<p>Porous boron nitride nanotubes (BNNTs) have been investigated as gas selective membranes for gas separation by classical molecular dynamics (MD) simulations. Nanopores were engineered by removing 9 to 14 atoms from the BNNT walls, and their performance was examined for CH₄/H₂, CO₂/H₂, and CH₄/CO₂ mixtures at equimolar conditions with total number of molecules ranging from 100 to 600. The results reveal a strong sensitivity of gas permeation to pore size and gas loading, leading to distinct transport mechanisms for each mixture. For the CO₂/H₂ system, both gases exhibit their highest flux and selectivity at the 12-atom pore. At moderate loading (200 molecules), CO₂ flux is at 789.32 mol/(s·m²) and H₂ does not pass, with infinite selectivity (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">$\:{S}_{\raisebox{1ex}{${CO}_{2}$}\!\left/\:\!\raisebox{-1ex}{${H}_{2}$}\right.}=\infty\:$</EquationSource> </InlineEquation>). With higher loading (300 molecules), H₂ flux is at 468.24 mol/(s·m²) with CO₂ suppressed to zero, resulting in infinite <InlineEquation ID="IEq2"> <EquationSource Format="TEX">$\:{S}_{\raisebox{1ex}{${H}_{2}$}\!\left/\:\!\raisebox{-1ex}{${CO}_{2}$}\right.}$</EquationSource> </InlineEquation>, demonstrating a loading-dependent reversal in the leading transport mechanism. In the CH₄/H₂ system, H₂ has peak flux of 494.99 mol/(s·m²) at loading 300 and pore size 14 with infinite selectivity for CH₄. CH however, has its peak flux of 588.64 mol/(s·m²) at loading 200 and pore size 12 with selectivity <InlineEquation ID="IEq3"> <EquationSource Format="TEX">$\:{S}_{\raisebox{1ex}{${CH}_{4}$}\!\left/\:\!\raisebox{-1ex}{${H}_{2}$}\right.}=11.1$</EquationSource> </InlineEquation>, indicating different size- and concentration-dependent permeation behavior. For CH₄/CO₂ mixtures, permeation is not possible through pores with diameter smaller than those formed by the removal of 11 atoms. For loading 200, CO₂ flux is 655.53 mol/(s·m²) with <InlineEquation ID="IEq4"> <EquationSource Format="TEX">$\:{S}_{\raisebox{1ex}{${CO}_{2}$}\!\left/\:\!\raisebox{-1ex}{${CH}_{4}$}\right.}=16.66$</EquationSource> </InlineEquation> for pore 11, while pore 12 slightly enhances CO₂ flux to 668.91 mol/(s·m²) but reduces selectivity to 5, showing that the bigger pores facilitate easier transport of CH₄ and reduce CO₂/CH₄ selectivity. When loadings are higher (250–300 molecules), there is no permeation of any of the BNNTs studied, demonstrating strong size- and loading-dependent transport. These findings demonstrate that porous BNNTs are capable of functioning as ultra-highly tunable membranes, where pore size and operating conditions can be controlled to be optimized for selective separation, particularly for hydrogen purification and for carbon dioxide management in nanoscale systems.</p>

Unveiling gas transport mechanisms in porous boron nitride nanotubes: A simulation study of CH₄/H₂, CO₂/H₂, and CO₂/CH₄ mixtures

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发表日期：

2025年11月19日

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Zabiholah Zabihi Lahrami,
Ali Afshar

摘要

Porous boron nitride nanotubes (BNNTs) have been investigated as gas selective membranes for gas separation by classical molecular dynamics (MD) simulations. Nanopores were engineered by removing 9 to 14 atoms from the BNNT walls, and their performance was examined for CH₄/H₂, CO₂/H₂, and CH₄/CO₂ mixtures at equimolar conditions with total number of molecules ranging from 100 to 600. The results reveal a strong sensitivity of gas permeation to pore size and gas loading, leading to distinct transport mechanisms for each mixture. For the CO₂/H₂ system, both gases exhibit their highest flux and selectivity at the 12-atom pore. At moderate loading (200 molecules), CO₂ flux is at 789.32 mol/(s·m²) and H₂ does not pass, with infinite selectivity ( $\:{S}_{\raisebox{1ex}{${CO}_{2}$}\!\left/\:\!\raisebox{-1ex}{${H}_{2}$}\right.}=\infty\:$ ). With higher loading (300 molecules), H₂ flux is at 468.24 mol/(s·m²) with CO₂ suppressed to zero, resulting in infinite $\:{S}_{\raisebox{1ex}{${H}_{2}$}\!\left/\:\!\raisebox{-1ex}{${CO}_{2}$}\right.}$ , demonstrating a loading-dependent reversal in the leading transport mechanism. In the CH₄/H₂ system, H₂ has peak flux of 494.99 mol/(s·m²) at loading 300 and pore size 14 with infinite selectivity for CH₄. CH however, has its peak flux of 588.64 mol/(s·m²) at loading 200 and pore size 12 with selectivity $\:{S}_{\raisebox{1ex}{${CH}_{4}$}\!\left/\:\!\raisebox{-1ex}{${H}_{2}$}\right.}=11.1$ , indicating different size- and concentration-dependent permeation behavior. For CH₄/CO₂ mixtures, permeation is not possible through pores with diameter smaller than those formed by the removal of 11 atoms. For loading 200, CO₂ flux is 655.53 mol/(s·m²) with $\:{S}_{\raisebox{1ex}{${CO}_{2}$}\!\left/\:\!\raisebox{-1ex}{${CH}_{4}$}\right.}=16.66$ for pore 11, while pore 12 slightly enhances CO₂ flux to 668.91 mol/(s·m²) but reduces selectivity to 5, showing that the bigger pores facilitate easier transport of CH₄ and reduce CO₂/CH₄ selectivity. When loadings are higher (250–300 molecules), there is no permeation of any of the BNNTs studied, demonstrating strong size- and loading-dependent transport. These findings demonstrate that porous BNNTs are capable of functioning as ultra-highly tunable membranes, where pore size and operating conditions can be controlled to be optimized for selective separation, particularly for hydrogen purification and for carbon dioxide management in nanoscale systems.