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Metal-Organic Frameworks

Vapor-phase method makes membranes for gas separation

Scalable deposition technique yields chemically selective film of zeolitic imidazolate framework (ZIF)

by Mitch Jacoby
September 10, 2018 | APPEARED IN VOLUME 96, ISSUE 36

 

09636-scicon9-reaction.jpg
Credit: Science
Exposing a two-component alumina substrate (left, gray) to gas-phase diethylzinc and water fills the substrate’s pores with zinc oxide (green spheres). An imidazole converts the oxide to an imidazolate (yellow), yielding a gas-selective membrane.

Permeable membranes could potentially lower the cost of industrial gas separations traditionally done via the energy-intensive process of cryogenic distillation. A number of metal-organic framework compounds known as ZIFs—zeolite-type structures consisting of transition metals connected by imidazolate linkers—show promise for this application, but the solution-based methods used to make them are tough to scale up reliably. So Xiaoli Ma, Michael Tsapatsis, and coworkers at the University of Minnesota came up with a simple vapor-phase ZIF-preparation method based on atomic layer deposition (ALD), a well-known film-growth technique. The group showed that the ZIF membranes they produced efficiently separate the gases propane and propylene (Science 2018, DOI: 10.1126/science.aat4123). The researchers started with a common substrate made by sandwiching two types of alumina. That material is gas permeable but does not separate propane from propylene. Using ALD, they deposited zinc oxide on the alumina, plugging its 2–5-nm pores and making it impermeable. They then treated the membrane with 2-methylimidazole, converting the zinc oxide to a partially crystallized ZIF. That step largely restored permeability, but only for propylene, yielding a stable, selective membrane.

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