The Role of Bimolecular Mechanism in the Skeletal Isomerisation ofn-Butene to Isobutene

Abstract

Several methods were used to evaluate the potential role of bimolecular mechanism in skeletal isomerisation ofn-butene to isobutene. Dimers ofn-butene were prepared and their reaction monitored. Product distributions of these reactions were compared with those obtained withn-butene. Comparison was performed with different catalysts at 1 bar total pressure in an apparatus which makes both continuous flow and pulse experiments possible. Compositions of products of the reactions ofn-butene and of dimers were found to be very different on catalysts selective to isobutene. On highly acidic catalysts, which form isobutene nonselectively and show a high formation of by-products (C3=and C5=), distributions were similar. Skeletal isomerisation issuppressedby a higher pressure of butene while the rate of formation of by-productsincreaseswith increasing pressure. Optimum temperatures of dimerisation and of skeletal isomerisation differ considerably. All these results indicate that bimolecular mechanism is responsible mainly for the by-products. It is concluded that the skeletal isomerisation ofn-butene to isobutene runs via a monomolecular mechanism whereas the by-products are formed by cracking of dimers ofn-butene. Another source of the by-products seems to be dimers arising from isobutene, the latter being first formed by a monomolecular skeletal isomerisation ofn-butene. The formation of by-products is suppressed when 10-membered ring zeolite is used as a catalyst.