A catalytic approach of blending CO2-activating MOF struts for cycloaddition reaction in a helically interlaced Cu(II) amino acid imidazolate framework: DFT-corroborated investigation

Abstract

In CO2 transformation catalysis, the synthesis of cyclic carbonates using two classes of MOF catalysts viz., zeolitic imidazolate frameworks (ZIF) and MOFs with carboxylate-capped SBUs have gained large attention. Herein we propose the strategy of employing a unified multifunctional framework formed in the metal-centered assembly of imidazole and amino-carboxylates for CO2 transformation, such as propylene carbonate (PC) by the cycloaddition of CO2 with propylene oxide. The framework {[Cu(L-asp)(1,4-bix)(0.5)]center dot 3H(2)O}(n) (CuAspBix) comprises of the amino acid building units, L-aspartic acid (L-Asp) and the flexible ligand, 1,4-bis(imidazole-1-yl methyl)benzene [1,4-Bix]. The 1,4-Bix ligand with imidazole terminals renders elongated M-M distances and flexibility in comparison with pristine ZIF materials. The cumbersome synthesis procedure poor phase purity of the bulk catalyst in solvothermal conditions were improved by a microwave-assisted synthesis, preserving the structural and physicochemical properties. Minimal energy input or room temperatures for the catalysis occurred via the synergistic participation of CuAspBix and quaternary ammonium bromide salt, demonstrated by density-functional theory computational studies to propose mechanistic pathway of the reaction. Reaction conditions were optimized by altering the parameters. The heterogeneous catalyst was reused four times without a significant change in activity.