OXIDATIVE DEHYDROGENATION OF 4-VINYLCYCLOHEXENE TO STYRENE CATALYZED BY PV2MO10O405- HETEROPOLYACIDS

The gas-phase oxidative dehydrogenation of 4-vinylcyclohexene (VCH) to styrene in high selectivities was successfully carried out at moderat e temperatures, 200-260 degrees C, using a vanadium substituted polyox ometalate, PV2Mo10O405-, supported on carbon as catalyst. The major co -product was ethylbenzene and only a small amount of over-oxidation to CO, was observed. Maximum conversions and selectivity were obtained a t a O-2/VCH ratio of similar to 1.9. The identity of the counter catio n also affected the results with activity and selectivity decreasing i n the following order H(5)similar to(NH4)(4)K>Cs(3)H(2)much greater th an(NH4)(5). Ethylbenzene and styrene are not formed by the same reacti on pathway. For ethylbenzene formation, oxydehydrogenation is preceded by isomerization of the exocylic double bond to an endocyclic positio n, whereas for styrene formation there is no such isomerization. A mec hanism 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 oxydeh ydrogenation. (C) 1998 Elsevier Science B.V. All rights reserved.