Selective gas-phase oxidation of polycyclic aromatic hydrocarbons on vanadium oxide-based catalysts

The selective gas-phase oxidation of fluorene, anthracene, phenanthrene and naphthalene over supported and unsupported vanadium oxide-based catalysts frequently modified with other oxides such as Fe2O3 or MoO3 is reviewed and the effect of doping with alkali metal compounds is discussed. In particular, the oxidation of fluorene to 9-fluorenone is used as a key reaction for the discussion of the results. Reaction schemes derived from catalytic and kinetic measurements show strong similarities for all polycyclic aromatic hydrocarbons investigated indicating that selective products of inner and outer-ring oxidation are formed in primary steps while their decomposition occurs consecutively particularly at high degrees conversion. In addition to the catalytic studies reaction engineering simulations with respect to fixed-bed and fluidized-bed reactor performance are discussed. By comparing textural and structural peculiarities of the catalysts (e.g. surface acidity, phase composition, redox properties, active sites and role of the support) with their catalytic performance it becomes evident that high oxygenate selectivities are favoured by moderate surface acidity and oxidation potentials which can be controlled by admiring alkali compounds or less acidic metal oxides like Fe2O3 to V2O5. In cesium-doped vanadium/iron oxides an amorphous phase was identified containing lattice-oxide vacancies occupied by electrons which, presumably, act as catalytically active sites. On the basis of these considerations essential common features for the different oxidation processes are pointed out and some rules for catalysts design are derived.