The roles of redox and acid-base properties of silica-supported vanadia catalysts in the selective oxidation of ethane

The redox and acid-base characters of silica-supported vanadium or alkali-modified vanadium catalysts by varying vanadium loading were studied with the methods of H-2-TPR, NH3-TPD and CO2-TPD combining other structural characterization techniques of ESR, UV-vis spectroscopy. The effects of these properties on their catalytic performances for the ethane oxidation by oxygen were investigated. For both the unpromoted V/SiO2 (V:Si = x:Si) and promoted Cs-V/SiO2 (CS:V:Si = 1:x:100) systems, the reduction extent and the reducibility increase with increasing vanadia loading and the presence of cesium enhances the reduction extent. The structures of the vanadyl species have large effect on the vanadia reducibility and the isolated surface vanadyl species are less reducible than the polymeric vanadyl species including microcrystalline vanadia. For the samples with high vanadia loading (greater than or equal to2.0%), the reducibility is high and thus the redox characteristic of the catalyst is one of the most important factors that govern the catalytic reactivity. In the samples with low vanadia loading (<0.5%), the reducibility is low. Therefore, the basicity or acidity characteristics of the catalyst become the major factors that control the catalytic reactivity. The cesium promoted samples CS-V/SiO2 (2.0 and 10.0%) exhibit high reducibilities, which qualitatively suggests that their oxygen mobility is very high and may result in deep oxidation of ethane. In contrast, those samples with low reducibility are selective for the oxidation of ethane including ODH to ethylene and oxygenate formation. (C) 2004 Elsevier B.V. All rights reserved.