UV Raman spectroscopic studies of V/θ-Al₂O₃ catalysts in butane dehydrogenation

To explore the coke formation mechanism and catalyst structure under alkane dehydrogenation (DH) conditions, the DH of butane on V/θ-Al ₂O₃ was explored by in situ UV Raman spectroscopy and reactivity tests. Studies of butane DH on V/θ-Al₂O₃ catalysts with various distributions of surface VO_x species identify a structure-coke relationship. The deactivation of the catalysts in butane DH is due mainly to the formation of coke species. Both the nature and amount of coke formed are related to the structure of VO_x species. Monovanadates make chain-like polyaromatics, whereas polyvanadates produce mainly sheet-like (two-dimensional) polyaromatics that are detrimental to catalytic activity. The amount of coke formed from butane DH follows this sequence: polymeric VO _x > monomeric VO_x > V₂O₅, Al₂O₃. Raman spectroscopy studies of butane, 1-butene, cis/trans-2-butenes, and 1,3-butadiene reactions on V/θ-Al ₂O₃ catalysts enable the formulation of a coke formation pathway for butane DH, in which polystyrene is found to be a key intermediate. Although the surface of V/θ-Al₂O₃ is partially reduced under butane DH conditions, the structure of VO_x species can be fully regenerated by oxidation of the coke species at temperatures up to 873 K.