Investigating the Intrinsic Ethanol/Water Separation Capability of ZIF-8: An Adsorption and Diffusion Study

Intrinsic ethanol/water separation capability of ZIF-8 is characterized by a detailed study of adsorption and diffusion of ethanol and water vapor in dodecahedral crystals with principle axis dimension of 324, 15.8, and 0.4 mu m. ZIF-8 exhibits extremely low water uptakes. At 35 degrees C and a relative pressure (P/P-o) of 0.95, the water uptakes for 324, 15.8, and 0.4 mu m ZIF-8 are 0.184, 0.197, and 0.503 mmol/g, respectively, all of which are less than 1 wt % increase relative to original sorbent mass (0.33, 0.35, 0.91 wt %). For ethanol adsorption, ZIF-8 exhibits an S-shape isotherm with low ethanol uptakes at P/P-o up to 0.08 and the cage filling phenomenon occurs at P/P-o higher than 0.08. The ethanol saturation uptake in ZIF-8 is as high as 30% of the sorbent weight. Because of the existence of the hydrophilic -N-H functionality introduced by the terminating imidazolate (Im) linker and the overall hydrophobicity of the inner network, the effect of outer surface area of ZIF-8 crystals is proved to be non-negligible as ZIF-8 crystals becomes smaller despite the extremely large inner surface area and pore volume, especially for water sorption. The variation of isosteric heats of adsorption for water reveals the existence of structural defect of ZIF-8 framework. Transport diffusivity and corrected diffusivity for water and ethanol in ZIF-8 are determined within the entire P/P-o range. The ethanol/water separation performance in ZIF-8 is evaluated in terms of vapor. phase sorption selectivity and permselectivity. While ZIF-8 exhibits ample ethanol/water sorption selectivity, it is not effective for ethanol extraction as a membrane material from dilute ethanol aqueous solutions due to the unfavorable diffusion selectivity and the competitive water uptakes in the adsorbed ethanol phase.