Weak-field ligands enable inert early transition metal oxides to convert methane to methanol: the case of ZrO

Abstract

Zirconium monoxide, ZrO, was studied by multi-reference configuration interaction (MRCI) and coupled cluster methods using large basis sets in conjunction with effective core potentials. Complete potential energy curves were constructed and bonding patterns are proposed for several electronic states. Numerical results include accurate equilibrium bond lengths, harmonic vibrational frequencies, anharmonicities, excitation energies, dipole moments, and binding energies for both ground and excited states. The application of a ZrO unit as the catalytic center for methane activation is explored through the reaction ZrO + CH₄ → Zr + CH₃OH. Optimal density functional structures combined with single-point MRCI energy calculations are obtained for the complete reaction pathway. It is found that the lower energy singlet and triplet multiplicities (oxo states) favor the [2+2] mechanism and the higher energy quintets (oxyl states) favor the radical mechanism, which is overall more efficient in producing methanol. We finally suggest proper ligands that stabilize the oxyl states. These include halogens or other weak-field ligands, which finally convert the inert early transition metal oxide units to efficient methane-to-methanol catalysts.