Molybdenum complexes oxidation catalysts

The formation of metallocyclic complexes by addition of ketones to dioxygen complexes resembles a three-step Criegee mechanism for ozonolysis. " " In each case one envisages the formation of a dipolar 0-0 bond that leads to electrocyclic ring closure with the ketone. No proven oxidation of aldehydes or ketones has yet been achieved with these metal-locycles. Nevertheless, in the catalytic conversion of cyclic ketones to lactones using hydrogen peroxide and a molybdenum complex as catalyst, it is believed that such an intermediate metallocycle is first formed and that it subsequently converts to lactone and molybdenyl products (16). " ... 

Oxidations of cyclic ketones to lactones by H2O2 take place in the presence of catalytic amounts of certain molybdenum complexes. The catalysts are peroxo-molybdenum complexes stabilized by picolinato- and pyridine-2,6-dicarboxyl-ato-ligands, e.g. [Mo(0)(02) CsH3N(C02)2 ]- Although the reaction is catalytic, turnover numbers are low (25 or less) and chemical yields are variable. Moreover, competing reactions are the formation of oligomeric peroxides and ring-opened products. 

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

Benzene-Based Catalyst Technology. The catalyst used for the conversion of ben2ene to maleic anhydride consists of supported vanadium oxide [11099-11-9]. The support is an inert oxide such as kieselguhr, alumina [1344-28-17, or sUica, and is of low surface area (142). Supports with higher surface area adversely affect conversion of benzene to maleic anhydride. The conversion of benzene to maleic anhydride is a less complex oxidation than the conversion of butane, so higher catalyst selectivities are obtained. The vanadium oxide on the surface of the support is often modified with molybdenum oxides. There is approximately 70% vanadium oxide and 30% molybdenum oxide [11098-99-0] in the active phase for these fixed-bed catalysts (143). The molybdenum oxide is thought to form either a soUd solution or compound oxide with the vanadium oxide and result in a more active catalyst (142). 

MAA and MMA may also be prepared via the ammoxidation of isobutylene to give meth acrylonitrile as the key intermediate. A mixture of isobutjiene, ammonia, and air are passed over a complex mixed metal oxide catalyst at elevated temperatures to give a 70—80% yield of methacrylonitrile. Suitable catalysts often include mixtures of molybdenum, bismuth, iron, and antimony, in addition to a noble metal (131—133). The meth acrylonitrile formed may then be hydrolyzed to methacrjiamide by treatment with one equivalent of sulfuric acid. The methacrjiamide can be esterified to MMA or hydrolyzed to MAA under conditions similar to those employed in the ACH process. The relatively modest yields obtainable in the ammoxidation reaction and the generation of a considerable acid waste stream combine to make this process economically less desirable than the ACH or C-4 oxidation to methacrolein processes. 

The selective oxidation is catalyzed by silver, which is the only good catalyst. Other olefins are not converted selectively to the epoxides in the presence of silver. However, propylene epoxidation is appHed commercially the catalysts are either molybdenum complexes in solution or soHd Ti02—Si02 (see... 

Epoxidation systems based on molybdenum and tungsten catalysts have been extensively studied for more than 40 years. The typical catalysts - MoVI-oxo or WVI-oxo species - do, however, behave rather differently, depending on whether anionic or neutral complexes are employed. Whereas the anionic catalysts, especially the use of tungstates under phase-transfer conditions, are able to activate aqueous hydrogen peroxide efficiently for the formation of epoxides, neutral molybdenum or tungsten complexes do react with hydrogen peroxide, but better selectivities are often achieved with organic hydroperoxides (e.g., TBHP) as terminal oxidants [44, 45],... 

The reaction scheme is rather complex also in the case of the oxidation of o-xylene (41a, 87a), of the oxidative dehydrogenation of n-butenes over bismuth-molybdenum catalyst (87b), or of ethylbenzene on aluminum oxide catalysts (87c), in the hydrogenolysis of glucose (87d) over Ni-kieselguhr or of n-butane on a nickel on silica catalyst (87e), and in the hydrogenation of succinimide in isopropyl alcohol on Ni-Al2Oa catalyst (87f) or of acetophenone on Rh-Al203 catalyst (87g). Decomposition of n-and sec-butyl acetates on synthetic zeolites accompanied by the isomerization of the formed butenes has also been the subject of a kinetic study (87h). 

Similar to molybdenum oxide catalyst the capability to emit singlet oxygen is inherent to Si02 doped by Cr ions as well. Similar to the case of vanadium oxide catalysts in this system the photogeneration occurs due to the triplet-triplet electron excitation transfer from a charge transfer complex to adsorbed oxygen. 

The formation of molybdenum complexes with diols (formed by olefin oxidation) was proved for the use of the molybdenum catalysts. Therefore, the participation of these complexes in the developed epoxidation reaction was assumed [242]. 

A catalytic asymmetric oxidation of mono-, di-, and tri-substituted alkenes using a chiral bishydroxamic acid (BHA) complex of molybdenum catalyst in air at room temperature leads to good to excellent selectivity. It has been suggested that the Mo-BHA complex combines with the achiral oxidant to oxidize the alkene in a concerted fashion by transfer of oxygen from the metal peroxide to the alkene.78 The chiral BHA-molybdenum complex has been used for the catalytic asymmetric oxidation of sulfides and disulfides, utilizing 1 equiv. of alkyl peroxide, with yields up to 83% and ees up to 86%. An extension of the methodology combines the asymmetric oxidation with kinetic resolution providing excellent enantioselectivity (ee = 92-99%).79... 

Molybdenum complexes are the most effective catalysts known for the selective epoxidation of olefins with alkyl hydroperoxides (210-212). Commonly known is the Arco or Halcon process for the large-scale manufacture of propylene oxide from propylene. This process uses t-BuOOH or ethyl benzene hydroperoxide (EBHP) as an oxidant and Mo(CO)6, for example, as a source of Mo. The Mo(CO)6 acts as a catalyst precursor, which is converted into a soluble active form by complexation with diols (3). Chemists have designed several supported versions of the catalysts for this epoxidation chemistry. A clear classification can be made on the basis of the nature of the support. 

Two research groups496 497 have recently studied the autoxidation of cyclohexene at 60° to 65°C in the presence of a mixture of a low-valent Group VIII metal complex, e.g., RhCl(Ph3P)3 or (Ph3P)2Pt02, and an epoxidation catalyst (molybdenum complexes). Cyclohexen-l-ol and cyclohexene oxide are formed in roughly equimolar amounts. The results could be explained by a scheme involving two successive catalytic processes ... 

A large range of different ionic liquids have been screened in the epoxidation of cyclooctene with dioxomolybdenum(VI) complexes and ferf-butyl hydrogenperoxide as oxidant, as shown in Table 5.2.[32] With the diazabutadiene complex, 48, as catalyst, inferior turnover frequencies were observed relative to the reaction in neat substrate or in dichloromethane and the recycling potential of the catalyst turned out to be only limited. Catalyst immobilisation was better with the cationic tris(methylaminomethyl)ethane complex, 49, however at the expense of selectivity. Of the ionic liquids tested, [C4Ciim][Tf2N] gave the best results for both molybdenum complexes. 

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