SKELETAL REACTIONS OF HYDROCARBONS ON PLATINUM SULFATED ZIRCONIA SUPERACID CATALYSTS HAVING AN OXIDATIVE REDISPERSION STAGE IN THEIR PREPARATION - A POLAR MECHANISM OF C-C BOND SCISSION ON PLATINUM AFFECTS SELECTIVITY

A 1% w/w Pt/sulphated zirconia derived from a zirconia hydrogel and ha ving high metal dispersion (arising from having an oxidative redispers ion stage in the preparation) is active for skeletal reaction of n-pen tane and of methylcyclopentane (MCP) at and above room temperature. A number of tests show that this high activity derives from the classica l dual-functionality 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 ope ning reaction much greater than the statistical value of 2.0. The effe ct derives from the Pt/zirconia combination in contrast with Pt on mor e usual supports (silica, alumina, magnesia, titania). The presence of the electronegative constituent sulphate does not lead to a significa ntly higher 2MP/3MP ratio than for Pt/zirconia itself. There is adequa te correlation of the high 2MP/3MP ratio feature with an initial produ ct pattern of major D-2-isomer (alongside D-5) in the exchange reactio n between cyclopentane and deuterium conducted at lower temperatures o n identical batches of the same catalysts so as to suggest the followi ng rationale. This deuterium pattern is known to be associated with th e action of electron deficient platinum sites. For MCP ring scission, requiring a site pair, such platinum sites may act catalytically in co ncert with normal Pt-0 sites. The electrical asymmetry of such a plati num neighbour-site pair leads to a polar transition state which favour s 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 pro posed that this electrical asymmetry in a pair site can be furnished b y a platinum atom atop a surface ensemble (and therefore topographical ly electron deficient) and a nearest-neighbour (electrically normal) p latinum atom. (C) 1997 Elsevier Science B.V.