Metal–Organic\nFramework-Encaged Monomeric Cobalt(III)\nHydroperoxides Enable Chemoselective Methane Oxidation to Methanol

Abstract

Developing highly efficient catalysts for chemoselective\noxidation\nof methane to methanol under mild conditions is a grand challenge.\nWe report the successful design and synthesis of a heterogeneous single-site\ncobalt hydroxide catalyst [Ce-UiO-Co(OH)] supported by the nodes of\na cerium metal–organic framework (Ce-UiO-66 MOF), which is\nefficient in partial methane oxidation using hydrogen peroxide at\n80 °C, giving an extraordinarily high methanol yield of 2166\nmmol g_cat^–1 in 99% selectivity with a\nturnover number of 3250. The Ce-UiO-Co catalyst is significantly more\nactive and selective than its iso-structural zirconium analogue Zr-UiO-Co\nin methane to methanol conversion. Experimental and computational\nstudies suggest the formation of the Co^III(η²-hydroperoxide) intermediate coordinating with one μ₄-O^– and two neutral carboxylate oxygens\nof Ce⁴⁺ oxo nodes within the pores of Ce-UiO-66, which\nundergoes σ-bond metathesis with the methane C–H bond\nin the turnover limiting step of the catalytic cycle. The significantly\nlower activation energy of Ce-UiO-Co than Zr-UiO-Co is due to the\nhighly electron-deficient nature of the cobalt ion of the Co(η²-O₂H) species supported by the Ce-UiO nodes, which\npromotes facile C–H activation of methane via σ-bond\nmetathesis. This MOF-based catalyst design holds promise in developing\nmolecular electrophilic abundant metal catalysts for chemoselective\nfunctionalization of saturated hydrocarbons.