Methane Partial Oxidation by Unsupported and Silica Supported Iron Phosphate Catalysts: Influence of Reaction Conditions and Co-Feeding of Water on Activity and Selectivity

摘要

The partial oxidation of methane to methanol and formaldehyde by molecular oxygen has been investigated over crystalline and silica supported FePO_4 at a pressure of 1 atm and in the temperature range of 723-973 K. The quartz Phase of FePO_4, as well as silica supported FePO_4 prepared by impregnations (5 wt%), were examined in a continuous flow reactor. Experiments carried out over FePO_4 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 FePO_4, formaldehyde selectivity did not fall off rapidly, exhibiting a formaldehyde selectivity of 12% at about 10% conversion (STY = 285 g/kg_(cat)-h). Quantifiable fields 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 (approx 25 g/kg cat/h with up to 3% selectivity) and formaldehyde (approx 487 g/kg cat/h with up to 94% selectivity) for the silica-supported FePO_4 catalyst. Steam addition had little effect on catalyst activity. Characterization results indicate the presence of FePO_4, as well as fivefold coordinate Fe~(3+) 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 freaction of the FePO_4 has been reduced to Fe_2P_2O_7. 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.