Reaction network and kinetics of o-xylene oxidation to phthalic anhydride over V 2O 5/TiO 2 (anatase) catalysts

Abstract

The reaction network of o-xylene oxidation to phthalic anhydride over V 2O 5/TiO 2 (anatase) catalysts was determined in the present investigation. The detailed o-xylene conversion/product selectivity data obtained demonstrate that phthalic anhydride is not produced from the direct oxidation of o-xylene over V 2O 5/TiO 2 (anatase) catalysts, but indirectly via a series of consecutive reaction steps involving o-tolualdehyde and phthalide intermediates. The maleic anhydride by-product is produced from the over-oxidation of phthalic anhydride. The influence of the two reactive sites in V 2O 5/TiO 2 (anatase) catalysts, surface vanadium oxide monolayer and V 2O 5 crystallites, upon the o-xylene oxidation reaction were obtained for moderate amounts of vanadium oxide by varying the crystalline V 2O 5 content of the catalysts. The initial reaction step of the partial oxidation of o-xylene to o-tolualdehyde occurs almost exclusively over the surface vanadium oxide monolayer. The o-tolualdehyde intermediate is directly converted to phthalic anhydride primarily over the surface vanadium oxide monolayer and to phthalide over both the surface vanadium oxide monolayer and the V 2O 5 crystallites. The partial oxidation of phthalide to phthalic anhydride mostly occurs over the monolayer of surface vanadium oxide. The over-oxidation of phthalic anhydride to maleic anhydride also occurs primarily over the monolayer of surface vanadium oxide. The non-selective formation of the combustion products, CO and CO 2, primarily occurs from the direct oxidation of o-xylene over the monolayer of surface vanadium oxide. The turnover rate for o-xylene oxidation over V 2O 5/TiO 2(anatase) catalysts is determined for the first time, and found to be ~1 x 10 −3 molecules site −1 s −1 for the experimental conditions used in the present study. The apparent activation energy for the conversion of o-xylene to all products is found to be ~32 kcal mole −1.