Delignification of Olive Tree Pruning Using a Ternary EutecticSolvent for Enhanced Saccharification and Isolation of a UniqueLignin Fraction

Abstract

This work aimed at exploring the potentialities of eutectic solvents (ES) in the fractionation of olive tree pruning (OTP) biomass within a biorefinery framework, targeting efficient separation of cellulose fibers and lignin and simultaneously producing high-quality fractions for further processing and application. In this sense, delignification performances of cholinium chloride:ethylene glycol, ChCl:EG (1:9) and cholinium chloride:p-toluenesulfonic acid:ethylene glycol, ChCl:pTSA:EG (1:1:9) as binary and ternary mixtures, respectively, were first evaluated. ChCl:EG demonstrated low efficacy for biomass delignification, while the highest lignin extraction (62.7%) was achieved with ChCl:pTSA:EG at 80 °C and 4 h. At the same conditions, the cellulose content (62.5%) of the resulting solid fraction increased almost three-fold compared to that of the raw OTP (22.3%), using ChCl:pTSA:EG. This ternary ES enabled the OTP matrix breakdown, which, combined with lignin extraction, enhanced the enzymatic hydrolysis of the cellulose-rich fraction to a maximum saccharification yield of 81.8%. The sample exhibited an impressive aliphatic OH group content of 5.2 mmol·g–1 lignin, one of the highest values among the state-of-the-art. The resulting phenomenon is explained by the ethylene glycol grafting onto the lignin structure (aliphatic region), as demonstrated by 31P and HSQC NMR, giving chemical functionality to the isolated lignin fraction. Finally, up to 90% of the initial mass of ChCl:pTSA:EG was recovered through the adsorption of impurities. NMR data validated the high purity and the same molar ratio (1:1:9) of recovered ES, two important outcomes to ensure a sustainable reutilization of this solvent.