256
T.V. Kovalchuk et al. / Journal of Catalysis 263 (2009) 247–257
Table 5
materials display good catalytic performance: high activity (activ-
ity per anion, up to 150 and 25 h−1, respectively), good selectivity
(around 80% and 100%, respectively) and relatively good structural
stability. Indeed the resistance to HPA leaching is significantly bet-
ter than for the silica-supported HPA materials, the best hydrolytic
Catalytic activity of imidazolium-HPA-silicas (0.2 g) in esterification of acetic acid
(8 mmol) at 60 C, 20 h. TON—turnover number, total mol product per anion. Values
in parentheses correspond to recycling tests.
◦
Sample
Conversion
(%)
Selectivity
(%)
Mol product
per KU
per h
TON
(ac
−1
)
stability being found for H4SiW12O40
.
The method used to graft the onium cation onto the silica and
the nature of the cation slightly influences catalyst performance.
Catalysts having a greater coverage (as shown by comparing two
pyridinium salts) and hydrophobic cations (as shown by comparing
two imidazolium salts) are preferable. Amongst the heteropoly-
acids studied, H4SiW12O40 is the most active and promising for
catalyst design.
MeIm-PMo
MeIm-PW
MeIm-SiMo
BuIm-PMo
BuIm-PW
15.5
36 (29)
45
29
73 (66)
68
100
100
100
100
100
100
7.2
144
335
327
200
479
494
16.7
16.4
10.0
23.9
24.7
BuIm-SiMo
immobilized on oniumsilicas, is highly active. This can result from
the greater number of protons per Keggin unit of the tetraprotic
References
acid [(MI)2.4–2.9H1.6–1.1SiMo12O
40] [1]. Indeed, 31P NMR chemical
shifts of TEPO adsorbed on MeIm-SiMo, BuIm-SiMo catalysts (as
well as for Py-1-SiW and Py-2-SiW catalysts) confirm that its acid-
ity strength is higher than that of the other acids. Furthermore, 3-
and 4-charged heteropolyanions are capable of providing an ad-
ditional stabilizing effect on the intermediate organic cations, and
this effect is more pronounced for the “softer” 4-charged anion.
The activities of HPAs immobilized on γ -propyl-N-butyl imi-
dazoilum silica are 1.5 times greater than when they are on γ -
propyl-N-methyl imidazoilum silica. Perhaps this is because the
bulkier and more hydrophobic cation contributes positively to the
diffusion of reactants towards the active site and increases the ac-
tivity per Keggin unit.
[1] T.V. Kovalchuk, Ph.D. thesis, Pierre et Marie Curie University, Paris, France,
2003.
[2] F. Cunill, M. Vila, J.F. Izquierdo, M. Iborra, J. Tejero, Ind. Eng. Chem. Res. 32
(1993) 564.
[3] K.W. Otto, Oil Gas, January 11, 1993.
[4] T. Nishina, Shokubai Catal. 186 (1994) 36.
[5] T. Deguchi, Petrotech 15 (1992) 874.
[6] C.P. Nicolaides, C.J. Stotijn, E.R.A. Van der Veen, M.S. Visser, Appl. Catal. A 103
(1993) 223.
[7] J.S. Kim, J.M. Kim, G. Seo, N.C. Park, H. Niiyama, Appl. Catal. 37 (1988) 45.
[8] G.M. Maksimov, I.V. Kozhevnikov, React. Kinet. Catal. Lett. 39 (1989) 317.
[9] S. Shikata, T. Okuhara, M. Misono, Sekiyu Gakkaishi 37 (1994) 632.
[10] G. Baronetti, L. Briand, U. Sedran, H. Thomas, Appl. Catal. A 172 (1998) 265.
HPA concentrations in the spent BuIm-PMo and MeIm-PMo
samples indicate partial loss of the acid during the reaction (after
two consecutive tests): 34.7 and 6.4%, respectively, of the amount
initially grafted (Tables 1 and 2). Recycling tests, performed with
the two most active catalysts, demonstrate that the activity de-
creased by 9–20%, whereas the concentration of grafted PW de-
creased by 5–6% (Tables 1 and 2).
[11] Y. Ono, T. Baba, in: Proc. 8th Int. Congr. Catal., Berlin, Germany, vol. 5, Verlag
Chemia-Dechema, Weinheim, 1984, p. 405.
[12] A. Agrachi, T. Matsuda, Y. Ogino, Sekiyu Gakkaishi 22 (1979) 331.
[13] A. Bielanski, R. Dziembaj, A. Malechka-Lubanska, J. Pozniczek, M. Hasik, M.
Drozdek, J. Catal. 185 (1999) 363.
[14] M.H. Matouq, S. Goto, Int. J. Chem. Kinet. 25 (1993) 825.
[15] O. Françoise, F.C. Thyrion, Chem. Eng. Process. 30 (1991) 141.
[16] N. Mizuno, M. Misono, Chem. Rev. 98 (1998) 199.
The obtained esterification results are comparable to those re-
ported for related immobilized HPA catalysts. The activities of the
catalysts studied here vary from 7 to 25 h−1. This is higher than
reported for carbon-immobilized H4SiW12O40 (activity per anion,
3.6 h−1) and H3PW12O40 (activity per anion, 6 h−1) in the es-
terification of propanoic acid with butanol and 2-ethylhexanol in
[17] P. Dupont, F. Lefebvre, J. Mol. Catal. 114 (1996) 299.
[18] M.A. Schwegler, H. Van Bekkum, N.A. Munck, Appl. Catal. 74 (1991) 191.
[19] M.J. Verhoef, Microporous Mesoporous Mater. 27 (1999) 365.
[20] W. Chu, X. Yang, Y. Schan, X. Ye, Y. Wu, Catal. Lett. 42 (1996) 201.
[21] J.P. Osegovic, R.S. Drago, J. Catal. 182 (1999) 1.
[22] H. Niiyama, Y. Saito, et al., in: Proc. 7th Int. Congress Catal., Kodansha, Japan,
Elsevier, Amsterdam, 1980, p. 1417.
◦
toluene at 60 C, respectively [17]. The results are, however poorer
[23] T. Baba, H. Watanabe, Y. Ono, J. Phys. Chem. 87 (1983) 2406.
[24] S. Shikata, T. Okuhara, M. Misono, J. Mol. Catal. A Chem. 100 (1995) 49.
[25] T. Okuhara, T. Nakato, Catal. Surv. Jpn. 2 (1998) 41.
than those obtained with MCM-41-immobilized H4SiW12O40 and
−1
H3PW12O40 (activity per anion varies from 200 to 250 h
in
the liquid-phase esterification of benzoic acid with propan-1-ol in
toluene) and parallel to the activity of MCM-41-immobilized HPAs
(TOF 40 h−1) in the gas-phase esterification of acetic acid with
butan-1-ol [36]. However, in this case the catalysts suffered from
serious HPA leaching.
[26] M. Kimura, T. Nakato, T. Okuhara, Appl. Catal. A 165 (1997) 227.
[27] B. Bardin, R. Davis, Top. Catal. 6 (1998) 77.
[28] T. Okuhara, T. Yamada, K. Seki, K. Johkan, T. Nakato, Microporous Mesoporous
Mater. 21 (1998) 637.
[29] N. Essayem, G. Coudurier, M. Fournier, J. Vedrine, Catal. Lett. 34 (1995) 223.
[30] M. Misono, in: B. Imelik (Ed.), Catalysis by Acids and Bases, Elsevier, Amster-
dam, 1985, p. 147.
4. Conclusion
[31] A. Corma, Chem. Rev. 95 (1995) 592.
[32] S. Damyanova, J.L.G. Fierro, I. Sobrados, J. Sanz, Langmuir 15 (1999) 469.
Keggin heteropolyacids (H3PMo12O40, H3PW12O40, H4SiMo12O40
and H4SiW12O40) were immobilized on functionalized silica as
their onium (propylpyridinium and alkylimidazolium) salts. Inter-
action between HPA and the surface-grafted onium cations affords
acid salts. Tetraprotic acids, H4SiW12O40 and H4SiMo12O40, give
better surface exchange yields and result in solids with greater
acid site number per anion and strength. The major part of im-
mobilized PW, SiW, PMo and SiMo anions in the as-prepared and
spent catalysts samples preserve Keggin structure. Immobilization
of H3PMo12O40 is accompanied by partial decomoposition (up to
ca. 0–5%).
[33] W. Yang, J. Billy, Y. Ben Taarit, J.C. Vedrine, N. Essayem, Catal. Today 73 (2002)
153.
[34] F. Babou, G. Coudurier, J.C. Vedrine, J. Catal. 152 (1995) 341.
[35] N. Essayem, A. Holmquist, G. Sapaly, J.C. Vedrine, Y. Ben-Taarit, Stud. Surf. Sci.
Catal. 135 (2001) 1991.
[36] M.J. Verhoef, P.J. Kooyman, J.A. Peters, H. Van Bekkum, Microporous Meso-
porous Mater. 27 (1999) 365.
[37] A. Bielanski, A. Lubanska, J.J. Pozniczek, A. Micek-Ilnicka, Appl. Catal. A 238
(2003) 239.
[38] V.N. Zaitsev, T.V. Kovalchuk, J. Fraissard, in: Proc. of the Congress “Functional-
ized Materials”, Kiev, Ukraine, 2002, p. 118;
T.V. Kovalchuk, V.N. Zaitsev, P. Batamack, H. Sfihi, J. Fraissard, in: Congress “Sil-
ica 2001”, Mulhouse, France, September 2001, CD-ROM Proceedings;
T.V. Kovalchuk, V.N. Zaitsev, J. Fraissard, in: Proc. 4th Int. Symp. Supported
Reagents and Catalysts in Chemistry, St. Andrews, UK, July 2000, p. 40.
In the two model reactions tested, the gas-phase synthesis of
ETBE and the liquid-phase esterification of AcOH with EtOH, the