Exploring a new silica-supported Brønsted-Lewis ionic liquid catalyst for the cycloaddition of CO2 and propylene oxide: A combined experimental and computational study

Abstract

The field of liquid phase catalysis has explored the replacement of traditional solvents with ionic liquids, as well as the use of ionic liquids as catalyst. Herein, a novel silica-supported Fe-based Brønsted-Lewis functionalized ionic liquid (BLsIL-Fe) was synthesized and used as a catalyst for the cycloaddition of CO₂ to propylene oxide, yielding propylene carbonate. The catalyst was prepared by grafting a sulfonic acid-functionalized imidazole with a FeCl₄^- complex anion onto a commercial silica support (silica gel 60). The effects of temperature, pressure, catalyst loading, and reaction time on the catalytic activity of BLsILs were investigated. Our findings reveal that our catalyst remained active at a low catalyst loading of 0.18 mol% (active sites per mol of propylene oxide) at 120 °C, achieving up to 90.5 % conversion with a propylene carbonate selectivity above 97.0 % after four hours. The catalyst demonstrated reusability through a simple filtration and washing procedure, maintaining its catalytic activity over five cycles. Plausible catalytic reaction pathways for the CO₂ valorization process using BLsILs were elucidated using density functional theory, revealing that the ring-closing step is the rate determining step. Overall, the work opens new avenues for designing dual-acid functionalized ILs, enhancing performance across a wide range of catalytic reactions.