(449c) Mild Alkaline Pretreatment for Isolation of Native-like Lignin and Lignin-Containing Cellulose Nanofibers (LCNF) from Crop Waste

Biomass, including various woods, grasses, bast fibers, and agriculture straw, is a promising feedstock for the development of sustainable materials by reducing the dependence on fossil energy and petrochemical-based polymers. Common reed (Phragmites australis), a commelinid monocot, is widespread with an annual production reaching millions of tons in Chinese wetlands. In addition to its applications for woven fabric, reed straws were used to block river canals or burned to generate low-value heat. As with most monocot stem tissues, the chemical composition of the reed straw cell wall consists of 35-50% cellulose, 25-30% hemicelluloses, and 10-20% lignin. Recently, there have been many studies focusing on the complete valorization of biomass. The main approach to biomass utilization is to separate the three plant cell wall components and use them separately. Nanocellulose is a high-value product from the cellulose stream. Conventionally, lignin removal from lignocellulose has been considered to be a necessary step in the preparation of cellulose nanofiber (CNF). However, the required pulping and bleaching of lignocellulose involves harsh treatment with hazardous chemicals, which causes environmental issues. It has recently become apparent that a certain amount of added lignin provides useful features to the CNF, such as reducing polarity, lowering hydrophilicity, and delivering additional thermal stability. CNF produced directly from the lignocellulose rather than purified fiber could therefore advance the material properties of the nanofibers.

Lignin, the third most abundant component in plant cell walls, is quite sensitive to chemical processing. Conventional biomass pretreatment methods using harsh acid/base conditions and/or high-temperature organic solvents not only have sustainability problems but can also completely destroy lignin’s native structure. The condensed lignin that results is almost impossible to characterize and contains C–C bonds that are difficult to cleave. Valuable platform monomers can be obtained from various lignin ether-cleaving depolymerization methods from native or native-like lignins in which the β-ethers are predominant. However, once lignin is condensed by a pretreatment process, monomers can only be obtained in low yields. Generally, the only option left for such condensed lignins is to burn them to generate low-value heat.

Here, we present a sustainable ball-milling process in aqueous mild-alkaline solution to produce high-quality lignin-containing cellulose nanofibers (LCNF). The composition, morphology and dispersion stability of LCNF, as well as the optical and contact angle properties of the LCNF films, were characterized in this study. LCNF films showed unique properties and extra mechanical strength compared to CNF. Concomitantly, a native-like high-yield lignin stream was obtained using this process. The lignin fraction, characterized by 2D HSQC NMR, showed no significant structural changes to the polymer backbone, nor condensation, after the mild-alkaline treatment. This native-like lignin is therefore suitable for downstream lignin valorization processes.