Heteropolyacids additives

Fujiwara et al. [94] found that, when present as a heteropolyacid complex, molybdenum(VI), germanium(IV), and silicon(IV) produced CL emission from the oxidation of luminol, and similar CL oxidation of luminol was observed for arsenic(V) and phosphorus(V) but with the addition of the metavanadate ion to the acid solution of molybdate. A hyphenated method was therefore proposed for the sensitive determination of arsenate, germanate, phosphate, and silicate, after separation by ion chromatography. The minimum detectable concentrations of arsenic(V), germanium(IV), phosphate, and silicon(IV) were 10, 50, 1, and 10... 

Since S03/H2S04 is clearly not the most desirable system for industrial applications, a formidable challenge is to find an oxidant that oxidizes Pt(II) much faster than S03 does, operates in an environmentally friendly solvent, and can be (like SVI/SIV) reoxidized by oxygen from air. Ideally, the reduced oxidant would get reoxidized in a continuous process, such that the oxidant acts as a redox mediator. In addition, the redox behavior has to be tuned such that the platinum(II) alkyl intermediate would be oxidized but the platinum(II) catalyst would not be completely oxidized. Such a system that efficiently transfers oxidation equivalents from oxygen to Pt(II) would be highly desirable. A redox mediator system based on heteropolyacids has been reported for the Pt-catalyzed oxidation of C-H bonds by 02, using Na8HPMo6V6O40... 

They present strong acidities (the pH values of aqueous solutious of heteropolyacids indicate that they are strong acids) both in solid and in liquid solution (Figure 13.2). In addition, they can be prepared in an wide range of surface areas (partially salified heteropolyoxometalates permit the modification of the surface areas of these materials) or be supported in metal oxides. 

It can be seen that the catal5dic activity strongly depends on the number and t) pe of the incorporated countercation, which determines the number and strength of acid sites. In addition to this, the existence of mesoporosity (which also depends on the countercation) is also a key factor in the catalytic behavior of these catalysts.In this way, Si02- or MCM-41-supported heteropolyacids also have been studied in order to increase catalytic activity, apparendy without modifying the acid strength. ... 

The addition of carboxylic acids to olefins proceeds in solution in the presence of 10—4— 10 2 mol dm-3 of H3PW12O40 at 293-413 K with a selectivity of 100% (7). H2S04 is a less active catalyst than H3PW12O40 by a factor of 30-90. Esterification of /j-nitrobenzoic acid with ethanol has been carried out by using H2SO4 catalyst in an industrial process 160). In the presence of H3PW12O40, this reaction takes place with a yield >99% 160). At the end of the reaction, the reaction solution separates into two phases. The upper layer contains ethyl-/ -nitrobenzoate, toluene, and ethanol, and the lower layer consists of a solution of the heteropolyacid in ethanol. Consequently, the catalyst can be readily separated and reused. 

The objective of the present study was to analyze the reasons for the deactivation effects observed in solid Wacker-type catalysts for 1-butene oxidation. For this purpose the catalytic behavior and characteristics of Pd-V205 on alumina catalysts, prepared using either a N PdC or a PdS04 salt, were compared with those of alternative catalysts prepared by substituting the V-oxide with Ce02 in order to obtain a better understanding of the role of V-oxide. In addition, the behavior of a Pd-doped V-heteropolyacid also is discussed to further extend the analysis. 

In the group of Backvall a method was developed involving palladium and benzoquinone as cocatalyst (Fig. 4.42) [103]. The difficulty of the catalytic reaction lies in the problematic reoxidation of Pd(0) which cannot be achieved by dioxygen directly (see also Wacker process). To overcome this a number of electron mediators have been developed, such as benzoquinone in combination with metal macrocycles, heteropolyacids or other metal salts (see Fig. 4.42). Alternatively a bimetallic palladium(II) air oxidation system, involving bridging phosphines, can be used which does not require additional mediators [115]. This approach would also allow the development of asymmetric Pd-catalyzed allylic oxidation. 

In addition to heteropolyacids there are isopolyacids, the commonest of which are the parent acids of the paramolybdates, Hg[Mo7024], and of the paratungstates, H5[HWg02i]. The structure of the [Mo7024] ion (Lindqvist, 1950) is like that of the 6-acid anions (above) but with a central, octahedrally co-ordinated Mo atom. 

Conversion of TIPB in dependence on W content, activation temperature (623 and 773 K) and time on stream (TOS) is shown in Figures 6 and 7. It follows that the impregnation of H-ZSM-5 with heteropolyacid has altered the catalytic properties of the external zeolite surface. Decomposition of TIPB distinctly differs in dependence on the activation temperature. All samples exhibited very low initial activities (TOS =15 min) after activation at 623 K. Activation at 773 K primarily enhanced the initial activity of the zeolite itself (sample OW-ZSM-5) but, nevertheless, activities of the modified samples were also considerably increased, even after 55 min time on stream. Obviously, blockage of acidic zeolite sites is mostly removed by calcination at 773 K, possibly due to aggregation of dispersed heteropolyacid but, additionally, this aggregated heteropolyacid should partly contribute to the TIPB conversion because the activity of the modified samples 2W-ZSM-5 and 4W-ZSM-5 exceeds that of the unmodified ZSM-5. 

In gas phase oxidations usually water is also added to the stream. This is supposed to guarantee stable catalytic performance and higher activity, likely because the presence of water can favour the surface reconstruction of the heteropolyacid even under conditions at which it would usually decompose. In addition, water may favour the desorption of the products, saving them from unselective eonsecutive combustion. 

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. 

A process for the coproduction of acetic anhydride and acetic acid, which has been operated by BP Chemicals since 1988, uses a quaternary ammonium iodide salt in a role similar to that of Lil [8]. Beneficial effects on rhodium-complex-catalyzed methanol carbonylation have also been found for other additives. For example, phosphine oxides such as Ph3PO enable high catalyst rates at low water concentrations without compromising catalyst stability [40—42]. Similarly, iodocarbonyl complexes of ruthenium and osmium (as used to promote iridium systems, Section 3) are found to enhance the activity of a rhodium catalyst at low water concentrations [43,44]. Other compounds reported to have beneficial effects include phosphate salts [45], transition metal halide salts [46], and oxoacids and heteropolyacids and their salts [47]. 

Heteropolyacids have been used as catalysts in a variety of acid-catalyzed and oxidation reactions.273 A few examples will be given to show the advantages of these catalysts. Showa Denko will use a heteropolyacid catalyst in a new plant that it is building for the addition of acetic acid to ethylene to produce ethyl acetate.274 Butyl acrylate can be made with 96% selectivity at 98% conversion in a flow system using H3PWi204o on carbon.275 The activity of immobilized... 

The catalyst systems employed are based on molybdenum and phosphorus. They also contain Various additives (oxides of bismuth, antimony, thorium, chromium, copper, zirconium, etc.) and occur in the form of complex phosphomolybdates, or preferably heteropolyacids deposited on an inert support (silicon carbide, a-alumina, diatomaceous earths, titanium dioxide, etc.). This makes them quite different from the catalysts used to produce acrylic acid, which do not offer sufficient activity in this case. With residence times of 2 to 5 s, once-through conversion is better than 90 to 95 per cent, and the molar yield of methacrylic acid is up to 85 to 90 per cent The main by-products formed are acetic add, acetone, acrylic add, CO, C02, etc. The major developments in this area were conducted by Asahi Glass, Daicel, Japan Catalytic Chemical, Japanese Gem, Mitsubishi Rayon, Nippon Kayaku, Standard Oil, Sumitomo Chemical, Toyo Soda, Ube, etc. A number of liquid phase processes, operating at about 30°C, in die presence of a catalyst based on silver or cobalt in alkaline medium, have been developed by ARCO (Atlantic Richfield Co,), Asahi, Sumitomo, Union Carbide, etc. 

Thus, the addition of silica and heteropolyacid modifies the relative ratio between crystalline and amorphous structure of ctist Nafion with respect to bare Ntifion. Yet, due to the low loading of silica and heteropolyacid in the membrane (3% silica, less than 1.5% PWA), it appears unlikely that the inorganic components are responsible of such significant modification in the Nafion structure. More probably, the observed changes have to be attributed to the final thermal treatment (160°C). 

Cobalt chloride in diglyme is a useful catalyst for benzylic [22a] and allylic [22b] oxidation under mild conditions. The addition of sodium azide to oxidations catalyzed by transition metal acetylacetonates, heteropolyacids, phtha-locyanines, bis-(pyridylimino)isoindolines, porphyrins and Schiff bases significantly enhances the rates of the low-temperature catalytic oxidation of alkanes [22c]. Ethylbenzene is slowly oxidized by air in MeCN in the presence of catalytic amount of chromium trioxide [23]. Complexes of Fe(III) and Co(II)... 

The method fails with heteropoly salts whose aqueous solutions exhibit a strong acid reaction. Additional complications arise if salt impurities (NaCl, NaNOa, etc.) are present in the solution, since these salts produce HCl, HNOa, etc., during passage through the column. A too strongly acid medium hinders the formation of free, crystalline heteropolyacids during concentration of the eluate. 

The one advantage of this method has already been mentioned. The disadvantages are that one must begin with pure, crystalline alkali salts (which in some cases can only be obtained by the roundabout route of first preparing the free acid by Drechsel s method), and, in addition, the heteropolyacid solutions obtained by ion exchange are often relatively dilute so that their concentration is time-consuming. 

Oxidative coupling of other arenes (e.g., toluene, benzene, methyl benzoate) has also been investigated. The activity of Pd catalysts is enhanced by various additives, including P-diketones [36], Zr [37], Mo02(acac)2 [38], Zr /Co /Mn [39], HPMoV-type heteropolyacids (HPA) [40], and triflic acid [41]. Acetic acid is a common solvent for these reactions. It was disclosed recently that supported nanoparticle Au catalysts can be applied as catalysts in reactions of this type [42]. 

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