Metallophthalocyanine-carbon Nitride Hybrid Catalyzes CO₂ Chemical Fixation To Cyclic Carbonate

Carbon dioxide（CO₂） and its increasing concentration in the atmosphere over the last century is an important issue in the media, where the current concentration of 386 ppm is a far cry from the pre-industrial levels of 280 ppm. Such a drastic increase in CO₂ concentration was deemed as the main culprit for the increase in earth’s temperature. For chemists, CO₂ is an attractive C building block inorganic synthesis as it is a highly functional, abundant, renewable carbon source and an environmental friendly chemical reagent. The utilization of CO₂ is indeed more attractive, especially if its conversion to useful bulk products. One of the most successful synthetic methods is the synthesis of cyclic carbonates from CO₂ and epoxides by the 100% atom economical reaction. Cyclic carbonates can serve as polar aprotic solvents, electrolytes for lithium-ion batteries, and valuable intermediates for polycarbonates and polyurethanes. To overcome the disadvantages of low activity and difficulty of separation of the catalyst in this cycloaddition process,in this study, we synthesized a series of catalysts by using a direct calcination of mixture of dicyandiamide and different kinds of metallophthalocyanine（CoPc,CuPc） under flowing-nitrogen atmosphere, and readily obtained bi-functional nucleophile–electrophile catalysts of metallophthalocyanine-carbon nitride hybrid [MPc/g-C₃N₄（M = Co, Cu）],which combining the both advantages of heterogeneous catalysts for their separation/recycling and homogenous catalysts for their higher activity.We used different characterization methods such as XPS 、 FT-IR 、 XRD to confirm the successfully synthesis of catalyst.Chemical fixation of carbon dioxide（CO₂） to cyclic carbonates were investigated by using MPc/g-C₃N₄（M = Co, Cu） in the absence of any co-catalysts and organic solvents.It was demonstrated that 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） under 130 °C for 24 h,when catalyzed by CoPc/g-C₃N₄-480（ 0.66） and CuPc/g-C₃N₄-480（0.78）respectively, the product yields were 97.6%、93.6%.Above all, the catalyst could be easily separated from the reaction system and reused at least for six times..For bi-functional catalyst MPc/g-C₃N₄, MPc species function as Lewis acidic center for ECH activation via electrophilic attack; while, g-C₃N₄ moiety, possessing abundant and uncondensed species with the forms of primary amine（–NH2） groups and secondary amine（–NH–） groups at the edges of graphitic sheets as edge defects, acts as 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 a facile separation and recycling in CO₂ fixation reaction.