Reactions involved in the alkylation of isobutane with 2-butene and with propene on a USHY zeolite Original Research Article

The transformation of an isobutane-2-butene mixture with a 41 molar ratio was carried out in liquid phase at 323 K on a USHY zeolite with a framework Si/Al ratio of 4.5. C5–C14 isoalkanes with about 70 wt.-% of C8 and a small amount of C8 alkenes were obtained. Moreover, about 7% of butene was transformed into undesorbed C10–C30 products containing three unsaturations or cycles. The C5–C14 products resulted from the following reactions: alkylation of isobutane with 2-butene, dimerization of butene, oligomerization-cracking and self-alkylation of isobutane. In order to determine the significance of this latter reaction, the transformation of an isobutane-propene mixture was investigated under similar operating conditions. Trimethylpentanes, and particularly the 2,2,4 isomer, were formed, which confirmed that isobutane self-alkylation was taking place. This reaction was about twice slower than the alkylation of isobutane with propene and twice faster than the dimerization of propene. From the hydrogen balance it was concluded that self-alkylation of isobutane supplied the main part of the hydrogen necessary for the saturation of the products of alkene dimerization and oligomerization-cracking. Hydride transfer would be the limiting step of the formation of isoalkanes. However, isomerization through hydride and methyl shifts of carbenium ion intermediates played also a significant role in the product distribution. This reaction occurred at a rate similar to that of hydride transfer to tertiary carbenium ions but much faster than hydride transfer to secondary carbenium ions.