Relationship between single-file diffusion of mixed and pure gases in dipeptide nanochannels by high field diffusion NMR

Abstract

High field NMR diffusometry reveals single-file diffusion of CO/CH4 mixture in dipeptide nanochannels with a coincident mobility for CO and CH4. In contrast to the relationship commonly observed for normal diffusion, this mixture mobility is only slightly smaller than that of pure CO which diffuses much faster than pure CH4.

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

“…11b) to test the applicability of Eq. 6 for SFD (Dutta et al 2015(Dutta et al , 2016. It is seen in the figure that the attenuation curves for each sorbate type in each sorbate/AV sample overlap across a broad range of diffusion times, which is consistent with Eq.…”

“…6 identical values of F are expected for mixtures as molecules are unable to pass one another. Qualitatively the same results were also obtained by PFG NMR for the diffusion of mixed and pure CO and CO 2 in AV (Dutta et al 2015(Dutta et al , 2016. Least-square fit of the combined data corresponding to all the attenuation curves measured in each sample (i.e.…”

“…In addition to investigations of normal and restricted selfdiffusion in ZIFs and mixed-matrix membranes, also studies of diffusion with anomalous diffusion mechanism in porous systems can significantly benefit from an application of high field and high gradient PFG NMR. One type of anomalous diffusion of interest is that of single-file diffusion (SFD; Kärger et al 2012;Nelson and Auerbach 1999;Adhangale and Keffer 2003;Dutta et al 2015). SFD occurs when nonintersecting channels are narrow enough such that they prohibit molecules from passing one another.…”

“…One major challenge in developing such SFDbased applications is related to the difficulty of studying molecular SFD experimentally, especially when SFD needs to be observed for molecular mixtures expected under typical conditions of separations and catalysis. Although experimental investigations of SFD of single component sorbates were reported more than two decades ago (Hahn et al 1996;Kukla et al 1996), the corresponding studies of SFD of mixtures of different types of molecules were performed only recently owing to the availability of high field and high magnetic field gradient PFG NMR (Dutta et al 2015(Dutta et al , 2016. In the latter studies, the high field of 17.6 T allowed observing sufficiently large signal of diffusing molecules even when using 13 C PFG NMR instead of a more traditional 1 H PFG NMR (Dutta et al 2015(Dutta et al , 2016).…”

“…It is important to note, however, that the signal-to-noise ratios in diffusion NMR studies performed with 13 C and 129 Xe at low-and midfield can become too low for obtaining meaningful diffusion data. We use high (up to 17.6 T) magnetic field in 13 C and 129 Xe pulsed field gradient (PFG) NMR diffusion studies to increase signal-to-noise ratios, which become larger with increasing field strength (Forman et al 2017(Forman et al , 2019(Forman et al , 2020Mueller et al 2016Mueller et al , 2015Baniani et al 2019;Berens et al 2018Berens et al , 2019Dutta et al 2015Dutta et al , 2016. As a result, using high field allows performing high-sensitivity diffusion studies with the 13 C and 129 Xe detection when there are no disturbing effects, such as chemical shift anisotropy relaxation effects, leading to a significantly faster NMR relaxation at a larger field.…”