Direction-specific enhanced diffusion of CO₂ in chiral hexagonal boron nitride nanotubes

To meet performance requirements, the next generation of gas separation membranes will need both high gas permeability and selectivity, attainable if we could coax adsorbates to minimize random Brownian motion and produce direction-specific diffusion along a desired axis. In this atomistic modeling study, we detail how direction-specific diffusion of CO₂ can be achieved in chiral hexagonal boron nitride nanotubes (hBNNTs) by means of a non-Knudsen diffusion mechanism. Our findings detail how this mechanism of diffusion is driven by interactions with the tube walls and enables the CO₂ molecules to diffuse along the nanotube's z-axis with minimized collisions and directional changes. hBNNTs with chiral indices exhibit CO₂ diffusion rates faster than non-chiral tubes of comparable and larger diameters. Of the hBNNTs studied, a (7,3) tube appears to be ideally sized (3.7 Å radius) exhibiting CO₂ diffusion that is 3.4 times faster than diatomic N₂. Applying this mechanism of diffusion to hypothetical sheet membranes prepared with aligned chiral (7,3) hBNNTs results in membranes with a calculated CO₂/N₂ permselectivity of 170 and a CO₂ permeability limit of nearly 1.35 ×10⁷ Barrer, readily surpassing the Robeson upper bound for CO₂/N₂ separations.