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 (Qst > 45 kJ/mol), and incredible CO2/N2 selectivity (SIAST = 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 (Vpore = 0.6 cc/g), and relatively small N2 accessible surface area (180 m2/g).