Mechanism of hydrogenolysis and isomerization of oxacycloalkanes on metals, XVI. Transformation of tetrahydrofuran on platinum catalysts

The hydrogenolysis of tetrahydrofuran (THF) on TiO₂, SiO₂ and Al₂O₃ supported Pt catalysts has been investigated in the gas phase at 423–623 K in a pulse reactor. The main reactions are hydrodeoxygenation to butane and decarbonylation to propane. The supports had a marked effect on the selectivity of conversion. On the Pt−TiO₂ catalyst mainly butane was formed while on the other two catalysts propane formation was dominant. Based on the results of earlier studies on oxacycloalkanes as well as on data in the literature and the experimental results presented here, a new reaction scheme is outlined for the interpretation of the chemical processes discussed. Part XV.: J. Mol. Catal. (in press)     

Stability of intermediates in the glycerol hydrogenolysis on transition metal catalysts from first principles

The hydrogenolysis reaction catalyzed by a transition metal solid catalyst is a potential way to transform glycerol to 1,2-propylene glycol or 1,3-propylene glycol, two important chemicals. We explore the thermodynamic profile of this reaction from first principle simulation, comparing Ni, Rh and Pd catalysts modeled by (111) surfaces. The stability of adsorbed reactants, dehydrated intermediates, and hydrogenated propylene glycol is compared, with a special focus on the factors controlling the selectivity of the reaction. From a global thermodynamic view point, the formation of 1,2-propylene glycol is favored, and in addition the most stable intermediates in the gas phase (acetol and 1,2-aldol) lead to the formation of this product. The metal catalyst has three roles. First it stabilizes the dehydrated intermediates and renders the dehydration more exothermic. Second, the adsorption on the surface modifies the relative stability of the dehydrated intermediates, with implications on the reaction selectivity. Third it catalyses the hydrogenation step, leading to propylene glycol.     

Recent development of heterogeneous catalysis in ring-opening,biocatalysis, and selective partial oxidation reactions on metal oxides

In this review article, we analyze the state of the art and future developments in three important domains of heterogeneous catalysis, namely ring opening, biocatalysis, and partial oxidation on metal oxides. After recollecting the scientific bases of each domain, we consider several examples, some recent improvements/developments, and some prospective views.     

Influence of the particle size of metal in the hydrogenolysis of n-butane on carbon supported iron catalysts

The hydrogenolysis of n-butane on different carbon supported iron catalysts has been studied. Changes in activity, product distribution, apparent activation energy and frequency factor were found to be a function of particle size. This behavior is explained as a change in the reaction mechanism, which also leads to the isomerization of n-butane in the case of catalysts with higher dispersity.

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Mechanism of skeletal reactions of hydrocarbon on metals

The mechanism and intermediates of the title reaction are described. The subjects discussed include the influence of carbonaceous layers, particle size, alloying etc., on the activity and selectivity of metals. The roles of chemical bond strengths and metal-carbon multiple bonds are also briefly discussed. Discussion on the selectivity of metals reviews the well established data and theories concerning the skeletal reactions.     

Comparison of chemisorption and hydrogenolysis of ethane on transition metals

Data on chemisorption of ethane on transition metal blacks have been correlated with hydrogenolysis activity measurements. On Pt and Pd the rupture of the C–C bond seems to be a hindered process. Desorption of methane requires larger activation on Co and Ni than on Ru, Rh or or Ir blacks.

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. Pt Pd C–C . Co Ni, Ru, Rh Ir.

     

Comparison of Ru- and Mo-based chiral olefin metathesis catalysts. Complementarity in asymmetric ring-opening/cross-metathesis reactions of oxa- and azabicycles

A comparative study of chiral Mo- and Ru-based catalysts to promote enantioselective synthesis of 2,6-disubstituted pyrans and piperidines through asymmetric ring-opening/cross-metathesis (AROM/CM) reactions is presented. These studies demonstrate the critical complementarity that exists between the two classes of chiral catalysts.     

Influence of organic reagents on nickel alumina supported catalysts. II. hydrogenolysis of n-butane

The influence of organic reagents on the rate, selectivity and activation energy of n-butane hydrogenolysis has been studied. Increasing of the functional groups and coordination place in used organic reagents the caused higher mean values of reaction rates, activation energy and selectivity towards methane.

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Mechanism of mechanochemical reactions in malachite-active metal systems

Mechanically activated mixtures of some hydroxocarbonates with powder, metallic aluminium caused chemical reactions, which result in alloy formation. The identification of phases formed during milling was successfully achieved by thermal analysis methods supplemented by X-ray diffraction measurement.     

Hydrogenation and hydrogenolysis of methylcyclohexane-1,4-dione on Pt and Pd catalysts

Relative hydrogenation reactivity of the two carbonyl groups of methylcyclohexane-1,4-dione can be estimated beside that of two related compounds, 2- and 3-methyl-cyclohexanones. The reaction is accompanied with hydrogenolysis on Pt and Pd. The less hindered 4-carbonyl group is selectively hydrogenolyzed.

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-1,4- , 2- 3-. Pt Pd. 4- .

     

On the interpretation of differences in mechanisms, kinetics and dynamics of oxidation reactions on metal and oxide catalysts

Langmuir-Hinshelwood type mechanisms with competitive adsorption are suggested to be more typical for metal catalysts, since, unlike on oxide systems, oxidizable substance and molecular oxygen can be adsorbed on the same (reduced) active sites. Therefore, multiple steady states and critical phenomena are more characteristic for catalysis on metals.

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Activation of noble metals on metal-carbide surfaces: novel catalysts for CO oxidation, desulfurization and hydrogenation reactions

This perspective article focuses on the physical and chemical properties of highly active catalysts for CO oxidation, desulfurization and hydrogenation reactions generated by depositing noble metals on metal-carbide surfaces. To rationalize structure-reactivity relationships for these novel catalysts, well-defined systems are required. High-resolution photoemission, scanning tunneling microscopy (STM) and first-principles periodic density-functional (DF) calculations have been used to study the interaction of metals of Groups 9, 10 and 11 with MC(001) (M = Ti, Zr, V, Mo) surfaces. DF calculations give adsorption energies that range from 2 eV (Cu, Ag, Au) to 6 eV (Co, Rh, Ir). STM images show that Au, Cu, Ni and Pt grow on the carbide substrates forming two-dimensional islands at very low coverage, and three-dimensional islands at medium and large coverages. In many systems, the results of DF calculations point to the preferential formation of admetal-C bonds with significant electronic perturbations in the admetal. TiC(001) and ZrC(001) transfer some electron density to the admetals facilitating bonding of the adatom with electron-acceptor molecules (CO, O(2), C(2)H(4), SO(2), thiophene, etc.). For example, the Cu/TiC(001) and Au/TiC(001) systems are able to cleave both S-O bonds of SO(2) at a temperature as low as 150 K, displaying a reactivity much larger than that of TiC(001) or extended surfaces of bulk copper and gold. At temperatures below 200 K, Au/TiC is able to dissociate O(2) and perform the 2CO + O(2)→ 2CO(2) reaction. Furthermore, in spite of the very poor hydrodesulfurization performance of TiC(001) or Au(111), a Au/TiC(001) surface displays an activity for the hydrodesulfurization of thiophene higher than that of conventional Ni/MoS(x) catalysts. In general, the Au/TiC system is more chemically active than systems generated by depositing Au nanoparticles on oxide surfaces. Thus, metal carbides are excellent supports for enhancing the chemical reactivity of noble metals.     

Ethylene polymerization catalysts from supported organotransition metal complexes: I. Pentadienyl derivatives of Ti,V, and Cr

A new class of ethylene polymerization catalysts, namely the bis-(2,4-dimethyl pentadienyl) derivatives of titanium, vanadium, and chromium, have been synthesized and tested. When supported on a variety of inorganic carriers, these compounds yielded 0.2–1.0 million g polymer/g metal/h under typical slurry conditions. Sensitivity to H₂ as a molecular weight regulator varied among the three metals, but in the absence of H₂ all produced ultrahigh-molecular-weight polyethylene. The molecular weight distribution varied from moderately narrow to very broad (bimodal) depending on the metal and the carrier. Catalyst synthesis and polymer properties are discussed.     

Light-mediated heterogeneous cross dehydrogenative coupling reactions: metal oxides as efficient, recyclable, photoredox catalysts in C-C bond-forming reactions

Let there be light: A heterogeneous photocatalytic system based on easily recyclable TiO(2) or ZnO allows cross dehydrogenative coupling reactions of tertiary amines. The newly developed protocols have successfully been applied to various C-C and C-P bond-forming reactions to provide nitro amines as well as amino ketones, nitriles and phosphonates.     

Fe/Zn double metal cyanide catalyzed ring-opening polymerization of propylene oxide: 2. Characterization of active structure of double metal cyanide catalysts

Double metal cyanide (DMC) complexes based on Zn₃[Fe(CN)₆]₂ were synthesized using different molar ratios of ZnCl₂ to K₃[Fe(CN)₆] and special complexing agents. IR spectroscopy, electron spectroscopy for chemical analysis, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and other analytical techniques were employed to characterize these catalysts. The morphology and structure of these DMC catalysts were attributed to the different complexing agents as well as to the different molar ratios of ZnCl₂ to K₃[Fe(CN)₆]. In addition, the catalytic activity was strongly correlated with the morphology and noncrystalline content of DMC catalysts. High-activity catalysts could be prepared by controlling the structure of DMC catalysts by incorporating complexing agents. The active species of DMC catalysts for ring-opening polymerization are Zn²⁺, [Fe(CN)₆]^3–, Cl^–, and the compound of their ligands.