A study of binuclear zirconium hydride catalysts of the hydrogenolysis of alkanes by the density functional theory method

Binuclear hydride centers containing two Zr(IV) atoms are suggested as promising catalysts for the hydrogenolysis of alkanes under mild conditions (T < 450 K, p ∼ 1 atm). Reactions of model compounds L₂(H)Zr(X)₂Zr(H)L₂ (X = H, L = OSi≡ (4a), X = L = OMe (4d)), L(H)Zr(O)₂Zr(H)L (L = OSi≡ (4b), Cp(4c)) and (≡SiO)₂(H)Zr-O-Zr(H)(OSi≡)₂ (4e and 4f) with the propane molecule were studied using the density functional theory method. The results show that centers of the 4a, 4e, and 4f types and especially 4b are promising catalysts of the hydrogenolysis of alkanes due to a high degree of unsaturation of two Zr atoms and their sequential participation in the splitting of the C-C bond and hydrogenation of ethylene formed as a result of splitting.     

Kinetic investigation of propane oxidation on diluted Mo1–V0.3–Te0.23–Nb0.125–Ox mixed-oxide catalysts

Reaction kinetics and proposed mechanism for the oxidation of propane over diluted Mo₁–V_0.3–Te_0.23–Nb_0.125–O _x are described. The kinetic study allowed determination of the orders of propane disappearance, propene formation, CO _x formation, and acids formation. The results show that selective oxidation of propane to propylene over this catalyst follows the Langmuir-Hinshelwood mechanism. Deep oxidation of propane to carbon dioxide is first order with respect to hydrocarbon, and partial order (0.21) with respect to oxygen. The selective oxidation of propane to acrylic acid is half order with respect to hydrocarbon and partial order (0.11) with respect to oxygen, while water does not participate directly in propane transformation. The result also shows that the overall reaction consists of three parallel process channels. One main sequence of consecutive reactions leads to the desired product.     

Three-step macrokinetic model of butane and propane steam conversion to methane-rich gas

Low-temperature steam conversion (LTSC) of a methane-butane mixture (95% methane and 5% butane) into a methane-rich gas over an industrial Ni-based catalyst has been studied with the following reaction conditions: temperature 200–320°C, pressure 1 bar, gas hour space velocity 1200–3600 h^–1, and steam to carbon ratio 0.64. A three-step macrokinetic model has been suggested based on the kinetic parameters found. The model includes the following reactions: (1) irreversible steam reforming; (2) CO₂ methanation, which occurs in a quasi-equilibrium mode at temperatures above 260°C; (3) hydrogenolysis of propane and butane, which is essential at temperatures below 260°C. Steam reforming was shown to limit the overall reaction rate, whereas hydrogenolysis and CO₂ methanation determined the product distribution in low- and high-temperature regions, respectively. Temperature dependencies of the product distribution for the LTSC of a model ternary methane-propane-butane mixture (85% methane, 10% propane, and 5% butane) have been successfully simulated using the three-step model suggested.     

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)     

Bridged cyanine dyes. Part 1. 1-(N-methyl-4-pyridinio)-3-(N-methylpyridylene)cyclopenta-1,4-dienes

1,3-Bis(N-methyl-4-pyridinio)propane is condensed with a series of α-diketones to form deep red monocyanine dyes 8 . In strongly acidic solutions C-protonation occurs at the central carbon atom. A similar condensation with α-ketoesters yields zwitterionic cyanines 12 with extinction coefficients exceeding 10⁵.     

Kinetic analysis of the hydrogenolysis of propane on ruthenium catalysts

The dependence of rates of propane hydrogenolysis on temperature and H₂ pressure have been modelled, using a rate expression based on Langmuir-Hinshelwood formalism. Dedicated to Professor Pál Tétényi on the occasion of his 70th birthday     

Synthesis of α-hydroxy-β-lactams

A convenient synthesis of α-hydroxy-β-lactams has been devised that involves the annelation of an inline with benzyloxyacetyl chloride and triethylamine and subsequent hydrogenolysis in the presence of palladium on carbon. In most cases a cis-β-lactam was obtained. A thioimidate can also be used as the imino component in the annelation reaction but the hydrogenolysis step fails. The annelation of the appropriate thiazoline to a 6-epi-penicillin derivative occurred much more readily with benzyloxyacetyl chloride than with azidoacetyl chloride.     

Micropolarity of sodium bis(2-ethylhexyl) sulfosuccinate reverse micelles prepared in supercritical ethane and near-critical propane

The micropolarity of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles prepared in supercritical ethane and near-critical propane has been determined in terms of a solvent polarity parameter, E _T(30) values, by using absorption probes, 1-ethyl-4-methoxycarbonyl pyridinium iodide and 2,6-diphenyl-4-(2,4,6-triphenylpyridinio)-phenolate as a functions of pressure and the molar ratio of water to AOT, W ₀, at a constant temperature of 310 K. The micropolarity comparable to that of ethanol was observed for reverse micelles containing water of W ₀ = 2. The micropolarity increased with the water content and became independent of pressure after the system changed to a one-phase reverse micelle solution. For a given W ₀ value, no difference in the micropolarity was noticed in the micelles prepared in ethane and propane. Phase behaviour investigations have revealed that complete dissolution of 50 mM AOT occurred at 20 MPa in supercritical ethane, while a much lower pressure of 1 MPa was required in near-critical propane. The amount of water solubilized in reverse micelles formed in supercritical ethane was relatively low, reaching a W ₀ value of 7 at 36 MPa. In contrast, the amount of water solubilized in near-critical propane reverse micelles was W ₀ = 11 at a much lower pressure of 6 MPa. A higher pressure was required to solubilize larger amount of water in reverse micelles prepared in both ethane and propane. Received: 9 October 1998 Accepted in revised form: 12 February 1999     

Deep oxidation in propane oxidative dehydrogenation to propene over V2O5/γ-Al2O3 studied by in-situ DRIFTS

In-situ DRIFTS was used to study the deep oxidation of propane, a side reaction during propane oxidative dehydrogenation to propene. Strong adsorption of propene was supposed to be the main reason for the deep oxidation. It was found that gaseous oxygen in the feed and the reaction temperature had great influence on the reaction. To obtain a relative high selectivity to propene, the reaction temperature should be maintained at 150250 °C with a proper content of gaseous oxygen in the feed for a certain catalyst and some modifiers which could weaken the adsorption of propene on the catalyst surface would be favorable.     

Distribution of Platinum Particles in the Bimodal Micropore System of Activated Carbon

Platinum (Pt)/activated‐carbon catalysts were prepared and characterized by pore‐size distribution (PSD), propane‐sorption dynamics, and activity of cyclohexane dehydrogenation to benzene. The batch‐type frequency‐response (FR) spectroscopic technique was applied to determine the mass‐transport rate of propane sorption. Two parallel sorption processes of different time constants were distinguished, suggesting that adsorption proceeds in smaller and larger micropores that are not interconnected. Increasing Pt loading affected the propane mobility, but increased the dehydrogenation activity only up to ca. 1 wt‐% of Pt content. It was concluded that clusters of metallic Pt‐atoms are located preferentially at the narrowest pores. Blocking these micropores, the Pt reduces the carbon surface available for sorption; also, a significant fraction of the metal becomes inaccessible for the reactant.     

Carbon Dynamics on the Molybdenum Carbide Surface during Catalytic Propane Dehydrogenation

The effect of the gas‐phase chemical potential on surface chemistry and reactivity of molybdenum carbide has been investigated in catalytic reactions of propane in oxidizing and reducing reactant mixtures by adding H₂, O₂, H₂O, and CO₂ to a C₃H₈/N₂ feed. The balance between surface oxidation state, phase stability, carbon deposition, and the complex reaction network involving dehydrogenation reactions, hydrogenolysis, metathesis, water‐gas shift reaction, hydrogenation, and steam reforming is discussed. Raman spectroscopy and a surface‐sensitive study by means of in situ X‐ray photoelectron spectroscopy evidence that the dynamic formation of surface carbon species under a reducing atmosphere strongly shifts the product spectrum to the C₃‐alkene at the expense of hydrogenolysis products. A similar response of selectivity, which is accompanied by a boost of activity, is observed by tuning the oxidation state of Mo in the presence of mild oxidants, such as H₂O and CO₂, in the feed as well as by V doping. The results obtained allow us to draw a picture of the active catalyst surface and to propose a structure–activity correlation as a map for catalyst optimization.     

Preparation of Some Benzyl D-Glucuronatesfrom 4-Methoxybenzylidene Derivatives of D-Glucuronic Acid

 Because of the low stability of the benzyl ester linkage in benzyl 1,2:3,5-di-O-benzylidene-α-D-glucofuranuronate during the removal of the benzylidene groups by acid hydrolysis and/or hydrogenolysis, 4-methoxybenzylidene groups were used to block the free hydroxyl groups of D-glucuronic acid. Several benzyl esters of D-glucuronic acid were prepared, and their relative rates of acid catalyzed hydrolysis were determined by liquid-chromatographic separation of the reaction mixture and subsequent diode array detection.     

催化剂表面分形结构对催化反应的影响

用溶胶－凝胶方法制备了ＳｉＯ２并以其及另一种ＳｉＯ２气凝胶作载体,用浸渍法制备了两种铑基催化剂。以ｎ－Ｃ５～Ｃ８烷烃为探针分子,测得两种催化剂的表面分形维数Ｄ分别为３和２。在两种催化剂上,ＣＯ加氢和丙烷氢解反应的选择性没有显著差别,但在Ｄ＝２的催化剂上,ＣＯ加氢和丙烷氢解反应的速度显著高于Ｄ＝３的催化剂     

Rh—CeO2／SiO2催化剂中金属—氧化物相互作用的研究

用Ｈ２化学吸附和丙烷氢解作为模型反应研究了Ｒｈ－ＣｅＯ２／ＳｉＯ２催化剂中Ｒｈ与ＣｅＯ２的相互作用。结果表明,催化剂还原后,ＣｅＯ２覆盖了Ｒｈ表面。以往用Ｈ２或ＣＯ化学吸附研究金属－氧化物相互作用的结果只是定性的,而以丙烷氢解作为模型反应可以定量地得出氧化物对金属粒子的覆盖成度     

A New Method for the Production of Carbon Nanotubes

It was found for the first time that carbon nanotubes are formed during the mechanical treatment of V₂O₅ powder in the presence of an organic solvent. The carbon content in the V₂O₅ sample, dispersed in alcohol, varies during milling and amounts to 2.1 wt.%. The unusually large increase in the catalytic activity of mechanically activated V₂O₅ in the selective oxidation of n-butane and also of benzene and propane is due to the formation of new active centers containing carbon inserted into the V₂O₅ structure.     

Preparation of Some Benzyl D-Glucuronatesfrom 4-Methoxybenzylidene Derivatives of D-Glucuronic Acid

Summary.  Because of the low stability of the benzyl ester linkage in benzyl 1,2:3,5-di-O-benzylidene-α-D-glucofuranuronate during the removal of the benzylidene groups by acid hydrolysis and/or hydrogenolysis, 4-methoxybenzylidene groups were used to block the free hydroxyl groups of D-glucuronic acid. Several benzyl esters of D-glucuronic acid were prepared, and their relative rates of acid catalyzed hydrolysis were determined by liquid-chromatographic separation of the reaction mixture and subsequent diode array detection. Received May 22, 2000. Accepted (revised) July 17, 2000     

Stabilization of nonclassical types of valence bond orientation at the carbon atom in organoboron compounds

The complete topological structure of the potential energy surface (PES) of methane in the inversion region was studied by theab initio CCD(full)/6-311++G^** method. The necessity of taking into account nuclear motions was shown. Penta- and hexacoordination of carbon atoms in boron-containing organic compounds was investigated by theab initio MP2(full)/6-31G^** and MP2(full)/6-311++G^** methods. The CB₄H₄Li₂, CB₆N₂H₂, and CB₆O₂ systems containing hexacoordinated carbon atoms correspond to rather deep minima on the relevant PES and can be the subject of synthetic studies. According to theab initio calculations, pyramidal boron-containing systems with hypercoordinated carbon atoms, which fulfill the “8e rule,” also correspond to rather deep PES minima and can be detected experimentally. Dedicated to Academician V. I. Minkin on the occasion of his 65th birthday. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 786–796, May, 2000     

Liquid phase hydrogenolysis of dibenzyl ether over metals of the VIIIth group

It has been shown that depending on the catalyst and the solvent used, during the conversion of dibenzyl ether to toluene hydrogenolysis of C-O ether bonds, dehydrogenation and hydrogenolysis of benzyl alcohol formed, hydrogenation and decarbonylation of benzaldehyde, and the recombination of benzyl radicals and the benzylation of toluene can take place. The activity of catalysts in hydrogenolysis reactions of dibenzyl ether and benzyl alcohol to produce toluene decreases as follows: Pd/C>Pd>Raney Ni>Rh, whereas the selectivity drops down in the order: Raney Ni>Pd>Pd/C>Rh. The reaction rates depend on the solvent and diminish in the order: ethanol>2-propanol>benzene.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1257–1261, July, 1993.