Exploring a new platform to separate Kraft lignin from choline chloride-based eutectic solvents using aqueous mixtures of 1-butanol or ethyl acetate

Abstract

Eutectic Solvents (ES) have emerged as sustainable media for lignocellulosic biomass delignification. However, the lack of efficient downstream strategies to separate the extracted lignin from the ES-rich mixtures remains a significant industrial bottleneck. This study investigates the potential of aqueous biphasic systems containing organic cosolvents (1- butanol or ethyl acetate) to recover lignin from choline chloride (ChCl)-based ESs. Experimental binodal curves and Kraft lignin partition coefficients (KL) were determined for 12 systems containing water, a cosolvent, and ES formed by ChCl and six hydrogen-bond donors (three carboxylic acids and three diols). Results indicate that 1-butanol induces larger immiscible regions compared to ethyl acetate, while larger biphasic regions were observed in systems containing acid-based ESs than in those containing diols. To complement the experimental studies, the Merchuk and Hu equations were used to correlate the binodal curves, with the Hu model yielding superior representations (R2 � 0.999). Remarkably, Kraft lignin preferentially partitioned into the organic-rich top phase (KL > 1) in all cases. Promising results were achieved for the (water þ 1-butanol þ ChCl:FA) and (water þ ethyl acetate þ acid-based ES) systems (11.3 � KL � 29.6), suggesting these water-cosolvent mixtures are auspicious routes for recovering lignin from acid-based ES media.