The role of -NH position in polyamide precursors for CMS adsorbents: A study on CO2/CH4 size-sieving separation

Abstract

Natural and biogas purification by CO₂/CH₄ separation is a promising route for reducing carbon emissions. Constructing carbon molecular sieves (CMS) that demonstrate size-exclusion of small gas molecules, such as CH₄, is desirable but is hindered by the wide pore size distribution of carbons. In this work, a facile interfacial polyamidation approach was employed to regulate the carbon pore structure by manipulating the position of NH functional groups on the aromatic ring of diamine monomers. During pyrolysis, NH groups at the ortho position experience an increased steric strain, resulting in a denser microdomain stacking due to melt polycondensation phenomenon. In contrast, meta and para positions result in a more stable polymer matrix that undergoes reduced graphitization and a higher sp³/sp² ratio. The CO₂ uptake of one sample (MPDA900) reached as high as 2.78 mmol/g at 298 K and 1 bar, while almost total exclusion was successfully attained for CH₄. The superior separation performance steered by the size-exclusion effect was further validated by the dynamic breakthrough and regeneration ability tests. These findings offer new insights for designing advanced carbon sorbents with controlled hybridized structure and precisely tuned pores for biogas upgradation.