Heteropolyacids, heteropolyanions

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... 

As excellent candidates for design at the atomic or molecular level, heteropoly catalysts have proven to be of value in fundamental studies as well as practical applications. But it is also true that much remains to be done. Efforts to establish methodologies for design of practical catalysts are still under way. The acid strength and acid site density can be controlled quite well both in solution and in the solid state, but the redox properties in the solid state are much less well understood because of the lack of sufficient thermal stability of mixed-metal (mixed-addenda) heteropolyanions. The acid strengths of some solid heteropolyacids have been suggested to reach the range of superacids, but they... 

For acid forms, polyacids = polyoxoacids, including heteropolyacids (e.g., H3PW12O40) and isopolyacids (e.g., H2Mo60, ) and for oxoanions, polyanions = polyoxoanions = polyoxometalates, including heteropolyanions (e.g., PW12O40") and isopolyanions (e.g., Mo60 9 ). 

Heteropolyacids are prepared in solution by acidifying and heating in the appropriate pH range (], 49-54). For example, 12-tungstophosphate is formed according to Eq. (1). Free acids are synthesized primarily by the following two methods (1) by extraction with ether from acidified aqueous solutions and (2) by ion exchange from salts of heteropolyacids. Dawson-type heteropolyanions,... 

Mixed addenda heteropolyanions with regiospecific substitution need careful preparation by use of lacunary heteropolyanions. If they are prepared from aqueous solutions of corresponding oxoanions, the products are usually mixtures of heteropolyanions having different compositions of addenda atoms. General procedures for the syntheses of various kinds of heteropolyacids are described in the literature (51-54). 

Particular attention should be paid to both the stability in solution and the thermal stability. The condesation-hydrolysis equilibria of heteropolyanions in aqueous media are shown in Fig. 8. Each heteropolyanion is stable only at pH values lower than the corresponding solid line (55). Some solid heteropolyacids are thermally stable and applicable in reactions with vapor-phase reactants conducted at high temperatures. The thermal stability is measured mainly by X-ray diffraction (XRD), thermal gravimetric analysis, and different thermal analysis (TG-DTA) experiments. According to Yamazoe et al. (56), the decomposition temperatures of H3PM012O40 and its salts depend on the kinds of cations Ba2 +, Co2+ (673 K) < Cu2+, Ni2+ (683 K)[Pg.127]

The strength and the number of acid centers as well as related properties of heteropolyacids can be controlled by the structure and composition of heteropolyanions, the extent of hydration, the type of support, the thermal pretreatment, etc. 

Heteropolyacids are more active catalysts for various reactions in solution than conventional inorganic and organic acids, and they are used as industrial catalysts for several liquid-phase reactions (6, 12). Important characteristics accounting for the high catalytic activities are the acid strength, softness of the heteropolyanion, catalyst concentration, and nature of the solvent (6, 7, 9, 116, 160, 161). 

A commercial process for the separation of isobutylene from a mixture of isobutylene and -butenes through direct hydration of isobutylene to give tert-butyl alcohol has been established by use of a concentrated solution of heteropolyacids (6, 163, 170). The reaction order in the heteropolyanion varies from 1 to 2 as the concentration of heteropolyanion increases from 0.05 to 10 mol dm-3. This increase corresponds to a change from the first to the second term in Eq. (11a). At concentration of the heteropolyanion greater than 0.5 mol dm-3, path B in Scheme 3 becomes dominant. 

Heteropolyacids catalyze the Prins reaction of alkenes [Eq. (14)] more efficiently than H2SO4 and /j-toluenesulfonic acid (PTS). For example, H3PW12O40 is 10-50 times more active than H2S04 or PTS (162). In this reaction, oxocarbo-cations may be stabilized through complexation with heteropolyanions ... 

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. 

It has been pointed out that these pillared intercalates are intrinsically difficult to synthesize in highly crystalline form because the layered hosts are basic, whereas most heteropolyacids are acidic and tend to decompose. Narita et al. (392) tried direct synthesis of a heteropolyanion-pillared layered double hydroxide by a coprecipitation reaction of Zn2+ and A1J+ ions in the presence of a moderately acidic lacunary Keggin anion, a-SiWn039 XRD of the product showed a basal spacing of 14.6 A, which corresponds to a gallery height of 9.9 A. The surface area was found to be 97 m2 g, which is three times that of the layered double hydroxide. 

The heteropolyanion stabilizes protonated intermediates by coordination in solution and the pseudoliquid phase as well as on the surface, thus lowering the activation energy and accelerating reactions. Several protonated intermediates including the protonated ethanol dimer and monomer [18], the protonated pyridine dimer [12, 19], and protonated methanol [20] have been detected in the pseudoliquid phase directly by use of X-ray diffraction (XRD), IR or solid-state NMR. In solid-state H NMR, the chemical shift for the protonated ethanol dimer, (C2HsOH)2H+ is 9.5 ppm down-field from tetramethylsilane, which lies in the range of supcracids reported by Olah et al. [18]. This fact also supports the strong acidity of heteropolyacids. 

Keggin-type heteropoly compounds have attractive and important characteristics in terms of catalysis. They consist of heteropolyanions and counter-cations such as H, Cs or NHT When the counter-cations are protons, they are called heteropolyacids (HPA). An important characteristic of HPAs, such as 12-tungstophos-phoric acid (H3PW12O40), is the presence of very strong Bronsted acid sites. But the characteristics of HPAs strongly depend on temperature and relative humidity. When they are used in heterogeneous catalysis, it is often necessary to support them on high-surface-area oxides or activated carbons, in order to increase the surface contact with the reactants. 

The number and strength of the acid centers of tungstic heteropolyacids have been determined by ammonia adsorption calorimetry. Ammonia is irreversibly absorbed, with the formation of the corresponding ammonium salts. An increase in the number of protons in Keggin heteropolyanions decreases the acidic strength. 

Heteropolyacids are polyoxometalates incorporating anions (heteropolyanions) having metal-oxygen octahedra as the basic structural units. Among a wide variety of heteropoly acids those belonging to the so-called Keggin series are the most important for catalysis. They include heteropolyanions where X is the central atoms (P, Si , etc.), y... 

Characteristic features of vanadium containing heteropoly catalysts for the selective oxidation of hydrocarbons have been described. MAA yield ftom isobutyric acid was successfully enhanced by the stabilization of the vanadium-substituted heteropolyanions by forming cesium salts. As for lower alkane oxidation by using vanadium containing heteropoly catalysts, it was found that the surface of (V0)2P207 was reversibly oxidized to the Xi (8) phase under the reaction conditions of n-butane oxidation. The catalytic properties of cesium salts of 12-heteropolyacids were controlled by the substitution with vanadium, the Cs salt formation, and the addition of transition metal ions. By this way, the yield of MAA from isobutane reached 9.0%. Furthermore, vanadium-substituted 12-molybdates in solution showed 93% conversion on H2O2 basis in hydroxylation of benzene to phenol with 100% selectivity on benzene basis. 

Heteropolyacids (HPA s) [27] and their salts are polyoxo compounds incorporating anions (heteropolyanions) having metal-oxygen octahedra (MO6) as the basic structural unit. They contain one or more heteroatoms (Si, Ge, P, As, etc.) that... 

Films of oxides can be produced by anodization of metal electrodes. For example, AI2O3 forms on an aluminum anode immersed in a solution of H3PO4. The thickness of the film can be controlled by the applied potential and the time of anodization. Such a film can be used as a support for other materials, such as poly(vinylpyridine) (PVP). Oxide films of other metals, such as Ti, W, and Ta, can be produced in a similar way. Oxide films can also be produced by CVD, vacuum evaporation and sputtering, and deposition from colloidal solution. Related inorganic films are those of polyoxometallates (iso- and heteropolyacids and their salts) (20). For example, the heteropolyanion P2W17M0O62K6 shows a number of reduction waves at a glassy carbon electrode. A wide variety of metallic polyanionic species (e.g., of W, Mo, V) exist and have a rich chemistry. Films of such materials are interesting for their electrocatalytic possibilities. 

In Table 4.2 the positions of the Keggin unit characteristic modes for crystalline heteropolyacids are listed and compared with the same modes observed for heteropolyanions inserted into polyaniline. Band assignments are taken from [82]. 

A class of catalysts that is being increasingly used in organic synthesis is the strongly acidic heteropolyacids (HPA). These catalysts are polymeric 0x0-metalates or metal oxide clusters incorporating heteropolyanions formed by the condensation of different oxoanions, for example. 

The etherate method requires a strongly acidified aqueous solution of the heteropolyanion (from an aqueous-soluble heteropoly-salt) that is shaken with diethyl ether in order to separate three phases an upper ether layer, an aqueous layer, and a heavy oily layer. This layer contains an etherate of the heteropolyacid whose nature is unknown. The etherate is decomposed with water, and the solution is evaporated... 

Raman spectroscopy allowed determining the purity of the heteropolyacids in the solid state. Figure 5.4 shows the spectra of the starting salts and the corresponding acids, both fuUy hydrated. The absence of the signals of WO3 (803, 713, 607, and 275 cm" ) and M0O3 (987, 815, 664, and 279 cm ) provides evidence of the purity of the fosfotungstic and fosfomolybdic heteropolyanions phases, respectively. 

During the reaction, the heteropolyacids were reduced, but maintained their structure intact [110]. By using heteropolyacids in different states of reduction (obtained by reaction of H3PM012O40 with L-ascorbic acid) as starting modifiers, it was determined that the catalytic activity of palladium increased with the extent of the reduction of the heteropolyanion until each anion was reduced with three electrons. This observation indicates that the active form of the heteropolymolybdate was [PMo Mo i2-n04o] " (n > 3), in line with the higher activity shown by more easily reduced heteropolyanions, which can more easily reach and maintain this oxidation state. It was also tested that the reduced form of the heteropolyanion is not able to reduce nitrobenzene in the absence of palladium (however, photoreduced heteropolytungstates can effect this reaction [114]). The complexity of the reaction mechanism is also indicated by the fact that the reaction was first order in CO pressure and catalyst concentration, but showed 0.43-order dependence on the concentration of nitrobenzene. 

Immobilization of Keggin-type and Dawson-type heteropolyanions (HPA) PMoi204o> SiWi204o and P2W18O62 in PMT is performed by electropolymerization of 3-methylthiophene in aqueous and nonaqueous solution in the presence of corresponding heteropolyacids and salts. It is possible to obtain electrodes modified with HPA molecules. These are stable in acidic solutions [696-699]. 

The use of heteropoly anions as catalysts, however, was not new for the Eni researchers involved in oxidation chemistry. Some very peculiar heteropolyanions had been discovered at the Istituto Donegani at the beginning of the 1980s. Basically, they were quaternary ammonium (or phosphonium) salts of a new class of peroxidic tungsto-phosphates or arsenates in which the W (or Mo) to P (or As) ratio was as low as four the best known, a typical example of this class, is the PW4O2J anion, which was also the first peroxidic derivative of a heteropolyacid whose structure was solved. Both the free acid and the reduced forms of these heteropolyanions are unstable, and it was not possible to isolate them. 

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