Quaternary ammonium salt grafted nanoporous covalent organic polymer for atmospheric CO2 fixation and cyclic carbonate formation

Abstract

Non-redox carbon dioxide utilization through cycloaddition of CO2 to epoxides offers great promise but suffers from lack of heterogeneous catalysts that don't need additives or pressure. Here we report a systematic postsynthetic modification procedure on a highly porous hydrocarbon framework for efficient grafting of quaternary ammonium salts. The active sites were tuned and characterized while maintaining the porous structure. The metal free and cost-effective catalysts showed quantitative selectivity and very high conversion yields in atmospheric pressure catalysis for cyclic carbonate formation from CO2 and epoxides. The reaction proceeded without additives or co-catalysts, and tolerant for a wide substrate scope. We found that the steric hindrance of the alkyl units on ammonium salts affect microporosity, CO2 binding and the kinetics of cycloaddition reactions. The catalysts were also recyclable, an attractive prerequisite for industrial implementation.