Rational engineering of single-component heterogeneous catalysts based on abundant metal centers for the mild conversion of pure and impure CO2 to cyclic carbonates

Abstract

• Novel, inexpensive heterogeneous catalysts for cyclic carbonate synthesis were prepared by readily accessible strategies. • The catalysts enabled the cocatalyst-free cycloaddition of CO 2 to several epoxides under mild conditions. • Besides pure CO 2 , diluted and impure CO 2 could successfully be used as feedstocks. • Recyclability and deactivation of the catalysts were studied. The development of fully heterogeneous catalysts for the widely investigated cycloaddition reaction of CO 2 to epoxides is crucial to achieve the sustainable synthesis of cyclic organic carbonates as useful emerging intermediates and building blocks. Nevertheless, relatively few fully heterogeneous catalytic systems exist for the title reaction with the ability to convert CO 2 to carbonates under atmospheric pressure and mild temperatures especially when using diluted and/or impure CO 2 as found in real case scenarios. Moreover, such catalysts are generally constituted by organic or metal–organic frameworks whose preparation generally involves the use or synthesis of expensive building blocks. In this work, we devised a convenient strategy to prepare single-component heterogeneous catalysts based on readily available metal halides and quaternary ammonium halide-based ionic liquids that started from a systematic evaluation of the corresponding homogeneous binary catalysts. This was followed by the preparation of silica-supported heterogeneous systems with different loadings of both catalytic components whose nature and composition were carefully studied through several characterization techniques (elemental analysis, FT-IR, ICP-OES, SEM and TEM microscopies, TGA analysis and XPS). The thus prepared catalysts showed the ability to carry out the cycloaddition of CO 2 to several epoxide under atmospheric pressure at mild temperatures (25–40 °C) including when using diluted CO 2 mixtures resembling low-calorific landfill gas with or without poisoning additives (H 2 S). Finally, the fate of the catalysts after reaction and their deactivation through the catalytic cycles is discussed.