Keggin-type

Keggin type H3PW)2O40 is a stable, recyclable and effective catalyst for H2S04-free liquid phase nitration of bezene, chlorobenzene and toluene with nitric acid as a nitration agent. Higher para-selectivity of nitrotoluene was obtained, and the result implies that HPA can effectively catalyze the liquid phase nitration of various aromatics as an environmentally friendly nitration process. 

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. 

Effect of the preparation method on physicochemical and catalytic Properties of transition metal M substituted Keggin type MPMoi2O40 heteropolyoxometalates... 

There is potential in the anti-HIV field. Polyoxometallates of the Keggin type bind to viral envelope sites on cell surfaces and interfere with virus adsorption. Metal-chelating macrocyclic bicyclam ligands are among the most potent inhibitors of HIV ever described, and there is considerable interest in the role of Zn proteins in the viral life cycle. 

Fig. 15. Crystal structure of the Keggin-type polyoxometallate ion [PMo427W7.73O40]6. Adapted from (608).

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). 

TABLE 14.2 Summary of Results Reported in the Scientific and Patent Literature for the Oxidation of Isobutane to Methacrolein and Methacrylic Acid Catalyzed by Keggin-Type POMs... 

Figure 14.4 Mechanism of the oxidation of isobutane to methacrylic acid catalyzed by Keggin-type POMs.

All Keggin-type POMs exhibit an initial unsteady catalytic behavior, which can last from a few hours up to 100 hours, depending on the composition of the POM and on the method employed for its preparation. The progressive variation of catalytic performance occurring during this equilibration period is shown in Figure 14.5, where the conversion of isobutane and the selectiviy to the products are plotted as functions of the reaction time. The catalyst was a... 

This chapter describes some aspects of the reactivity of POMs as catalysts for the selective oxidation of isobutane to methacrylic acid. If developed at the industrial level, this reaction could represent an alternative to the current production method via the ACH route. P/Mo Keggin-type POMs are active and selective catalysts for this reaction. 

Figure 1. Ball and stick (A) and polyhedral (B) representation of the monosubstituted Keggin-type polyanion [XWl lM(L)039]n-. Reprinted from J. Mol. Catal. A Chem. 262, Kholdeeva, O. A. Maksimovskaya, R. L, 7-24, Copyright (2007), with permission from Elsevier.

In 2002, Ichihara and coworkers reported on the employment of Keggin-type phos-phometalates dispersed on fluorapatite (FAp) solid phase for the epoxidation of alkenes in the presence of urea hydrogen peroxide (Scheme 78) . While the phosphotungstate (NH4)3PWi204o was superior to phosphomolybdate (NH4)3PMoi2O40 in the liquid phase, the opposite was the case in the solid-phase system. The solid-phase method is very... 

In 2002, Ichihara and coworkers reported on the utilization of a Keggin-type phos-phomolybdate (NH4)3PMoi2O40 on apatite as catalyst for the solvent-free epoxidation of olefins (see Section III.A.3.c) and the oxidation of sulfldes and sulfoxides in the presence of urea hydrogen peroxide (Scheme 104) . Chemoselectivities of the oxidation of the sulfides were good [product ratios (sulflde/sulfoxide) 84/16 up to 91/9] depending on the substrate, temperature and reaction time. 

Oxygenation of alkanes with hydrogen peroxide in homogeneous reactions can be obtained also with Keggin-type iron-substituted polyoxometalates. Evidence against radical reactions has been offered. 

Herve et al. (57) investigated the thermal changes of structures by means of XRD and TG-DTA for Keggin-type heteropolyacids and proposed Scheme 2. Infrared spectroscopy of H4PMo, VO40 showed the release of vanadium atoms to form H3PM012O40 and vanadium phosphate species (55). Exposure to water vapor induces the decomposition of the latter (indicated by the disappearance of a band at ca. 1037-1030 cm -1) (58). 

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