Synergistic Effect of Brønsted and Lewis Acids in Aromatic Alkylation Catalyzed by Chloroaluminate Ionic Liquids

Abstract

Introduction: Chloroaluminate Ionic Liquids (CILs) are highly effective catalysts for the alkylation of aromatics with long-chain olefins, yet the individual and synergistic effects of Brønsted and Lewis acid species remain contentious. Herein, 1-methylnaphthalene and 1-decene are used as model compounds to elucidate the synergistic catalysis mechanism through an integrated experimental and theoretical approach. Methods: Pyridine adsorption infrared spectroscopy was used to characterize acid species. Catalytic performance was evaluated under various reaction temperatures, times, water contents (50–350 mg/L), and catalyst dosages. Concentrations of Brønsted and Lewis acids were quantitatively correlated with water content and catalyst dosage. Density Functional Theory (DFT) calculations were performed to compute energy barriers for three catalytic pathways (Brønsted alone, Lewis alone, and combined). Results: Pyridine adsorption infrared spectroscopy confirms the in situ generation and coexistence of both Brønsted and Lewis acid species in the reaction system. Catalytic performance evaluations demonstrate that efficient reaction at low temperatures requires not only a sufficient concentration of Lewis acid (> 0.88 mol/L) but also an appropriate amount of Brønsted acid (> 12.5 mmol/L), which is generated controllably via hydrolysis. Under optimized conditions (30°C, 20 minutes), nearly complete olefin conversion is achieved. Most significantly, theoretical calculations reveal that the synergy of Brønsted and Lewis acids dramatically reduces the rate-determining energy barrier from 244.5 kJ/mol (Lewis alone) to 24.3 kJ/mol (for the synergy). Discussion: The synergy of Brønsted and Lewis acids shifts the rate-determining step from highenergy dehydrogenation to σ-complex formation, enabling low-temperature, high-efficiency alkylation. Controlled water addition offers a practical method to generate Brønsted acid in situ. Conclusion: Both Brønsted and Lewis acids are essential. Optimized conditions (30°C, 20 minutes, [Lewis acid] ≥ 0.88 mol/L, [Brønsted acid] ≥ 12.5 mmol/L) achieve near-complete conversion. The proposed synergistic mechanism offers strategic insights for optimizing CILs-catalyzed alkylation processes.