A. E. Gekhman

Catalysis with a palladium giant cluster: phenol oxidative carbonylation to diphenyl carbonate conjugated with reductive nitrobenzene conversion

Giant palladium-561 clusters were found to be effective in catalysis of the oxidative carbonylation of phenol to diphenyl carbonate, conjugated with the reductive carbonylation and reduction of nitrobenzene under CO pressure.

Free radical/singlet dioxygen system under the conditions of catalytic hydrogen peroxide decomposition

Hydrogen peroxide decomposition and the oxidation of unsaturated compounds (anthracenes, alkenes, etc.) in the H2O2/V(V)/AcOH system occur via a molecular mechanism. H2O2 decomposes to yield singlet dioxygen1O2(1Δg). Among VV peroxo complexes of different compositions, coordinated Superoxide radical anions V(V)(O2/∸) are found in a steady-state concentration in the system under investigation. Styrene oxidation in the H2O2V(V)/AcOH system unusually accelerates in the presence of 2,6-di-tert-butyl-4-methylphenol (BHT), which is an inhibitor of radical chain reactions. This is explained by a decrease in the V(V)(O2∸) concentration and an increase in the concentration of dissolved1O2 in the presence of ionol. A new phenomenon in the chemistry of singlet dioxygen is found: the ESR signal from the paramagnetic system upon its interaction with1O2 broadens in an unusually drastic manner (up to 10 G). This broadening is virtually independent of the nature of the radicals, the acidity of the medium, and the nature of the metal catalysts used for the generation of1O2(1Δg).

Cocatalytic effect of palladium and zinc in the condensation of alcohol carbon backbones into hydrocarbons

The results of the direct conversion of ethanol and its mixture with glycerol into a C4–C10+ alkane and olefin fraction in the presence of Pd-, Zn-, and Pd-Zn-containing catalysts, which were prepared by supporting homo- and heterometallic acetate complexes onto the surface of γ-Al2O3, are reported. It was found that, in the presence of mono- and bicomponent Pd-ZnO (Pd; ZnO)/γ-Al2O3 systems, selectivity in the formation of alkanes, olefins, or their mixtures in the target fraction can be controlled as a result of the cocatalytic effect of active components that are responsible for the catalyst activity in condensation and hydrogenation reactions. The structures of the active components were studied and the genesis of the catalytic systems was characterized using XAFS, XPS, and XRD analysis. It was found that the addition of glycerol considerably increased the yield of the target hydrocarbon fraction.

Alumina-platinum catalyst in the reductive dehydration of ethanol and diethyl ether to alkanes

The reductive dehydration of ethanol and diethyl ether selectively occurs with the formation of alkanes to C10+ on an AP-64 alumina-platinum catalyst (0.6 wt % Pt/γ-Al2O3) after its reduction with hydrogen at 450°C for 12 h in Ar. It was found that one of the main reaction paths is the insertion of ethylene into substrate intermediates with the predominant formation of normal alkanes. It was found by XAFS spectroscopy that Pt2Al intermetallide particles were formed along with platinum metal clusters after long reduction. The ammonia TPD data indicated a change in the acid properties of the surface after the long reduction of the catalyst: the concentration of medium-strength surface aprotic acid sites increased by a factor of 2. It was found that the interaction of aprotic sites with water vapor resulted in the formation of strong proton acid sites. It is likely that these latter are responsible for the growth of a carbon skeleton in the course of alkane formation from ethanol.

Metal complex catalyzed oxidations with hydroperoxides: Inner-sphere electron transfer

Abstract The reactions involving electron transfer between peroxidic oxidant and a substrate capable of coordination to a metal atom are reviewed. A metal ion can act as a mediator between the coordinated reactants in such a redox reaction. Oxidation of acetylacetonate and related compounds, singlet dioxygen formation and decomposition of H 2 O 2 to form ozone are discussed as examples.

Isotope effects in the oxidative functionalization of methane in the presence of rhodium-containing homogeneous catalytic systems

The combined oxidation of carbon monoxide, CH4, and CD4 by molecular oxygen (16O2 and 18O2) in aqueous solutions of trifluoroacetic acid labeled with 18O in the presence of rhodium and copper compounds and potassium iodide has been studied. The distribution of 18O in isotopically substituted products suggests that oxygen entered into the methane molecule from an active oxidizing agent. This oxidizing agent was produced from molecular oxygen under the action of reagents and catalytic system components. The kinetic isotope effect observed for methane (kH/kD=3.9−4.3) suggests a nonradical character of the step at which the oxygen atom passes from an active oxidizing agent to the methane molecule or its fragments—transition-state components of the corresponding step.

Unexpected participation of nucleophiles in the reaction of palladium(II) acetate with divalent 3d metals

The kinetics of reactions of palladium(II) acetate with cobalt(II), nickel(II), and copper(II) acetates were studied by spectrophotometry. These reactions produce heterobimetallic complexes PdII(μ-OOCMe)4MII(OH2)(HOOCMe)2, where M = Co, Ni, or Cu. These reactions are very slow in carefully dehydrated (<0.01% H2O) acetic acid, but are considerably enhanced by water or acetonitrile. Our data indicate that the activation of the kinetically inert ring structure of the initial palladium complex Pd3(μ-OOCMe)6 by means of the nucleophilic attack of an H2O or acetonitrile molecule is the key step of the reaction mechanism.

Hydroperoxide Oxidation of Difficult-to-Oxidize Substrates: An Unprecedented C–C Bond Cleavage in Alkanes and the Oxidation of Molecular Nitrogen

In the V(V)H2O2/AcOH system, C5–C20n-alkanes, isooctane, and neohexane undergo oxidation to ketones and alcohols; the oxidation products of branched alkanes are indicative of a C–C bond cleavage in these substrates. A concept is developed, according to which the peroxo complexes of vanadium(V) are responsible for alkane oxidation. These complexes can transfer the oxygen atom or the O+· radical cation to a substrate. The formation of nitrous oxide was found in the oxidation of molecular nitrogen in the H2O2/V(V)/CF3COOH system.

Decomposition of hydrogen peroxide catalyzed by vanadium(v) compounds: the pathways for the formation of ozone

Reactions of H2O2 in trifluoroacetic acid catalyzed by vanadium(v) compounds were studied. The system under study exhibits unusual behavior: along with oxygen, large quantitaties (10–15 %) of ozone are found in the products of hydrogen peroxide decomposition; difficultly oxidizable compounds (alkanes, arenes, and perfluoroalkenes) are oxidized under mild conditions. The rates of the oxidation of individual substrates are commensurable. However, when two compounds are simultaneously present in the reaction mixture, cyclohexane stops the oxidation of all of the other substrates, arenes suppress the oxidation of perfluoroolefins, and perfluoro-1-octene stops the consumption of internal perfluoroolefins. The effect of the oxidizable substrates on the amount of ozone evolved was studied. Based on the kinetic data obtained, a mechanism that involves the consecutive formation of several active complexes of vanadium(v) responsible for the oxidation of substrates and for the formation of ozone is suggested. In terms of the scheme suggested, the inner-sphere oxidation of the peroxo ligand by the coordinated peroxotrifluoroacetic acid affords a complex incorporating O32− as a ligand. The latter acts as the precursor of the ozone. A mathematical model that adequately describes the experimental data is proposed.

Oxidations in the H2O2VvAcOH system. Evidence of vanadium (V) complexes with superoxide anion and singlet dioxygen molecule as ligands

Anthracene, its alkyl derivatives, and alkenes have been found to undergo oxidation by the VvH2O2AcOH catalytic system via a polar mechanism involving the transfer of singlet dioxygen from H2O2 to the unsaturated substrate molecule. The vanadium (V) complex with a singlet dioxygen molecule as ligand was suggested to be the only oxidant species active in the reaction. The Vv(O2•−) complex radical was detected as the only paramagnetic species in the system. Unusual acceleration of the styrene oxidation caused by the 2,6-di-tert-butyl-4-methylphenol additive (BHT) was observed and attributed to lowering the concentration of the Vv(O2•−) complex radical, which is assumed to be an active reductant toward the Vv complex with singlet dioxygen.