Delignification of unbleached pulp by ternary deep eutectic solvents

The processing of lignocellulosic materials is very limited because of their low solubility in water and some organic solvents. Fifteen ternary deep eutectic solvents (DESs) were prepared and tested as solvents suitable for delignification of unbleached pulp. The selected ternary DESs are composed of quaternary ammonium salts and amino acids as hydrogen-bond acceptors, and organic acids and polyvalent alcohols as hydrogen bond donors, with molar ratios varying for the individual components. The delignification efficiency is significantly influenced by the degree of penetration of the solvent into the pulp fibre structure. Therefore, the density and viscosity analysis of individual solvents was performed. Unbleached beech pulp with initial kappa number 13.9 was treated with the prepared DESs. The kappa number indicates the residual lignin content or the bleachability of pulp. The efficiency of the solvents ranged from 1.4% to 28.1%. The most suitable for the pretreatment of fibres, based on lignin removal efficiency, were found to be three DESs, in the following order: malonic acid/choline chloride/1,3-propanediol (1:1:3) > choline chloride/acetamide/lactic acid (1:2:3) > choline chloride/urea/lactic acid (1:2:3).