Surface Barriers of Hydrocarbon Transport Triggered by Ideal Zeolite Structures

Zimmermann, Nils; Balaji, Sayee Prasaad; Keil, Frerich J.

doi:10.1021/jp2112389

Cited by 49 publications

(66 citation statements)

References 29 publications

(72 reference statements)

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“…This is because the importance of surface barriers scales inversely with membrane width. 20 As a final consequence, simple design protocols, such as membrane selectivity formulas on the mere basis of adsorption data and diffusion coefficients, are becoming increasingly inappropriate for the new generation of ultrathin membranes, calling for development of more accurate design models. …”

Section: ■ Conclusionmentioning

confidence: 99%

Predicting Local Transport Coefficients at Solid–Gas Interfaces

2012

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The regular nanoporous structure make zeolite membranes attractive candidates for separating molecules on the basis of differences in transport rates (diffusion). Since improvements in synthesis have led to membranes as thin as several hundred nanometers by now, the slow transport in the boundary layer separating bulk gas and core of the nanoporous membrane is becoming increasingly important. Therefore, we investigate the predictability of the coefficient quantifying this local process, the surface permeability α, by means of a two-scale simulation approach. Methane tracer-release from the one-dimensional nanopores of an AFI-type zeolite is employed. Besides a pitfall in determining α on the basis of tracer exchange, we, importantly, present an accurate prediction of the surface permeability using readily available information from molecular simulations. Moreover, we show that the prediction is strongly influenced by the degree of detail with which the boundary region is modeled. It turns out that not accounting for the fact that molecules aiming to escape the host structure must indeed overcome two boundary regions yields too large a permeability by a factor of 1.7−3.3, depending on the temperature. Finally, our results have far-reaching implications for the design of future membrane applications. ■ INTRODUCTIONMolecular exchange between a gas reservoir and a nanoporous crystalline solid (e.g., a zeolite membrane or crystal) represents a key design process in applications, such as adsorption, molecular-sieving, catalysis, and ion-exchange. Over the last few decades, a good understanding has been developed with regard to the role 1 and dependence 2−7 of guest diffusion in such regular host structures, that is, the transport mechanism of molecules inside the nanopores far away from the interface to the fluid phase. 8,9 For example, gas diffusion in zeolites is known to be an activated process 4−7 where molecules need to overcome a series of regularly distributed internal diffusion barriers which arise from nanopore shape in the unit cell alone. Many phenomena, including the loading-dependence of the self-diffusion coefficient, can be explained by the variation of such (free) energy barriers. In this context, molecular simulations have been proven to be invaluable, 4,5,10 owing to improved agreement with experiments.11−13 Despite these accomplishments, there are still unresolved problems, 14 many of which are related to the boundary layer separating the gasphase region from the core zeolite space.While cases exist in which the boundary layer may accelerate molecular exchange between the reservoir and the porous host, 15,16 it usually slows down the transport rate close to the surface, 6,7,17−20 leading to the name of this phenomenon: surface barriers. Exciting insights into their nature have been unraveled only recently. 6,7,20 Microscopy experiments 6 in conjunction with mesoscopic modeling 7 evidenced that exceptionally few accessible pore entrances together with a large number of lattice defects (i.e...

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Section: ■ Conclusionmentioning

confidence: 99%

Predicting Local Transport Coefficients at Solid–Gas Interfaces

2012

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“…An energy barrier exists when a molecule hops out of a pore mouth, because the molecule experiences distinct force fields at the sites on the external surface and in a pore mouth [18,19,22]. The molecule needs to overcome this high energy barrier before adsorbing on the external surface, which can cause surface barriers even for zeolites free of structural imperfections.…”

Section: The Nature Of External Surface Barriers In Zsm-5mentioning

confidence: 99%

Probing the Nature of Surface Barriers on ZSM‐5 by Surface Modification

Guo

Sun

et al. 2017

Chemie Ingenieur Technik

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The nature of external surface barriers in the zeolite ZSM-5 was explored by comparing mass transfer of n-heptane in samples with different crystal sizes, surface structure, and composition. These different surface properties were achieved by HF acid etching and SiO 2 deposition on an as-synthesized ZSM-5 crystal sample. HF etching does not reduce surface barriers. SiO 2 deposition on the same sample reduces surface barriers significantly. These insights into the nature of surface barriers could be used to guide the rational design of zeolitic materials with better mass transport properties.

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“…c) Intracrystalline diffusivities of ethane in the purely microporous specimen (NaCaA-0, circles) and in mesoporous NaCaA (NaCaA-2, triangles and NaCaA-5, diamonds), both with only ethane adsorbed (open symbols) and with co-adsorbed cyclohexane, (filled symbols) and a guest loading of about three molecules per supercage. [187] The broken line is a guide for the eyes representing the variation of the contribution of diffusion in the (unblocked) mesopores to the overall intracrystalline diffusivity.…”

Section: Pore Hierarchies: Interpenetrating Micro-and Mesoporesmentioning

confidence: 99%

“…In this context, molecular simulations of the rate of mass transfer through the interface between the genuine micropore space and the gas phase attain particular attention. [187]…”

Section: Pore Hierarchies: Interpenetrating Micro-and Mesoporesmentioning

confidence: 99%

Transport Phenomena in Nanoporous Materials

2014

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Diffusion, that is, the irregular movement of atoms and molecules, is a universal phenomenon of mass transfer occurring in all states of matter. It is of equal importance for fundamental research and technological applications. The present review deals with the challenges of the reliable observation of these phenomena in nanoporous materials. Starting with a survey of the different variants of diffusion measurement, it highlights the potentials of "microscopic" techniques, notably the pulsed field gradient (PFG) technique of NMR and the techniques of microimaging by interference microscopy (IFM) and IR microscopy (IRM). Considering ensembles of guest molecules, these techniques are able to directly record mass transfer phenomena over distances of typically micrometers. Their concerted application has given rise to the clarification of long-standing discrepancies, notably between microscopic equilibrium and macroscopic non-equilibrium measurements, and to a wealth of new information about molecular transport under confinement, hitherto often inaccessible and sometimes even unimaginable.

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Surface Barriers of Hydrocarbon Transport Triggered by Ideal Zeolite Structures

Cited by 49 publications

References 29 publications

Predicting Local Transport Coefficients at Solid–Gas Interfaces

Predicting Local Transport Coefficients at Solid–Gas Interfaces

Probing the Nature of Surface Barriers on ZSM‐5 by Surface Modification

Transport Phenomena in Nanoporous Materials

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