Carbon neutral via catalytic transformation of CO2 into cyclic carbonates by an imidazolium-based ionic zeolitic imidazolate frameworks
On the way toward a carbon–neutral future, anthropogenic chemical recycling of captured carbon dioxide (CO 2 ) into value-added chemicals and reusable materials provides an attractive and promising pathway. A multifunctional zeolitic imidazolate framework (ZIF-9-ImBr) consisting of electrophilic cobalt ions, CO 2 -philic nitrogen-rich units and nucleophilic imidazolium-based ionic liquids (ILs) was synthesized by simple substitution reaction of amino-functionalized ZIF (ZIF-9-NH 2 ) with 1-(4-bromobutyl)-3-methylimidazolium bromide ([BrBMIM]Br). The obtained ZIF-9-ImBr was employed as heterogeneous catalyst for the solvent-free synthesis of cyclic carbonates from CO 2 and epoxides in the absence of any cocatalyst. A certain amount of micropores and abundant nitrogen atoms could facilitate CO 2 enrichment and its activation, while the mesopores could enhance the mass transfer of reactant and product molecules. Moreover, the synergistic effect between Lewis-acid cobalt and Lewis-base bromine ions of ZIF-9-ImBr made it promising in the CO 2 cycloaddition reaction. As expected, ZIF-9-ImBr displayed good catalytic performance of 93.8 % yield and 98.9 % selectivity of propylene carbonate at 120 °C and 2.0 MPa for 3.5 h. Additionally, the reactions proceeded with a broad substrate scope and high selectivity to give the corresponding cyclic carbonates in good to excellent yields (77.9–95.8 %). Even for the challenging internal epoxide, cyclohexene oxide, a satisfactory 71.4 % yield was obtained by extending the reaction time to 24 h. More importantly, ZIF-9-ImBr can be easily recycled and maintains its reactivity and selectivity after five successive cycles. This work highlights the potential of functional ZIF-based materials for targeting CO 2 capture and conversion.
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