Abstract
A standard X-observe NMR probe was equipped with a z-gradient coil to enable high-sensitivity pulsed field gradient NMR diffusion studies of and cations of aqueous salt solutions in a high-porosity mesocellular silica foam (MCF) and of adsorbed in metal-organic frameworks (MOF). The coil design and the necessary probe modifications, which yield pulsed field gradients of up to , are introduced. The system was calibrated at resonance frequency and successfully applied for diffusion studies at , , and frequencies. Significant reductions of the diffusivities of the cations in and solution introduced into MCFs are observed. By comparison of the diffusion behavior with the bulk solutions, a tortuosity of the silica foam of was derived. Single component self-diffusion of and (measured by NMR) as well as self-diffusion of the individual components in / mixtures was studied in the MOF CuBTC. The experimental results confirm high mobilities of the adsorbed gases and trends for diffusion separation factors predicted by MD simulations.
Highlights
Diffusion processes of liquids and gases under the influence of internal surfaces of porous materials are of fundamental importance for understanding host-guest-interaction and of practical relevance for the prediction of material properties
We introduce a modification of a Bruker X-observe NMR probe with a Maxwell-pair z-gradient coil to enable high-sensitivity pulsed field gradient NMR self-diffusion studies with a wide range of X-nuclei
The self-diffusion of Li+ and Cs+ cations in aqueous solutions of LiCl and CsCl introduced into a high-porosity mesocellular silica foam (MCF) [18,19] are measured and compared to the bulk diffusivities revealing the restricting influence of the solid silica host matrix on the diffusion pathway of the cations
Summary
Diffusion processes of liquids and gases under the influence of internal surfaces of porous materials are of fundamental importance for understanding host-guest-interaction and of practical relevance for the prediction of material properties. The self-diffusion of Li+ and Cs+ cations in aqueous solutions of LiCl and CsCl introduced into a high-porosity mesocellular silica foam (MCF) [18,19] are measured and compared to the bulk diffusivities revealing the restricting influence of the solid silica host matrix on the diffusion pathway of the cations. These studies serve both the experimental test of the developed probe with a porous material and the characterization of the internal diffusion resistance of the silica matrix of the MCF. The data are compared to MD simulations of diffusion and of diffusion separation factors [21,22] and provide experimental verifications of the results of the computer simulations
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