Spaced functionalization of poly(ionic liquid)s for boosting the catalytic conversion of CO2 into cyclic carbonates

Abstract

Preparation of cyclic carbonates from CO₂ and epoxides via cycloaddition is a well-established synthetic route, which is not only economical but also in line with the atomic economy. To overcome the problem of the cluster formation of hydrogen-bond donors in functionalized poly(ionic liquid)s, which reduces their catalytic activity, a series of spacer-functionalized poly(ionic liquid) catalysts were developed. In poly(ionic) liquids, the hydrogen-donating effect enhances the intrinsic catalytic performance of the active sites and the long-chain alkyl groups prevent interactions between hydrogen-bond-donor groups, thus increasing the utilization of the active sites. Among the developed poly(ionic liquid) catalysts, the poly(ionic liquid) P[AC₁₂VIM][Br] containing amino groups demonstrated the highest catalytic activity (propylene oxide conversion up to 99%), which was comparable with that of bulky ILs. The best catalytic performance of P[AC₁₂VIM][Br] was attributed owing to its multiple functions in not only activating CO₂ and epoxides but also stabilizing Br⁻. Furthermore, the amphiphilicity of amino-functionalized poly(ionic liquid)s boosted their catalytic suitability for epoxide substrates with lipophilic edge groups, which was better than that of conventional poly(ionic liquid)s. Through XPS and NMR analyses, a mechanism of operation is proposed in which imidazole and hydrogen donor groups act co-operatively in epoxy during the reaction to assist in ring-opening. Thus, this study provides a new approach for improving the catalytic performance of poly(ionic liquid) catalysts from the viewpoint of an intrinsic reaction and utilization of the active sites.