Metal–Organic Framework-Encaged Monomeric Cobalt(III) Hydroperoxides Enable Chemoselective Methane Oxidation to Methanol
Neha Antil, Manav Chauhan, Naved Akhtar, Rajashree Newar, Wahida Begum, Jaideep Malik, Kuntal Manna
Abstract
Developing highly efficient catalysts for chemoselective oxidation of methane to methanol under mild conditions is a grand challenge. We report the successful design and synthesis of a heterogeneous single-site cobalt hydroxide catalyst [Ce-UiO-Co(OH)] supported by the nodes of a cerium metal–organic framework (Ce-UiO-66 MOF), which is efficient in partial methane oxidation using hydrogen peroxide at 80 °C, giving an extraordinarily high methanol yield of 2166 mmol gcat–1 in 99% selectivity with a turnover number of 3250. The Ce-UiO-Co catalyst is significantly more active and selective than its iso-structural zirconium analogue Zr-UiO-Co in methane to methanol conversion. Experimental and computational studies suggest the formation of the CoIII(η2-hydroperoxide) intermediate coordinating with one μ4-O– and two neutral carboxylate oxygens of Ce4+ oxo nodes within the pores of Ce-UiO-66, which undergoes σ-bond metathesis with the methane C–H bond in the turnover limiting step of the catalytic cycle. The significantly lower activation energy of Ce-UiO-Co than Zr-UiO-Co is due to the highly electron-deficient nature of the cobalt ion of the Co(η2-O2H) species supported by the Ce-UiO nodes, which promotes facile C–H activation of methane via σ-bond metathesis. This MOF-based catalyst design holds promise in developing molecular electrophilic abundant metal catalysts for chemoselective functionalization of saturated hydrocarbons.