Mixed-Linker Zr-Metal-Organic Framework with Improved Lewis Acidic Sites for CO₂ Fixation Reaction Catalysis

Applying the mixed-linker strategy in synthesizing metal-organic frameworks (MOFs) has drawn considerable attention as a heterogeneous catalyst owing to their easy synthesis and different functional ligands in their frameworks. Following this strategy, we have developed a mixed linker Zr(IV)-based MOF, [Zr₆O₄(OH)₄(FUM)(PZDC-NO₂)_6-] (PZDC-NO₂ = 4-nitro-3,5-pyrazoledicarboxylic acid, FUM = fumaric acid) denoted as MOF-801(PZDC-NO₂) synthesized via this strategy which possess an electron-withdrawing group (-NO₂) on secondary linkers. The MOF-801(PZDC-NO₂) has been fully characterized via various analyses, such as Fourier transform infrared, powder X-ray diffraction, ¹³C/¹H nuclear magnetic resonance, XPS, TGA, and N₂ adsorption/desorption, SEM, EDX, etc. By considering the concurrent existence of acid-base active sites and the synergistic role of these sites, this mixed-linker MOF was used as a catalyst for the cycloaddition reaction of CO₂ and epoxides under mild without-solvent conditions. MOF-801(PZDC-NO₂) displays significant catalytic performance by producing the highest catalytic conversion of epoxide to cyclic carbonate (93%) with a turnover number of 130.7 in 8 h reaction time and 100 °C temperature under low-pressure CO₂ pressure. The mixed-linker Zr-MOF exhibits exceptional stability and reusability, maintaining its structure and functionality after consecutive cycles of utilization. Finally, the reaction mechanism was further investigated by density functional theory calculations. The total energy of the reactants, intermediates, and products involved in the process.