Application of microimaging to diffusion studies in nanoporous materials

Abstract

Microimaging on the basis of, respectively, interference microscopy and IR microscopy permit the observation of the distribution of guest molecules in nanoporous solids and their variation with time. Thus attainable knowledge of both concentration gradients and diffusion fluxes provides direct access to the underlying diffusion phenomena. This includes, in particular, the measurement of transport diffusion under transient, i. e. under non-equilibrium conditions, and of self- or tracer diffusion on considering the rate of tracer exchange. Correlating the difference in guest concentration close to the external surface to its equilibrium value with the influx into the nanoporous solid, microimaging does as well allow the direct determination of surface resistances. Examples illustrating the variety of information thus attainable include the comparison of mass transfer under equilibrium and non-equilibrium conditions, single- and multicomponent diffusion and chemical reactions. They, finally, introduce into the potentials of microimaging for an in-depth study of mass transfer in mixed-matrix membranes. This tutorial review may serve as first introduction into the topic. Further references are linked for the interested reader.

Diffusion in Nanoporous Solids in the Focus of IUPAC – A Tribute to Jens Weitkamp

“…Its key information, the probability distribution of molecular displacements during an observation time of milliseconds to seconds, is based on the observation of incredibly large molecular ensembles (on the order of 10 10 ), in contrast to single-molecule tracking (SMT [90]), by which one is able to trace diffusion paths of the individual molecules. Changes in the [96], NMR Imaging (MRI) [97], X-Ray Computed Tomography (XCT) [98] concentration profiles of the guest molecules within the individual crystals (rather than the positions of the individual molecules) are the focus of microimaging [91].…”

“…It is by this very reasoning that these differences should be expected to vanish in the limit of sufficiently small concentrations, i.e., under conditions where the interaction between the guest molecules becomes negligibly small -and distinction between equilibrium and non-equilibrium conditions becomes anyway meaningless [112]. Exactly this behavior has been observed in comparative studies using uptake and PFG NMR [75] and microimaging [91,113]. Refs.…”

“…Refs. [91,113], moreover, refer to the exceptional case where it is found that the (different) loading dependences of the coefficients of self-and transport diffusion can be derived from the system's equilibrium data, namely the adsorption isotherms. The prediction is based on the classical transition state theory [114], which may become applicable to determine the rate of mass transfer in nanoporous systems as soon as, for narrow-pore materials, the passage through the ''windows'' between adjacent cages becomes the limiting process [115][116][117].…”

“…Recording of the spatial distribution of the molecules involved in catalytic reactions and of their temporal dependence necessitates the combination of NMR spectroscopy with imaging techniques as presented and discussed in [97]. Similar information can be provided by the application of microimaging [91,127] via IR microscopy [91,127,128]. More extensive information about the process of catalytic conversion is provided by SPT [90], enabling monitoring of even the individual reaction path ways [129][130][131].…”

Evidence of Combined Heat-Transfer and Surface Barrier Resistances for Propane in Commercial ZIF-8 Crystals Probed by Pressure-Swing Frequency Response

“…With the recent advance of microimaging techniques, such as interference microscopy and IR microscopy (IRM) to follow the evolution of guest concentrations, it is possible to “observe” a surface barrier in the uptake and release by single large crystals/particles to provide a direct measurement of surface resistances. − The importance of external surface barriers for diffusion has been reported experimentally for n -hexane on mordenite and hydrocarbons on SAPO systems , and can become increasingly complicated due to variation in surface permeability within a single batch of particles/crystals …”