Diffusion phenomena of propane and propylene in colloidal zeolitic imidazolate Framework-8 particles

Zeolitic imidazolate framework-8 (ZIF-8) enables a kinetic separation of propane and propylene with high efficiency, although the detailed mechanism has not yet been clarified. In the present study, we measure the adsorption rates of propane and propylene in ZIF-8 particles and in-situ X-ray diffraction patterns of ZIF-8 during the gas adsorption process to investigate the effect of the gas pressure and the flexibility of ZIF-8 framework on the diffusion process of propane and propylene in ZIF-8 particles. The adsorption rates of propane and propylene depend on the gas pressure, which would be due to the enlargement of the pore window diameter of the ZIF-8 framework during the gas adsorption as suggested by in-situ XRD measurements. This result is also supported by the calculation of interaction potential profiles between a gas molecule and the ZIF-8 framework, in which an energy barrier at the pore window limits the gas diffusion and the height of the barrier decreases with the increase in the pore window diameter. Furthermore, we find that the adsorption rates of propane and propylene depend on the particle size of ZIF-8, and the selectivity of propylene over propane increases with the particle size. (C) 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.