Mixed-Linker\nZr-Metal–Organic Framework with\nImproved Lewis Acidic Sites for CO<sub>2</sub> Fixation Reaction Catalysis

Applying the mixed-linker strategy in synthesizing metal–organic\nframeworks (MOFs) has drawn considerable attention as a heterogeneous\ncatalyst owing to their easy synthesis and different functional ligands\nin their frameworks. Following this strategy, we have developed a\nmixed linker Zr(IV)-based MOF, [Zr<sub>6</sub>O<sub>4</sub>(OH)<sub>4</sub>(FUM)<sub><i>n</i></sub>(PZDC-NO<sub>2</sub>)<sub>6‑<i>n</i></sub>] (PZDC-NO<sub>2</sub> = 4-nitro-3,5-pyrazoledicarboxylic\nacid, FUM = fumaric acid) denoted as MOF-801(PZDC-NO<sub>2</sub>)\nsynthesized via this strategy which possess an electron-withdrawing\ngroup (–NO<sub>2</sub>) on secondary linkers. The MOF-801(PZDC-NO<sub>2</sub>) has been fully characterized via various analyses, such\nas Fourier transform infrared, powder X-ray diffraction, <sup>13</sup>C/<sup>1</sup>H nuclear magnetic resonance, XPS, TGA, and N<sub>2</sub> adsorption/desorption, SEM, EDX, etc. By considering the concurrent\nexistence of acid–base active sites and the synergistic role\nof these sites, this mixed-linker MOF was used as a catalyst for the\ncycloaddition reaction of CO<sub>2</sub> and epoxides under mild without-solvent\nconditions. MOF-801(PZDC-NO<sub>2</sub>) displays significant catalytic\nperformance by producing the highest catalytic conversion of epoxide\nto cyclic carbonate (93%) with a turnover number of 130.7 in 8 h reaction\ntime and 100 °C temperature under low-pressure CO<sub>2</sub> pressure. The mixed-linker Zr-MOF exhibits exceptional stability\nand reusability, maintaining its structure and functionality after\nconsecutive cycles of utilization. Finally, the reaction mechanism\nwas further investigated by density functional theory calculations.\nThe total energy of the reactants, intermediates, and products involved\nin the process.