Atomic layer deposition yields highly selective filtration membranes
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iltration technology is essential for many industrial processes. In particular, oil refinery systems must effectively separate paraffins (such as propane) from olefins (such as propylene) in order to conserve and recover the latter, more precious commodity. To date, most large-scale endeavors have relied on distillation to split these organic molecules. These approaches are expensive and energy-intensive. Membrane-based separations, when combined with distillation, can more effectively separate hydrocarbons than conventional distillation-only industrial systems. A metal– organic framework (MOF) called ZIF-8 (zeolitic imidazolate framework-8), when made into a thin membrane film on a support structure, has been found to effectively separate propylene. The biggest hurdle to implementing this hybrid membranedistillation process is cost: solution-based solvothermal methods used to form these membranes are difficult and expensive to scale up. An all-vapor membrane synthesis method can eliminate the complexities of a liquid- or gel-based processing approach and is much more attractive. However, existing vapor-based atomic layer deposition (ALD) methods have yet to demonstrate converting precursor components into comprehensive, fully functional membranes. Now researchers from the University of Minnesota (UMN) have adjusted the synthesis method of the zeolitic imidaz-
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Atomic layer deposition yields highly selective filtration membranes
olate framework to comprehensively reconstruct the selectively permeable membrane and yield a more industrially scalable method that can be used to separate paraffins from olefins. Xiaoli Ma, Michael Tsapatsis, and their colMicrostructure of the manufactured membrane is revealed through (left to l e a g u e s f r o m right) cross-sectional analysis, annular dark-field scanning transmission UMN’s Depart- electron microscopy, and spatial distribution of aluminum (orange) and zinc (green) for the ZIF-8 nanocomposite membrane. Credit: Xiaoli Ma. ment of Chemical Engineering and Materials Science deposited the sieving materials within were able to study the penetration of zinc a porous substrate. Key to their efforts oxide into the porous alumina support at was a two-step vapor-based process that the nanoscale,” says Prashant Kumar, a first used ALD with diethyl zinc water member of the research team. “This alvapor to deposit ZnO into a layer of lowed us to tune our ALD method for opmesoporous γ-alumina on a macropotimal membrane performance,” he added. rous α-alumina support. They then trans The resulting membranes yielded formed the ZnO into a MOF by expossignificantly higher propane/propylene ing it to 2-methylimidazole (a
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