Unveiling Modifications of Biomass Polysaccharides during Thermal Treatment in Cholinium Chloride : Lactic Acid Deep Eutectic Solvent

• Published in: ChemSusChem Vol. 14; no. 2; pp. 686 - 698
• Main Authors: Morais, Eduarda S., Da Costa Lopes, André M., Freire, Mara G., Freire, Carmen S. R., Silvestre, Armando J. D.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 21.01.2021
• Subjects: Acids; Biomass; Bleaching; Cellulose; Cellulose esters; Cellulose fibers; Chlorides; Crystalline cellulose; deep eutectic solvents; Esterification; Fractionation; Furfural; green chemistry; Heat treatment; Kraft pulp; Lactic acid; Polysaccharides; Recyclability; solvent stability; Solvents; polysaccharides; solvent stability; biomass; deep eutectic solvents; green chemistry
• ISSN: 1864-5631; 1864-564X
• DOI: 10.1002/cssc.202002301

A deep analysis upon the chemical modifications of the cellulose and hemicelluloses fractions that take place during biomass delignification with deep eutectic solvents (DES) is lacking in literature, being this a critical issue given the continued research on DES for this purpose. This work intends to fill this gap by disclosing a comprehensive study on the chemical modifications of cellulose (microcrystalline cellulose and bleached kraft pulp) and hemicelluloses (xylans) during thermal treatment (130 °C) with cholinium chloride/lactic acid ([Ch]Cl/LA) at molar ratio 1 : 10, one of the best reported DES for biomass delignification. The obtained data revealed that [Ch]Cl/LA (1 : 10) has a negative impact on the polysaccharides fractions at prolonged treatments (>4 h), resulting on substantial modifications including the esterification of cellulose with lactic acid, shortening of fibers length, fibers agglomeration and side reactions of the hemicelluloses fraction (e. g., humin formation, lactic acid grafting). Wood delignification trials with [Ch]Cl/LA (1 : 10) at the same conditions also corroborate these findings. Moreover, the DES suffers degradation, including the formation of lactic acid derivatives and its polymerization. Therefore, short time delignification treatments are strongly recommended when using the [Ch]Cl/LA DES, so that a sustainable fractionation of biomass into high quality cellulose fibers, isolated lignin, and xylose/furfural co‐production along with solvent recyclability could be achieved. An in‐depth investigation of the thermal treatment of biomass polysaccharides with cholinium chloride/lactic acid ([Ch]Cl/LA; 1 : 10) deep eutectic solvent allowed the identification of relevant physicochemical and morphological modifications of polysaccharides, such as cellulose fiber shortening, cellulose esterification, and lactic acid grafting from xylan. The degradation of [Ch]Cl/LA (1 : 10) at prolonged treatment was also observed.