CO2 Capture over Chitosan Biochars: Tailoring the Properties for Highly Efficient Adsorbents

Chitosan biochars were prepared via two different pathways to obtain N-functionalized adsorbents. The direct calcination of chitosan in the presence of K₂CO₃ afforded CHIT-CO3-XXX materials (XXX refers to the temperature of calcination), whereas the initial hydrothermal carbonization (HTC) of chitosan, followed by calcination in the presence of K₂CO₃, afforded HTC48-CO3-XXX materials. The biochars obtained by direct carbonization of chitosan presented a lower BET area and nitrogen content than the carbons obtained from HTC chitosan, considering the same temperature of calcination. Among the biochars, CHIT-CO3-700 presented the highest CO₂ uptake (5.3 mmol·g^–1), whereas HTC48-CO3-600, prepared upon calcination of HTC chitosan, presented 4.7 mmol·g^–1 of CO₂ uptake, at 25 °C and 1 bar. These materials were characterized by different techniques and tested for the adsorption of CO₂ at different temperatures. The adsorption profiles were better fitted by the Freundlich isotherm and the second-order kinetic model. The selectivity for CO₂/N₂ gas mixtures, taken from the Henry constant, was 1.7 and 10 for CHIT-CO3-700 and HTC48-CO3-600, respectively. In situ FTIR drift studies also indicated that the HTC48-CO3-600 biochar can retain some adsorbed CO₂ at 150 °C. These results may be explained by the higher total basicity of the HTC48-CO3-600 biochar, indicating that the initial hydrothermal carbonization of chitosan produces a biochar with improved properties for selective CO₂ capture.