PFG NMR self-diffusion of propylene in ITQ-29, CaA and NaCaA: Window size and cation effects

Abstract

Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy was used to measure and rationalize the self-diffusion coefficients of propylene in various LTA materials having different framework composition and varying proportions of charge balancing cations: ITQ-29 (Si/Al = ∞), CaA (Si/Al = 1), and six NaCaA (Si/Al = 1) samples at greater than 50% Na + exchange. The critical role of the framework Si/Al ratio on the self-diffusivity of propylene is demonstrated. Changes in self-diffusion driven by changes in framework composition are well documented in the literature. Diffusion frequency response experiments in the pure silica and alumino-phosphate forms of the CHA structure are included in this work to illustrate such situation. For the LTA structures, propylene was chosen as a very sensitive probe molecule because its size is comparable to the size of the 8-ring window apertures in the materials. A clear increase in self-diffusivity with the size of the window apertures, which increases with Al content in the framework, is shown. Thus, ITQ-29 and CaA exhibit the lowest and highest self-diffusivities, respectively. The self-diffusivities in the NaCaA samples lie in between those of ITQ-29 and CaA and increase with the Ca 2+ content. Thus, the presence of extra-framework balancing cations in either CaA or NaCaA does not lead to lower self-diffusivities than in the cation-free ITQ-29. The lower self-diffusivities in ITQ-29, instead, can be largely attributed to its smaller window size, which is a more dominant effect than any reduction in self-diffusivity caused by the cations, at the levels of Na + exchange studied.