Oxidative dehydrogenation of 4-vinylcyclohexene to styrene catalyzed by PV₂Mo₁₀O^5-₄₀ heteropolyacids

The gas-phase oxidative dehydrogenation of 4-vinylcyclohexene (VCH) to styrene in high selectivities was successfully carried out at moderate temperatures, 200-260°C, using a vanadium substituted polyoxometalate, PV₂Mo₁₀O^5-₄₀, supported on carbon as catalyst. The major co-product was ethylbenzene and only a small amount of over-oxidation to CO_x was observed. Maximum conversions and selectivity were obtained at a O₂/VCH ratio of ∼1.9. The identity of the counter cation also affected the results with activity and selectivity decreasing in the following order H₅∼(NH₄)₄K>Cs₃H ₂≫(NH₄)₅. Ethylbenzene and styrene are not formed by the same reaction pathway. For ethylbenzene formation, oxydehydrogenation is preceded by isomerization of the exocylic double bond to an endocyclic position, whereas for styrene formation there is no such isomerization. A mechanism is proposed whereby the active catalyst is a polyoxometalate - carbon support complex, which yields in the presence of oxygen quinone/hydroquinone or aroxy/phenol redox couples responsible for the oxydehydrogenation.