Diffusion Monte Carlo Study of Methane-to-Methanol Conversion on Fe– and Mn–MOF Catalysts

The direct partial oxidation of methane to methanol is an important catalytic challenge, and metal–organic frameworks (MOFs) with open transition-metal sites have emerged as promising candidates for this reaction. However, the complex electronic structure of their reactive intermediates often limits the reliability of standard density functional theory (DFT) predictions. In this work, we use fixed-node diffusion Monte Carlo (FNDMC) to establish accurate reaction energetics for the complete methane-to-methanol conversion pathway using two representative cluster models, Fe2–BTC(Mg) and Mn2–HTPA(Mg). These models represent an Fe-based MOF constructed from 1,3,5-benzenetricarboxylate (BTC) linkers and an Mn-based MOF constructed from tris((4-carboxyl)phenylduryl)amine (HTPA) linkers, respectively. FNDMC predicts mildly endothermic O atom adsorption on both systems, with adsorption energies of 0.66(5) and 0.71(4) eV for Fe2–BTC(Mg) and Mn2–HTPA(Mg), respectively. For Fe2–BTC(Mg), FNDMC identifies a septet ground state for the oxygen-bound intermediate, consistent with Fe(III)–oxyl character, whereas all tested DFT functionals favor a quintet state. FNDMC predicts that the C–H activation step is more exothermic for Fe2–BTC(Mg), with a reaction energy of −0.51(6) eV, than for Mn2–HTPA(Mg), for which it is only slightly exothermic at −0.16(6) eV. Methanol desorption is 0.97(7) eV more endothermic on Mn2–HTPA(Mg) than on Fe2–BTC(Mg), indicating stronger product retention despite their similar O atom adsorption energies. These results show that FNDMC captures differences in reaction energetics and electronic structure that standard DFT may miss, providing a robust reference for reactive MOF intermediates.