The vanadium pentoxide-titanium dioxide system: Structural investigation and activity for the oxidation of butadiene

Abstract

The interactions of various anatase and rutile samples with V 2O 5, and their products, have been studied by thermal analysis, by ir and ESR spectroscopy and by X-ray diffraction. The V 2O 5 was introduced either by mechanical admixture or by impregnation of the TiO 2 with NH 4VO 3 and subsequent calcination. With anatase samples containing only low levels of P 2O 5 and K 2O (< 0.6%) or containing SO 4 = (6.3%), heating to 450 to 600 °C leads only to supported V 2O 5: but heating at 750 °C produces (i) a change in color to black, (ii) an irreversible loss of oxygen, (iii) a transformation of the anatase into rutile, and (iv) a substantial decrease in surface area. The results of experiments performed with various V 2O 5 concentrations ( 1 to 60% w w ) suggest that about 8% w w V 2O 5 becomes incorporated as V 4+ ions into the rutile lattice during the transformation, through the formation of a compound having the composition V 0.04Ti 0.96O 2. Corresponding processes with rutile take place only at higher temperatures. Anatase containing Na + (1.4%) behaves quite differently, and there is evidence for the formation of sodium vanadium bronzes on heating to 450 or 750 °C. Catalysts containing supported V 2O 5 oxidize butadiene to maleic anhydride at 260 °C with a selectivity which increases with V 2O 5 contents between 1 and 10%, and thereafter remains constant ( S ∼- 0.45). Those containing the compound V 0.04Ti 0.96O 2 are less active but more selective ( S ∼- 0.57 for 10 to 30% V 2O 5). With both types, the selective reaction is zero order in butadiene and in O 2, and both show activation energies for selective and for nonselective oxidation of 20 to 25 kcal · mol −1. Selective oxidation is believed to require lattice oxygen, and selectivity correlates with difficulty of reduction by CO.