X-Nuclei NMR Self-Diffusion Studies in Mesoporous Silica Foam and Microporous MOF CuBTC

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 Li⁺ and Cs⁺ cations of aqueous salt solutions in a high-porosity mesocellular silica foam (MCF) and of CO₂ adsorbed in metal-organic frameworks (MOF). The coil design and the necessary probe modifications, which yield pulsed field gradients of up to ±16.2Tm^-1, are introduced. The system was calibrated at ²H resonance frequency and successfully applied for diffusion studies at ⁷Li, ²³Na, 13C and ¹³³Cs frequencies. Significant reductions of the diffusivities of the cations in LiCl_ac and CsCl_ac solution introduced into MCFs are observed. By comparison of the diffusion behavior with the bulk solutions, a tortuosity of the silica foam of 4.5 ± 0.6 was derived. Single component self-diffusion of CO₂ and CH₄ (measured by ¹H NMR) as well as self-diffusion of the individual components in CO₂/CH₄ 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.