THE OXIDATIVE DEHYDROGENATION OF ETHANE ON SILICA-SUPPORTED METAL-OXYGEN CLUSTER COMPOUNDS

The oxidative dehydrogenation of ethane with nitrous oxide and oxygen on silica-supported metal-oxygen cluster compounds (MOCC) has been investigated. The effects of several variables such as reaction temperature, partial pressure of reactants, nature of the oxidants (N2O and O-2), residence time, loading of the catalysts, and pretreatment environment, on the conversion, product distribution and the kinetics have been studied. With nitrous oxide, on unsupported H3PMo12O40 and on the silica support no acetaldehyde was observed in the product stream while significant amounts of acetaldehyde are found with the supported MOCC. With nitrous oxide as an oxidant, acetaldehyde and ethylene were the principal products, while carbon monoxide and ethylene were the predominant products with oxygen. On H3PMo12O40 the conversion of ethane and the yield of acetaldehyde have maximum values at a loading of 20 wt.-%. The results from studies of the effect of contact time suggest that acetaldehyde and ethylene are primary products. Increases in the relative amounts of the oxidants produce changes in the selectivities which are strongly dependent on the nature of the oxidant. For catalyst pretreatment temperatures greater than 500 degrees C, the conversion of ethane and selectivity to acetaldehyde decrease. At partial pressures of ethane between 0.1 and 0.8 atm the rate of ethane conversion is of approximate order 0.8 and 0.6 in ethane and nitrous oxide, respectively and 0.7 and 0.4 in ethane and oxygen, respectively. The results show striking correspondences with those reported earlier for methane with MOCC and thus suggest that the mechanisms for the two processes may depend on the lability of the terminal oxygen atoms of the MOCC anion, the production of oxygen vacancies and the ability of the oxidant to regenerate the active oxygen sites in the anion.