Random walk through two-channel networks: a simple means to correlate the coefficients of anisotropic diffusion in ZSM-5 type zeolites

Kaerger, Joerg

doi:10.1021/j100167a036

Cited by 132 publications

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“…Computer simulations (June et al, 1990;Hufton, 1991) and PFG-NMR measurements (Karger, 1991) have shown that the amount of transport in the z direction is relatively small. As a rough approximation, assume that the resistance to transport along z is infinite.…”

Section: Interpretation Of Experimental Resultsmentioning

confidence: 99%

“…Comparison of the shape of this response with a suitable mathematical model allows determination of the micropore diffusion coefficient. This technique has been criticized because the rate of heat transfer resulting from the exothermic adsorption process frequently controls the system kinetics (Ruthven, 1984, p. 189;Karger andRuthven, 1981, 1989).…”

Section: Introductionmentioning

confidence: 99%

“…The pulsed field gradient nuclear magnetic resonance (PFG-NMR) experiment developed by Karger andPfeifer (1987, 1988) and Karger et al (1983) is the most common microscopic technique for measuring micropore diffusion coefficients. This method utilizes NMR principles to monitor the displacement of molecules in an equilibrated system.…”

Section: Introductionmentioning

confidence: 99%

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Chromatographic study of alkanes in silicalite: Transport properties

Hufton

Danner

1993

AIChE Journal

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Section: Interpretation Of Experimental Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Chromatographic study of alkanes in silicalite: Transport properties

Hufton

Danner

1993

AIChE Journal

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“…In these simulations, diffusion is considered as a hopping process on a loading-independent lattice of adsorption sites. Previous studies have shown that in three-dimensional systems, such as MFI, geometric correlations are present that are difficult to capture in a lattice model [11,22,23]. Here we have shown that, on top of these problems, as the effective topology changes as a function of loading, these correlations change too, making lattice model simulations highly impractical.…”

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confidence: 83%

Molecular Understanding of Diffusion in Confinement

2005

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We introduce a computational method to directly relate diffusivities to the microscopic behavior of the adsorbed molecules. We apply this method to gases in an MFI-type molecular sieve, the reference system in this field. Transitions in the number and nature of adsorption sites result in temporary local increases in the diffusion. This occurs at different loadings in each of the x, y, and z directions, giving rise to the complex loading behavior found experimentally. Our method can be applied to any adsorbent-adsorbate system, and provides a fundamental understanding of diffusion in confinement on a molecular level.

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“…This is particularly true for loading effects in materials with different channels and͞or cages in the x, y, and z direction. Anisotropic single-component diffusion in silicalite has been known for a long time (8)(9)(10)(11)(12). In general, the diffusion coefficients in the different directions can have different dependencies on temperature and loading.…”

mentioning

confidence: 99%

Molecular path control in zeolite membranes

Dubbeldam

Beerdsen

Calero

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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We report molecular simulations of diffusion in confinement showing a phenomenon that we denote as molecular path control (MPC); depending on loading, molecules follow a preferred pathway. MPC raises the important question to which extent the loading may affect the molecular trajectories in nanoporous materials. Through MPC one is able to manually adjust the ratio of the diffusivities through different types of pores, and as an application one can direct the flow of diffusing particles in membranes forward or sideward by simply adjusting the pressure, without the need for mechanical parts like valves. We show that the key ingredient of MPC is the anisotropic nature of the nanoporous material that results in a complex interplay between different diffusion paths as a function of loading. These paths may be controlled by changing the loading, either through a change in pressure or temperature.anomalous diffusion ͉ nanoporous materials ͉ statistical physics A mong other emerging membrane technologies like polymerinorganic composites, carbon films, and micro-and mesoporous silica films, zeolite membranes offer outstanding potential for molecular recognition at the subnanometer level and the ability to operate at high temperatures (1, 2). Zeolites are crystalline structures made up of ''T-atoms,'' where T is an aluminum or silicon atom, which are tetrahedrally bonded to each other with oxygen bridges. Because of the regularity of the crystalline structure and the pores with angstrom-size dimensions, these crystals, when grown together to form a membrane, can operate as separation devices for gas and liquid mixtures. From a scientific point of view zeolites are ideal systems to study the effect of confinement on the properties of the adsorbed molecules.Transport of adsorbates in nanoporous adsorbents such as zeolites is determined by a complex interplay between adsorbent-adsorbate and adsorbate-adsorbate interactions. Molecules diffuse through the pores via various diffusion mechanisms (3). Although interesting effects like single-file diffusion (4), incommensurate diffusion (5, 6), and levitation effects (7) are well known, most of the effects of confinement on diffusion remain poorly understood. This is particularly true for loading effects in materials with different channels and͞or cages in the x, y, and z direction. Anisotropic single-component diffusion in silicalite has been known for a long time (8-12). In general, the diffusion coefficients in the different directions can have different dependencies on temperature and loading. A limited number of studies deal with nonzero loading. Bussai et al. (13) found little change in anisotropy for water in silicalite as a function of loading. In this article, we report a reversal of anisotropy, i.e., at low loading the diffusivity in the z direction is two times faster than in the xy direction for both the self and collective diffusivity, whereas for higher loadings this changes into a z diffusivity that is more than two times slower. This behavior is due to a complete ...

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Random walk through two-channel networks: a simple means to correlate the coefficients of anisotropic diffusion in ZSM-5 type zeolites

Cited by 132 publications

References 0 publications

Chromatographic study of alkanes in silicalite: Transport properties

Chromatographic study of alkanes in silicalite: Transport properties

Molecular Understanding of Diffusion in Confinement

Molecular path control in zeolite membranes

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