Designing oxidation catalysts

Abstract

A simplified approach is described for the design of oxidation catalysts. It is based on three components and for selective oxidation the approach involves identifying catalysts that (a) do not catalyse the oxidation of the required product under the reaction conditions to be tested, (b) activate the oxidant and (c) activate the substrate. This approach is initially demonstrated to identify components of novel methane oxidation catalysts for the formation of methanol. The approach seeks to identify oxides that are capable of activating methane and oxygen, but do not destroy methanol, the desired product. From the perspective of methanol stability the oxides MoO 3, Nb 2O 5, Ta 2O 5 and WO 3 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 considered to be suitable catalyst components. The activation of oxygen was probed using the 16 O 2/ 18 O 2 exchange reaction. The activation of methane has been probed by the exchange reaction with deuterium under non-oxidative conditions. The most active catalyst was Ga 2O 3 which exhibited normalised exchange rates several orders of magnitude greater than the other catalysts. On the basis of these results we tested a simple two component oxide catalyst for methane partial oxidation based on a Ga 2O 3/MoO 3 physical mixture, which 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 Ga 2O 3 and MoO 3 oxide phases. The design approach has also been extended to the design of total oxidation of hydrocarbons for VOC destruction using benzene as a model VOC. In this case the design approach seeks to identify catalysts that can activate the substrate and the oxidant and also destroy the possible partial oxidation products. This approach is used to demonstrate that uranium oxide catalysts are extremely active for the oxidation destruction of hydrocarbons and chlorohydrocarbons. The degree of success from this approach indicates the validity of this novel approach in both the identification of selective and total oxidation catalysts.