Wood delignification with aqueous solutions of deep eutectic solvents

Abstract

The development of a sustainable wood fractionation has been a goal for the pulp and paper industries to develop a bioeconomy. The use of green solvents, in particular deep eutectic solvents (DES) has been explored to accomplish such goal. This work unveils the potential of DES aqueous solutions (at 50 wt% water content) as green solvents for the Eucalyptus globulus Labill. wood delignification. Aqueous solutions of propionic acid:urea - PA:U (2:1), urea:choline chloride - U:Ch[Cl] (2:1), lactic acid:choline chloride - LA:Ch[Cl] (10:1) and p-toluenesulfonic acid:choline chloride - PTSA:Ch[Cl] (1:1) were investigated. The delignification process was assisted by the addition of mineral (H2SO4 and HCl) or organic (PTSA) acids to efficiently promote wood matrix disruption and further lignin extraction. This process enabled solid pulp and lignin as separated fractions, which were characterized by analytical, optical and spectrometric methods. The influence of each DES on the morphology of cellulose fibers in solid pulp and the chemical structure of isolated lignin fractions was evaluated. The best system allowing delignification while preserving the cellulose fibers (comparable to Kraft cellulose pulp) and lignin structure was PA:U (2:1) aqueous solution with 25 wt% PTSA. A mild wood delignification process (363.15 K for 8 h) using this DES allowed to obtain a cellulose pulp yielding 59.50 ± 0.51 wt% of the initial wood mass and containing only 3.86 ± 0.10 wt% residual Klason lignin content. This innovative approach led to 80.64 wt% lignin extraction from E. globulus wood at mild conditions showing its potential for industrial application. Furthermore, 40.73 wt% of the initial lignin was recovered from DES liquor by a simple precipitation with water.