Cross section of membrane

Passing an organic vapour over a membrane formed by layering zinc (green) onto an aluminium-based substrate (red) makes the membrane permeable to small molecules. Credit: Prashant Kumar

Chemistry

‘Molecular sieve’ offers inexpensive way to sift plastic’s raw materials

A membrane with minuscule pores could help to replace bulky, energy-hungry infrastructure.

Low-cost membranes pierced with pores so small that they admit only certain molecules could serve as environmentally friendly filters.

The industrial technology used to separate many chemical mixtures into their constituents is energy-intensive, and membranes that can perform such separations have so far proved expensive to produce.

To make a modestly priced membrane, Xiaoli Ma, Michael Tsapatsis and their colleagues at the University of Minnesota in Minneapolis added an impervious layer of zinc oxide to an aluminium-oxide base. The researchers then wafted an organic vapour over the zinc. The zinc and vapour combined to create a type of porous material called a zeolitic imidazolate framework (ZIF).

The team passed a mixture of propylene — a feedstock for plastics — and propane over the ZIF face of the membrane. Large amounts of propylene passed through the ZIF’s pores, but propane could not do the same.

The process could also be used to produce other types of industrial membrane, the authors say.