POROUS MATERIALS

A carbon dioxide screw

Chem. Sci. https://doi.org/10.1039/c8sc04376k (2018)

Permanent porous materials can be designed from molecular building blocks and their pores made to have a defined size and chemical functionalization. These materials have been used for gas sorption and separation applications, but a common challenge is to achieve a high enough selectivity for quantitative separations. Now, Xing et al. describe a porous material that shows high selectivity for carbon dioxide versus nitrogen and that is capable of siphoning CO2 molecules with a mechanism analogous to that of small molecules moving through certain transmembrane channels.

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Royal Society of Chemistry

The researchers start by assembling the porous material using molecules consisting of tetrasulfonate anions and diaminium cations. The channels are subnanometre-size and perfectly fit for CO2 sorption. The positive and negative charges of the ionic building blocks that face the inside of the pores generate a strong electrostatic interaction with the electric dipoles of a CO2 molecule. As a result, the selectivity of the material for CO2 versus N2 is greater than 500-fold at room temperature. Interestingly, Xing et al. were also able to elucidate the dynamics of CO2 permeation through the channels. Using temperature-dependent 13C nuclear magnetic resonance, they find that full rotation of CO2 on the spot is sterically hindered; rather, the molecule can move from one adsorption site to the next by a 90° rotation at a rate of 106 steps per second, in a screw-like fashion.

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Correspondence to Alberto Moscatelli.

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Moscatelli, A. A carbon dioxide screw. Nature Nanotech 13, 1092 (2018). https://doi.org/10.1038/s41565-018-0332-7

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