A superacid is a known class of acidic material that has an acid strength, measured by Hammett acidity function H.sub.0, greater than that of 100% H.sub.2 SO.sub.4, which has an H.sub.0 value of -12. A superacid, therefore, has an H.sub.0 value of less than -12 or an acid strength greater than -12. Superacids are useful for reactions that are generally catalyzed by strong acid, such as paraffin isomerization.
M. Hino and K. Arata describe the synthesis of a solid superacid having an acid strength of up to H.sub.0 .ltoreq.-16.04 by exposing hydroxides or oxides of Fe, Ti, Zr and Hf, prior to crystallization, to sulfate ions, followed by calcination in air at over 500.degree. C. in J. Chem. Soc., Chem. Commun., 1148 (1979). K. Arata and M. Hino also describe the synthesis of a solid superacid having an acid strength of H.sub.0 .ltoreq.-14.52 by impregnating Zr(OH).sub.4 or amorphous ZrO.sub.2 with aqueous ammonium metatungstate, followed by calcining in air at 800.degree. to 850.degree. C. in J. Chem. Soc., Chem. Commun., 1259 (1988) and in "Synthesis of Solid Superacid of Tungsten Oxide Supported on Zirconia and Its Catalytic Action", Proceedings 9th International Congress on Catalysis, Volume 4, pages 1727-1734 (1988).
The superacid described by K. Arata and M. Hino in "Synthesis of Solid Superacid of Tungsten Oxide Supported on Zirconia and Its Catalytic Action" is particularly useful as a catalyst in the isomerization of butane to isobutane and of pentane to isopentane. However, in order to obtain maximum catalytic activity, K. Arata and M. Hino report calcination temperatures of from 800.degree. to 850.degree. C. for the tungsten-modified zirconia catalyst.
Soled et al describe, in U.S. Pat. No. 5,113,034, a solid acid catalyst having an H.sub.0 value ranging from -14.5 to about -16.5, comprising a sulfate or tungstate-modified Group IVB oxide, which is useful to dimerize C.sub.3 or C.sub.4 containing feedstreams. A calcination temperature range for the tungstate-modified zirconia catalyst of 450.degree. to 800.degree. C. is given. Specifically, the Examples use an initial calcination temperature of 600.degree. C., followed by a calcination temperature of 800.degree. C. prior to charging.
U.S. Pat. No. 5,198,403 (Brand et al, herinafter Brand) discloses a catalyst for the selective reduction of nitrous oxide with ammonia which contains, in addition to titanium oxide as component A, at least one oxide of tungsten, silicon, boron, aluminum, phosphorus, zerconium, barium, yttrium, lanthanum, cerium as component B1 and at least one oxide of vanadium, niobium, molybdenum, iron and copper as B2, with an atomic ratio between the elements of components A and B from 1:0.001 up to 1. The catalyst of Brand requires the presence of titanium oxides, presumably because it is effective in the reduction of nitrous oxides. Reduction of nitrous oxides is the intended purpose of Brand. The catalysts of the instant invention do not contain titanium and are not drawn to nitrous oxide reduction.
U.S. Pat. No. 4,918,041 (Hollstein et al, hereinafter Hollstein) is directed to a sulfated calcined solid catalyst. The catalysts of the instant invention are not sulfated. Group VI B metals such as tungsten are employed in the superacid, rather than Group VI A metals such as sulfur. Group VI B metals are preferred in the instant invention due to their stability and ease in regeneration.
The generation of acid activity in solid oxide catalysts in general, and in the tungsten-modified zirconia catalyst specifically, requires calcination of the catalyst at temperatures of about 800.degree. C. This extreme temperature, however, causes significant loss of catalyst surface area. For example, K. Arata and M. Hino report surface areas of 35.3 and 29.5 m.sup.2 /g for catalysts calcined at 800.degree. and 900.degree. C. in "Synthesis of Solid Superacid of Tungsten Oxide Supported on Zirconia and Its Catalytic Action". By contrast, the surface areas for catalysts calcined at 600.degree. and 700.degree. C. are reported as 44.2 and 38.5 m.sup.2 /g. Hence, it is clear that as the calcination temperature is increased, the surface area of the catalyst is decreased. Further, the extreme calcination temperature required to generate acid activity results in a more difficult manufacturing process.