CaCl ₂ -Activated Carbon Nitride: Hierarchically Nanoporous Carbons with Ultrahigh Nitrogen Content for Selective CO ₂ Adsorption

Bulk carbon nitride (C3N4) was transformed into hierarchically nanoporous, nitrogen-rich (N-rich) carbons via calcium chloride (CaCl2)-mediated thermal activation. By systematically varying the annealing time and CaCl2:C3N4 weight ratio, we describe the fragmentation-recombination porogen mechanism and show that Ca2+ effectively stabilizes pyridinic N species through high-temperature solvent–solute interactions. The resulting N-rich nanocarbons were applied as CO2 sorbents over the pressure range of 0–1.0 bar. For these relatively low surface area materials, surface chemistry has the dominant impact on the affinity for CO2 adsorption, primarily at low pressures relevant for carbon capture. An optimal sample with a gravimetric CO2 uptake of 1.9 mmol/g at 25 °C and 0.1 bar and large associated isosteric heat of adsorption (Qst > 45 kJ/mol) resulted in incredible CO2/N2 selectivity (SIAST = 105) for a simulated binary gas feed of 10% CO2 (1.0 bar, 25 °C). We attribute these attractive properties to a unique combination of dipole-rich surface chemistry (43 at % N), moderate porosity (Vpore = 0.6 cc/g), and relatively small N2 accessible surface area (180 m2/g).