Uphill diffusion and overshooting in the adsorption of binary mixtures in nanoporous solids

Abstract

Under certain conditions, during binary mixture adsorption in nanoporous hosts, the concentration of one component may temporarily exceed its equilibrium value. This implies that, in contrast to Fick's Law, molecules must diffuse in the direction of increasing rather than decreasing concentration. Although this phenomenon of ‘overshooting' has been observed previously, it is only recently, using microimaging techniques, that diffusive fluxes in the interior of nanoporous materials have become accessible to direct observation. Here we report the application of interference microscopy to monitor ‘uphill' fluxes, covering the entire period of overshooting from initiation until final equilibration. It is shown that the evolution of the profiles can be adequately predicted from the single-component diffusivities together with the binary adsorption equilibrium data. The guest molecules studied (carbon dioxide, ethane and propene) and the host material (ZSM-58 or DDR) are of practical interest in relation to the development of kinetically selective adsorption separation processes.

Tracing the origins of transient overshoots for binary mixture diffusion in microporous crystalline materials

“…29 For describing the unary transport of bound moisture in wood, Babbitt 30,31 suggested the use of the gradient of the 19,20 for the transient development of molar loadings of (a) CO 2 (1), and (b) C 2 H 6 (2) along the radius of the DDR crystal monitored at different times from the start of the uptake. 29 For describing the unary transport of bound moisture in wood, Babbitt 30,31 suggested the use of the gradient of the 19,20 for the transient development of molar loadings of (a) CO 2 (1), and (b) C 2 H 6 (2) along the radius of the DDR crystal monitored at different times from the start of the uptake.…”

“…28 The hexagonal-shaped crystals of DDR zeolites, used in the Binder and Lauerer 19,20 experiments, are modelled as spherical crystals of radius r c . 28 The hexagonal-shaped crystals of DDR zeolites, used in the Binder and Lauerer 19,20 experiments, are modelled as spherical crystals of radius r c .…”

“…19,20 From the experimental data on the q 1 (r,t), and q 2 (r,t) in Fig. 19,20 From the experimental data on the q 1 (r,t), and q 2 (r,t) in Fig.…”

Multi‐Component Uptake of Dye Molecules by Films of Nanoporous Metal–Organic Frameworks

“…For instance, binary diffusion experiments were realized by using infrared microscopy analysing orthogonal vibration bands, for example, deuterated molecules were used to investigate self‐exchange in MOFs . In zeolites, another class of nanoporous crystalline materials, interference microscopy combined with an intelligent data analysis was used to investigate binary mixture diffusion and uphill‐diffusion effects were explored . Infrared microscopy data of the molecular uptake combined with theoretical simulations were used to determine the binary‐mixture diffusion properties and enable the comparison with experimental MOF‐membrane separation data .…”

IR Microimaging of Direction‐Dependent Uptake in MFI‐Type Crystals

“…Deviations of the host material from a regular crystal structure like the presence of stacking faults in the interior [14,15] or anomalies (surface barriers [16,17]) on their external surface are often found to be rate determining for mass transfer [18]. Knowledge about the influences of such structural peculiarities became accessible with the advent of the microscopic techniques of diffusion measurement [26 -30], notably with the recent introduction of microimaging by interference and IR microscopy [17,31,32]. Knowledge about the influences of such structural peculiarities became accessible with the advent of the microscopic techniques of diffusion measurement [26 -30], notably with the recent introduction of microimaging by interference and IR microscopy [17,31,32].…”