Windowed carbon nanotubes for efficient CO₂ removal from natural gas

We show from molecular dynamics simulations that windowed carbon nanotubes can efficiently separate CO₂ from the CO₂/CH₄ mixture, resembling polymeric hollow fibers for gas separation. Four CO ₂/CH₄ mixtures with 10, 30, 50, and 80% CO₂ are investigated as a function of applied pressure from 80 to 180 bar. In all simulated conditions, only CO₂ permeation is observed; CH₄ is completely rejected by the nitrogen-functionalized windows or pores on the nanotube wall in the accessible time scale, while maintaining a fast diffusion rate along the tube. The estimated time-dependent CO₂ permeance ranges from 10⁷ to 10⁵ GPU (gas permeation unit), compared with ∼100 GPU for typical polymeric membranes. CO₂/CH ₄ selectivity is estimated to be ∼10⁸ from the difference in free-energy barriers of permeation. This work suggests that a windowed carbon nanotube can be used as a highly efficient medium, configurable in hollow-fiber-like modules, for removing CO₂ from natural gas.