Support Effects of SiO2 and Al2O3 on Ga3+ Lewis Acid Catalysts for Olefin Oligomerization

High-metal-loading 12% Ga/SiO₂ and 12% Ga/Al₂O₃ catalysts were synthesized to investigate the effect of the support on the structures and catalytic performance for ethylene and propylene oligomerization. X-ray absorption spectroscopy analysis revealed that Ga³⁺ ions with four Ga–O bonds at 1.84 Å interact differently with SiO₂ and Al₂O₃ supports, with two Ga–O–Si bonds at 3.12 Å and four Ga–O–Al bonds at 3.23 Å. Pyridine infrared spectra confirmed the presence of Lewis acid sites without Bro̷nsted acid sites on both catalysts, with Ga/SiO₂ exhibiting a stronger Lewis acidity. While Lewis acids are not typically known to react with hydrocarbons, isolated Ga³⁺ sites catalyze oligomerization and secondary reactions including hydrogen transfer, olefin isomerization, and C–C bond fragmentation, leading to nonoligomer products. While the reaction rates for olefin oligomerization were similar for both catalysts, there were significant differences in product distributions. For instance, under high-pressure ethylene oligomerization at ∼20% conversion, Ga/Al₂O₃ produced higher-molecular-weight olefins and more nonoligomer products, including propylene and pentenes, which were absent with Ga/SiO₂. These findings highlight the dual role of the support material, serving both as a host for active sites and as a critical factor influencing catalytic performance.