Thermal stability of wood cell walls determined by lignin and exchangeable cations: Insights from holocellulose-based analysis

The thermal reactivity of wood components is influenced by their supramolecular environment within the cell wall. This study investigated the effects of lignin and exchangeable cations on the thermal reactivity of wood polysaccharides using holocelluloses prepared by delignification of five softwood and five hardwood species, followed by exchange with H⁺, Na⁺, K⁺, Mg²⁺, or Ca²⁺. Thermal decomposition was examined by thermogravimetric analysis, hydrolysable sugar analysis of pyrolysis residues, and TG-MS. Delignification markedly increased the thermal reactivity of cellulose and hemicelluloses, particularly cellulose. The large cation-dependent differences observed in non-delignified wood were substantially reduced after delignification, although Ca²⁺-exchanged holocelluloses remained relatively thermally stable. Despite exchangeable cations being associated primarily with uronic acid residues in xylan, their effects were greater on cellulose and glucomannan than on xylan. Approximately 10% of lignin remained after delignification, and its decomposition temperature shifted with cation identity in parallel with that of cellulose; glucomannan showed a similar but smaller response. These coordinated changes suggest close supramolecular associations among cellulose, glucomannan, and a portion of lignin. The results demonstrate the importance of both supramolecular organization and exchangeable cations in determining the thermal reactivity of wood components, providing insights relevant to wood cell-wall structure and thermochemical conversion.