Development of zeolite catalysts for isobutane/butene alkylation enabling production of high-octane gasoline

Abstract

Gasoline is petroleum products derived from fractional distillation and refinery of crude oil, which is widely used in automobile, aircraft, and industrial applications. In gasoline market, fuel for automobile with internal combustion engine accounts for the largest portion. To avoid knocking, MTBE was generally used as octane boosters. However, the MTBE was designated as water pollutant in clean air act, 1970. Thus, the importance of isobutane/butene alkylation process to produce alkylates (ca. isooctanes) have been increased. Until now, liquid acid (H2SO4 and HF) are used as catalysts for isobutane/butene alkylation. However, they always suffer from toxicity, causticity, and environmental hazard. To overcome the limitations, several big oil companies suggested solid acid catalyzed isobutane/butene alkylation processes. Zeolite is the representative solid acid catalysts, which is crystalline aluminosilicate materials have ordered micropore and channel. However, previous researches failed to commercialize zeolite catalyzed isobutane/butene alkylation due to extremely fast deactivation resulted from carbon deposition in micropore. To realize utilization of zeolite in isobutane/butene alkylation, elongation of catalyst lifetime as well as high selectivity of trimethylpentane should be achieved. In the present study, a series of BEA zeolites having different secondary pore structures were synthesized to investigate the effects of facilitated molecular transport on trimethylpentane selectivity and catalyst deactivation. The results showed that the hierarchical BEA zeolite containing trimodal micro-/meso-/macroporosity synthesized by the pseudo-solid-phase crystallization of diatomaceous earth exhibited the superior selectivity to trimethylpentanes and remarkably enhanced catalyst lifetime. The excellent catalytic properties were attributed to the enhanced diffusion of a hydride donor (isobutane) and bulky alkylate product to or from the zeolite micropores in the presence of hierarchical pore structures. After supporting a trace amount of Pt, the zeolite catalysts could be fully regenerated by hydrogenative treatments if regeneration was carried out before heavy coke formation. The hierarchical BEA zeolite with trimodal porosity enabled 4 times less frequent regeneration compared to solely microporous BEA zeolites, which will greatly contribute to the reduction of process operating costs.