Oxidative dehydrogenation of ethylbenzene over vanadia-alumina catalysts in the presence of nitrous oxide: structure-activity relationship

Abstract

A series of vanadia-alumina catalysts with different vanadia contents were prepared by a wet impregnation method. The influence of the local structure of vanadia in these catalysts on the oxidative dehydrogenation of ethylbenzene with nitrous oxide was investigated. The use of N₂O as a co-feed remarkably enhanced the styrene yield compared with the use of N₂. Characterization of these vanadia catalysts by XRD, FTIR, UV-vis, TPR, XPS, and ⁵¹V NMR techniques suggests that the nature of the VO_x species depends on the vanadia loading; the predominant species are monomeric vanadia at lower loadings, two-dimensional polyvanadates at intermediate loadings, and bulk-like V₂O₅ and AlVO₄ at higher loadings. The rate of oxidative dehydrogenation (ODH) of ethylbenzene per vanadium atom increases with vanadia loading and reaches a maximum at 10 wt%, the loading at which the surface predominantly contains polyvanadate species. The observed variation in the selectivity of products with vanadium loading indicates that the monomeric V⁵⁺ species favors dehydrogenation, whereas bulk-like V₂O₅ preferentially participates in the dealkylation of ethylbenzene. The vanadium species remains at a higher oxidation state in the presence of N₂O, leading to a higher styrene yield, than in a N₂ atmosphere. The ODH turnover rates increased with decreasing energy of the absorption edge in the UV-vis spectrum, at low VO_x coverages of less than one monolayer on the Al₂O₃ surface.