Diol Functionalization of Lignin during Biomass Pretreatment by Deep Eutectic Solvents for Controlled Morphology Evolution at Nanoscale

Lignin chemistry plays a fundamental role in determining the properties of lignin nanoparticles (LNPs). However, it remains challenging to extract lignin with the desired chemistry/characteristics during biomass pretreatment for controlled LNP synthesis while yielding digestible pulp. In this study, diol-based deep eutectic solvents (DESs) were rationally designed to not only effectively pretreat lignocellulose but also enable in situ lignin functionalization. Most lignin (>90%) was solubilized from switchgrass along with substantial dissolution of xylan. The lignin was endowed with new attributes such as new aliphatic side chains and abundance of hydroxyl groups as well as narrower-ranged molecular weight distribution. LNPs were readily synthesized from the DES lignin with different morphologies including compact, hollow structure, and mixed. The morphology evolution can be controlled by the selection of diol. Branched diols (i.e., 1,2-propanediol, 2,3-butanediol, and 1,3-butanediol) resulted in more hollow structures and bigger nanospheres than C3–C6 linear diols. In addition to surface chemistry, the mixing rate and initial lignin concentration also influenced the formation and abundance of cavities in the nanospheres. The proportion of hollow LNPs can be tuned from 0 to 85%. Hydrophilic structural modification of lignin during diol-based DES pretreatment and particle growth-dominant assembly kinetics should be mainly involved in assembling unique hollow structures. This work provided new insights into simultaneous biomass pretreatment and lignin functionalization and its implications in LNP synthesis. The work also showcased the rationale design of diol-based DESs for having multiple functions for cost-effective and sustainable biomass conversion and upgrading.