Heterogeneous epoxidation of olefins

The search for new environmentally-friendly epoxidation methods using 02 as a sole oxidant has attracted much interest. Although there has been some success with 02 and homogeneous catalyst systems in the liquid phase without the use of reducing reagents, there have been few reports concerning heterogeneous epoxidation of olefins [45, 46]. 

Ueno, S., Yamaguchi, K., Yoshida, K., Ebitani, K. and Kaneda, K. Hydrotalcite catalysis heterogeneous epoxidation of olefins using hydrogen peroxide in the presence of nitriles, Chem. Commun., 1998, 295-296. 

A. Klust, R. J. Madix, Selectivity limitations in the heterogeneous epoxidation of olefins Branching reactions of the oxametallacycle intermediate in the partial oxidation of styrene, /. Am. Chem. Soc. 128 (2006) 1034. 

The tert-huty hydroperoxide is then mixed with a catalyst solution to react with propylene. Some TBHP decomposes to TBA during this process step. The catalyst is typically an organometaHic that is soluble in the reaction mixture. The metal can be tungsten, vanadium, or molybdenum. Molybdenum complexes with naphthenates or carboxylates provide the best combination of selectivity and reactivity. Catalyst concentrations of 200—500 ppm in a solution of 55% TBHP and 45% TBA are typically used when water content is less than 0.5 wt %. The homogeneous metal catalyst must be removed from solution for disposal or recycle (137,157). Although heterogeneous catalysts can be employed, elution of some of the metal, particularly molybdenum, from the support surface occurs (158). References 159 and 160 discuss possible mechanisms for the catalytic epoxidation of olefins by hydroperoxides. 

The search for a new epoxidation method that would be appropriate for organic synthesis should also, preferably, opt for a catalytic process. Industry has shown the way. It resorts to catalysis for epoxidations of olefins into key intermediates, such as ethylene oxide and propylene oxide. The former is prepared from ethylene and dioxygen with silver oxide supported on alumina as the catalyst, at 270°C (15-16). The latter is prepared from propylene and an alkyl hydroperoxide, with homogeneous catalysis by molybdenum comp e ts( 17) or better (with respect both to conversion and to selectivity) with an heterogeneous Ti(IV) catalyst (18), Mixtures of ethylene and propylene can be epoxidized too (19) by ten-butylhydroperoxide (20) (hereafter referred to as TBHP). 

Heterogeneous Asymmetric Epoxidation of Olefins over Jacobsen s Catalyst Immobilized in Inorganic Porous Materials... 

Different immobilization methods were employed, and the new heterogenized catalysts were applied in asymmetric hydrogenation and epoxidation of olefins, as well as in ring-opening reactions of epoxides. In some cases, the typical catalytic properties of homogeneous catalysts could be transferred to the heterogeneous systems. The immobilization of complexes on solid supports facilitated recovery and recycling of the catalysts. 

It is well known that homogeneous compounds of Mo(VI) with carboxylic acids are powerful catalysts for the epoxidation of olefins. In order to heterogenize the Mo(VI) derivatives, a commercial carboxylated resins has been used by Ivanov et al. (ref. 1). A weak point of using a commercial resin is that it is not possible to control the number and distribution of carboxylic groups. This results in a poor flexibility of structure and properties of the final catalyst. 

The epoxidation of olefins plays an important role in the industrial production of several commodity compounds, as well as in the synthesis of many intermediates, fine chemicals, and pharmaceuticals. The scale of production ranges from millions of tons per year to a few grams per year. The diversity of catalysts is large and encompasses all the known categories of catalyst type homogeneous, heterogeneous, and biological. 

Although the conditions of catalysis have not yet been optimized, the results are interesting in the sense that these heterogeneous molecular catalysts all show properties different from their molecular counterparts, in terms of either activity (epoxidation of olefins, MPVO reactions, silane activation) or selectivities (epoxidation for example). These features need to be further investigated, by varying the number of solid ligands in the coordination sphere of Zr preliminary experiments with epoxidation of olefins show that this parameter is important for the activity of Zr. 

In Parton et al., a new type of heterogeneous catalyst was proposed consisting of a solid catalyst (iron phthalocyanine zeolite Y) dispersed in a dense PDMS (polydimethylsiloxane) polymer matrix.[l] The system resulted in strongly increased catalytic activities in the oxidation of cyclohexane.[2] Other systems, such as Mn(bipy)2-Y (mangtuiese bipyridine zeolite Y) were also proven to benefit from such incorporation.[3,4] The results presented here using Ti-MCM-41 confirm this for the epoxidation of olefins, an important route for the production of fine chemicals.[5] The influence of the polymer on the reaction activity and selectivity is shown by using different oxidants and solvent conditions in the epoxidation of 1-octene. It will enable the deduction of the advantages and limitations of the reported membrane occluded catalyst system. 

Epoxides are reactive, versatile intermediates in organic chemistry [1,2], The epoxidation of olefins and subsequent rearrangement in the presence of acidic or basic catalysts is widely used for the preparation of aldehydes, ketones, ethers, or alcohols. The reactions are used for the synthesis of a variety of fine chemicals and are conventionally catalyzed with homogeneous catalysts such as metal halides or mineral acids. Substitution of these homogeneous systems by heterogeneous catalysts is, however, becoming increasingly attractive because they are readily separated from the product and do not usually cause environmental problems. 

In this review we shall focus on the use of heterogeneous catalysts for the liquid phase epoxidation of olefins with alkyl hydroperoxides or hydrogen peroxide. The latter is generally the oxidant of choice for fine-chemicals production owing to a better availability and lower price. Emphasis is placed on methods with a broad scope in organic synthesis. 

Peroxo and alkylperoxo metal conqilexes are presumed intermediates in the homogeneous and heterogeneous catalytic epoxidations of olefins. Both solid and soluble versions containing high valent, d transition metal ions activate peroxides heterolytically, thereby facihtating oxygen atom transfer to electron-rich substrates... 

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