The Role of Redox and Acid-base Characters of Silica-supported Vanadia Catalysts Playing in the Selective Oxidation of Ethane

摘要

Direct conversion of low alkanes to useful oxygenates is an important industrial objective and also as one of the main challenges for the efficient utilization of natural gas. It has been generally recognized that acid-base character and redox property are two of the important factors governing the catalysis of metal oxides. These two factors are often dealt with separately in hydrocarbon oxidation. In the most case the redox property is closely related to the catalytic activity, while basic and acidic nature commonly to the product selectivity. Very recently, we reported that silica- supported alkali metal and a small amount of Fe, V or Bi can catalyze the selective oxidation of ethane to acetaldehyde and even to acrolein by oxygen. Both the ethane conversion and total selectivity to aldehydes were enhanced by the addition of alkali metal during ethane oxidation. Although some roles of alkali metal have been classified, however, there are still some unknown points. For example, what states do alkali metals exist on the SiO_2? What kind of alkali metal is favorable for the formation of acetaldehyde or for the aldol condensation of the aldehydes with short carbon chains to give higher aldehydes. Especially, why the reactivity was enhanced by the addition of alkali metals? In this work the redox property and acid-base nature of several systems of silica-supported vanadia catalysts with or without alkali-metal were studied and the effects of these characteristics on their catalytic performances for ethane oxidation were investigated. The reason why the reactivity was enhanced by the addition of alkali metals was discussed.