Copper Phosphate Nanostructures as Catalysts for the Direct Methane Oxidation

The development of heterogeneous catalysts for the selective direct transformation of methane (CH4) remains a challenge because of the difficulty in activating the strong C–H bond and controlling selectivity to target products. The effect of various metal phosphate catalysts (37 examples) on the direct oxidation of CH4 to formaldehyde (HCHO) with molecular oxygen (O2) as the sole oxidant was studied using a fixed-bed flow reactor, and the effectiveness of the copper phosphate catalysts was confirmed. Four crystalline copper phosphates (Cu2P2O7, Cu3(PO4)2, Cu2(P4O12), and Cu4O(PO4)2) with different Cu coordination geometries and Cu/P ratios were synthesized from Cu(OAc)2·H2O and (NH4)2HPO4, and the dependence of CH4 oxidation on their structures, as well as that on the structure of CuO, was investigated. The Cu/P molar ratio strongly affected the oxidation catalysis; CH4 conversion increased with increasing Cu/P molar ratio, although the selectivity to HCHO decreased. Among the investigated Cu-based catalysts and metal phosphate nanoparticles (FePO4 and BiPO4), monoclinic Cu2P2O7, which has a Cu/P ratio of 1/1, exhibited the highest HCHO yield. The catalytic activity of Cu2P2O7 was improved by changing the copper source to Cu(NO3)2·3H2O due to the surface nanostructure control. On the basis of mechanistic studies that include catalyst effect, kinetics, isotope-labeling, and pulse reaction experiments, as well as infrared spectroscopic analyses of adsorbed probe molecules, (i) surface lattice oxygen species of Cu2P2O7 possibly react with CH4 to give HCHO as the primary product and (ii) the surface redox-active Lewis acidic Cu2+ sites and weakly basic phosphate units on Cu2P2O7 play important roles in the C–H activation and the suppression of overoxidation to CO2, respectively. Density functional theory calculations revealed that the vacancy formation energies at oxygen sites in β-Cu2P2O7, which was formed by the phase transition of α-Cu2P2O7 under the catalytic conditions, were lower than those in α-Cu2P2O7. Such a superior oxygen-transfer ability likely contributes to the high catalytic performance and durability of Cu2P2O7 for the oxidation of CH4 to HCHO.