Abstract
Titania samples were prepared by precipitation at pH 3.0 (to provide ‘Ta’ samples) or at pH 9.0 (to provide ‘Tb’ samples). These materials were then impregnated with NH4VO3 to obtain vanadia/titania catalysts of different V2O5 content. These V2O5/TiO2 samples were subsequently calcined at 400°C or 600°C to obtain two series. The oxidation of o-xylene was undertaken at 250°C, 300°C or 350°C using a conventional flow apparatus at atmospheric pressure. The reaction products were detected and separated via a gas chromatograph fitted with a flame ionization detector. The chemical composition of the catalyst, the calcination temperature, the reaction mixture employed and the reaction temperature were among the variables considered.
Catalysts calcined at 400°C were more active than those calcined at 600°C, thereby indicating that anatase was the proper titania phase for supporting oxidation catalysts. Titania samples precipitated at pH 3.0 were more effective as a support for vanadia than those precipitated at pH 9.0 despite the fact that they possessed lower surface areas. This demonstrated that surface area was not a determining factor in catalyst performance.
The conversion of o-xylene to phthalic anhydride increased with increasing vanadia content in the catalysts employed and with increasing reaction temperature. Surface acidity was one factor amongst others contributing to the catalytic activity. It is thought that acid sites enhance the reduction of V5+ to lower oxidation states to provide the active oxidation sites. Mechanisms for the partial oxidation of o-xylene were advanced and explained.