ReviewEpoxidation of olefins with hydrogen peroxide catalyzed by polyoxometalates
Introduction
The epoxidation of olefins is an important reaction in the laboratory as well as in chemical industry [1], [2], [3], because epoxides are widely used as raw materials for epoxy resins, paints, surfactants, and are intermediates in organic syntheses. For example, over 5,000,000 and ∼70,000 tonnes of propylene and butene oxides, respectively, are produced per year [4]. Although a number of epoxidation processes use various catalysts and oxidants, a chlorine-using non-catalytic process (the chlorohydrin process) and catalytic processes based on organic peroxides and peracids are still used extensively (Scheme 1) [5]. These processes have disadvantages from the economical viewpoint because they are very capital intensive. Furthermore the chlorohydrin process has environmental disadvantages due to the large output of chloride laden sewage. The co-oxidation processes are environmentally acceptable, but the coupling of two products (propylene oxide and styrene or methyl t-butyl ether) is commercially undesirable.
In contrast to such classical processes, the catalytic epoxidation with hydrogen peroxide as an oxidant might offer some advantages because (i) it generates only water as a by-product and (ii) it has a high content of active oxygen species [6], [7], [8], [9]. Although transition-metal compounds such as metalloporphyrin [10], titanosilicates [11], methyltrioxorhenium [12], tungsten compounds [13], [14], [15], polyoxometalates [16], [17], manganese complexes [18], [19], and non-heme iron complexes [20], [21] have been used as effective catalysts for homogeneous and heterogeneous epoxidation with hydrogen peroxide, these systems have some disadvantages. The use of an excess amount of hydrogen peroxide and low pH in an aqueous phase in the case of the biphasic systems lead to the low efficiency of hydrogen peroxide utilization, selectivity to epoxides, especially for the water-soluble shorter-chain epoxides, and stereospecificity [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21] or the kinds of olefins are limited, e.g., due to the small pore size of TS-1 [22]. In these contexts, effective catalysts for epoxidation of a wide range of olefins with hydrogen peroxide are still desired.
The versatility and accessibility of polyoxometalates have led to various applications in the fields of structural chemistry, analytical chemistry, surface science, medicine, electrochemistry, and photochemistry [23]. Especially, the catalytic function of polyoxometalates has attracted much attention [24], [25], [26], [27], [28] because their acidic and redox properties can be controlled at atomic or molecular levels. Various catalytic systems for H2O2-based epoxidation catalyzed by polyoxometalates have been developed. These systems can be classified into two groups according to the structural and mechanistic aspects of polyoxometalates (Scheme 2).
- (i)
Catalyst precursors of peroxotungstate or peroxomolybdate species: the monomeric, dimeric, and tetrameric peroxo species are generated by the reaction of polyoxometalates with hydrogen peroxide, and the peroxo species can catalyze the epoxidation. The polyoxometalates act as catalyst precursors.
- (ii)
Transition-metal-substituted polyoxometalates: transition-metal-substituted polyoxometalates are oxidatively and hydrolytically stable, and various kinds of catalytically active site can be introduced. The sites influence the catalytic activity and selectivity for the epoxidation.
This review focuses on the H2O2-based epoxidation of olefins catalyzed by polyoxometalates including our recent investigation on lacunary polyoxometalates, [γ-SiW10O34(H2O)2]4− [29], [30]. Recent review articles describe transition-metal-catalyzed epoxidations in more detail [6], [7], [8], [9].
Section snippets
Catalyst precursors of peroxotungstate or peroxomolybdate species
Tungsten-based epoxidation systems with hydrogen peroxide have attracted much attention because of their high reactivities compared with molybdenum analogues and inherent poor activity for decomposition of hydrogen peroxide. Ishii et al. found effective H2O2-based epoxidation of terminal olefins catalyzed by H3PW12O40 combined with cetyl pyridinium chloride (CPC) as a phase transfer agent [14] (Scheme 3(a)). Other polyoxometalates such as H3PMo12O40, H4SiW12O40, and H3PMo6W6O40 were much less
Conclusion and future opportunities
Polyoxo- and peroxo-tungstates are predominant among simple, soluble metal oxide salts for H2O2-based epoxidation, but are sometimes unsuitable for the production of acid-sensitive and/or water-soluble shorter-chain epoxides due to the low pH in an aqueous phase. Although Fe-, Mn-, Co-, Zn-, Ni-substituted polyoxometalates exhibit the high turnover numbers, the efficiency of hydrogen peroxide utilization, selectivity to epoxides, and activity for the epoxidation of non-reactive terminal olefins
Acknowledgments
This work was supported by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency (JST) and a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Science, Sports and Technology of Japan.
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