Adsorption data extracted from kinetic equations of some hydrocarbon conversions on nickel

Adsorption equilibrium constants and adsorption enthalpies have been calculated from kinetic data of cyclohexane dehydrogenation and ethane hydrogenolysis on metallic nickel. From these data the bond strength of hydrogen and carbon with the active sites of nickel and the activation barrier of C–H and H–H bond dissociation were calculated by the recently developed method of Shustorovich. The calculated data indicate that the active sites for dehydrogenation differ (or in part differ) from those of hydrogenolysis.     

Trapping-mediated dissociative chemisorption of cycloalkanes on Ru(001) and Ir(111): influence of ring strain and molecular geometry on the activation of C-C and C-H bonds.

We have measured the initial probabilities of dissociative chemisorption of perhydrido and perdeutero cycloalkane isotopomers on the hexagonally close-packed Ru(001) and Ir(111) single-crystalline surfaces for surface temperatures between 250 and 1100 K. Kinetic parameters (activation barrier and preexponential factor) describing the initial, rate-limiting C-H or C-C bond cleavage reactions were quantified for each cycloalkane isotopomer on each surface. Determination of the dominant initial reaction mechanism as either initial C-C or C-H bond cleavage was judged by the presence or absence of a kinetic isotope effect between the activation barriers for each cycloalkane isotopomer pair, and also by comparison with other relevant alkane activation barriers. On the Ir(111) surface, the dissociative chemisorption of cyclobutane, cyclopentane, and cyclohexane occurs via two different reaction pathways: initial C-C bond cleavage dominates on Ir(111) at high temperature (T > approximately 600 K), while at low temperature (T < approximately 400 K), initial C-H bond cleavage dominates. On the Ru(001) surface, dissociative chemisorption of cyclopentane occurs via initial C-C bond cleavage over the entire temperature range studied, whereas dissociative chemisorption of both cyclohexane and cyclooctane occurs via initial C-H bond cleavage. Comparison of the cycloalkane C-C bond activation barriers measured here with those reported previously in the literature qualitatively suggests that the difference in ring-strain energies between the initial state and the transition state for ring-opening C-C bond cleavage effectively lowers or raises the activation barrier for dissociative chemisorption via C-C bond cleavage, depending on whether the transition state is less or more strained than the initial state. Moreover, steric arguments and metal-carbon bond strength arguments have been evoked to explain the observed trend of decreasing C-H bond activation barrier with decreasing cycloalkane ring size.     

Hydrogenolysis of cycloalkanes on a tantalum hydride complex supported on silica and insight into the deactivation pathway by the combined use of 1D solid-state NMR and EXAFS spectroscopies

Hydrogenolysis of cyclic alkanes is catalysed by [(triple bond)SiO)(2)Ta-H] (1) at 160 degrees C and leads to lower alkanes and cyclic alkanes including cyclopentane. The turnover number is correlated with the number of carbon atoms of the cyclic alkanes, and therefore while cycloheptane is readily transformed, cyclopentane does not give any product (<1 %). The mechanism of ring contraction probably involves carbene de-insertion as a key carbon-carbon bond-cleavage step. The reluctance of cyclopentane to undergo hydrogenolysis was further studied: under the reaction conditions cyclopentane reacts with 1 to give the corresponding cyclopentyl derivative [(triple bond)SiO)(2)Ta-C(5)H(9)] (13), which evolves towards cyclopentadienyl derivative [(triple bond)SiO)(2)Ta(C(5)H(5))] (14) according to both solid-state NMR and EXAFS spectroscopies. This latter complex is inactive in the hydrogenolysis of alkanes, and therefore the formation of cyclopentane in the hydrogenolysis of various cyclic alkanes is probably responsible for the de-activation of the catalyst by formation of cyclopentadienyl complexes.     

Heterolytic activation of hydrogen as a trigger for iridium complex promoted activation of carbon-fluorine bonds

The cationic iridium(III) complex [IrCF(3)(CO)(dppe)(DIB)][BARF](2) where DIB = o-diiodobenzene, dppe = 1,2-bis(diphenylphosphino)ethane, and BARF = B(3,5-(CF(3))(2)C(6)H(3))(4)(-) undergoes reaction in the presence of dihydrogen to form [IrH(2)(CO)(2)(dppe)](+) as the major product. Through labeling studies and (1)H and (31)P[(1)H] NMR spectroscopies including parahydrogen measurements, it is shown that the reaction involves conversion of the coordinated CF(3) ligand into carbonyl. In this reaction sequence, the initial step is the heterolytic activation of dihydrogen, leading to proton generation which promotes alpha-C-F bond cleavage. Polarization occurs in the final [IrH(2)(CO)(2)(dppe)](+) product by the reaction of H(2) with the Ir(I) species [Ir(CO)(2)(dppe)](+) that is generated in the course of the CF(3) --> CO conversion.     

Untersuchungen an oxidischen Katalysatoren. XXXIX. Der Einfluß acider Träger auf die katalytische Wirksamkeit von Metallen

Studies on Oxide Catalysts. XXXIX. Influence of Acidic Carriers on the Catalytic Effect of Metals The catalytic behaviour of modified zeolite catalysts containing metal in the isomerization of n-hexane as well as in the dehydrogenation of cyclohexane and in the hydrogenolysis of ethane shows that the catalytic action of the metal can be continuously and reversibly varied by modifying the carrier acidity. From the concept of an electronic interaction between metal and (acid) carrier conclusions can be derived for a directed modification of the catalytic properties of metal/carrier catalysts.     

Chemoselective alkane oxidation by superoxo-vanadium(V) in vanadosilicate molecular sieves

Electron paramagnetic resonance (EPR) spectroscopy of reactive superoxo-vanadium(V) species in vanadosilicate molecular sieves (microporous VS-1 and mesoporous V-MCM-41) generated on contact with H2O2, tert-butyl hydroperoxide (TBHP), or (H2+O2) is reported for the first time. By suitable choice of the silicate structure, solvent, and oxidant, we could control the vanadium-(O2-*) bond (i.e., the V-O bond) covalency, the mode of O-O cleavage (in the superoxo species), and, therefore, chemoselectivity in the oxidation of n-hexane: Oxidation by TBHP over V-MCM-41, for example, yielded 27.2% of (n-hexanol+n-hexanal+n-hexanoic acid), among the highest chemoselectivities for oxidation of the terminal -CH3 in a linear paraffin reported to date. Over these vanadosilicates, oxidation of the primary C-H bond occurs only via a homolytic O-O bond cleavage; the secondary C-H bond oxidations may proceed via both the homo- and heterolytic O-O cleavage mechanisms.     

Ir/SiO2上甲烷部分氧化制合成气反应的原位时间分辨红外和原位Raman光谱表征

采用原位时间分辨红外光谱和原位显微Raman光谱技术对Ir/SiO₂上甲烷部分氧化(POM)制合成气反应的初级产物和反应条件下催化剂表面物种进行了跟踪考察,实验结果表明,在H₂预还原的新鲜Ir/SiO₂表面,CO是V(CH₄):V(O₂):V(Ar)=2:1:45混合气反应的初级产物,因而甲烷的直接氧化过程是CO生成的主要途径;而在稳态反应条件下,CO生成的途径可能主要来自CO₂和H₂O与催化剂表面积碳物种(CHx)和/或CH₄的反应.催化剂上生成的积碳可能是导致稳态条件下Ir/SiO₂上POM反应机理不同于H₂预还原的新鲜催化剂的主要原因.     

铑催化合成气制乙醇反应中CO断键途径的研究

利用程序升温表面反应─红外（ＴＰＳＲ－ＩＲ）动态技术考察ＣＯ吸附物种对氢的反应性能并检验表面反应生成的中间物，结果表明线式ＣＯ对氢的反应性能高于桥式ＣＯ，即线式ＣＯ更可能是活性吸附态；表面反应生成了ＨＣＯ、ＣＨ２等中间物．用键级守恒（ＢＯＣ）－Ｍｏｒｓｅ势方法计算比较了ＣＯ→ＣＨ２过程中各可能基元步骤在Ｒｈ（１１１）面上的反应活化能和反应热，结果表明ＣＯ经其部分氢化物种（如Ｈ２ＣＯ、ＨＣＯＨ）的氢解反应断裂Ｃ─Ｏ键在能量上最有利．根据这些实验结果，提出铑基催化剂上合成气转化反应主要按缔合式机理进行；ＣＯ的优势断键途径为先部分氢化，而后氢助断键．     

Ni2P/SiO2 和Ni/SiO2 催化剂甘油氢解反应性能比较:催化剂活性及产物选择性影响因素的探讨（英文）

采用浸渍法及程序升温还原法制备了Ni2P/SiO2和Ni/SiO2催化剂,利用N2吸附-脱附、X射线衍射、X射线荧光、CO化学吸附、氢气程序升温脱附及氨气程序升温脱附等手段对催化剂进行了表征并用于甘油氢解反应.结果表明,Ni2P/SiO2和Ni/SiO2具有相近的表面Ni密度,但前者表面酸中心和表面氢物种(包括吸附氢和溢流氢)密度明显更高,且在甘油氢解反应中的活性也更高,这与其酸性中心与金属中心之间的协同作用有关.Ni2P/SiO2催化剂上主要产物为1,2-丙二醇及1-丙醇,而Ni/SiO2催化剂上主要产物为1,2-丙二醇、乙二醇和乙醇.提高反应温度和H2压力不能促进Ni2P/SiO2上乙醇和乙二醇的生成,但促进了1,2-丙二醇进一步氢解转化为1-丙醇.由此可见,Ni2P/SiO2具有较强的C-O键断裂活性及较弱的C-C键断裂活性,这可能分别与其较多酸性中心和电子及几何结构性质密切相关.     

Microkinetic Modeling of Ethane Hydrogenolysis on Metals

The microkinetic analysis of ethane hydrogenolysis in the framework of the Sinfelt-Taylor mechanism is carried out using UBI-QEP data on the energetics of elementary steps. On single-crystalline surfaces of Pd(111), Pt(111), and Ni(111), the reaction predominantly occurs via ethyl formation in which the C-C bond is cleaved. The surface concentration of ethylidyne is very low, and ethyl has the highest concentration among all hydrocarbon fragments. For the conditions studied in this work, the activity in ethane hydrogenolysis increases in the series Pd(111) < Pt(111) < Ni(111). According to the results of kinetic modeling, the reaction occurs via quasi-equilibrium steps of adsorption and dissociation of hydrogen and ethane and further, practically irreversible formation of methane.     

Mechanism of hydroxyl radical generation from a silica surface: molecular orbital calculations

The interaction of an H(2)O molecule with cluster models of fractured silica surfaces was studied by means of quantum mechanical calculations. Two clusters representing homolytic cleavage (triple bond Si(*) and triple bond SiO(*)) and two representing heterolytic cleavage (triple bond Si(+) and triple bond Si-O(-)) of silica surfaces were modeled. Vibrational frequencies of the reactants and products of these silica surfaces reacting with H(2)O have been calculated and compare favorably with experiment. Comparisons of the Gibbs free and potential energies for the model ionic and radical states were made, and the radical pair of sites was predicted to be more stable by approximately -70 to -85 kJ/mol, depending on the computational methodology. These calculations suggest that when silica is fractured in a vacuum homolytic cleavage is favored. Reaction pathways were investigated for these four model surface sites interacting with H(2)O. The reaction of H(2)O with triple bond SiO(*) was predicted to generate OH(*). Rate constants for these reactions were also calculated and predict a rapid equibrium for the reaction triple bond SiO(*) + H(2)O --> triple bond SiOH + OH(*). Stability of a finite number of triple bond SiO(*) sites at equilibrium in the above reaction with H(2)O was also predicted, which implies a long-term ability of silica surfaces to produce OH(*) radicals if the sites of the broken bonds do not repolymerize to form siloxane groups.     

Molecular understanding of the formation of surface zirconium hydrides upon thermal treatment under hydrogen of [([triple bond]SiO)Zr(CH2tBu)3] by using advanced solid-state NMR techniques

The reaction of [([triple bond]SiO)Zr(CH(2)tBu)(3)] with H(2) at 150 degrees C leads to the hydrogenolysis of the zirconium-carbon bonds to form a very reactive hydride intermediate(s), which further reacts with the surrounding siloxane ligands present at the surface of this support to form mainly two different zirconium hydrides: [([triple bond]SiO)(3)Zr-H] (1a, 70-80%) and [([triple bond]SiO)(2)ZrH(2)] (1b, 20-30%) along with silicon hydrides, [([triple bond]SiO)(3)SiH] and [([triple bond]SiO)(2)SiH(2)]. Their structural identities were identified by (1)H DQ solid-state NMR spectroscopy as well as reactivity studies. These two species react with CO(2) and N(2)O to give, respectively, the corresponding formate [([triple bond]SiO)(4-x)Zr(O-C(=O)H)(x)] (2) and hydroxide complexes [([triple bond]SiO)(4-x)Zr(OH)(x)] (x = 1 or 2 for 3a and 3b, respectively) as major surface complexes.     

Untersuchungen an oxidischen Katalysatoren. XXXVIII. Zum bifunktionellen Zusammenwirken metallischer und acider Zentren in Metall/HNaY-Zeolith-Katalysatoren

Studies on Oxide Catalysts. XXXVIII. On the Bifunctional Action of Metal Sites and Acid Sites in Metal/HNaY Zeolite Catalysts The catalytic activity and selectivity of HNaY zeolites containing nickel were investigated in the isomerization of n-hexane. In order to characterize independently the catalytic properties of the supported metal, the catalytic behaviour of these samples was determined in the dehydrogenation reaction of cyclohexane and in the hydrogenolysis of ethane. The catalytic properties of the metal in metal/HNaY zeolite catalysts are strongly modified by acid sites of the zeolite. This modification of the catalytic properties is attributed to electronic interactions between metal and zeolite.     

钡修饰Ir/SiO2催化剂对氨分解促进作用的研究

研究了浸渍法制备的Ba修饰S iO2担载型铱(Ir)基催化剂(Ba-Ir/S iO2)在氨催化分解模型反应的作用。结果表明Ba的添加显著提高了催化剂的活性和稳定性。采用H2-TPR和H2-TPD对Ba-Ir/SiO2进行分析和研究,结果显示助催化剂Ba和活性催化组分Ir之间发生了强相互作用。Ba-Ir/SiO2在氨催化分解模型反应中的主要活性物种是零价态的Ir。     

Energetics of C–C Bond Scission in Ethane Hydrogenolysis: A Theoretical Study of Possible Intermediates and Reaction Pathways

C–C bond scission steps, which are often considered as rate-determining in ethane hydrogenolysis, are studied by the Unity Bond Index–Quadratic Exponential UBI–QEP method. The binding energies of atomic carbon with Group VIII and IB metal surfaces Ni(111), Pd(111), Pt(111), Rh(111), Ru(001), Ir(111), Fe(110), Cu(111), and Au(111) are estimated using experimental data on the adsorption of various species on these surfaces. These estimates are corrected using data from density functional theory (DFT) on the adsorption heats of the CH _x species. Metal surfaces are arranged in the following series according to the binding strength of a carbon atom: Cu(111) < Au(111) < Pd(111) < Ru(001) Pt(111) < Ni(111) Rh(111) < Ir(111) < Fe(110). The values of chemisorption heats range from 121 kcal/mol for Au(111) to 193 kcal/mol for Fe(110). The activity of these surfaces toward C–C bond scission increases in the same series. The results of this work suggest that the most probable C–C bond scission precursors are ethyl, ethylidyne, adsorbed acetylene, CH₂CH, CH₂C, and CHC. Theoretical data obtained by different methods are compared and found to agree well with each other. An overview of experimental data on ethane hydrogenolysis mechanisms is given.     

Effects of adsorbates on supported platinum and iridium clusters: Characterization in reactive atmospheres and during alkene hydrogenation catalysis by X-ray absorption spectroscopy

MgO-, SiO2-, and gamma-Al2O3-supported platinum clusters and particles (with average diameters ranging from 11 to 45 A) and zeolite-supported Ir4 clusters (approximately 6 A in diameter) were characterized by extended X-ray absorption fine structure spectroscopy in the presence of H2, O2, ethene, propene, and ethane, as well as under conditions of alkene hydrogenation catalysis. The results indicate that under various atmospheres, the presence of adsorbates affects the smaller platinum clusters (11 A) on gamma-Al2O3 more substantially than the larger platinum particles (i.e., those greater than approximately 21 A in average diameter) on MgO or SiO2. When Pt/gamma-Al2O3 was exposed to H2, the platinum morphology did not change, although the Pt-Pt bond distance increased. In contrast, when the same sample was exposed to O2, complete oxidative fragmentation took place. This processes was reversed following subsequent treatment with H2. Exposure to alkenes changed both the morphology and electron density (as indicated by X-ray absorption near-edge spectra) of the gamma-Al2O3-supported platinum clusters. Under conditions of alkene hydrogenation catalysis at room temperature, the electronic properties and the structure of the platinum clusters were found to depend on the reactant composition and the nature of molecules involved in the reaction process. The effects of the reactant gases on the smaller iridium clusters (Ir4) were substantially less pronounced, apparently as a consequence of the extremely small number of atoms in each iridium cluster.     

Hydrogenolysis of β-O-4 lignin model dimers by a ruthenium-xantphos catalyst

Hydrogenolysis reactions of so-called lignin model dimers using a Ru-xantphos catalyst are presented (xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene). For example, of some nine models studied, the alcohol, 2-(2-methoxyphenoxy)-1-phenylethanol (), with 5 mol% Ru(H)(2)(CO)(PPh(3))(xantphos) () in toluene-d(8) at 135 °C for 20 h under N(2), gives in ～95% yield the C-O cleavage hydrogenolysis products, acetophenone () and guaiacol (), and a small amount (<5%) of the ketone, 2-(2-methoxyphenoxy)-1-phenylethanone (), as observed by (1)H NMR spectroscopy. The in situ Ru(H)(2)(CO)(PPh(3))(3)/xantphos system gives similar findings, confirming a recent report (J. M. Nichols et al., J. Am. Chem. Soc., 2010, 132, 12554). The active catalyst is formulated 'for convenience' as 'Ru(CO)(xantphos)'. The hydrogenolysis mechanism proceeds by initial dehydrogenation to give the ketone , which then undergoes hydrogenolysis of the C-O bond to give and . Hydrogenolysis of to and also occurs using the Ru catalyst under 1 atm H(2); in contrast, use of 3-hydroxy-2-(2-methoxyphenoxy)-1-phenyl-1-propanone (), for example, where the CH(2) of has been changed to CHCH(2)OH, gives a low yield (≤15%) of hydrogenolysis products. Similarly, the diol substrate, 2-(2-methoxyphenoxy)-1-phenyl-1,3-propanediol (), gives low yields of hydrogenolysis products. These low yields are due to formation of the catalytically inactive complexes Ru(CO)(xantphos)[C(O)C(OC(6)H(4)OMe)[double bond, length as m-dash]C(Ph)O] () and/or Ru(CO)(xantphos)[C(O)CH[double bond, length as m-dash]C(Ph)O] (), where the organic fragments result from dehydrogenation of CH(2)OH moieties in and . Trace amounts of Ru(CO)(xantphos)(OC(6)H(4)O), a catecholate complex, are isolated from the reaction of with . Improved syntheses of and lignin models are also presented.     

Dihydroceramide delta(4) desaturase initiates substrate oxidation at C-4.

The intermolecular primary deuterium isotope effects on the individual C-H bond cleavage steps involved in dihydroceramide Delta(4) desaturation have been determined for the first time by incubating rat liver microsomes with 1:1 mixtures of nonlabeled substrate and the appropriate regiospecifically dideuterated analogue. Analysis of the enzymatic products via gas chromatography coupled to mass spectrometry showed that the introduction of the (E) double bond between C-4 and C-5 occurs in two discrete steps: cleavage of the C4-H bond was found to be very sensitive to isotopic substitution (k(H)/k(D) = 8.0 +/- 0.8), while a negligible isotope effect (k(H)/k(D) = 1.02 +/- 0.07) was observed for the C5-H bond-breaking step. According to a mechanistic model that we have previously proposed, these results suggest that initial oxidation for this desaturation reaction occurs at C-4. This finding correlates nicely with the observation that 4-hydroxylated products are produced from a similar substrate by a closely related oxidative enzyme in yeast.     

Selective cleavage of the C-C bonds of aminoethyl groups, via a multistep pathway, by a pincer iridium complex

A pincer-ligated iridium complex is found to react with N-ethylamines, HN(Et)R (R = cyclohexyl, tert-butyl, ethyl), to give the corresponding iridium isocyanide complexes (PCP)Ir(CH3)(H)(CNR) (PCP = kappa3-2,6-(tBu2PCH2)2C6H3). This novel, regioselective C-C bond cleavage reaction occurs readily under mild conditions (25-45 degrees C). The reaction is shown to proceed via initial dehydrogenation of the amine to give the corresponding imine (N-ethylidenealkylamine). The ethylidene sp2 C-H bond then undergoes addition to iridium, followed by methyl migration.     

Hydrogenolysis of benzylic alcohols on rhodium catalysts

Alumina-supported catalysts from various sources and with different rhodium dispersions predominantly yield the hydrogenation product perhydro-1-indanol in the liquid-phase hydrogenation of 1-indanol, while carbonsupported catalysts mainly give the C-O bond scission - hydrogenation product perhydroindane. Addition of organic or inorganic bases to the reaction mixture suppresses C-O bond scission. To distinguish between the two possible pathways for C-O bond scission of direct hydrogenolysis or dehydration followed by hydrogenation, deuteration studies have been carried out with carbon-supported catalysts. Not only 1-indanol but also indane and indene (the two possible mechanistic intermediates in the C-O bond scission routes) were deuterated. Information about the actual pathway has been obtained by determining the degree of deuteration and the positions at which deuterium is incorporated in the resulting perhydroindane product by means of mass spectrometry and 13C NMR spectroscopy. The results prove that C-O bond scission takes place primarily through the direct hydrogenolysis pathway on the carbon-supported catalysts. Direct hydrogenolysis occurs on the carbon support because of the formation of a better leaving group (OH2+) from the benzylic hydroxy group and its subsequent substitution by spillover hydrogen.