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.