Carbon pathways for the partial oxidation of methane

Abstract

The partial oxidation of methane to formaldehyde was studied at 1-atm pressure using a 2% V[sub 2]O[sub 5]/SiO[sub 2] catalyst with molecular oxygen as an oxidant. As methane conversion increased, formaldehyde selectivity decreased rapidly while CO[sub x] selectivity increased, indicating that formaldehyde was a primary product of the reaction, while CO[sub x] formation occurred apparently in large part by secondary reaction. Using steady-state isotopic transient kinetic analysis technique, carbon transients were obtained by switching from [sup 12]CH[sub 4] to [sup 13]CH[sub 4] at steady state, permitting the determination of surface residence times and surface concentrations of various carbon-containing intermediates. It was possible to observe detectable reversible chemisorption of methane on the catalyst studied and other reference oxides only at conditions where significant reaction took place. For the partial oxidation of methane over the 2% V[sub 2]O[sub 5]/SiO[sub 2] catalyst, it was found that HCHO and CO did not exhibit significant reversible readsorption under the reaction conditions used. It is suggested that the formation of CO involves a sequential pathway wherein an irreversible readsorption of HCHO is involved. A carbon pathway consistent with the observations is proposed. It is speculated that different types of sites involved in the formation of HCHO,more » CO and C[sub 2]H[sub 6] may correspond to vanadium oxide species in different redox states. 30 refs., 6 figs., 5 tabs.« less