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 self-diffusion 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.
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Ethane diffusion in mixed linker zeolitic imidazolate framework-7-8 by pulsed field gradient NMR in combination with single crystal IR microscopy
Pulsed field gradient (PFG) NMR was used in combination with single crystal IR microscopy (IRM) to study diffusion of ethane inside crystals of a mixed linker zeolitic imidazolate framework (ZIF) of the type ZIF-7-8 under comparable experimental conditions. These crystals contain 2-methylimidazolate (ZIF-8 linker) and benzimidazolate (ZIF-7 linker). It was observed that the PFG NMR attenuation curves measured for ethane in ZIF-7-8 exhibit deviations from the monoexponential behaviour, thereby indicating that the ethane self-diffusivity in different crystals of a crystal bed can be different. Measurements of the ethane uptake curves performed by IRM under the same conditions in different ZIF-7-8 crystals of the bed yield different transport diffusivities thus confirming that the rate of ethane diffusion is different in different ZIF-7-8 crystals. The IRM observation that the fractions of ZIF-8 and ZIF-7 linkers are different in different ZIF-7-8 crystals allowed attributing the observed heterogeneity in diffusivities to the heterogeneity in the linker fraction. The quantitative comparison of the average ethane self-diffusivities measured by PFG NMR in ZIF-7-8 with the corresponding data on corrected diffusivities from IRM measurements revealed a good agreement between the results obtained by the two techniques. In agreement with the expectation of smaller aperture sizes in ZIF-7-8 than in ZIF-8, the average ethane self-diffusivities in ZIF-7-8 were found to be significantly lower than the corresponding self-diffusivities in ZIF-8.
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- Award ID(s):
- 1561347
- NSF-PAR ID:
- 10079254
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 20
- Issue:
- 37
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 23967 to 23975
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Graphical Abstract -
Archaean orbicular granitoids from western Australia were investigated to better understand crystal growth processes. The orbicules are dioritic to tonalitic spheroids dispersed in a granitic host magma. Most orbicules have at least two to three concentric bands composed of elongate and radially oriented hornblendes with interstitial plagioclase. Each band consists of a hornblende-rich outer layer and a plagioclase-rich inner layer. Doublet band thicknesses increase, crystal number density decreases, and grain size increases from rim to core, suggesting crystallization was more rapid on the rims than in the core. Despite these radial differences, mineral mode and bulk composition of each band are similar, indicating limited crystal-melt segregation during crystallization. These observations lead us to suggest that the orbicules represent slowly quenched blobs of hot dioritic to tonalitic liquids injected into a cooler granitic magma. The oscillatory bands in the orbicules can be explained by rapid, disequilibrium crystallization (supercooling). In particular, a linear correlation between bandwidth and radial distance from orbicule rim can be explained by transport-limited crystallization, wherein crystallization timescales are shorter than chemical diffusion timescales. The slope of this linear relationship corresponds to the square root of the ratio between effective chemical diffusivity in the growth medium and thermal diffusivity, resulting in effective chemical diffusivities of 3 × 10−8 m2/s. These high effective diffusivities require static diffusion through a free volatile phase (fluid) and/or a strong advective/convective component in the fluid. Regardless of the mechanisms, these effective diffusivities can be used to estimate growth rates of ~10−6 m/s or 0.4 cm/hr. Our results indicate that crystals can grow rapidly, possibly facilitated by fluids and dynamic conditions. These rapid growth rates suggest that centimetre or larger crystals, such as in porphyritic and pegmatitic systems, can conceivably grow within days.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c8cp04889d
