Catalysts heteropolyacid

Red-ox type catalysts are mostly used in oxidation or related types of reactions. For instance, vanadium catalysts containing ions of different valence state are used in the oxidation of benzene to maleic anhydride. Bismuth molybdate catalyst can be used both for the oxidation or ammoxidation of propene. Anchored metal-complex catalysts combine the advantage of both homogeneous and heterogeneous catalysts, however in these catalysts the molecular character of the active sites is maintained. In the last generation of this type of catalysts, heteropolyacids are fixed first to the support and in the second step different metal-complexes are anchored to the heteropolyacid. In this way highly active and stable catalyst have been prepared for different reactions. ... 

Actually, the large demand for cleaner environment has forcing the chemical industy to develop less environmental hazardous acid catalysts. Heteropolyacid catalysts have attracted considerable amount of interest due to the its easy handling in addition of having higher acidity. They have been widely used in line and chemicals synthesis [6-8], There are so much information related to heteropolyacid synthesis methods, nevertheless, however, only the Keggin-structure heteropolyacids (HPAs) are well described in respect of their physicochemical and catalytic properties [9]. 

Essayem, N., Ben Taarit, Y., Feche, C., Gayraud, P.Y., Saopaly, G., and Naccache, C. Comparative study of n-pentane isomerization over solid acid catalysts, heteropolyacid, sulphated zirconia, and mordenite dependence on hydrogen and pblatinum addition. J. Catal 2003, 219, 97-106. 

In the synthesis of AGE with an acid as the catalyst, aHyl alcohol is added to the epoxy group of epichlorohydrin, yielding 3-aHyloxy-l-chloro-2-propanol [4638-03-3], which then undergoes cyclization with alkaU to yield AGE. Catalysts such as H2SO4, SnCl, BE (C2H )20 (33), heteropolyacids, HQO, andy -CH CgH SO H (34) are used. 

Sodium tungstate is used in the manufacture of heteropolyacid color lakes, which are used in printing inks, plants, waxes, glasses, and textiles. It is also used as a fuel-ceU electrode material and in cigarette filters. Other uses include the manufacture of tungsten-based catalysts, for fireproofing of textiles, and as an analytical reagent for the deterrnination of uric acid. 

Heteropolyacids (HPA) are the unique class of inorganic complexes. They are widely used in different areas of science in biochemistry for the precipitation of albumens and alkaloids, in medicine as anticarcinogenic agents, in industry as catalysts. HPA are well known analytical reagents for determination of phosphoms, silica and arsenic, nitrogen-containing organic compounds, oxidants and reductants in solution etc. 

Preparation, characterization, and catalytic activity of H3PW12O40 heteropolyacid catalyst supported on mesoporous Y-AI2O3... 

Heteropolyacids are also beginning to emerge from academic laboratories and find commercial applications. Showa Denko, for example, claim to have a process [14] for the direct oxidation of ethylene to acetic acid employing a bifunctional Pt/heteropolyacid catalyst system. 

As was stated above, the very strong acidity (and probably together with the organophilicity of the pore wall) makes these salts very active catalysts in liquid-solid organic reaction systems. We wish to emphasize that this is the first example for the shape selective catalysis of heteropolyacids at least to our knowledge. 

Friedel-Crafts alkylations are among the most important reactions in organic synthesis. Solid acid catalysts have advantages in ease of product recovery, reduced waste streams, and reduction in corrosion and toxicity. In the past, people have used (pillared) clays (18), heteropolyacids (19) and zeohtes (20) for Friedel-Craft alkylations, with mixed success. Problems included poor catalyst stabihty and low activity. Benzylation of benzene using benzyl chloride is interesting for the preparation of substitutes of polychlorobenzene in the apphcation of dielectrics. The performance of Si-TUD-1 with different heteroatoms (Fe, Ga, Sn and Ti) was evaluated, and different levels of Fe inside Si-TUD-1 (denoted Fei, Fe2, Fes and Feio) were evaluated (21). The synthesis procedure of these materials was described in detail elsewhere (22). 

Pd(II) Wacker-type catalysts were also studied.146 Selective oxidation of benzene to phenol by molecular oxygen in the presence of Pd and heteropolyacids have been published.147... 

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... 

Cesium salts of 12-tungstophosphoric acid have been compared to the pure acid and to a sulfated zirconia sample for isobutane/1-butene alkylation at room temperature. The salt was found to be much more active than either the acid or sulfated zirconia (201). Heteropolyacids have also been supported on sulfated zirconia catalysts. The combination was found to be superior to heteropolyacid supported on pure zirconia and on zirconia and other supports that had been treated with a variety of mineral acids (202). Solutions of heteropolyacids (containing phosphorus or silicon) in acetic acid were tested as alkylation catalysts at 323 K by Zhao et al. (203). The system was sensitive to the heteropoly acid/acetic acid ratio and the amount of crystalline water. As observed in the alkylation with conventional liquid acids, a polymer was formed, which enhanced the catalytic activity. 

Polymers have served roles in PEM fuel cell cathodes such as modifiers to macrocycle-based electrodes to improve conductivity and stability,165 composite materials with heteropolyacids,166 and as precursors to pyrolyzed catalysts.38,112,132,133 However, as discussed in the previous section, the activity of nitrogen-containing carbon raises the possibility of non-metal electrodes functioning in a cathode environment. Likewise, researchers have noted ORR activity for various conducting polymers containing nitrogen, and recently studies on their potential use in PEM fuel cell cathodes have been reported. 

Magnetic Resonance Studies of Metal Deposition on Hydrotreating Catalysts and Removal with Heteropolyacids... 

The applications of IR spectroscopy in catalysis are many. For example, IR can be used to directly characterize the catalysts themselves. This is often done in the study of zeolites, metal oxides, and heteropolyacids, among other catalysts [77,78], To exemplify this type of application, Figure 1.11 displays transmission IR spectra for a number of Co Mo O (0 < x < 1) mixed metal oxides with various compositions [79]. In this study, a clear distinction could be made between pure Mo03, with its characteristic IR peaks at 993, 863, 820, and 563 cm-1, and the Mo04 tetrahedral units in the CoMo04 solid solutions formed upon Co304 incorporation, with its new bands at 946 and 662 cm-1. These properties could be correlated with the activity of the catalysts toward carburization and hy-drodenitrogenation reactions. 

M. Fournier, C. Fuemi-Jantou, C. Rabia, G. Herve, and S. Launay, Polyoxometalates Catalyst Materials X-Ray Thermal Stability Study of Phosphorus-containing Heteropolyacids Hj xPMi2. xVx04o13-14H20 (M = Mo, W x = 0-1), J. Mater. Chem., 2, 971-978 (1992). 

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... 

Thus, if the incorporation of some metal oxides indicated a notable improvement in the catalytic activity (permitting it to operate at lower reaction temperatures),the incorporation of metals, especially Pt and working in the presence of H2, has prolonged the hfe of the catalysts. However, new catalyst formulations have recently increased the resistance of these catalysts to such poisons as water or sulfur during the isomerization of n-C5 and n-C6 paraffins. Nevertheless, the use of other anions, by supporting WO3 or MoOf or heteropolyacids,which have higher thermal stability, can also be interesting alternative routes to develop new catalytic systems. 

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 first examples of an electrostatie attachment of POMs on solid supports are dated to the mid 1980. Baba et al. reported immobilization of heteropolyacids on the anion-exchange resin Amberlist-15 [68]. Later on, this type of the support was successfully used by Jacob s group to immobilize the Venturello complex P04[W0(02)2]4, well known as highly selective homogeneous catalyst for H202-based epoxidation of alkenes... 

In this context, much effort has also been invested in controlling the nuclearity of the catalyst ensemble through the selection of its precursor. One area in which considerable progress has been made involves the adsorption of polynuclear clusters onto supports [33]. Examples involving the immobilization of small, preformed polynuclear clusters on supports are the reactions of carbonyl clusters of the late metals [16, 34], the binding of polyoxometalates (POMs) and their neutral alkoxy analogues [35] and heteropolyacids such as the Keggin cluster [36, 37]. 

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