Bifunctional MOF heterogeneous catalysts based on the synergy of dual functional sites for efficient conversion of CO2 under mild and co-catalyst free conditions

Abstract

We reported herein a strategy for combining CUS-based MOF (CUS = coordinatively unsaturated metal sites) with ionic liquid (IL) functional sites to form bifunctional heterogeneous catalysts with extra high activity for CO₂ fixation. Based on this strategy, two quaternary ammonium salt and quaternary phosphorus salt ionic liquid functionalized CUS-containing MOF heterogeneous catalysts, MIL-101-N(n-Bu)₃Br and MIL-101-P(n-Bu)₃Br, have been prepared for the first time by a post-synthesis modification method. Due to the synergetic role of dual functional sites including Lewis acid sites in the MOF framwork and Br⁻ ions in the IL functional sites, MIL-101-N(n-Bu)₃Br and MIL-101-P(n-Bu)₃Br exhibit high catalytic activity for the cycloaddition of CO₂ and epoxide under mild and co-catalyst free conditions, which significantly outperforms other benchmark MOF catalysts. Moreover, such bifunctional catalysts can be easily recovered and recycled several times without leaching and loss of activity. Our work thus paves a way for the development of IL functionalized MOFs as heterogeneous catalysts for CO₂ fixation.