Direction-Specific Diffusion Enhances Gas Separations

The Science

The next generation of gas separations membranes will need to be more selective for target gases without compromising on permeability. Meeting these goals requires innovative solutions, including altering the diffusion of the gas molecules. Researchers used atomistic modeling to study carbon dioxide (CO₂) diffusion through chiral hexagonal boron nitride nanotubes (hBNNTs). They found that the CO₂ exhibited direction-specific diffusion in the hBNNTs, driven by a newly identified molecular-level “precession” through the hBNNT. This altered diffusion could be leveraged in hypothetical sheet membranes to create a CO₂/N₂ separation system with high selectivity for CO₂.

The Impact

Separations are both commonly used and highly energy intensive. Separating target molecules from dilute gas streams is particularly challenging. This work centers on understanding how chirality and motion within a separations system can enhance the diffusion of CO₂ relative to other gases. This process could enable the separation of CO₂ from nitrogen and other molecules without requiring additional energy inputs. The separated CO₂ can then be captured and used as a feedstock for commodity chemicals or other processes. Beyond CO₂, the process could potentially be applied to separate other gases in different systems.

Summary

To meet performance requirements, the next generation of gas separation membranes need both high gas permeability and selectivity. One potential avenue for attaining these properties is by manipulating adsorbates into moving in direction-specific diffusion along a desired axis, minimizing random Brownian motion. In a theoretical atomistic modeling study, researchers showed how direction-specific diffusion of CO₂ can be achieved in chiral hBNNTs. In chiral hBNNTs, the bond angle of the CO₂ molecules is distorted and leads to lowered symmetry. The CO₂ molecules then exhibit molecular-level “precession,” avoiding interactions with the electron-rich clouds of nitrogen along the pore wall and resulting in increased directional diffusion. The chiral hBNNTs exhibit CO₂ diffusion rates faster than non-chiral tubes of both similar and larger diameters. Of the hBNNTs studied, the (7,3) tube appears to be ideally sized, with CO₂ diffusion 3.4 times faster than N₂. Calculations of hypothetical sheet membranes prepared with aligned chiral (7,3) hBNNT have a CO₂/N₂ permselectivity of 170 and a CO₂ permeability limit with the potential to surpass the Robeson upper bound for CO₂/N₂ separation.

Contact

David Heldebrant, Pacific Northwest National Laboratory, david.heldebrant@pnnl.gov 

Funding

This work was supported by the Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Harnessing Confinement Effects, Stimuli, and Reactive Intermediates in Separations (FWP 81462). This research used resources of the National Energy Research Scientific Computing Center, a Department of Energy Office of Science user facility.