Skeletal reactions of hydrocarbons on platinum/sulphated zirconia superacid catalysts having an oxidative redispersion stage in their preparation A polar mechanism of C—C bond scission on platinum affects selectivity Original Research Article

A 1% w/w Pt/sulphated zirconia derived from a zirconia hydrogel and having high metal dispersion (arising from having an oxidative redispersion stage in the preparation) is active for skeletal reaction of n-pentane and of methylcyclopentane (MCP) at and above room temperature. A number of tests show that this high activity derives from the classical dualfunctionality of the catalyst. Implicitly, therefore, spillover of alkene or cycloalkene to and from the metal surface takes place at or near room temperature. Experiments with this catalyst and with Pt/zirconia show values of the 2MP/3MP product ratio for the MCP ring opening reaction much greater than the statistical value of 2.0. The effect derives from the Pt/zirconia combination in contrast with Pt on more usual supports (silica, alumina, magnesia, titania). The presence of the electronegative constituent sulphate does not lead to a significantly higher 2MP/3MP ratio than for Pt/zirconia itself. There is adequate correlation of the high 2MP/3MP ratio feature with an initial product pattern of major D2-isomer (alongside D5) in the exchange reaction between cyclopentane and deuterium conducted at lower temperatures on identical batches of the same catalysts so as to suggest the following rationale. This deuterium pattern is known to be associated with the action of electron deficient platinum sites. For MCP ring scission, requiring a site pair, such platinum sites may act catalytically in concert with normal Pt0 sites. The electrical asymmetry of such a platinum neighbour-site pair leads to a polar transition state which favours the asymmetric ring scission of MCP to 2MP, rather than to 3MP, akin to other well-known examples among retro-[2+2] reactions. It is proposed that this electrical asymmetry in a pair site can be furnished by a platinum atom atop a surface ensemble (and therefore topographically electron deficient) and a nearest-neighbour (electrically normal) platinum atom.