Photocatalytic cyclohexane oxidehydrogenation on sulphated MoOx/γ-Al2O3 catalysts

The photocatalytic oxidative dehydrogenation of cyclohexane on sulphated MoOx/γ-Al2O3 catalysts has been studied in a two-dimensional fluidized bed photoreactor. The influence of Mo loading at similar sulphate content and the effect of catalyst preparation method have been investigated. Considering the influence of Mo loading at similar sulphate content, the highest photoactivity at 2.4 SO4 wt% was found at MoO3 loading of 8 wt%. Selectivity to cyclohexene was 100%, irrespective of the Mo content. At fixed MoOx content, in particular at 50% of theoretical monolayer coverage, the preparation method of catalysts strongly affected the photocatalysts performances, showing in addition a slight decrease in selectivity to cyclohexene due to side-production of benzene. All the catalysts showed a similar equivalent band gap energy. Thermogravimetric analysis evidenced the presence of surface sulphate species of different thermal stabilities. A linear correlation of photoactivity with the surface sulphates amount of lower thermal stability has been found for all sub-monolayer MoOx sulphated catalysts. The neighboring of surface sulphates to octahedral polymolybdate species appears to be a key parameter for the photoactivity of the catalysts. The catalyst selectivity was related to surface acidity. Higher acidity resulted in increased cyclohexene dark adsorption and consequently in enhanced benzene formation.