CaCl2-Activated Carbon Nitride: Hierarchically Porous Carbons with Ultrahigh Nitrogen Content for Selective CO2 Adsorption

Bulk carbon nitride (C₃N₄) was transformed into hierarchically porous, ultrahigh nitrogen content porous carbon materials with calcium chloride (CaCl₂) mediated thermal activation. By systematically varying two synthesis parameters (annealing time and CaCl₂:C₃N₄ weight ratio) and analyzing both solids and volatile species produced during the synthesis process, we investigated the fragmentation-recombination porogen mechanism and show that Ca²⁺ effectively stabilizes pyridinic nitrogen species through high temperature solvent-like interactions. Additionally, we characterized the physicochemical properties of the resulting porous carbons with X-ray photoelectron spectroscopy (XPS) and nitrogen (N₂) adsorption and tested their performance for CO₂ adsorption from 0 – 1.0 bar. We concretely show that, for these relatively low surface area materials, surface chemistry has a strong impact on the affinity for CO₂ adsorption, especially at low pressures relevant for carbon capture. The best performing sample, 200-0, exhibited large gravimetric CO₂ uptake at 25 °C and 0.1 bar (~1.9 mmol/g), large isosteric heat of adsorption (Q_st > 45 kJ/mol), and incredible CO₂/N₂ selectivity (S_IAST = 105) for a simulated binary gas feed of 10% CO₂ (1.0 bar, 25 °C) due to its unique combination of dipole-rich surface chemistry (43 at% N), moderate porosity (V_pore = 0.6 cc/g), and relatively small N₂ accessible surface area (180 m²/g).