Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Hydrogenation catalysis”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 recordsLinked to original sources
PubMed2005

Effects of adsorbates on supported platinum and iridium clusters: Characterization in reactive atmospheres and during alkene hydrogenation catalysis by X-ray absorption spectroscopy.

MgO-, SiO2-, and gamma-Al2O3-supported platinum clusters and particles (with average diameters ranging from 11 to 45 A) and zeolite-supported Ir4 clusters (approximately 6 A in diameter) were characterized by extended X-ray absorption fine structure spectroscopy in the presence of H2, O2, ethene, propene, and ethane, as well as under conditions of alkene hydrogenation catalysis. The results indicate that under various atmospheres, the presence of adsorbates affects the smaller platinum clusters (11 A) on gamma-Al2O3 more substantially than the larger platinum particles (i.e., those greater than approximately 21 A in average diameter) on MgO or SiO2. When Pt/gamma-Al2O3 was exposed to H2, the platinum morphology did not change, although the Pt-Pt bond distance increased. In contrast, when the same sample was exposed to O2, complete oxidative fragmentation took place. This processes was reversed following subsequent treatment with H2. Exposure to alkenes changed both the morphology and electron density (as indicated by X-ray absorption near-edge spectra) of the gamma-Al2O3-supported platinum clusters. Under conditions of alkene hydrogenation catalysis at room temperature, the electronic properties and the structure of the platinum clusters were found to depend on the reactant composition and the nature of molecules involved in the reaction process. The effects of the reactant gases on the smaller iridium clusters (Ir4) were substantially less pronounced, apparently as a consequence of the extremely small number of atoms in each iridium cluster.

Journal Article↗
PubMed2006

Allosteric effects in asymmetric hydrogenation catalysis? Asymmetric induction as a function of the substrate and the backbone flexibility of C1-symmetric diphosphines in rhodium-catalysed hydrogenations.

The new unsymmetrical, optically active ligands 1,2-C(2)H(4)(PPh(2))(2'R,5'R-2',5'-dimethylphospholanyl) (L(a)) and 1,3-C(3)H(6)(PPh(2))(2'R,5'R-2',5'-dimethylphospholanyl) (L(b)) form complexes of the type [Rh(L)(cyclooctadiene)][BF(4)] where L = L(a) (1a) or L(b) (1b), [PtCl(2)(L)] where L = L(a) (2a) or L(b) (2b) and [PdCl(2)(L)] where L = L(a) (3a) or Lb (3b). The crystal structures of 2a and 2b show the chelate ligand backbones adopt delta-twist and flattened chair conformations respectively. Asymmetric hydrogenation of enamides and dehydroaminoesters using 1a and 1b as catalysts show that the ethylene-backboned diphosphine L(a) gives a more efficient catalyst in terms of asymmetric induction than the propylene-backboned analogue L(b). The greatest enantioselectivities were obtained with 1a and enamide substrates with ees up to 91%. Substrate-induced conformational changes in the Rh-diphosphine chelates are proposed to explain some of the ees observed in the hydrogenation of enamides.

Journal Article↗
PubMed2006

Testing electrostatic complementarity in enzyme catalysis: hydrogen bonding in the ketosteroid isomerase oxyanion hole.

A longstanding proposal in enzymology is that enzymes are electrostatically and geometrically complementary to the transition states of the reactions they catalyze and that this complementarity contributes to catalysis. Experimental evaluation of this contribution, however, has been difficult. We have systematically dissected the potential contribution to catalysis from electrostatic complementarity in ketosteroid isomerase. Phenolates, analogs of the transition state and reaction intermediate, bind and accept two hydrogen bonds in an active site oxyanion hole. The binding of substituted phenolates of constant molecular shape but increasing pK(a) models the charge accumulation in the oxyanion hole during the enzymatic reaction. As charge localization increases, the NMR chemical shifts of protons involved in oxyanion hole hydrogen bonds increase by 0.50-0.76 ppm/pK(a) unit, suggesting a bond shortening of 0.02 A/pK(a) unit. Nevertheless, there is little change in binding affinity across a series of substituted phenolates (DeltaDeltaG = -0.2 kcal/mol/pK(a) unit). The small effect of increased charge localization on affinity occurs despite the shortening of the hydrogen bonds and a large favorable change in binding enthalpy (DeltaDeltaH = -2.0 kcal/mol/pK(a) unit). This shallow dependence of binding affinity suggests that electrostatic complementarity in the oxyanion hole makes at most a modest contribution to catalysis of 300-fold. We propose that geometrical complementarity between the oxyanion hole hydrogen-bond donors and the transition state oxyanion provides a significant catalytic contribution, and suggest that KSI, like other enzymes, achieves its catalytic prowess through a combination of modest contributions from several mechanisms rather than from a single dominant contribution.

Anions↗
PubMed2002

A new conceptual framework for enzyme catalysis. Hydrogen tunnelling coupled to enzyme dynamics in flavoprotein and quinoprotein enzymes.

Recent years have witnessed high levels of activity in identifying enzyme systems that catalyse H-transfer by quantum tunneling. Rather than being restricted to a small number of specific enzymes as perceived initially, it has now become an accepted mechanism for H-transfer in a growing number of enzymes. Furthermore, H-tunneling is driven by the thermally induced dynamics of the enzyme. In some of those enzymes that break stable C-H bonds the reaction proceeds purely by quantum tunneling, without the need to partially ascend the barrier. Enzymes studied that fall into this category include the flavoprotein and quinoprotein amine dehydrogenases, which have proved to be excellent model systems. These enzymes have enabled us to study the relationship between barrier shape and reaction kinetics. This has involved studies with "slow" and "fast" substrates and enzymes impaired by mutagenesis. A number of key questions now remain, including the nature of the coupling between protein dynamics and quantum tunneling. The wide-ranging implications of quantum tunneling introduce a paradigm shift in the conceptual framework for enzyme catalysis, inhibition and design.

Catalysis↗
PubMed2001

Direct Comparison of Hydrogenation Catalysis by Intact versus Fragmented Triruthenium Clusters D.B. acknowledges financial support from the ORS Award Scheme. S.B.D. is grateful to the EPSRC, the JREI Scheme, and Bruker UK for financial support. P.J.D. thanks the Royal Society for a University Research Fellowship. Discussions with Prof. R. Eisenberg (NATO support), Prof. B. F. G. Johnson, Prof. R. N. Perutz, and Dr. R. J. Mawby are gratefully acknowledged.

Explore the source record for details and available documents.

Journal Article↗
PubMed2005

Iridium(0) nanocluster, acid-assisted catalysis of neat acetone hydrogenation at room temperature: exceptional activity, catalyst lifetime, and selectivity at complete conversion.

Acetone hydrogenation catalysis is important in applications such as heat pumps and fuel cells or in fulfilling the sizable demand for the product of selective acetone hydrogenation, 2-propanol. Reported herein is the discovery of a superior acetone hydrogenation catalyst--superior in terms of activity at low temperature, selectivity at complete conversion, and total catalyst lifetime. The new catalyst system consists of Ir(0)(n) nanoclusters plus HCl easily and reproducibly formed from commercially available [(1,5-COD)IrCl](2) under H(2). The resultant, room temperature, high activity, and highly selective (2/n)Ir(0)(n) plus 2HCl catalyst system hydrogenates acetone at 22 degrees C and 40 psig of H(2) pressure to 95% 2-propanol and the rest diisopropyl ether at 100% conversion with 16400 total catalytic turnovers and with an initial turnover frequency of 1.9 s(-1) at 22 degrees C. When molecular sieves are added, the catalyst system becomes even more selective and long-lived, providing the complete and selective conversion of acetone to 100% 2-propanol with 188000 total turnovers. Also reported are initial kinetic, D-labeling and other mechanistic studies, a summary section detailing the four main findings, the "green chemistry" aspects, and the current main drawback (a limited catalytic lifetime due to nanocluster precipitation) of the present invention. A review of the extensive literature of acetone hydrogenation is also tabulated as part of the Supporting Information.

Journal Article↗
PubMed2006

Polymers of intrinsic microporosity (PIMs): organic materials for membrane separations, heterogeneous catalysis and hydrogen storage.

This tutorial review describes recent research directed towards the synthesis of polymer-based organic microporous materials termed Polymers of Intrinsic Microporosity (PIMs). PIMs can be prepared either as insoluble networks or soluble polymers with both types giving solids that exhibit analogous behaviour to that of conventional microporous materials such as activated carbons. Soluble PIMs may be processed into thin films for use as highly selective gas separation membranes. Preliminary results also demonstrate the potential of PIMs for heterogeneous catalysis and hydrogen storage.

Catalysis↗
PubMed1970

Directional character of proton transfer in enzyme catalysis.

Hydrogen bonding can facilitate proton transfer in certain directions and retard proton transfer in certain other directions. By assuming that directed proton transfer along strategically oriented hydrogen bonds in the enzyme-substrate complex plays an important role in determining the efficiency and specificity of the enzyme, we present a unified interpretation for the reported observations on carbonic anhydrase and alpha-chymotrypsin.

Binding Sites↗
PubMed1984

Hydrogen bond catalysis of mononucleotide ethylation supports non-random DNA alkylations by N-ethyl, N-nitrosourea.

The axial (ax.) and equatorial (eq.) diastereomeric forms of phosphate triesters resulting from reactions of N-ethyl, N-nitrosourea with 3 cyclic mononucleotides were analyzed by column liquid chromatography (CLC). Evidence is presented that the 2'OH group of 3', 5'cAMP essentially contributes to the stereoselective eq. alkyl substitution, most probably by hydrogen bonding catalysis. The neighboring group direction of ethylation gives substantial support to non-random DNA alkylations by NEU.

Adenine Nucleotides↗
PubMed1978

Rotating disk membrane oxygenator based on transmembrane catalysis of hydrogen peroxide.

1) Oxygen transfer in the catalyst membrane oxygenator is not limited by diffusion through the blood-membrane boundary layer. Instead, oxygen transfer occurs at a rate independent of blood flow conditions and is evenly distributed over the membrane surface. Shear-induced mixing combined with high boundary O2 layer gradients are the mechanisms of transfer. 2) The rotating disk design provides very high level oxygen transfer. However, H2O2 concentration, blood flow rate, and disk rotation rate must be adjusted so that excess O2 supersaturation in the blood-membrane boundary does not lead to significant bubble formation. 3) The catalyst membranes used in the hemodialysis-based oxygenator have been industrially produced. 4) Effective CO2 transfer can be carried out across a hemodialysis type of membrane as previously reported. 5) The catalyst membrane oxygenator as studied in the dog performs well for 3-5 hrs, at which time inhibition of the catalyst leads to H2O2 breakthrough which causes methemoglobin formation. The mechanisms of catalyst inhibition and ways for its prevention are under study. 6) The catalyst membrane oxygenator offers the advantages of simultaneous artificial kidney treatment. This feature may be of help in maintaining acid-base balance and treating edema states.

Animals↗
PubMed2002

Anisole hydrogenation with well-characterized polyoxoanion- and tetrabutylammonium-stabilized Rh(0) nanoclusters: effects of added water and acid, plus enhanced catalytic rate, lifetime, and partial hydrogenation selectivity.

Following a comprehensive look at the arene hydrogenation literature by soluble nanocluster catalysts, six key, unfulfilled goals in nanocluster arene hydrogenation catalysis are identified. To begin to address those six goals, well-characterized polyoxoanion- and tetrabutylammonium-stabilized Rh(0) nanoclusters have been synthesized by the reduction of the precisely defined precatalyst [Bu(4)N](5)Na(3)[(1,5-COD)Rh small middle dotP(2)W(15)Nb(3)O(62)] with H(2) in propylene carbonate solvent. These Rh(0) nanoclusters are characterized by their stoichiometry of formation, transmission electron microscopy, and the two rate constants which characterize their mechanism of formation; previous studies in our laboratories have provided additional characterization of polyoxoanion-stabilized Rh(0) nanoclusters. Propylene carbonate solutions of the Rh(0) nanoclusters catalyze the hydrogenation of anisole (methoxybenzene) under mild conditions (22-78 degrees C, 30-40 psig H(2)). Proton donors such as water or HBF(4) small middle dotEt(2)O are discovered to affect both nanocluster formation and nanocluster arene hydrogenation catalysis. Under identical conditions, the Rh(0) nanoclusters are 10-fold more active than a commercially available, oxide-supported 5% Rh/Al(2)O(3) catalyst of the same average metal-particle size. A series of lifetime experiments shows that the Rh(0) nanoclusters are capable of at least 2600 total turnovers (TTO), a lifetime significantly longer than the approximately 100 TTO often seen for nanocluster arene hydrogenation catalysts, and a lifetime slightly better than the prior record of 2000 TTO for a literature nanocluster system. The present polyoxoanion-stabilized Rh(0) nanoclusters also display a record, albeit modest, 30% selectivity for the partial hydrogenation of anisole to 1-methoxycyclohexene with an overall yield of up to 8% at higher temperatures. In comparison to the 5% Rh/Al(2)O(3) catalyst, the polyoxoanion-stabilized nanoclusters yield a 4.7-fold higher maximum yield of 1-methoxycyclohexene. Finally, the seven main findings of the present work are summarized, including how they address five of the aforementioned six main goals in nanocluster arene hydrogenation.

Journal Article↗