'Molecular trapdoor' opens only for CO₂

A family of nanoporous materials well known for their gas separation properties can sort molecules with much more sophistication than previously thought. Researchers in Australia have discovered that certain zeolites don’t act as simple molecular sieves, but rather separate molecules according to their ability to open ‘molecular trapdoors’ within the zeolite structure. Carbon dioxide molecules are particularly adept at slipping through these trapdoors, making it a promising discovery for industrial gas separation technologies such as carbon capture.

The trapdoor mechanism was discovered by Paul Webley at the University of Melbourne, Jefferson Liu at Monash University and their colleagues. The researchers were investigating new zeolite structures as part of their work for the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) when they noticed some unusual behaviour. The particular zeolite they were studying, a chabazite, wasn’t just taking up carbon dioxide in preference to larger molecules – expected behaviour for a molecular sieve – it was also taking up carbon dioxide in preference to smaller ones.

Key to this behaviour, the researchers found, was the behaviour of free metal ions in their chabazite structure. These cations balance the negative charge of the zeolite framework and sit within the oxygen-rich nanopores that act as doorways through which gases enter the structure. ‘It’s a bit like when you want to go into a bar and there’s a bouncer at the door,’ says Webley. ‘If you sweet-talk the bouncer then he’ll move aside and let you through – and that’s what the CO₂ molecule is able to do.’

Carbon dioxide can slip past the cation bouncers because of its electron-rich oxygen atoms. By interacting with the cation and partly stabilising it, the cation becomes less tightly bound within the doorway, moving aside enough for the carbon dioxide molecule to slip past. The cation then snaps back into place, preventing other gases from riding on the carbon dioxide’s coat tails. Webley suspects that other, known zeolites might also work in the same way.

The material's behaviour is promising for two key industrial gas separation processes: separating carbon dioxide from nitrogen in flue gases and removing carbon dioxide from natural gas. ‘We see a nice take-up of CO₂ and almost no take-up at all of nitrogen or methane,’ says Webley. ‘Those selectivities are right in the ballpark for what we are looking for.’

Randy Snurr, who researches nanoporous materials for gas separation at Northwestern University, US, is impressed by the research. ‘It provides a new insight into what people thought they had understood for a long time, this old idea of molecular sieving,’ he says. ‘They have used a whole array of techniques to back up this picture, but then they do the very practical thing and pass gas mixtures through the material and they see this very nice selectivity.’

While the zeolite’s selectivity is high, its CO₂ uptake capacity is modest, admits Webley. However, the team’s future work will include efforts to grow the zeolites in the form of membranes through which only CO₂ can pass, circumventing the capacity issue.