Heteropoly compounds as catalyst

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... 

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%. 

It is increasing interested in the utilization of heteropoly compounds as catalysts for the oxidation of various organic compounds. Recently, we have found that the mixed addenda heteropolyoxometallates such as (NH4)5H4PV0Mo0O4o 6H2O (PV0MO0) was efficient catalysts for the aerobic oxidation of olefinic compounds in the presence of isobutyraldehyde. We now present here the direct oxidation of amines and hydrocarbons with molecular oxygen by PV0MO0 catalyst. 

The first attempts at the use of heteropoly compounds as catalysts for the partial oxidation of propane can be dated back to the early 1980s with the research conducted by Rohm and Haas company [40] (Table 13.3). Since then, many and very different HPCs are studied for this reaction, but none can achieve acryUc acid yields higher than 15%. 

The catalytic properties of heteropoly compounds have drawn wide attention in the preceding two decades owing to the versatility of these compounds as catalysts, which has been demonstrated both by successful large-scale applications and by promising laboratory results. 

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. 

Some research groups have used heteropoly compounds as Mo-P precursors for preparation of hydrotreating catalysts (22, 25, 42). As de-... 

Izumi, Y., Ono, M., Kitagawa, M., Yostiida, M., and Urabe K. 1995. Silica-included heteropoly compounds as solid acid catalysts. Microporous Mater. 5 255-262. 

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... 

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... 

Reviews on solid acids289,290and heteropoly compounds, applied in both the heterogeneous289-292 and homogeneous phases,293 briefly discuss the use of these materials as catalysts in various isomerization processes. 

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... 

Catalytic oxidation of propylene to acrolein was first discovered by the Shell group in 1948 on Cu20 catalyst (/). Both oxidation and ammoxidation were industrialized by the epoch-making discovery of bismuth molybdate catalyst by SOHIO (2-4). The bismuth molybdate catalyst was first reported in the form of a heteropoly compound supported on Si02, Bi P,Mo,2052/Si02 having Keggin structure but it was not the sole active species for the reactions. Several kinds of binary oxides between molybdenum trioxide and bismuth oxide have been known, as shown in the phase... 

One of the authors of this chapter has previously reviewed heterogeneous catalysis by heteropoly compounds (4-6). Catalysis in solution has also been described (7-10). In this chapter, we critically survey the literature and attempt to describe the essence of the catalytic chemistry of heteropoly compounds in solution and in the solid state. We have attempted to highlight the advantages of heteropoly catalysts as described in Table I. 

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. 

Hence, to a first approximation, the rate of reduction of these heteropoly compounds by CO expresses the oxidizing ability of the surface, whereas, as described above, the rate of reduction by H2 reflects the oxidizing ability of the catalyst bulk. If the former rate is divided by the surface area and the latter normalized to the catalyst mass, both oxidizing abilities decrease monotonically with the extent of neutralization with alkali (Figs. 53a and 53c). Although it is not shown in Fig. 53, CS2 5H0.5PM012O40, a class B salt that has a high surface area, is reduced exceptionally rapidly. 

The mixed-addenda atoms affect the redox properties mixed-addenda heteropoly compounds are used as industrial oxidation catalysts. For example, the rate of reduction by H2 is slower and less reversible for solid PMO 2-,VJto m+, than for solid PM012O40, although the former are stronger oxidants than the latter in solution (279, 280). The effects of substituting V for Mo on the catalytic activity are controversial (279, 281-284). Differences in redox processes between solutions and solids, the thermal or chemical stability of the heteropoly compounds, and the effects of countercations in solids have been suggested to account for the discrepancies. 

Hydrogen peroxide and alkyl hydroperoxides are important oxidants in organic synthesis, but they usually need to be activated by catalysts such as tungsten, molybdenum, and titanium oxides. Heteropoly compounds are good catalysts for oxygenation of olefins or paraffins and oxidative cleavage of vic-diols. 

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. 

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). 

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). 

Oxyfunctionalization of lower paraffins such as methane, ethane, propane, and butanes has recently attracted much attention (5, 330, 331, 347-350). Oxidation of -butane to maleic anhydride is an industrial example (346, 351). The oxidation of propane and isobutane with heteropoly catalysts was first reported in 1979 (352). Ai (324a) and Centi et al. (324b, 324c) reported that heteropoly compounds catalyze the oxidation of lower paraffins, especially propane, isobutane, and pentane (324). 

Heteropoly compounds are useful as catalysts due to the following unique characteristics ... 

Several acid-catalyzed reactions are used as test reactions to demonstrate the shape selectivity of the microporous heteropoly compound, Cs2.1, having only micropores. Catalytic activities of Pt-Cs2.1 and Pt/Si02 toward the oxidation of various molecules are summarized in Table 12. Two catalysts are active for the oxidation of CH4, CO, and... 

POMs are promising catalysts for acid, redox and bifunctional catalysis. In many structures, the transition metal addenda atoms such as Mo or W exist in two oxidation states, which results in different redox properties as determined by polarog-raphy. The exceptional ability of heteropolyanions to act as electron reservoirs has been demonstrated by the preparation and characterization of numerous reduced derivatives [32]. They also exhibit high solubility in polar solvents, which means that they can be used in homogeneous catalysis. The wide range of applications of heteropoly compounds are based on their unique properties which include size, mass, electron and proton transfer (and hence storage) abilities, thermal stability. 

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) ... 

The order of stability also depends on the nature of the central atom (Si > Zr, Ti > Ge > P > As) and on the nature of the surrounding anion groups (W > Mo > V). This classification has some relevance to hydrotreating catalysts if, for example, introduction of Si into alumina as a support (giving silica-alumina, zeolite, etc.) leads to the formation of Mo —Si or W —Si heteropoly compounds, they will not be degraded during calcination. 

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