SKELETAL ISOMERIZATION OF N-BUTENES .1. MECHANISM OF N-BUTENE TRANSFORMATION ON A NONDEACTIVATED H-FERRIERITE CATALYST

The transformation of n-butenes and of isobutene was carried out at 35 0 degrees C on a fresh H-FER zeolite (Si/Al = 13.8) with various conta ct times so as to obtain a large range of conversion (from 5 to 65%). With both reactants the skeletal isomerization was accompanied by the formation of various products. The skeletal isomer, propene, pentenes, octenes (traces), and n-butane (from n-butenes) or isobutane (traces from isobutene) appeared as primary products. The simultaneous formati on of the skeletal isomer and of propene and pentenes proved that we w ere in the presence of a dimerization-cracking process. Moreover, the low amount of octenes in the products showed that dimerization was the limiting step of the process. The dimerization step involved two seco ndary carbenium ions, which explained its slow rate. The dimer was rap idly isomerized through Type A (alkyl shift) or Type B (via protonated cyclopropanes) isomerization into the other octenes. Lastly octenes w ith a trimethylpentane or a dimethylhexane skeleton were rapidly crack ed into isobutene, propene, and pentenes through Type A (involving two tertiary carbenium ions) and Type B (involving one tertiary and one s econdary carbenium ions) mechanisms. The transformation of n-pentenes and of n-octenes confirmed the high rate of the Type A and B isomeriza tion and cracking steps. The slow rate of octene formation from butene s was also due to an inhibition of the diffusion of branched octenes i n the narrow pores of H-FER. The shape selectivity of H-FER was also r esponsible for the very slow formation of isobutane by hydrogen transf er from coke precursors to isobutene and for the unexpected slow forma tion of propene and of pentenes from isobutene. (C) 1996 Academic Pres s, Inc.