Direct conversion of methane and ethane to the corresponding alcohols using nitrous oxide over iron phosphate catalysts

Abstract

The partial oxidation of methane and ethane by nitrous oxide over iron phosphate catalysts has been studied at 573–723 K. Methanol and ethanol were obtained as the primary products from methane and ethane, respectively. Further oxidation of methanol leads to the formation of formaldehyde, CO and CO₂ as the consecutive oxidation products. In the case of ethane oxidation, the primary product, ethanol, undergoes dehydration and oxidation to ethene and acetaldehyde, respectively.

An excess of either iron or phosphorus in the FePO₄ catalysts was disadvantageous to their catalytic activities. It was suggested that an iron site together with the surrounding phosphates was important for the reactions. The most appropriate Fe/P ratio on the surface was 0.4–0.5. X-Ray photoelectron spectroscopy (XPS) studies suggested that the active site was Fe²⁺ on the FePO₄ surface. The pulse experiments showed that the pre-reduction of the catalyst surface by either hydrogen or carbon monoxide greatly increased the conversion of methane and the yield of methanol. Co-feeding of hydrogen in a reactant gas flow also increased the yields of methanol and of ethanol for the oxidations of methane and ethane, respectively. In the case of methane oxidation, the yield of methanol was decreased markedly with the addition of water. In the case of ethane oxidation, the yield of ethanol increased at a low partial pressure of water, owing to the suppression of the dehydration of ethanol to ethene.

It is suggested that an active oxygen species (O*) formed on the surface Fe²⁺ site is responsible for the oxidation of both methane and ethane. Methane or ethane is activated by the O* in cooperation with the neighbouring phosphates, forming a methoxide or ethoxide as an intermediate. Methanol or ethanol is then produced through the reaction of the intermediate with a highly acidic proton on the neighbouring phosphate unit.