Toward Stable Solid Acid Catalysts for 1-Butene + Isobutane Alkylation: Investigations of Heteropolyacids in Dense CO2 Media

Heteropolyacid (HPA) catalysts and their cesium salts were synthesized, characterized and evaluated for the alkylation of isobutane with 1-butene. Steady butene conversion and C8 alkylates production were demonstrated for over 50 h in dense CO2-enhanced reaction media in both a fixed bed reactor and a CSTR operated at 368 K, 80 bar, and molar feed isobutane/olefin ratio of 5 with 70 mol % CO2. The C8 alkylates selectivity is highest (~25%) on HPA/SiO2, observed in a CSTR operated at an olefin space velocity of 0.05 h-1. This value is comparable to that obtained at similar operating conditions with Nafion/SiO2 (SAC-25), an ion-exchange resin. The cesium salts of HPA (either supported or unsupported) show high selectivity toward dimerization (>60%). The estimated effective alkylation rate constant is highest for SAC-25 closely followed by 70% HPA/SiO2. On all the catalysts tested, effective alkylation rate constants are 2 to 3 orders of magnitude lower than the dimerization rate constant. Hence, the design of solid acid catalysts that provide stable, industrially attractive C8 alkylates selectivity (70+%) remains a challenge.