Isomerization of -butane was studied on silica-supported heteropolyacids. The activity depends strongly on the solvent used for the preparation and on the polyoxometalate loading. This behavior was explained by different proportions of isolated and bulk heteropolyacids. Silicotungstic acid is inherently more selective than phosphotungstic acid. In presence of platinum and hydrogen the catalysts are stable and highly selective (more than 97%) to isobutane. A study as a function of temperature shows that there is an optimal temperature for performing the reaction. 1. Introduction Skeletal isomerization of low carbon -paraffins to isoparaffins is of great importance for the petroleum industry as it allows preparation of fuel mixtures of improved performance, low-octane-number compounds being replaced by high-octane-number ones [1]. As a consequence, the use of lead-containing fuel additives could be suppressed completely, resulting in economic and environmental benefits. In particular, -butane can be transformed to isobutane, which is used in the synthesis of MTBE (methyl-tert-butyl ether, another fuel additive) or in the alkylation of butenes [2]. Various alternatives have been tested so far in order to replace the currently employed commercial system (which among other inconveniences is water sensitive and requires constant addition of a small quantity of organic chloride precursor to the feed gas) or Pt/mordenite (less active than the former but resistant to poisoning). Sulfated zirconia was, for instance, reported to catalyze butane-isobutane isomerization already at room temperature [3]. Although it deactivated rapidly, its activity was higher than those of zeolites and is rather similar to mordenite [4, 5]. The presence of in the feed gas was shown to increase and stabilize the activity even in the absence of a noble metal cocatalyst and was ascribed to the suppression of the agglomeration of polymeric cracking products at the surface of the catalyst. However, when the concentration of hydrogen was too high a inhibiting effect was noted [4]. Some other proposed systems for this reaction were based on sulfated titania [6] or tungstated zirconia [7]. Polyoxometalates can be perceived as discreet analogues of metal oxide surfaces [8]. Particularly, the Keggin family members are widely used in homogenous catalysis, because of their pronounced and tuneable acidic and redox properties [9]. There are however only few examples of applications of pure solid heteropoly oxometalates in heterogeneous catalysis, due to their very low specific surface area (only
References
[1]
J. N. Armor, “A history of industrial catalysis,” Catalysis Today, vol. 163, no. 1, pp. 3–9, 2011.
[2]
A. Galadima, J. A. Anderson, and R. P. K. Wells, “Solid acid catalysts in heterogeneous n-alkanes hydroisomerisation for increasing octane number of gasoline,” Science World Journal, vol. 4, no. 3, pp. 15–22, 2009.
[3]
M. Hino, S. Kobayashi, and K. Arata, “Reactions of butane and isobutane catalyzed by zirconium oxide treated with sulfate ion. Solid superacid catalyst,” Journal of the American Chemical Society, vol. 101, no. 21, pp. 6439–6441, 1979.
[4]
F. Garin, D. Andriamasinoro, A. Abdulsamad, and J. Sommer, “Conversion of butane over the solid superacid ZrO2/SO42？ in the presence of hydrogen,” Journal of Catalysis, vol. 131, no. 1, pp. 199–203, 1991.
[5]
F. R. Chen, G. Coudurier, J. F. Joly, and J. C. Vedrine, “Superacid and catalytic properties of sulfated zirconia,” Journal of Catalysis, vol. 143, no. 2, pp. 616–626, 1993.
[6]
M. Hino and K. Arata, “Reactions of butane and isobutane catalysed by titanium oxide treated with sulphate ion. Solid superacid catalyst,” Journal of the Chemical Society, pp. 1148–1149, 1979.
[7]
M. Hino and K. Arata, “Synthesis of solid superacid of tungsten oxide supported on zirconia and its catalytic action for reactions of butane and pentane,” Journal of the Chemical Society, pp. 1259–1260, 1988.
[8]
C. L. Hill, “Introduction: polyoxometalates multicomponent molecular vehicles to probe fundamental issues and practical problems,” Chemical Reviews, vol. 98, pp. 1–2, 1998.
[9]
I. V. Kozhevnikov, “Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions,” Chemical Reviews, vol. 98, pp. 171–198, 1998.
[10]
N. Mizuno and M. Misono, “Heterogeneous catalysis,” Chemical Reviews, vol. 98, no. 1, pp. 199–217, 1998.
[11]
S. Suzuki, K. Kogai, and Y. Ono, “Isomerization of alkanes over a palladium salt of heteropolyacid,” Chemistry Letters, vol. 13, no. 5, pp. 699–702, 1984.
[12]
K. Na, T. Okuhara, and M. Misono, “Skeletal isomerization of n-butane catalyzed by an acidic cesium salt of 12-tungstophosphoric acid,” Chemistry Letters, vol. 22, no. 7, pp. 1141–1144, 1993.
[13]
K. Na, T. Okuhara, and M. Misono, “Isomerization of n-butane over bifunctional platinum-heteropoly compounds in the presence of hydrogen,” Journal of the Chemical Society, pp. 1422–1423, 1993.
[14]
K. Na, T. Okuhara, and M. Misono, “Skeletal isomerization of n-butane over caesium hydrogen salts of 12-tungstophosphoric acid,” Journal of the Chemical Society, vol. 91, pp. 367–373, 1995.
[15]
T. Suzuki and T. Okuhara, “Mechanism of skeletal isomerization of n-butane using 1,4-13C2-n-butane on solid strong acids,” Chemistry Letters, vol. 29, no. 5, pp. 470–471, 2000.
[16]
Z. N. Ma, W. M. Hua, Y. Ren, H. Y. He, and Z. Gao, “n-butane isomerization over Cs-salts of H3PW12O40: a mechanistic study by 1C MAS NMR,” Applied Catalysis A, vol. 256, pp. 243–250, 2003.
[17]
K. Na, T. Okuhara, and M. Misono, “Catalysis by heteropoly compounds: 34. Skeletal isomerization of n-butane over Pt- or Pd-promoted cesium hydrogen salts of 12-tungstophosphoric acid,” Journal of Catalysis, vol. 170, no. 1, pp. 96–107, 1997.
[18]
K. Na, T. Iizaki, T. Okuhara, and M. Misono, “Molecular design of solid acid catalysts. Isomerization of n-butane catalyzed by acid cesium salts of 12-tungstophosphoric acid combined with platinum,” Journal of Molecular Catalysis A, vol. 115, no. 3, pp. 449–455, 1997.
[19]
K. Nowinska, R. Fiedorov, and J. Adamiec, “Catalytic activity of supported heteropoly acids for reactions requiring strong acid centres,” Journal of the Chemical Society, vol. 87, pp. 749–753, 1991.
[20]
Y. Liu, G. Koyano, and M. Misono, “Hydroisomerization of n-hexane and n-heptane over platinum-promoted Cs2.5H0.5PW12O40 (Cs2.5) studied in comparison with several other solid acids,” Topics in Catalysis, vol. 11-12, no. 1–4, pp. 239–246, 2000.
[21]
A. V. Ivanov, T. V. Vasina, V. D. Nissenbaum, L. M. Kustov, M. N. Timofeeva, and J. I. Houzvicka, “Isomerization of n-hexane on the Pt-promoted Keggin and Dawson tungstophosphoric heteropoly acids supported on zirconia,” Applied Catalysis A, vol. 259, no. 1, pp. 65–72, 2004.
[22]
R. Hubaut, B. Ouled Ben Tayeb, W. Kuang, A. Rives, and M. Fournier, “Mechanical mixtures of Me (Ni, Pd) Ce oxides and silica-supported heteropolyacids: role and optimal concentration of each active species in n-hexane isomerization,” Kinetics and Catalysis, vol. 47, no. 1, pp. 20–24, 2006.
[23]
A. Gherib, A. Aouissi, A. Rives, M. Fournier, and R. Hubaut, “Isomerization of n-hexane over silica-supported heteropolyoxometallates promoted by Pt-Ce oxides,” Chinese Journal of Catalysis, vol. 28, no. 12, pp. 1041–1046, 2007.
[24]
A. Oulmekki and F. Lefebvre, “n-Hexane cracking over heteropoly acids, 1. Treatment of data and temperature dependence of the catalyitc activity of H3PW12O40,” Reaction Kinetics and Catalysis Letters, vol. 48, pp. 593–600, 1992.
[25]
A. Oulmekki and F. Lefebvre, “n-hexane cracking over heteropoly acids, 2. Catalytic activity and thermal stability of H4SiW12O40, H6P2W18O62 and H6P2W21O71(H2O)3,” Reaction Kinetics and Catalysis Letters, vol. 48, pp. 601–606, 1992.
[26]
A. Oulmekki and F. Lefebvre, “n-Hexane cracking over heteropoly acids, 3. Silica supported heteropoly acids,” Reaction Kinetics & Catalysis Letters, vol. 48, no. 2, pp. 607–612, 1992.
[27]
F. Lefebvre, “31P MAS NMR study of H3PW12O40 supported on silica: formation of ( SiOH2+)(H3PW12O40–),” Journal of the Chemical Society, Chemical Communications, pp. 756–757, 1992.
[28]
S. M. Kulikov, M. N. Timofeeva, I. V. Kozhevnikov, V. I. Zaikovskii, L. M. Plyasova, and I. A. Ovsyanikova, “Adsorption of the heteropolyacid H4SiW12O40 by porous substrates,” Bulletin of the Academy of Sciences of the USSR Division of Chemical Science, vol. 38, no. 4, pp. 687–691, 1989.
[29]
A. D. Newman, D. R. Brown, P. Siril, A. F. Lee, and K. Wilson, “Structural studies of high dispersion H3PW12O40/SiO2 solid acid catalysts,” Physical Chemistry Chemical Physics, vol. 8, no. 24, pp. 2893–2902, 2006.
[30]
A. Thomas, C. Dablemont, J. M. Basset, and F. Lefebvre, “Comparison of H3PW12O40 and H4SiW12O40 heteropolyacids supported on silica by 1H MAS NMR,” Comptes Rendus Chimie, vol. 8, no. 11-12, pp. 1969–1974, 2005.
[31]
N. Legagneux, A. De Mallmann, E. Grinenval, J. M. Basset, and F. Lefebvre, “Grafting reaction of organotin complexes on silica catalyzed by tungstic heteropolyacids,” Inorganic Chemistry, vol. 48, no. 18, pp. 8718–8722, 2009.
[32]
N. Legagneux, E. Jeanneau, A. Thomas et al., “Grafting reaction of platinum organometallic complexes on silica-supported or unsupported heteropolyacids,” Organometallics, vol. 30, no. 7, pp. 1783–1793, 2011.
[33]
G. Caiero, R. H. Carvalho, X. Wang et al., “Activation of C2–C4 alkanes over acid and bifunctional zeolite catalysts,” Journal of Molecular Catalysis A, vol. 255, pp. 131–158, 2006.
[34]
M. Guisnet, P. Bichon, N. S. Gnep, and N. Essayem, “Transformation of propane, n-butane and n-hexane over H3PW12O40 and cesium salts. Comparison to sulfated zirconia and mordenite catalysts,” Topics in Catalysis, vol. 11-12, no. 1–4, pp. 247–254, 2000.
[35]
S. S. Chen, R. C. Wilhoit, and B. J. Zwolinski, “Thermodynamic properties of normal and deuterated methanols,” Journal of Physical and Chemical Reference Data, vol. 6, no. 1, pp. 105–112, 1977.
[36]
C. Travers, N. Essayem, M. Delage, and S. Quelen, “Heteropolyanions based catalysts for paraffins isomerization,” Catalysis Today, vol. 65, no. 2–4, pp. 355–361, 2001.
