Researchers at the University of Melbourne (Australia) have uncovered a new mechanism for molecular separation by a particular family of zeolites that use a “molecular trapdoor”. The new mechanism, reported in an open access paper in the Journal of the American Chemical Society, could pave a new route to design materials for high performance adsorption, membrane, and catalysis processes.
Separation of molecules based on molecular size in zeolites with appropriate pore aperture dimensions has given rise to the definition of “molecular sieves” and has been the basis for a variety of separation applications. We show here that for a class of chabazite zeolites, what appears to be “molecular sieving” based on dimension is actually separation based on a difference in ability of a guest molecule to induce temporary and reversible cation deviation from the centre of pore apertures allowing for exclusive admission of certain molecules.
This new mechanism of discrimination permits “size-inverse” separation: we illustrate the case of admission of a larger molecule (CO) in preference to a smaller molecule (N2). Through a combination of experimental and computational approaches, we have uncovered the underlying mechanism and show that it is similar to a “molecular trapdoor”. Our materials show the highest selectivity of CO2 over CH4 reported to date with important application to natural gas purification.
The molecular trapdoor chabazite materials exhibit record high selectivity for separation of important industrial gas mixtures such as CO2/CH4, and counter-intuitive size-inverse “sieving” of CO/N2, and are currently under investigation for industrial pressure swing adsorption processes, the researchers said.
|A trapdoor mechanism for molecular separations. Credit: ACS, Shang et al. Click to enlarge.|
The findings suggest that this new material has important applications to natural gas purification, said research leader Professor Paul Webley. Because the process allows only carbon dioxide molecules to be captured, it will reduce the cost and energy required for separating carbon dioxide.
Jin Shang, Gang Li, Ranjeet Singh, Qinfen Gu, Kate M. Nairn, Timothy J. Bastow, Nikhil V. Medhekar, Cara M. Doherty, Anita J. Hill, Jefferson Z. Liu, and Paul A. Webley (2012) Discriminative Separation of Gases by a “Molecular Trapdoor” Mechanism in Chabazite Zeolites. Journal of the American Chemical Society doi: 10.1021/ja309274y