Heteropoly acids HPAs

HPAs are Brpnsted acids composed of heteropoly anions and protons as countercations the most commonly utilized HPA is the phosphotungstic acid (PW). HPAs are stronger than many 

The acylation of toluene and anisole with aUphatic carboxyUc acids can be carried out with 

Cs2 5H(,j[PWj20 (,], affording the corresponding products in 41-71% yield [93]. These soUd acids are superior in activity to the conventional acid catalysts such as sulfuric acid and zeoUtes and can be reused after a simple work-up, albeit with reduced activity. As already underlined, the activity of this kind of catalyst can be improved by carrying out the reaction in the presence of trifluoroacetic anhydride [94]. The acylation of anisole is conducted by acetic or benzoic acid in the presence of trifluoroacetic anhydride catalyzed by AlPW jO j with excellent yields (94—96%). Under the same reaction conditions, 2-methyl- and 4-methylanisoles are also acylated, giving the corresponding ketones in 90-97% yield. 

A hybrid material based on aluminum tungstophosphate can be utilized in the acylation of anisole with acetic anhydride [95]. The catalyst is prepared by immobilizing aluminum tungstophosphate in a polymeric blend formed by polyvinyl alcohol and polyethylene glycol by the freeze-thawing method. The salt retains the Keggin stmeture of the heteropolyanion during the immobilization procedure. para-Methoxy acetophenone is obtained in 90% yield in 30 min at 100°C accompanied by 10% of ortho-methoxyacetophenone. 

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. 

BBA cycloacylafion has been studied in the presence of differenf polyoxometalate cafalysfs with strong acid character, using both a batch and a pseudocontinuous reactor. Batch runs have been performed for screening fhe catalysis, whereas the continuous reactor was used with the best catalyst to obtain detailed information regarding kinetics, catalyst 

Three zones in this reactor are working at different temperatures  

It is well known that one of the major problems associated with HPAs in the bulk form is their low efficiency due to low surface area 

Zirconia-supported PW (PW/Zr02) catalyst shows good activity in the acylation of diphenyl efher wifh BC. The maximum conversion of BC is 60%, and 4-benzoyldiphenyl efher is obfained wifh a selectivity higher than 97%. The catalyst can be efficiently recycled three times, affording the same BC conversion. 

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Chloramine-T also functions as a nitrene source in the presence of heteropoly acids (HPAs) such as phosphomolybdic and phosphotungstic acids. The aziridination of alkenes by treatment with the combination of HPA and chloramine-T is... 

Chromatographic method The determination of silicate based upon the adsorption of preconcentrated phosphomolybdic and silicomo-lybdic heteropoly acid (HPA) in the dynamic mode on a microcolumn packed with an Amberlite XAD-8 polyacrylate adsorbent is reported [49]. The method is based on the adsorption of HPAs followed by desorption with acetonitrile and determination by reversed-phase HPLC. Another highly sensitive HPLC method for the simultaneous determination of soluble silicate and phosphate in environmental waters is used in ion-pair liquid... 

Various additives have been added to PFSA membranes to either replace the water or to retain the water at higher temperatures. Nafion has been swollen with phosphoric acid and a conductivity of 0.05 S/cm has been achieved at 150°C. Unfortimately, the anode fails after a short period of time in these membranes so no successful FC tests have been run. 1-Butyl,3-methyl imidazolium triflate and tetrafluoroborate have also been used to swell Nafion giving a conductivity of 0.1 S/cm at 180°C. Nafion has also been swollen with heterocycle solutions, imidazole, and imidazolium salts in trifluoroacetate and trifluoro methane sulfonate solution, although the reported conductivities are more modest, 10 S/cm at 100°C. Acetic acid and tetra-n-butylammonium chloride solutions of the heteropoly acid (HPA), 12-phosphotungstic acid (PTA), have all been used with Nafion giving improved FC performance at 110°C and 120°C, respectively, vs. the undoped PFSA. ... 

Observation of reactive oxygen species on oxides is significantly more difficult. Figure 3 shows an isotope exchange experiment with the molecular oxide H4PVM011O40 14 H2O, a heteropoly acid (HPA) [26,27] and oxygen 18 in the gas... 

In this paper we would like to communicate the details of our investigation in the acylation of 2-MON with acetic anhydride using different solid acid catalysts such as zeolites, clays, pillared clays, unsupported heteropoly acids (HPA) and HPA/KIO, sulphated zirconia euid ion-exchange resins. 

Heteropoly acids (HPAs) are promising solid acid catalysts for the Friedel - Crafts They are stronger than many conventional solid acids such as mixed... 

An increasing interest exists in adding heteropoly acids (HPA) as protonconducting components to the sulfonated polymers [35]. The addition will enhance the proton conductivity and the acceptance of CO. Because of their structural... 

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... 

Methanol Carbonylation. Metal-ion exchanged heteropoly acids (HPAs) of the general formula M[PWi204o] (M = a Group VIII metal) supported on Si02 have been found to be excellent catalysts for the vapor-phase carbonylation of methanol or DME to methyl acetate at 225°C and 1 atm total operating pressure (84). Experiments witii H3PM12O40 (M = Mo,W) carried out in a conventional flow reactor at 1 atm CO and 200-275°C have shown that methanol is converted into DME and small amount of C1-C5 saturated hydrocarbons (HCs) and C2-C4 olefins. No carbonylation products are detected under these... 

Heteropoly acids (HPA) are strong acids of a general formula (XxM ,Oy] , where ... 

Even though the catalytic systens described iu the preceding sections provide greater practicality than stoichiometric systens, their application in industrial synthesis is not widespread. However, in more recent times, maximum effort has been directed toward the use of solid acid catalysts. In fact, heterogeneous catalysts can be easily separated from the reaction mixture and reused they are generally not corrosive and do not produce problematic side products. Different classes of materials have been studied and utilized as heterogeneous catalysts for Friedel-Crafts acylations these include zeolites, (acid-treated) metal oxides, and heteropoly acids (HPAs) already utilized in the hydrocarbon reactions [24]. Moreover, the application of clays, perfluorinated resin-sulfonic acids, and supported (fluoro) sulfonic acids, mainly exploited in the production of fine chemicals, is the subject of intensive studies in this area. 

Khenkin and Neumann discovered that SET from polycyclic arenes to a mixed-addenda heteropoly acid (HPA), Hj[PV2M0jj 0 (j], can be followed by oxygen atom transfer from the HPA to the radical cation (Scheme 14.7) [38], This type of reactivity is well known in the area of heterogeneous gas-phase oxidation as a Mars-van Krevelen mechanism, whereby a metal oxide at high temperature transfers oxygen atom from the lattice to an activated hydrocarbon substrate, but in homogeneous catalysis, it is veiy rare. 

Another alternative solid state electrolyte under study is the use of solid-state proton conductors such as heteropoly acid (HPA) electrolytes. The two most common HPAs are H4SiWi2O40 (SiWA) and H3PW12O40 (PWA) [121]. The HPA materials have high proton conductivities at room temperature (solid form of pure SiWA = 27 mS.cm i). The traditional problem with solid state proton conductors is their poor film making properties that make forming a separator difficult. 

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