Elementary mechanistic steps and the influence of process variables in isobutane alkylation over H-BEA

Liquid phase conversion of n-butene in excess iso-butane was investigated over zeolite BEA as catalyst in a continuously operated slurry reactor. Single and multiple alkylation and cracking were the main reaction pathways. Only saturated products were observed indicating that hydride transfer reactions were fast, compared to desorption of olefins under the experimental conditions chosen. An optimal reaction temperature of 350 K was identified for single alkylation, higher reaction temperatures favoring cracking of intermediately formed products and lower temperatures, multiple alkylation. Under optimum reaction conditions over 80 wt% butene was converted via single alkylation. The C₈ product selectivity is determined by the balance between isomerization of the C₈ alkoxy and the hydride transfer to release iso-octanes from the acid sites. Once desorbed from an acid site, the alkanes do not isomerize. Independent of the space velocities the catalyst and, hence, the individual acid sites deactivate after approximately 30 butene turnovers at 350 K by deposition of polyalkylates. A simplified reaction model for alkylation over solid acid catalysts and the implications for catalyst design are discussed.