Potentials and challenges of high-field PFG NMR diffusion studies with sorbates in nanoporous media

Abstract

High magnetic fields (up to 17.6 T) in combination with large magnetic field gradients (up to 25 T/m) were successfully utilized in pulsed field gradient (PFG) NMR studies of gas and liquid diffusion in nanoporous materials. In this mini-review, we present selected examples of such studies demonstrating the ability of high field PFG NMR to gain unique insights and differentiate between various types of diffusion. These examples include identifying and explaining an anomalous relationship between molecular size and self-diffusivity of gases in a zeolitic imidazolate framework (ZIF), as well as revealing and explaining an influence of mixing different linkers in a ZIF on gas self-diffusion. Different types of normal and restricted selfdiffusion were quantified in hybrid membranes formed by dispersing ZIF crystals in polymers. High field PFG NMR studies of such membranes allowed observing and explaining an influence of the ZIF crystal confinement in a polymer on intra-ZIF self-diffusion of gases. This technique also allowed measuring and understanding anomalous single-file diffusion (SFD) of mixed sorbates. Furthermore, the presented examples demonstrate a high potential of combining high field PFG NMR with single-crystal infrared microscopy (IRM) for obtaining greater physical insights into the studied diffusion processes.

Guidelines for Techno-Economic Analysis of Adsorption Processes

“…ROA data can usually be obtained simultaneously with isotherm data on most commonly-available isotherm measurement apparatus. PFG NMR is also a valid alternative (Hwang and Karger, 2019;Baniani et al, 2020), however, it is currently not a readily accessible technique.…”

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

“…Their spatial resolution is, correspondingly, in the range of micrometers as a typical size of nanoporous crystals. Their most prominent representative is the pulsed field gradient (PFG) NMR technique [88,89]. 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.…”

“…The thus attainable information may indeed be referred to as meso-rather than macroscopic. Vice versa, on considering, e.g., sufficiently large observation times, PFG NMR diffusion studies [88,89] must, more properly, be referred to as mesoor even macroscopic rather than as microscopic techniques. It is, most importantly, a particular virtue of QENS [87] to monitor, under appropriate conditions, not only the elementary steps of diffusion but also their superposition over distances providing clear evidence of Fickian diffusion, i.e., on displacements over ''microscopic'' dimensions.…”

“…This concerns, in particular, a decision about the limiting steps of mass transfer and their evaluation as a function of the time-on-stream in the industrial device. In this context, the message provided by PFG NMR [88,89] has proven to be especially informative. Examples of its benefit include the investigation of A-type zeolites [18] where the industrial process of pellet formulation was found to give rise to the formation of transport resistances on the external surfaces of the individual crystals (surface barriers [105]).…”

Diffusion and reaction in pore hierarchies by the two-region model

“…Such a possibility is provided within the frame of the so-called two-region model of diffusion. Originally introduced for quantifying PFG NMR diffusion measurements (see [3,40] in this Thematic Issue) in beds of zeolite crystallites [37,38,43], it has meanwhile found widespread application in compartmented systems quite in general [1,5,71,90]. This refers, in particular, to its application to diffusion studies with assemblages of cells in organic tissues [27,66,93], where the term "Kärger model" [22,70,92] has been coined for this approach.…”