Nitrogen doped activated carbon derived from chitosan/hexamethylenetetramine: structural and CO2 adsorption properties

Abstract

Chitosan and chitosan/hexamethylenetetramine (HMT) macrospheres were prepared respectively by dropping the solution of chitosan and chitosan/HMT (in wt/wt ratios of 1:1 and 1:3) in aq. NaOH solution. Here, HMT served as an additional nitrogen precursor for in situ N-doping with chitosan derived activated carbon (AC). The as-formed macrospheres were impregnated using ZnCl2, freeze-dried and carbonized at 500 °C under inert atmosphere to yield ACs of size ranging from 2.5 to 2.8 mm. All the samples were characterized using SEM, EDS, CHNS Analyzer, TGA, FT-IR, Raman, XRD and BET surface area analyzer. All the samples showed mesoporous characteristics. The surface area of the AC without HMT, with 1:1 HMT and 1:3 HMT were 391.502, 259.017 and 111.717 m2/g respectively. Similarly, the pore volumes of AC without HMT, with 1:1 HMT and 1:3 HMT were 0.138, 0.095 and 0.075 cc/g respectively. The chitosan/HMT derived AC possessed higher N-content and better thermal stability, however exhibited lower surface properties with increasing HMT content. The CO2 adsorption capacity of chitosan derived AC was 97.98 mg/g while that of AC with 1:1 HMT and 1:3 HMT were 72.95 mg/g, 55.11 mg/g respectively at 25 °C and 1 bar. Deterioration in adsorption capacity of chitosan/HMT derived AC may be attributed to the physical cross-linking of chitosan polymer chains induced by increase in pH with addition of HMT and also the intermolecular H-bonding interaction between HMT and chitosan, which in turn reduces the surface area of as-formed N-doped ACs progressively.