Rapid Hydrogenation Research Articles

Hydrogenation of olefins in supercritical CO(2) catalyzed by palladium nanoparticles in a water-in-CO(2) microemulsion.

Nanometer-sized metallic palladium particles can be synthesized by hydrogen reduction of Pd2+ ions dissolved in the water core of a water-in-CO2 microemulsion. The Pd nanoparticles, stabilized by the micromeulsion and uniformly dispersed in the supercritical fluid phase, are effective catalysts for hydrogenation of olefins. Examples of rapid and efficient hydrogenation of water-soluble and CO2-soluble olefins catalyzed by the Pd nanoparticles in supercritical CO2 are given.

Selectivity in hydrocarbon catalytic reforming: a surface chemistry perspective

A brief overview of the recent advances in the understanding of the reaction mechanisms of hydrocarbon reforming processes is provided. Emphasis is placed on the knowledge developed by studies using model single-crystal metals and modern surface analytical techniques. An argument is presented for the early definition of reaction selectivities in these processes because of subtle relative changes in dehydrogenation rates. Specifically, the ability of platinum to promote γ-hydride elimination steps from chemisorbed alkyl groups is proposed to be the reason for its unique behavior in isomerization and cyclization reactions. Nickel, in contrast, facilitates dehydrogenation preferentially at the α-position, and catalyses hydrogenolysis instead. Nevertheless, by far the most favorable reaction of alkyl moieties on transition metals is β-hydride elimination. It is this step the one that accounts for the fast equilibrium reached between alkanes and alkenes under most reaction conditions. Additional mechanistic complications in hydrocarbon reforming under catalytic conditions are introduced by the strongly bonded carbonaceous deposits that cover the surface of the active catalyst. The role that those deposits play in reforming is not yet entirely clear, but it is believed to involve passivation of the surface to facilitate the adsorption of π-bonded olefins and other weakly adsorbed intermediates, and also the provision of a reservoir for hydrogen. The working reforming catalysts is therefore likely to display bifunctional character, with rapid hydrogenation–dehydrogenation steps taking place on the hydrocarbon-covered surface, and more demanding skeletal rearrangement steps occurring on patches of bare metal.

Catalytic aspects in the transformation of pinenes to p-cymene

The reaction investigated in this work is the dehydrogenation of α-pinene to p-cymene over carriers impregnated with Pd. An optimal acid strength is required to cleave selectively the CC bond in the cyclobutane ring of α-pinene. Too strong acid sites such as in zeolites favor side reactions like oligomerization and cracking. Too weak acid sites fail to cleave the aforementioned CC bond and rapid hydrogenation of the α-pinene is a consequence. Hydrogenolysis is also a major side reaction leading to tetramethylcyclohexanes. A reaction mechanism is proposed in which first isomerization is involved followed by hydrogenation/dehydrogenation to stabilize the components. The catalyst has a dual-functionality with the acid sites in charge of isomerization and the metallic sites responsible of hydrogenation/dehydrogenation. The use of crude sulfate turpentine (CST) as raw material shows that β-pinene has a similar reactivity as α-pinene and high yields of p-cymene can be obtained from this cheap starting material. The sulfur remains however a major drawback.

Hydrogenation of oleochemicals at supercritical single-phase conditions: influence of hydrogen and substrate concentrations on the process

Fatty alcohols can be produced by catalytic hydrogenation of fatty acid methyl esters. This heterogeneous catalytic reaction is normally performed in a multi-phase system. In such a system, with a low hydrogen solubility in the liquid substrate and a large mass transport resistance, the hydrogen concentration at the catalyst is low and limits the reaction rate. To overcome this limitation, we have used the unique properties of supercritical fluids, properties which are in between those of liquids and gases, making them a very suitable medium for reactions. By adding propane to the reaction mixture of hydrogen and fatty acid methyl esters (C 18) we have created supercritical single-phase conditions. At these single-phase conditions the concentrations of all the reactants at the catalyst surface can be controlled, and an excess of hydrogen becomes possible. In this way, extremely rapid hydrogenation can be combined with a high product selectivity. In our lab-scale experiments the catalyst performance was studied as a function of hydrogen concentration, substrate concentration and temperature. Complete conversion of the liquid substrate was reached in a few seconds. As long as single-phase conditions remain, we have, in our experiments, tested up to 15 wt.% substrate, vapor-phase like reaction rates can be maintained. However, at these high substrate concentrations, mass transport becomes important again. Our results show that performing hydrogenation at supercritical single-phase conditions has a large potential for this and other catalytic processes where the hydrogen concentration at the catalyst is the limiting factor.

Hydrogenation of Aromatic Ketones Catalyzed by (η5-C5(CH3)5)Ru Complexes Bearing Primary Amines

The rapid hydrogenation of simple aromatic ketones is accomplished by using a combination of (η5-C5(CH3)5)Ru complexes and primary amines as a catalyst in 2-propanol. In particular, 1,2-diamines with primary and tertiary amino groups at both ends exhibit a significant ligand acceleration effect. Isotope-labeling experiments using D2 and 2-propanol-dn reveal that 2-propanol participates in the activation of H2 based on a metal/NH bifunctional effect to facilitate the hydrogenation. Asymmetric hydrogenation of prochiral simple ketones with the chiral version of the catalysts provides optically active secondary alcohols with up to 95% ee.

Asymmetric Catalysis by Architectural and Functional Molecular Engineering: Practical Chemo- and Stereoselective Hydrogenation of Ketones

Hydrogenation is a core technology in chemical synthesis. High rates and selectivities are attainable only by the coordination of structurally well-designed catalysts and suitable reaction conditions. The newly devised [RuCl(2)(phosphane)(2)(1,2-diamine)] complexes are excellent precatalysts for homogeneous hydrogenation of simple ketones which lack any functionality capable of interacting with the metal center. This catalyst system allows for the preferential reduction of a C=O function over a coexisting C=C linkage in a 2-propanol solution containing an alkaline base. The hydrogenation tolerates many substituents including F, Cl, Br, I, CF(3), OCH(3), OCH(2)C(6)H(5), COOCH(CH(3))(2), NO(2), NH(2), and NRCOR as well as various electron-rich and -deficient heterocycles. Furthermore, stereoselectivity is easily controlled by the electronic and steric properties (bulkiness and chirality) of the ligands as well as the reaction conditions. Diastereoselectivities observed in the catalytic hydrogenation of cyclic and acyclic ketones with the standard triphenylphosphane/ethylenediamine combination compare well with the best conventional hydride reductions. The use of appropriate chiral diphosphanes, particularly BINAP compounds, and chiral diamines results in rapid and productive asymmetric hydrogenation of a range of aromatic and heteroaromatic ketones and gives a consistently high enantioselectivity. Certain amino and alkoxy ketones can be used as substrates. Cyclic and acyclic alpha,beta-unsaturated ketones can be converted into chiral allyl alcohols of high enantiomeric purity. Hydrogenation of configurationally labile ketones allows for the dynamic kinetic discrimination of diastereomers, epimers, and enantiomers. This new method shows promise in the practical synthesis of a wide variety of chiral alcohols from achiral and chiral ketone substrates. Its versatility is manifested by the asymmetric synthesis of some biologically significant chiral compounds. The high rate and carbonyl selectivity are based on nonclassical metal-ligand bifunctional catalysis involving an 18-electron amino ruthenium hydride complex and a 16-electron amido ruthenium species.

Asymmetric hydrogenation via architectural and functional molecular engineering

Abstract RuCl2 (phosphine) 2 (1,2-diamine) complexes, coupled with an alkaline base in 2-propanol, allows for preferential hydrogenation of a C=O function over coexisting conjugated or nonconjugated C=C linkages, a nitro group, halogen atoms, and various heterocycles. The functional group selectivity is based on the novel metal-ligand bifunctional mechanism. The use of appropriate chiral diphosphines and diamines results in rapid and productive asymmetric hydrogenation of a range of aromatic, hetero-aromatic, and olefinic ketones. The versatility of this method is manifested by the asymmetric synthesis of various biologically significant chiral compounds.

Liquid-Phase Hydrogenation of Citral over Pt/SiO2 Catalysts: I. Temperature Effects on Activity and Selectivity

Liquid-phase hydrogenation of citral (3,7-dimethyl-2,6-octadienal) over Pt/SiO2 catalysts was studied in the temperature and pressure ranges 298–423 K and 7–21 atm, respectively. The reaction kinetics were shown to be free of artifacts arising from transport limitations and poisoning effects. The reaction rate in hexane as the solvent exhibited an activity minimum at 373 K. The initial turnover frequency for citral disappearance over 1.44% Pt/SiO2 catalyst at 20 atm H2 pressure decreased from 0.19 s−1 at 298 K to 0.02 s−1 at 373 K, but exhibited normal Arrhenius behavior between 373 and 423 K with an activation energy of 7 kcal/mol. Reaction at 298 K produced substantial deactivation, with the rate decreasing by more than an order of magnitude during the first 4 h of reaction; however, reaction at temperatures greater than 373 K exhibited negligible deactivation and a constant rate up to citral conversions greater than 70%. These unusual temperature effects were modeled using Langmuir–Hinshelwood kinetics invoking dissociative adsorption of hydrogen, competitive adsorption between hydrogen and the organic compounds, and addition of the second hydrogen atom to each reactant as the rate-determining step. Decomposition of the unsaturated alcohol (either geraniol or nerol) was proposed to occur concurrently with the hydrogenation steps to yield adsorbed CO and carbonaceous species which cause the deactivation, but at higher temperatures these species could be removed from the Pt surface by desorption or rapid hydrogenation, respectively. The activity minimum observed in the present study is attributed to the relative rates of the alcohol decomposition reaction and CO desorption, with the decomposition reaction having an activation barrier lower than that for CO desorption.

Extraction of Taiheiyo coal with supercritical water–HCOOH mixture

Taiheiyo coals were extracted with supercritical toluene (SC-toluene) (653K, 20MPa), supercritical water (SCW) (653K, 35MPa) and formic acid (HCOOH)–SCW mixed solvents (653K, 35MPa) using a semi-batch type system. The results clearly indicate that the coal conversion (=1−weight of residual coal (daf)/weight of loaded coal (daf)) and the liquid yield (=weight of liquid extracts/weight of loaded coal) in HCOOH–SCW were higher than those in SCW and SC-toluene. A considerable portion of coal is thus converted into light oils probably through hydrolysis and hydrogenation in HCOOH–SCW. We conducted another series of experiments for direct in situ observation of coal conversion in SCW and HCOOH–SCW using a diamond anvil cell (DAC). In HCOOH–SCW, the effective and rapid hydrogenation of coal with HCOOH occurs mainly at the early stage of the reaction.

ChemInform Abstract: A Chiral Rhodium Complex for Rapid Asymmetric Transfer Hydrogenation of Imines with High Enantioselectivity.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Hydrogenation of fatty acid methyl esters to fatty alcohols at supercritical conditions

AbstractExtremely rapid hydrogenation of fatty acid methyl esters (FAME) to fatty alcohols (FOH) occurs when the reaction is conducted in a substantially homogeneous supercritical phase, using propane as a solvent, over a solid catalyst. At these conditions, the limitations of hydrogen transport are eliminated. At temperatures above 240°C, complete conversion of the starting material was reached at residence times of 2 to 3 s, which is several orders of magnitude shorter than reported in the literature. Furthermore, formation of by‐products, i.e., hydrocarbons, could be prevented by choosing the right process settings. Hydrogen concentration turned out to be the key parameter for achieving the above two goals. As a result of the supercritical conditions, we could control the hydrogen concentration at the catalyst surface independently of the other process parameters. When methylated rapeseed oil was used as a substrate, the hydrogenation catalyst was deactivated rapidly. However, by using methylated sunflower oil, a catalyst life similar to that obtained in industrial processes was achieved. Our results showed that the hydrogenation of FAME to FOH at supercritical conditions is a much more efficient method than any other published process.

Rapid, productive and stereoselective hydrogenation of ketones

RuCl2(phosphine)2(1,2-diamine) complexes act as excellent precatalysts for homo- geneous hydrogenation of simple ketones which lack any functionality capable of interacting with the metallic center. This newly devised catalytic system allows preferential saturation of a CaO function over a coexisting CaC linkage in 2-propanol containing an alkaline base. Furthermore, the stereoselectivity of the reaction is easily controlled by the electronic and steric properties (bulkiness and chirality) of the ligands as well as the reaction conditions. Selective reduction of carbonyl compounds to alcohols has relied heavily on the use of metal hydride reagents. Our goal is to replace such stoichiometric reductions by catalytic hydrogenation, because the latter is obviously more beneficial, particularly for large-scale reactions. Here we describe a very practical homogeneous hydrogenation of ketones catalyzed by RuCl2(phosphine)2(1,2-diamine) and an inorganic base. The precatalysts can be preformed or generated in situ by mixing the phosphine-RuCl2 complex and an appropriate 1,2-diamine in 2-propanol. The reaction is normally conducted at room temperature and at < 8 atm of hydrogen. The new method shows promise for the synthesis of a wide range of achiral and chiral alcohols from ketonic substrates. This chemistry is entirely different from the BINAP-Ru catalyzed asymmetric hydrogenation of functionalized ketones developed earlier in our laboratories (1). RAPID HYDROGENATION OF SIMPLE KETONES

A Chiral Rhodium Complex for Rapid Asymmetric Transfer Hydrogenation of Imines with High Enantioselectivity

[formula: see text] A chiral rhodium complex, (R)-Cp*RhCl[(1S,2S)-p-TsNCH(C6H5)CH(C6H5)NH2] (1a, (S,S)-Cp*RhClTsDPEN), generated from [Cp*RhCl2]2 and (1S,2S)-N-p-toluenesulfonyl-1,2-diphenylethylenediamine [(S,S)-TsDPEN], and its enantiomer 1b were found to provide superior catalysts for the rapid, high-yielding, asymmetric transfer hydrogenation of some heterocyclic imines, using an HCO2H-Et3N azeotrope as the hydrogen source.

Hydroxycinnamic acids in the digestive tract of livestock and humans

Hydroxycinnamic acids are consumed as water-soluble conjugates and in larger amounts bound to plant cell walls. Bound acids are primarily released by microbial action in the modified forestomach of ruminants and the hindgut of non-ruminant species, including humans. In the rumen, rapid hydrogenation of p-coumaric, ferulic and caffeic acids, followed by dehydroxylation at C4 and more slowly at C3 yields 3-phenylpropionic acid. Phenylpropionate is absorbed and undergoes β-oxidation in the liver to benzoic acid which is then excreted predominately (75-95%) as its glycine conjugate (hippuric acid), but also as the free acid or glucuronide. In non-ruminants, hydroxycinnamates may be absorbed unchanged in the upper digestive tract via a Na + -dependent saturable transport system or escape to the hindgut where they are subject to microbial transformations with further absorption of metabolites. Metabolites of p-coumaric acid found in rat urine are the unchanged compound and its glycine conjugate, the reduced derivative and the β-oxidation product, 4-hydroxybenzoic acid. Caffeic acid and its methyl ethers (ferulic and iso-ferulic acids) are interconvertable and share metabolites. As in the rumen, reduction of the C 3 side-chain, demethylation of ferulate and dehydroxylation at C4 are products of microbial action. Dehydroxylation at C3 is more rarely encountered. The resulting 3-hydroxyphenylpropionic acid is commonly found in the urine of all species and is the major metabolite in rats where relatively little chain-shortening occurs. A larger range of metabolites including C 6 -C 1 compounds have been detected in human urine. Metabolism of hydroxycinnamate dimers found as cross-links between polysaccharide chains has been little studied although evident differences in the ability to metabolise such compounds exist between the human and rumen microflora.

Benzenethiol Reaction on the Clean and Hydrogen Pretreated Ni(100) Surface

Benzenethiol reactions on clean and hydrogen precovered Ni(100) surfaces have been studied in order to characterize C-S bond breaking. Benzenethiol adsorbs at 90 K as phenylthiolate and surface hydrogen. The dominant benzene formation pathway for large coverages of benzenethiol occurs at 270 K. C-S bond breaking involves direct reaction of phenylthiolate with the nickel surface; hydrogen does not appear to be directly involved. The rate-limiting C-S bond breaking step is followed by rapid hydrogenation of adsorbed phenyl to form benzene which desorbs from the sulfur covered surface. Deuterium incorporation studies support this mechanism since single hydrogen addition dominates as expected for hydrogenation of an adsorbed phenyl intermediate. For the benzenethiol saturated surface (0.30 monolayer), hydrogen preadsorption increases the temperature of benzene formation by up to 20 K and increases the benzene yield up to 37%. Reorientation of the phenylthiolate away from the surface in the presence of large coadsorbed hydrogen coverages is indicated by vibrational characterization. Above 500 K dehydrogenation of adsorbed phenylthiolate derived species results in formation of surface bound polyaromatic hydrocarbons (PAHs) coadsorbed with sulfur; the hydrogen formed in the process desorbs. For low coverages of benzenethiol, no 270 K benzene is observed due to increased dehydrogenation bymore » the surface. A small amount of benzene is formed by disproportionation above the temperature of free hydrogen desorption (400 K). A comparison of benzenethiol reactions on the three low Miller index surfaces of nickel clearly indicates that hydrogen availability at reaction temperature has a large influence on benzene formation. Hydrogen desorption prior to C-S bond activation on the Ni(100) surface limits benzene formation, while substantial hydrogen availability on the Ni(111) and Ni(110) surfaces facilitates benzene formation.« less

Catalytic hydrogenation of polyunsaturated biological membranes: effects on membrane fatty acid composition and physical properties

The relationship between phospholipid saturation and membrane physical structure in a complex, highly polyunsaturated biological membrane (trout liver microsomes) has been studied by the graded and specific hydrogenation of polyunsaturated fatty acids. The homogeneous catalyst Pd(QS) 2 caused rapid and effective hydrogenation, increasing the proportion of saturated fatty acids from 20–30% up to 60%, without loss or fragmentation. Long chain, polyunsaturated fatty acids (20:5 ω3, 22:6 ω3) were rapidly converted to a large number of partially hydrogenated isomers, and ultimately to the fully saturated C20 or C22 fatty acids. C18 mono- and di-unsaturates showed slower rates of hydrogenation. Increased saturation was closely associated with an increased membrane physical order as determined by the fluorescence anisotropy probe, 1,6-diphenyl-1,3,5-hexatriene. However, extensive hydrogenation led to highly ordered membranes exhibiting a gel-liquid crystalline phase transition between 30 and 60°C. Polyunsaturated membranes can thus be converted into partially or substantially saturated membranes with measurable phase structure without direct alteration of other membrane components. This offers a less equivocal means of assessing the influence of polyunsaturation upon membrane structure and function.

Rapid hydrogenation of unsaturated sterols and bile alcohols using microwaves

This paper describes an operationally simple, rapid hydrogenation of unsaturated sterols and bile alcohols in a domestic microwave oven. This has been achieved by the addition of catalytic amounts of Pd C in methylene chloride/propylene glycol solvents in the presence of ammonium formate followed by microwave irradiation. It is suggested that this methodology will be helpful in the identification of saturated and unsaturated sterols with different side-chain structures in rare diseases: sitosterolemia, cerebrotendinous xanthomatosis (CTX), as well as atherosclerosis and diabetes mellitus. Sterols, such as cholesterol, campesterol, sitosterol, and bile alcohols with unsaturated side chains, were converted to their reduced congeners with high yield and purity.

Solid-State NMR Investigation of Ethylbenzene Reactions over HMOR and Pt–HMOR Catalysts

Ethylbenzene reactions on HMOR and Pt–HMOR have been studied by means of solid-state MAS NMR. These experimental findings clearly indicate that disproportionation via the intermediacy of diphenylethane structure constitutes the first steps of this catalytic process over both HMOR and Pt–HMOR. In addition, ethylbenzene is suggested to undergo nonbranching rearrangements via the intermediacy of a two-electron–three-center bond leading to a13C-label transfer from α- to β-position of the side chain. This appears to be a major catalytic event over both catalysts. At later stages, disproportionation of ethylbenzene via dealkylation/alkylation steps accompanied by secondary reactions of ethylene proceeds freely on HMOR, whereas it is very much suppressed on Pt–HMOR. While dealkylation is very pronounced over Pt–HMOR, alkylation is very much limited by rapid hydrogenation of ethylene to ethane on Pt sites. Based on the spectroscopic evidence of this investigation, a number of schematic representations are illustrated to address the mechanistic approaches taken in this contribution.

Homogeneous Catalysis in Supercritical Fluids: Hydrogenation of Supercritical Carbon Dioxide to Formic Acid, Alkyl Formates, and Formamides

Rapid, selective, and high-yield hydrogenation of CO2 can be achieved if the CO2 is in the supercritical state (scCO2). Dissolving H2, a tertiary amine, a catalyst precursor such as RuH2[P(CH3)3]4 or RuCl2[P(CH3)3]4, and a promoting additive such as water, CH3OH, or DMSO in scCO2 at 50 °C leads to the generation of formic acid with turnover frequencies up to or exceeding 4000 h-1. In general, experiments in which a second phase was formed by one or more reagents or additives had lower rates of reaction. The high rate of reaction is attributed to rapid diffusion, weak catalyst solvation, and the high miscibility of H2 in scCO2. The formic acid synthesis can be coupled with subsequent reactions of formic acid, for example, with alcohols or primary or secondary amines, to give highly efficient routes to formate esters or formamides. With NH(CH3)2, for example 420 000 mol of dimethylformamide/mol of Ru catalyst was obtained at 100 °C. The demonstrated solubility and catalytic activity of complexes of tertiary...

In Situ Rapid Thermal Hydrogenation Pretreatment of Ti for Salicide RTA

AbstractAn in situ rapid thermal hydrogenation (RTH) pretreatment of titanium prior to rapid thermal annealing (RTA), or RTH/RTA, is proposed as a silicide formation annealing in a CMOS self-aligned silicide (salicide) process. The in situ RTH is found to enhance silicidation, to reduce nitridation, and even to lower the resultant sheet resistance of titanium silicide.During in situ RTH (e.g., at 550°C), amorphous Ti silicide (e.g., 15-nm thick) grows selectively on Si. Furthermore, Ti nitridation during subsequent RTA (690°C, N2, 10 Torr, 30 s) is reduced depending on RTH (H2, 10 Torr, 30 s) temperature. Accordingly, for 550°C RTH and an initial Ti thickness of 15 nm, the sheet resistance obtained at the 0.27-μm-wide n+ poly-Si gate after a phase transition annealing (800°C, Ar, 10 s) was lower (11.7 Ω /□, st. dev. = 6%) than that of conventional Ti silicide (15.8 Ω/□, st. dev. = 10%). The silicidation enhancement and nitridation reduction are related to crystal structure metamorphosis or to hydrogen interstitial incorporation in the Ti layer during RTH as observed by x-ray diffraction analysis. It is concluded that in situ RTH pretreatment before RTA is very promising as a sub-quarter-micron CMOS salicide process.