THE ROLE OF BIMOLECULAR MECHANISM IN THE SKELETAL ISOMERIZATION OF N-BUTENE TO ISOBUTENE

Several methods were used to evaluate the potential role of bimolecula r mechanism in skeletal isomerisation of n-butene to isobutene, Dimers of n-butene were prepared and their reaction monitored. Product distr ibutions of these reactions were compared with those obtained with n-b utene. Comparison was performed with different catalysts at 1 bar tota l pressure in an apparatus which makes both continuous dow and pulse e xperiments possible. Com positions of products of the reactions of n-b utene and of dimers were found to be very different an catalysts selec tive to isobutene, On highly acidic catalysts, which form isobutene no nselectively and show a high formation of by-products (C-3(=) and C-5( =)), distributions were similac. Skeletal isomerisation is suppressed by a higher pressure of butene while the rate of formation of by-produ cts increases with increasing pressure. Optimum temperatures of dimeri sation and of skeletal isomerisation differ considerably. All these re sults indicate that bimolecular mechanism is responsible mainly for th e byproducts. It is concluded that the skeletal isomerisation of n-but ene to isobutene runs via a monomolecular mechanism whereas the by-pro ducts are formed by cracking of dimers of n-butene. Another source of the by-products seems to be dimers arising from isobutene, the latter being first formed by a monomolecular skeletal isomerisation of n-bute ne. The formation of by-products is suppressed when 10-membered ring z eolite is used as a catalyst. (C) 1996 Academic Press, Inc.