A ‘trapdoor’ filtering method that admits large molecules and excludes small ones could significantly reduce the cost of separating and storing carbon dioxide.

Most uses of molecular sieves (zeolites) depend on these materials being able to discriminate on the basis of size, letting small molecules move into the zeolite pores and keeping large molecules out. However, Australian researchers recently discovered a new separation mechanism in one particular kind of molecular sieve, where the target molecules – in this case carbon dioxide – induce some of the atoms in the zeolite structure to move aside temporarily so that the target molecules can enter the structure.

Webley_imageforLightspeed_web.jpgThe ‘trapdoor’ material is a type of chabazite zeolite developed by Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) researchers Professor Paul Webley and colleagues, including PhD student Jin Shang and research fellow Gang Li from the University of Melbourne School of Engineering.

Paul and his colleagues used powder diffraction at the Australian Synchrotron as part of a combination of experimental and computational approaches to discover the underlying mechanism, which he describes as “similar to a molecular trapdoor”.

The materials discriminate between carbon dioxide and methane (natural gas) with the highest selectivity so far reported. This has important potential implications for natural gas purification, as many natural gas fields contain excess carbon dioxide that must be removed before the natural gas can be liquefied and shipped. The discovery could also lead to more efficient supply of carbon dioxide for chemical manufacturing processes.

Image above: 'Molecular trapdoors’ in this zeolite structure admit carbon dioxide and exclude methane molecules. Image: Paul Webley, University of Melbourne

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