Riveting Hydroxyl Ionic Liquids onto Melem Oligomers for CO₂ Cycloaddition into Cyclic Carbonates

The product of cyclic carbonates from carbon dioxide (CO₂) and epoxides under solvent-free and co-catalyst-free conditions is an environmental-friendly and economical route. However, the core of this route lies in the design and preparation of efficient and low-cost catalysts. In this work, hydroxyl ionic liquids (HEIMBr) modified melem oligomers (CN-HEIMBr) was fabricated and used for the cycloaddition of CO₂ into cyclic carbonates. The characterizations for these catalysts, such as XPS, TGA, SEM, FT-IR, etc, all confirmed that HEIMBr was successfully grafted onto melem oligomers. The synergistic effects of edge defects (−NH and −NH₂) of melem oligomers, hydroxyl groups and halogen anions of ionic liquids were conductive to promoting CO₂ cycloaddition. The influences of CO₂ pressure, catalyst dosage, reaction time and temperature on the conversion and yield were investigated in detailed. In the conditions of co-catalyst-free and solvent-free, the conversion of epichlorohydrin (ECH) was 99 % and the yield of chloropronene carbonate (CPC) was 97 % under optimal conditions (140 °C, 2.0 MPa, 2 h, catalyst 0.1 g, ECH 30 mmol). Moreover, the catalyst was reused at least five runs without any significantly loss of catalytic performance. Furthermore, a credible multi-synergetic catalytic mechanism was proposed. An originality idea was provided for designing high-efficiency catalysts for coupling of CO₂ and epoxides, and put forward the multi-synergistic effects of catalysts on CO₂ conversion.