The effect of water content on lignin solubilization in deep eutectic solvents

Highly efficient, reversible and green dissolution of lignin is still a challenge, which hinders its wide scale-up applications. In this study, amino acid-polyol deep eutectic solvents (DESs) by employing l-arginine (l-Arg) and l-proline (l-Pro) as HBA and ethylene glycol (EG) and glycerol (Gly) as HBD were prepared as well as applied for dealkaline lignin (DAL) dissolution, to explore the efficient system for lignin dissolution and unveil the solubilization mechanism. The mostly used choline chloride (ChCl) based DESs were also evaluated as controlled groups. It was found that the higher value of α (hydrogen-bond donating ability) of DESs was conducive for DAL dissolution, due to the high content of β-O-4 bond in DAL. Among all the investigated DESs at water content of 2 wt%, ChCl/LA with the highest value of α (2.35) presented the best lignin dissolving ability (29.09 wt%). The effects of water content on the lignin dissolving ability of hydrated DESs were tested and discussed detailly, from a microcosmic perspective by using Fourier transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (1H NMR), solvatochromic parameters and quantum chemistry calculations. It was remarkable to find that added water greatly improved the mass transfer and activated the reaction sites in both l-Pro and l-Arg based DESs, thus enhancing or maintaining the lignin solubilities at high water concentrations. The hydrated l-Arg/EG DESs allowed a solubility enhancement of 3.49 and 2.16 times at 70 and 90 wt% water concentration, compared to l-Arg/EG/2 wt%H2O at 60 °C, which was environmentally friendly and economically attractive. However, the addition of water was a negative factor for ChCl based DESs, because water preferred to interact with Cl− with strong electronegativity, resulting in stronger interaction energy between hydrogen bond donors (HBD) and Ch+. This work provided a bridge connecting the microstructure and dissolving behavior of hydrated DESs, indicating that the addition of water could make DESs more versatile and flexible, thus boosting various potential applications such as gas capture, the dissolution and delivery of medicine.