Pillared-bilayer metal-organic framework membranes for dehydration of isopropanol

Metal-organic frameworks (MOFs) possess ordered micropores with high variety in surface functionality, and are considered excellent candidates for membrane pervaporation. However, only very few successful examples of pure MOF membranes for pervaporation have been reported to date. In this work we report on an emerging pillared-bilayer MOF, Zn-aip-azpy, with a pore limiting diameter of 3.57 Å for the pervaporation of water-IPA mixture. We find that the seeding procedure is crucial to the formation of high-quality dense membrane comprising Zn-aip-azpy. The deposition of low-molecular weight PVA is also found to be an effective approach to form a protective layer on the pure Zn-aip-azpy membrane. The optimized Zn-aip-azpy membrane presents a water-to-IPA separation factor of as high as ~2000 with a mass flux of 0.043 kg-m−2-h−1. This is for the first time that the water-to-alcohol selectivity of a pure MOF membrane can exceed 1000. A correlation between air permeance of a membrane and its separation performance is developed in this work, which can be applied to diagnose the pinhole type defects in pure MOF membranes. Molecular simulations conducted herein suggest that, while the interior surface of Zn-aip-azpy is largely alcohol selective, the narrow bottleneck in its one-dimensional channel imposes a large diffusion barrier for IPA and thus renders this membrane high water-to-IPA selectivity.