Highly-selective MOF-303 membrane for alcohol dehydration

Metal-organic frameworks (MOFs) are an emerging class of crystalline microporous materials, which have drawn considerable attention for separation applications. While a number of successful examples of MOF-based membranes for gas separation have been reported, only a few of pure MOF membranes presented high performance in pervaporation. This work reports on the application of a highly hydrophilic MOF, MOF-303, for dehydration of ethanol as well as isopropanol (IPA) via pervaporation. Dense MOF-303 membranes are fabricated with either a sodium hydroxide or urea solution. The latter recipe renders MOF-303 crystals a low quantity of missing linker; and it also yields a membrane with fewer pinhole-type defects. The MOF-303 membrane prepared with urea presents a relatively low air permeance and much higher separation performance for of water-ethanol and water-IPA mixtures, as compared to that synthesized with sodium hydroxide. The MOF-303-urea membrane possesses high separation factors for water/ethanol (55349) and for water/IPA (3801) at 303 K. At a higher temperature of 343 K, this membrane still offers a good water/ethanol separation factor of 1874. A 7-day pervaporation operation on the MOF-303-urea membrane demonstrates that the separation performance drops gradually during the test, but it can be restored via a thermal treatment on the membrane. Molecular simulations are performed to shed light on the transport property of water, ethanol, and IPA in MOF-303. The computational results suggest that the dehydration capability of this MOF can be attributed to both of its water-selective adsorption and diffusion, particularly the latter. Specifically, a relatively high diffusion barrier to the alcohols in MOF-303 results in the high selectivity of water over ethanol or IPA.