Heteropoly compounds acid-catalyzed reactions

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... 

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%. 

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. 

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... 

The latter property is important for the catalytic activity of heteropoly compounds in acid-catalyzed reactions. With certain heteropoly salts and polar reactants, catalytic reactions such as alcohol dehydration occur in the bulk of the solid catalyst, not only on its surface. The solid behaves like a very concentrated acid solvation medium for the reactant, which allows these solid acids to be called pseudoliquids [118],... 

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... 

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. 

Fe(III), Ce(IV), Ru(II) and monomeric V species (e.g., [V0 0-i-Pr 3]) also lead to cyclohexanone conversions that are as good as the heteropoly-compounds in this class of reactions, but with lower selectivity to the diacids. However, better performance is obtained when the reaction is catalyzed by Cu(N03)2 [131]. At 110 ° C and 8 h reaction time, 95% cyclohexanone conversion with 72% yield to AA, 8% to glutaric and 10% to succinic acid were obtained in an acetic acid-water solvent. A similar performance was reported with Mn(OAc)2 in acetic acid-CFsCOOH solvent, at 65 °C after a 3-h reaction time 99.8% conversion, 75% yield of AA, 9% to glutaric acid and 1% to succinic acid [14jj. 

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