Methane partial oxidation by silica-supported iron phosphate catalysts. Influence of iron phosphate content on selectivity and catalyst structure

Selective oxidation of methane to methanol and formaldehyde at atmospheric pressure was studied over a series of silica-supported FePO4 catalysts, with iron phosphate content ranging from 2 to 16 wt%. Performance was evaluated over the range T=773-963 K, GHSV=25,000-65,000 h(-1), and CH4 : O-2=1. The main products were formaldehyde, carbon monoxide and carbon dioxide. Small, but quantifiable amounts of methanol were also observed. Catalytic activity exhibited a clear dependence on the iron phosphate content. The highest selectivity and space time yield (STY) to formaldehyde and methanol were observed for 2 wt% FePO4 on silica (STY of 622 and 25 g/kg(cat) h, respectively). The selectivity-conversion pattern suggests that methane is oxidized directly to methanol and formaldehyde, and sequentially to carbon oxides. Characterization was performed by X-ray powder diffraction, X-ray photoelectron spectroscopy, and Mossbauer spectroscopy. Crystalline FePO4 is observed at all loading levels, however, a significant fraction of the iron (58% at 2 wt% FePO4) is present in an X-ray amorphous phase. Mossbauer spectra suggest that this phase contains iron in five-fold coordination, and with a higher electron density relative to bulk FePO4. The amount of this five-coordinate phase present is roughly 1 wt% Fe, independent of total iron loading. XPS confirms the lower effective oxidation state of iron, and indicates that at low loading the surface is enriched in phosphorus relative to bulk FePO4. It is proposed that iron in five-fold coordinate sites, isolated by phosphate groups, more selectively activates methane than crystalline FePO4. As loading increases, so does the amount of crystalline FePO4, which is proposed to more rapidly catalyze sequential oxidation of the selective products.