Abstract
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-hydroxyethylammonium 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 (HEAF) 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 (HEAO) 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.
Highlights
The current concerns about the environmental impact associated to fossil fuel consumption, the high energy demand and the growing need for commodities have been pushing the interest to use renewable resources, such as biomass, to provide biobased and sustainable products (Chang et al, 2017)
The water content was measured after this step using a Metrohm 831 Karl Fischer coulometer and determined values were considered in the preparation of Protic Ionic Liquids (PILs) aqueous solutions
Neat and aqueous solutions of hydroxyethylammonium propionate (HEAP), bis (2-hydroxyethyl)ammonium propionate (BHEAP) and tris(2-hydroxyethyl)ammonium propionate (THEAP) that share propionate anion were examined for Kraft lignin dissolution
Summary
The current concerns about the environmental impact associated to fossil fuel consumption, the high energy demand and the growing need for commodities have been pushing the interest to use renewable resources, such as biomass, to provide biobased and sustainable products (Chang et al, 2017). The state-of-the-art shows good results in the application of neat PILs or their aqueous solutions to dissolve lignin and to achieve efficient and sustainable biomass delignification. Limited information regarding the structural modifications of the recovered lignin has been provided in literature In this sense, the present work contributes to suppress all these gaps by giving a comprehensive understanding of the solubility of Kraft lignin (as a model macromolecule) in PILs aqueous solutions composed of alkanolammonium cations and carboxylate anions with different aliphatic chain lengths. The recovery and reuse of PIL was addressed in this work
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