Mixed-Metal Pyrophosphates

Several hundred mixed-metal pyrophosphates are known in the solid state. Many of these may be prepared by melting together two pyrophosphates (5.125), or orthophosphoric acid with the appropriate mixture of oxides, or a mixture of metal nitrates with dianunonium phosphate. When solutions of equivalent quantities of a soluble metal salt and sodium pyrophosphate are mixed, a precipitate of a mixed-metal salt is produced. These precipitates are often amorphous and reluctant to crystallise, and sometimes ill defined with a variable water content. In other cases, however, a crystalline compound with a definite composition can be isolated (5.126). Molten nitrates will react with some acid salts to produce double salts [30] (5.127). 

These mixed-metal compounds show a range of colours and water solnbilities, and complex anions may be present in some of their aqueous solutions  

Na6Cu(PA)2 I6H2O dark blue, soluble KM11P2O7 - 5H2O violet 

Na4Cu8(PA)5 17H,0 pale blue, insoluble KMnPjO, - 3HjO red 

Types of crystalline mixed-metal pyrophosphates are numerons and they include many examples of isomorphism and polymorphism. Known series include 

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A large number of pyrophosphate salts have been prepared (Table 10). In addition to individual metal salts, ammonium pyrophosphates and many mixed-metal pyrophosphates are known. Pyrophosphates of notable commercial importance include sodium, potassium, and calcium salts. 

Many studies have noted weak cooperativity between the sites during iron release (3). One recent analysis used mixed-metal transferrins, with kinetically inert Co3+ in one site and Fe3+ in the other (221,224). With pyrophosphate, release of iron from the C-site was accelerated by the presence of a metal in the N-site, but no corresponding effect was seen for iron release from the N-site. The cooperative effects were also weaker and somewhat different for different chelators (221). 

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. 

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. 

It can be argued that some mixed metal oxides can also be technically considered as supported metal oxide catalysts because the surface is discernibly different from the underlying mixed metal oxide in terms of composition and molecular structure. For example, the vanadium phosphorus oxide (VPO) catalyst is used in the commercial production of maleic anhydride from butane [12]. The most active crystal phase is the vanadium pyrophosphate (VO)2P207, and the surface structure proposed to be the active phase is a nanometer-thick amorphous VPO layer enriched in phosphorus [12,15]. As another example, Wachs and coworkers [16]... 

The use of mixed complex baths is interesting since the concentration of one free metal ion may be altered by varying the amount of one ligand. Thus, in a copper—tin bath, cyanide content may be varied to alter the activity of copper ions, with little or no effect on tin which is present as stannate or as a pyrophosphate complex. It is evident that some knowledge of the coordination chemistry will reduce the degree of empiricism in developing alloy plating baths. 

Sodium perborate is a stable material when mixed with other dry ingredients. However, the presence of traces of water and certain heavy metals will catalyze the decomposition of the perborate. Therefore, magnesium sulphate or silicate, or tetrasodium pyrophosphate is added to adsorb traces of water and metal to prolong the storage life of the powders [3, 4]. 

A 0.4987-g sample containing only ZnS and CdS is dissolved, and the metals are quantitatively precipitated as MNH4PO4 6H2O, which is ignited to yield 0.6987 g of mixed pyrophosphates (M2P2O7). Calculate the weight of ZnS in the sample. 

The aim of this review is to describe the reactivity of three catalytic systems whieh have been widely studied in recent years for the oxidative tran.sformation of light paraffins i) vanadyl pyrophosphate, which is the industrial catalyst for the oxidation of /i-butane, but has also been elaimed to be selective in the oxidation of n-pentane to maleie and phthalic anhydrides (18-22), ii) heteropolyeompounds, whieh are currently being studied for the oxidation of isobutane and propane to the corresponding unsaturated acids (methacrylic acid and acrylic acid) (5,23-29), and whose composition can be tuned to change the acidic and oxidizing properties and iii) rutile-based mixed oxides, which can act as the matrix to host various metal components, and whieh have been claimed as optimal eatalysts for the ammoxidation of propane to acrylonitrile (15,30-33). 

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