Selective sorption

A cage-like structure that is rapidly assembled from simple organic components is shown to selectively adsorb carbon dioxide gas.

Credit: © 2010 Wiley

Ultimately, a move away from fossil fuels will reduce the amount of carbon dioxide in the atmosphere but, in the short term at least, methods to selectively remove it from, for example, flue gas will be important. Much work has focused on the development of metal–organic frameworks and covalent organic frameworks; the low weight and stability of the latter being particularly advantageous. A third, far less studied, group of potential adsorbents is that of porous organic molecules — cage-like discrete molecular structures with an internal void. Now, Wei Zhang and co-workers from the University of Colorado have prepared¹ such a cage that shows high selectivity for adsorption of CO₂ over N₂.

The cage is constructed by imine formation between three amine groups attached to each of two triphenylbenzene panels, and two aldehyde groups attached to each of three anthracene-derived linkers. Imine formation is a dynamic equilibrium, so mixing these two components in the correct ratios will eventually lead to the most thermodynamically stable product. In this case the cage is both enthalpy favoured — because it has the least bond-angle strain — and entropy favoured, because it consists of the minimum number of building units. Reduction of the imines locks the cage structure.

At standard temperature and pressure the cage compound adsorbed 73 times more CO₂ than N₂ on a volume per gram basis. Processability will be an important factor in the development of such structures for CO₂ sequestration, and the cage's solubility in simple solvents will make it amenable to being solution-processed into adsorbent films or gas-separation membranes.