Heterogenization of Polyoxometalates

The di-copper-substituted y-Keggin silicotungstate [y-H2SiWio036Cu2( J--l, 1-N3)2] could act as an effective homogeneous catalyst for the oxidative homocoupling of various kinds of alkynes, including aromatic, aliphatic and heteroatom-containing ones (6.8) [108]  

Although terminal 0x0 complexes of the late-transition-metal elements have been proposed as possible intermediates for oxidations catalyzed by these elements, late-transition-metal-oxo complexes were scarcely known. Hill and coworkers reported the synthesis and characterization of Pt -, Pd - and Au -0x0 complexes, (M(0)(OH2) WO(0H2) (PW9O34)2] (M = Pt, Pd and Au, n = 0-2), stabilized by electron-accepting polyoxotungstate ligands [109-lllj. The stoichiometric reaction of the Au-oxo complex [Au(0)(OH2) WO(OH2) 2 (PW9034)2] with triphenylphosphine led to the formation of triphenylphosphine oxide. 

To date, many soluble transition metal-based homogeneous catalysts including POMs have been developed for H2O2- and 02-based green oxidations. They are usually dissolved in reaction solutions, making all catalytic sites accessible to 

B H2O2 NIPAM- [PWi204o] (NIPAM = N-isopropylacrylamide-based polymer) 

198 6 Liquid-Phase Oxidations with Hydrogen Peroxide and Molecular Oxygen 

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Fontananova, E., Donato, L., Drioli, E., Lopez, L.C., Favia, P. and d Agostino, R. 2006. Heterogenization of polyoxometalates on the surface of plasma-modified polymeric membranes. OimL MMenJ 1561-1568. 

In the book, the section on homogeneous catalysis covers soft Pt(II) Lewis acid catalysts, methyltrioxorhenium, polyoxometallates, oxaziridinium salts, and N-hydroxyphthalimide. The section on heterogeneous catalysis describes supported silver and gold catalysts, as well as heterogenized Ti catalysts, and polymer-supported metal complexes. The section on phase-transfer catalysis describes several new approaches to the utilization of polyoxometallates. The section on biomimetic catalysis covers nonheme Fe catalysts and a theoretical description of the mechanism on porphyrins. 

Monteleone, F., Cavani, F., Felloni, C., et al (2004). A Redox Process for the Production of Menadione and Use of Polyoxometalates as Oxidizing Agents, International Patent Application WO 2004014832 Strukul, G., Somma, F., BaUarini, N., et al. (2009). The Oxidation of 2-methyl-1-naphthol to Menadione with H2O2, Catalyzed by Nb-based Heterogeneous Systems, Ap/iZ. Catal. A Gen., 356, pp. 162-166 Zalomaeva, O., Ivanchikova, I., Kholdeeva, O., et al (2009). Kinetics and Mechanism of the Oxidation of Alkyl Substituted Phenols and Naph-thols with TBuOOH in the Presence of Supported Iron Phthalocyanine, New J. Chem., 33, pp. 1031-1037. 

Polyoxometalate-catalyzed oxidations are carried out in homogeneous or heterogeneous systems. Write advantages and disadvantages of homogeneous and heterogeneous systems, respectively. 

Liquid-phase oxidations catalyzed by polyoxometalates are covered by Mizuno, Kamata, and Yamaguchi (Chapter 13). Finally, the developing field of enantioselective heterogeneous catalysis is presented by Coman, Poncelet, and Parvulescu (Chapter 14). 

Iu search for efficieut aud greeuer processes over the past few years various heterogeneous catalysts such as titanium incorporated mesoporous molecular sieves [45,46], Schiff-base complexes supported on zeolite [47] and Zn(II)-Al(III) layered double hydroxide (LDH) [48], oxomolybdenum(VI) complexes supported on MCM-41 and MCM-48 [49], polyoxometallate supported materials [50], Co and Mn-AlPO s [51] etc. have been developed and studied for the catalytic epoxidatiou of a-pinene. Many of these processes suffer from drawbacks and limited applicability due to the poor conversion and also the selectivities. Sacrificial aldehydes are often used as an oxygen acceptor in such processes to achieve reasonable yield and selectivities but, this procedure leads to an increase in the E-factors and decrease in the atom economy [51]. 

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