Insights into Cis-Amide-Modified Carbon Nanotubes for Selective Purification of CH₄ and H₂ from Gas Mixtures: A Comparative DFT Study

Among a plethora of mixtures, the methane (CH₄) and hydrogen (H₂) mixture has garnered considerable attention for multiple reasons, especially in the framework of energy production and industrial processes as well as ecological considerations. Despite the fact that the CH₄/H₂ mixture performs many critical tasks, the presence of other gases, such as carbon dioxide, sulfur compounds like H₂S, and water vapor, leads to many undesirable consequences. Thus purification of this mixture from these gases assumes considerable relevance. In the current research, first-principle calculations in the frame of density functional theory are carried out to propose a new functional group for vertically aligned carbon nanotubes (VA-CNTs) interacting preferentially with polar molecules rather than CH₄ and H₂ in order to obtain a more efficient methane and hydrogen separations The binding energies associated with the interactions between several chemical groups and target gases were calculated first, and then a functional group formed by a modified ethylene glycol and acetyl amide was selected. This functional group was attached to the CNT edge with an appropriate diameter, and hence the binding energies with the target gases and steric hindrance were evaluated. The binding energy of the most polar molecule (H₂O) was found to be more than six times higher than that of H₂, indicating a significant enhancement of the nanotube tip’s affinity toward polar gases. Thus, this functionalization is beneficial for enhancing the capability of highly packed functionalized VA-CNT membranes to purify CH₄/H₂ gas mixtures.