Heteropoly acids, catalysis

The first attempt to synthesize and characterize Kegj -type heteropoly acid supported on various mesoporous silicas and its application to add catalysis in the formation of acetic anhydride via dehydration of acetic acid were described in this study. A variety of characterization techniques such as Na adsorption, TEM and XRD were applied... 

Heteropoly acids can be synergistically combined with phase-transfer catalysis in the so-called Ishii-Venturello chemistry for oxidation reactions such as oxidation of alcohols, allyl alcohols, alkenes, alkynes, P-unsaturated acids, vic-diols, phenol, and amines with hydrogen peroxide (Mizuno et al., 1994). Recent examples include the epoxidations of alkyl undecylenates (Yadav and Satoskar, 1997) and. styrene (Yadav and Pujari, 2000). 

Y. Izumi, R. Hasebe, and K. Urabe, Catalysis by heterogeneous supported heteropoly acid, J. Catal., 84 (1983) 402 -09. 

There has been an enormous technological interest in tertfa/j-butanol (tBA) dehydration during the past thirty years, first as a primary route to methyl te/f-butyl ether (MTBE) (1) and more recently for the production of isooctane and polyisobutylene (2). A number of commercializable processes have been developed for isobutylene manufacture (eq 1) in both the USA and Japan (3,4). These processes typically involve either vapor-phase tBA dehydration over a silica-alumina catalyst at 260-370°C, or liquid-phase processing utilizing either homogenous (sulfonic acid), or solid acid catalysis (e.g. acidic cationic resins). More recently, tBA dehydration has been examined using silica-supported heteropoly acids (5), montmorillonite clays (6), titanosilicates (7), as well as the use of compressed liquid water (8). 

Savinov E. N., Catalytic and photocatalytic properties of heteropoly acids of 12th series in the reaction of hydrogen extraction from aqueous and water-alcohol solutions, Ph.D. Thesis, Institute of Catalysis, Novosibirsk (1983) (in Russian). 

I. V. Kozhevinikov, Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions, Chem. Rev. 98 (1998) 171. 

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... 

Catalysis by heteropoly compounds. III. The structure and properties of 12-heteropoly-acids of molybdenum and tungsten (HjPMOjj and their salts pertinent to heteroge-... 

Selective oxidation catalysis over heteropoly acid supported on polymer... 

Nitroethene is sufficiently electrophilic to substitute indole without the need for acid catalysis. Despite this, it has been shown that siUca-gel-supported CeCl3.7H20/NaI brings about such reactions at room temperature under solvent-free conditions or, to take another solvent extreme, the reaction occurs in water with a catalytic amount of a heteropoly acid (H3PWi204o). The employment of 2-dimethylamino-l-nitroethene in trifluoroacetic acid leads to 2-(indol-3-yl)nitroethene - the reactive species is the protonated enamine and the process is similar to a Mannich condensation (20.1.1.9). The use of 3-trimethylsilyl-indoles, with tpxo-substitution of the silicon, is an alternative means for effecting alkylation, avoiding the need for acid catalysis. 

The use of HPAs and multicomponenf polyoxometalates as catalysts in liquid-phase reactions was reviewed by Kozhevnikov. Moreover, an interesting minireview was published concerning the Friedel-Crafts acylation of arenes and the Fries rearrangement catalyzed by HPA-based solid acids. The results show that HPA-based solid acids, including bulk and supported heteropoly acids as well as heteropoly acid salts, are efficient and environmentally friendly catalysis for all reacfions analyzed. 

Izumi, Y, Matsuo, K., and Urabe, K. 1983. Efficient homogeneous acid catalysis of heteropoly acid and its characterization through ether cleavage reactions. /. Mol. Catal. 18 299-314. 

Misono M. 2001. Unique acid catalysis of heteropoly compounds (hetero-polyoxometalates) in the solid state. Chem. Commun. 1141-1152. 

I. V. Kozhevnikov, Catalysis by Heteropoly Acids and Multicomponent Polyoxometalates in... 

It is important to note that the synthesis of many heterocycles is often carried out under acid-catalyzed reactions, so much effort has been put into the search for solid acid catalysts (Rosati et al., 2007 Dhakshinamoorthy et al., 2011 Sreekumar and Padmakumar, 1998 Kandarpa et al., 2011 Krishnakumar and Swaminathan, 2011 Huang et al., 2008). From this point of view, catalysis by heteropoly acids (HPAs) and related compounds is a field of increasing importance worldwide. To avoid the use of conventional acid catalysts (sulfuric, phosphoric, and hydrofluoric acids and boron trifluoride) and the related environmental pollution and corrosion problems (Vdzquez et al., 2002), insoluble solid acid catalysts such as HPAs can be used. HPAs are mixed oxides composed of a central ion or heteroatom, generally P, As, Si or Ge, bonded to an appropriate number of oxygen atoms and surrounded by a shell of octahedral MOg units. HPAs with Keggin structure and related polyoxometalates are quite common and are represented by the formula Hg [XM,204o], where X is the... 

Da Silva Rocha, K.A., Kozhevnikov, I.V., Gusevskaya, E.V. 2005. Isomerisation of a-pinene oxide over silica supported heteropoly acid H3PW,204q. Applied Catalysis A General 294 106-110. 

Kaur, J., Griffin, K., Harrison, B., Kozhevnikov, I.V. 2002. Friedel-Crafts acylation catalysed by heteropoly acids. Journal of Catalysis 208(2) 448-455. 

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