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.
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.