Efficient acid-base bifunctional catalysts for the fixation of CO ₂ with epoxides under metal- and solvent-free conditions

A series of acid-base bifunctional catalysts (ABBCs) that contain one or two BrÃnsted 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 CO₂ 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 [{(CH₂)₃COOH}₂im]Br under mild reaction conditions (398 K and 2 MPa). 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 CO₂ into value-added chemicals.