Biggest zeolites

C&I Issue 8, 2015

Researchers have discovered a new family of complex zeolites – including some of the largest zeolites ever made – that promise to be useful for carbon dioxide capture.

Zeolites are solid crystalline alumina-silicates – with a Swiss-cheese arrangement of channels and holes the size of molecules. Scientists from Korea, the UK and Sweden used electron diffraction to determine the structure of a known zeolite, ZSM-25, and predicted larger and more complex zeolite structures of the same symmetry (Nature, doi:10.1038/nature14575).

For now, new zeolites are discovered mainly using an empirical trial-and-error approach, using extensive syntheses.  ‘It is difficult to know in advance exactly which new zeolite structure the synthesis may produce,’ explains senior author Xiaodong Zou at Stockholm University, Sweden. ‘In this paper we show, for the first time, that it is possible to experimentally synthesise target zeolite structures that have been predicted but not observed.’

The new zeolites were found to have attractive properties as adsorbents and separated out carbon dioxide more effectively from fuel gases such as biogas and natural gas (mixtures of methane and CO2 with other gases) than previous zeolites.

‘These are the largest zeolites made thus far,’ Zou says. ‘The larger the structures, the more possible variations of atomic sites. This gives us the possibility to fine-tune the chemical environments of the zeolite cages and thus their selectivity.’

‘[Zeolites] are of intense interest for practical applications such as for selective absorbents and as catalysts, but their structures can be complex and hard to determine,’ comments organometallic chemist Robert Crabtree at Yale University, US.  ‘[This paper] is important in combining classical structural with new computational methods to resolve hard structural problems in the most complex zeolites. Their work should greatly help in the design and synthesis of new zeolites.’

Removal and capture of CO2 is an important strategy for reducing CO2 emissions from combustion sources, adds Prabir Dutta, materials chemist at Ohio State University, US. ‘Clearly, it is worthwhile exploring why the kinetics of CO2 adsorption is facilitated in this family of zeolites.’

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