Ionic Liquid-Functionalized Covalent Organic Frameworks for CO2 to Cyclic Carbonate Conversion under Solvent, Metal, and Co-Catalyst Free Conditions

The efficient and sustainable conversion of carbon dioxide (CO2) into value-added chemicals represents a promising approach for addressing environmental and energy challenges. Herein, we report the design and synthesis of a series of ionic liquid-functionalized covalent organic frameworks, [DD]X%-PB-COF (X = 25, 50, 75, 100), via postsynthetic modification of hydroxyl-containing COFs using triethylenediamine-based ionic liquids. The resulting materials exhibit high crystallinity, tunable active site density, and well-defined one-dimensional channels conducive to CO2 mass transport. Among the series, [DD]75%-PB-COF showed the highest catalytic activity for the cycloaddition of CO2 with epoxides under metal-free, solvent-free, cocatalyst-free, and pressure-free conditions, affording cyclic carbonates in excellent yields with >99% selectivity. The catalyst demonstrated broad substrate scope, including aliphatic, aromatic, and functionalized epoxides, and retained its activity and structural integrity over multiple recycling cycles. Mechanistic studies and in situ IR analysis revealed a synergistic dual-center catalytic mode, involving both electrophilic activation and nucleophilic ring-opening, enabled by the ionic liquid moieties and the COF architecture. This work presents a robust and recyclable heterogeneous catalyst platform for green CO2 fixation, offering new insights into the integration of ionic liquids and crystalline porous frameworks for sustainable catalysis.