Solid-Phase Synthesis of Metal-Free Melamine-Based Catalysts for CO2 Utilization

Recent advancements in heterogeneous catalysis have increased the interest in the synthesis of metal-free polymer-based catalysts. This work presents a novel approach for the solvent- and additive-free synthesis of a nitrogen-rich catalyst. Our unique procedure yields a non-porous organic polymer (NPOP) with a wide range of functional groups on the surface, attributed to the incomplete polymerization inherent to our solvent-free method. Detailed analysis revealed significant differences between NPOP and its Covalent Organic Framework counterpart. Remarkably, the absence of a high surface area did not hinder the efficiency of NPOP as a catalyst for the CO2 cycloaddition. The performance of NPOP exceeded that of its COF counterpart, with a conversion rate of 99% for NPOP and 35% for the COF. An observation attributed to the abundance of nitrogen functional groups on the surface of NPOP. A combination of characterizations and density functional theory (DFT) calculations was employed to thoroughly understand the working mechanism of NPOP. The imines and secondary amines on the surface function as the active sites for the ring-opening of epichlorohydrin. This study supports existing theories that N atoms can serve as nucleophiles by donating their free electron pairs. Furthermore, the distinctive synthesis procedure reported here can serve as inspiration for further design of polymer-based catalysts.