Effect of Headspace CO₂ in Hydrothermal Carbonization of Loblolly Pine for In Situ Functionalization and Its Impact...

The steady rise in anthropogenic CO₂ emissions from industrial processes and fossil fuel combustion highlights the urgent demand for efficient carbon capture technologies. In this study, loblolly pine was utilized to develop a CO₂-responsive adsorbent through hydrothermal carbonization (HTC) under pressurized CO₂ atmospheres. By incorporating CO₂ as a reactive medium during HTC (200-260 °C), the process simultaneously promoted in situ surface functionalization and carbon retention. The resulting hydrochars were subsequently superactivated with KOH (2:1 ratio) at 800 °C to create ultraporous functionalized activated hydrochar for CO₂ uptake applications. Significant enhancements in BET surface area and micropore volume were observed with increasing CO₂ pressure across three HTC temperatures. CO₂ pressurization facilitated surface carboxylation while suppressing decarboxylation, leading to higher oxygen content and enriched functional groups, particularly carboxylic groups, on the functionalized activated hydrochar hydrochars. These modifications improved both physical adsorption via porosity and chemical adsorption via surface reactivity. The optimized material (H200 P150 psi) achieved a maximum CO₂ adsorption capacity of 17.25 mmol/g at 4 bar and 25 °C. However, at elevated HTC temperatures (>230 °C), the influence of CO₂ pressure on hydrochar properties diminished, indicating a transition to a temperature-dominated reaction regime. Thermal decomposition and condensation processes override the functionalizing role of CO₂, reducing its effectiveness as a reactive medium. Overall, this work presents a dual-benefit approach in which CO₂ serves as both a reactant and target which enables sustainable production of functionalized activated hydrochar while contributing to CO₂ mitigation.