Porphyrin-based bifunctional catalysts and their catalytic activities for the formation of cyclic carbonates from carbon dioxide and epoxides

Abstract

Utilization of carbon dioxide has been the goal of the century, as demands for the reduction of greenhouse gas grows larger than ever. Several chemical routes have been discovered to convert carbon dioxide into valuable chemicals. One of the best routes that have been discovered so far is the fixation of CO2 with epoxides to form 5-membered cyclic carbonates, as it is 100% atom efficient and can be conducted in a solvent-free manner. For feasible reaction kinetics, many catalysts have been developed so far, such as salens, ionic liquids, metal organic frameworks, and porphyrins. Porphyrin-based catalysts have shown to possess high converting abilities in mild reaction conditions, and many attempts have been made to modify the structure of porphyrins for enhanced catalytic activities. In this work, we have newly synthesized four different bifunctional porphyrin-based catalysts with imidazolium-halogen based side chains attached to its structures. The structures of the porphyrin-based catalysts differed in the presence/absence of an alkyl chain spacer, the substitution place of the alkyl spacers, and steric hindrance near the metal atom center. All porphyrin-based catalysts were subjected to CO2 utilization reactions, and the catalytic activities of the catalysts were evaluated using the convergence of the reaction. The newly synthesized bifunctional porphyrin-based catalysts are expected to be utilized in CO2-epoxide cycloaddition reactions as a highly efficient catalyst with easy synthesis routes and short purification steps after the reaction.