In situ investigation of the oxidative addition in homogeneous Pd catalysts by synchronised time resolved UV-Vis/EXAFS

Simultaneous structuro-kinetic information obtained via time resolved stopped-flow/UV-Vis spectroscopy/dispersive EXAFS (EDE) experiments elucidated a two-step process for the addition of iodobenzene to [(Ph3P)2Pd(dba)].     

Insights into reaction mechanisms in heterogeneous catalysis revealed by in situ NMR spectroscopy

This tutorial review intends to show the possibilities of in situ solid state NMR spectroscopy in the elucidation of reaction mechanisms and the nature of the active sites in heterogeneous catalysis. After a brief overview of the more usual experimental devices used for in situ solid state NMR spectroscopy measurements, some examples of applications taken from the recent literature will be presented. It will be shown that in situ NMR spectroscopy allows: (i) the identification of stable intermediates and transient species using indirect methods, (ii) to prove shape selectivity in zeolites, (iii) the study of reaction kinetics, and (iv) the determination of the nature and the role played by the active sites in a catalytic reaction. The approaches and methodology used to get this information will be illustrated here summarizing the most relevant contributions on the investigation of the mechanisms of a series of reactions of industrial interest: aromatization of alkanes on bifunctional catalysts, carbonylation reaction of methanol with carbon monoxide, ethylbenzene disproportionation, and the Beckmann rearrangement reaction. Special attention is paid to the research carried out on the role played by carbenium ions and alkoxy as intermediate species in the transformation of hydrocarbon molecules on solid acid catalysts.     

Activation in homogeneous catalysis

The topic is discussed in relation to the definition of activators, which are classified in accordance with their mode of action; particular attention is given to orienting and polarizing effects, activated electron transfer, formation of intermediate compounds, and displacement of equilibria in intermediate stages. Criteria are proposed for assigning activators to these four groups.     

Deactivation processes of homogeneous Pd catalysts using in situ time resolved spectroscopic techniques

UV-Vis, combined with ED-XAFS shows, for the first time, the evolution of inactive Pd dimers and trimers, that are a possible first stage in the deactivation process of important palladium catalysed reactions, leading to larger palladium clusters and eventually palladium black.     

Charge-transfer complexes in homogeneous catalysis

It is shown that these complexes can be formed between the reactants in redox reactions. Oxidizing and reducing agents are classified on the basis of charge-transfer capacity, which is defined by the integral for the overlap between the filled orbital of the reductant and the free one of the oxidant. Unfilled diffuse d-orbitals most readily give such complexes. Homogeneous catalysis of a redox reaction involves charge transfer in either direction between the catalyst (which has partly filled d-orbitals) and one of the reagents. The transfer is very much dependent on the orientation, so activators play a large part.     

Recycling of homogeneous hydrogenation catalysts by dialysis coupled catalysis

Although transition-metal complexes are very attractive as homogeneous catalysts in fine chemistry, their high prices often limit their applications. A means to recycle those catalysts would solve this problem and would simultaneously facilitate the downstream purification of the product. This is now realized in a new concept in which homogeneous catalysis is coupled to dialysis. The advantages of homogeneous catalysis (off-the-shelf catalysts, high activities and selectivities) are thus combined with those of heterogeneous catalysis (easy catalyst separation from product solution, reuse of catalyst, and possibility for continuous operation). Since the heart of the process is the membrane, self-prepared membranes were preferred as they allow a better control and understanding of the separation characteristics. Rhodamine B was used as a probe molecule to define the working conditions of the membrane. The concept is proven to work for two relevant chiral reactions: a hydrogenation with Ru-BINAP and a hydrogen transfer reaction with Ru-TsDPEN [BINAP=(1,1'-binaphthalene)-2,2'-diylbis(diphenylphosphine); TsDPEN= tosyl-N,N'-diphenyl-1,2-ethanediamine].     

Carboxylic acids as substrates in homogeneous catalysis

In organic molecules carboxylic acid groups are among the most common functionalities. Activated derivatives of carboxylic acids have long served as versatile connection points in derivatizations and in the construction of carbon frameworks. In more recent years numerous catalytic transformations have been discovered which have made it possible for carboxylic acids to be used as building blocks without the need for additional activation steps. A large number of different product classes have become accessible from this single functionality along multifaceted reaction pathways. The frontispiece illustrates an important reason for this: In the catalytic cycles carbon monoxide gas can be released from acyl metal complexes, and gaseous carbon dioxide from carboxylate complexes, with different organometallic species being formed in each case. Thus, carboxylic acids can be used as synthetic equivalents of acyl, aryl, or alkyl halides, as well as organometallic reagents. This review provides an overview of interesting catalytic transformations of carboxylic acids and a number of derivatives accessible from them in situ. It serves to provide an invitation to complement, refine, and use these new methods in organic synthesis.     

In situ NMR spectroscopy in heterogeneous catalysis: Kinetic study of hydrocarbon conversion mechanisms

The potential of high-resolution solid-state NMR spectroscopy for kinetic and mechanistic studies of hydrocarbon conversion on solid acid catalysis between 20 and 300°C is considered. The use of this technique is illustrated by the elucidation of the mechanisms of hydrogen exchange and ¹³C label transfer in alkanes and olefins, n-butane isomerization on sulfated zirconia, and ethane aromatization on zinc-containing zeolite beta. The kinetic parameters determined in these studies provide a basis for quantum chemical calculations of possible hydrocarbon activation and conversion pathways and for evaluating the reliability and accuracy of these theoretical calculations.     

Determination of cyanide ion by homogeneous catalysis and gas chromatography

A simple procedure for the determination of trace levels of cyanide ions is described and evaluated. The procedure is based on the cyanide-catalyzed cleavage of benzil in the presence of methanol to produce benzaldehyde and methyl benzoate. The concentrations of these products are determined by gas chromatography. Linearity between gas chromatographic response and cyanide concentration is observed from 0.05 to 3 ppm cyanide; a detection limit of 1 ppb is calculated. A series of anions is shown not to interfere.     

Identification of the active species generated from supported Pd catalysts in Heck reactions: an in situ quick scanning EXAFS investigation

Quick scanning extended X-ray absorption fine structure (QEXAFS) studies in the subsecond time scale have been performed to gain insight into the reaction mechanism of Heck-type C-C coupling reactions in the presence of supported Pd-based catalysts. Using a specially designed in situ EXAFS cell, both the solid catalyst and the liquid reaction mixture during the reaction of phenyl bromide (PhBr) with styrene were monitored. Soluble Pd species were only, but rapidly, detected in the liquid reaction phase once the reaction temperature of 150 °C was reached. At the same time, the conversion of PhBr started, and during the following "active phase" of the catalyst hardly any changes in the corresponding EXAFS and XANES spectra were observed. The present species could be identified as colloidal Pd(0) clusters with a size of ～2 nm estimated from the corresponding EXAFS spectra. The QEXAFS mode not only allowed monitoring rapid changes in the second time scale but also permitted minimization of effects caused by the heterogeneity of the systems. When the reaction rate started to decrease, pronounced changes in the EXAFS spectra were observed, which were attributed to an increased formation of bromo-palladates ([PdBr(4)](2-), [Pd(2)Br(6)](2-)). In addition to the liquid-phase species, significant changes were observed for the solid catalyst that was also probed in situ during the reaction. The originally oxidized Pd catalyst was efficiently reduced upon heating. Additionally, growth of the supported Pd particles was observed by both EXAFS and STEM. The above results confirm the role of the soluble molecular Pd species as the catalytically active species and clarify their conjunction with the in situ formed Pd colloids. Furthermore, the investigation demonstrates the potential of the QEXAFS not only for monitoring rapid changes during catalysis but also for gaining deeper insight into the mechanism of such complex industrially important systems under relevant reaction conditions.     

Kinetics of esterification of succinic anhydride with methanol by homogeneous catalysis

Kinetic data on the esterification of succinic anhydride with methanol catalyzed by sulfuric acid have been obtained using a stirred batch reactor. In addition to get a precise ascertainment of the parameters the esterification of monomethyl succinate with methanol has been studied separately. Several experiments have been carried out with different initial molar ratios and different amounts of sulfuric acid at various temperatures. The conversion to dimethyl succinate at 30–65°C follows a first-order rate expression with respect to each component. A theoretical rate equation is derived by the following reaction mechanism: (1) succinic anhydride is protonated by sulfuric acid to form a reaction intermediate, (2) irreversible esterification to monomethyl succinate, (3) protonation of the monoester, and (4) the esterification to dimethyl succinate proceeds reversibly. The resultant kinetic equation fitted the experimental data quite well and is given by the expression: © 1994 John Wiley & Sons, Inc. © 1994 John Wiley & Sons, Inc.     

In situ studies of methanol decomposition and oxidation on Pd(111) by PM-IRAS and XPS spectroscopy

Methanol decomposition and oxidation on Pd(111) at millibar pressure were studied by in situ polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), on-line gas chromatography and pre- and postreaction X-ray photoelectron spectroscopy (XPS). Various dehydrogenation products such as methoxy CH3O, formaldehyde CH2O, formyl CHO, and CO could be spectroscopically identified. Methanol oxidation proceeds via dehydrogenation to formaldehyde CH2O, which either desorbs or is further dehydrogenated to CO, which is subsequently oxidized to CO2. Carbonaceous overlayers that are present during the reaction may favorably affect the selectivity toward CH2O. The reaction takes place on metallic Pd, and no indications of an involvement of Pd surface oxide were observed.     

Intramolecular oxycyanation of alkenes by cooperative Pd/BPh3 catalysis

The cooperative catalysis by palladium and triphenylborane effects the intramolecular oxycyanation of alkenes through the cleavage of O-CN bonds and the subsequent insertion of double bonds. The use of 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) as a ligand for palladium is essential for allowing the transformation to proceed with high chemo- and regioselectivity. Variously substituted dihydrobenzofurans with both a tetra-substituted carbon and cyano functionality are accessed by the newly developed methodology.     

Half-sandwich iridium complexes for homogeneous water-oxidation catalysis

Iridium half-sandwich complexes of the types Cp*Ir(N-C)X, [Cp*Ir(N-N)X]X, and [CpIr(N-N)X]X are catalyst precursors for the homogeneous oxidation of water to dioxygen. Kinetic studies with cerium(IV) ammonium nitrate as primary oxidant show that oxygen evolution is rapid and continues over many hours. In addition, [Cp*Ir(H(2)O)(3)]SO(4) and [(Cp*Ir)(2)(μ-OH)(3)]OH can show even higher turnover frequencies (up to 20 min(-1) at pH 0.89). Aqueous electrochemical studies on the cationic complexes having chelate ligands show catalytic oxidation at pH > 7; conversely, at low pH, there are no oxidation waves up to 1.5 V vs NHE for the complexes. H(2)(18)O isotope incorporation studies demonstrate that water is the source of oxygen atoms during cerium(IV)-driven catalysis. DFT calculations and kinetic experiments, including kinetic-isotope-effect studies, suggest a mechanism for homogeneous iridium-catalyzed water oxidation and contribute to the determination of the rate-determining step. The kinetic experiments also help distinguish the active homogeneous catalyst from heterogeneous nanoparticulate iridium dioxide.     

Advances in homogeneous and heterogeneous catalysis with metal-containing silsesquioxanes

Metal-containing silsesquioxane derivatives provide new catalysts with both homogeneous and heterogeneous applicability. The steric and electronic properties of silsesquioxane silanolate ligands render metal centers more Lewis acidic than conventional alkoxide or siloxide ligands do. This concept has been exploited in newly developed catalysts for alkene metathesis, polymerization, epoxidation, and Diels-Alder reactions of enones. Other applications are envisioned in the near future.     

Pyridine and related ligands in transition metal homogeneous catalysis

This review provides a broad overview of the literature related to the importance of pyridine and related ligands in homogeneous catalysis. In particular, it describes the various ways by which this ligand can stabilised the metal within a complex for homogeneous catalysis. We surveyed the important transition metal homogenous catalysts containing pyridine and related ligand acting as backbone for other ligands in homogeneous catalytic reactions explicitly from 2011 up to early 2014 and summarized their comparative catalytic activities.     

通过表面钨掺杂大幅提升钯纳米立方体的燃料电池催化性能（英文）

发展兼具高活性和高稳定性的规整非铂电化学催化剂无论对于燃料电池的推广应用还是基础研究都具有重要意义.我们将钯纳米立方体(Pd nanocubes)作为晶种,使用表面掺杂的手段制备了一种表面结构规整的钨掺杂钯纳米立方体(W-doped Pd nanocubes).通过改变合成过程中所加入羰基钨前驱体的量以调控表面钨的原子比例,继而获得了钨原子比例分别为0%,0.8%,1.2%,1.5%的纳米立方体.所制W-doped Pd nanocubes/C催化剂在碱性条件下的氧还原反应中表现出优异性能,其中1.2%W-doped Pd nanocubes/C催化剂性能最佳,在0.9 VRHE时比活性达1.18 mA cm~(-2),质量活性达0.25 A mg~(-1)Pd,分别是商业Pt/C催化剂的4.7倍和2.5倍.研究表明,随着钨的掺杂量从0%增至1.5%,钨掺杂钯纳米立方体的d带中心从-2.49 eV逐渐降至-3.08 eV.同时,光电子能谱结果表明,随着钨掺杂量的增加,钯的3d峰位向低能逐渐偏移,说明了钨掺杂导致了电荷由钨转向钯.而d带中心的下移能够将更多的反键态拉下费米能级,继而导致反应中间体的吸附减弱.因此,由钨到钯的电荷转移导致的d带中心的下移,继而引起的反应中间体对催化剂的吸附作用变弱是氧还原催化活性增强的原因.而过高的W掺杂(1.5%)导致活性的降低也可以用Sabatier规则解释.在循环测试10000圈之后,1.2%W-doped Pd nanocubes/C催化剂的质量活性仅仅减少了14.8%,而商业Pt/C催化剂减少了40%,可见其具有极佳的稳定性.而且循环测试之后的透射电镜表征显示,相比于团聚严重的商业Pt/C催化剂,1.2%W-doped Pd nanocubes/C催化剂仍然分散良好,其形貌也几乎没有发生变化.此外,该催化剂对乙醇氧化反应也表现出优异的性能.在1.0 mol L~(-1)氢氧化钾和1.0 mol L~(-1)乙醇混合溶液中,测试峰电流达6.6 A mg~(-1)Pd,是Pd nanocubes/C催化剂的2.2倍,商业Pd/C催化剂的5.1倍.这同样得益于适量钨掺杂所导致的催化剂d带中心—下移引起的含碳中间体吸附的削弱.经过1000 s的稳定性测试,1.2%W-doped Pd nanocubes/C同样表现出高于商业Pd/C催化剂的稳定性.优异的氧还原和乙醇氧化性能表明所制1.2%W-doped Pd nanocubes/C是一种极具潜力的双功能燃料电池催化剂.     

Rapid thermal hydrogen production from 2,3-butanediol catalyzed by homogeneous rhodium catalysis

Thermal production of hydrogen from butane-2,3-diol is achieved with rates of up to 125 catalyst turnovers per hour using the homogeneous rhodium catalysts, [Rh(bipy)₂]Cl, [RhH(PⁱPr₃)₃] and [RhCl(PPh₃)₃]. (bipy = 2,2′-bipyridyl).