Deep eutectic solvent fractionation of biomass and development of a multi analytical tools strategy for the characterization of obtained extracts

Abstract

The increasing demand to replace chemicals derived from fossil fuels with sustainable ones has positioned lignocellulosic biomass as a promising alternative. Among its components, lignin is the most underutilized despite being rich in aromatics. In this work, the extraction of lignin from brewery spent grain using deep eutectic solvents (DES) and its electrochemical depolymerization is studied. Two extraction temperatures (100 or140°C) were investigated. The DESs studied were choline chloride/lactic acid (ChCl/LA) at a molar ratio of 1:1 and 1:10, and a ternary DES composed of choline chloride, ethylene glycol, and oxalic acid (ChCl/EG/OA). Kamlet-Taft parameters of of the DESs were also determined using solvatochromic dyes to. for the ChCl/LA based extraction, the results revealed that both acidity (high LA content) and high temperature favored solid recovery whereas their effect on lignin purity wasn’t as visible. The ternary DES system yielded the least pure lignin despite its high solid recovery. These observations were in line with the Kamlet-Taft parameters of the DES. The difference between the acidity and basicity (α – β) of DESs as determined using solvatochromic dyes gives an insight into lignocellulose solvation potential. High values mean better the DES dissolves lignin better. The α – β value for the three DESs were: ChCl/LA 1:10) > ChCl/LA (1:1) > ChCl/EG/OA (1:2:10 w%). The structural analysis of the recovered lignin was also analyzed using FTIR and 2D HSQC NMR. FTIR spectra showed characteristic bands for aromatic skeletal vibrations (~1510 and 1630 cm⁻¹). NMR analysis confirmed the presence of guaiacyl (G) and syringyl (S) units as well as linkages such as β–O–4, β–β. Finally, electrochemical depolymerization of the extracted lignin in alkaline media using a nickel working electrode was researched. The lignin sample that was extracted with ChCl/LA (1:1) at 140°C for 3h was studied for depolymerization, and preliminary results demonstrated that lignin breakdown under mild, reagent-free conditions, as observed by its color change and reduced UV absorbance after electrolysis. This is an interesting starting point to proceed further with the identification and quantification of the monomers.