Selective precipitation of ionosolv lignins: Replacing water with low enthalpy of vaporization antisolvents

Abstract

Ionic liquids (ILs) have shown promising results in the extraction of lignin in biorefineries using the Ionosolv process. Isolating lignin from the IL-lignin solution to enable IL recycling is a critical step for the viability of this technology. The use of water as a lignin antisolvent is a well-established practice. However, the subsequent distillation of the antisolvent inevitably results in considerable energy expenditure due to the high enthalpy of vaporization of water. In this context, the substitution of water by solvents with low enthalpy of vaporization has been investigated. Lignin was extracted from eucalyptus wood using the IL 2-hydroxyethylammonium acetate or monoethanolammonium acetate ([MEA][OAc]). Alternative antisolvents tert-butanol, 2-propanol, and 1-butanol were tested for lignin precipitation and their energy recovery cost, was calculated and compared with the base case (water as antisolvent). The recovered lignins were characterized by a combination of techniques, including gel permeation chromatography (GPC), two-dimensional nuclear magnetic resonance (2D‐NMR),and thermogravimetric analysis (TGA), with the objective of elucidating the extracted lignin chemical structure and thermal stability. Lignins precipitated with alcohols had higher average molecular weights and lower polydispersity indexes compared to those precipitated with water. Furthermore, lignins precipitated with alcohols exhibited remarkable thermal stability at high temperatures and were characterised by an abundance of β-O-4’ linkages and S units. Consequently, this work proposes an alternative approach to the separation of ionosolv lignins from ILs with lower associated energy consumptions, while simultaneously delivering lignins with enhanced characteristics in comparison to the conventionally accepted Ionosolv process.