A quantum-chemical study of hydride transfer in catalytic transformations of paraffins on zeolites. Pathways through adsorbed nonclassical carbonium ions

A quantum-chemical investigation of the hydride transfer reaction in catalytic transformations of hydrocarbons on zeolites has been performed. Ab initio calculations at the MP2/6-31++G^**//HF/6-31G^* level demonstrated that the activated complexes of hydride transfer reaction in catalytic transformations of paraffins on zeolites very much resemble adsorbed nonclassical carbonium ions. However, these transient species are strongly held at the surface active sites by the Coulomb interaction. The calculated activation energies for reactions involving propane and isobutane are in a reasonable agreement with the experimental data. This revised version was published online in July 2006 with corrections to the Cover Date.     

Air‐Stable,Nitrile‐Ligated (Cyclopentadienone)iron Dicarbonyl Compounds as Transfer Reduction and Oxidation Catalysts

A series of air‐stable, nitrile‐ligated (cyclopentadienone)iron dicarbonyl compounds was synthesized and their activities as catalysts in the transfer reduction of acetophenone were explored. While all were active catalysts, the acetonitrile adduct was chosen for further study and was found to be active in the transfer reduction of aldehydes and ketones and in the Oppenauer‐type oxidation of secondary alcohols. The acetonitrile catalyst exhibited activities similar to those of an analogous air‐sensitive iron hydride, but unlike the iron hydride it was unreactive in carbonyl reductions using hydrogen gas.     

Effect of pH on hydride transfer by Escherichia coli dihydrofolate reductase

The kinetic isotope effect (KIE) on hydride transfer in the reaction catalysed by dihydrofolate reductase from Escherichia coli (EcDHFR) is known to be temperature dependent at pH 7, but essentially independent of temperature at elevated pH. Here, we show that the transition from the temperature-dependent regime to the temperature-independent regime occurs sharply between pH 7.5 and 8. The activation energy for hydride transfer is independent of pH. The mechanism leading to the change in behaviour of the KIEs is not clear, but probably involves a conformational change in the enzyme brought about by deprotonation of a key residue (or residues) at high pH. The KIE on hydride transfer at low pH suggests that the rate constant for the reaction is not limited by a conformational change to the enzyme under these conditions. The effect of pH on the temperature dependence of the rate constants and KIEs for hydride transfer catalysed by EcDHFR suggests that enzyme motions and conformational changes do not directly influence the chemistry, but that the reaction conditions affect the conformational ensemble of the enzyme prior to reaction and control the reaction though this route.     

耦合相变储热的金属氢化物反应器吸氢过程模拟

基于金属氢化物储氢反应,建立了相变材料蓄热的固体储氢反应器模型,模拟研究了吸氢压力等操作参数及相变材料的相变温度、固(液)态导热系数、相变潜热等物性参数对固体储氢反应器工作过程的影响. 结果表明,相变材料的固态导热系数和相变潜热对固体储氢反应器性能的影响较小,相变温度和液态导热系数对反应器性能影响较大. 相变温度越低,液态导热系数越大,储氢反应器性能越好. 在使用最优的相变材料储能时,提高充入氢气的压力可加快反应速率,强化相变材料的传热,有助于进一步优化反应器的储氢性能.     

Theoretical examination of the mechanism of aldose-ketose isomerization

Two mechanisms for an aldose–ketose isomerization havebeen examined using high level ab initio and semiempirical molecularorbital methods. The proton transfer pathway via an enediolintermediate is shown to be favored in the absence of a metalion, while the hydride transfer pathway becomes favored in thepresence of a metal ion. Our calculations explain why the protontransfer pathway is operative in most aldose–ketose isomerizationreactions. These calculations also provide further support forthe previously proposed metal ion-mediated hydride transfermechanism for xylose isomerase.     

Synthesis of Halogen-Free Polyisobutylene by in situ Hydride Transfer to Living Polyisobutylene from Tributylsilane

Summary Hydride transfer reaction between living polyisobutylene cation (PIB⁺) and tributylsilane in hexanes/methyl chloride 60/40 (v/v) solvent mixtures at -80, -70 and -60 °C was studied to synthesize halogen-free polyisobutylene (PIB). The hydride transfer reaction between tributylsilane and living PIB capped with 1,1-ditolylethylene (PIB-DTE⁺) have also been carried out under similar conditions at -80 °C. The rate of hydride transfer reaction increases with the increase of tributylsilane concentration for the reaction of both PIB⁺ and PIB-DTE⁺ with tributylsilane. Gel Permeation Chromatography and NMR Spectroscopy suggested practically complete capping of the polymeric cation and the absence of side reactions.     

Catalytic cycles for isobutane isomerization over sulfated-zirconia catalysts

Reaction kinetic studies of isobutane isomerization over sulfated-zirconia catalysts were performed at 423 K. These catalysts show sustained rates of dihydrogen production that follow the same trend with time-on-stream as the rate of hydrocarbon production. Lower rates of dihydrogen production and correspondingly lower rates of hydrocarbon production are observed for catalysts dried at 773 K compared to samples dried at 588 K. It appears that sulfated-zirconia has the ability to generate olefins under reaction conditions, and this dehydrogenation reaction appears to be reversible, because increasing levels of dihydrogen in the feed suppress catalytic activity. These kinetic data can be described by a reaction scheme involving oligomerization/β-scission, isomerization and hydride transfer steps over acid sites, combined with dehydrogenation steps that generate olefins. The values of the rate constants for oligomerization steps over sulfated-zirconia are several orders of magnitude higher than for the corresponding steps over H-mordenite, whereas the rate constants for hydride transfer steps are similar for sulfated-zirconia and H-mordenite. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of polymers containing acetamide structure and their use as phase transfer catalyst

Summary N-Methyl-N-(p-vinylbenzyl) acetamide was synthesized by the reaction of p-chloromethylstyrene with N-methylacetamide in the presence of sodium hydride. This monomer was readily polymerized or copolymerized with styrene by AIBN. The resulting polymers served as effective phase transfer catalysts for several nucleophilic substitution reactions under liquid-liquid biphase conditions. The catalytic activity was discussed on the terms of cation binding ability of the polymers and activation of anions caused by the desolvation on hydrophobia region of the polymers.     

相转移催化合成三溴季铵盐

介绍了相转移催化合成三溴季铵盐的 3种方法 ,讨论了氢溴酸、季铵盐和液溴对反应的影响。反应条件简单、实用 ,应用前景十分广泛。     

Catalytic transfer hydrogenation on a hydrogen-absorbing alloy (CaNi5) using hydriding–dehydriding properties

By using the characteristics of a hydrogen-absorbing alloy, the hydrogen produced by catalytic dehydrogenation of saturated compounds can be absorbed to form metal hydrides, and, vice versa, the resulting metal hydrides are able to hydrogenate efficiently unsaturated compounds upon dehydriding. Gas-phase reactions between 2-butene and 2-propanol on a hydrogen-absorbing alloy CaNi₅ have been studied in the temperature range of 393–473 K. CaNi₅ showed interesting characteristics as an active catalyst for the catalytic transfer hydrogenation of butene from propanol as a hydrogen donor. 2-propanol was effectively dehydrogenated at 423 K to yield acetone in which the dissociated hydrogen was completely absorbed by CaNi₅ to form the metal hydride. When the alloy was hydrided to some extent, butene was hydrogenated by the absorbed hydrogen in the metal hydride to produce butane. The overall reaction on CaNi₅ was expressed as catalytic transfer hydrogenation of 2-butene from 2-propanol through intermediate formation of metal hydrides, rather than the direct reaction between butene and propanol on the alloy. Thus, CaNi₅ effectively repeated hydriding–dehydriding cycles: hydriding of CaNi₅ by 2-propanol dehydrogenation with subsequent dehydriding for the hydrogenation of 2-butene. The use of hydrogen-absorbing CaNi₅ provides a novel reaction system for the catalytic transfer hydrogenation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Redox-active silica nanoparticles : Part 2. Photochemical hydrosilylation on a hydride modified silica particle surface for the covalent immobilization of ferrocene

The surface of spherical nonporous monodisperse silica particles (diameter: 200 nm) is covalently modified with hydride by means of hydrosilanization. A direct silicon-carbon link between the hydride modified silica surface and 10-undecylenic acid is obtained by photochemical radical initiated hydrosilylation. A ferrocene unit is attached through an amide link to the carboxylic acid modified surface. Solid-state NMR and DRIFT spectroscopy indicate success of the reactions. The electrochemical oxidation and reduction of the ferrocene modified silica particles are observed after their adsorption on a platinum electrode. The cyclic voltammograms indicate, in addition to the charge transfer between ferrocene units on the particle surface, an interparticle charge transfer.     

Comparative catalytic study on butene/isobutane alkylation over LaX and CeX zeolites: The influence of calcination atmosphere

Lanthanum-containing (LaX) and cerium-containing X zeolites (CeX) were prepared by a double-exchange, double-calcination method. By changing the calcination atmospheres between nitrogen and air, the Ce^IV contents in CeX zeolites were adjusted and their impacts on physicochemical properties and catalytic performance in isobutane alkylation were established. The crystallinity of CeX zeolite was found to be negatively correlated with the Ce^IV content. This i s believed to be due to the water formed during the oxidation of Ce^III, which facilitates the framework dealumination. As a consequence, calcining in air resulted in a great elimination of strong Brønsted acid sites while under nitrogen protection, this phenomenon was mostly hindered and the sample's acidity was preserved. When tested in a continuously flowed slurry reactor, the catalyst lifetime for isobutane alkylation was found to be linearly related to the strong Brønsted acid concentration. In addition, Ce³⁺ was found more benefit for the hydride transfer compared with La³⁺, which is ascribed to the stronger polarization effect on the CH bond of isobutane. Moreover, the decline of hydride transfer activity can be slowed down by the catalytic cracking of the bulky molecules. Based on the product distribution, a new catalytic cycle of dimethylhexanes (DMHs) involving a direct formation of isobutene rather than tert-butyl carbocation was proposed in isobutane alkylation.     

复合固化吸附储氢材料的传热性能测试及仿真

针对目前金属氢化物储氢技术中面临的粉化后传热性能不佳等问题,采用膨胀石墨为基质配制固化的金属氢化物吸附剂。使用Hotd isk热物性分析仪,测试了固化吸附剂的热物性参数,并对固化吸附床的传热及相应的储氢特性进行了仿真。研究结果表明,固化混合吸附剂相对于散装的储氢合金,轴向导热系数最高提高了52倍左右,径向导热系数最高提高了12倍左右。通过仿真发现,在相同的80℃热源温度下,固化混合吸附床所需要的加热时间相对于散装储氢合金吸附床可以缩短17%,吸氢速率可以提高24%。     

Hydride Transfer: A Dominating Reaction of Ozone with Tertiary Butanol and Formate Ion in Aqueous Solution

This article provides evidences that hydride transfer is an important primary step in ozone reactions of formate and tertiary butanol in aqueous media. In both systems, one argument is the fact that the free hydroxyl radical yields are relative low ((40 ± 4)% and (7 ± 0.8)% for formate and tertiary butanol, respectively). Another hint is the high exergonicity of these reactions: ΔG = –249 kJ mol^?1 for formate/ozone system and ΔG = –114 kJ mol^?1 for hydride transfer followed by a methyl shift in the reaction between tertiary butanol and ozone. In addition, the main product of tertiary butanol ozonolysis is butan-2-one [(89 ± 3)%], a compound that is formed only via hydride transfer. For the reaction of ozone with formate an activation energy of (54.6 ± 1.2) kJ mol^?1 and a pre-exponential term of (2.5 ± 1.2) × 10¹¹ were determined (in the presence of tertiary butanol as ^?OH scavenger) whereas for tertiary butanol the two activation parameters were (68.7 ± 1.9) kJ mol^?1 and (2.0 ± 1.5) × 10⁹, respectively.     

Tandem Catalysis: Access to Ketones from Aldehydes and Arylboronic Acids via Rhodium‐Catalyzed Addition/Oxidation

Direct cross‐coupling reactions of aromatic aldehydes with arylboronic acids afforded ketones in high yields and under mild conditions in the presence of a rhodium catalyst, acetone and a base. This new reaction, involving a formal aldehyde C H bond activation, is believed to proceed via a tandem process involving addition of the organometallic species to the aldehyde followed by oxidation by β‐hydride transfer.     

Application of Danckwerts-type boundary conditions to the modeling of the thermal behavior of metal hydride reactors

The paper presents a model-based investigation of a metal hydride reactor applied as a solid state hydrogen storage device. The elements of a metal hydride reactor are hydrogen supply duct, internal hydrogen distribution, hydride bed, reactor shell and the flow domain of the heat transfer fluid. Internal hydrogen distribution and hydride bed are porous media. Therefore, hydrogen flows through non-porous and porous regions during its reversible exothermic absorption and endothermic desorption, respectively. The interface between porous and non-porous regions is a discontinuity with respect to energy transport mechanisms. Hence, Danckwerts-type boundary conditions for the energy balance equation are introduced. Application of the first and second law of thermodynamics to the interface reveals that temperature jumps may occur at the hydrogen inlet but are not allowed at the hydrogen outlet. Exemplarily the loading behavior of a metal hydride storage tank based on sodium alanate is analyzed. It is demonstrated and experimentally validated that only Danckwerts-type boundary conditions predict the important cooling effect of the inlet hydrogen on the exothermic absorption process correctly.     

Acid strength and solvation effects on methylation,hydride transfer,and isomerization rates during catalytic homologation of C1 species

Dimethyl ether (DME) homologation forms isobutane and triptane (2,2,3-trimethylbutane) with supra-equilibrium selectivities within C₄ and C₇ hydrocarbons on both mesoporous solid acids (SiO₂–Al₂O₃, H₃PW₁₂O₄₀/SiO₂) and the acid forms of various zeolites (BEA, FAU, MFI) via methylation and hydride transfer steps that favor isobutane and triptane formation because of the relative stabilities of ion-pairs at transition states for chains along the preferred growth path. The stabilities of ion-pair transition states increase as acid sites become stronger and energies for charge separation decrease and as van der Waals interactions within pores become stronger, which respectively lead to higher rates on H₃PW₁₂O₄₀/SiO₂ and aluminosilicate zeolites than on amorphous SiO₂–Al₂O₃. Solid acids with different strengths and abilities to solvate ion-pairs by confinement differ in selectivity because strength and solvation influence transition states for the hydride transfer, methylation, and isomerization steps to different extents. Stronger acid sites on H₃PW₁₂O₄₀/SiO₂ favor isomerization and hydride transfer over methylation; they lead to higher selectivities to n-butane and non-triptane C₇ isomers than the weaker acid sites on BEA, FAU, and mesoporous SiO₂–Al₂O₃. This preference for hydride transfer and isomerization on stronger acids reflects transition states with more diffuse cationic charge, which interact less effectively with conjugate anions than more localized cations at methylation transition states. The latter recover a larger fraction of the energy required to form the ion-pair, and their stabilities are less sensitive to acid strength than for diffuse cations. Large-pore zeolites (BEA, FAU) form triptane with higher selectivity than SiO₂–Al₂O₃ because confinement within large pores preferentially solvates the larger transition states for hydride transfer and methylation, which preserve the four-carbon backbone in triptane, over smaller transition states for alkoxide isomerization steps, which disrupt this backbone and cause growth beyond C₇ chains and subsequent facile β-scission to form isobutane. MFI forms isobutane and triptane with much lower selectivity than mesoporous acids or large-pore zeolites, because smaller pores restrict the formation of bimolecular methylation and hydride transfer transition states required for chain growth and termination steps to a greater extent than those for monomolecular alkoxide isomerization. These data and their mechanistic interpretations show that the selective formation of isobutane and triptane from C₁ precursors like DME is favored on all acids as a result of the relative stability of methylation, hydride transfer, and isomerization transition states, but to a lesser extent when small confining voids and stronger acid sites preferentially stabilize monomolecular isomerization transition states. The observed effects of acid strength and confinement on rates and selectivities reflect the more effective stabilization of all ion-pairs on stronger acids and within solvating environments, but a preference for transition states with more diffuse charge on stronger acids and for ion-pairs with the appropriate solvation within voids of molecular dimensions.     

Evolution of a Transition State: Role of Lys100 in the Active Site of Isocitrate Dehydrogenase

An active site lysine essential to catalysis in isocitrate dehydrogenase (IDH) is absent from related enzymes. As all family members catalyze the same oxidative β‐decarboxylation at the (2R)‐malate core common to their substrates, it seems odd that an amino acid essential to one is not found in all. Ordinarily, hydride transfer to a nicotinamide C4 neutralizes the positive charge at N1 directly. In IDH, the negatively charged C4‐carboxylate of isocitrate stabilizes the ground state positive charge on the adjacent nicotinamide N1, opposing hydride transfer. The critical lysine is poised to stabilize—and perhaps even protonate—an oxyanion formed on the nicotinamide 3‐carboxamide, thereby enabling the hydride to be transferred while the positive charge at N1 is maintained. IDH might catalyze the same overall reaction as other family members, but dehydrogenation proceeds through a distinct, though related, transition state. Partial activation of lysine mutants by K⁺ and NH₄⁺ represents a throwback to the primordial state of the first promiscuous substrate family member.     

Numerical Modeling of Differential Kinetics in the Asymmetric Hydrogenation of Acetophenone by Noyori's Catalyst

A combination of numerical integration of the rate equations and experimental measurement of the time‐dependence of rates rather than concentrations is described for the analysis of the catalytic cycle by which Noyori's catalyst, trans‐RuCl₂[(S)‐binap][(S,S)‐dpen], hydrogenates acetophenone. The method is simpler in practice than usual kinetic methods and yields furthermore rate constants for activation, dihydrogen cleavage, and hydride transfer to substrate under realistic catalytic conditions. The specific results for the asymmetric hydrogenation of acetophenone show that the turnover‐limiting step changes from dihydrogen cleavage to hydride transfer if the H₂ pressure is increased, but also during the course of the reaction when the hydrogenation is performed under typical conditions.     

Enhanced benzene formation on Pt/H-mordenite and Pd/H-mordenite

The distribution of methylcyclopentane (MCP) ring enlargement (RE) products between benzene (Bz) and cyclohexane (CH) provides information on the reaction mechanism. The Bz/CH ratio is in excess of the calculated and experimentally determined equilibrium ratio. This implies that benzene is a primary product; it eliminates the possibility of direct hydride ion transfer to carbenium ions as the prevailing mechanism. The results are consistent with the concept that metal-proton adducts are instrumental in bifunctional catalysis.