Urea/amide-functionalized melamine-based organic polymers as efficient heterogeneous catalysts for CO2 cycloaddition

The development of highly efficient and metal- and halogen-free heterogeneous catalysts for the chemical conversion of CO2 to cyclic carbonates plays a critical role in the reduction of greenhouse gas emissions. Herein, two melamine-based organic polymers namely UM-OP and AM-OP with abundant N sites and –NH- groups were synthesized by a facile method and their activity for cycloaddition of CO2 with epoxides was investigated. The structural analysis shows that the UM-OP and AM-OP catalysts were composed of specially designed urea and amide group linkages, respectively. UM-OP and AM-OP are metal-free, halogen-free, and can efficiently catalyze CO2 cycloaddition under solvent-free and cocatalyst-free conditions. Moreover, the UM-OP catalyst exhibited higher catalytic activity than AM-OP, good structural stability, recyclability, separability from the products, and shown high activity to a series of epoxides. The density functional theory (DFT) calculations revealed the activation of epoxides by –NH- groups and CO2 by N sites in the samples, respectively, and N sites can also act as nucleophiles to realize ring opening of epoxides. Combining the results of experiments and calculations, it was further found that the double –NH- (urea group) is more efficient than single –NH- (amide group) to activate the epoxides. This paper reveals the mechanism of N sites and –NH- groups synergistically catalyzing the CO2 cycloaddition, and further supports the theory that N atoms acts as nucleophile for ring opening, providing a reference for the design of metal- and halogen-free heterogeneous catalysts.