Vanadyl pyrophosphate structure

Literature descriptions of active sites on oxide catalysts are often speculative and very often just generate a picture of the surface active site by extrapolation of the bulk structure. In general they envisage approach of the starting material to the active site in a preferred orientation without any indication of how the preferred orientation is established. In addition, the description of the active site is usually restricted to a small number of molecules. For example, the vanadium phosphorus oxide catalysts used for n-butane oxidation to maleic anhydride is based on the vanadyl pyrophosphate structure and an active site architecture is... 

It is clear that strong differences are observed between VPO and MoVTeNbO catalysts, which are responsible for their particular catalytic behavior. Thus, VPO catalysts with a vanadyl pyrophosphate structure present i) one electron redox couple (V +A " +, V +ZV b) ii) two electron redox couples, V +A + iii) bridging oxygen, V-O-V, V-O-P, or VO(P)V iv) terminal oxygen (V " " = O and V "b = O) and activated molecular oxygen, ri -peroxo and T -superoxo species and v) Lewis (probably V + and V +) and Brpnsted (probably — POH groups) acid sites. On the other hand, selective MoVTe(Sb)NbO catalysts are characterized hy b>89-93... 

The selective oxidation of ra-butane to give maleic anhydride (MA) catalyzed by vanadium phosphorus oxides is an important commercial process (99). MA is subsequently used in catalytic processes to make tetrahydrofurans and agricultural chemicals. The active phase in the selective butane oxidation catalyst is identified as vanadyl pyrophosphate, (V0)2P207, referred to as VPO. The three-dimensional structure of orthorhombic VPO, consisting of vanadyl octahedra and phosphate tetrahedra, is shown in Fig. 17, with a= 1.6594 nm, b = 0.776 nm, and c = 0.958 nm (100), with (010) as the active plane (99). Conventional crystallographic notations of round brackets (), and triangular point brackets (), are used to denote a crystal plane and crystallographic directions in the VPO structure, respectively. The latter refers to symmetrically equivalent directions present in a crystal. 

It is now considered, by most groups working in this area, that vanadyl pyrophosphate (VO)2P207 is the central phase of the Vanadium Phosphate system for butane oxidation to maleic anhydride (7 ). However the local structure of the catalytic sites is still a subject of discussion since, up to now, it has not been possible to study the characteristics of the catalyst under reaction conditions. Correlations have been attempted between catalytic performances obtained at variable temperature (380-430 C) in steady state conditions and physicochemical characterization obtained at room temperature after the catalytic test, sometimes after some deactivation of the catalyst. As a consequence, this has led to some confusion as to the nature of the active phase and of the effective sites. (VO)2P207, V (IV) is mainly detected by X-Ray Diffraction. 

The proposed mechanisms are linked to the researcher s hypothesis of the active site, and it is widely accepted that the (100) plane of vanadyl pyrophosphate, (VO)2P207, (referred to as the (020) plane by some authors) plays an important role in the selective oxidation of butane. The structure has been determined by X-ray diffraction and consists of edge-sharing VO5... 

The Raman spectroscopy investigation mentioned above (118) showed that the presence of the products is important in controlling the structural transformations occurring during catalyst activation and that a highly disordered structure can be important in selective butane oxidation. The techniques and findings obtained through in situ analysis of the vanadyl pyrophosphate surface have been reviewed by Bluhm et al. (151), who discussed the state of current techniques used to probe the surface... 

The preparation procedure employed is known to lead to the formation of VOPO4, rather than (VO)2P207. The presence of Sb, however, may lead to a modification of the structural features. Indeed, the authors claim the presence of vanadyl pyrophosphate as the major phase present in catalysts, with a minor amount of vanadium phosphate. The atomic ratio between the components of the y-alumina-supported active phase was V/Sb/P 1/1.9/1.18. The reaction conditions were 425 °C (at which the best yields were reported), and a feed ratio of reactant/ air/ammonia of 0.6-1.0/4.2/1.5. The following results were claimed under these conditions ... 

Recently, Carreon and Guliants reported novel hexagonal, cubic and lamellar VPO phases, which displayed improved thermal stability, desirable chemistries (i.e. the P/V ratios and vanadium oxidation states), and pore structures for the partial oxidation of n-butane [143-145]. These novel VPO phases displayed the selectivities to maleic anhydride up to 40 mol. % at 673K at 10 % n-butane conversion [146]. A conventional organic VPO catalyst containing well-crystallized vanadyl(IV) pyrophosphate, the proposed active and selective phase for n-butane oxidation to maleic anhydride, displayed the selectivities to maleic anhydride 50 mol. % under the same reaction conditions. The low yields observed for mesoporous VPO catalysts confirmed the critical role of the vanadyl pyrophosphate phase (VO)2P207 in catalyzing the oxidation of -butane to maleic anhydride. Therefore, the amorphous nature of the mesoporous VPO... 

The availability of surface arrays of active centres able to cooperate in such a way as to avoid the desorption of olefin intermediates, and to allow the rapid transformation of the latter to the final oxygenated compounds. This is a property of the vanadyl pyrophosphate and of heteropolycompounds for these compounds the "intrinsic multifunctionality" arises from the molecular-type organization of the structure into well-defined moieties, each one characterized by specific properties. 

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. 

Both charge neutral and cation incorporating V/P/O phases exhibit as a recurring theme the isolation of high-temperature, dense-phase materials based on vanadium polyhedra and pyrophosphate P207 " subunits. Such materials have attracted considerable attention as components or models for industrial catalysts. While considerable controversy surrounds the nature of the active component of such catalysts (180-186), in the case of vanadyl pyrophosphate the active phase involved in the redox cycle for the organic oxidation appears to be structurally related to 8-VO(PO)4 (187). 

These findings, together with analogies of the two structures, allowed Bordes et al. (66) to propose that the transformation from (VO)HPO40.5H2O to (VO)2P2C>7 is topotactic. Recently Thompson et al. (67) have suggested, on the basis of different symmetries of the two structures, that this transformation is not a topotactic one. In addition, it is difficult to assume a topotactic reaction for the formation of an active phase when an intermediate amorphous phase has been identified which transforms very slowly to vanadyl pyrophosphate. 

Linde S.A. and Gorbunova Yu.E. (1979). Crystal structure of vanadyl pyrophosphate, Dokl. Akad. Nauk SSSR, 245, pp. 584-591. 

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.

With respect to the catalysts, i.e. mixed-metal oxides and/or heteropolyacids, those presenting well-known structures — as is the case for vanadyl pyrophosphate, orthorhombic bronzes, or Keggin-type heteropolyacids — have been developed in well-known catal5dic processes. However, the roles of amorphous and/or quasicrystalline phases present in the more active and selective catalysts are still unclear and therefore should also be considered. 

Thompson, M. R., Hess, A. C., Nicholas, J. B., White, J. C., AncheU, J., and Ebner, J. R. A concise description of the bulk structure of vanadyl pyrophosphate and implications for n-butane oxidation. Stud SurfSci Catal 82,167-181 (1994). 

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