Other Heteropoly Compounds

The heteropoly anions that do not belong to any of the classifications just discussed are presented below. 

---

In 1826 J. J. Berzelius found that acidification of solutions containing both molybdate and phosphate produced a yellow crystalline precipitate. This was the first example of a heteropolyanion and it actually contains the phos-phomolybdate ion, [PMoi204o] , which can be used in the quantitative estimation of phosphate. Since its discovery a host of other heteropolyanions have been prepared, mostly with molybdenum and tungsten but with more than 50 different heteroatoms, which include many non-metals and most transition metals — often in more than one oxidation state. Unless the heteroatom contributes to the colour, the heteropoly-molybdates and -tungstates are generally of varying shades of yellow. The free acids and the salts of small cations are extremely soluble in water but the salts of large cations such as Cs, Ba" and Pb" are usually insoluble. The solid salts are noticeably more stable thermally than are the salts of isopolyanions. Heteropoly compounds have been applied extensively as catalysts in the petrochemicals industry, as precipitants for numerous dyes with which they form lakes and, in the case of the Mo compounds, as flame retardants. 

Niobic acid displays a much less pronounced tendency than vanadie acid to form heteropoly-compounds with other acids, but oxaloniobates are known. It reacts with hydrogen peroxide to form perniobic add, HNb04.a,H20, salts of which are known. The double niobium oxy-fluorides also take up active oxygen. 

As will be described in more detail in later sections, in acid and oxidation catalysis by solid heteropoly compounds, that is, gas-solid and liquid-solid systems, there are three different classes of catalysis (1) surface catalysis, (2) bulk type 1 (pseudoliquid catalysis), and (3) bulk type II catalysis, as shown in Fig. 1. The latter two have been specifically demonstrated for heteropoly catalysts, and they could be found for other solid catalysts as well. 

Heteropolyanions and isopolyanions are polymeric oxoanions (polyoxometalates) (2, 3, 5, 6). The structure of a heteropolyanion or polyoxoanion molecule itself is called a primary structure (5, 6, 77). There are various kinds of polyoxoanion structure (Section II.A. 1). In solution, heteropoly anions are present in the unit of the primary structure, being coordinated with solvent molecules and/ or protonated. Most heteropolyanions tend to hydrolyze readily at high pH (Section 1I.C). Protonation and hydrolysis of the primary structure may be major structural concerns in solution catalysis. Heteropoly compounds in the solid state are ionic crystals (sometimes amorphous) consisting of large polyanions, cations, water of crystallization, and other molecules. This three-dimensional arrangement is called the secondary structure. For understanding catalysis by solid heteropoly compounds, it is important to distinguish between the primary structure and the secondary structure (5, 6, 17). Recently, it has been realized that, in addition... 

Keggin-type heteropoly compounds having Mo and V as addenda atoms are usually used for such oxidations. The catalysts reported in patents often contain several elements other than Mo, V, and P. An excess amount of P is added to stabilize the structure, and the presence of additional transition elements like Cu improves redox reversibility. Supported heteropoly catalysts are also important for industrial applications and have been characterized (69, 325, 326). 

Heteropoly compounds may be classified according to the ratio of the number of central atoms to the peripheral molybdenum or other such atoms. Compounds with the same number of atoms in the anion usually are isomoiphous and have similar chemical properties. Usually, the heteropolymolybdates and heteropolytungstates containing nontransition elements as central atoms have more structural analogues than those that contain transition elements as central atoms. Table 1 lists all elements... 

Other studies on heteropoly anions include electron paramagnetic resonance on the reduced forms of heteropoly compounds196, 198 and thermal and in situ X-ray studies of some heteropoly compounds200. No thermodynamic data are available on heteropoly compounds. 

VII. Other Sorbents and Their Unique Adsorption Properties Carbon Nanotubes, Heteropoly Compounds, and Pillared Clays 118... 

Association of molybdates (tungstates) with phosphate-like structures leads to a class of compounds called heteropoly compounds or heteropoly acids of Mo or W, hereafter schematically written as Mo —P or W—P heteropoly compounds. The heteropoly anions, which may contain Mo, W, P, and other elements, are paired, both in the solid state and in solution, with cations such as H+, NH4, and Na+. Here, only some Mo —P heteropoly compounds playing an important role in hydrotreating catalysts during the catalyst preparation (impregnation) or as supported oxidic phases are discussed. The Mo — P heteropoly compounds have the following properties 18,19) ... 

Studies of Solid Heteropoly Compounds by Means Other Than X-rays. .. 150... 

At a pH near 6, manganese(II) metavanadate precipitates and does not react further, whereas at a pH near 2, other heteropoly species are formed. Inasmuch as commercial potassium metavanadate is not suitable for the syntheses of the 13-vanadomanganate(IV) and nickelate(IV), directions for the preparation of this compound are included in the syntheses below, which are modified from the method of Flynn and Pope.5 The preparation of the yellow-tinged green reduced form of potassium 13-vanadomanganate(IV) is also included.5... 

Hydroxylation of benzene to phenol using hydrogen peroxide in the presence of heteropoly compounds was observed in this study. Other aromatic hydrocarbons that were used as substrates were toluene, ethylbenzene, o,p-xylene and isopropyl benzene under homogeneous conditions. Both side chain oxidation and ring hydroxylation were observed in presence of hydrogen peroxide. For example, toluene gave benzyl alcohol, benzaldehyde, o,p-cresols in presence of hydrogen peroxide, whereas benzaldehyde and benzyl alcohol were observed in presence of t-BuOOH. 

The other acidic oxides such as WO3-, M0O3-, and V205-based mixed oxides, and heteropoly compounds [H3PM0J2O40, H3PW12O40] are not effective. 

It is concluded that acidic oxides that possess a certain extent of basic property are favorable for promoting the condensation reaction a more basic property is required than in the case of the reaction with carboxylic acid. The acidic oxides such as H-zeolite, Si-Al, B, Mo-P, and W-P oxides, and heteropoly compounds are lacking in a basic property as catalyst for the reaction. On the other hand, alkali and alkaline earth metal oxides and amphoteric oxides combined with K2O are lacking in acidic property. 

Activated carbons are also adequate supports for HPA since the acidity of the heteropoly compounds is preserved [P3j. In the case of microporous carbons the HPA will be deposited on the external surface. However, in any support, if liquid phase processes are used, leaching of the supported HPA can be a serious problem [37]. Conjugated polymers such as polyacetylene, polypyrrole and polyaniline have been used as HPA supports. In the case of polyacetylene the HPA can be introduced on the surface of the polymer, while retaining its acidity [38]. However, unless a very special behaviour could be achieved, this will nor be able to compete with other less expensive inorganic supports. 

Briand, L.E., Baronetti, G.T., and Thomas, H.J. The state of the art on WeUs-Dawson heteropoly-compounds. A review of their properties and applications. Appl. Catal. A Gen. 2003, 256, 37-50 and other papers inclnded in this special issue dedicated to heteropoly-oxo compounds. 

Heteropoly compounds in the solid state consist of heteropolyanions, cations, (protons, and metal or onium ions) and water of crystallization and/or other molecules. This three-dimensional arrangement of polyanions, etc. may be called the secondary structures and the heteropolyanions are denoted the primary structures. It is important for the understanding of heteropoly compounds in the solid state to make a clear distinction between the primary and the secondary structure. 

---