PNNL

Abstract

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 overcome Brownian motion into direction-specific diffusion down a desired axis. In this first-principles computational study, we detail how direction-specific diffusion of CO2 can be achieved in chiral hexagonal boron nitride nanotubes (hBNNT) where the chirality introduces a molecular-level “spin” on CO2 molecules to minimizes collisions and direction changes. hBNNTs with chiral rifling patterns exhibit CO2 diffusion rates faster than non-chiral tubes of comparable and larger diameters. Of the hBNNTs studied, (7,3) tubes appear to be ideally sized (3.7 Å radii) and exhibit an optimal “twist rate,” enabling rapid diffusion with a predicted selectivity (CO2/N2 = 633). Calculations of two hypothetical sheet membranes prepared with aligned chiral hBBNTs have potential to surpass the Robeson upper bound for CO2.