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

ZZ Lahrami and A Afshar, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 131, 1001 (2025).

DOI: 10.1007/s00339-025-09137-y

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/H-2, CO2/H-2, 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/H-2 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/(sm(2)) and H-2 does not pass, with infinite selectivity (\documentclass12ptminimal \usepackageamsmath \usepackagewasysym \usepackageamsfonts \usepackageamssymb \usepackageamsbsy \usepackagemathrsfs \usepackageupgreek \setlength\oddsidemargin-69pt \begindocument$$\:S_\raisebox1ex $CO_2$\!\left/\:\!\raisebox- 1ex$H_2$\right.=\infty\:$$\enddocument). With higher loading (300 molecules), H-2 flux is at 468.24 mol/(sm(2)) with CO2 suppressed to zero, resulting in infinite \documentclass12ptminimal \usepackageamsmath \usepackagewasysym \usepackageamsfonts \usepackageamssymb \usepackageamsbsy \usepackagemathrsfs \usepackageupgreek \setlength\oddsidemargin-69pt \begindocument$ $\:S_\raisebox1ex$H_2$\!\left/\:\!\raisebox- 1ex$CO_2$\right.$$\enddocument, demonstrating a loading- dependent reversal in the leading transport mechanism. In the CH4/H-2 system, H-2 has peak flux of 494.99 mol/(sm(2)) at loading 300 and pore size 14 with infinite selectivity for CH4. CH however, has its peak flux of 588.64 mol/(sm(2)) at loading 200 and pore size 12 with selectivity \documentclass12ptminimal \usepackageamsmath \usepackagewasysym \usepackageamsfonts \usepackageamssymb \usepackageamsbsy \usepackagemathrsfs \usepackageupgreek \setlength\oddsidemargin-69pt \begindocument$ $\:S_\raisebox1ex$CH_4$\!\left/\:\!\raisebox- 1ex$H_2$\right.=11.1$$\enddocument, 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/(sm(2)) with \documentclass12ptminimal \usepackageamsmath \usepackagewasysym \usepackageamsfonts \usepackageamssymb \usepackageamsbsy \usepackagemathrsfs \usepackageupgreek \setlength\oddsidemargin-69pt \begindocument$ $\:S_\raisebox1ex$CO_2$\!\left/\:\!\raisebox- 1ex$CH_4$\right.=16.66$$\enddocument for pore 11, while pore 12 slightly enhances CO2 flux to 668.91 mol/(sm(2)) 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.

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