Catalyst Design for the Selective Oxidation of Methane to Formaldehyde over Metal Phosphates

Abstract

Selective oxidation of methane (CH₄) to formaldehyde (HCHO) using molecular oxygen (O₂) is an attractive route for the direct utilization of abundant natural gas resources but remains challenging because of the inertness of the C–H bond and the ease of subsequent overoxidation of the oxygenated products. This review article summarizes our recent studies on the development of iron, copper, and bismuth phosphate catalysts for the selective oxidation of CH₄ and discusses how catalyst design has evolved through mechanistic understanding of methane C–H bond activation. FePO₄ promotes CH₄ oxidation through a Mars–van Krevelen mechanism involving reactive surface lattice oxygen, whereas the surface acid–base properties suppress the adsorption of oxygenated products. Cu₂P₂O₇ undergoes dynamic structural evolution under reaction conditions which enhances the reactivity of lattice oxygen, leading to improved catalytic performance. In contrast, BiPO₄ follows a fundamentally different reaction pathway, in which surface oxygen species generated in the presence of gaseous O₂, rather than lattice oxygen, are responsible for CH₄ activation. Despite these distinct reaction mechanisms, phosphate incorporation reduces the surface basicity relative to the corresponding metal oxides, so suppressing product adsorption and consecutive overoxidation but maintaining efficient CH₄ oxidation. These findings establish a rational framework for designing highly selective phosphate-based oxidation catalysts and provide mechanistic insights into the selective oxidation of CH₄ and other hydrocarbons containing inert C–H bonds.