Effect of Different Parameters on the Microstructure and Performance of ZIF-8 Membranes
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Metal-Organic Frameworks (MOFs) are nanoporous materials comprising of metal nodes coordinated to organic ligands. They exhibit regular crystal lattice structures with well-defined pores. Owing to the ease with which the organic groups can be given different chemical functionalities, MOFs display highly flexible surface chemistry and pore sizes. This makes them suitable for their application as gas separation membranes. Zeolitic-Imidazolate Frameworks (ZIFs), a sub-class of MOFs, have pore sizes in the range of C3-C6 hydrocarbons, and in particular ZIF-8 (pore size 3.4Å), can be used for the industrially important propylene/propane separation. Because heterogeneous nucleation and growth of ZIF-8 crystals on the ceramic supports is not favored, their synthesis as membranes is complicated and involves multiple steps. Of the different synthesis techniques, the microwave-assisted seeding followed by the secondary growth is one of the most effective because of strong attachment of seed crystals to the supports achieved. The microstructure, and consequently, performance of membranes grown by this method is highly sensitive to different parameters and a systematic study of some of these will help to identify the best microstructure possible, reduce synthesis time, and improve reproducibility. Some of the parameters studied are different metal salt sources, aging of precursor solutions, post-synthetic activation time and membrane performance under high pressure and temperature. ZIF-8 seed layers and membranes were synthesized using the same recipe but different zinc salts to investigate the effect of different zinc metal sources on both seed layer and membrane performance and morphology. It was found that ZIF-8 membranes from zinc nitrate based seed layers had the best performance possibly due to better grain boundaries. Zinc nitrate-based ZIF-8 membranes showed good propylene/propane separation performance without any structural degradation when tested under harsher pressure and temperature (~ 7 bar and 100 ⁰C) conditions. Reduction in total synthesis time along with significant improvement in membrane performance was achieved by reducing the post synthesis activation time and using aged precursor solutions.
Zeolitic Imidazolate Frameworks
Zinc salt effect
High pressure performance
Ramu, Gokula Krishnan (2016). Effect of Different Parameters on the Microstructure and Performance of ZIF-8 Membranes. Master's thesis, Texas A & M University. Available electronically from