Selective Oxidation of Methane to Formaldehyde Catalyzed by Phosphates: Kinetic Description by Bond Strengths and Specific Total Acidities

Overview & Synopsis

A kinetic model predicts the experimental activity and selectivity patterns obtained in oxidation of methane for divided unsupported orthophosphates (M II 3(PO4)2, M III PO4,) and pyrophosphates (M II 2P2O7, M IV P2O7) catalysts spanning extended ranges of M-O bond strength (calculated from first principles) and specific total acidity H 0. A volcano curve is obtained when total methane oxidation rates are plotted against M-O bond strength. M IV P2O7 Pyrophosphates are the most acidic catalysts (H 0 >10 µmol.m-2) and remarkably selective to formaldehyde (>90%). For this subset of catalysts, experimental oxidation rates deviate however from predictions in correlation with their acidity, suggesting a significant deactivation phenomenon. H 0 is shown to be described, and therefore predictable for these phosphates, by a structural parameter, the average P-Oa bond length, Oa being a bridging atom between M and P. Differences in acidity are interpreted as a consequence of the compression or elongation of the Oa-P bond inside the M-Oa-P bridges.