The nature and the role of oxygen species and vanadium oxidation states on the activation of n-butane for selective oxidation to maleic anhydride were investigated. Bi-Fe doped and undoped vanadium phosphate catalysts were used a model catalyst. XRD revealed that Bi-Fe mixture dopants led to formation of a II -VOPO 4 phase together with (VO) 2 P 2 O 7 as a dominant phase when the materials were heated in n-butane/air to form the final catalysts. TPR analysis showed that the reduction behaviour of Bi-Fe doped catalysts was dominated by the reduction peak assigned to the reduction of V 5? species as compared to the undoped catalyst, which gave the reduction of V 4? as the major feature. An excess of the oxygen species (O 2-) associated with V 5? in Bi-Fe doped catalysts improved the maleic anhydride selectivity but significantly lowering the rate of n-butane conversion. The reactive pairing of V 4? -O -was shown to be the centre for n-butane activation. It is proposed that the availability and appearance of active oxygen species (O -) on the surface of vanadium phosphate catalyst is the rate determining step of the overall reaction.

No abstract

No abstract

VOHPO 4 Á0.5H 2 O synthesized via the alcohol reduction of VOPO 4 Á2H 2 O was mechanochemical treated for 30 min in three different media, i.e. cyclohexane, ethanol and air. XRD results revealed that their structure became less crystalline compared to the unmilled material. SEM showed that the particles for the milled materials become smaller and unique features were observed in the different type of media used. The reactivity of the oxygen species linked to V 5? and V 4? were also affected by the milling process. The selectivity to maleic anhydride from n-butane oxidation were observed to increase in line with the increase in the oxygen species associated with V 5? and the presence of isolated V 5? phase. A correlation was observed between the crystallite size of the pyrophosphate phase at (020) plane with the maleic anhydride selectivity.

The physico-chemical and catalytic properties of three ways of modified catalysts were studied, i.e. (i) the addition of both Bi and Fe (nitrate form) during the refluxing VOPO4•2H2O with isobutanol (Catalyst A), (ii) the simultaneous addition of BiFe oxide powder in the course of the synthesis of precursor VOHPO4•0.5H2O (Catalyst B) and (iii) the mechanochemical treatment of precursor VOHPO4•0.5H2O and BiFe oxide in ethanol (Catalyst C). It was found that surface area of the modified catalysts has increased except Catalyst B. The reactivity of the oxygen species linked to V5+ and V4+ was studied by using H2-TPR, which also affected the catalytic performance of the catalyst. The conversion of nbutane decreases with an increment of oxygen species associated with V5+.