Vanadyl pyrophosphate, alkane oxidation

It is well established [1-3] that vanadyl pyrophosphate (VO)2P207 is an essential component of the most selective VPO catalysts. For example, structural and chemical characterization studies of "reactor equilibrated" VPO catalysts indicate that the predominate crystalline phase is vanadyl pyrophosphate (VO)2P207 [1-3], that the bulk P/V ratio is close to 1.0, and that the average vanadium oxidation state is close to -1-4.0 [3-5]. A number of studies [2,5] have indicated that alkane oxidation primarily involves oxygen adspecies adsorbed at vanadium surface sites, and relatively little bulk lattice oxygen. 

Centi, G. and Busca, G. (1989). Surface dynamics of adsorbed species on heterogeneous catalysts Evidence from the oxidation of C4 and C5 alkanes on vanadyl pyrophosphate, J. Am. Chem. Soc., Ill, pp. 46-54. 

Indeed, more complex catalysts are required for partial oxidation reactions. Although several catalytic systems have been studied in the last twenty years, a very limited number of catalysts have been reported for industrial or pre-industrial use. In fact, in addition to V-P-0 catalysts (based on vanadyl pyrophosphate), the unique catalyst used for an alkane oxidation industrialized process, only V-Sb- and MoVTe(Sb)NbO-based mixed-metal oxides have been proposed as sufficiently effective catalysts for the propane ammoxidation process. In both cases pilot plants using the latter catalysts have been announced on the bases of their catalytic results. 

Figure 24.2. Structure of the most studied catalysts in alkane partial oxidation (a) vanadyl pyrophosphate (VPO) (b) VSbO rutile phase (c) orthorhombic bronze, so-called Ml phase, i.e MoVTe(Sb)NbO (d) Keggin molybdophophoiic acid structure.

In either VPO or MoVTeNbO catalysts, V-sites are directly involved in the selective oxidative activation of paraffins, while the presence of a second element is generally required in order to facilitate the formation of a defined crystalline phase (i.e. vanadyl pyrophosphate or orthorhombic metal oxide bronze, respectively) (Figs. 24.2a and 24.2c), but also for facilitating the multifuctionality of the catalysts. However, although both catalytic systems are active for the oxidative activation in alkanes, different selectivities are achieved depending on the alkane feed. Thus, a... 

Centi, G., Colinelli, G., and Busca, G. Modification of the surface pathways in alkane oxidation by selective doping of Broensted acid sites of vanadyl pyrophosphate.,/ Phys. Chem. 1990, 94, 6813-6819. 

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