Ionic Liquid Microenvironment Engineering in HKUST-1 for Efficient Photothermal CO₂ Cycloaddition

A novel composite catalyst for photothermal CO₂ cycloaddition was developed by integrating the ionic liquid 1-ethylpyridinium bromide (EPB) with a copper-based metal-organic framework (HKUST-1). HKUST-1 was synthesized via a hydrothermal method and functionalized with EPB through a wet impregnation strategy to enhance its catalytic performance. Under xenon lamp irradiation and optimized conditions (80 °C, 1 MPa CO₂ pressure, 12 h, and 0.07% mol of TBAB bromide as a co-catalyst), the HK@EPB composite exhibited outstanding performance in catalyzing the conversion of CO₂ and various epoxides into cyclic carbonates. The exceptional catalytic activity arises from a synergistic multicomponent mechanism: the incorporation of EPB not only enhances CO₂ adsorption capacity but also provides photothermal energy for the reaction; simultaneously, EPB dissociates bromide ions to effectively initiate epoxide ring-opening. In particular, propylene oxide achieved a selectivity of 95% for the desired cyclic carbonate, surpassing most previously reported MOF-based catalysts. This system enables efficient catalysis under mild conditions through the synergistic contributions of the high CO₂ adsorption capacity and Cu²⁺/Cu⁺ redox-mediated electron transfer of HKUST-1, the provision of nucleophilic Br-species from EPB to promote epoxide ring-opening, and the cooperative effect of TBAB. This study demonstrates that ionic-liquid-functionalized MOF composites can serve as sustainable and versatile catalytic platforms, offering an environmentally friendly pathway for large-scale CO₂ utilization.