Liquid Phase Oxidation Reactions Catalyzed by Polyoxometalates

Abstract

j315 and recycling of the catalyst. Following a short introduction to polyoxometalates, their structure and general properties, the major parts of this review deal with three different types of oxidants: (i) mono-oxygen donors, (ii) peroxides and (iii) molecular oxygen. Finally, various systems for catalyst engineering designed to aid in catalyst manipulation, recovery and recycling are discussed. Polyoxometalates (POMs)Research applications of polyoxometalates have over the past two decades become very apparent and important, as reflected by the recent publication of a special volume of Chemical Reviews [1] devoted to these compounds. The diversity of research in the polyoxometalate area is significant and includes their application in many fields, including structural chemistry, analytical chemistry, surface chemistry, medicine, electrochemistry, and photochemistry. However, the most extensive research on the application of polyoxometalates appears to have been in the area of catalysis, where their use as Brønsted acid catalysts and oxidation catalysts has been going on since the late 1970s. Research published over the past decade or two has firmly established the significant potential of polyoxometalates as homogeneous oxidation catalysts. Through the development of novel ideas and concepts, polyoxometalates have been shown to have significant diversity of activity and mechanism that in the future may lead to important practical applications. In recent years, a number of excellent general reviews on the subject of catalysis by polyoxometalates have already been published [2], and while some repetition is inevitable we will attempt to keep the redundancy in the present review to a minimum.A basic premise behind the use of polyoxometalates in oxidation chemistry is the fact that these compounds are oxidatively stable. In fact, as a class of compounds they are thermally stable generally to at least 350-450 C in the presence of molecular oxygen. 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. Polyoxometalates, previously also called heteropolyanions, are oligooxide clusters of discrete structure with a general formula [X x M m O y ] qÀ (x m) where X is defined as the heteroatom and M are the addenda atoms. The addenda atoms are usually either molybdenum or tungsten in their highest oxidation state, 6 þ , while the heteroatom can be any number of elements, transition metals and main group elements phosphorus and silicon being the most common heteroatoms. A most basic polyoxometalate, [XM 12 O 40 ] qÀ where (X ¼ P, Si, etc.; M ¼ W, Mo), is that possessing the Keggin structure, Figure 9.1. Such Keggin type polyoxometalates, commonly available in their protic form, are significant for catalysis only as Brønsted acid catalysts.However, since polyoxometalate synthesis is normally carried out in water by mixing ...

Cubic Cesium Hydrogen Silicododecatungstate with Anisotropic Morphology and Polyoxometalate Vacancies Exhibiting Selective Water Sorption and Cation-Exchange Properties

“…Design of such materials requires use of molecular building blocks with well-defined sizes, shapes, compositions, and electronic configurations. Polyoxometalates (POMs) are well-defined inorganic metal–oxygen anionic clusters, and complexation of POMs with appropriate counter cations forms solid compounds exhibiting unique properties in broad fields of sciences such as catalysis, electrochemistry, photochemistry, magnetism, and medicine. − …”

Reusable Magnetite Nanoparticle (Fe3O4 NP) Catalyst for Selective Oxidation of Alcohols under Microwave Irradiation

“…For the aforestated reasons, a new simple, selective, and cost-effective method that combines the beneficial characteristics of the iron oxide nanocatalyst with the high selectivity of conventional oxidation methods is urgently needed. 41 In this study, we have developed a new selective oxidation of primary and secondary alcohols catalyzed by unsupported iron oxide NPs, specifically magnetite nanoparticles (Fe 3 O 4 NPs) under MW irradiation. Target aldehydes and ketones are produced in a medium yield with a high selectivity by adding N-methylmorpholine N-oxide (NMO) as a terminal oxidant, which is also employed as a commodity chemical by the pulping industry.…”

“…To the best of our knowledge, two previous reports by Beller et al have been so far the most notable examples of methods taking advantage of unsupported iron oxide nanocatalysts for primary and secondary alcohol oxidation in the presence of hydrogen peroxide as an oxidant. , The aforementioned catalytic systems afforded the target oxidation products in low to medium yields and with a medium to good selectivity. For the aforestated reasons, a new simple, selective, and cost-effective method that combines the beneficial characteristics of the iron oxide nanocatalyst with the high selectivity of conventional oxidation methods is urgently needed …”

“…For the aforestated reasons, a new simple, selective, and cost-effective method that combines the beneficial characteristics of the iron oxide nanocatalyst with the high selectivity of conventional oxidation methods is urgently needed. 41 …”

Synergistic Effect of Polyoxometalate and Single Walled Carbon Nanotubes on Peroxidase‐like Mimics and Highly Sensitive Electrochemical Detection of Hydrogen Peroxide

“…Polyoxometalates (POMs) are composed of nanosized anionic transitional metal-oxide clusters [1][2][3][4]. They are excellent candidates for electrocatalysis due to their multiple redox properties [5][6][7][8]. The generic structure of Keggin type POMs is XM 12 O 40 n-, X = P, Si/M = W, Mo, V [9,10].…”