Materials science

Controlling gas with the flip of a switch

A metal–organic framework can separate gas molecules that differ by just two hydrogen atoms.

Electric fields can be used to control the flow of gas through a membrane.

Porous materials known as metal–organic frameworks (MOFs) can be used to separate gas molecules of different sizes. But their scaffold-like structures, composed of metals linked by organic struts, are often flexible, and struggle to separate molecules whose sizes differ by only a small amount.

Jürgen Caro and Alexander Knebel at the Leibniz University of Hanover, Germany, and their colleagues investigated the effect of an electric field on a MOF called ZIF-8, made up of zinc ions and imidazolate (C3H3N2-) struts. They found that applying the field made a ZIF-8 membrane less permeable, allowing it to better sieve propane from propene — molecules that differ by just two hydrogen atoms.

X-ray studies suggested that this was due to the MOF’s crystal structure polarizing under the electric field and becoming more rigid, which barred the larger, propane molecules from passing through while permitting propene to pass. The reversible effect could be harnessed in a range of MOFs to create controllable materials for use in drug delivery and sensors, the authors suggest.