Separating carbon dioxide from dilute gas streams remains challenging due to high costs, and energy intensive sorbents. Adsorbents and membranes are preferable to sorbents due to a lower energy demand, however, there exists a tradeoff between permeability and selectivity representing the Robeson upper bound, which is a threshold that has not yet been broken since 2008. CO₂ molecules move inside adsorbents or membranes via Brownian Motion, meaning their diffusion is non-directional (without stimuli) due to random collisions between CO₂ molecules or the interface of the adsorbent. Each collision of CO₂ into a wall or an adjacent molecule results in a loss of kinetic energy or a change in direction, making transport within media slower and less efficient than theoretically possible. Separations could be more efficient if were possible to minimize any collisions and losses of kinetic energy by coaxing molecules into directional motion down a desired axis of a sorbent, but it was not initially clear as to how this could be done without the energy demand and complexity of applied external stimuli.

Applying the principles of ballistics at the molecular/nano level, enabled the design of a material that achieves direction-specific diffusion of CO₂ without the need of external stimuli. This first-principles computational study introduces a newly discovered phenomenon that we define as “nano-rifling” where 10 or 11-right polygonal geometries in chiral hexagonal boron-nitride nanotubes can impart a spin upon CO₂ molecule like a bullet in a rifled barrel. This new discovery assesses how the chiral indices impart a spin on CO₂ at the molecular level, allowing the molecule to retain angular momentum and thereby minimizing kinetic losses due to collisions as CO₂ twists down the barrel of the tubes. This finding suggests potential for significantly enhanced diffusion of CO₂ over non-chiral tubes of comparable dimensions (3.7 Å radii) while retaining a high calculated CO₂/N₂ selectivity of 300. We then detail a theoretical performance assessment of the calculated permeability coefficients of two hypothetical membrane assemblies made from these tubes to compare against other leading CO₂-separating media. We close with a discussion of how the principles of nano-rifling could be introduced to membranes and porous materials and the potential for this phenomenon to be used to design more advanced media that could potentially exceed or shift the Robeson upper bound for CO₂.

Jian Liu

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