V/P/O phases

Vanadium phosphates have been established as selective hydrocarbon oxidation catalysts for more than 40 years. Their primary use commercially has been in the production of maleic anhydride (MA) from n-butane. During this period, improvements in the yield of MA have been sought. Strategies to achieve these improvements have included the addition of secondary metal ions to the catalyst, optimization of the catalyst precursor formation, and intensification of the selective oxidation process through improved reactor technology. The mechanism of the reaction continues to be an active subject of research, and the role of the bulk catalyst structure and an amorphous surface layer are considered here with respect to the various V-P-O phases present. The active site of the catalyst is considered to consist of V and V couples, and their respective incidence and roles are examined in detail here. The complex and extensive nature of the oxidation, which for butane oxidation to MA is a 14-electron transfer process, is of broad importance, particularly in view of the applications of vanadium phosphate catalysts to other processes. A perspective on the future use of vanadium phosphate catalysts is included in this review. 

C. Quaternary M /V/P/O Phases with Negatively Charged V—P—O Covalent Linkages and Charge Compensating M Cations... 

Figure 1. Views of two V/P/O phases containing the ethylenediammonium cations. Polyhedral representations of the structures of [H3NCH2CH2NH3]2[H3NCH2CH2NH2][V(H20)2(VO)8 (OH)4 (HP04)4(P04)4] 2H20, viewed parallel to the crystallographic c axis (a) and of [H3NCH2CH2 NH3](V0)3(H20)2(P04)2(HP04)], viewed parallel to the b axis (b).

The polyhedral geometries associated with the oxidation states observed for vanadium in V/P/O phases exhibit several noteworthy characteristics. As noted... 

While compositionally the simplest of the inorganic phases, the vanadium phosphates with neutral V—P—O covalent structures display many of the structural motifs common to the larger family. Thus, ID, 2D, and 3D structures with a variety of polyhedral connectivities have been described. The layered neutral V/P/O phases have received considerable attention as precursors to the catalytically active (VO)2(P207) phase (36). 

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). 

The first attempts to introduce AIO4 tetrahedra into vanadium phosphate stmctures were predicated on the assumption that such incorporation would increase the thermal stability of the V/P/O phases. Furthermore, the diverse coordination environments and stmctural connectivities of both aluminophosphates and vanadium phosphate systems suggest that introduction of aluminum stmctural units into the V/P/O phase would lead to unusual materials. 

While layered V/P/O phases are common and often exhibit stmctural similarities to the network motif of MOPO4 prototypes, V/P/0 skeletal frameworks... 

V/P/O phases discussed above, in this application the organic component serves as a ligand to a secondary metal site, a first-row transition or posttransition metal cation (Tables Vlll and DC). Consequently, a coordination complex cation is assembled that serves to provide charge-compensation, space-filling, and structure-directing roles. 

Bordes E. and Courtine P. (1979). Some selectivity criteria in mild oxidation catalysis V-P-O phases in butene oxidation to maleic anhydride, J. Catal., 57, pp. 237-252. 

Bordes E. (1987a). Crystallochemistry of V-P-O phases and application to catalysis, Catal. Today, 1, 499-526 Bordes E. (1987b). Reactivity and crystal chemistry of V-P-O phases related to C4 -hydrocarbon catalytic oxidation, Catal. Today, 3, pp. 163-174. 

Bordes, E. Crystallochemisiry of V-P-O phases and application to catalysis. Catal Today 1987,1, 499-526. 

Bordes, E. Reactivity and crystal chemistry of V-P-O phases related to C4-hydrocarbon catalytic oxidation. Catal. Today 1987, 3, 163-174. 

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