Keggin structure, heteropoly compounds

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). 

The following syntheses involve some of the most common heteropoly compounds with the Keggin structure. These compounds can help students develop a good understanding of how subunits can be connected together to form condensed structures. In addition, the preparations expose the students to a variety of useful laboratory techniques such as vacuum filtration, solvent... 

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... 

Early attempts to use heteropoly compounds as catalysts are summarized in reviews published in 1952 (//) and 1978 (7). The first industrial process using a heteropoly catalyst was started up in 1972 for the hydration of propylene in the liquid phase. The essential role of the Keggin structure in a solid heteropoly catalyst was explicitly shown in 1975 in a patent concerning catalytic oxidation of methacrolein. Systematic research in heterogeneous catalysis with these materials started in the mid-1970s and led to the recognition of quantitative relationships between the acid or redox properties and catalytic performance... 

Fig. 2. Primary, secondary, and tertiary structures of heteropoly compounds, (a) Primary structure (Keggin structure, XM 204o) (b) secondary structure (H3PW 204o 6H20) (c) secondary structure (Cs3PW 204o) (d) tertiary structure [Cs25Ho5PW 204o, cubic structure as in (c)].

H3PW12O40 is one of the most common heteropoly compounds. A distinct X-ray diffraction pattern is seen for H3PWi2O40.6H2O, where the Keggin units are linked by H+(H20)2 bridges, resulting in a body-centered cubic structure and hence the X-ray diffraction pattern. The water molecules can be easily replaced by a number of polar molecules such as alcohols and amines (Misono, 1987). 

NMR has also been widely used for structural elucidation of vanadium-containing mixed addenda heteropoly compounds, owing to the large natural abundance of nuclei. The number of structural isomers increases with the increase in the number of addenda atoms and their position in the Keggin structure is confirmed by NMR spectroscopy [138]. 

A single sharp pak at -3.2 ppm (Fig. la) was observed for H3PM012O40, on addition of hydrogen peroxide this peak completely disappeared and a new peak at 4.1 ppm (Fig.lb) was observed. This peak was shown to be due to the formation of molybdenum peroxospecies [5] whose structure was different from the Keggin structure of the heteropoly compound. 

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... 

A variety of methods to study the acidity of heteropoly compounds are reported in the literature in line with the study of the catalytic applications of the HPAs. These methods, mainly applied on Keggin type structures, evaluate the acid strength and the amount of molecules adsorbed with probe molecules (ammonia, pyridine) different from actual reactants. The studies that used actual reactant molecules, such as alcohols, determine the total amount of adsorption sites rather than the number of active acid sites at a certain temperature of reaction. In fact, the studies reported in the literature focused on the intermediate adsorbed species rather than on the in situ identification and quantification of the products of... 

Moffat, J.B. Cation-anion effects in heteropoly compounds with Keggin structures. Polyhedron 1986, 5, 261-269. 

A variety of polyanion structures are known. For example, the structure of the polyanion of so-called Keggin structure is shown in Fig. 3.74a. The heteropoly compounds having the Keggin structure are thermally more stable and rather e y obtained, so that investigations have been devoted mostly to this group. They are used as acid catalysts as well as oxidation catalysts in both hereto- and homogenous systems (cf. comprehensive review on heterogenous catalysis ). The principal merits of heteropoly compounds when they are used as solid acid catalysts are as follows. ... 

Fig. 3.2 Structural hierarchy of heteropoly compounds (a) Keggin cluster (primary particle), (b) secondary structure and (c) tertiary structure (texture particle size, porosity, surface area, etc.) (Adapted with permission fiorn Ref. [16]. Copyright 2001, Royal Society of Chemistry)...

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