Microwave-assisted deep eutectic solvent extraction of lignin from spruce heartwood and sapwood, targeting the comparison of different biorefinery concepts

Lignin-first fractionation (microwave-assisted deep eutectic solvent fractionation, MW-DES) and carbohydrate-first fractionation (hot water extraction followed by MW-DES, HWE-MW-DES), were compared while pretreating spruce heartwood (HW) and sapwood (SW) for lignin extraction. The lignin recovery yield was 80 % for HW and 74 % for SW, while SW demonstrated high selectivity in delignification, achieving 86 % removal of lignin with only minor cellulose loss (3.6 %) treated with ChCl/LA (1:2) at 145 °C for 10 min. The carbohydrate-first approach gives an opportunity to obtain lignin with a lower carbohydrate content (∼0.5 %) and higher lignin purity (88 %). The structure of lignin exhibited no significant differences after two different fractionation approaches, with the phenolic hydroxyl content in lignin in the amount of 2.59/2.73 mmol/g (MW-DES) and 2.54/2.74 mmol/g (HWE-MW-DES) from HW/SW. The above provides a theoretical basis for developing and optimizing the valorization of lignin from spruce HW/SW, employing either lignin-first or carbohydrate-first fractionation approaches of the sustainable biorefinery processes.