Acidity heteropoly compounds

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. 

It is clear that the V-P oxide and Mo-P heteropoly compound catalysts are not effective for the production of citraconic anhydride. The acidic catalysts such as... 

Acidity, 27 284, 285 catalytic performance, 30 121 crystalline titanium silicates, 41 319-320 estimating, 37 166 heteropoly compounds, 41 139-150 ion exchange and, zeolites, 31 5-6 sulfate-supported metal oxides, 37 186-187 surface, monolayer dispersion, 37 34-35 tin-antimony oxide, 30 114-115, 125-1256 Acids, see also specific compounds adsorption of, on oxide surfaces, 25 243-245... 

Heterogenous reactions, Sh/Nu ratio, 27 64 Heteroligand complex, 32 260-262 Heteropolyacids defined, 41 117 heteroatoms, 41 118, 120, 121 Prins reaction, 41 156 supported, 41 149-150 Heteropolyanions, 41 113, 117, 119-121 Heteropoly blues, 41 191 Heteropoly compounds absorption, 41 179-180, 190-191 acid-catalyzed reactions heterogeneous, 41 161-178 liquid phase, 41 150-161 acidic properties in solid state, 41 141-150 in solution, 41 139—14] catalysis, 41 114, 116-117, 190-191 as catalyst, 41 113-116, 117, 223-232... 

In addition to large-scale industrial applications, solid acids, such as amorphous silica-alumina, zeolites, heteropoly acids, and sulfated zirconia, are also versatile catalysts in various hydrocarbon transformations. Zeolites are useful catalysts in fine-chemical production (Friedel-Crafts reactions, heterosubstitution).165-168 Heteropoly compounds have already found industrial application in Japan, for example, in the manufacture of butanols through the hydration of butenes.169 These are water tolerant, versatile solid-phase catalysts and may be used in both acidic and oxidation processes, and operate as bifunctional catalysts in combination with noble metals.158,170-174 Sulfated zirconia and its modified versions are promising candidates for industrial processes if the problem of deactivation/reactivation is solved.175-178... 

The constitution of many of the heteropoly-acids and of their salts may thus be explained, although direct proof of the correctness of some of these formulae is lacking. Some of the heteropoly-compounds of vanadium are, however, very complex, and cannot be represented by application of the foregoing scheme. To take an extreme instance, Rogers1 prepared a black, crystalline compound to which he ascribed the following formula —... 

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. 

Early attempts to use heteropoly compounds as catalysts are summarized in reviews published in 1952 (//) and 1978 (7). The first industrial process using a heteropoly catalyst was started up in 1972 for the hydration of propylene in the liquid phase. The essential role of the Keggin structure in a solid heteropoly catalyst was explicitly shown in 1975 in a patent concerning catalytic oxidation of methacrolein. Systematic research in heterogeneous catalysis with these materials started in the mid-1970s and led to the recognition of quantitative relationships between the acid or redox properties and catalytic performance... 

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. 

Results from TG and DTA show the presence of two types of water in heteropoly compounds, i.e., water of crystallization and constitutional water molecules (59). Loss of the former usually occurs at temperatures below 473 K. At temperatures exceeding 543 K for H3PMoI204o or 623 K for H3PW12O40, the constitutional water molecules (acidic protons bound to the oxygen of the polyanion) are lost. Data obtained by in situ XRD, 31P NMR, and thermoanalysis show that thermolysis of H3PM012O40 proceeds in two steps, as shown by Eq. (6) (60). 

Acidic properties of heteropoly compounds in the solid state are sensitive to countercations, constituent elements of polyanions, and tertiary structure. Partial hydrolysis and inhomogeneity of composition brought about during preparation are also important in governing the acidic properties. There are several possible types of origins of acidity (5) ... 

In this section, these influences will be described. Besides the acidic properties, the absorption properties of solid heteropolyacids for polar molecules are often critical in determining the catalytic function in pseudoliquid phase behavior. This is a new concept in heterogeneous catalysis by inorganic materials and is described separately in Section VI. With this behavior, reactions catalyzed by solid heteropoly compounds can be classified into three types surface type, bulk type I, and bulk type II (Sections VII and IX). Softness of the heteropolyanion is important for high catalytic activity, although the concept has not yet been sufficiently clarified. 

Cs2.5 for the acylation. Anisole and /j-xylene are acylated with benzoic anhydride and acetic anhydride in the presence of Cs2.5 without the dissolution of this catalyst. Carboxylic acids are much less reactive as acylating agents than the corresponding anhydrides because of the liberation of water. But when the water is removed, the acylation proceeds smoothly 214). Although the reaction of benzene with acetic acid is attractive in prospect, there is no report of heteropoly compounds as catalysts for this reaction. 

A wide variety of acid-catalyzed reactions besides those described above have been investigated with heteropoly compounds as catalysts. Al203-supported H3PW12O40 (probably decomposed) catalyzed propylene-ethylene codimerization at 573 K to form pentenes with a selectivity of 56% (butenes 17%, hexenes 27%) (224). Propylene oligomerization proceeded on various kinds of salts of H3PWl204o (225). The activities of the salts decrease in the order A1 > Co > Ni, NH4 > H, Cu > Fe, Ce > K. The A1 salt gave trimers with 90% conversion at 503 K. The selectivities to trimer are about 40% for Al, Ce, Co, and Cu, while that of the acid form is 25%. 

Solid heteropoly compounds are suitable oxidation catalysts for various reactions such as dehydrogenation of O- and N-containing compounds (aldehydes, carboxylic acids, ketones, nitriles, and alcohols) as well as oxidation of aldehydes. Heteropoly catalysts are inferior to Mo-Bi oxide-based catalysts for the allylic oxidation of olefins, but they are much better than these for oxidation of methacrolein (5). Mo-V mixed-oxide catalysts used commercially for the oxidation of acrolein are not good catalysts for methacrolein oxidation. The presence of an a-methyl group in methacrolein makes the oxidation difficult (12). The oxidation of lower paraffins such as propane, butanes, and pentanes has been attempted (324). Typical oxidation reactions are listed in Table XXXI and described in more detail in the following sections. 

To understand oxidation catalysis by solid heteropoly compounds, the contrast between surface and bulk type II catalysis, and acid-redox bifunctionality... 

This reaction is another possible route for the production of methacrylic acid, since isobutyric acid can be obtained by an oxo process from propene and CO. Heteropoly compounds and iron phosphates are so far the most efficient catalysts for the reaction. The favorable role of the presence of an a-methyl group is remarkable for oxidative dehydrogenation, as the heteropoly compounds are not good catalysts for the dehydrogenation of propionic acid (338, 339). 

Okuhara, Mizuno, and Misono report the catalytic properties of heteropoly compounds as exemplified by H,PWl3O40 and the anion [PW,2O40p. Some of these compounds are strongly acidic, and some have redox properties the large-scale applications involve acid-catalyzed reactions. The heteropoly compounds are metal oxide clusters, used as both soluble and solid catalysts. Their molecular character provides excellent opportunities for incisive structural characterization and for tailoring of the catalytic properties. Physical properties also affect catalytic performance. Catalysis sometimes occurs on the surface of the solid material, and sometimes it occurs in the swellable bulk. 

The 6-heteropoly compounds of molybdenum have been studied extensively and are well understood. Typical examples are the 6-molybdotellurate anion, [TeMo6024]-6, and the 6-molybdochromate(III) anion, [CrMo6024H6]-3. These are usually prepared by the reaction of ammonium paramolybdate with a soluble salt of the central atom7. Some of these are stable only in very mild acid solution and caution should thus be exercised in preparing them. 

Mild acidification of molybdate-phosphate solutions yielded the colorless [P2Mos023]"6 anion, isolated as the sodium salt48,49 The heteropoly acid containing pentavalent antimony, H3[SbMo12O40] 48 H20, was reported to have been prepared by refluxing M/20 molybdic acid with M/75 potassium pyroantimonates°), however, more work is necessary to elucidate to exact nature of molybdenum heteropoly compounds containing antimony. 

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