Supported heteropoly compounds

Soeda et al. (358) used Pd-supported heteropoly compounds for a heterogeneous Wacker-type reaction and found that Pd/Cs2 5H0 5PW6Mo6C)4o was active for oxidation of cyclohexene to produce cyclohexanone and cyclohex-enone. The active sites are assumed to be Pd2+ and Pd° for the two products, respectively. Homogeneous Wacker-type reactions are described in Section XI. 

Acidity, 27 284, 285 catalytic performance, 30 121 crystalline titanium silicates, 41 319-320 estimating, 37 166 heteropoly compounds, 41 139-150 ion exchange and, zeolites, 31 5-6 sulfate-supported metal oxides, 37 186-187 surface, monolayer dispersion, 37 34-35 tin-antimony oxide, 30 114-115, 125-1256 Acids, see also specific compounds adsorption of, on oxide surfaces, 25 243-245... 

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... 

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... 

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%. 

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). 

A weak support effect, as shown in Fig. 59, is another indication of bulk type II behavior (327). With an increase in the loading of the heteropoly compound on the support, the rate of bulk type II catalysis increases to high loading levels, whereas the rate of surface catalysis shows saturation at relatively low loadings because of the decrease in the dispersion of the heteropoly compound on the support (327). 

Heteropoly compounds, nanostructured adsorbents, 119 High-powered ultrasound metal oxides and supported metal catalysts, 3... 

Supported hcteropoly compounds, where heteropoly compounds are dispersed on the surface of porous solids, are important for applications, since the surface area of heteropolyacids is usually low. The structure, stabil-... 

Keggin-type heteropoly compounds have attractive and important characteristics in terms of catalysis. They consist of heteropolyanions and counter-cations such as H, Cs or NHT When the counter-cations are protons, they are called heteropolyacids (HPA). An important characteristic of HPAs, such as 12-tungstophos-phoric acid (H3PW12O40), is the presence of very strong Bronsted acid sites. But the characteristics of HPAs strongly depend on temperature and relative humidity. When they are used in heterogeneous catalysis, it is often necessary to support them on high-surface-area oxides or activated carbons, in order to increase the surface contact with the reactants. 

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. 

Supported heteropoly acids have been shown to be active catalysts for the acylation of aromatic compounds using organic acids as the acylating agents. Their activity being far superior to Ce and La exchanged zeolites, that have previously been reported to be active for these reactions. ... 

Shikata, S., Nakata, S., Okuhara, T., Misono, M. 1997. Catalysis by heteropoly compounds. 32. Synthesis of methyl tert-butyl ether catalyzed by heteropolyacids supported on silica. Journal of Catalysis 166(2) 263-271. 

Zhang, F.M., Wang, J., Yuan, C.S., Ren, X.Q. 2005. A pronounced catalytic activity of heteropoly compounds supported on dealuminated USY for liquid-phase esterification of acetic acid with n-butanol. Catalysis Letters 102(3-4) 171-174. 

Activated carbons are also adequate supports for HPA since the acidity of the heteropoly compounds is preserved [P3j. In the case of microporous carbons the HPA will be deposited on the external surface. However, in any support, if liquid phase processes are used, leaching of the supported HPA can be a serious problem [37]. Conjugated polymers such as polyacetylene, polypyrrole and polyaniline have been used as HPA supports. In the case of polyacetylene the HPA can be introduced on the surface of the polymer, while retaining its acidity [38]. However, unless a very special behaviour could be achieved, this will nor be able to compete with other less expensive inorganic supports. 

---