Synthesis, Characterization, and CO₂ Uptake of Adsorbents Prepared by Hydrothermal Carbonization of Chitosan

Chitosan, a heteropolysaccharide obtained from the N-deacetylation of chitin, has stood out as a raw material to produce CO₂ adsorbents. In this work, we report the hydrothermal carbonization (HTC) of chitosan for different times and the potential of the materials for CO₂ adsorption. Elemental analysis indicated that the carbon weight content increases, whereas the relative amount of oxygen atoms decreases upon increasing the time of HTC. The relative nitrogen content was almost constant, indicating that HTC did not lead to significant loss of nitrogenated compounds. FTIR and ¹³C MAS/NMR spectra suggest that the structure of the sorbents becomes more aromatic with the increase of HTC time. The thermal properties of HTC materials were similar to that of chitosan, whereas their basicity was less compared to that of the parent chitosan. SEM images did not show significant porosity, which was confirmed by the BET area of the materials, around 2 m²·g^-1, similar to that of the parent chitosan. The materials were tested for CO₂ capture at 25 °C and 1 bar; the HTC chitosan adsorbents showed CO₂ uptakes about 4-fold higher than that of the parent chitosan. The adsorption process was better described by the Freundlich isotherm and the pseudo-second-order kinetic model.