Melamine-based nitrogen-heterocyclic polymer networks as efficient platforms for CO2 adsorption and conversion

Constructing green and efficient porous material platforms for carbon dioxide (CO2) adsorption and chemical conversion can effectively solve a series of environmental problems caused by the greenhouse effect. Herein, four melamine-based polymer networks PANs (PAN-P, PAN-PY, PAN-PZ, and PAN-IM) with abundant N sites and hydrogen bond donor (HBD) groups were synthesized by simple polymerization. The chemical structure of PANs was determined by various characterizations, and then the CO2 uptake of PANs and their catalytic activity for the cycloaddition of CO2 with epoxides were investigated. Among them, PAN-P has the largest CO2 adsorption capacity, reaching values of 73.5 cm3/g at 273 K. Meanwhile, PAN-IM exhibits the best catalytic activity with 93 % yield and 99 % selectivity under metal-, halogen-, solvent-, and cocatalyst-free conditions (120 ◦C, 1.0 MPa CO2, 8 h), which mainly attribute to the multi-site synergistic activation of the imidazole heterocycle. In addition, PAN-IM also has satisfactory activity to various epoxides and exhibited good recyclability. Density functional theory (DFT) and FT-IR analysis were used to further reveal the interaction of PANs towards CO2 and epoxides. Finally, a possible mechanism was suggested, including activation of CO2 and epoxide, ring-opening of epoxide, insertion of CO2 and ring-closing to form cyclic carbonate. The research in this paper will provide a reference for the development of a green and efficient CO2 adsorption conversion platform.