Bifunctional catalyst of a metallophthalocyanine-carbon nitride hybrid for chemical fixation of CO2 to cyclic carbonate

Abstract

Chemical fixation of carbon dioxide (CO₂) to cyclic carbonates was investigated by using bifunctional nucleophile–electrophile catalysts of a metallophthalocyanine–carbon nitride hybrid [MPc/g-C₃N₄ (M = Co, Cu)] in the absence of any co-catalysts and organic solvents. MPc/g-C₃N₄ was readily obtained by direct calcination of a mixture of dicyandiamide and metallophthalocyanine under a flowing-nitrogen atmosphere, and the MPc/g-C₃N₄ prepared at 480 °C (MPc/g-C₃N₄-480) showed the highest catalytic performance toward the cycloaddition reaction of CO₂ to epichlorohydrin (ECH). For bifunctional MPc/g-C₃N₄, the MPc species function as Lewis acidic centers for ECH activation via electrophilic attack; while, the g-C₃N₄ moiety, possessing abundant and uncondensed species with the forms of primary amine (NH₂) groups and secondary amine (C–NH–C) groups at the edges of graphitic sheets as edge defects, acts as an organic base for CO₂ activation through nucleophilic attack. The developed MPc/g-C₃N₄ is stable and insoluble in any commonly used organic solvents and behaves as heterogeneous catalyst, leading to facile separation and recycling in a CO₂ fixation reaction.