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 350 degrees C on a fresh H-FER zeolite (Si/Al = 13.8) with various contact 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 formation of the skeletal isomer and of propene and pentenes proved that we were 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 secondary carbenium ions, which explained its slow rate. The dimer was rapidly isomerized through Type A (alkyl shift) or Type B (via protonated cyclopropanes) isomerization into the other octenes. Lastly octenes with a trimethylpentane or a dimethylhexane skeleton were rapidly cracked into isobutene, propene, and pentenes through Type A (involving two tertiary carbenium ions) and Type B (involving one tertiary and one secondary carbenium ions) mechanisms. The transformation of n-pentenes and of n-octenes confirmed the high rate of the Type A and B isomerization and cracking steps. The slow rate of octene formation from butenes was also due to an inhibition of the diffusion of branched octenes in the narrow pores of H-FER. The shape selectivity of H-FER was also responsible for the very slow formation of isobutane by hydrogen transfer from coke precursors to isobutene and for the unexpected slow formation of propene and of pentenes from isobutene. (C) 1996 Academic Press, Inc.