Dual-active-site engineering of urea-based ionic polymer towards enhanced synergistic catalytic performance of carbon dioxide fixation

Abstract

Bifunctional catalysts featuring synergistic hydrogen-bond donor (HBD) and halogen anion nucleophile functionalities hold substantial promise for the catalytic conversion of carbon dioxide (CO₂). However, the rational design of such solid catalysts at the molecular level is yet to be comprehensively elucidated. In this proof-of-concept study, we synthesized a series of porous ionic polymers (PIPs). These PIPs integrate urea groups and pyridinium bromide as cooperative moieties to promote the conversion of CO₂ with epoxides. Through meticulous adjustment of the chemical structure of urea-based monomers, we were able to regulate the relative spatial positioning between neighboring urea and pyridinium salt units within the PIPs. Experimental results revealed that the spatial arrangement of these partners within PIPs not only influences the spatial distance between dual-active sites, but also affects the HBD ability of the urea groups, which subsequently impacts catalytic performance. The optimal catalyst, UPIP-1, distinguished by the placement of the urea group in the ortho-position of the pyridinium salts, showcased the highest HBD ability and the closest dual-active-site distance, resulting in the highest catalytic activity. Specifically, the activity of UPIP-1 was 2.1-fold higher than that of PIPs with the longest dual-site distance and 2.4-fold higher than that of PIPs lacking HBD functionalities. Moreover, UPIP-1 displayed exceptional catalytic performance, broad substrate tolerance, high stability, and recyclability under mild conditions. As a highly effective solid catalyst, UPIP-1 also efficiently catalyzed the cycloaddition reaction, achieving a remarkable yield of 94.3 % under simulated flue gas conditions (15 % CO₂ and 85 % N₂). This study elucidates the molecular-level correlation between spatial arrangement of cooperative sites and catalytic performance in urea-based bifunctional systems, thereby offering a valuable reference for the rational design of high-efficiency bifunctional catalysts for CO₂ conversion.