Japanese scientists have found a new type of gas storage based on restraining gas molecules within narrow tunnels. 

Gas storage in microporous materials normally relies on physisorption - a type of adsorption that relies on weak Van der Waals interactions - to fill the micropores with gas, explains Toshimasa Katagiri, from Okayama University, who led the team behind the research. Their new method involves physically restraining the gas within narrow tunnels (less than 1 nm diameter) with a serrated internal structure. The structure and flexibility of these tunnels in the nanoporous material trifluorolactate crystal creates an induced fit - meaning that as the gas molecules are adsorbed they cause the tunnels to swell. Katagiri and the team suggest that this new type of gas adsorption may be useful to store gaseous molecules with weak physisorption, such as hydrogen.

The serration on the internal surface of the tunnels comes from the trifluoromethyl groups protruding into the tunnel cavity, and it is these protrusions that physically restrain the gas molecules within the tunnel. 'The unique adsorption-desorption properties of these materials are very inspiring as they show the great potential of engineered hybrid systems where hydrocarbon and fluorocarbon domains alternate,' comments Giuseppe Resnati an expert in nano-structured materials at the Polytechnic of Milan, Italy.

The tunnel diameter can be altered by changing the length of the organic chain in the trifluorolactates, meaning materials properties can be optimised to improve storage of a specific gas. 'We are now trying to grow a perfect single crystal with tunnels. They could act as true molecular sieves for the separation of gaseous molecules by their size, at room temperature.  Such a system would be a key technology for the realisation of a hydrogen fuel cell vehicle with a methanol reforming system,' says Katagiri. 

Russell Johnson