The Experts below are selected from a list of 141 Experts worldwide ranked by ideXlab platform
Enrique Iglesia - One of the best experts on this subject based on the ideXlab platform.
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elementary steps in acetone condensation reactions catalyzed by aluminosilicates with diverse Void Structures
Journal of Catalysis, 2017Co-Authors: Stanley Herrmann, Enrique IglesiaAbstract:The unselective nature and ubiquitous deactivation of Bronsted acids in aldol condensations have precluded their practical use and unequivocal mechanistic assessments. H2 and a Pt function are used here to confer stability by scavenging unsaturated intermediates to form stable products, thus allowing kinetic, spectroscopic, and isotopic assessments of elementary steps and their kinetic relevance, confirmed by density functional theory (DFT), for acetone condensation on microporous and mesoporous aluminosilicates (FER, TON, MFI, BEA, FAU, MCM-41). The selective titration of protons with 2,6-di-tert-butyl pyridine during catalysis shows that condensations occur exclusively on protons; the number of titrants required to suppress reactivity measures accessible sites and allows reactivity to be rigorously reported as turnover rates. Infrared spectra show that H-bonded acetone is present at saturation coverages during condensation catalysis. Taken together with rates that depend linearly on acetone pressure and with the absence of H/D kinetic isotope effects, these data indicate that condensation turnovers are mediated by the kinetically-relevant formation of a CC bond via reactions of H-bonded acetone with another acetone molecule, a conclusion confirmed by DFT-derived free energies along the reaction coordinate. Measured rate constants reflect free energy differences between this transition state and its relevant precursors, a H-bonded and a gaseous acetone. These rate constants (per H+) depend sensitively on size and shape of the confining Voids among aluminosilicates of similar acid strength but diverse framework structure. Confinement effects are mediated by van der Waals contacts and are accurately described by energies derived from Lennard-Jones potentials of DFT-derived transition state Structures; these energies are ensemble-averaged over all accessible configurations and T-site locations in each aluminosilicate framework. These energy descriptors replace incomplete metrics based solely on the sizes of Voids and transition states, which fail to capture differences in reactivity among different confining frameworks.
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elementary steps in acetone condensation reactions catalyzed by aluminosilicates with diverse Void Structures
Journal of Catalysis, 2017Co-Authors: Stanley Herrmann, Enrique IglesiaAbstract:The unselective nature and ubiquitous deactivation of Bronsted acids in aldol condensations have precluded their practical use and unequivocal mechanistic assessments. H2 and a Pt function are used here to confer stability by scavenging unsaturated intermediates to form stable products, thus allowing kinetic, spectroscopic, and isotopic assessments of elementary steps and their kinetic relevance, confirmed by density functional theory (DFT), for acetone condensation on microporous and mesoporous aluminosilicates (FER, TON, MFI, BEA, FAU, MCM-41). The selective titration of protons with 2,6-di-tert-butyl pyridine during catalysis shows that condensations occur exclusively on protons; the number of titrants required to suppress reactivity measures accessible sites and allows reactivity to be rigorously reported as turnover rates. Infrared spectra show that H-bonded acetone is present at saturation coverages during condensation catalysis. Taken together with rates that depend linearly on acetone pressure and with the absence of H/D kinetic isotope effects, these data indicate that condensation turnovers are mediated by the kinetically-relevant formation of a CC bond via reactions of H-bonded acetone with another acetone molecule, a conclusion confirmed by DFT-derived free energies along the reaction coordinate. Measured rate constants reflect free energy differences between this transition state and its relevant precursors, a H-bonded and a gaseous acetone. These rate constants (per H+) depend sensitively on size and shape of the confining Voids among aluminosilicates of similar acid strength but diverse framework structure. Confinement effects are mediated by van der Waals contacts and are accurately described by energies derived from Lennard-Jones potentials of DFT-derived transition state Structures; these energies are ensemble-averaged over all accessible configurations and T-site locations in each aluminosilicate framework. These energy descriptors replace incomplete metrics based solely on the sizes of Voids and transition states, which fail to capture differences in reactivity among different confining frameworks.
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from rays to Structures representation and selection of Void Structures in zeolites using stochastic methods
Microporous and Mesoporous Materials, 2013Co-Authors: Andrew J Jones, Enrique Iglesia, Christopher Ostrouchov, Maciej HaranczykAbstract:Abstract Voids within crystalline microporous solids are represented here using stochastic distributions of rays placed and oriented randomly within the accessible Void space, represented using Voronoi decompositions. This algorithm is provided in the Zeo++ software for open use. In this method, ray lengths are depicted as two-dimensional histograms that complement alternate descriptors, such as free and included sphere diameters. We illustrate the specific use of these methods as a tool to narrow the range of zeolites useful for a given catalytic application because of the shape and size of Voids. DAC, AFS, AFY, SFO and EON zeolites contain Void spaces similar, as suggested by Euclidean distance values between histograms, to those within MOR 8-MR side pockets, which stabilize the transition states that mediate dimethyl ether carbonylation to methyl acetate; these alternate Structures offer different connecting Void environments, which can enhance or restrict molecular access and influence the effectiveness of the 8-MR protons. NES, EON and USI zeolites exhibit histogram features similar to those of 12-MR MOR channels, where protons selectively catalyze alkylation of biphenyl and naphthalene to 4,4′-diisopropylbiphenyl and 2,6-diisopropylnaphthalene, respectively, with propene. SBT, FAU and SBS contain Voids similar in topology to the 12-MR channels of LTL zeolites, within which Pt clusters remain active and stable during the dehydrocyclization of light alkanes, but without the one-dimensional nature of LTL channels. The approach and implementation of these methods are applicable to any microporous or mesoporous solids and to adsorption processes driven by van der Waals contacts between hosts and guest molecules.
Patrick A.c. Gane - One of the best experts on this subject based on the ideXlab platform.
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porometry porosimetry image analysis and Void network modelling in the study of the pore level properties of filters
Chemical Engineering Science, 2011Co-Authors: Christopher M. Gribble, G P Matthews, Giuliano M. Laudone, Andrew Turner, Cathy J. Ridgway, Joachim Schoelkopf, Patrick A.c. GaneAbstract:We present fundamental and quantitative comparisons between the techniques of porometry (or flow permporometry), porosimetry, image analysis and Void network modelling for seven types of filter, chosen to encompass the range of simple to complex Void structure. They were metal, cellulose and glass fibre macro- and meso-porous filters of various types. The comparisons allow a general reappraisal of the limitations of each technique for measuring Void Structures. Porometry is shown to give unrealistically narrow Void size distributions, but the correct filtration characteristic when calibrated. Shielded mercury porosimetry can give the quaternary (sample-level anisotropic) characteristics of the Void structure. The first derivative of a mercury porosimetry intrusion curve is shown to underestimate the large number of Voids, but this error can be largely corrected by the use of a Void network model. The model was also used to simulate the full filtration characteristic of each sample, which agreed with the manufacturer’s filtration ratings. The model was validated through its correct a priori simulation of absolute gas permeabilities for track etch, cellulose nitrate and sintered powder filters. & 2011 Published by Elsevier Ltd.
G P Matthews - One of the best experts on this subject based on the ideXlab platform.
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porometry porosimetry image analysis and Void network modelling in the study of the pore level properties of filters
Chemical Engineering Science, 2011Co-Authors: Christopher M. Gribble, G P Matthews, Giuliano M. Laudone, Andrew Turner, Cathy J. Ridgway, Joachim Schoelkopf, Patrick A.c. GaneAbstract:We present fundamental and quantitative comparisons between the techniques of porometry (or flow permporometry), porosimetry, image analysis and Void network modelling for seven types of filter, chosen to encompass the range of simple to complex Void structure. They were metal, cellulose and glass fibre macro- and meso-porous filters of various types. The comparisons allow a general reappraisal of the limitations of each technique for measuring Void Structures. Porometry is shown to give unrealistically narrow Void size distributions, but the correct filtration characteristic when calibrated. Shielded mercury porosimetry can give the quaternary (sample-level anisotropic) characteristics of the Void structure. The first derivative of a mercury porosimetry intrusion curve is shown to underestimate the large number of Voids, but this error can be largely corrected by the use of a Void network model. The model was also used to simulate the full filtration characteristic of each sample, which agreed with the manufacturer’s filtration ratings. The model was validated through its correct a priori simulation of absolute gas permeabilities for track etch, cellulose nitrate and sintered powder filters. & 2011 Published by Elsevier Ltd.
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an improved simulation of Void structure water retention and hydraulic conductivity in soil with the pore cor three dimensional network
European Journal of Soil Science, 2003Co-Authors: A Johnson, I M Roy, G P Matthews, D PatelAbstract:Summary We present a new method of characterizing the Void Structures of soils from water retention curves as the primary source of data. The method aVoids the problems of other current approaches, which use smoothing curves and can miss the subtleties of soil structure, and usually ignore the shielding of large pores by the small connecting throats surrounding them. In the new method, software we have named ‘Pore-Cor’ is used to generate simple three-dimensional networks of Voids that have the same water retention characteristics and porosities as the soils. To find the geometry of the required networks, we have introduced a Boltzmann-annealed simplex which works in four parametric and three Boolean dimensions of parameter space. Also, a more robust measure of the difference between the experimental and simulated water retention curves has been developed. The method is applied to water retention curves for a wide range of English and Welsh soils, both experimental and generated from a pedotransfer function. The resulting simulated Void Structures have Void sizes that change as expected across the soil texture diagram, have different Structures as highlighted by the locations of retained water, but have connectivities (number of connecting throats per pore) that vary little. A wide range of other calculations of wetting and non-wetting fluid transport properties, and calculations of the behaviour of fluid-borne pollutants, are now possible. The main bar to further progress is a lack of sufficiently accurate and comprehensive data for water retention, and for saturated and unsaturated hydraulic conductivity.
Christopher M. Gribble - One of the best experts on this subject based on the ideXlab platform.
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characterisation of the porous structure of gilsocarbon graphite using pycnometry cyclic porosimetry and Void network modeling
Carbon, 2014Co-Authors: Giuliano M. Laudone, Christopher M. Gribble, Peter G MatthewsAbstract:Abstract The cores of the fourteen Advanced Gas-cooled nuclear Reactors (AGRs) within the UK comprise Gilsocarbon graphite, a manufactured material surrounded predominantly by CO 2 at high pressure and temperature to provide heat exchange. The intense ionising radiation within the reactors causes radiolytic oxidation, and the resulting mass loss is a primary factor in determining reactor lifetime. The Void structure of the porous Gilsocarbon graphite affects the permeability and diffusion of the carbon dioxide, and the sites of oxidation. To model this Void structure, the porosities and densities of ten virgin Gilsocarbon graphite samples have been measured by powder and helium pycnometry. For comparison, results are also presented for highly ordered pyrolytic graphite (HOPG), and a fine-grained Ringsdorff graphite. Samples have been examined at a range of magnifications by electron microscopy. Total porosities and percolation characteristics have been measured by standard and cyclic mercury porosimetry up to an applied mercury pressure of 400 MPa. Inverse modelling of the cyclic intrusion curves produces simulated Void Structures with characteristics which closely match those of experiment. Void size distributions of the Structures are presented, together with much Supplementary Information. The simulated Void networks provide the bases for future simulations of the radiolytic oxidation process itself.
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porometry porosimetry image analysis and Void network modelling in the study of the pore level properties of filters
Chemical Engineering Science, 2011Co-Authors: Christopher M. Gribble, G P Matthews, Giuliano M. Laudone, Andrew Turner, Cathy J. Ridgway, Joachim Schoelkopf, Patrick A.c. GaneAbstract:We present fundamental and quantitative comparisons between the techniques of porometry (or flow permporometry), porosimetry, image analysis and Void network modelling for seven types of filter, chosen to encompass the range of simple to complex Void structure. They were metal, cellulose and glass fibre macro- and meso-porous filters of various types. The comparisons allow a general reappraisal of the limitations of each technique for measuring Void Structures. Porometry is shown to give unrealistically narrow Void size distributions, but the correct filtration characteristic when calibrated. Shielded mercury porosimetry can give the quaternary (sample-level anisotropic) characteristics of the Void structure. The first derivative of a mercury porosimetry intrusion curve is shown to underestimate the large number of Voids, but this error can be largely corrected by the use of a Void network model. The model was also used to simulate the full filtration characteristic of each sample, which agreed with the manufacturer’s filtration ratings. The model was validated through its correct a priori simulation of absolute gas permeabilities for track etch, cellulose nitrate and sintered powder filters. & 2011 Published by Elsevier Ltd.
G E Morfill - One of the best experts on this subject based on the ideXlab platform.
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dust and dust Void Structures in the presence of the ion diffusion
NEW VISTAS IN DUSTY PLASMAS: Fourth International Conference on the Physics of#N#Dusty Plasmas, 2005Co-Authors: V N Tsytovich, S V Vladimirov, G E MorfillAbstract:The structure of the dust Void boundary and the distribution of the dust and plasma parameters in the dust structure bordering the Void is analyzed taking into account effects of dissipation due to the ion diffusion on plasma neutrals. Numerical calculations demonstrate various sorts of diffusive dust Void Structures; the possibility of singularities in the balance equations caused by the diffusion process inside the dust Structures is investigated. These singularities can be responsible for a new type of shock‐like Structures.
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theory of dust and dust Void Structures in the presence of the ion diffusion
Physical Review E, 2004Co-Authors: V N Tsytovich, S V Vladimirov, G E MorfillAbstract:A dust Void is a dust-free region inside the dust cloud that often develops for conditions relevant to plasma processing discharges and complex plasma experiments. A distinctive feature of the Void is a sharp boundary between the dust and dust-free regions; this is manifested especially clear when dissipation in the plasma is small and discontinuity of the dust number density appear. Here, the structure of the dust Void boundary and the distribution of the dust and plasma parameters in the dust structure bordering the Void is analyzed taking into account effects of dissipation due to the ion diffusion on plasma neutrals. The sharp boundary between the dust and Void regions exists also in the presence of the ion diffusion; however, only derivatives of the dust density, dust charge, electron density and electric field are discontinuous at the Void boundaries, while the functions themselves as well as derivatives of the ion drift velocity and the ion density are continuous. Numerical calculations demonstrate various sorts of diffusive dust Void Structures; the possibility of singularities in the balance equations caused by the diffusion process inside the dust Structures is investigated. These singularities can be responsible for a new type of shocklike Structures. Other Structures are typically self-organized to eliminate the singularities. Numerical computations in this case demonstrate a set of thin dust layers separated by high density thin dust clouds similar to the multiple-layer dust Structures observed in the laboratory and in the upper ionosphere. The possibility for existence of a few equilibrium positions of the Void boundary is discussed.
