Keggin heteropoly acids

The Friedel-Crafts alkylation of indoles could also be performed in water, as presented in Figure 21, with yields as high as 97% in the presence of Keggin heteropoly acids, solid superacid catalysts [40]. 

The role of special catalysts is played by well known Keggin heteropoly acids. The catalysts of a regular structure (zeolites, clays and heteropoly acids) as well as mechanism of the reactions in them, can be designed and investigated by means of the Monte Carlo, molecular dynamics and quantum mechanics methods... 

New acid catalyst comprising Keggin-type heteropoly acid supported on mesoporous silica for dehydration of acetic acid... 

The selectivity for acetic anhydride in the catalytic dehydration of acetic acid could be controlled by the pore size of pure mesoporous silica SBA-15. New acid catalyst comprising Keggin-type heteropoly acid supported on SBA-15 enhanced the activity etfectively when tungstophosphoric acid was highly dispersed on the silica substrate. 

I0M.S. Kaba, I.K. Song, and M.A. Barteau, Investigation of Framework and Cation Substitutions in Keggin-type Heteropoly acids Probed by Scanning Tunneling Microscopy and Tunneling Spectroscopy,/ Vac. Sci. Technol. A, 15,1299-1304(1997). 

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... 

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... 

Firouzabadi, H., Iranpoor, N., Amani, K. 2003. Keggin-type heteropoly acids revealed high catalytic activity for swift and selective oxidation of various hydroxy functionalities to the corresponding carbonyl groups using ferric nitrate as an oxidant under mild and solvent-free conditions. Synthesis 408 12. 

Fig. 1 Heteropoly acid structures (a) Ball and stick model of the silicotungstic Keggin anion (SiWi2O40 " ). (b) Space filling model of the secondary crystalline structure of Keggin HPA with base-centered cubic arranged cavities containing H502 cations (red = O, gray = W, blue = H, purple = P or Si)...

A tungstophosphoric acid (HPWO) grafted Pt/MCM-41 catalyst was also synthesized and tested in paraffin isomerization [19]. The Keggin structure of the supported heteropoly acid HjPWjjO was rather stable after calcination. The conversion of n-heptane in the isomerization reaction over Pt/H3PW,204(/MCM-41 catalyst linearly increased from 5.4%... 

Figure 3.30. Heteropoly molybdate catalysts (a) Keggin anion (PMoi204o) (the central PO4 tetrahedron is shaded and the M0O6 octahedra are unshaded) and (b) selectivity (S) against conversion (C) data for acrolein to acrylic acid over KjcH3 PMoi2O40 (with x = 1-3) at " 350 °C show high selectivities. (After Black et al 1987, Gai et al 1983.)...

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... 

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. 

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