Copper Phosphate Nanostructures as Catalysts for the Direct Methane Oxidation

Abstract

The development of heterogeneous catalysts for the selective direct transformation of methane (CH 4 ) 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 CH 4 to formaldehyde (HCHO) with molecular oxygen (O 2 ) 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 (Cu 2 P 2 O 7 , Cu 3 (PO 4 ) 2 , Cu 2 (P 4 O 12 ), and Cu 4 O(PO 4 ) 2 ) with different Cu coordination geometries and Cu/P ratios were synthesized from Cu(OAc) 2 •H 2 O and (NH 4 ) 2 HPO 4 , and the dependence of CH 4 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; CH 4 conversion increased with increasing Cu/P molar ratio, although the selectivity to HCHO decreased. Among the investigated Cu-based catalysts and metal phosphate nanoparticles (FePO 4 and BiPO 4 ), monoclinic Cu 2 P 2 O 7 , which has a Cu/P ratio of 1/1, exhibited the highest HCHO yield. The catalytic activity of Cu 2 P 2 O 7 was improved by changing the copper source to Cu(NO 3 ) 2 •3H 2 O 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 Cu 2 P 2 O 7 possibly react with CH 4 to give HCHO as the primary product and (ii) the surface redox-active Lewis acidic Cu 2+ sites and weakly basic phosphate units on Cu 2 P 2 O 7 play important roles in the C−H activation and the suppression of overoxidation to CO 2 , respectively. Density functional theory calculations revealed that the vacancy formation energies at oxygen sites in β-Cu 2 P 2 O 7 , which was formed by the phase transition of α-Cu 2 P 2 O 7 under the catalytic conditions, were lower than those in α-Cu 2 P 2 O 7 . Such a superior oxygen-transfer ability likely contributes to the high catalytic performance and durability of Cu 2 P 2 O 7 for the oxidation of CH 4 to HCHO.