Uncovering the potentialities of protic ionic liquids based on alkanolammonium and carboxylate ions and their aqueous solutions as non-derivatizing solvents of Kraft lignin


The present study scrutinized in depth the ability of alkanolammonium-based Protic Ionic Liquids (PILs) with carboxylate anions to dissolve Kraft lignin at 323.15 K. A focus was put on understanding the role of both PIL. ions and water on the dissolution process. The results demonstrated that the anion plays a more important role in lignin dissolution than the cation. Furthermore, lignin dissolution was favored by increasing the alkyl chain of the carboxylate anion, while a smaller cation with lower number of hydroxyalkyl groups performed better. Among the studied solvents, the 2-hydroxyethylarnmonium hexanoate (HEAH) displayed the highest lignin solubility (37 wt%). In general, the addition of water had a negative influence on lignin solubility with the tested PILs. A sharp decrease in lignin solubility curves of 2-hydroxyethylammonium formate (HEAP) and acetate (HEAA) was observed, while a more softly effect was observed for 2-hydroxyethylammonium propionate (HEAP) and HEAH with the addition of water. However, a distinct behavior was observed for 2-hydroxyethylammonium octanoate (HEAD) that acted as hydrotrope enhancing lignin solubility in aqueous solutions to a maximum value at 40 wt% water content. Furthermore, by increasing the temperature, the lignin solubility was favored due to endothermic behavior of lignin dissolution process. The dissolution of Kraft lignin was also performed at 393.15 K to unravel any lignin modification unleashed by PILs. GPC, FTIR-ATR and 2D NMR were employed for lignin characterization and the changes observed between native lignin and recovered lignin samples were negligible demonstrating the non-derivatizing character of the PILs. Moreover, the recycle of 2-hydroxyethylammonium propionate (HEAP) was successfully demonstrated for at least 3 cycles. In this way, PILs are herein revealed as promising solvents to apply in lignin valorization towards more efficient and eco-friendly processes.



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Dias, RM; Lopes, AMD; Silvestre, AJD; Coutinho, JAP; da Costa, MC

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This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, FCT Ref. UID/CTM/50011/2019, financed by national funds through the FCT/MCTES. The work was also funded by Fundacdo para a CMncia e Tecnologia (FCT) through the projects DeepBiorefinery [PTDC/AGR-TEC/1191/2014] and MultiBiorefinery [POCI-01-0145-FEDER-016403]. Authors acknowledge ISPT for funding A. M. da Costa Lopes postdoctoral grant. This study was financed in part by the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior - Brasil (CAPES) - Finance Code 001. The authors would like also to thank national funding agencies the Banco Santander S. A., FAPESP [2014/21252-0, 2016/08566-1], CNPq [169743/2018-7, 310272/2017-3] and FAEPEX/UNICAMP for financial support. The Suzano Papel & Celulose was gratefully acknowledged for isolated Kraft lignin. The authors also thank Professor Dmitry Evtyugin for his kindly support in the analysis of lignin samples in GPC equipment.

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