Synchronized lignocellulosic pyrolysis for activator-free carbon foams: Role of selective hemicellulose removal

The direct conversion of lignocellulosic biomass into carbon foam is constrained by the divergent pyrolytic behaviors of its three primary components, whose temporal mismatch during thermochemical transformation disrupts pore network development. This study reveals that hemicellulose undergoes rapid volatilization at low temperatures (150–250 ℃), prior to the softening of lignin, creating a gas–liquid phase mismatch that causes pore collapse. Lignin serves as scaffold, cellulose as micropore enhancer, and hemicellulose as pore inhibitor. Guided by these mechanistic insights, an acid-hydrothermal pretreatment (AHP) was developed to selectively remove 85.5% of hemicellulose, synchronizing the pyrolytic phase evolution of the remaining lignin-cellulose framework. The resulting rice straw-derived carbon foam possesses a hierarchical pore structure with a high surface area (581.87 m²/g) and microporous volume (0.165 cm³/g)—2.0-fold and 2.7-fold higher than those of the untreated sample, respectively. This superior porosity endows the material with an excellent CO 2 adsorption capacity of 2.86 mmol/g (STP), outperforming commercial activated carbons (2.0–2.2 mmol/g). Notably, the adsorbent exhibits outstanding cycling stability, retaining > 96% of its initial capacity after 25 cycles with minimal loss (3.504%, desorption at 150 ℃). This work establishes a mechanism-driven, activator-free framework for rational carbon foam design from agricultural residues, grounded in the pyrolytic phase synchronization of lignocellulosic components.