Correlating Surface Permeability with Intracrystalline Diffusivity in Nanoporous Solids

Heinke, Lars; Kärger, Jörg

doi:10.1103/physrevlett.106.074501

Cited by 92 publications

(105 citation statements)

References 32 publications

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“…Previous IFM studies [31][32][33][34][35] have shown that, for many zeolite systems, the sorption kinetics are significantly affected by surface resistance. It was not possible to investigate this aspect in the present study due to the shape of the DDR crystals which precludes accurate measurements close to the external surface.…”

Section: Discussionmentioning

confidence: 99%

Micro-imaging of transient guest profiles in nanoporous host systems of cylindrical symmetry

Binder

Hibbe

Chmelik

et al. 2012

The Journal of Chemical Physics

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Nanoporous host materials giving rise to transient guest profiles of cylindrical symmetry during molecular uptake and release are shown to provide particularly advantageous conditions for the study of guest diffusion by micro-imaging. Considering zeolites of structure type DDR (Deca-dodecasil 3R) as a host system and short-chain length hydrocarbons as guest molecules, the benefits thus attainable in micro-imaging studies using interference microscopy are shown to include the determination of transient concentration profiles with improved accuracy, the option to overcome the disturbing impact of surface imperfections, and easy access to concentration-dependent diffusivities.

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Section: Discussionmentioning

confidence: 99%

Micro-imaging of transient guest profiles in nanoporous host systems of cylindrical symmetry

Binder

Hibbe

Chmelik

et al. 2012

The Journal of Chemical Physics

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“…Previous studies 35,36 have shown that the transport resistance of surface barriers results from a total blockage of the pore entrances and that the uptake and release occurs by detours via unblocked pores, which remain fully opened. This means water molecules destroy the MOF structure close to the crystal surface, resulting in blocked pore entrances that have to be bypassed by the guest molecules during the mass transfer, causing the additional mass transfer resistance referred to as surface barriers.…”

Section: Discussionmentioning

confidence: 99%

The surface barrier phenomenon at the loading of metal-organic frameworks

2014

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Loading with guest molecules is a crucial step for most applications of porous materials. For metal-organic frameworks, which are one of the most intensely investigated classes of porous materials, the experimentally determined rate of mass transfer into the material may vary by several orders of magnitude for different samples of the same material. This phenomenon is commonly attributed to the presence of so-called surface barriers, which appear to be omnipresent but poorly understood. Here we quantitatively study this phenomenon with a quartz crystal microbalance, using well-defined, highly crystalline, epitaxially grown thin films of metal-organic frameworks as a model system. Our results clearly demonstrate that surface barriers are not an intrinsic feature of metal-organic frameworks, as pristine films do not exhibit these limitations. However, by destroying the structure at the outer surface, for instance by exposure to air or water vapour, surface barriers are created and the molecular uptake rate is reduced.

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“…In addition, we provide a linear fit to the data points by which means we have determined the mean surface permeability to 0.383 m/s and we show also a surface permeability prediction on the basis of the self-diffusion coefficient 43 (crosses). It is furthermore instructive to mention that the corrected diffusivity is identical to the self-diffusion coefficient in the concentration range of Figure S11.…”

Section: S22mentioning

confidence: 99%

Transport into Nanosheets: Diffusion Equations Put to Test

2013

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Ultrathin porous materials, such as zeolite nanosheets, are prominent candidates for performing catalysis, drug supply, and separation processes in a highly efficient manner due to exceptionally short transport paths. Predictive design of such processes requires the application of diffusion equations that were derived for macroscopic, homogeneous surroundings to nanoscale, nanostructured host systems. Therefore, we tested different analytical solutions of Fick’s diffusion equations for their applicability to methane transport into two different zeolite nanosheets (AFI, LTA) under instationary conditions. Transient molecular dynamics simulations provided hereby concentration profiles and uptake curves to which the different solutions were fitted. Two central conclusions were deduced by comparing the fitted transport coefficients. First, the transport can be described correctly only if concentration profiles are used and the transport through the solid–gas interface is explicitly accounted for by the surface permeability. Second and most importantly, we have unraveled a size limitation to applying the diffusion equations to nanoscale objects. This is because transport-diffusion coefficients, D T, and surface permeabilities, α, of methane in AFI become dependent on nanosheet thickness. Deviations can amount to factors of 2.9 and 1.4 for D T and α, respectively, when, in the worst case, results from the thinnest AFI nanosheet are compared with data from the thickest sheet. We present a molecular explanation of the size limitation that is based on memory effects of entering molecules and therefore only observable for smooth pores such as AFI and carbon nanotubes. Hence, our work provides important tools to accurately predict and intuitively understand transport of guest molecules into porous host structures, a fact that will become the more valuable the more tiny nanotechnological objects get.

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Correlating Surface Permeability with Intracrystalline Diffusivity in Nanoporous Solids

Cited by 92 publications

References 32 publications

Micro-imaging of transient guest profiles in nanoporous host systems of cylindrical symmetry

Micro-imaging of transient guest profiles in nanoporous host systems of cylindrical symmetry

The surface barrier phenomenon at the loading of metal-organic frameworks

Transport into Nanosheets: Diffusion Equations Put to Test

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