW) was prepared according to Ref. 25 and identified by IR spectroscopy, ICPꢀAES
and 31P NMR spectrum. Preparation of Magnetic Fe3O4 nanoparticles were prepared by a
coꢀprecipitation method. 26 MNPs functionalized by silica (SiO2ꢀMNPs) were fabricated according to
Ref. 27
2.1.1 Preparation of Magnetic Fe3O4 nanoparticles
26
Preparation of Magnetic Fe3O4 nanoparticles were prepared by a coꢀprecipitation method as
follows: 10.3 mL of 1M HCl and 25 mL of purified, distilled water were mixed. 8.65 g of
FeCl3 ⋅6H2O and 3.18 g of FeCl2 ⋅ 4H2O (ratio of Fe(II)/Fe(III) = 1/2) were dissolved in the
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solution with vigorous stirring. The above solution was dropped into 250 mL of 1.5 M NaOH
solution under vigorous stirring for 1 h. Precipitate was separated by maganitic and washed with
purified deoxygenated water repeatedly to pH = 7.0, then dried at room temperature.
2.1.2 Preparation of the SiO2ꢀMNPsꢀ1ꢀHPW
The obtained SiO2ꢀMNPs (0.25 g) were suspended in 200 mL of anhydrous ethanol containing 0.5
mL of aminopropyl trimethoxy silane (APTES) under dry nitrogen atmosphere. The mixture was
refluxed for 12 h and then aged for another one night. The resulting solid was magnetically
separated, washed with ethanol three times to remove the unreacted residue of silylating agent and
then dried under vacuum at 353 K. APTESꢀfunctionalized SiO2ꢀMNPs (SiO2ꢀMNPsꢀ1) were
obtained with yield of 0.34 g and suspended in hydrochloric acid (pH = 4) to acidify the amino group
to ammonia group. To the above mixture, HPW solution (1 g/10 mL) was added and stirred for 4 h to
confirm the loading effect of HPW on SiO2ꢀMNPsꢀ1. Then the mixture was separated by magnetic
separation and washed with deionized water three times, then dried under vacuum at 353 K to give
SiO2ꢀMNPsꢀ1ꢀHPW material with the yield of 0.51 g.
2.1.3 Preparation of hydrophobic hybrids SiO2ꢀMNPsꢀ2ꢀHPW
The prepared procedure is as the same as SiO2ꢀMNPsꢀ1ꢀHPW, only the mixture of 1 mL
octyltrimethoxysilane (OTMS) and 0.5 mL APTES were used instead of APTES only.
2.1.4 Preparation of SiO2ꢀMNPsꢀHPW
SiO2ꢀMNPsꢀHPW was synthesized as follows: Tetraethoxysilane (TEOS) was dissolved in
ethanol, then an aqueous H3PW12O40 solution was added to the above solution with the molar ratio of
the H3PW12O40: H2O: TEOS: EtOH 0.04: 13.4: 1: 2. The mixture was stirred vigorously for 1 h at
room temperature to form HPW silica solution. Then 1 g of SiO2ꢀMNPs was added to 10 g of HPA
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silica sol with vigorous stirring at 45 °C for 3 h to form hydrogel. The hydrogel was cooled to room
temperature and then dehydrated for one day. The gels were further dried in vacuum at 150 °C for 2
h.
2.2. Catalytic experiments
The esterification of palmitic acid with methanol was carried out in a 50 mL threeꢀnecked round
bottomed flask with mechanical stirring at atmospheric pressure and 338 K. In a typical experiment,
palmitic acid (5.12 g), anhydrous methanol (6.4 mL) and certain amounts of catalyst were mixed and
refluxed for some time. After reaction, magnetic separated the catalyst for reuse. The residual
methanol was gathered using rotary evaporator at 323 K. The conversion of free fatty acid into ester
was estimated by measuring the acid value of the product and the yield of the product was measured
by gas chromatography.
2.3. Instrument
FTIR spectra (4000ꢀ400 cmꢀ1) of different catalysts were recorded as KBr discs on a Nicolet
Magna 560 IR spectrometer. Elemental analysis was carried out using a Leeman Plasma Spec (I)
ICPꢀES and a PE 2400 CHN elemental analyzer. The 31P MAS spectrum was recorded on a Bruker
AVANCE III 400 WB spectrometer equipped with a 4 mm standard bore CP MAS probe head whose
X channel was tuned to 162 MHz for 31P, using a magnetic field of 9.39 T at 297 K. Xꢀray
diffraction (XRD) patterns were collected on a Japan Rigaku Dmax 2000 Xꢀray diffractometer with
Cu Kα radiation (λ = 0.154178 nm). TEM micrographs were recorded on a Hitachi Hꢀ600
transmission electron microscope. The yield of monoester was determined on Shimazu GCꢀ14C
fitted with a HPꢀINNOWAX capillary column and flame ionization detector and HPꢀINNOWAX
capillary column (30 m × 0.32 mm; 0.50ꢁm film). The oven temperature was set at 220 °C and the
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temperature of the detector and injector were set at 250 °C and 250 °C, respectively. The conversion
was calculated on a GC using ethyl laurate as an internal standard.
2.4 Analytical analyses
The acid values (AV) of were measured following standards of PRC: GB/T14489.3ꢀ93, calculated
by the following equation (AV):
AV（mg KOH gꢀ1）=（VKOH×CKOH×56.1）/macid
KOH : the volume of KOH standard solution (mL),
CKOH : KOH standard solution concentration (mol Lꢀ1).
(1)
V
AV: the number of milligrams of KOH needed for the neutralization of 1 g free fatty acids. The
AV reduction efficiency (a) was calculated by the following equation:
a = (AVrꢀ AVE)/AVr×100%
(2)
AVr : the acid values in raw material (mg KOH gꢀ1)
AVE : the acid values after esterification (mg KOH gꢀ1).
3 Results and discussion
3.1. Preparation strategy of SiO2ꢀMNPꢀ1ꢀHPW and SiO2ꢀMNPꢀ2ꢀHPW
The common strategy for preparation of MNPꢀsupported HPAs is through the wellꢀapplied
‘‘acid–base’’ strategy. Since MNPs are acid sensitive, they were first protected with a 1ꢀ2 nm
thickness silica layer. The obtained SiO2ꢀMNP was then treated with the commercial (3ꢀaminopropyl)
triethoxysilane in refluxing ethanol to afford SiO2ꢀMNPꢀ1 (Scheme 1), which the surface of silica
was functionalized by aminoropropyl groups. The next step involves reaction of amine
groups of SiO2ꢀMNPꢀ1 with H3PW12O40 to yield SiO2ꢀMNPꢀ1ꢀHPW through the electrostatical
interaction of protonation of amine groups and HPAs. Following the same procedure, SiO2ꢀMNPꢀ2
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was also prepared only using the mixture of APTES and OTMS, which the surface of the silica was
modified by aminoropropyl groups and octyl groups. The immobilization of SiO2ꢀMNPꢀ2ꢀHPW was
achieved by stirring of a mixture of SiO2ꢀMNPꢀ2 and H3PW12O40.
Scheme 1
3.2. Characterization of catalysts
HPW was identified by ICPꢀAES (H: 0.03%, P: 1.06%, W: 76.4%, O: 22.4%) and 31P NMR
spectrum (Fig. S1). IR spectra (Fig. S2) shows four characteristic peaks of Keggin structure at 1080,
9979, 887 and 814 cmꢀ1, which are attributed to the asymmetric vibrations PꢀOa, WꢀOd, WꢀOb and
WꢀOc, respectively. The above results indicated that HPW was in high purity.
The loadings of HPW on SiO2ꢀMNPꢀHPW, SiO2ꢀMNPꢀ1ꢀHPW and SiO2ꢀMNPꢀ2ꢀHPW measured
by ICPꢀAES were 34.8, 30.3, and 29.2%, respectively. The contents of N and Si in
SiO2ꢀMNPꢀ1ꢀHPW and SiO2ꢀMNPꢀ2ꢀHPW were 0.64%，0.60%，21.3% and 32.9%, respectively.
The IR spectra of different complexes including Fe3O4, SiO2ꢀMNPꢀ1ꢀHPW, and
SiO2ꢀMNPꢀ2ꢀHPW were shown in Fig. 1. The infrared spectra of SiO2ꢀMNPꢀ1ꢀHPW (Fig. 1b) and
SiO2ꢀMNPꢀ2ꢀHPW (Fig. 1c) in the 1080 cmꢀ1, 984 cmꢀ1, 893 cmꢀ1, 804 cmꢀ1 are consistent with
parent HPW, which suggested that HPW maintained integrity Keggin structure in hybrid complexes.
The bands at 580 cmꢀ1 and 1200ꢀ1100 cmꢀ1 should be attributed to FeꢀO and SiꢀOꢀSi, respectively.
The bands at 3378ꢀ3370 cmꢀ1 which are attributed to NꢀH stretching vibration proved the presence of
aminopropyl silane. The bands at 2920 cmꢀ1 and 2860 cmꢀ1 were attributed to the CꢀH stretching
vibration of CH3 and the CꢀH stretching vibration of CH2, respectively, which demonstrated that the
organic group of octyltrimethy has been introduced into the hybrids. 28
Fig. 1
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The magnetic core was analyzed by XRD and the observed diffraction pattern at 18.3°, 30.1°,
35.5°, 43.1°, 56.9°, 62.6°, and 74.0°, respectively, is consistent with the JCPDS no. 851436 for
magnetite (Fig. 2a). The XRD spectrum of silica coated Fe3O4 (Fig. 2b) gives the same diffraction
peaks with that of Fe3O4. The diffraction peaks of SiO2ꢀMNPꢀ1ꢀHPW and SiO2ꢀMNPꢀ2ꢀHPW (Fig.
2c, 2d) could be indexed to H3PW12O40 (JCPDS no. 76ꢀ1815) with one peak at 8°.
Fig. 2
The SEM image of Fe3O4 (a) and TEM of SiO2ꢀMNPꢀ1ꢀHPW (b) were shown in Fig. 3. It could be
known that the size of Fe3O4 microspheres were around 30 nm. When functionalized by organic
group and HPW, the size was around 45 nm and SiO2ꢀMNPꢀ1ꢀHPW has coreꢀshell structure. The
thickness of silica layer was about 15 nm.
Fig. 3
3.2. Catalytic activity
7
In our previous studies, it was found that the heteropolyacids were efficient catalysts for
esterification of FFAs or transterification of triglycerides. However, only a few solid HPA acids are
known to demonstrate acceptable performance in the presence of water 8, 29 including SiO2 supported
HPAs and Ta2O5 supported HPAs. Try our best to evaluate the MNP supported HPAs in esterification
of FFAs is meaningful for the solid acid catalysis, not only for reusable FFA esterification but also
alternative for other acid catalytic reactions.
The catalytic activity for different catalysts had been done including HPW, SiO2ꢀMNPs,
SiO2ꢀMNPꢀHPW, SiO2ꢀMNPsꢀ1ꢀHPW, and SiO2ꢀMNPsꢀ2ꢀHPW (Table 1). It can be seen that the
conversion of palmitic acid was only 41.7% catalyzed by SiO2ꢀMNP due to the absence of acid sites.
The FFA conversions were enhanced catalyzed by SiO2ꢀMNPꢀHPW, SiO2ꢀMNPsꢀ1ꢀHPW, and
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SiO2ꢀMNPsꢀ2ꢀHPW attributed to the immobilization of HPAs on SiO2ꢀMNPs. To evaluate the
catalytic activity per mole of HPW, the catalytic activity decreased in range of SiO2ꢀMNPsꢀ2ꢀHPW >
SiO2ꢀMNPsꢀ1ꢀHPW > SiO2ꢀMNPꢀHPW > HPW. When considering the reaction turnover frequency
(TOF) based on acidic protons of the catalyst, the activity decreased in the same trend, while the
30
amount of acidic protons was measured by titration method.
The results show that HPW is
homogeneous catalyst with high FFA converision, but the activity is restricted by the absence of
waterꢀtolerant property. SiO2ꢀMNPsꢀHPW is only grafting HPW on MNPs, there is no interaction
between HPW and supports resulting in the leaching problem. Moreover, HPW was enbedded by
silica, and the substrates were hindered to access the HPW sites with low activity. HPW was
solidified on the surface of SiO2ꢀMNPsꢀ1ꢀHPW through the acidꢀbase interaction of NH3+ and HPW,
therefore the leaching of HPW was avoided. From Table 1, the octylꢀgrafted catalyst
SiO2ꢀMNPsꢀ2ꢀHPW showed a high activity of 2.12×102 hꢀ1, which was attributed to the tolerance to
water produced during the esterification reaction. To our knowledge, the activity of
SiO2ꢀMNPsꢀ2ꢀHPW is the highest observed in solid acid catalysts. This point could also be observed
in the experiment of the reaction time (Fig. 4).
Table 1
One can be seen from Fig. 4 conversion of FFA depends on methods of HPAs immobilization in
solids, amount of octylꢀ and aminoꢀgroups in solids. For SiO2ꢀMNPsꢀHPW and SiO2ꢀMNPsꢀ1ꢀHPW,
the highest conversions of FFA were obtained at 4 h for 84.5%, and 10 h for 91.4%, respectivley.
Further increasing the reaction time could decrease the FFA conversion. This trend might be
attributed to the adsorption of reagents by the catalysts. Therefore, the adsorption of ester and water
on the catalysts had been done. The results showed the ester decomposition under the reaction
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conditions. It is known that esterification of fatty acid is reversible reaction, which water might
caused the ester to decompose catalyzed by acid catalyst. For SiO2ꢀMNPsꢀ2ꢀHPW, the conversion
kept increase as enhancement of the reaction time with highest conversion of 98.4% at 10 h. This
results also confirm that grafting of organic group on the surface of the support could remove the
water molecules from the catalyst. 28
Fig. 4
The molar ratios of methanol/acid of esterification reaction is very important parameters in real
application. The previous reports used HPA loading on the pore of the SiO2 or Ta2O5 carriers, which
might hinder the reaction due to the diffusion to the catalytic acitive sites resulting in high usage of
methanol. Our strategy is to loading HPW on the surface of the magnetic nanoparticles overcoming
the limitation of diffusion (Fig. 5). From the difference activity of SiO2ꢀMNPsꢀHPW and
SiO2ꢀMNPsꢀ1ꢀHPW at different molar ratio of methanol and FFA, it can be seen that at low molar
ratio of 4:1, the conversions of FFA were lower by SiO2ꢀMNPsꢀHPW than by SiO2ꢀMNPsꢀ1ꢀHPW.
When increasing of molar ratio to 1:8, the conversion of FFA increased by two catalysts. However,
the significant increase by SiO2ꢀMNPsꢀ1ꢀHPW was observed attributing to the easy access of
substrates to catalytic sites. Therefore, the lower molar ratio of methanol to acid is required by
SiO2ꢀMNPsꢀ1ꢀHPW, by which the drawbacks of diffusion to pore structure could be overcome. From
Fig. 5, it also can be seen that increase of molar ratio could enhance the FFA conversion.
Fig. 5
The other problem of solid acid catalysts in esterification or transesterification is the sensitivity to
water from feedstocks and the byꢀproduct of esterification reaction, which could easily cause the
deactivation and lead to phase segregation to influence the activity of the catalysts. Most inorganic
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solid acid catalysts do not work in water, while the exceptions include HꢀZSMꢀ5, and a hydrophobic
zeolite, which are also believed to have hydrophobic surfaces. Therefore, inorganic solid acid
catalysts with waterꢀtolerant property are required to develop. Compared to SiO2ꢀMNPsꢀ1ꢀHPW,
more the octyl groups were grafted on the surface of silica. Therefore, SiO2ꢀMNPsꢀ2ꢀHPW was
further modified by grafting of organic group around the HPW, which provides hydrophobic
surroundings available for elimination of water molecules from the catalyst leading to waterꢀtolerant
property. 29 In order to assess the waterꢀtolerance of SiO2ꢀMNPsꢀ2ꢀHPW, some extra water had been
added to the mixture under the same conditions and the result was given in Fig. 6. In addition,
SiO2ꢀMNPsꢀ1ꢀHPW also had been tested for its waterꢀtolerant property.
Fig. 6
It can be seen that the effect of water on the catalytic activity is related to water content and the
different components of catalysts. As for SiO2ꢀMNPsꢀ1ꢀHPW, the water content gives a significant
influence on the conversion of FFAs, while the conversion decreased to half as the water content of
0.4 wt% and the FFA conversion decreased to 6.5% in presence of 1 wt% of water. As using
SiO2ꢀMNPsꢀ2ꢀHPW, the conversion was slightly reduced to 80.6% when the water content was
below 0.4 wt%. The high amount of water of 1 wt% did not influence the FFA conversion sharply.
When 1% of water was added, the conversion was significantly reduced to 33.9%. Therefore, the
grafting of octyl on the surface of MNPs would protect the HPW from being destroy by water
molecules. In addition, SiO2ꢀMNPsꢀ2ꢀHPW is relatively more tolerant of water than other solid acid
2ꢀ
32
catalysts such as H2SO4, 31 SO4 /ZrO2.
Acting as heterogeneous catalysts, the regeneration of SiO2ꢀMNPs HPA hybrids is of practical,
economic importance. The regeneration is easily achieved by magnetic separation by the external
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magnetic field and reused only by calcinating at 373 K for 2 h (Fig. 7). As presented in Fig. 8,
SiO2ꢀMNPsꢀ1ꢀHPW and SiO2ꢀMNPsꢀ2ꢀHPW demonstrated excellent stability and reusability,
maintaining 97.7% and 95.7% conversion throughout five reaction cycles. As the catalysts are stable,
there is no regeneration step needed to restore the acid capacity of the catalyst, thus decreasing the
cost of catalyst production and hence the cost of biodiesel production.
Fig. 7
Fig. 8
4. Conclusions
Active, stable, waterꢀtolerant and recyclable magnetic nanoꢀsize HPA catalysts have been
developed, for the first time, as efficient catalysts for the esterification of FFA with methanol. A novel
acid catalyst SiO2ꢀMNPsꢀ2ꢀHPW consisting of a core of magnetic iron oxide MNPs, silica shell,
aminoꢀ silica and organic groups on the surface was was fabricated. SiO2ꢀMNPsꢀ2ꢀHPW catalyzed
the esterification of palmitic acid with methanol at 8.2 wt% loading to give 90.4% FFA conversion
within 2 h and high TOF of 4.95×102 hꢀ1 at lower methanol to acid ratio. The catalytic performance
was good among the other solid acid catalysts, demonstrating the advantage of the small size of
catalyst particles in reducing massꢀtransfer limitation and providing better accessibility of the
catalyst sites on the surface of carriers for the substrates. Moreover, the organic groups on the surface
of the supports provide the resistence to water molecules, allowing the esterification reaction occurs
under the conditions in the presence of high content of water. The catalysts are easily separable from
the reaction mixture under a magnetic field and showed high stability and recyclability, with no
significant loss of productivity after five cycles of esterification.
Acknowledgments
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This work was supported by the National Natural Science Foundation of China (20871026), “the
Fundamental Research Funds for the Central Universities” (10JCXK011) and the major projects of
Jilin Provincial Science and Technology Department (20100416).
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Figure Captions
Fig. 1 IR spectra of (a) Fe3O4, (b) SiO2-MNP-1-HPW and (c) SiO2-MNP-2-HPW.
Fig. 2 XRD patterns of (a) Fe3O4, (b) SiO2-MNP, (c) SiO2-MNP-1-HPW and (d)
SiO2-MNP-2-HPW.
Fig. 3 The SEM image of (a) Fe3O4 and (b) TEM image of SiO2-MNP-1-HPW.
Fig. 4 Kinetic data of esterification of palmitic acid with methanol over
SiO2-MNP-HPW systems. Reaction conditions: molar ratio of methanol to acid 6:1,
amount of catalyst 8.2 wt%, 65 ºC.
Fig. 5 The effect of methanol/acid molar ratio on the ester conversion over different
catalysts. Reaction conditions: amount of catalyst 8.2 wt%, 65 ºC for 2 h.
Fig. 6 Effect of water on activity of SiO2-MNPs-1-HPA and SiO2-MNPs-2-HPA.
Reaction conditions: molar ratio of methanol to acid 6:1, amount of catalyst 8.2 wt%,
65 ºC for 2 h.
Fig. 7 Photographs of magnetic separation after the esterification of palmitic acid with
methanol, a: without, b: with the external magnetic field.
Fig. 8 Catalytic cycles of SiO2-MNPs-1-HPA and SiO2-MNPs-2-HPA recycling in
esterification of palmitic acid with methanol. Reaction conditions: molar ratio of
methanol to acid 6:1, amount of catalyst 8.2 wt%, 65 ºC for 2 h.

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Fig. 1 The IR spectra of (a) Fe3O4, (b) SiO2-MNP-1-HPW and (c) SiO2-MNP-2-HPW.

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a
b
c
d
20
40
60
80
2 Theta / degree
Fig. 2 XRD patterns of (a) Fe3O4, (b) SiO2-MNP, (c) SiO2-MNP-1-HPW and (d)
SiO2-MNP-2-HPW.

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(a)
(b)
Fig. 3 The SEM image of (a) Fe3O4 and (b) TEM image of SiO2-MNP-1-HPW.

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Fig. 4 Kinetic data of esterification of palmitic acid with methanol over
SiO2-MNP-HPW systems. Reaction conditions: molar ratio of methanol to acid 6:1,
amount of catalyst 8.2 wt%, 65 ºC.

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Fig. 5 The effect of methanol/acid molar ratio on the ester conversion over different
catalysts. Reaction conditions: amount of catalyst 8.2 wt%, 65 ºC for 2 h.

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Fig. 6 Effect of water on activity of SiO2-MNPs-1-HPA and SiO2-MNPs-2-HPA.
Reaction conditions: molar ratio of methanol to acid 6:1, amount of catalyst 8.2 wt%,
65 ºC for 2 h.

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a
b
Fig. 7 Photographs of magnetic separation after the esterification of palmitic acid with
methanol, a: without, b: with the external magnetic field.

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100
SiO2-MNPs-1-HPW
SiO2-MNPs-2-HPW
80
60
1
2
3
4
5
Catalytic cycles
Fig. 8 Catalytic cycles of SiO2-MNPs-1-HPA and SiO2-MNPs-2-HPA recycling in
esterification of palmitic acid with methanol. Reaction conditions: molar ratio of
methanol to acid 6:1, amount of catalyst 8.2 wt%, 65 ºC for 2 h.

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OEt
OEt
OEt
Si
NH2
+
O
O
O
O
+
Si
OEt
H3PW12O40
NH2
Si
OEt
HPW
NH2
SiO2-
P-1-HPW
OEt
OEt
OEt
Si
NH2
OEt
OEt
OEt
Si
O
Si
OEt
NH2HPW
O
Si
NH2
O
O
H3PW12O40
+
OEt
O
O
O
Si
O
Si
OEt
OEt
SiO2-
P-2-HPW
Fe3O4
SiO2
Scheme 1 Preparation strategy of SiO2-MNP-1-HPW and SiO2-MNP-2-HPW.