Low-transition temperature mixtures pretreatment and hydrothermal carbonization of corncob residues for CO2 capture materials

Activated char is one of the most cost-effective absorbents for low-temperature CO₂ capture. Rather than disposing of corncob residues by open-air burning, this study focused on improving corncob residues through pretreatment with low-transition temperature mixtures (LTTM) of choline chloride and glycerol combined with hydrothermal carbonization before being converted to CO₂ capture activated char. Compared to raw corncob residues, cellulose-rich materials (CRMs) from the pretreatment had less thermal stability and substantial changes in cellulose crystallinity and surface functional groups relative to cellulose and lignin, thus differently resulting in surface morphologies of activated biochars and hydrochars. The crystallinity of cellulose was still maintained in CRMs but not when the CRMs were hydrothermally carbonized. All hydrochars mainly contained aromatic compounds of lignin. The presence of choline chloride in LTTM pretreatment caused activated hydrochar to increase the specific surface area (from 25.4 to 64.0 m²/g) while reducing the total pore volume (from 1.2 to 0.7 m³/g) and pore size (from 184.3 to 34.3 nm). The activated CRM hydrochar had a CO₂ adsorption capacity of over ten times that of the activated biochar (no pretreatment and hydrothermal carbonization were applied). After 20 cycles of regeneration, it maintained >97 % of the initial CO₂ adsorption capacity. This finding pointed out that pretreatment and hydrothermal carbonization can potentially enhance the CO₂ adsorption performance of activated hydrochar from corncob residues.