Design of halide-free and metal-free carbon nitrogen catalyst for the cycloaddition of CO2 into epoxides

• S-doped carbon nitrogen (CNS) catalyst is synthesized from the self-assemble of melem. • CNS catalyst containing Lewis basic and Lewis acid active sites. • The CNS material can be used as an efficient bifunctional catalyst to synthesize cyclic carbonates. • The catalyst is reusable and stable against leaching of active sites in the cycloaddition reaction. The cycloaddition reaction of carbon dioxide (CO 2 ) with epoxides represents one of the most promising industrial pathways for CO 2 conversion. Herein, a novel halide- and metal-free carbon nitrogen (CN) catalyst was synthesized through the self-assembly of an oligomer derived from melamine as a precursor, employing a programmed heating calcination method. This catalytic system effectively circumvents the issue of halogen-induced corrosion in process equipment, which is commonly associated with halogen-based co-catalysts, thereby significantly reducing the environmental burden of halogen waste. The prepared catalyst is characterized using various characterization techniques, such as CO 2 -TPD, TEM and XPS. The characterization results revealed that the CN catalyst contains abundant pyridine nitrogen and hydroxyl species, which function as Lewis basic and Lewis acid active sites to facilitate the cycloaddition reactions of CO 2 . Additionally, Crystal Orbital Hamilton Population (COHP) analysis demonstrated that the CN catalyst exhibits superior activation of epichlorohydrin (ECH) compared to other catalysts, highlighting its enhanced catalytic efficiency. Catalytic performance evaluations demonstrated that the CN-300 catalyst achieved an exceptional cyclic carbonate yield of 99 %. Remarkably, the CN-300 catalyst also exhibited outstanding recyclability, maintaining high catalytic performance even after five consecutive cycles. CNS materials containing both Lewis base sites and Lewis acid sites are prepared from the self-assemble of melem and thiourea by facile pyrolysis method, which are used as efficient bifunctional catalyst in cycloaddition of CO 2 and epoxides.