Microwave-assisted deep eutectic solvent extraction of lignin from spruce heartwood and sapwood, targeting the comparison of different biorefinery concepts
Jiayun Xu, Chuanling Si, Lin Dai, Jarl Hemming, Andrey Pranovich, Chunlin Xu
Abstract
• Up to 80% of lignin yield from HW, up to 86% of delignification from SW obtained. • Hot water extraction enhances lignin purity but does not improve delignification. • The cellulose loss of HW was about 3.8 times higher than that of SW. • The PhOH contents in lignin obtained through MW-DES and HWE-MW-DES are similar. 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.