The Nature of Surface Barriers on Nanoporous Solids Explored by Microimaging of Transient Guest Distributions

Abstract

Nanoporous solids are attractive materials for energetically efficient and environmentally friendly catalytic and adsorption separation processes. Although the performance of such materials is largely dependent on their molecular transport properties, our fundamental understanding of these phenomena is far from complete. This is particularly true for the mechanisms that control the penetration rate through the outer surface of these materials (commonly referred to as surface barriers). Recent detailed sorption rate measurements with Zn(tbip) crystals have greatly enhanced our basic understanding of such processes. Surface resistance in this material has been shown to arise from the complete blockage of most of the pore entrances on the outer surface, while the transport resistance of the remaining open pores is negligibly small. More generally, the revealed correlation between intracrystalline diffusion and surface permeation provides a new view of the nature of transport resistances in nanoporous materials acting in addition to the diffusion resistance of the regular pore network, leading to a rational explanation of the discrepancy which is often observed between microscopic and macroscopic diffusion measurements.

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

“…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.…”

Transport into Nanosheets: Diffusion Equations Put to Test

“…The simulation set-up mimics situations encountered in diffusion measurements, such as uptake experiments, as well as interference 1 and infrared microscopy, 1 where the zeolite crystal is initially empty and then starts to be filled with gas molecules as time proceeds. In the experiments, the huge surrounding that is several magnitudes larger than the pore volume of the small crystal sample forms practically an infinite reservoir of gas molecules that can enter the pores of the solid.…”

“…Every ten MD steps, five molecule insertion as well as five molecule deletion trials were performed inside the control volume. The acceptance probability of S4 an insertion trial to the control volume, acc(N cv → N cv + 1), reads 2,3 acc(N cv → N cv + 1) = min 1, V cv Λ 3 (N cv + 1) exp β µ cv −U (N cv + 1) +U (N cv ) (1) where N cv denotes the number of molecules in the control volume, V cv its size, Λ the thermal de Broglie wavelength which equals h 2 /(2πmk B T ), h is Planck constant, m the mass of the just inserted particle, k B Boltzmann constant, T the temperature imposed, U (N cv ) and U (N cv + 1) the total potential energy of the old (N cv ) and new (N cv +1) state, respectively, as well as β = 1/(k B T ).…”

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

“…For silicalite-1 with MFI structure, Dauenhauer et al [7,8,13] attributed its surface barriers primarily to the blockage of surface pores and estimated that over 99.9 % of its surface pores could be blocked, using ZLC and FR experiments, as well as molecular simulations. For Zn(tbip), a nanoporous metal-organic framework (MOF), Kärger et al [14,15] reached a similar conclusion that the surface barriers on this material arise from the complete blockage of surface pores, using microscopic diffusion measurements and mesoscopic modeling. The exact nature of surface pore blockage is still uncertain.…”