Multicomponent Linkage Engineering of Quinoline-Linked Covalent Organic Frameworks for Atmospheric-Pressure Metal-Free CO2 Cycloaddition.

Abstract

Developing efficient metal-free porous catalysts for the cycloaddition of CO2 with epoxides under atmospheric-pressure conditions remains challenging because strong CO2 affinity, efficient epoxide activation, and framework stability are difficult to integrate within a single material. Herein, we report a chemically robust bifunctional quinoline-linked covalent organic framework, Quinoline-COFCOOH, synthesized via a three-component Doebner multicomponent reaction. This approach enables the direct incorporation of CO2-philic triazine units and Brønsted acidic carboxylic acid groups into an ordered porous framework without post-synthetic modification. Owing to the fused quinoline linkage, Quinoline-COFCOOH exhibits high crystallinity, permanent porosity, excellent chemical stability, and a surface area of 1296 m2 g-1. Compared with the imine-linked analogue, Quinoline-COFCOOH shows significantly enhanced CO2 uptake and higher isosteric heat of adsorption (Qst = 43.2 kJ mol-1), indicating strong framework-CO2 interactions. Benefiting from the synergistic effect of triazine units and Brønsted acidic sites, Quinoline-COFCOOH efficiently catalyzes the cycloaddition of epoxides with CO2 under atmospheric-pressure and metal/solvent-free conditions, achieving >99% conversion and selectivity within 6 h. Mechanistic studies and DFT calculations reveal that the carboxylic acid groups promote epoxide activation, while the triazine units facilitate CO2 adsorption and insertion. This work highlights multicomponent linkage engineering for constructing a bifunctional COF for sustainable CO2 conversion.