Alkylation of isobutane with light olefins catalyzed by zeolite beta

Abstract

Alkylation of isobutane with ethene and propene was studied over an H-BEA catalyst in a well-stirred reactor. Under similar conditions of space velocity and paraffin-to-olefin feed ratio, lower initial olefin conversions were observed with ethene or propene than those reported earlier for butene. The sole presence of saturated hydrocarbons indicates the dominance of the alkylation over alkene oligomerization. The product distribution indicates that the rate of hydride transfer in the reaction sequence of alkylation is significantly lower than the rate of methyl shift isomerization. For ethene, significant amounts of C 8 alkanes, especially trimethylpentanes were found in the products indicating multiple alkylation or dimerization followed by alkylation to dominate. With propene, iso-heptanes dominated the products, as expected for single alkylation. Next to single and multiple alkylation, evidence for cracking of surface alkoxy-species by β-scission, iso-butane self-alkylation and olefin dimerization was observed. The product distribution suggests that dimerization over weaker acidic sites allowing desorption of olefins occurred also during the period of stable catalytic activity, but the resulting olefins were immediately added to alkoxy species present and thus contributed to the alkylation route. Once these alkoxy species become large, either olefin addition or hydride transfer is blocked, and olefins are detected in the products.