- Effects of rhodium dispersion on catalytic behavior of Rh/active-carbon catalysts for H/D exchange reaction between and CH4 and D2
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The H/D exchange reaction between CH4 and D2 was carried out over Rh/active-carbon catalysts, which were prepared from RhCl3 and Rh(NO3)3. In the case of the catalysts prepared from RhCl3, Rh species were homogeneously dispersed on the support from external surface to the inside of pores. Metallic particles of Rh were found to be the predominant species on the catalysts prepared from Rh(NO3)3 in the low Rh-loading region of 2 wt.%. The reaction rate per unit gram of catalyst and the product distribution in methane reflected well the Rh-dispersion on the catalysts. The catalysts which contained the highly dispersed Rh species as predominant species were found to be more active for the H/D exchange reaction than the catalysts with relatively large metal particles of Rh. On the former, the ratio of CH3D/CD4 was observed to be much higher than that on the latter.
- Sakagami,Ogata,Takahashi,Matsuda
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- High-temperature Shilov-type methane conversion reaction: Mechanistic and kinetic studies
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Traditional Shilov reactions (performed in aqueous solution with a PtCl2 catalyst) for methane conversion suffer from catalyst deactivation at high temperatures (> 100 °C), therefore only very low conversion rates have been achieved. In this paper, we show that Shilov-type C-H activations are achievable at much higher temperatures (～200 °C) by addition of concentrated aqueous solutions of Cl- to inhibit Pt catalyst precipitation. Various chloride-based ionic liquids also stabilized the Pt catalyst at mild reaction temperatures (～140 °C). Under high-pressure conditions (> 25.5 MPa), achieved using a specially designed sealed gold-tube reactor, very high methane conversion rates (> 90%) were obtained; this is attributed to the improved methane solubility in aqueous solution. Deuterium isotope (H/D) exchange between methane and water was used to examine the reaction reactivity and selectivity. Multiply D-substituted products were observed, indicating that multiple C-H activations occurred. A comprehensive network reaction that included all the chain reactions was set up to clarify the reactivities and product selectivities of the methane activation reactions. The reaction network consisted of a series of parallel first-order reactions, which can be described by the Arrhenius equation. The kinetic parameters such as the frequency factor, activation energies, and stoichiometric coefficients were obtained by fitting the experimental data. Because all four C-H bonds in a methane molecule are equivalent, multiple substitutions during methane conversion cannot be avoided. Our studies indicate that mono-substituted and di-substituted methane isotopologue generations have similar activation energies, suggesting that the highest mono-substitution selectivity cannot be greater than 50%.
- Kang, Shujuan,Ma, Qisheng,Chen, Weiqun,Chen, Guanyi,Tang, Yongchun
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- Methane hydrogenation and confirmation of CHx intermediate species on NaY encapsulated cobalt clusters and Co/SiO2 catalysts: EXAFS, FTIR, UV characterization and catalytic performances
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The adsorption of Co2(CO)8 onto dehydrated NaY powder under an N2 atmosphere predominantly yielded supported Co4(CO)12. The molecular cobalt carbonyl clusters and their decarbonylated products have been structurally characterized by in situ IR, extended X-ray absorption fine structure (EXAFS) and diffuse reflectance spectroscopies. The IR spectrum assigned to the species Co4(CO)12/NaY is shifted significantly from that observed for externally supported analogues on NaY and for this cluster in solution, which indicates that the cobalt carbonyl clusters occurring on the NaY are similar to those occurring in weakly basic solution. EXAFS coordination numbers (N) show that the successively decarbonylated samples maintain small cluster sizes, which depend on the temperature-programmed oxidation of the precursor. Methane hydrogenation was carried out on intrazeolitic cobalt clusters using a two-step process. It showed a relatively higher activity and selectivity to C2+ hydrocarbons in comparison with Co/SiO2 catalysts. On the basis of IR (νC-H 2960, 2880 cm-1 and δC-H 1520, 1393 cm-1) spectroscopy, mass spectrometry and reaction studies, one can conclude that CHx (x = 0, 1 or 2) surface carbonaceous species were generated by CH4 dissociation on the activated cobalt catalyst; the CH2 species was quite reactive, and propagated higher hydrocarbons.
- Shen, Guo-Cheng,Ichikawa, Masaru
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- Effect of Pt Particle Size on H/D Exchange of Methane over Alumina- and Zeolite-supported Catalysts
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Deuterium exchange in methane was studied over a series of Pt/Al2O3 and Pt/mazzite catalysts.The reaction proceeds by a simple stepwise mechanism with CH3D being the primary product.Large and small metal particles located in zeolite and alumina catalysts have different catalytic activity with respect to deuterium exchange in methane.The rate of reaction per Pt atom was the same on the catalysts investigated for particles >15-20 Angstroem and considerably lower than the rate of reaction on samples with smaller Pt clusters.The activity of small clusters of Pt (diameter 15-20 Angstroem) depended on the nature of the support.
- Khodakov, Andrei,Barbouth, Nisso,Berthier, Yves,Oudar, Jacques,Schulz, Philippe
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- Platinum catalyzed c-h activation and the effect of metal-support interactions
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Catalytic C-H bond activation of methane and ethane on a series of silica supported platinum catalysts (Pt/SiO2) was studied by using hydrogen/deuterium (H/D) exchange. Kinetic experiments demonstrate that under the reaction conditions studied, the rate of C-H bond activation shows approximate first order dependence in alkane and inverse first order dependence in D2. The rate of C-H activation is affected by the presence of sodium on the silica support, where sodium-free supports have the fastest rates of C-H activation, as assessed by H/D exchange. CO adsorption and FTIR studies indicate that the Pt particles on the sodium-free support are more electron-deficient, having the most blue-shifted linear CO stretch, while sodium-containing supports are more electron-donating, having the most red-shifted linear CO stretch. It is proposed, based on the results described in this article and previous work in the literature, that more electron-donating supports cause the Pt particles to be more electron-rich and to adsorb D? (or H*) more strongly, thereby stabilizing the ground state and resting state of the catalyst, resulting in a decreased rate of C-H activation.
- Sattler, Aaron,Paccagnini, Michele,Lanci, Michael P.,Miseo, Sabato,Kliewer, Chris E.
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- Exchange Reactions of Hydrocarbons on Silica-supported Rh-Pt Bimetallic Catalysts
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A series of silica-supported rhodium-platinum catalysts containing 10E-1 mol metal g-1 silica have been shown to be well dispersed by temperature-programmed reduction.Exchange reactions with deuterium of methane (463 K), 2,2-dimethylpropane (323 K) and n-butane (311 K) were followed using most of the catalysts in the series.Two patterns of activity against composition curve were found.For methane and 2,2-dimethylpropane, i.e. molecules containing only primary C-H bonds which cannot form α,β-diadsorbed species, platinum was as effective if not more effective than rhodium.However, for the exchange of n-butane (and also propane) rhodium was some 20-30 times more active than platinum.The extent of multiple exchange increased with rhodium content.There was some evidence for the importance of electronic effects from the variations of activity and selectivity with metal composition.
- Oliver, James A.,Kemball, Charles,Brown, Ronald,Jamieson, Edward W.
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- Isotopic Excange in the Sonolysis of Aqueous Solutions Containing D2 and CH4
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Water was insonated under an argon atmosphere which contained various amounts of a D2-CH4 (2:1 vol percent) mixture.Maximum yield for the formation of CH3D, CH2D2, CHD3 and CD4 was observed at 40 vol percent argon.Ethane, ethylene, acetylene, and many higher hydrocarbons were also produced with about one third of the deuterated methane yield.In addition, some H/D exchange took place between D2 and H2O, and H2 was formed from water.Carbon monoxide and carbon were produced, too.A mechanism involving free radicals and atoms is discussed to explain these observations.
- Hart, Edwin J.,Fischer, Ch.-Herbert,Henglein, Arnim
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- Reactions of Hydrocarbons over Ru/SiO2: Exchange with Deuterium and the Onset of Hydrogenolysis
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Exchange reactions with deuterium of a number of hydrocarbons, chosen to provide evidence about the nature of the intermediates, have been followed mass spectrometrically over a ruthenium-silica catalyst.The selected hydrocarbons were methane, propane, 2-methylpropane (2MP), 2,2-dimethylpropane (DMP), and 2,2,3,3-tetramethylbutane (TMB).Products from the reaction of propane and 2MP were analysed by deuterium NMR spectroscopy.The extent of multiple exchange increased with temperature and mechanisms involving αα- or αδ-adsorbed intermediates were favoured.The hydrogenolyses of DMP and TMB were followed at higher temperature with emphasis on the selectivity of the reaction and the change of the pattern of products with contact time.Subsidiary experiments to learn more about the course of the reactions with TMB were carried out with 2,2,3-trimethylbutane (TriMB) and 2,3-dimethylbutane (23DMB).There was evidence for a strong hydrocarbon-ruthenium interaction leading to a sequence of events with increase of temperature, relatively easy exchange but with self-poisoning, followed by moderately easy hydrogenolysis, again subject to self-poisoning.The sequence suggested the formation in turn of reversibly adsorbed hydrocarbon intermediates, more strongly adsorbed species leading to hydrogenolysis and finally carbonaceous residues.
- Brown, Ronald,Kemball, Charles
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- Catalytic activity of systems based on supported potassium salts of transition metal carbonyl hydrides in hydrogen-deuterium exchange of hydrocarbons
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The deposition of K2[Ru4(CO)13], K 2[Os3(CO)11], K2[Fe 2(CO)8], and K[Re(CO)5] onto graphite-like carbon Sibunit followed by the thermal decomposition of the supported carbonylmetallate in a flow of dihydrogen or argon affords systems capable of activating C-H bonds of methane, ethylene, and acetylene and of introducing them into hydrogen-deuterium exchange reactions. In the case of ethylene and acetylene, the isotope exchange proceeds at room temperature, while in the case of methane reaction temperatures not lower than 150 C are needed.
- Yunusov,Rummel,Herrmann,Kalyuzhnaya,Shur
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- Dynamics of the formation of CD4 from the direct reaction of incident D atoms with CD3/Cu(111)
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Using molecular beam techniques we find that incident D atoms can abstract CD3 from a Cu(111) surface to yield CD4 in a direct (Eley-Rideal) gas-surface reaction with a cross section of ～10-16 cm2/D atom. Dynamical evidence for a direct reaction includes the observation of an extremely sharp angular distribution that is clearly displaced from the surface normal, and the determination of a very high translational energy of the product, Ef, which is ～2 eV. For a 0.25 eV D-atom beam incident at 45° on a 95 K surface, this energy varies with the detection angle, θf, as Ef(θf) = (1.8+θf/45) eV, where θf0° in the "backscattering'' direction. For these conditions, the angular distribution approximately follows the function cos70(θf-5.5), being peaked 5.5° from the normal with a full width at half maximum of 60(θf-1.5). The reaction with 0.25 eV H incident at 45° gives a similar distribution peaked at ～3.5° from the normal. The shifts in the angular distributions are approximately consistent with parallel momentum conservation. The CD3/Cu(111) surface was prepared by thermal dissociation of CD3I on the surface or by adsorbing CD3 directly from a CD3 beam produced by the pyrolvsis of azomethane.
- Rettner,Auerbach,Lee
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- Heterolytic Oxidative Addition of sp2and sp3C-H Bonds by Metal-Ligand Cooperation with an Electron-Deficient Cyclopentadienone Iridium Complex
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Oxidative addition reactions of C-H bonds that generate metal-carbon-bond-containing reactive intermediates have played essential roles in the field of organometallic chemistry. Herein, we prepared a cyclopentadienone iridium(I) complex 1 designed for oxidative C-H bond additions. The complex cleaves the various sp2 and sp3 C-H bonds including those in hexane and methane as inferred from their H/D exchange reactions. The hydroxycyclopentadienyl(nitromethyl)iridium(III) complex 2 was formed when the complex was treated with nitromethane, which highlights this elementary metal-ligand cooperative C-H bond oxidative addition reaction. Mechanistic investigations suggested the C-H bond cleavage is mediated by polar functional groups in substrates or another iridium complex. We found that ligands that are more electron-deficient lead to more favorable reactions, in sharp contrast to classical metal-centered oxidative additions. This trend is in good agreement with the proposed mechanism, in which C-H bond cleavage is accompanied by two-electron transfer from the metal center to the cyclopentadienone ligand. The complex was further applied to catalytic transfer-dehydrogenation of tetrahydrofuran (THF).
- Higashi, Takuya,Kusumoto, Shuhei,Nozaki, Kyoko
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supporting information
p. 12999 - 13004
(2021/08/16)
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- Hydrogenation of CO2 to Methanol by Pt Nanoparticles Encapsulated in UiO-67: Deciphering the Role of the Metal-Organic Framework
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Metal-organic frameworks (MOFs) show great prospect as catalysts and catalyst support materials. Yet, studies that address their dynamic, kinetic, and mechanistic role in target reactions are scarce. In this study, an exceptionally stable MOF catalyst consisting of Pt nanoparticles (NPs) embedded in a Zr-based UiO-67 MOF was subject to steady-state and transient kinetic studies involving H/D and 13C/12C exchange, coupled with operando infrared spectroscopy and density functional theory (DFT) modeling, targeting methanol formation from CO2/H2 feeds at 170 °C and 1-8 bar pressure. The study revealed that methanol is formed at the interface between the Pt NPs and defect Zr nodes via formate species attached to the Zr nodes. Methanol formation is mechanistically separated from the formation of coproducts CO and methane, except for hydrogen activation on the Pt NPs. Careful analysis of transient data revealed that the number of intermediates was higher than the number of open Zr sites in the MOF lattice around each Pt NP. Hence, additional Zr sites must be available for formate formation. DFT modeling revealed that Pt NP growth is sufficiently energetically favored to enable displacement of linkers and creation of open Zr sites during pretreatment. However, linker displacement during formate formation is energetically disfavored, in line with the excellent catalyst stability observed experimentally. Overall, the study provides firm evidence that methanol is formed at the interface of Pt NPs and linker-deficient Zr6O8 nodes resting on the Pt NP surface.
- Gutter?d, Emil S.,Lazzarini, Andrea,Fjermestad, Torstein,Kaur, Gurpreet,Manzoli, Maela,Bordiga, Silvia,Svelle, Stian,Lillerud, Karl P.,Skúlason, Egill,?ien-?Degaard, Sigurd,Nova, Ainara,Olsbye, Unni
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supporting information
p. 999 - 1009
(2020/02/20)
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- Dimension-matched plasmonic Au/TiO2/BiVO4 nanocomposites as efficient wide-visible-light photocatalysts to convert CO2 and mechanistic insights
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Dimension (D)-matched plasmonic Au/TiO2/BiVO4 nanocomposites have been successfully constructed by first preparing BiVO4 nanoflakes via an ion exchange process from BiOCl nanosheets, followed by coupling (001) facet-exposed anatase TiO2 nanosheets and then modifying plasmonic Au nanorods. The well-designed 1D/2D/2D nanocomposites exhibit exceptional visible-light photocatalytic activities for CO2 conversion, and these nanocomposites indicate ～30-fold and ～60-fold enhancements compared to the resulting BiVO4 nanoflakes and the previously-reported BiVO4 nanoparticles, respectively. Based on steady-state surface photovoltage spectroscopy, transient-state surface photovoltage responses, wavelength-dependent photocurrent action spectra and fluorescence spectra related to the amount of produced OH species, it is confirmed that the exceptional photocatalytic activity can be comprehensively ascribed to the following phenomena which result from the fabrication of 2D nanoflakes: increased specific surface area of BiVO4, coupling of TiO2 nanosheets to form a new proper-energy platform, which can accept photogenerated electrons from BiVO4 and plasmonic Au so as to greatly enhance the charge separation, and modified plasmonic Au with the extended wide-visible-light absorption to 660 nm, which can act as the co-catalyst for the transferred electrons to induce reduction reactions. Moreover, dimension matching in the fabricated nanocomposite is very favorable for photogenerated charge transfer and separation. Furthermore, isotope experiments of 13CO2 and D2O along with electrochemical measurements suggest that the produced H atoms are dominant active radicals, which can initiate the conversion of CO2.
- Bian, Ji,Qu, Yang,Zhang, Xuliang,Sun, Ning,Tang, Dongyan,Jing, Liqiang
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supporting information
p. 11838 - 11845
(2018/07/05)
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- Elemental Boron for Efficient Carbon Dioxide Reduction under Light Irradiation
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The photoreduction of CO2 is attractive for the production of renewable fuels and the mitigation of global warming. Herein, we report an efficient method for CO2 reduction over elemental boron catalysts in the presence of only water and light irradiation through a photothermocatalytic process. Owing to its high solar-light absorption and effective photothermal conversion, the illuminated boron catalyst experiences remarkable self-heating. This process favors CO2 activation and also induces localized boron hydrolysis to in situ produce H2 as an active proton source and electron donor for CO2 reduction as well as boron oxides as promoters of CO2 adsorption. These synergistic effects, in combination with the unique catalytic properties of boron, are proposed to account for the efficiency of the CO2 reduction. This study highlights the promise of photothermocatalytic strategies for CO2 conversion and also opens new avenues towards the development of related solar-energy utilization schemes.
- Liu, Guigao,Meng, Xianguang,Zhang, Huabin,Zhao, Guixia,Pang, Hong,Wang, Tao,Li, Peng,Kako, Tetsuya,Ye, Jinhua
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supporting information
p. 5570 - 5574
(2017/05/05)
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- Highly efficient and stable photocatalytic reduction of CO2 to CH4 over Ru loaded NaTaO3
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An efficient and stable photocatalytic activity was obtained over NaTaO3 by introducing an electron donor (H2) into the CO2 reduction process with water. Ru/NaTaO3 demonstrated the best activity (CH4 51.8 μmol h-1 g-1) and product selectivity in converting CO2 to CH4.
- Li, Mu,Li, Peng,Chang, Kun,Wang, Tao,Liu, Lequan,Kang, Qing,Ouyang, Shuxin,Ye, Jinhua
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supporting information
p. 7645 - 7648
(2015/06/16)
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- Methylene migration and coupling on a non-reducible metal oxide: The reaction of dichloromethane on stoichiometric α-Cr2O3(0001)
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The reaction of CH2Cl2 over the nearly-stoichiometric α-Cr2O3(0001) surface produces gas phase ethylene, methane and surface chlorine adatoms. The reaction is initiated by the decomposition of CH2Cl2 into surface methylene and chlorine. Photoemission indicates that surface cations are the preferred binding sites for both methylene and chlorine adatoms. Two reaction channels are observed for methylene coupling to ethylene in temperature-programmed desorption (TPD). A desorption-limited, low-temperature route is attributed to two methylenes bound at a single site. The majority of ethylene is produced by a reaction-limited process involving surface migration (diffusion) of methylene as the rate-limiting step. DFT calculations indicate the surface diffusion mechanism is mediated by surface oxygen anions. The source of hydrogen for methane formation is adsorbed background water. Chlorine adatoms produced by the dissociation of CH2Cl2 deactivate the surface by simple site-blocking of surface Cr3 + sites. A comparison of experiment and theory shows that DFT provides a better description of the surface chemistry of the carbene intermediate than DFT+U using reported parameters for a best representation of the bulk electronic properties of α-Cr2O3.
- Dong, Yujung,Brooks, John D.,Chen, Tsung-Liang,Mullins, David R.,Cox, David F.
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- CO2 methanation by Ru-doped ceria: The role of the oxidation state of the surface
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Ru0.05Ce0.95Ox is an active catalyst for methanation of CO2 with H2. Under reaction conditions one expects that oxygen vacancies are present on the oxide catalyst surface and that their steady-state concentration depends upon the relative ratio of the oxidant (CO2) to the reductant (H2). We show that the activity of the catalyst is sensitive to the degree of surface reduction: a surface that is too reduced or too oxidized loses activity. Exposing the oxidized surface to CO2 and then to H2 produces no methane, while on a reduced surface methane is produced by exposure to CO2 followed by H2. If the reaction is carried out at the steady state, purged, and then exposed to only hydrogen, methane is produced. Methane is formed through the reaction of hydrogen with surface species, whose infrared spectrum is associated with a variety of surface carbonates, and not through CO or a formate intermediate.
- Upham, D. Chester,Derk, Alan R.,Sharma, Sudanshu,Metiu, Horia,McFarland, Eric W.
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p. 1783 - 1791
(2015/04/27)
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- Differential Reduction of CO2 by Molybdenum and Vanadium Nitrogenases
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The molybdenum and vanadium nitrogenases are two homologous enzymes with distinct structural and catalytic features. Previously, it was demonstrated that the Vnitrogenase was nearly 700 times more active than its Mo counterpart in reducing CO to hydrocarbons. Herein, a similar discrepancy between the two nitrogenases in the reduction of CO2 is reported, with the Vnitrogenase being capable of reducing CO2 to CO, CD4, C2D4, and C2D6, and its Mocounterpart only capable of reducing CO2 to CO. Furthermore, it is shown that the Vnitrogenase may direct the formation of CD4 in part via CO2-derived CO, but that it does not catalyze the formation of C2D4 and C2D6 along this route. The exciting observation of a Vnitrogenase-catalyzed C-C coupling with CO2 as the origin of the building blocks adds another interesting reaction to the catalytic repertoire of this unique enzyme system. The differential activities of the V and Monitrogenases in CO2 reduction provide an important framework for systematic investigations of this reaction in the future.
- Rebelein, Johannes G.,Hu, Yilin,Ribbe, Markus W.
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supporting information
p. 11543 - 11546
(2016/02/19)
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- The decomposition of methanol on Au-Pt bimetallic clusters supported by a thin film of Al2O3/NiAl(100)
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With various techniques to probe a surface, we studied the decomposition of methanol on Au-Pt bimetallic clusters, of diameter ≤6.0 nm, formed by sequential deposition of Au and Pt evaporated onto thin-film Al 2O3/NiAl(100). The surface of the bimetallic clusters comprised both Au and Pt, but the decomposition, through dehydrogenation to CO and scission of the C-O bond, proceeded primarily on the surface Pt. Alloying of Pt with Au altered little the dehydrogenation on the Pt sites. The CO and hydrogen produced from dehydrogenated methanol increased with the extent of Pt sites; the production per surface Pt was comparable to that of Pt clusters. The temperature of the onset of dehydrogenation resembled that of Pt clusters. Little methanol decomposed to CO on the Au sites. Varying the surface structure and composition of the bimetallic clusters affected these properties insignificantly. In contrast to the dehydrogenation, scission of the C-O bond in methanol did not depend exclusively on the concentration of Pt atoms at the surface, given that production of methane from this second channel did not increase with the extent of Pt surface sites. The modified electronic structure of the alloyed Pt controlled the probability of the C-O bond scission. The bimetallic clusters restructured during the reaction such that the Au atoms in the clusters aggregated and decorated the Pt surface, leading to fewer surface Pt and increased mean coordination of surface Au.
- Li,Liao,Wang,Chao,Hung,Ho,Luo,Lai, Yu-Ling,Hsu, Yao-Jane
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p. 31602 - 31613
(2014/08/18)
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- On-stream regeneration of a sulfur-poisoned ruthenium-carbon catalyst under hydrothermal gasification conditions
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Catalytic processes that employ Ru catalysts in supercritical water are capable of converting organics, such as wood waste or biosolids, into synthetic natural gas (CH4) with high efficiencies at relatively moderate temperatures of around 400 °C. However, Ru catalysts are prone to S poisoning and are quickly deactivated. As S is ubiquitous in raw biomass and technologies to remove S from hydrothermal biomass feeds are lacking, regeneration protocols that efficiently reactivate S-poisoned catalysts are required to realize efficient conversion processes and long catalyst lifetimes. In this work, we developed a method to remove S from a S-poisoned Ru catalyst under hydrothermal conditions through an oxidative treatment in the aqueous phase. By using in situ X-ray absorption spectroscopy under the reaction conditions, we show that Ru is oxidized by dilute H2O2 at low temperatures, which leads to the removal of adsorbed S species from the catalyst surface. By optimizing the regeneration conditions, it was possible to prevent oxidation of the catalyst carbon support, as revealed by ex situ TEM. This treatment led to a reactivation of the Ru catalyst with a significant increase in carbon-to-gas conversion and methane selectivity. Don't preach, bleach! Dilute hydrogen peroxide effects the reactivation of a sulfur-poisoned ruthenium catalyst under hydrothermal conditions. This mild oxidative treatment efficiently removes adsorbed sulfur from the ruthenium surface and restores the catalytic activity without corroding the catalyst support. Copyright
- Dreher, Marian,Steib, Matthias,Nachtegaal, Maarten,Wambach, Joerg,Vogel, Frederic
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p. 626 - 633
(2014/03/21)
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- Reactions of d0 tungsten alkylidyne complexes with O2 or H2O. Formation of an oxo siloxy complex through unusual silyl migrations
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(Me3SiCH2)3(Me3SiC) W←OPMe3 (1), an adduct between (Me3SiCH 2)3WCSiMe3 (2) and OPMe3, reacts with O2 to give OW(OSiMe3)(CH
- Morton, Laurel A.,Miao, Maozhong,Callaway, Tabitha M.,Chen, Tianniu,Chen, Shu-Jian,Tuinman, Albert A.,Yu, Xianghua,Lu, Zheng,Xue, Zi-Ling
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supporting information
p. 9555 - 9557
(2013/10/08)
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- Cleavage of carbon monoxide promoted by a dinuclear tantalum tetrahydride complex
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The reaction of one equivalent of carbon monoxide with the dinuclear tetrahydride complex ([NPN]Ta)2(μ-H)4 (where NPN = PhP(CH2SiMe2NPh)2) results in the cleavage of the CO triple bond and formation of ([NPN]Ta)(μ-O)(μ-H)(Ta[NPN′]) (where NPN′ = PhP(CH2SiMe2NPh)(CH 2SiMe2NC6H4)) via extrusion of CH4. The identity of the ditantalum complex was confirmed by single-crystal X-ray analysis, isotopic labeling studies, and GC-MS analysis of the methane released. DFT calculations were performed to provide information on the initial adduct formed and likely transition states for the process.
- Ballmann, Joachim,Pick, Fraser,Castro, Ludovic,Fryzuk, Michael D.,Maron, Laurent
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p. 8516 - 8524
(2013/02/23)
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- FTIR matrix-isolation study of the reaction products of vanadium atoms with propene: Observation of allylvanadium hydride as a precursor to sacrificial hydrogenation of propene
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Vanadium atoms have been reacted with different partial pressures of propene in Ar under matrix-isolation conditions, and the products have been observed using Fourier transform infrared (FTIR) spectroscopy. Under dilute propene in Ar conditions, new features are observed in the IR spectra corresponding to a C-H insertion product, identified here as H-V-(η3-allyl). Use of d3-propene (CD 3-CH=CH2) demonstrates that the initial V-atom insertion occurs at the methyl group of the propene molecule, and DFT calculations have been used to support the identity of the initial product. Upon increasing the partial pressure of propene, additional features corresponding to propane (C3H8) are observed, with the hydrogen-atom source for the observed hydrogenation demonstrated to be additional propene units. Analysis of a systematic increase in the partial pressure of propene in the system demonstrates that the yield of propane correlates with the decrease of the allyl product, demonstrating the H-V(allyl) species as a reactive intermediate in the overall hydrogenation process. An overall mechanism is proposed to rationalize the formation of the insertion product and ultimately the products of hydrogenation, which agrees with previous gas-phase and matrix-isolation work involving propene and the related system, ethene.
- Thompson, Matthew G. K.,Walker, Stephen W. C.,Parnis, J. Mark
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experimental part
p. 7317 - 7323
(2011/10/03)
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- Decarbonylation of ethanol to methane, carbon monoxide and hydrogen by a [PNP]Ir complex
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The putative three-coordinate Ir(i) PNPPri(PNPPri = [N{2-P(CHMe2)2-4-MeC6H3} 2]-) pincer complex decarbonylates ethanol to yield methane, hydrogen and [PNPPri]Ir(CO). The mechanism involves the isolable trans-[PNPPri]Ir(H)(Me)(CO), which is susceptible to photochemical reductive elimination of methane.
- Melnick, Jonathan G.,Radosevich, Alexander T.,Villagran, Dino,Nocera, Daniel G.
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supporting information; experimental part
p. 79 - 81
(2010/04/01)
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- Hydrogen-deuterium exchange of methane on nickel and potassium promoted nickel prepared by the reduction of nickel oxide
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The hydrogen-deuterium exchange reactions of methane in a deuterium stream were studied by a pulse experiment over a reduced nickel (Ni1373 prepared from nickel oxide calcined at 1373 K, and Ni773 prepared from nickel oxide calcined at 773 K) and K2O promoted reduced nickel (K-Ni1373 and K-Ni773). Ni1373 had a higher exchange activity than Ni773. The effects of the addition of K2O resulted in the decrease in the exchange activity due to the inhibition of step defect sites and/or the promotion of the change of nickel crystal structure. High activity of Ni1373 could be due to that the Ni1373 had the surface of a higher Ni(1 0 0)/Ni(1 1 1) ratio.
- Osawa, Tsutomu,Futakuchi, Takashi,Imahori, Tomoharu,Lee, I-Yin Sandy
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experimental part
p. 68 - 71
(2010/06/20)
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- Kinetic and theoretical study of the hydrodechlorination of CH 4- xClx (x = 1-4) compounds on palladium
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The reaction kinetics of hydrodechlorination (HDCl) for a series of CH 4-xClx (x = 1-4) compounds were measured on a Pd/carbon catalyst. The rate of HDCl correlated with the C-Cl bond energy, suggesting scission of this bond in the molecularly adsorbed molecule is rate-determining. The measured reaction kinetics of the CH4-xClx compounds support a previously proposed Langmuir-Hinshelwood type reaction mechanism. Kinetic and isotope exchange experiments demonstrated the following: gas phase H2 and HCl are in equilibrium with surface H and Cl; adsorbed Cl is the most abundant surface intermediate; and irreversible scission of the first C-Cl bond is rate-determining. The overall hydrodechlorination reaction rate can be written as kKR-Cl[R-Cl]/(1 + KHCl[HCl]/KH2 1/2[H2]1/2). The activation energy of the rate-determining step was related linearly to the dissociation energy of the first C-Cl bond broken in a Broensted-Evans-Polanyi relationship. This behavior is in agreement with a previous study of CF3CF 3-xClx compounds. During the reaction of CH3Cl, CH2Cl2, and CHCl3 with deuterium, H-D exchange occurred in only 2%, 6%, and 9% of products, respectively. The increasing H-D exchange with Cl content suggests the steps which determine selectivity in these multipath, parallel reactions. The density functional theory (DFT)-calculated activation energies for the dissociation of the first C-Cl bond in the family of chlorinated methane compounds are in good agreement with the values extracted from kinetic modeling, suggesting that parameters estimated from DFT calculations may be used to estimate the reactivity of a particular chlorinated compound within a family of chlorocarbons.
- Chen, Nan,Rioux, Robert M.,Barbosa, Luis A. M. M.,Ribeiro, Fabio H.
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experimental part
p. 16615 - 16624
(2011/10/30)
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- Isothermal pyrolysis of iodomethanes in gases
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The fact was established that the pyrolysis of gaseous iodomethanes RI yields methane and non traces of recombination products R2. A pyrolysis mechanism was proposed and rate constants of limiting stages of the pyrolysis of iodomethane, trideuteroiodomethane, and diiodomethane over the range of 500-1500 K were determined. Pleiades Publishing, Ltd., 2009.
- Skorobogatov,Khripun,Rebrova
-
scheme or table
p. 2641 - 2651
(2010/06/14)
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- Photoinduced ethane formation from reaction of ethene with matrix-isolated Ti, V, or Nb atoms
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The reactions of matrix-isolated Ti, V, or Nb atoms with ethene (C 2H4) have been studied by FTIR absorption spectroscopy. Under conditions where the ethene dimer forms, metal atoms react with the ethene dimer to yield matrix-isolated ethane (C2H6) and methane. Under lower ethene concentration conditions (～1:70 ethene/ Ar), hydridic intermediates of the types HMC2H3 and H2MC 2H2 are also observed, and the relative yield of hydrocarbons is diminished. Reactions of these metals with perdeuterioethene, and equimolar mixtures of C2H4 and C2D 2, yield products that are consistent with the production of ethane via a metal atom reaction involving at least two C2H4 molecules. The absence of any other observed products suggests the mechanism also involves production of small, highly symmetric species such as molecular hydrogen and metal carbides. Evidence is presented suggesting that ethane production from the ethene dimer is a general photochemical process for the reaction of excited-state transition-metal atoms with ethene at high concentrations of ethene.
- Thompson, Matthew G.K.,Parnis, J. Mark
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p. 9465 - 9470
(2008/10/09)
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- H/D isotope exchange between methane and magic acid (HSO 3F-SbF5): An in situ NMR study
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The kinetics of hydron exchange between methane and a series of DSO 3F-SbF5 superacids were measured by in situ 2H decoupled 1H NMR spectroscopy. The rates of exchange showed a strong dependence on antimony pentafluoride concentration, with the free energy of activation ΔG# (30°C) decreasing from 97 to 84 kJ mol -1 over the range of concentration 19 to 49 mol % SbF5. The constant free enthalpy of activation ΔH# (ca. 65 kJ mol-1) and the decreasing entropy of activation ΔS# seem to indicate that an increase in acidity of the superacid system does not substantially change the nature of the transition state but rather acts on its solvation.
- Walspurger, Stephane,Goeppert, Alain,Haouas, Mohamed,Sommer, Jean
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p. 266 - 269
(2007/10/03)
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- Structure and reactivity studies of CoHNO+ in the gas phase
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CoHNO+ was generated in the gas phase by collision-induced dissociation (CID) or sustained off-resonance irradiation (SORI)-CID of CoH(CO)3NO+, formed from chemical ionization of Co-(CO)3NO with CH5+. Among the three isomeric structures, HCo(NO)+ (1), Co(HNO)+ (2), and Co(NOH)+ (3), density functional calculations found that 2 was the global minimum on the potential energy surface with 1 and 3, 14.1 and 15.4 kcal/mole higher in energy. Co(HNO)+ reacted with methane by dehydrogenation to yield CoCH3NO+, while CID of CoCH3NO+ yielded competitive formation of CoCH3+ and CoNO+, SORI-CID of CoCH3NO+ yielded CoCH3+ exclusively. These results suggested CoCH3NO+ had structure CH3CoNO+ rather than Co(CH3NO)+. A mechanism was proposed for the reaction with methane, where a four-centered transition state is involved for C-H bond activation as opposed to direct C-H insertion.
- Chen, Huiping,Jacobson, Denley B.,Freiser, Ben S.
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p. 10884 - 10892
(2007/10/03)
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- Photoproduct Characterization and Dynamics in the 248 nm Photlysis of CH3I Thin Films on Ag(111)
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The 248 nm photochemistry of methyl iodide thin films was studied using reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), and time-of-flight quadrupole mass spectrometry (TOF-QMS).The formation of predominantly CH2I2 and CH4 and some C2H6, CH3CH2I, CHI3 and I2 photoproducts retained in the film was characterized by RAIRS and TPD.The integrated areas of the IR absorption bands for the two major photoproducts, CH2I2 and CH4, increase to a maximum and then decrease as photolysis of the film proceeds.A cross section for the loss of CH3I by 248 nm photolysis of the film was measured to be (1.0+/-0.1)E-19 cm2, approximately 1 order of magnitude lower than the gas-phase cross section.At all laser fluences used in this study, CH3, I, and CH3I were ejected into the gas phase.The CH3 TOF distribution showed the signature of the gas-phase CH3I photodissociation dynamics-two sharp peaks corresponding to the production of iodine atoms in the I(2P3/2) and I*(2P1/2) states.The TOF distributions of I and CH3I were fit by Maxwell-Boltzmann distributions corresponding to temperatures of 1400 and 1170 K, respectively.Three other species-CH4, I2 and CH2I2-were observed in TOF-QMS, but only at higher laser fluences.It was determined that the I2 and CH2I2 species are most likely fragments of a larger molecule, perhaps a cluster species, that photodesorbs as the film becomes enriched with photoproducts.The mechanism for CH4 photoejection appears to be of a different nature.The photochemistry of methyl iodide thin films can be understood in terms of a combination of photoproceses occuring in the film and at the film surface.
- Coon, S. R.,Myli, K. B.,Grassian, V. H.
-
p. 16416 - 16424
(2007/10/02)
-
- Influence of Strong Metal-Support Interaction on Exchange with Deuterium and other Reactions of Hydrocarbons. Part 1. - Studies with Rh/TiO2 and Rh/SiO2
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The changes in the catalytic properties of Rh/TiO2 caused by raising the reduction temperature from 473 to 773 K have been investigated for four reactions, the exchange with deuterium of methane and cyclopentane, and the hydrogenolysis of 2,2-dimethylpropane and methylcyclopentane.The so-called strong metal-support interaction (SMSI) brought about by the high-temperature reduction had least effect on the exchange of methane but reduced the rate of hydrogenolysis of 2,2-dimethylpropane by factors of 1E4 or 1E5.The SMSI was reversible and its influence on each of the reactions was eliminated by oxidation of the catalyst followed by low-temperature reduction.No marked changes in the catalytic behaviour of Rh/SiO2 resulted from increasing the reduction temperature.The results provide further evidence for the existence of a number of kinds of catalytic sites on Rh/TiO2.The sites for the more structure-sensitive reactions tend to be more seriously affected by SMSI and probably involve more metal atoms than the sites responsible for methane exchange.
- Costa Faro, Arnaldo da,Kemball, Charles
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p. 741 - 748
(2007/10/02)
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- Coenzyme F430 from Methanogenic Bacteria: Mechanistic Studies on the Reductive Cleavage of Sulfonium Ions Catalyzed by F430 Pentamethyl Ester
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Mechanistic questions regarding the reductive cleavage of sulfonium ions by the NiI form of coenzyme F430 pentamethylester (F430M) were adressed in a series of kinetic studies and isotope labeling experiments.In neat DMF, methane formation from Dialkyl(methyl)sulfonium ions consistently showed a delay time of ca. 1 h.In the presence of excess propanethiol, no delay was observed and methane formation followed pseudo-first-order kinetics with a logarithmic dependence of the initial rate on the concentration of propanethiol.From the temperature dependence of the reaction rate, an estimate for the activation parameters ΔH(excit.) = 49 kJ mol-1 and (apparent) ΔS(excit.) = -114 J K-1 mol-1 was derived.The observation of deuterium incorporation into methane from (CH3)2CHOD, but not from (CH3)2CDOH, indicates that the fourth H-entity is introduced into CH4 as a proton, and that free CH3 radicals are not involved.In contrast to the reaction with the homogeneous one-electron reductant sodium naphthalide, the F430M-catalyzed reduction of mixed dialkyl(methyl)sulfonium ions showed a pronounced selectivity for the cleavage of Me-S over that of alkyl-S (alkyl Me) bonds.Mechanisms that are consistent with these results, as well as possible explanations for the time delay and the apparent highly negative entropy of activation, are discussed.
- Lin, Shu-Kun,Jaun, Bernhard
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p. 1478 - 1490
(2007/10/02)
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- Heterogenous rediction of (diolefin)dialkylpatinum(ii) complexes on platinum black in ethyl alcohol: Kinetics isotopic interchange of hydrogen between coadsorbed surface alkyls, and comparison of surface alkyls generated from the platinum complexes and from olefins
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This paper reports an investigation of the heterogeneous hydrogenations of (diolefin)dialkylplatinum(II) complexes ((DO)PtR2) catalyzed by platinum black in ethyl alcohol The organic of (DO)PtR2 complexes are converted to via intermediate surface alkyls, and the platinum(II) is ipcorporated into the surface of the catalyst as platinum(0). These reductions exhibit two kinetic regimes: in the first, the rate of reaction is limited by the mass transport of hydrogen to the surface of the catalyst (the mass-transport-limited regime, MTL); in the second, the rate is limited by a reaction on the surface of the catalyst (the reaction-rate-limited regime, RRL), In reductions of (DO)PtRz complexes in n-hexane, interchange of H/D occurs between the surface alkyls derived from the diolefin and those derived from the R groups; in reductions in ethyl alcohol (EtOH), this interchange is eliminated by rapid exchange between D* and (EtOH). Under RJRL conditions, the distributions of ethanes-dn, produced from the reductions of (1,5-cyclooctadiene)diethylplatinumCII) ((COD)PtEtj) and of ethylene that the R* from and from platinum have relative of (and thus of C-H bond activation) and reductive elimination as alkane. Comparison of the distributions of propanes-dn, produced from the reductions under RRL conditions of (1,5-cyclooctadiene)di-n-propylplatinum(II), of (1,5-cyclooctadiene)diisopropylplatinum(ll), and of propylene to the conclusion. Under MTL conditions, the Et* moieties derived from (COD)PtEt2 have a slower rate of C-H bond activation (relative to the rate of reductive elimination) than these derived from ethylene, of the R* to be that of the cyclooctyl* in.
- Randell Lee,Whitesides, George M.
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p. 2576 - 2585
(2007/10/02)
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- Structure and Catalytic Activity of Alumina-Supported Pt-Co Bimetallic Catalysts. 2. Chemisorption and Catalytic Reactions
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A series od Pt1-xCox/Al2O3 bimetallic catalysts have been characterized by temperature-programmed reduction (TPR), chemisorption of hydrogen and CO, deuterium exchange using both methanol and methane, and activity for the CO/H2 reaction.A Pt-assisted reduction mechanism over the entire range of composition was established by the TPR studies as well as by the chemisorption results.An enhanced metallic dispersion for the Pt-rich catalyst and formation of bimetallic particles on the Co-rich side was also indicated.In the CO hydrogenation over the Pt-rich catalysts the predominant products are methanol and dimethyl ether whereas on the Co-rich samples hydrocarbons and higher alcohols are produced.The mechanisms of deuterium exchange with methane and methanol are significantly different, the former being catalyzed solely by metallic sites while the latter utilizes both oxide and metallic sites for stepwise and multiple exchange, respectively.On the basis of the XPS data (preceding article) as well as the chemisorption results reported here, a surface model is introduced for interpretation of the catalytic results.
- Guczi, Lazlo,Hoffer, Tamas,Zsoldos, Zoltan,Zyade, Souad,Maire, Gilbert,Garin, Francois
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p. 802 - 808
(2007/10/02)
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- Construction of heterobimetallics bridged by (oxyalkyl)phosphines: Syntheses of trans-Me2Ta(μ-CH2)(μ-OCMe2CH 2Ph2P)2PtMe and (TMEDA)Ta(μ-CH2)(μ-Me)(μ-OCMe2CH 2Ph2P)2Ni
-
Precursors to potentially bridging (oxyalkyl)phosphine ligands, μ-OCR2(CH2)nPh2P (R = tBu, n = 1; R = Me, n = 1, 2), were synthesized through the addition of LiCH2PPh2·TMEDA and LiPPh2 to tBu2C=O and Me2CCH2O. Treatment of ZrCl4 with 2.0 equiv of LiOCtBu2CH2Ph2P·xTHF provided (Ph2 PCH2CtBu2O)2ZrCl2 (1), which could be alkylated with MeLi to generate pseudo-Oh trans-(Ph2PCH2CtBu2O) 2ZrMe2 (2) and with tBuCH2Li to give pseudo-Td (Ph2PCH2CtBu2O) 2Zr(CH2tBu)2 (3). The fluxional, five-coordinate tris(alkoxide) (Ph2PCH2CtBu2O) 2Zr(OCtBu2CH2Ph2P)Cl (4), prepared from 1 and LiOCtBu2CH2Ph2P·xTHF, was methylated to afford four-coordinate (Ph2PCH2CtBu2O)3ZrMe (5). No heterobimetallics were produced when 1-5 were exposed to several substitutionally labile late-metal complexes. The tBu groups apparently place severe conformational contraints on the potential bridging ligands. Alcoholysis of Zr(CH2Ph)4 by HOCMe2(CH2)2Ph2P provided (Ph2P(CH2)2CMe2O) 2Zr(CH2Ph)2 (6) and (Ph2P(CH2)2CMe2O)4Zr (7), which was shown to conproportionate with Zr(CH2Ph)4 to give 6. Complex mixtures were obtained when 6 was utilized in the preparation of heterobimetallics. Metathesis of Me3TaCl2 with 2 equiv of LiOCMe2CH2Ph2P afforded (PPh2CH2CMe2O)2TaMe3 (8). Treatment of (COD)PtMe2 with 8 produced trans-Me2Ta(μ-CH2)(μ-OCMe2CH 2Ph2P)2PtMe (10, 21%) via thermolysis of an intermediate oligomer, ([-(Me)3Ta(μ-OCMe2CH2Ph 2P)Pt(Me)2PPh2CH2CMe 2O-]n)1/n (9). Similarly, (TMEDA)NiMe2 and 8 yielded (TMEDA)Ta(μ-CH2)(μ-Me)(μ-OCMe2CH 2Ph2P)2Ni (11, 30%). During the formation of 10 and 11, bimetallic Me/Me exchange reactions were prevalent, as shown via labeling experiments and isotopic shifts observed in the 195Pt{1H} NMR spectra of 10-dn, a mixture of isotopomers prepared from (CD3)3TaCl2 (8-d9). Plausible mechanisms rationalizing the generation of the μ-CH2 and μ-CH3 ligands of 10 and 11 are also discussed.
- Baxter, Steven M.,Wolczanski, Peter T.
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p. 2498 - 2509
(2008/10/08)
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- Elimination processes for alkyl, hydride, and hydroxy derivatives of permethyltungstenocene
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Elimination processes for alkyl, hydride, and hydroxy derivatives of permethyltungstenocene have been examined. The alkyl-hydride derivatives Cp*2W(R)H (Cp* = η5-C5Me5; R = CH3, CH2C6
- Parkin, Gerard,Bercaw, John E.
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p. 1172 - 1179
(2008/10/08)
-
- Spectroscopic and chemical studies of nickel(II) hydrides
-
The trans influence of X ligands on the spectroscopic properties of the Ni-H bond is reported for the series of square-planar nickel hydrides trans-HNi(X)(PCy3)2 (X = Me, Ph, CN, SCN, I, Br, Cl, SPh, S(p-tol), SH, OAc, O2CH, O2CPh, O2CCF3, OPh) prepared by oxidative addition of HX to [Ni(PCy3)2]2N2 or derivatives of subsequent products. The infrared-derived parameter v(Ni-H) shows a similar ligand dependence as the proton chemical shift of the hydride ligand, with more covalently bound ligands such as methyl or phenyl producing lower v(Ni-H) and smaller upfield chemical shifts as compared to those ligands that bind to Ni(II) with more electrostatic character such as anionic O-donor ligands. Comparisons with other ligand influenced, spectroscopic scales are made. Carbon dioxide and iodomethane were used as chemical probes of reactivity at the Ni-X or Ni-H bond. The derivatives with stronger Ni-H bonds (S and O donors) show no reactivity at the hydride while the C-bond derivatives exhibit CO2 insertion at the hydride. The Ni-H functionality is active toward iodomethane in the C-donor derivatives. A mechanism of hydrogen atom abstraction by methyl radicals is consistent with literature precedents as well as the Ni-H bond strengths determined by spectroscopies.
- Darensbourg, Marcetta Y.,Ludwig, Maria,Riordan, Charles G.
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p. 1630 - 1634
(2008/10/08)
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- An Investigation of the Reaction Between CH3 Radicals and Acetaldehyde-d4 at High Temperatures
-
The reaction between CH3 radicals and acetaldehyde-d4 was investigated behind incident shock waves at temperatures between 1050 and 1190 K and total densities about 2E-6 mol cm-3. - The consumption of CH3 was followed by time-resolved UV-absorption measurement at λ = 216.5 nm.The measurements were carried out with perdeuterated acetaldehyde. - A rate constant of 1.1E11 cm3 mole-1 s-1 for the reaction CH3 + CD3CDO --> CH3D + CD3CO (1) at temperatures around 1100 K was determined. - Key words: Chemical Kinetics / Elementary Reactions / Radicals / Shock Waves / Spectroscopy, Ultraviolet
- Klatt, M.,Wagner, H. Gg.
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p. 841 - 846
(2007/10/02)
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- A Stepwise Mechanism for Gas-Phase Unimolecular Ion Decompositions. Isotope Effects in the Fragmentation of tert-Butoxide Anion
-
Infrared multiple photon (IRMP) photochemical activation of gas-phase ions trapped in an cyclotron resonance (ICR) spectrometer has been used to the mechanism of a gas-phase negative ion unimolecular decomposition.Upon irradiation with a CO2 laser (both high-power pulsed and low-power continous wave (CW)), tert-butoxide anion, trapped in a pulsed ICR spectrometer, decomposes to yield acetone enolate anion and methane.The mechanism of this formal 1,2-elimination reaction was probed by measuring hydrogen isotope effects (both primary and secondary) in the IR laser photolysis of 2-methyl-2-propoxide-1,1,1-d3 (1) and 2-methyl-2-propoxide-1,1,1,3,3,3-d6 (2) anions.Unusually large secondary isotope effects (pulsed laser, 1.9 for 1 and 1.7 for 2; cw laser, 8 for 1) and small primary isotope effects (pulsed laser, 1.6 for 1 and 2; cw laser, 2.0 for 1) were observed.These isotope effects, particularly the large difference in energy dependence of the primary and secondary effects, are consistent only with a stepwise mechanism involving initial bond cleavage to an intermediate ion-molecule complex followed by a hydrogen transfer within the intermediate complex.The observed secondary isotope effects have been modelled by using statistical reaction rate (RRKM) theory.The implications of this study for several previously reported unimolecular ion decompositions are also discussed.
- Tumas, William,Foster, Robert F.,Pellerite, Mark J.,Brauman, John I.
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p. 961 - 970
(2007/10/02)
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- Gas-Phase Studies of Alkane Oxidation by Transition-Metal Oxides. Selective Oxidation by CrO+
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The gas-phase reactions of CrO+ with alkanes have been studied by using ion beam reactive scattering techniques.CrO+ undergoes facile reactions with alkanes larger than methane.CrO+ selectively oxidizes ethane to form ethanol.In addition to the possibility of alcohol formation, reactions with larger alkanes are more complex, yielding products in which dehydrogenation and loss of alkenes and alkanes occur.In reactions with cyclic alkanes, cyclopropane and cyclobutane yield products characterictic of C-C bond cleavage.In contrast, reactions with cyclopentane and cyclohexane mainly involve dehydrogenation and elimination of H2O.A series of hydrogen abstraction reactions are examined to determine the bond dissociation energy D0(CrO+-H) = 89 +/- 5 kcal/mol-1.This bond energy has implications for the reaction mechanisms of CrO+ with alaknes, leading to the suggestion of a multicenter reaction intermediate, in which alkyl C-H bonds add across the Cr+-O bond as an initial step.This is supported by an examination of the reactions of Cr+ with alcohols.
- Kang, H.,Beauchamp, J. L.
-
p. 7502 - 7509
(2007/10/02)
-
- Mechanistic and Kinetic Study of Alkane Activation by Ti+ and V+ in the Gas Phase. Lifetimes of Reaction Intermediates
-
The reactions of Ti+ and V+ with several deuterium-labeled alkanes are studied by using an ion beam apparatus.The dominant reactions observed for both of these metal ions are single and double dehydrogenations.Alkane loss recations are also observed for Ti+ but may be due to electronically excited states.The dehydrogenation mechanisms are investigated by using partially deuterated alkenes.The results are consistent with 1,2-eliminations for both V+ and Ti+, where deuterium scrambling may occur in the latter case.It is proposed that some 1,3-elimination of hydrogen also occurs in the reaction of Ti+ with n-butane.Although the dehydrogenation reactions of V+ and Ti+ appear to be similar to those of Ru+ and Rh+, there are some important differences in the reactivity of V+.Extensive adduct formation and large deuterium isotope effects are consistent with reaction intermediates which are relatively long-lived for V+ in comparison to Ti+, Ru+, and Rh+.Collisional stabilization studies are used to estimate dissociation rates of reaction intermediates formed when Ti+ and V+ interact with n-butane.The measured upper limits to the unimolecular decomposition rates are 1.47 X 105 s-1 and 1.23 X 107 s-1 for V+ and Ti+ respectively.Model RRKM calculations are able to reproduce these rates and provide an explanation of isotope effects observed when n-butane-d10 is employed as the neutral reactant.The slower rate for V+ is suggested to arise from the inability of V+ to form two strong ? bonds due to the 3d4 electronic configuration of the ground-state ion.This renders C-H bond insertion energetically much less favorable for V+ than for the other metal ions and limits the excitation energy of reaction intermediates.
- Tolbert, M. A.,Beauchamp, J. L.
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p. 7509 - 7517
(2007/10/02)
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- Reactions of Hydrocarbons on Alumina-supported Pt-Ir Bimetallic Catalysts - Part 1. - Exchange of Methane and Cyclopentane with Deuterium, and Hydrogenolysis of Butane, Pentane and Cyclopentane
-
Exchange reactions with deuterium of cyclopentane (303 K) and of methane (473 K) and the hydrogenolysis of n-butane, n-pentane and cyclopentane have been studied on a series of alumina-supported Pt-Ir catalysts.Metal composition had little effect on the activity of the catalysts for the exchange reactions but influenced the character of the reactions in a regular manner.Rates of hydrogenolysis increased with iridium content, and the change was >100 for n-butane but only 10 for cyclopentane.The main reaction with cyclopentane was ring-opening, but the extent of further hydrogenolysis increased with iridium content in a manner that suggested the possible requirement of ensembles of three iridium atoms.
- Garden, David,Kemball, Charles,Whan, David A.
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p. 3113 - 3124
(2007/10/02)
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- NOVEL OXIRANE FORMATION VIA 1-METALLA-2-OXACYCLOBUTANES
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1-Metalla-2-oxacyclobutane complex, which was formed by intramolecular γ-hydrogen elimination of alkoxy(alkyl)metal complex of Ni or Pd, yields mainly oxirane, while its Ti complex affords olefins resulting from C-O bond cleavage.
- Miyashita, Akira,Ishida, Jun-ya,Nohira, Hiroyuki
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p. 2127 - 2130
(2007/10/02)
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- Alkylperoxy and Alkyl Radicals. 1. Infrared Spectra of CH3O2 and CH3O4CH3 and the Ultraviolet Photolysis of CH3O2 in Argon + Oxygen Matrices
-
Methyl radicals, generated by the pyrolysis of azomethane and/or methyl iodide, were allowed to interact with matrices of Ar + 10percent O2 and the products isolated.IR spectra were obtained for species containing the following isotopically labeled groups: CH3, 13CH3, CD3, 16O2, 18O2 and 16O(18)O.From these spectra, the methylperoxy radical and its dimer, dimethyltetroxide, were identified and vibrational assignments made.Irradiation of CH3O2 at ca. 254 nm resulted in its photodissociation.The nature of this process in the matrix is discussed.
- Ase, P.,Bock, W.,Snelson, A.
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p. 2099 - 2109
(2007/10/02)
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- COMPARATIVE STUDY OF cw CO2 LASER INDUCED AND SF6 SENSITIZED DECOMPOSITION OF METHYL IODIDES CH3I AND CD3I
-
The decomposition of methyl iodide-d3 initiated by the irradiation of a cw CO2 laser in the presence of SF6 proceeds under comparable conditions faster, particularly in the shorter wavelength region of the CO2 laser spectrum, than the decomposition of methyl iodide.The differences can be explained by the different distribution of energy levels of both reactants.Observable differences are also in the composition of the reaction products.All these facts indicate the participation of at least two reaction mechanisms, a radical one and a molecular one, which canprobably proceed via the collision of two vibrationally excited molecules of CX3I.
- Rejnek, Jaroslav,Engst, Pavel,Jakoubkova, Marie,Horak, Milan
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p. 215 - 222
(2007/10/02)
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- Photocatalytic Hydrogenation of Alkynes and Alkenes with Water over TiO2. Hydrogenation Accompanied by Bond Fission
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Photocatalytic reactions of alkynes and alkenes with water have been investigated over TiO2 powder.The major photoformed products are those formed by hydrogenation accompained by C=C or CC bond fission.Oxidation products such as CO and CO2 are also formed.From the change in the product yields with the remaining amount of water adsorbed as well as the pressure of water vapor it is concluded that water molecules and not surface OH- groups are responsible for the reaction and also that vacant sites for the alkyne (or alkene) adsorption are necessary for the occurrence of the reaction.The fission of the C=C or CC bond observed with the hydrogenation is attributed to the interaction of the alkynes (or alkenes) with the trapped e-(Ti3+) and h+(.OH) pairs.It is suggested that a close association of photoformed electron and hole pairs plays a significant role in the bond fission.
- Anpo, Masakazu,Aikawa, Norikazu,Kodama, Sukeya,Kubokawa, Yutaka
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p. 2569 - 2572
(2007/10/02)
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- METHYLENE SPECIES AS PIVOTAL PRECURSORS FOR HYDROCARBON FORMATION REACTIONS ON COBALT CATALYSTS.
-
The presence of CH//2 species on a carbon-deposited cobalt catalyst is demonstrated by the disproportionate formation of CH//2D//2 in the reaction with D//2. Hydrogenation of a **1**3C-deposited Co/C catalyst at 430 degree C yielded methane and a trace amount of ethylene, and it was found that the **1**3C content was substantially different in methane (31%) from that in ethylene (83%). If ethylene was brought into contact with the **1**3C-deposited Co/C catalyst at 180 degree C, (**1**3C//1)-propene was predominantly formed via a homologation reaction.
- Tanaka,Nakagawa,Yaegashi,Aomura
-
p. 239 - 246
(2008/10/08)
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- ELECTRONIC EFFECTS OF ADDITIVES ON THE SELECTIVITY IN CO/H2 REACTION OVER NICKEL CATALYSTS
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It was found with CO/H2 reaction over unsupported Ni catalysts that the C2+/CH4 selectivity decreased in the order; Ni-boride > Raney-Ni > Decomposed-Ni ca.Ni-phosphide.This suggests that both the activity and selectivity increase as the electron density on Ni metal increases.The H/C atomic ratio of the surface carbon species deposited during the reaction was also dependent on the catalyst.
- Okamoto, Yasuaki,Matsunaga, Etsuo,Imanaka, Toshinobu,Teranishi, Shiichiro
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p. 565 - 568
(2007/10/02)
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