Covalent Immobilization of Quaternary Ammonium Salts on Covalent Organic Framework: Sustainable Intensification Strategy for the Synthesis of Cyclic Carbonates from CO₂

Abstract

Sustainable metal-free catalytic conversion of carbon dioxide (CO2) via cycloaddition of epoxides with CO2 has shown great promise but suffers from a lack of recyclability because of the homogeneous nature, limiting their use. Heterogeneous organocatalysts have gained immense attention in the last decade because of superior application potential and their important characteristics, and they also play pivotal roles in making environmentally friendly processes a reality. Herein, we describe an unprecedented postsynthetic modification approach for efficient covalent immobilization of quaternary ammonium salts to a microporous covalent organic framework (COF). More interestingly, no noticeable loss in crystallinity occurred after postsynthetic modification (PSM) and the quaternary ammonium salt-decorated COF (MA-PDA IL@COF) consists of only micropores (around 6–15 Å), which are smaller than most of the reported COF-based catalysts. Detailed investigations on CO2 chemical fixation reveal that ionic liquid-based COF is a promising metal-free catalyst to promote the coupling of CO2 with epoxides under very mild conditions (metal-free/solvent-free/cocatalyst-free/additive-free and 1 atm of CO2 pressure). The metal-free COF displayed quantitative selectivity, and more intriguingly, the cycloaddition reaction with CO2 occurred with a high efficiency, broad scope, and functional group tolerance without additives or cocatalysts. The catalytic system can be recovered for repeated use at least five times with almost similar catalytic performance and a promising prerequisite for industrial implementation.