Reactivity Enhancement by Molecular Traffic Control

Neugebauer, N.; Bräuer, Peter; Kärger, Jörg

doi:10.1006/jcat.2000.2963

Cited by 38 publications

(20 citation statements)

References 16 publications

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“…It is thought that the origin of MTC lies in the mutual correlation in the movement of a multicomponent fluid through two types of pores (34). MTC has never been convincingly established and has remained a controversial subject for over two decades now, although recently some theoretical progress has been achieved (35)(36)(37). The current work demonstrates only how the diffusivity of one component may vary between pore systems in the same zeolite.…”

Section: Discussionmentioning

confidence: 76%

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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Section: Discussionmentioning

confidence: 76%

Molecular path control in zeolite membranes

Dubbeldam

Beerdsen

Calero

et al. 2005

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

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“…The diffusivities of the molecules were assumed to have an Arrhenius dependence on temperature and it was indeed possible to obtain quantitative agreement with the experimental desorption profile within this simple model [22]. Starting with the work of Kärger and coworkers [3,4,10] the conditions of reactivity enhancement of the catalyst using MTC were investigated by a series of dynamical Monte Carlo simulations carried out on stochastic model systems involving networks of channels with NBK topology. This is a two-dimensional network of perpendicular sets of intersecting channels where the vertical channels carry reactant molecules (a-channels) and the horizontal ones carry product molecules (b-channels) as shown in Fig.…”

Section: Earlier Studies On Molecular Traffic Control and Single-filementioning

confidence: 99%

“…When a reaction occurs, the reactant and product molecules mutually block each other and this results in a low output of product molecules from the catalytic grain. To avoid this adverse side-effect of single-file diffusion, the concept of reaction enhancement by molecular traffic control (MTC) was suggested by Kärger and collaborators [3,4]. The main idea of MTC, originally proposed in [5,6], is that inside the catalytic grain the reactant and product molecules diffuse through different channels, thereby avoiding the mutual suppression of self-diffusion.…”

Section: Introductionmentioning

confidence: 98%

Strong Reactivity Enhancement through Molecular Traffic Control in Zeolites

Chatterjee

Harish²,

Schütz

2013

Chemie Ingenieur Technik

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In the past, the investigation of catalytic reactivity enhancement through molecular traffic control (MTC) by means of dynamical Monte Carlo simulations of catalysis was initiated in simple, idealized zeolite channel networks. These results are reviewed here and, emphasizing more recent work, the conditions of reactivity enhancement through MTC are examined for a realistic channel topology based on the pore structure of the zeolite TNU-9. For a wide range of reaction rates a very strong MTC effect can be found that increases with grain size. This effect is argued to be a generic feature of a certain class of zeolite pore topologies.

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“…15 Moreover, considering arrays of mutually intersecting sets of parallel single-file systems, 16 in Ref. 17 for the first time a model system has been established which allowed a quantitative estimation of the possible benefit of MTC for catalytic performance.…”

Section: Introductionmentioning

confidence: 99%

“…17 for the first time a model system has been established which allowed a quantitative estimation of the possible benefit of MTC for catalytic performance. 16 As a most simple case, the unidirectional, monomolecular reaction A → B was considered, where the reaction was assumed to take place exclusively on intersection sites. In the so-called reference system ͑REF͒, the molecular species A and B were accommodated by all channels with equal probability.…”

Section: Introductionmentioning

confidence: 99%

Two-component desorption from anisotropic pore networks

Bräuer

Brzank²,

Kärger³

2006

The Journal of Chemical Physics

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Dynamic Monte Carlo simulations are performed to explore molecular desorption of a two-component mixture from a mode adsorbent of adsorbate-related anisotropy. The intrinsic dynamics of the adsorbent-adsorbate host-guest system is described by assuming a situation typical of a "molecular-traffic-control" system, where the hopping rates of the two components in two directions, perpendicular to each other, are identical, while for each component, perpendicular to that direction of preferential propagation, the hopping rates are reduced. The resulting desorption patterns are discussed in terms of the jump probabilities and are shown to approach the corresponding analytical solutions in the limiting cases of isotropic and unidirectional diffusions.

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Reactivity Enhancement by Molecular Traffic Control

Cited by 38 publications

References 16 publications

Molecular path control in zeolite membranes

Molecular path control in zeolite membranes

Strong Reactivity Enhancement through Molecular Traffic Control in Zeolites

Two-component desorption from anisotropic pore networks

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