Dissolution of Lignocelluloses with a High Lignin Content in a N-Methylmorpholine-N-oxide Monohydrate Solvent System via Simple Glycerol-Swelling and Mechanical Pretreatments

Lignocelluloses (LCs) with various amounts of lignin (even as high as 18.4%) were successfully dissolved in N-methylmorpholine-N-oxide monohydrate (NMMO/H2O) solution with stirring at 85 °C within 5 h. For the developmental dissolution methods of LCs with a high lignin content in NMMO/H2O solution, the following two pretreatment steps of LCs were necessary: (1) glycerol swelling and (2) mechanical extrusion. The mechanical extrusion pretreatment under glycerol swelling dissociated the fiber bundles of LCs to thinner fibers and, thus, enhanced the accessibility and solubility of the LCs in NMMO/H2O. The crystal structure of the pretreated LCs had no significant transformation during pretreatment, while the diameters of the fiber bundles were reduced from 50–60 to 10–12 μm, as investigated by X-ray diffraction and scanning electron microscopy. After the dissolution–regeneration process of LCs, the fiber bundles of the LCs disappeared and the crystal type of cellulose in the LCs was transformed from cellulose I to cellulose II, which indicated the complete dissolution of LCs.