The Experts below are selected from a list of 84 Experts worldwide ranked by ideXlab platform
Robert J Davis - One of the best experts on this subject based on the ideXlab platform.
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evidence for the bifunctional nature of pt re catalysts for selective glycerol hydrogenolysis
ACS Catalysis, 2015Co-Authors: Derek D Falcone, John H Hack, Alexander Yu Klyushin, Axel Knopgericke, Robert Schlogl, Robert J DavisAbstract:Rhenium substantially promotes the rate of Pt-catalyzed glycerol hydrogenolysis to propanediols and shifts the product selectivity from 1,2-propanediol to a mixture of 1,2 and 1,3-propanediols. This work presents experimental evidence for a tandem dehydration–hydrogenation mechanism that occurs over a bifunctional Pt–Re catalyst. Infrared spectroscopy of adsorbed pyridine and the rate of aqueous-phase hydrolysis of propyl acetate were used to identify and quantify Bronsted acid sites associated with the Re component. Near-ambient-pressure XPS revealed a range of Re oxidation states on the Pt–Re catalysts after reduction in H2 at 393 and 493 K, which accounts for the presence of Bronsted acidity. A mechanism involving acid-catalyzed dehydration followed by Pt-catalyzed hydrogenation was consistent with the negative influence of added base, a primary kinetic isotope effect with deuterated glycerol, an inverse isotope effect with Dideuterium gas, and the observed orders of reaction.
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Evidence for the Bifunctional Nature of Pt–Re Catalysts for Selective Glycerol Hydrogenolysis
2015Co-Authors: Derek D. Falcone, John H Hack, Alexander Yu Klyushin, Axel Knop-gericke, Robert Schlögl, Robert J DavisAbstract:Rhenium substantially promotes the rate of Pt-catalyzed glycerol hydrogenolysis to propanediols and shifts the product selectivity from 1,2-propanediol to a mixture of 1,2 and 1,3-propanediols. This work presents experimental evidence for a tandem dehydration–hydrogenation mechanism that occurs over a bifunctional Pt–Re catalyst. Infrared spectroscopy of adsorbed pyridine and the rate of aqueous-phase hydrolysis of propyl acetate were used to identify and quantify Brønsted acid sites associated with the Re component. Near-ambient-pressure XPS revealed a range of Re oxidation states on the Pt–Re catalysts after reduction in H2 at 393 and 493 K, which accounts for the presence of Brønsted acidity. A mechanism involving acid-catalyzed dehydration followed by Pt-catalyzed hydrogenation was consistent with the negative influence of added base, a primary kinetic isotope effect with deuterated glycerol, an inverse isotope effect with Dideuterium gas, and the observed orders of reaction
Donald G Truhlar - One of the best experts on this subject based on the ideXlab platform.
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quantum effects on h2 diffusion in zeolite rho inverse kinetic isotope effect for sieving
Journal of the American Chemical Society, 2019Co-Authors: Rui Ming Zhang, Xuefei Xu, Donald G TruhlarAbstract:We use canonical variational theory (CVT) with small-curvature tunneling (SCT) contributions to investigate quantum effects on the H2 diffusion process in the pure-silica zeolite RHO. At low temperature we find an inverse kinetic isotopic sieving effect in that the heavier isotopic species diffuses faster than the lighter one. Three quantum effects contribute to this kinetic isotope effect (KIE). The first one is quantum mechanical tunneling; this—on its own—would lead to a normal kinetic isotopic sieving effect, in which lighter diprotium diffuses faster than Dideuterium. The second factor, which we find to dominate in the present case, is zero-point energy (ZPE). Deuterium has a lower ZPE, which leads to a smaller effective barrier for tunneling because the transition state has a larger ZPE than the precursor stable state; this results in an inverse KIE. The third factor, the thermal vibrational energy (computed from the quantized vibrational partition function), also favors a normal KIE, but it is outw...
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free energy surface reaction paths and kinetic isotope effect of short chain acyl coa dehydrogenase
Journal of Physical Chemistry B, 2003Co-Authors: Tina D Poulsen, Mireia Garciaviloca, Jiali Gao, Donald G TruhlarAbstract:We have applied molecular dynamics umbrella-sampling simulations and ensemble-averaged variational transition state theory with multidimensional tunneling (EA-VTST/MT) to explore the free energy surface, reaction paths, and Dideuterium kinetic isotope effect (KIE) of the β-oxidation of butyryl-coenzyme A by short-chain acyl-CoA dehydrogenase. The potential energy surface is obtained by combined quantum mechanics−molecular mechanics (QM/MM) with specific reaction parameters and a simple valence bond term. The calculations include determination of the potential of mean force (PMF) in both two dimensions (2D) and one dimension (1D) by using the weighted histogram analysis method. The 2D PMF indicates that the hydride transfer is the rate-limiting step of the mechanism, and the 1D PMFs are used to calculate rate constants. We include full molecular dynamics of a 7824-atom reaction zone with stochastic boundary conditions in a first approximation of the quasiclassical rate constant. This first approximation is...
Gerd Buntkowsky - One of the best experts on this subject based on the ideXlab platform.
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Coherent D2 rotational tunneling and incoherent D2 dynamics in a solid non-classical RuD2 complex studied by 2H solid state NMR spectroscopy
Physical Chemistry Chemical Physics, 1999Co-Authors: Frank Wehrmann, Tina P. Fong, Robert H. Morris, Hans-heinrich Limbach, Gerd BuntkowskyAbstract:The 2H solid state NMR spectra and T1 relaxation data of a transition metal η2-Dideuterium complex, namely trans-[Ru(D2)Cl(PPh2CH2CH2PPh2)2]PF6, have been measured in the temperature regime from 5.4 to 320 K. In the Ru–D2 sample, coherent and incoherent exchange processes on the time scale of the quadrupolar interaction have been found, leading at low temperatures to a tunnel splitting of the 2H NMR spectrum. With increasing temperature a slight increase of the tunnel splitting is observed, in conjunction with a strong increase of the incoherent exchange process, which finally, at temperatures above 20 K, destroys the tunnel splitting and determines the spectral line shape. For the description of the experimental spectra a Liouville formulation of the Alexander–Binsch NMR line shape theory, adapted for exchanging deuterons, is employed. It is shown that the whole evaluation of the 2H magnetization takes place in four 2D and two 4D subspaces of the 81D Liouville space, leading to a drastic simplification of the numerical efforts in the simulation of the spectral line shapes. The height of the tunnel barrier calculated from the value of the tunnel splitting is 270 meV (6.22 kcal mol-1). The incoherent exchange rates extracted from the spectra and from T1 relaxation data are analyzed in terms of a Bell tunneling model, with a temperature dependent effective potential.
Derek D Falcone - One of the best experts on this subject based on the ideXlab platform.
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evidence for the bifunctional nature of pt re catalysts for selective glycerol hydrogenolysis
ACS Catalysis, 2015Co-Authors: Derek D Falcone, John H Hack, Alexander Yu Klyushin, Axel Knopgericke, Robert Schlogl, Robert J DavisAbstract:Rhenium substantially promotes the rate of Pt-catalyzed glycerol hydrogenolysis to propanediols and shifts the product selectivity from 1,2-propanediol to a mixture of 1,2 and 1,3-propanediols. This work presents experimental evidence for a tandem dehydration–hydrogenation mechanism that occurs over a bifunctional Pt–Re catalyst. Infrared spectroscopy of adsorbed pyridine and the rate of aqueous-phase hydrolysis of propyl acetate were used to identify and quantify Bronsted acid sites associated with the Re component. Near-ambient-pressure XPS revealed a range of Re oxidation states on the Pt–Re catalysts after reduction in H2 at 393 and 493 K, which accounts for the presence of Bronsted acidity. A mechanism involving acid-catalyzed dehydration followed by Pt-catalyzed hydrogenation was consistent with the negative influence of added base, a primary kinetic isotope effect with deuterated glycerol, an inverse isotope effect with Dideuterium gas, and the observed orders of reaction.
Carl D Hoff - One of the best experts on this subject based on the ideXlab platform.
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why does d2 bind better than h2 a theoretical and experimental study of the equilibrium isotope effect on h2 binding in a m η2 h2 complex normal coordinate analysis of w co 3 pcy3 2 η2 h2
Journal of the American Chemical Society, 1997Co-Authors: Bruce R Bender, Gregory J Kubas, Llewellyn H Jones, Basil I Swanson, Juergen Eckert, Kenneth B Capps, Carl D HoffAbstract:Vibrational data (IR, Raman and inelastic neutron scattering) and a supporting normal coordinate analysis for the complex trans-W(CO)3(PCy3)2(η2-H2) (1) and its HD and D2 isotopomers are reported. The vibrational data and force constants support the well-established η2-bonding mode for the H2 ligand and provide unambiguous assignments for all metal−hydrogen stretching and bending frequencies. The force constant for the HH stretch, 1.3 mdyn/A, is less than one-fourth the value in free H2 and is similar to that for the WH stretch, indicating that weakening of the H−H bond and formation of W−H bonds are well along the reaction coordinate to oxidative addition. The equilibrium isotope effect (EIE) for the reversible binding of dihydrogen (H2) and Dideuterium (D2) to 1 and 1-d2 has been calculated from measured vibrational frequencies for 1 and 1-d2. The calculated EIE is “inverse” (1-d2 binds D2 better than 1 binds H2), with KH/KD = 0.78 at 300 K. The EIE calculated from vibrational frequencies may be resolve...