Wilhelm F. Maier

Carbon-hydrogen activation on rhodium: reaction mechanism and the role of carbonaceous residues.

Evidence for two C-H activation mechanisms, the oxidative addition of a single C-H bond and the concerted oxidative addition of two adjacent C-H bonds, is presented. The two mechanisms show different sensitivity to surface structure or particle size. While the first process (E/sub a/ approx. 21 kcal/mol) is insensitive to the hydrocarbon structure, the second process (E/sub a/ approx. 5-9 kcal/mol) is sensitive to the hydrocarbon structure. C-H activation is found to be 0.6 order in hexane and zero order in deuterium, which is consistent with oxidative addition as the rate-determining step. The d/sub 2/ maximum obtained by H/D exchange of linear alkanes on Rh results from surface olefins, as indicated by high-resolution deuterium NMR. Evidence against the participation of carbonaceous residues in the H/D exchange process is presented, indicating that carbonaceous material is not part of the active site. Two types of carbonaceous residues are detected. One forms at lower temperatures and does not interfere with the reaction; the other forms at higher temperatures and acts to poison the catalyst.

How facile is CH activation?—A semi-empirical study

Abstract CH activation is studied by reaction path calculations for the CH insertion reactions of methane with BU + , B, B − , C, and Al atoms, by semi-empirical calculations. Low activation barriers are predicted for all atoms with p electrons in agreement with qualitative MO arguments and experimental results.

Mechanism of 2-hexyne hydrogenation on heterogeneous palladium

Abstract Relative rates of reaction, orders of reaction, barriers of activation and patterns of deuteration are used to study the mechanism of 2-hexyne hydrogenation in solution on selective and unselective bulk Pd. The selectivity of cis semihydrogenation of disubstituted alkynes is dominated by the difference in surface affinity of alkyne and alkenes. Trans-2-hexene and n-hexane form directly from the alkyne. The intrinsic selectivity for cis semihydrogenation of 95% is unaffected by added quinoline, indicating that quinoline only suppresses the hydrogenation of the product olefin. While there is no difference in intrinsic selectivity, the rate of hydrogenation of intermediate alkenes is much higher on defect-rich Pd surfaces, while the rate of 2-hexyne hydrogenation is not affected by surface structure. The Arrhenius plot shows a drastic change in the rate-determining step at temperatures above 30 °C.

Problems in Selective Catalysis with Molecular Imprints in Silica - Selective Lactones Formation from Hydroxyesters in Micropores -

Selective lactone formation from hydroxy esters can be obtained with microporous glass catalysts. By a sol-gel polycocondensation of TEOS and a cyclic phosphonate an imprinted microporous silica catalyst was prepared. The imprint molecule selected is a transition state analogue for the lactone formation from hydroxy esters. Although selective γ- and δ-lactone formation from the respective hydroxy esters was observed, control experiments revealed, that the catalytic selectivity of the lactone formation is already obtained by the micropores of the material and not associated to the imprint. The study illustrates the importance of proper control experiments for the evaluation of imprint effects in selectively catalyzed reactions.

Reactions of methane with unsaturated hydrocarbons in the presence of nickel catalysts

Methane, when used to activated supported nickel catalysts, react at ca. 300 °C with benzene, cyclopentene, methylenecyclopentane, and simple alkenes to give modest but definite amounts of higher hydrocarbons.

Heterogeneous Catalysis, VI Direct Reduction of Alcohols to Hydro- carbons [1]

Abstract Secondary and tertiary alcohols are converted cleanly to the parent hydrocarbons in the gas phase with H2 in the presence of a Ni/Al2O3 catalyst at 190 °C. Primary alcohols are de-hydroxymethylated; e.g., 1-hydroxymethylada-mantane (4) gives adamantane (1). Alumina alone, in the absence of Ni, results in the formation of homoadamantane (5) from 4.

Hinweise zur thermodynamischen Kontrolle von relativen Hydriergeschwindigkeiten an Platin Indications to a Thermodynamic Control of the Relative Rate of Hydrogenation on Platinum

The relative rate of hydrogenation of various alkenes has been determined by competition experiments in the gas phase on platinum films. Despite big structural differences in the alkenes there is a good correlation of the relative rate of hydrogenation with the calculated heat of hydrogen atom addition to the double bond, indicating thermodynamic control of this reaction.

Reaction selectivity as a test for catalysis on exposed metal

Abstract The metal sensitive selectivities of hydrogenation reactions are employed to differentiate between metal catalysis and spillover catalysis. Two catalyst metal specific reactions, dihydrogen addition to 2-hexyne and 1,6-dimethylcyclohexene, are studied on silicon supported palladium and platinum films covered with silica overlayers. At temperatures near 200°C selectivities observed are characteristic of the metal in the underlayer. Since such films have been shown not to contain transition metal in the silica layer, the results suggest that cracks in the silica are present during reaction. In contrast, the selectivity observed at temperatures below 100°C is not that of the metal underlayer and is therefore consistent with a spillover phenomenon. The method represents a general test which may also prove useful to rule out transition metal catalysis in studies invoking a hydrogen spillover mechanism.

C ‐ H Activation on Platinum, a Mechanistic Study.

C-H activation on heterogeneous Pt catalysts is found to be most facile at low Pt dispersion and surface structure insensitive at dispersions above 10%. Two distinct processes have been identified which lead to the formation of mono- and polydeuterated products, respectively. The ratio of mono-/polydeuteration processes increases with increasing dispersion. Evidence is provided for the formation of ..pi..-intermediates as the initiator of polydeuteration. The most facile mechanism of exchange propagation in a single molecule is through a ..pi..-allyl-type interconversion. The deuterium in monodeuterated n-hexane is located at the terminal position by high-resolution deuterium NMR. NMR spectroscopy also provides evidence for steric effects which prevent significant H/D exchange at positions close to quaternary centers. C-H activation appears to be sensitive to steric hindrance, the C-H bond energy, and the partial pressure of the substrate.

Intramolecular hydrogen exchange in 2,3-dimethylbutane catalysed by nickel

A cross experiment with a specifically labeled hydrocarbon reveals that intramolecular hydrogen exchange is an important process for saturated hydrocarbons chemisorbed on a nickel catalyst.