(419d) Diffusion of Pure and Mixed Light Gases in a Commercial 5A Zeolite By High Field NMR Diffusometry

Design of advanced microporous sorbents, membranes, catalysis, and sensors requires detailed, fundamental transport studies to unravel complexities of molecular diffusion in such systems. Here we report a study of self-diffusion of pure and mixed light gases (CO₂, CH₄, and C₂H₆) inside crystals and crystal aggregates of a commercial 5A zeolite. The diffusion measurements were performed using pulsed field gradient (PFG) NMR at high magnetic fields of 14 T and 17.6 T with high magnetic field gradients up to 18 T/m. The diffusion measurements were carried out for different diffusion times and temperatures. ¹³C PFG NMR was used for a majority of diffusion measurements to take advantage from larger T₂ NMR relaxation times of ¹³C nuclei of confined gas molecules in comparison to those of protons of the same molecules. Complementary diffusion measurements were also carried out for selected samples using ¹H PFG NMR under identical experimental conditions to confirm the absence of any measurement artifacts. Utilization of the high magnetic fields enabled obtaining sufficiently high signal-to-noise ratios in ¹³C PFG NMR gas diffusion studies where the PFG NMR signal suffers from a relatively low sensitivity of ¹³C NMR detection. The large magnetic field gradients allowed for gas diffusion measurements to be performed for length scales of displacements smaller than or comparable with the sizes of crystals or crystal aggregates. The diffusion data for one component gases will be reported and compared with the corresponding data for selected two-component gas mixtures. Our diffusion data demonstrate a significant influence of electrostatic field in zeolite crystals on the diffusion of polarizable molecules (CO₂). We will show evidence that such influence can also significantly impact diffusion of non-polar and non-polarizable molecules (C₂H₆) when these molecules are mixed with CO₂ molecules in the same zeolite crystals.