(447b) Microscopic Diffusion Studies of Pure and Mixed Methane and Carbon Dioxide in ZIF-11 By NMR

Zeolitic imidazole frameworks (ZIFs), a subclass of metal organic frameworks, have shown promise in challenging gas separations. Specifically, the high surface area and pore volume, along with great chemical and thermal stability and great control over the pore sizes has already made ZIFs an excellent choice material for both adsorption-based and diffusion-based separations. The tunable ZIF pore aperture, which is in the range of about 0.2 â?? 2 nm, can result in molecular sieving phenomena, allowing for great diffusion-based selectivity.

Fundamental understanding of gas diffusion in ZIFs is of crucial importance for their potential applications in separations. Here we report microscopic studies of diffusion of CO₂, CH₄, and a CO₂/CH₄ mixture inside ZIF-11 particles. These gases are commonly found in natural and coal bed gases, where CO₂ is an impurity. Diffusion measurements were performed using ¹H and ¹³C pulsed field gradient (PFG) NMR utilizing a high field (17.6 T) and high field gradients (up to 30 T/m) to obtain diffusivities for the length scales of displacements in the range of about 1-20 mm. The diffusivities for one component gases are correlated with those in the corresponding gas mixture. The role of particle external surface in the measured transport properties is discussed.