Acidic depolymerization vs ionic liquid solubilization in lignin extraction from eucalyptus wood using the protic ionic liquid 1-methylimidazolium chloride

Abstract

Protic ionic liquids have been proposed as effective solvents for the selective extraction of lignin from wood. In this work, the protic ionic liquid 1-methylimidazolium chloride has been used to extract lignin at different biomass loadings, temperatures, and times to understand the influence of treatment severity on the lignin dissolution mechanism. Themaximumlignin recovery (82.35 g lignin/100 g biomass lignin) was achieved at 10% (w/w) biomass loading, 135 °C, and 6 h. An increase in treatment severity leads to an acid cleavage of ether linkages, which increases the average molecular weight and thermal stability of lignins due to C-C repolymerization. HSQC-NMR analysis showed the effect of operating conditions on the predominant mechanismof lignin depolymerization. Atmild conditions, there is a preferential degradation of G units (the typical depolymerization mechanism of ionic liquid treatments); but at the most severe conditions, S units are predominantly removed, as usually occurs in acidic treatments. This work contributes to better understanding the different lignin extraction mechanisms occurring with a protic ionic liquid depending on different operating conditions.