A novel approach to the scientific design of oxide catalysts for the partial oxidation of methane to methanol

Abstract

The approach adopted for the scientific design of methane partial oxidation catalysts has identified oxides that are capable of activating methane and oxygen, but do not destroy methanol, the desired product. From the perspective of methanol stability Sb2O3 showed the best performance, destroying only 3 of the methanol feed at 500°C. The majority of oxides totally combusted methanol below 400°C. The oxides MoO3, Nb2O5, Ta2O5 and WO3 all produced high methanol conversion, however, high selectivities towards formaldehyde and dimethylether were obtained with only low levels of carbon oxides throughout the range of conversions. The products formaldehyde and dimethylether were desirable by-products from a methane partial oxidation process hence these oxides are suitable catalyst components. The activation of methane has been probed by the exchange reaction with deuterium under non-oxidative conditions. The most active catalyst was Ga2O3 which exhibited normalised exchange rates several orders of magnitude greater than the other catalysts. A relationship between the rate of methane activation and the predicted basic strength of the rare earth sesquioxides was established. This relationship indicates that methane activation took place via the abstraction of H+ to form a surface methyl carbanion species. On the basis of these results we have designed and tested a limited range of two component oxide catalysts for methane partial oxidation. The best catalyst was Ga2O3/MoO3 prepared by physically mixing the component oxides. This catalyst showed an increased methanol yield compared with the homogeneous gas phase reaction in the reactor tube packed with quartz chips. The increased methanol yield has been attributed to the development of a co-operative effect between the Ga2O3 and MoO3 oxide phases. The degree of success from this approach indicates the validity of this novel study.