Phosphate tetrahedrons

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. 

Fig. 17. Structure of complex (VO)2P2C>7 in (010), viewed down the fr-axis. Vanadium octahedra and phosphate tetrahedra link together forming a three-dimensional network. Front (bold) and back (faint) layers are shown.

Fig. 20. (a) Active sites observed by in situ atomic-resolution ETEM structural modification of VPO in n-butane along (201) indicates the presence of in-plane anion vacancies (active sites in the butane oxidation) between vanadyl octahedra and phosphate tetrahedra. (b) Projection of (010) VPO (top) and generation of anion vacancies along (201) in n-butane. V and P are denoted. Bottom model of novel glide shear mechanism for butane oxidation catalysis the atom arrowed (e.g., front layer) moves to the vacant site leading to the structure shown at the bottom. 

Figure 3.23. Model for topotactic transformation of the precnrsor to the active catalyst (a) face-sharing VOe octahedra and H-phosphate tetrahedra (H atoms are shaded) (b) removal of bonded water (resulting in VO5 which rotate, and transfer of H-atoms from H-phosphates) and (c) final reconnected VPO after removal of water.

A further group, the ultra-polymeric or cross-linked-phosphates, which were first mentioned by Kroll (165), are characterized by the fact that tertiary phosphate tetrahedra are present in addition to the ordinary P—O-—P groupings. These are joined with other tetrahedra by three oxygen atoms and not by two, as in the meta- and poly-phosphates. They will be discussed in Section V. 

All three groups come under the heading of condensed phosphates, for the simplest method for their preparation is still by condensation reactions in which phosphate tetrahedra are condensed by elimination of water. 

Fig. 3. Model of DNA (schematic) showing metal-coordinated phosphate groups. A sodium ion is bonded by ionic forces between two phosphate tetrahedra (after Matheja and Degens48 )...

Fig. 18. Average environment of calcium ions in amorphous calcium phosphates as determined by X-ray absorption spectroscopy near the K absorption edge of calcium. The diagram shows two bidentate and two monodentate phosphate tetrahedra and two water molecules contributing to the eight oxygens of the first coordination sphere of the calcium ions. A third shell, possibly comprising the two phosphorus atoms of the monodentate phosphate ions, can be seen in some of the preparations. The positions of the protons are not established (Holt et al., 1988, 1989b Holt and Hukins, 1991).

The two-dimensional structures are extended networks formed by the linking of the metal-oxygen polyhedra and the phosphate tetrahedra. These are sheet structures and often resemble those of naturally occurring clay minerals. The sheets are usually anionic and the protonated (cationic) amine molecules, located between the two sheets, render the framework neutral. The two-dimensional structures are intermediates between the one-dimensional chains and the three-dimensional structures, and the literature on phosphate networks contains descriptions of several layered materials, owing to the wide compositional diversity exhibited by them [22-24]. The layered materials are of interest because they act as precursors for the three-dimensional structures. 

Figure 7.14. Figure showing the large elliptical tunnels. The layer-type arrangement (Fig. 7.13) gets crosslinked by phosphate tetrahedra (Choudhury et al. [29]). 

Coordination of metal ions with neighbors through phosphate tetrahedra in heavy metal phosphate minerals. 

Catenaphosphate the infinite chain phosphate, (P03 )oo Cyclophosphates cyclic anions of two-connected phosphate tetrahedra, P 03 " ... 

Linking phosphate tetrahedra into chains through two vertices results in polyphosphate anions, also... 

Phosphoms pentoxide may be regarded as a neutral ultraphosphate containing only three-connected phosphate tetrahedra. Two extended forms of P2O5 are known, one containing infinite sheets, and the other a three-dimensional... 

Solid phosphates show a huge variety of crystal structures, and it is not practical to classify them in terms of structural types as is done with simple oxides, halides, etc. However, some general classes of metal phosphate structures will be considered three-dimensional frameworks of linked phosphate tetrahedra and tetrahedrally or octahedrally coordinated cations, layered phosphates, and phosphate glasses. In all of these materials the size and topology of pores within the structure are of importance, as these determine the ability of ions and molecules to move within the structure, giving rise to useful ion exchange, ionic condnction, or catalytic properties. Ion exchange can also be nsed to modify the properties of the host network, for example, the nonlinear optical behavior of potassium titanyl phosphate (KTP) derivatives. 

Rapid cooling of phosphate melts gives vitreous materials in which no long-range order is present, but with similar local environments and connectivity of phosphate tetrahedra... 

The stmcture of UP04C1(H20)2 is also adopted by its Br analog [33]. From the Wyckoff positions of the atoms, the actinide site in the stmcture appears to contain UO6X2 (X = Cl, Br) polyhedra that approximate a Johnson snub disphenoid, also known as a Siamese dodecahedron, but as the halogen site is half-occupied, the actual coordination number is seven [35]. These polyhedra share vertices with phosphate tetrahedra to form an open framework with voids that extend parallel to the [100] direction. The halogen atoms that coordinate uranium line these voids (Fig. 5). 

The structure of vitusite-(Ce) consists of edge-shared dimers of CeOg that are linked into chains by edge- and vertex-sharing with phosphate tetrahedra (Fig. 10). These chains extend along the [010] direction (in space group Pcali). The... 

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