METHANE PARTIAL OXIDATION OVER VANADYL PYROPHOSPHATE AND THE EFFECT OF FE AND CR PROMOTERS ON SELECTIVITY

Partial oxidation of methane by molecular oxygen over Cr-and Fe-promot ed, as well as unpromoted, vanadyl pyrophosphate catalysts has been st udied in the temperature range of 573-698 K and atmospheric pressure. Carbon monoxide, carbon dioxide, and water were found to be the princi pal reaction products over unpromoted vanadyl pyrophosphate, Over both the Fe-acid Cr-promoted catalysts high formaldehyde selectivity was o bserved at very low methane conversion levels with HCHO space time yie lds in the range of 0.5 to 2.0 g/kg-h. As the extent of the reaction w as increased selectivity to formaldehyde decreased rapidly and changes in selectivities with conversion indicate a sequential conversion of methane to formaldehyde, CO, then CO2, Macrokinetic parameters for the methane oxidation reaction were determined for the unpromoted catalys t. A simple power law rate expression fit the rate data well over the whole temperature range. The rate of reaction of methane was 0.08 +/- 0.02 order in oxygen partial pressure, 0.73 +/- 0.07 order in methane partial pressure, and the activation energy was found to be 102 +/- 6 kJ/mole. Activation energy was unchanged by promotion with Fe and Cr. Analysis by XPS and ICP-kcl indicates that promoters were incorporated equally into the bulk and surface of these catalysts, XPS indicates a n increase in the average surface oxidation state of vanadium in the p romoted catalysts and XRD shows that promotion causes oxidation of a s mall fraction of the pyrophosphate to form alpha(n)-VOPO4. P-31 NMR sp in-echo mapping confirms the enhanced formation of V5+ in the promoted samples. The presence of V5+ may therefore be required for the format ion of selective products. It is proposed that the role of promoters i s to increase the rate of formation of V5+ during activation, or to st abilize V5+ containing domains under the highly reducing methane oxida tion conditions. (C) 1997 Academic Press.