Effects of zinc salts on the microstructure and performance of zeolitic-imidazolate framework ZIF-8 membranes for propylene/propane separation

Zeolitic-imidazolate frameworks (ZIFs), a sub-class of metal-organic frameworks (MOFs), show a lot of promise for membrane-based gas separation applications. In particular, a prototypical ZIF-8 has attracted tremendous research interests as a promising membrane material for the industrially important propylene/propane separation. Of the different synthesis techniques developed for polycrystalline ZIP membranes, the microwave assisted seeding followed by the secondary growth has been shown effective to obtain high quality polycrystalline ZIF-8 membranes by achieving strong attachment of seed crystals to ceramic supports. However, as with any polycrystalline films, the performance of membranes synthesized by this method is determined by membrane microstructures (i.e., grain boundary structure, the size/shape/orientation of grains, etc.) which, in turn, are affected by processing conditions. Here, we report the effects of the nature of zinc salts on the microstructures of ZIF-8 seed layers prepared under microwave irradiation as well as subsequent secondarily grown ZIF-8 membranes. Significant differences in the microstructures of both seed layers and membranes, consequently binary propylene/propane separation performance of membranes, were observed with different zinc salts as metal sources. It was found that likely due to its fast nucleation as compared to other zinc salts, zinc nitrate was most effective as a zinc source in both the microwave-assisted seeding and the secondary growth step, resulting in both seed layers and membranes with better microstructures. The resulting ZIF-8 membranes showed the excellent propylene/propane separation factor of similar to 105.