Boosting CO₂ conversion to cyclic carbonates: Bimetallic Ni/Zn-PTA MOFs composite with Mim-6 ionic liquid for solvent-free catalysis

The escalating atmospheric CO₂ concentration necessitates its conversion into highvalue- added products like cyclic carbonates, yet existing catalysts face challenges such as poor recoverability (homogeneous ionic liquids, ILs) and insufficient activity (monometallic metal–organic frameworks, MOFs). To address this, a series of bimetallic Ni_x/Zn_1-x-PTA MOFs with different Ni/Zn molar ratios were synthesized via a solvothermal method, and then composite catalysts (Ni_x/Zn_1-x-PTA@Mim-6) were prepared by impregnating the MOFs with Mim-6 ILs. This work provides a promising approach for efficient and sustainable CO₂ capture and conversion by integrating the advantages of bimetallic MOFs (high porosity, recyclability) and ILs (high catalytic activity). Characterizations via FT-IR, XRD, TEM, STEM-EDS, and TGA confirmed the successful synthesis, well-defined structure (e.g., nanoflower-like morphology of Ni_0.2/Zn_0.8-PTA MOFs), homogeneous elemental distribution, and good thermal stability of the composites. Catalytic tests showed that Ni_0.2/Zn_0.8-PTA@Mim-6 exhibited optimal performance: under solvent-free and co-catalyst-free mild conditions, the yield and selectivity of styrene carbonate both exceeded 99%, which was significantly superior to monometallic Ni-PTA@Mim-6 (46% yield) and Ni_0.2/Zn_0.8- PTA MOFs with co-catalyst (76% efficiency). The catalyst also demonstrated excellent recyclability (no significant activity loss after 5 cycles, conversion 69% and selectivity 99%) and universality for various epoxides (e.g., high yields for 1,2-butylene oxide and epichlorohydrin). Mechanistic studies revealed that the synergistic effect of multiple active sites—Lewis acid sites (unsaturated Ni/Zn) for epoxide activation, Lewis base sites (O in Ni-O/Zn-O) for CO₂ activation, and Br^- from Mim-6 for ring opening—lowered the reaction energy barrier (activation energy 35.80 kJ/mol) and enhanced catalytic efficiency.