Partial oxidation of ethane over monolayers of vanadium oxide. effect of the support and surface coverage

Abstract

The interaction between the metal oxide overlayer and the underlying support similarly determines the performance of the catalyst that may also be affected by the exposed sites of the support. A series of supported vanadium oxide catalysts at monolayer and submonolayer coverage have been prepared. This chapter focuses on determining the relevance of the specific oxide support and of the surface vanadia coverage on the nature and activity of the supported vanadia species for the oxidation of ethane. The selectivity conversion trends suggest that ethane is initially oxidized to ethylene and that ethylene is further oxidized to CO. The acidic supports, alumina, and niobia also yield CO as the main oxidation products. Only silica-supported vanadium oxide shows higher selectivites for ethylene. Acetaldehyde and formaldehyde are also produced son 12VSi and 12VSi–H2O. The terminal V = O bond does not appear to be directly involved in the reaction. However, the bridging V–O–V or V–O-support bonds appear to be critical for the oxidation of ethane. The nature of the V–O-support bond is determined by the specific support. Bonding to a reducible support metal ion yields active catalysts. The ternary V2O5/TiO2–SiO2 catalyst shows interesting structural and catalytic properties. The surface vanadium oxide species are isolated and must have a different coordination environment that yields an activity similar to that on 12VSi, but more selective as no CO2 is formed and the selectivity of ethylene increases. The lower selectivity of oxygen-containing products suggest that vanadia species on the very much dispersed titania-on-silica supports may be less reducible than on the pure constituting oxide supports.