Tailoring the Active-Site Spacing of a Single-Atom Catalyst for CH₄-to-CH₃OH Conversion: The Co₁/UiO-66 MOF as an Exemplary Model

Abstract

Developing catalysts for the mild and selective oxidation of methane to methanol (CH4-to-CH3OH) is a challenging yet crucial endeavor for the industry. In this study, we propose a strategy to enhance the selective methane oxidation reaction, focusing on the crucial step of metal-oxo active-site formation. Our findings demonstrate that optimizing the spacing between active metal sites can facilitate efficient charge transfer from the metal sites to O2, thereby kinetically enhancing O2 activation and leading to the formation of highly reactive O (oxo) species capable of activating the methane C–H bond. Through the screening of different metals at varying metal site spacings, we find that the Co single atom exhibits favorable properties and performance, characterized by the duality of a low O2 activation energy and the radical character of the formed metal-oxo species. Utilizing a Co single-atom catalyst dispersed on the UiO-66 MOF as a probe catalyst, we have showcased the potential of tailoring the active-site spacing to enhance the activity and selectivity toward methane oxidation to methanol. Notably, the newly designed catalyst surpasses the activity of a known catalyst, achieving an observable turnover frequency (>1 s–1 site–1) at 350 K while also enhancing its selectivity by inhibiting continuous methane dehydrogenation. This strategic approach could provide valuable insights for further exploration of MOF-supported single-atom catalysts or other support morphologies for the selective oxidation of CH4 to CH3OH with excellent activity in the gas phase.