A Br?nsted-Lewis-surfactant-combined heteropolyacid as an environmental benign catalyst for esterification reaction

Article abstract of DOI:10.1016/j.catcom.2012.01.014

A Br?nsted-Lewis-surfactant-combined heteropolyacid (HPA) catalyst (C₁₆TA)H₄TiPW₁₁O₄₀ (C₁₆TA = cetyltrimethyl ammonium) had been designed and used as a water-tolerantly heterogeneous catalyst in esterification of free fatty acid with a higher conversion (94.7%) and excellent efficiency (91.8% yield) due to its acidic properties and structures. This micellar polyoxometalate catalyst was stable during the reaction and can be recycled by a simple separation process.

Full text of DOI:10.1016/j.catcom.2012.01.014

Catalysis Communications 20 (2012) 103–106
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
A Brønsted–Lewis-surfactant-combined heteropolyacid as an environmental benign
catalyst for esterification reaction
b,
Jing Zhao a, Hongyu Guan a, Wei Shi a, Mingxing Cheng a, Xiaohong Wang a, , Shiwu Li
⁎
⁎
a
Key Lab of Polyoxometalate Science of Ministry of Education, Northeast Normal University, Changchun 130024, PR China
b
Traffic College, Jilin University, Changchun 130025, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A Brønsted–Lewis-surfactant-combined heteropolyacid (HPA) catalyst (C16TA)H4TiPW11O40 (C16TA=cetyl-
trimethyl ammonium) had been designed and used as a water-tolerantly heterogeneous catalyst in esterifi-
cation of free fatty acid with a higher conversion (94.7%) and excellent efficiency (91.8% yield) due to its
acidic properties and structures. This micellar polyoxometalate catalyst was stable during the reaction and
can be recycled by a simple separation process.
Received 11 November 2011
Received in revised form 31 December 2011
Accepted 13 January 2012
Available online 25 January 2012
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Heteropolyacids
Micellar catalysis
Esterification
Water-tolerance
1. Introduction
surfactant cerium trisdodecylsulfate could be used as a catalyst for
transesterification and esterification [14]. The advantages of these
Fatty acid alkyl ester (FAAE), which is called biodiesel, is well
known as green energy resource and renewable liquid fuel instead
of fossil fuel. Biodiesel is produced from transesterification of vegeta-
ble oil or animal fats and esterification of free fatty acids (FFA)
with short chain alcohols such as methanol using a chemical method
[1–3]. Waste cooking oil is a cheap starting material that can help
in improving the economic feasibility of biodiesel [4]. However, bio-
diesel from such low quality oils is unsuitable for base catalysts be-
cause of the FFA and water content. Fatty acids could be converted
into FAAE by esterification with short chain alcohols in the presence
of acid catalysts, which is also an important process for biodiesel.
Among the acid catalysts, heteropolyacids (HPAs) represent promis-
ing catalysts for esterification reaction [5–10].
catalysts are: (1) concentrating the substrate molecules around the
catalysts through the interaction between lipophilic tail and ester
molecules. Meanwhile, methanol molecules could be absorbed by
HPAs through hydrogen bonds; (2) providing hydrophobic surround-
ings for separation the water molecules from the catalyst. This
catalyst exhibits some water-tolerant property available for the ester-
ification reaction. By now, there are no reports in the literature on the
application of Brønsted–Lewis-surfactant-combined heteropolyacids
in esterification reactions.
2. Methods
2.1. Material and reagent
Among the HPA catalysts, H3PW12O40 (HPW) as a Brønsted acid
3−
has higher strength than H2SO4 [11]. Metal salts of PW12
O
poten-
All chemicals were obtained from commercial suppliers. They were
of AR grade and used without further purification. Na7PW11O39 was
synthesized according to the Ref. [15].
40
tially show the Lewis acidity originating from the metal cations as
the electron pair acceptors as well as Brønsted acidity of protons
[12]. Therefore, Lewis acidic HPAs could be introduced by modular
grafting Lewis cations onto the phosphotungstic backbone [13].
As a continuation of our work, we designed a Brønsted–Lewis-
2.2. Preparation of catalysts
surfactant-combined heteropolyacid (C16TA)H4TiPW11O40
,
which
1 mL solution of Ti(SO4)2 (6 mmol in 2 M H2SO4) was added to
Na7PW11O39 (6 mmol) aqueous solution (20 mL). Adjusting the
pH to 5.6 by adding NaHCO3, the product of K5TiPW11O40 was precip-
itated by adding excess of KCl. Then separated by filtration and
recrystallized for three times. And K+ was replaced by H+ using cat-
ion exchange resins. A certain amount of hexadecyltrimethylammo-
nium bromide was added into the above solution with stirring.
Immediately the white precipitate formed and collected by filtration
was used in esterification of fatty acids. It is known that this kind of
complexes could not only act as acid catalysts, but also surfactants
in the organic reactions. It had been reported that Lewis acid/
⁎
Corresponding authors. Fax: +86 431 85099759.
E-mail address: wangxh665@nenu.edu.cn (X. Wang).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.01.014

104
J. Zhao et al. / Catalysis Communications 20 (2012) 103–106
then calcined at 100 °C for 3 h. The yield of (C16TA)H4TiPW11O40
was 50%.
2.3. Esterification reaction
A 100 mL three-necked glass flask charged with cooled condenser
with 5.0 g of palmitic acid, a certain amount of anhydrous methanol
and catalyst in it were vigorously stirred and refluxed for the required
reaction time. After reaction, the mixture was rotary evaporated at
50 °C to separate the excess of methanol. Then the mixture was left
divided into two layers by centrifugation with 4000 rpm. The upper
layer was methyl esters of palmitic acid and the lower layer was the
catalyst with small amount of water. The conversion of ester was
calculated by measuring the acid value of the product and the yield
was detected by gas chromatography (GC). The catalyst was decanted
at the bottom of centrifuge, which was easy to be separated, washed
by methanol to remove the remaining reactants and water, then dried
in air and reused for the next experiment.
Fig. 1. The IR spectra of the (C16TA)H4PW11TiO40 before (a) and after the reaction (b).
2.4. Critical micelle concentration (CMC) determination
The CMC of (C16TA)H4TiPW11O40 was determined by break points
of two nearly straight-line portions of the specific conductivity versus
concentration plot [16].
The result of micelles tested by cryo-TEM image (Fig. 2) shows
mostly single 200–300 nm particles. And the elemental analyses
of (C16TA)H4TiPW11O40 are: W, 66.69; Ti, 1.6; P, 0.98; C, 7.35; H,
1.53; N, 0.48%, respectively. EDX measurement results suggested the
molar ratio of C: P: W: Ti=19:1:11:1.
2.5. Analytical analyses
ꢀ
ꢁ
The CMC of (C16TA)H4TiPW11O40 given in Fig. S2 showed that the
CMC of (C16TA)H4TiPW11O40 was 0.92 mM, which also confirms the
formation of micelle in aqueous solution [17].
acid values in raw material
acid values after esterification
Converisionð%Þ ¼ 1−
ꢀ 100%
The acidity of the catalyst was studied by Hammett indicator
method [18] using paranitroaniline as the indicator. The acidity of
(C16TA)H4TiPW11O40 was 0.34 mmol/g, which was lower than HPW
(0.80 mmol/g) and H5PW11TiO40 (0.97 mmol/g). The surface acidity
of (C16TA)H4TiPW11O40 was determined by pyridine adsorption infra-
red spectroscopy (Fig. S3). The spectrum exhibits absorption bands
attributed to Brønsted (1540 cm–1) and Lewis (about 1450 cm−1
and 1620 cm−1) acid sites, which is confirmed by the reference
[19]. Actually, we observed absorption bands of Brønsted acid site
2.6. Instrument
FTIR spectra (4000–500 cm−1) were recorded in KBr discs on a
Nicolet Magna 560 IR spectrometer. The elemental analysis was carried
out using a Leeman Plasma Spec (I) ICP-ES and a P-E 2400 CHN elemen-
tal analyzer. X-ray diffraction (XRD) patterns of the sample were
collected on a Japan Rigaku Dmax 2000 X-ray diffractometer with Cu
Kα radiation (λ=0.154178 nm). DR-UV–vis spectra (200–800 nm)
were recorded on a Cary 500 UV–vis-NIR spectrophotometer. TEM
micrographs were recorded on a Hitachi H-600 transmission electron
microscope. The yield of monoester was determined on Shimazu GC-
14C fitted with a HP-INNOWAX capillary column and flame ionization
detector and HP-INNOWAX capillary column (30 m×0.32 mm;
0.50 μm film). Acetone was used as the solvent while nitrogen was
used as the carrier gas. The oven temperature was set at 220 °C and
the temperature of the detector and injector were set at 250 °C and
250 °C, respectively. Methyl laurate was used as the internal standard.
at 1539 cm−1 and Lewis acid site at 1469 cm−1 and 1629 cm−1
.
This indicates that the Lewis acid sites were successfully introduced
to the catalyst.
3. Results and discussion
3.1. Catalyst characterization
Fig. 1a. shows the IR spectrum of the (C16TA)H4TiPW11O40
.
Obviously, the characteristic bands of the Keggin structure are ob-
served at 1075, 970, 891, and 808 cm−1 corresponding to vas(P-Oa),
vas(W-Od), vas(W-Ob) and vas(W-Oc), respectively. The C–H stretching
vibrational peaks at 2923 and 2853 cm−1 and C–N at 1473 cm−1
show the existence of C16TA in the catalyst. The IR spectrum shows a
pronounced band at 655 cm−1 which indicated a Ti–O bond.
Powder X-ray diffraction patterns were used to confirm the struc-
ture of (C16TA)H4TiPW11O40. Compared with the diffraction peaks of
H5PW11TiO40 (Fig. S1a) at 8.4°(110), 23.4°(222), and 29.5°(332),
(C16TA)H4TiPW11O40 shows the similar diffraction peaks (Fig. S1b).
This result indicates the original structure of H5PW11TiO40 being
attained after forming the micelles.
Fig. 2. The cryo-TEM image of (C16TA)H4PW11TiO40 micellar catalyst.

J. Zhao et al. / Catalysis Communications 20 (2012) 103–106
105
3.2. Effect of catalyst
Different catalysts had been chosen to compare their activity,
including H5TiPW11O40, (C16TA)H4TiPW11O40 and H3PW12O40 (Fig. 3).
The catalytic activity was in the rank of (C16TA)H4TiPW11O40>H5-
TiPW11O40>H3PW12O40. Although H3PW12O40 is a homogeneous and
efficient acid catalyst, the separation is a problem. The acidity of
H5TiPW11O40 is higher than pure Brønsted acid H3PW12O40 owing to
its Brønsted and Lewis acidic sites. Therefore, H5TiPW11O40 exhibited
a higher activity than H3PW12O40. The introduction of CTAB into
H5TiPW11O40 makes (C16TA)H4TiPW11O40 equip with amphiphilic
properties and shows superior performance. The amphiphilic proper-
ties of (C16TA)H4TiPW11O40 could act both catalyst and surfactant to
assemble miceller in methanol solvent. It is known that this kind of
amphiphilic HPA catalysts could concentrate substrate molecules
around the catalysts [20] through the interaction between lipophilic
tail and ester molecules. Meanwhile, methanol could be absorbed
by HPAs through hydrogen bonds. Therefore, the catalytic activity of
(C16TA)H4TiPW11O40 was higher than H5TiPW11O40
.
Fig. 4. The effect of water on the catalytic activity. Reaction conditions: molar ratio of
methanol/acid/catalyst=4856:243:1, 65 °C, 0.25 g of catalyst, 6 h.
In this micellar catalytic system, the catalytic activity was not
significantly influenced by the lipophilic tail length. The different cat-
alysts containing [C12TA]H4TiPW11O40, [C14TA]H4TiPW11O40, [C16TA]
H4TiPW11O40 and [C18TA]H4TiPW11O40 (Fig. 3). The rank of activity
was [C18TA]H4TiPW11O40 (89.0%)~[C12TA]H4TiPW11O40 (90.1%)~
[C14TA]H4TiPW11O40 (91.0%)b[C16TA]H4TiPW11O40 (94.7%). The
length of carbon chain of palmitic acid is 16, which is similar to
[C16TA]+. Therefore, [C16TA]H4TiPW11O40 performed the highest
activity.
It is known that the water content in feedstocks may easily cause
the deactivation and lead to phase segregation. The side reaction
decreased the activity of catalyst and increased the purification cost.
Therefore, a water-tolerant catalyst is required.
be seen that an increase in the conversion of palmitic acid from
61.7% to 94.7% when the mass of (C16TA)H4TiPW11O40 increased
from 0.05 to 0.30 g and the acid–catalyst process attains the maxi-
mum conversion at 0.25 g of the catalyst. This can be attributed to
an increase in the availability of the number of catalytically active
site. The molar ratio of methanol/acid/catalyst was 4856:243:1.
Then the catalytic reaction reaches equilibrium.
3.3. Effect of the methanol/acid molar ratio
(C16TA)H4TiPW11O40 as a water-tolerant catalyst could easily iso-
late water molecules from catalysts. To investigate the effect of water,
some water had been added to the mixture under the same condi-
tions and the result was given in Fig. 4.
It can be seen that the effect of water on the catalytic activity is
related to water content. When the water content was below
0.2%, the conversion was slightly reduced to 83.8%. However, the con-
version was significantly reduced to 6.31% when 1% of water was
added. So our catalyst is relatively more tolerant of water than
other acid catalysts such as H2SO4, [21] SO42−/ZrO2. [22]
Generally, excess methanol is necessary in order to obtain a higher
yield of esters [23]. Here, we selected a molar ratio of methanol/acid
from 1:1 to 24:1 (Fig. S5). Obviously, the conversion reached maxi-
mum of 94.7% at the methanol/acid ratio of 20. When increasing
the molar ratio continually, no further improvement was found.
This may be attributed to that the high amount of methanol would
decrease the relative amount of catalytic sites and the availability of
acid molecules and hence to decrease the conversion of acid. As a
result, methanol/acid ratio of 20 is the best option. The molar ratio
of methanol/acid/catalyst was 4856:243:1.
Experiments were carried out by varying the mass of the catalyst
from 0.05 to 0.30 g keeping methanol/acid ratio 20:1(Fig. S4). It can
3.4. Effect of temperature
Esterification of fatty acid is normally performed near the boiling
point of the alcohol. We studied the esterification of palmitic acid at
45 °C, 50 °C, 55 °C and 65 °C in order to determine the effect of reac-
tion temperature on the methyl ester formation (Fig. S6). It shows
the temporal evaluation of the results and it can be seen that under
(C16TA)H4TiPW11O40 catalyzing, the esterification of acid to fatty
acid ester can be occurred at lower temperature, the conversion
being 90% at temperature 50 °C. In order to obtain the highest conver-
sion of acid, the reaction at higher temperature of 65 °C had been
done. The conversion was increased to 94.7%.
3.5. Effect of reaction time
In order to research the effect of reaction time, we choose the dif-
ferent reaction time with methanol/acid/catalyst 4856:243:1 at 65 °C
(Fig. S7). The conversion of the product increased with increasing
reaction time. It was found that the conversion increased from 1 h
to 6 h and reached a maximum of 94.7% at 6 h. After 6 h, the conver-
sion did not vary significantly. Thus the optimum reaction time is 6 h.
Fig. 3. Esterification of palmitic acid using different types of catalysts. Reaction condi-
tions: molar ratio of methanol/acid/catalyst=4856:243:1, 65 °C, 0.25 g of catalyst, 6 h.

106
J. Zhao et al. / Catalysis Communications 20 (2012) 103–106
The optimum reaction parameters were found to be molar ratio of
methanol/acid/catalyst 4856:243:1, 65 °C and reaction time of 6 h.
The maximum conversion was 94.7% under this reaction conditions.
The catalyst showed high activity because of double acid sites, amphi-
philic property and water-tolerant property. This micellar HPA
catalyst could be used as a heterogeneous acid catalyst for esterifica-
tion of fatty acid.
Acknowledgments
This work was supported by the National Natural Science Founda-
tion of China (20871026), “the Fundamental Research Funds for
the Central Universities” (10JCXK011) and the major projects of Jilin
Provincial Science and Technology Department (20100416).
Appendix A. Supplementary data
Fig. 5. The life span of catalyst for esterification. Reaction conditions: molar ratio of
methanol/acid/catalyst=4856:243:1, 65 °C, 0.25 g of catalyst, 6 h.
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2012.01.014.
Reference
3.6. Reusability of the catalyst
[1] S. Lestari, P. Mäki-Arvela, J. Beltramini, G.Q. Max Lu, D.Y. Murzin, ChemSusChem 2
(2009) 1109.
[2] A.A. Kiss, A.C. Dimian, G. Rothenberg, Advanced Synthesis and Catalysis 348
(2006) 75.
[3] D.Y.C. Leung, Y. Guo, Fuel Processing Technology 87 (2006) 883.
[4] M.G. Kulkarni, A.K. Dalai, Industrial and Engineering Chemistry Research 45
(2006) 2901.
[5] B.Y. Giri, K. Narasimha Rao, B.L.A. Prabhavathi Devi, N. Lingaiah, I. Suryanarayana,
R.B.N. Prasad, P.S. Sai Prasad, Catalysis Communications 6 (2005) 788.
[6] W.H. Zhang, Y. Leng, D.R. Zhu, Y.J. Wu, J. Wang, Catalysis Communications 11
(2009) 151.
[7] C.F. Oliveira, L.M. Dezaneti, F.A.C. Garcia, J.L. de Macedo, J.A. Dias, S.C.L. Dias, K.S.P.
Alvim, Applied Catalysis A: General 372 (2010) 153.
[8] C.S. Caetano, I.M. Fonseca, A.M. Ramos, J. Vital, J.E. Castanheiro, Catalysis Commu-
nications 9 (2008) 1996.
[9] L.H. Wee, S.R. Bajpe, N. Janssens, I. Hermans, K. Houthoofd, C.E.A. Kirschhock, J.A.
Martens, Chemical Communications 46 (2010) 8186.
[10] Y. Leng, J. Wang, D.R. Zhu, Y.J. Wu, P.P. Zhao, Journal of Molecular Catalysis A:
Chemical 313 (2009) 1.
[11] M.N. Timofeeva, Applied Catalysis A: General 256 (2003) 19.
[12] J. Li, X.H. Wang, W.M. Zhu, F.H. Cao, ChemSusChem 2 (2009) 177.
[13] N. Dupré, P. Rémy, K. Micoine, C. Boglio, S. Thorimbert, E. Lacôte, B. Hasenknopf,
M. Malacria, Chemistry A European Journal 16 (2010) 7256.
[14] G.F. Ghesti, J.L. de Macedo, V.C.I. Parente, J.A. Dias, S.C.L. Dias, Applied Catalysis A:
General 355 (2009) 139.
[15] C. Brevard, R. Schimpf, G. Tourne, C.M. Tournet, Journal of the American Chemical
Society 105 (1983) 7059.
[16] P. Mukherjee, K.J. Mysels, NSRDS-NBS, 1971, p. 36.
[17] N. Nazir, M.S. Ahanger, A. Akbar, Journal of Dispersion Science and Technology 1
(2009) 51.
[18] T. Okayasu, K. Saito, H. Nishide, M.T.W. Hearn, Green Chemistry 12 (2010) 1981.
[19] B.R. Jermy, A. Pandurangan, Journal of Molecular Catalysis A: Chemical 237
(2005) 146.
[20] S. Zhao, X.H. Wang, M.X. Huo, Applied Catalysis B: Environmental 97 (2010) 127.
[21] D. Kusdiana, S. Saka, Bioresource Technology 91 (2004) 289.
[22] J. Jitputti, B. Kitiyanan, P. Rangsunvigit, K,. Bunyakiat, L,. Attanatho, P,. Jenvanitpanjakul,
Chemical Engineering Journal 116 (2006) 61.
The catalyst was easily separated from the mixture by centrifuga-
tion, and treated with methanol to remove the polar compounds.
Then was dried in the air for reusing. After six reactions cycles,
there is no considerable change in the catalytic activity and the result
was shown in Fig. 5. The catalyst keeps the Keggin structure which
can be proved by the IR (Fig. 1b). At the end of the reaction, the cat-
alyst decanted at the bottom of the reactor and was used one more
time without any treatment. As the catalyst (C16TA)H4TiPW11O40
was able to decant from the mixture of fatty acid methyl ester and
glycerin into the bottom of the reactor, so the upper phase fatty
acid methyl ester did not contain any (C16TA)H4TiPW11O40 solid
(This can be determined by IR spectra of the fatty acid methyl ester).
In addition, the nature of (C16TA)H4TiPW11O40 in the inverse-
micellar reaction system had been tested. The test had been per-
formed as following: (C16TA)H4TiPW11O40 was contacted with palmi-
tic acid at 65 °C (without methanol) during 60 min and afterwards
the solid was separated from liquid phase by an ultrafiltration mem-
brane; then, to the liquid phase, methanol was added and the reaction
was monitored during 5 h at 65 °C. The conversion was only 13.6%.
From the result, (C16TA)H4TiPW11O40 is confirmed as heterogeneous
one. To test for leaching, the catalyst was filtered after a reaction
time of 90 min (ca. 66.17% ester conversion) and the filtrate reacted
further 6 h at the same temperature of 65 °C. From the results, it
can be seen that the conversion only 66.84%, which shows a very
slight leaching of [C16TA]H4TiPW11O40. The UV spectrum of the mix-
ture exhibited two absorption bands at 222 nm and 260 nm, which
are attributed to the Keggin structure. The total amount of [C16TA]
H4TiPW11O40 leaching through six runs of the reaction reached
1.93% of the starting amount of [C16TA]H4TiPW11O40, showing that
the leaching amount of (C16TA)H4TiPW11O40 is little.
[23] A.I. Tropecêlo, M.H. Casimiro, I.M. Fonseca, A.M. Ramos, J. Vital, J.E. Castanheiro,
Applied Catalysis A: General 390 (2010) 183.
4. Conclusion
In this paper, we investigated the optimum reaction conditions
for (C16TA)H4TiPW11O40 catalyzing esterification of palmitic acid.