Phase-controllable polymerized ionic liquids for CO2 fixation into cyclic carbonates

Abstract

A catalyst with activity comparable with homogeneous catalysts and easy separation like heterogeneous catalysts would be attractive for CO₂ cycloaddition. Here, a series of polymerized bis-imidazolium based ionic liquids (PBIL-m) were synthesized and could act as homogeneous catalysts during the CO₂ cycloaddition to epoxide process. They could be separated as heterogeneous catalysts after the cycloaddition reaction. PBIL-m was highly active for the cycloaddition reaction due to functional groups such as the imidazole ring, amino group and Br⁻. Specifically, the solid–liquid transition behavior endowed the PBIL-m with comparable activity to its homogeneous monomer catalysts (BIL-m). Among these PBIL-m catalysts, poly(1-vinyl imidazole-3-hexyl-1-imidazole-3-aminopropyl)dibromide (PBIL-3) exhibited superior catalytic performance due to the appropriate bridge chain compared with other PBIL-m. Under the conditions of 80 °C, 1.0 MPa and 24 h, 99% propylene carbonate yield and 99% selectivity were obtained. The PBIL-3 also showed excellent universality and recyclability. A reasonable reaction mechanism was deduced that the imidazole ring, amino group and Br⁻ promoted the cycloaddition reaction under metal-, solvent-, and cocatalyst-free conditions. Therefore, the polymerized bis-imidazolium based ionic liquid with solid–liquid transition behavior is a promising candidate for smooth catalysis of CO₂ conversion and utilization.