Strategies for oxidation catalyzed by polyoxometalates at the interface of homogeneous and heterogeneous catalysis

Abstract

A basic premise behind the use of polyoxometalates in oxidation chemistry is the fact that polyoxometalates are oxidatively stable. This, a priori, leads to the conclusion that for practical purposes polyoxometalates would have distinct advantages over widely investigated organometallic compounds that are vulnerable to decomposition due to oxidation of the ligand bound to the metal center. Since polyoxometalate synthesis is normally carried out in water by mixing the stoichiometrically required amounts of monomeric metal salts and adjusting the pH to a specific acidic value many structure types are accessible by variation of the reaction stoichiometry, replacement of one or more addenda atoms with other transition or main group metals, and pH control. The structural variety available has enabled the use of polyoxometalates as catalysts for oxidation of hydrocarbons and functionalized organic substrates (alcohols, amines, sulfides, etc.) with a wide range of oxygen donors ranging from molecular oxygen, hydrogen peroxide, nitrous oxide, ozone, alkyl hydroperoxides, periodate, sulfoxide and others. The wide purview of oxidation reactions is enabled because the structural variety leads to oxidation through a number of different mechanistic motifs.From a synthetic organic point of view, the most applicable uses of polyoxometalates as catalysts involve the ''green'' oxygen donors -hydrogen peroxide and molecular oxygen. Since practical applications are in hand in this area, practical considerations concerned with catalyst recycle and/or recovery and the elimination of environmentally problematic solvents are also coming to the forefront. In this paper, we will present some of our activities in the area of ''catalyst engineering'' for catalytic synthetic applications by polyoxometalates including: (a) catalytic mesoporous solids from organic-polyoxometalate hybrid materials, (b) fluorous phase polyoxometalates with and without fluorous solvents and (c) the use of aqueous biphasic media for oxidation with hydrogen peroxide.

Solvent-free oxidation of benzyl alcohol with oxygen using zeolite-supported Au and Au–Pd catalysts

“…The results presented in table 1 (entries [1][2][3][4][5] show that the activity of the Au catalyst is influenced by the type of support used for the oxidation of benzyl alcohol (see figure 1). The initial activity of the catalysts decreases in the order ZSM-5 > b > Y > TiO 2 > TS-1.…”

“…This is due to the importance of selective oxidation as a unit process in the manufacture of chemical intermediates [1]. The oxidation of alcohols to aldehydes is a fundamentally important laboratory and commercial procedure [1][2][3][4][5][6][7][8] since the products are valuable both as intermediates as well as being highvalue components for the perfumery industry [1,9,10]. Many selective oxidation reactions are carried out without the use of modern catalytic technology and in such cases these oxidations are effected using stoichiometric oxygen donors, such as chromate or permanganate.…”

Solvent-Free Oxidation of Primary Alcohols to Aldehydes Using Au-Pd/TiO 2 Catalysts

“…The addition of Au to Pd nanocrystals improved the overall selectivity and, using scanning transmission electron microscopy combined with x-ray photoelectron spectroscopy, we showed that the Au-Pd nanocrystals were made up of a Au-rich core with a Pd-rich shell, indicating that the Au electronically influences the catalytic properties of Pd. S elective oxidation is important in the synthesis of fine chemicals and intermediates (1); and, in particular, the oxidation of primary alcohols to aldehydes is a fundamentally important laboratory and commercial procedure (1)(2)(3)(4)(5)(6)(7)(8). Aldehydes are valuable both as intermediates and as highvalue components for the perfume industry (1,9,10).…”

Selective Oxidation of Phenol in the Presence of Transition-Metal Substituted Polyoxometalates

“…The behaviour of TSMP was studied in the number of papers by: Bregeault, Cavani, Kozhevnikov, Kuznetsova, Mizuno, and Vazylyev, who found that TSMP show catalytic activity in oxidation of hydrocarbons in both heterogeneous and homogeneous reactions. However, the majority of studies were concerned with mixed addenda systems [9][10][11][12][13][14][15]. Much less is known about behaviour of TSMP in homogeneous reactions.…”