Adsorption and Diffusion of Small Alcohols in Zeolitic Imidazolate Frameworks ZIF-8 and ZIF-90

Abstract

We examine the adsorption and diffusion of small alcohols in ZIF-8 and ZIF-90 with a combined experimental and modeling approach. Our Grand Canonical Monte Carlo (GCMC) simulations predict that both ZIFs exhibit a slight adsorption selectivity for ethanol over methanol, in good agreement with previous experimental data. The adsorption uptake of the alcohols at low pressures is found to be significantly higher in ZIF-90 than ZIF-8. Our simulations indicate that this is due to hydrogen bonding between the alcohols and the carbonyl group of ZIF-90 but that this effect is not strong enough to cause appreciable flexibility of the ZIF-90 framework during adsorption. We also report alcohol self-diffusivities and Arrhenius parameters measured using pulsed field gradient NMR (PFG-NMR) and molecular dynamics (MD) simulations. The diffusivities measured using PFG-NMR indicate that the diffusion selectivity of methanol over ethanol is significantly higher in ZIF-8 (S = 229) than in ZIF-90 (S = 6) at T = 25 °C. Qualitative agreement is obtained between experimental and simulated diffusivities using the generalized AMBER (GAFF) force field including framework flexibility.