Epoxidation heterogeneous catalysts

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. 

It is carried out in the Hquid phase at 100—130°C and catalyzed by a soluble molybdenum naphthenate catalyst, also in a series of reactors with interreactor coolers. The dehydration of a-phenylethanol to styrene takes place over an acidic catalyst at about 225°C. A commercial plant (50,51) was commissioned in Spain in 1973 by Halcon International in a joint venture with Enpetrol based on these reactions, in a process that became known as the Oxirane process, owned by Oxirane Corporation, a joint venture of ARCO and Halcon International. Oxirane Corporation merged into ARCO in 1980 and this process is now generally known as the ARCO process. It is used by ARCO at its Channelview, Texas, plant and in Japan and Korea in joint ventures with local companies. A similar process was developed by Shell (52—55) and commercialized in 1979 at its Moerdijk plant in the Netherlands. The Shell process uses a heterogeneous catalyst of titanium oxide on siHca support in the epoxidation step. Another plant by Shell is under constmction in Singapore (ca 1996). 

Very few examples have been described for the non-covalent immobilization of chiral porphyrin complexes (Fig. 26). In the first case, the porphyrin-dichlororutheninm complex was encapsulated in silica, which was prepared around the complex by a sol-gel method [78], in an attempt to prevent deactivation observed in solution in the epoxidation of different alkenes with 2,6-dichloropyridine N-oxide. In fact, the heterogeneous catalyst is much more active, with TON up to 10 800 in the case of styrene compared to a maximum of 2190 in solution. Enantioselectivities were about the same imder both sets of conditions, with values aroimd 70% ee. 

By complexation of MnNaY with 1,4,7-trimethyltriazacyclononane, a new heterogeneous catalyst was obtained for olefin epoxidation with H202. Excellent epoxide selectivities were obtained, with limited epoxide solvolysis. The oxygenation appears to go through a radical intermediate. The manganese trizacyclononane epoxidation catalyst was also heterogenized via surface gly-cidylation.103... 

Similarly, heterogeneous catalyst prepared by immobilization of POMs on chemically modified hydrophobic Si02 has been applied to the selective epoxidation of various alkenes with 15% aqueous H202 without organic solvents [168],... 

In the challenge to develop more environmentally benign routes to useful epoxidized fatty derivatives, the exploitation of heterogeneous catalysts is one of the... 

Methylrheniumtrioxide (MTO) can straightforwardly be immobilized using copolymers consisting of polystyrene and polyvinyl pyridine. The authors proposed two possible structures 18 shown in Scheme 4.10. In the presence of H2O2 these catalysts are efficient and selective heterogeneous catalysts for the epoxidation of alkenes. It was shown that their activity is maintained for at least five recycling experiments [23 dj. In analogy to this work, bipyridyl-functionalized mesoporous silica can be employed for the immobilization of MTO [75]. 

Figure 18. The synthesis of heterogeneous catalyst by grafting of Cp Mo(CO)3Cl (3) and [(-)-menthylCp]Mo(CO)3Cl (4) complexes in aluminated mesoporous materials for olefin epoxidation reactions.

Aiming at easier workup conditions, immobilization of several transition metal catalysts, which show activity for the epoxidation of allylic alcohols, on polymer support has been investigated. For example, Suzuki and coworkers incorporated an oxo-vanadium ion into cross-linked polystyrene resins functionalized with iminodiacetic acid or diethylenetri-amine derivatives (Scheme 57), which afforded a heterogeneous catalyst that can promote... 

The same authors later on presented their results of the epoxidation of various cyclic and acyclic olefins employing a heterogeneous catalyst with an oxovanadium(IV) ion incorporated on a sulfonic acid ion-exchange resin and TBHP as oxidant . Selectivities... 

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