Anomalous Relationship between Molecular Size and Diffusivity of Ethane and Ethylene inside Crystals of Zeolitic Imidazolate Framework-11

Abstract

Multinuclear pulsed field gradient (PFG) NMR was used to study the self-diffusion of ethane and ethylene inside loosely packed beds of zeolitic imidazolate framework-11 (ZIF-11) crystals. Diffusion measurements were performed at different temperatures under conditions where the length scales of displacements were smaller than or comparable with the mean size of the ZIF-11 crystals. For the crystal beds loaded with a single sorbate, these studies showed a larger intracrystalline diffusivity for ethane than that for ethylene under the same or comparable experimental conditions, an unexpected result due to the larger size of ethane compared to that of ethylene. PFG NMR diffusion studies of ZIF-11 beds loaded with ethane/ethylene mixtures revealed that substituting a fraction of ethane molecules by ethylene molecules decreases the intracrystalline diffusivity of ethane. These results in combination with an observation of a higher-for-ethylene-than-for-ethane activation energy of intracrystalline self-diffusion in single-sorbate systems suggest a hindering effect of ethylene molecules on the intra-ZIF diffusion. To further confirm and investigate the anomalous relationship between ethane and ethylene intracrystalline diffusivities in ZIF-11, uptake curves for ethane and ethylene were measured in single crystals using IR microscopy and in a thin crystal bed using a volumetric technique. The diffusion data obtained from these complementary uptake studies under the same or comparable conditions were found to be consistent with the PFG NMR measurements. The observed anomalous relationship between ethane and ethylene diffusivities in ZIF-11 is discussed in the context of the flexibility of the ZIF-11 framework.

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

“…Owing to the application of high field, it often becomes possible to verify our diffusion data by comparing the results obtained for the same samples and sorbates with different types of nuclei ( 13 C and 1 H) (Forman et al 2019(Forman et al , 2020(Forman et al , 2017Baniani et al 2019;Berens et al 2018). The diffusion results can also be routinely verified by comparing data measured with the same type of nuclei at high and low/midrange magnetic field strengths (Forman et al 2017(Forman et al , 2020(Forman et al , 2019Baniani et al 2019;Berens et al 2018Berens et al , 2019). An observation of an agreement of the diffusion data measured with different nuclei and/or field strengths results in a high degree of reliability of our data.…”

“…Owing to this interest, we have carried out a self-diffusion study of mixed and pure C 2 H 6 (~ 4.4 Å Li et al 2009) and C 2 H 4 (~ 4.2 Å Li et al 2009) in ZIF-11 by 13 C and 1 H PFG NMR at 14 and 17.6 T (Baniani et al 2019). The PFG NMR measurements were performed with different batches of ZIF-11 crystals for a broad range of root MSDs at different temperatures and gas concentrations.…”

“…Complementary diffusion measurements were carried out by IR microscopy (IRM) and standard volumetric uptake technique on large (~ 100 µm) ZIF-11 crystal batch. Due to technique limitations, only large crystal batch was used in IRM and volumetric uptake measurements (Baniani et al 2019).…”

“…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.…”

Molecular Diffusion in a Flexible Mesoporous Metal-Organic Framework over the Course of Structural Contraction

“…The diffusion of fluids in the pores of MOFs can be evaluated by experimental and theoretical approaches. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Amongst these, the combination of pulsed field gradient (PFG) NMR and molecular dynamics (MD) simulations represents a powerful complimentary approach to enable the exploration of diffusion at time-and length scales differing by orders of magnitude. 14,27,28 MD simulations can track molecular mobility within a single pore of ideal structures, while the PFG NMR is capable of direct probing of molecular displacements ranging from hundreds of nanometers to microns, i.e.…”