Facile integration of hydroxyl ionic liquid into Cr-MIL-101 as multifunctional heterogeneous catalyst for promoting the efficiency of CO2 conversion

Rational integration of functional ionic liquids (ILs) into metal-organic frameworks (MOFs) is an intriguing and challenging strategy for design of fully heterogeneous catalytic systems for the synthesis of cyclic carbonates from cycloaddition of CO2 and epoxides. This study demonstrates a facile and versatile post-synthetic modification approach for incorporating hydroxyl-functionalized imidazolium IL into Cr-MIL-101 (HImBr@Cr-MIL-101) via coordinative bonding interaction between Cr CUS (coordinative unsaturated metal sites) in MOF and electron-rich –SH group in IL. The resultant HImBr@Cr-MIL-101 possesses high surface area, profitable porosity and good CO2 adsorption capacity, rendering it as an efficient heterogeneous catalyst for the cycloaddition reaction under co-catalyst and solvent free conditions. Compared to either pure Cr-MIL-101 or hydroxyl IL, HImBr@Cr-MIL-101 shows largely enhanced catalytic activity, attributing to the synergistic effect of the integrated multiple active sites, involving Cr CUS as Lewis acid, hydroxyl group as hydrogen bond donor, and bromide anion as nucleophilic reagent for ring-opening of epoxide, as well as tertiary N atom as Lewis basic site for activating CO2. This is also evidenced by the activation energy Ea catalyzed by the Cr-MIL-101-based catalysts with different functions. Notably, HImBr@Cr-MIL-101 also significantly outperforms other benchmark MOF-based catalysts. Moreover, benefited from the superb coordinative bonding interaction of the built Cr (III)–S bond, this catalyst exhibits good chemical stability and recyclability, which can be reused for at least seven runs without obvious catalytic activity depletion. This work provides a promising strategy for constructing multifunctional heterogeneous catalyst for CO2 cycloaddition reaction by highlighting the synergistic effect enabled by one host.