Cation-exchangeable metal cations govern the thermal reactivity of hemicellulose and cellulose across wood species: Implications for cell wall ultrastructure

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Abstract

The effects of cation-exchangeable alkali (Na⁺, K⁺) and alkaline earth (Mg²⁺, Ca²⁺) metal cations on the thermal reactivity of cellulose, hemicellulose, and lignin—the main components of wood cell walls—were investigated using five softwood and five hardwood species. Uronic acid groups in pectin and xylan are likely to form salts with these cations. The thermal reactivity of cellulose and hemicellulose was evaluated based on the retention of hydrolyzable sugars in pyrolysis residues, while lignin degradation was analyzed by thermogravimetric-mass spectrometry (TG-MS). The results showed that the type of metal cation, rather than the wood species, primarily determines the reactivity of cellulose and hemicellulose. Exchange with Na⁺ or K⁺ caused cellulose to degrade about 40 °C lower than with Mg²⁺, Ca²⁺, or H⁺. Glucomannan and lignin showed similar cation-dependent trends, while xylan was largely unaffected. These findings offer new insights into the ultrastructural organization of wood cell walls, suggesting that glucomannan, xylan, and lignin coagulate in a specific manner on the surface of cellulose microfibrils. This understanding also contributes to the development of efficient thermochemical and biochemical wood biorefinery processes.

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