Construction of hierarchical porous carbon for efficient CH4/N2 and CO2/N2 separation: Synergistic effect of hydrothermal treatment on regulating pore formation and adsorption performance

Rising CH₄ and CO₂ emissions from coalbed methane and flue gas underscore the urgent need for efficient adsorbents for CH₄/N₂ and CO₂/N₂ separation. In this study, a hierarchically porous carbon adsorbent with abundant ultra-micropores was synthesized from long-flame coal using KOH activation and hydrothermal treatment. The synergistic effect of both treatments on coal structure and activation behavior was investigated. Results indicate that hydrothermal treatment promoted interlayer expansion and the formation of carbonyl groups in the coal precursor, which facilitated deeper KOH penetration and enhanced the development of a hierarchical pore architecture rich in ultra-micropores. Moreover, carbonyl groups partially transformed into carboxyl groups during activation, increasing surface polarity and improving the adsorption affinity for CH₄ and CO₂. The hydrothermal treatment also induced the formation of carbon nanotubes, which served as efficient channels for molecular transport. The optimal sample, C-HTP-300 (derived from raw coal via hydrothermal treatment at 300 °C followed by activation), exhibited CH₄ and CO₂ adsorption capacities of 1.75 mmol/g and 3.65 mmol/g, respectively, at 298 K and 100 kPa. This performance was attributed to micropores <0.7 nm, surface-enriched carboxyl groups, and π-π∗ interactions. The material also showed high selectivity of 5.88 for CH₄/N₂ and 14.82 for CO₂/N₂. Toth model fitting revealed a heterogeneous sub-monolayer adsorption behavior.