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

A kinetic model predicts the experimental activity and selectivity patterns obtained in oxidation of methane for divided unsupported orthophosphate (MII3(PO4)2, MIIIPO4,) and pyrophosphate (MII2P2O7, MIVP2O7) catalysts spanning extended ranges of M–O bond strength (calculated from first principles) and specific total acidity H0. A volcano curve is obtained when total methane oxidation rates are plotted against M–O bond strength. MIVP2O7 pyrophosphates are the most acidic catalysts (H0 > 10 μmol m–2) and are remarkably selective to formaldehyde (>90%). For this subset of catalysts, however, experimental oxidation rates deviate from predictions in correlation with their acidity, suggesting a significant deactivation phenomenon. H0 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 in...