Boosting low-concentration CO2 conversion via heterogeneous catalysts with hydrogen-bonding interactions

To address the challenges associated with global carbon emissions, this study developed a novel class of porous ionic polymer catalysts. Using a bipyridine-based porous organic polymer (POP-Bpy) as the framework, two Ionic liquid-functionalized materials, POP-Bpy-Et and POP-Bpy-OH, were successfully synthesized via quaternization with 1,2-dibromoethane and 2,3-dibromopropanol, respectively. Comprehensive structural characterization (including 13 C NMR, FTIR, and XPS) confirmed the successful quaternization and formation of the desired ionic polymer architectures. Catalytic evaluation revealed that POP-Bpy-OH exhibits outstanding performance in the cycloaddition of CO 2 with epoxides. Under mild conditions (60 °C, 1 atm CO 2 , 48 h), POP-Bpy-OH achieved 94.3% conversion of epichlorohydrin with 99% selectivity toward the corresponding cyclic carbonate, significantly outperforming the hydroxyl-free counterpart POP-Bpy-Et (78.4% conversion). Moreover, under industrially relevant low-CO 2 conditions (15 %CO 2 , 96 h), POP-Bpy-OH maintained a high conversion of 83.3%, far exceeding that of POP-Bpy-Et (64.2%). This enhanced activity is attributed to a pronounced synergistic effect between the hydroxyl groups (acting as hydrogen-bond donors) and the bromide anions (Br − ) in POP-Bpy-OH, which not only boosts reaction efficiency but also affords good substrate compatibility for epoxides with low to moderate steric hindrance. These results underscore that incorporating hydroxyl functionalities into the polymer backbone is a key strategy for improving CO 2 cycloaddition catalysis. In addition to high activity, POP-Bpy-OH demonstrated excellent structural stability and recyclability. After five consecutive reaction cycles, its catalytic activity decreased by only about 3%, highlighting its potential for practical industrial application. To elucidate the origin of its superior performance, density functional theory (DFT) calculations were employed to probe the reaction mechanism. The simulations show that the hydroxyl groups lower the ring-opening energy barrier of the epoxide via hydrogen-bonding interactions, while the adjacent Br − facilitates the nucleophilic attack. This cooperative action efficiently drives the cycloaddition reaction forward, providing an atomic-level understanding of the catalyst's high activity and selectivity.