Search

A new and efficient system for the allylic acetoxylation of olefins has been developed, which consists of a palladium (II) catalyst and hydrogen peroxide as oxidant in acetic acid. The acetoxylation reaction competes with the epoxidation by the peracetic acid generated in situ in this system. Increase of the concentration of palladium catalyst and addition of benzoquinone (BQ), enhance the acetoxylation considerably. For the Pd(OAc)2BQH2O2 system, more than 1000 turnover number could be achieved in the acetoxylation of cyclohexene. Most internal and cyclic olefins tested gave the corresponding allylic acetates in fair to high yields. 1,3-Cyclohexadiene gave 1,4-diacetoxy-cyclohex-2-ene while 1,5-hexadiene afforded 3-acetoxy-methylenecyclopentane; 1,7-octadiene produced 1-octen-7-one and octan-2,7-dione. Terminal olefins gave methyl ketones in good yield with high turnover number both in the presence and absence of BQ. Possible mechanisms and involvement of palladium peroxidic species in both the catalytic acetoxylation and ketonization of olefins are discussed.

The acetoxylation of toluene to benzyl acetate was found to be a purely heterogeneous catalyzed reaction which occurred on supported metallic palladium particles or on suspended palladium particles in the absence of a carrier material. An optimized catalytic system prepared in situ was found to be composed of [Pd(OAc)2, Sn(octoate)2, KOAc and Norit PKDA 1–3 charcoal] where the Pd/Sn/K mole ratio was 1/5/10. High toluene conversion (100%) and benzyl acetate selectivity (77%) were obtained after 180 minutes at 100°C. The addition of Sn2+, K+ and charcoal all led to an increased rate of Pd(OAc)2 decomposition to metallic palladium. The addition of tin(II) lowered the activation energy of the reaction by over 25%. Tin(II) may also serve as a reducing agent in the initial stages of Pd(0) particle formation. The addition of several potential co-catalysts other than tin, some of which possessed accessible redox potentials, had no effect or a negative effect on catalytic activity. The addition of various acetate counter ions had little effect on catalytic activity with the exceptions of K+ and Rb+.

X-Ray photoelectron spectroscopic studies of lanthanum oxide based oxidative coupling of methane catalysts, indicate a high binding energy O(1s) peak at 532.6–533.2 eV, which is strong evidence for surface superoxide (O2−) or peroxycarbonate (CO4=).

The IFP oxypyrolysis process for the conversion of natural gas to olefins combines the oxidative coupling of methane, a heterogeneously catalysed reaction producing ethane as a primary product, with the steam cracking of ethane, a homogeneous gas-phase reaction producing ethylene and hydrogen as primary products. In the presence of added ethane, the oxidative coupling of methane proceeds at lower methane conversion and C2+ selectivity. However, if the ethane is added to the post-catalyst bed region of the reactor after the majority of molecular oxygen has been consumed, both methane conversion and C2+ selectivity remain high while a substantial portion of the added ethane is converted to ethylene plus hydrogen. This process is therefore able to upgrade all the alkane components, not only the methane, in natural gas.

Considerable disagreement often exists regarding the conversion and activity of many oxidative coupling catalysts. The wide discrepancy in results, obtained on a given catalyst, is essentially due to differences in temperature and space velocity as well as the use of inert gas diluents. We have thus used three dimensional graphics in order to locate the region of maximum C2+ yield as a function of catalyst bed hot spot temperature and linear space velocity. The conclusion of our study, involving pure La2O3, is that much of the research in this field is being carried out in the wrong region of catalyst activity.

Die Pd(II)-aoetate (I) reagieren mit tert.-Butylhydroperoxid (II) zu den tetrameren Titelkomplexen (III).

The influence of reaction temperature, space velocity and methane and oxygen partial pressures were studied for the oxidative coupling of methane reaction (OCM) over a lithium doped lanthanum oxide catalyst. The kinetic data obtained with this catalyst, at temperatures between 650 and 750°C, indicate that oxygen is adsorbed non-dissociatively and non-competitively. Product selectivities extrapolated to zero percent methane conversion are similar to those obtained with Li/MgO, suggesting that both rare earth and alkaline earth based catalysts involve similar mechanisms. Carbon monoxide and ethylene were found to be secondary products exclusively.

In this progress report an attempt is made to rationalize, from a mechanistic point of view, the different ways in which oxygen is transferred from inorganic and organic peroxides to nucleophilic substrates, particularly olefins. Oxygen transfer from transition-metal peroxides, which is relevant to catalytic oxidations using O2, H2O2 or ROOH, occurs via a cyclic or “pseudocyclic” peroxymetalation in which a dioxametallacycle is formed. Owing to the wide discrepancy between peroxymetalation and the conventional oxidation mechanism, i.e. nucleophilic attack of the substrate at the electrophilic “active oxygen”, we propose an alternative mechanism involving dioxiranes as the reactive species. The generation of dioxiranes appears to be a common denominator in the reactions of most organic peroxides e.g. peroxy acids, the reaction of electrophilic ketones with H2O2, or ozonizations. Oxygen transfer from dioxirane reagents probably involves the formation of a charge-transfer π-complex between the substrate and the carbon atom of the dioxirane, and the subsequent formation of a cyclic peroxidic intermediate.

When mixtures of methane, ethane and oxygen are cofed over oxidative coupling of methane catalysts, oxygen is preferentially consumed for the oxidative dehydrogenation of ethane, resulting in low m...

The role of d o metal peroxides as intermediates in catalytic oxidations involving H 2 O 2 and ROOH as the oxygen source is studied through the stoichiometric oxidizing properties of the isolated d o metal peroxidic complexes. Whereas heterolytic epoxidation of olefins requires the coordination of olefins as a prerequisite for a cyclic or a pseudocyclic peroxymetalation mechanism, homolytic oxidation of hydrocarbons involves a bimolecular mechanism with radical intermediates, and is less selective. A complete elucidation of the mechanism of the Halcon epoxidation process has been made possible by studying the reactivity of the complexes VO(OPhsal)(OOtBu) towards olefins.

Novel oxoperoxomolybdenum(VI) complexes with the general formula MoO(O2)L2X2 (III, L = DMF, HMPT) and MoO(O2)Cl(ON)L(IV, ON) = pyridin-2-carboxylate (Pic), 8-hydroxyquinolinate (Quin) were prepared from the reaction of Ph3COOH or H2O2 with the corresponding cis-dioxo complexes. In the reaction with Ph3COOH both oxygen atoms of the peroxo moiety were found, by 18O labeling experiments, to come from the hydroperoxide. The X-ray crystal structure of MoO(O2)Cl(Pic)(HMPT) revealed a bipyramidal pentagonal surounding with a rather short OO distance (1.41 A). Complexes III were found to be more reactive than MoO(O2)2,HMPT for the epoxidation of olefins (oxidative cleavage products are consecutively formed) but react by the same cyclic peroxymetalation mechanism. The absence of reaction in the case of complexes IV illustrates the necessity for the metal to possess an equatorial releasable coordination site adjacent to the peroxo group for the oxygen transfer to occur. Catalytic oxidation of olefins using Ph3COOH gave a selectivity in oxygenated products very different from that using t-BuOOH, and 18O labeling studies showed that alkyl-peroxidic rather than peroxo species are intermediates in this latter reaction. The mechanism of epoxidation of olefins by alkyl hydroperoxides catalyzed by d0 metal complexes is discussed.

A practical procedure for the epoxidation of alkenes is described. The reaction is performed between 30 and 50°C in 1,2-dichloroethane with 30% hydrogen peroxide and a catalytic amount of a quaternary phosphonium or ammonium pertungstate. Allylic alcohols are very reactive and require lower temperatures for their epoxidation.

An attempt is made to correlate the catalytic activity of d0 metal complexes in the oxidation of hydrocarbons by H2O2 and ROOH with the oxidizing properties of their peroxidic derivatives. The triangular structure of bound peroxo and alkylperoxidic groups is a common characteristic of d0 metal peroxides. The heterolytic or homolytic nature of the catalytic oxidation is governed by the heterolytic or homolytic cleavage of the triangular peroxidic moiety. The mechanism of selective heterolytic epoxidation involves the peroxymetalation of the coordinated olefin. Homolytic oxidations are less selective and involve the non-stereospecific epoxidation of olefins and the hydroxylation of alkanes and arenes by biradical peroxidic species.