Tailoring a Pyridylimine-Functionalized Rh–Hydride Single-Site in Metal–Organic Framework UiO-68 for Enhanced CO₂ Hydrogenation to CH₃OH: Mechanistic Insights from DFT Calculations

Metal–organic frameworks (MOFs) have emerged as highly promising candidates for catalytic CO2 conversion into value-added chemicals, owing to their tunable porous architectures and versatile physicochemical properties. Notably, embedding active metal sites into MOF linkers can further enhance the CO2 hydrogenation efficiency by synergistically promoting H2 dissociation and carbon insertion. Herein, we systematically investigated the reaction mechanisms of hydrogenation of CO2 to methanol (CH3OH) on a Rh–hydride single-site catalyst supported by pyridylimine-functionalized UiO-68 (pyrim-UiO-RhH) using density functional theory (DFT) calculations. The results demonstrate that the pyrim-UiO-RhH catalyst exhibits superior catalytic activity with a free energy span of 37.1 kcal/mol for the rate-determining state involving H2 cleavage concurrent with simultaneous HCHO and H2O generation. The modulation of the microstructure environment of the active site primarily influences the frontier orbital energy levels of the catalyst, thereby altering the overall catalytic activity. In particular, the 3-NO2-pyrim-UiO-RhH catalyst, derived from the substitution of an electron-withdrawing group (−NO2) for the H atom at the outer C-site of the pyridylimine ring, exhibits a notably reduced free energy span of 31.0 kcal/mol and 5 orders of magnitude-enhanced turnover frequency of 1.17 × 10–10 s–1 for CH3OH production, compared to the pyrim-UiO-RhH. The present work highlights how precise linker functionalization in MOFs can optimize the active site, offering a viable strategy to boost CO2-to-CH3OH conversion.