Biomass-based Acidic Deep Eutectic Solvents for Efficient Dissolution of Lignin: Towards Performance and Mechanism Elucidation

Lignocellulose utilization has the unique feature of net-zero carbon emissions. Thus, the utilization of lignocellulose as a renewable alternative to fossil-based carbon resources is one of the most promising strategies to achieve "carbon neutrality". Lignin has been recognized as the most abundant aromatic biopolymer on Earth; hence, it could be the most promising alternative to fossil-based aromatics. The efficient dissolution of lignin is crucial for lignin upgrading, which relies on the design of innovative and robust solvents. Herein, we designed several biomass-derived acidic deep eutectic solvents (DESs) using choline chloride, betaine, and L-carnitine as hydrogen bond acceptors (HBAs), and four protic compounds as hydrogen bond donors (HBDs), namely, oxalic acid, benzoic acid, ethyl gallate, and 5-methoxysalicylic acid. The designed DESs can dissolve different types of lignin, including alkali lignin (AL), dealkaline lignin (DAL), enzymatic hydrolysis lignin (EHL), and Kraft lignin (KL). Lignin dissolution was found to be affected by the relative contents of three phenylpropanoid monomers in lignin: syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units. More S and fewer H units in lignin could result in higher solubility. G-, S-, and H-type structural units were found in EHL, while AL, KL, and DAL had only G-type structural units. Therefore, EHL could be more easily dissolved than AL, KL, and DAL in the most developed DESs. The hydroxyl group content of the four lignin samples had a significant impact on lignin dissolution. AL (1.98 mmol center dot g-1) and EHL (1.93 mmol center dot g-1) had much higher contents of phenolic hydroxyl groups than DAL (0.62 mmol center dot g-1), implying that AL and EHL had higher polarity than DAL. This resulted in different dissolution behaviors in different DESs with varying polarities. However, the sulfonate groups afforded KL with much higher polarity, thus resulting in the special dissolution behavior of KL. It is to be noted that not all cases of dissolving lignin in the developed DESs conformed to the above rules. Therefore, it is necessary to further explore the effect of the properties of the DESs on the dissolution of different lignins. Choline chloride was the preferred HBA to construct DESs with good performance and adaptability to lignin dissolution, whereas suitable acidity enabled benzoic acid and ethyl gallate to be favorable HBDs. Systematic investigation revealed that an efficient DES for lignin dissolution should possess stronger hydrogen-bonding acidity (alpha > 0.95) and appropriate polarity matching with the dissolved lignin. In addition, the pKa value of the HBD and the acidity of the DESs were also efficient indices for estimating the performance of an acidic DES in dissolving lignin, and the pKa value and acidity could be well correlated with the polarity. Generally, HBDs (e.g., BA and EG in this study) with moderate pKa values can be employed to construct robust DESs to dissolve lignin with satisfactory solubility. Additionally, the viscosity of the DESs should have an impact on lignin dissolution, and a lower viscosity is helpful for dissolving lignin. Therefore, the better performance of the developed ChCl-based DESs than the Betaine-and L-Carnitine-based DESs was partially because of the lower viscosity of the ChCl-based DESs.