Methane partial oxidation by unsupported and silica supported iron phosphate catalysts - Influence of reaction conditions and co-feeding of water on activity and selectivity

The partial oxidation of methane to methanol and formaldehyde by molecular oxygen has been investigated over crystalline and silica supported FePO4 at a pressure of 1 atm and in the temperature range of 723-973 K. The quartz phase of FePO4, as well as silica supported FePO4 prepared by impregnation (5 wt%), were examined in a continuous how reactor, Experiments carried out over FePO4 show high selectivity to formaldehyde at low conversion and suggest that formaldehyde is the primary reaction product, but selectivity decreased rapidly as conversion was increased. The highest space-time yield of formaldehyde observed for this catalyst was 59 g/kg(cat)-h. Above 5% methane conversion,carbon oxides were the only products. For silica-supported FePO4, formaldehyde selectivity did not fall off rapidly, exhibiting a formaldehyde selectivity of 12% Bt about 10% conversion (STY = 285 g/kg(cat)-h). Quantifiable yields of methanol were observed at very low conversion levels, i.e. below 3% (STY = 11 g/kg(cat)-h). Addition of steam (up to 0.1 atm partial pressure) into the feed stream increased the selectivity to methanol ( similar to 25 g/kg cat/h with up to 3% selectivity) and formaldehyde (similar to 487 g/kg cat/h with up to 94% selectivity) for the silica-supported FePO4 catalyst. Steam addition had little effect on catalyst: activity. Characterization results indicate the presence of FePO4, as well as fivefold coordinate Fe3+ in silica supported catalyst samples, and this species is proposed to be responsible for methane activation. After catalysis in the presence of steam, the fivefold coordinate iron is present, but a significant fraction of the FePO4 has been reduced to Fe2P2O7. Enhanced selectivity in the presence of steam is attributed in part to the ease of the reversible formation of surface hydroxyl groups (P-OH) from pyrophosphate (P-O-P) groups. (C) 1999 Academic Press.