Role of porosity and polarity of nanoporous carbon spheres in adsorption applications

Abstract

The role of porosity (pore size distribution (PSD) and specific surface area (SSA)) and polarity (N-doping) of carbonaceous materials for selective separation of CH₄ over N₂ and adsorption of heavy metal ions are presented herein. Two different carbons (resorcinol-formaldehyde carbon (RFC) and highly N-doped melanin carbon (MC)) with different N-doping levels are utilized and further activated by hot CO₂ treatment to finely control PSD and SSA. For CH₄ adsorption, the accumulated ultramicropore (<1 nm) volume of both carbons is strongly correlated with CH₄ adsorption regardless of surface polarity of carbons (R² = 0.94). This is probably due to the high polarizability and nonpolar nature of CH₄. Carbons with the highest ultramicropore volumes (RFC_C60 and MC_C55) show ultrahigh CH₄ uptake capacities of 2.64 and 2.41 mmol g⁻¹ at 273 K under 1 bar, respectively; these carbons also have superb CH₄ over N₂ selectivity of 6.8 and 7.4 obtained at 298 K, respectively. RFC_C60 and MC_C55 present excellent CH₄ adsorption capacities and selectivities for CH₄ over N₂, which are comparable with the best values reported from various porous materials. In addition, heavy metal ion (Fe²⁺, Sb³⁺, and Sb⁵⁺) adsorption was achieved to identify the importance of SSA and the polarity of carbons. The SSA of RFC samples is highly correlated with Fe²⁺ metal ion adsorption capacity (R² = 0.98). Conversely, highly N-doped MC series are located on the upper region of the plotted line of RFC samples due to their basic nature, which is caused by high loading of N-doping within the carbon framework. Furthermore, RFC_C60 and MC_C55 samples with similar porosity but different N-doping levels are utilized for selective adsorption of Sb³⁺ over Sb⁵⁺. The more basic MC_C55 has higher selectivity of 3.3 compared to that of its less basic counterpart (2.0), strongly inferring that carbon polarity is involved in metal ion adsorption. Distinct correlations between carbon porosity and polarity, adsorption capacity for CH₄, ultrahigh selectivity for CH₄ over N₂, adsorption capacity, and selectivity of metal ions are exclusively elucidated here, providing design principles of nanoporous carbonaceous materials for specific adsorption applications.