Catalytic Acidolysis: Impact on In Situ Pine-Lignin Repolymerization

Biofuels research has substantially improved our understanding of lignin reactivity, and this knowledge has broader application potential in carbon-storing materials. The abundance and reactivity of β-aryl ether linkages underly lignin’s natural tendency to cleave and self-heal via repolymerization––this reaction might feasibly be manipulated to create novel advanced composite materials. To assess this potential, in situ lignin cleavage and repolymerization were promoted by heating water-saturated Pinus taeda wood, with and without acid catalyst. Evidence of in situ lignin cross-linking was detected in the corresponding isolated milled wood lignin; the respective molecular weights and glass-transition temperatures (T_g) increased in the catalytic order of no added catalyst, HCl, and H₂SO₄, as expected. In the corresponding whole tissue samples, oxidative thermogravimetric analysis detected in situ lignin cross-linking but with detection limitations at extreme cross-linking levels. Solvent submersion dynamic mechanical analysis of whole tissues revealed that all heating conditions caused a reduction in the wood T_g, even without catalyst, where the lignin reaction and sugar degradation were the lowest. These observations indicated that in situ lignin reactions occur readily and that some degree of catalytic control is possible, and lignin-specific reactions merit study toward new carbon-storage opportunities in lignocellulosic composite materials.