Direct lignin depolymerization process from sulfur-free black liquors

Abstract

Agricultural residues (olive tree pruning and almond shell) were subjected to different delignification treatments (organosolv and soda) and the obtained liquors were treated in a high-pressure reactor at 300 °C for 80 min to depolymerize the dissolved lignin. In this way, the step of precipitating lignin from the liquor was avoided. The phenolic oil obtained after liquors treatment was around 20% of the organic matter contained in previous liquors in all cases. However, phenolic monomeric compounds varied in function of the liquor source. Soda black liquors produced higher quantity of catechols, phenol and cresols whereas using organosolv black liquors, more guaiacol and syringol were obtained, highlighting the higher potential enabled by base catalyst for demethoxylation, demethylation and dealkylation reactions. Furthermore, the NaOH present in soda black liquors prevented undesirable repolymerization reactions by inhibiting the char formation and noticeably dropping the molecular weight of residual lignin. However, organosolv liquors presented a significant higher yield of phenolic monomers, about three times higher than the one obtained in the soda process. Residual lignin, which was not only unconverted lignin, was proved to be different from the initial lignin, pointing out the totally conversion of the initial lignin samples.