Adamantane-Based Cation and [MFn]- Anion Synergistically Enhanced Catalytic Performance of Sulfuric Acid for Isobutane Alkylation

A series of adamantane-based ionic liquids (ADM-ILs) with [MFn](-) anions were synthesized as cocatalysts for the alkylation of isobutane and butene. By systematically tuning the structures of the cation and anion and their combination, we obtained the optimized ionic liquid, ADM-C-12-SbF6, which exhibited significant enhancements in the C-8 selectivities [especially to trimethylpentanes (TMPs)], the research octane number (RON) of the alkylate products, and the lifetime of sulfuric acid. The selectivity to TMPs was improved from 81.9% to 84.5%, and the alkylate RON was improved from 96.6 to 98.6 upon the addition of the ADM-ILs. In addition, the lifetimes of the ADM-IL/H2SO4 systems were increased to twice that of H2SO4 alone. Based on experimental measurements and DFT calculations, all of these enhancements were attributed to the multifunctions cooperatively integrated into the task-specific ADM-ILs, such as surfactant action, improving the interfacial properties of the acid/hydrocarbon biphases; buffer action, stabilizing the acidity changes during the reaction process; and hydride donor action, increasing the H- transfer rate, which promoted the production of TMPs. This study is beneficial for improving the isobutane alkylation process catalyzed by concentrated sulfuric acid.