Efficient Acid–Base Bifunctional Catalysts for the Fixation of CO2 with Epoxides under Metal‐ and Solvent‐Free Conditions

Abstract

A series of acid–base bifunctional catalysts (ABBCs) that contain one or two Brønsted acidic sites in the cationic part and a Lewis‐basic site in the anionic part are used as efficient catalysts for the synthesis of cyclic carbonates by cycloaddition of CO 2 to epoxides, without the use of additional co‐catalyst or co‐solvent. The effects of the catalyst structures and various reaction parameters on the catalytic performance are investigated in detail. Almost complete conversion can be achieved in 1 h for propylene oxide using [{(CH 2)3 COOH}2 im]Br under mild reaction conditions (398 K and 2 M Pa). Furthermore, the catalyst can be recycled over five times without substantial loss of catalytic activity. This protocol is found to be applicable to a variety of terminal epoxides, producing the corresponding cyclic carbonates in good yields and high selectivities. A synergistic effect of the acidic and the basic sites as well as suitable hydrogen‐bonding strength of ABBCs are considered crucial for the reaction to proceed smoothly. The activities of the ABBCs increase remarkably with increasing carboxylic‐acid chain length of the cation. This metal‐ and solvent‐free process thus represents an environmentally friendly process for BTC‐catalyzed conversion of CO 2 into value‐added chemicals.