Fe---P---0 catalysts for methane utilization--catalyst development and identification

Abstract

Fe---P---0 catalyst formulations show great promise for oxidative coupling of methane. They attain yields of C2 hydrocarbons comparable to the best in the literature at a temperature which is far below those reported. The catalysts are active in both supported (on Cab-O-Sil) and unsupported (bulk) forms. Both these forms are active around Fe: P ratios of 0.6, suggesting that the pathway in general is unaltered by the support. The first gas-phase product of the reaction appears to be ethane, which is subsequently converted to ethylene. CO and CO2 seem to be formed by oxidation of the C2 hydrocarbons. Activity testing also revealed that a catalyst of this "active composition" was not always active, indicating a possible bifurcation in the reaction rate. The existence of rate hysteresis is confirmed by temperature-programming experiments. ESCA and FTIR studies indicate that the active catalysts are composed of a mixture of various hydrated and unhydrated iron phosphates. These are known to consist of octahedrally coordinated iron atoms alternating with tetrahedrally coordinated phosphorus in phosphate groups. The octahedral coordination of iron includes some direct oxygen links from the phosphates and some indirect Fe---O---H---O---P structures. We speculate that a possible active site for the dehydrogenation is the P---O---P bridge structure of the polyphosphates. We find that on supported catalysts, dehydrogenating species actually build up on the surface during the reaction. These species are specific to dehydrogenation and appear to be incapable of effecting any further oxidation of the products. Thus, gas-phase oxygen alone is responsible for the formation of carbon monoxide and dioxide, while the dehydrogenating oxidic species on the surface are primarily responsible for the formation of C2 hydrocarbons. This suggests that forced concentration cycling would be a good way of separating products of the reaction on supported catalysts.