No abstract

Softwood mechanical pulps have proven to be quite recalcitrant to enzymatic hydrolysis. However, the unhydrolyzed, residual fibers might have potential as nanofibrillated cellulose feedstocks. In the work reported here, a bleached softwood chemithermomechanical pulp (CTMP) was neutrally sulfonated (S-BCTMP) in an attempt to enhance fiber accessibility and enzymatic hydrolysis. A 12 h hydrolysis at 10% solid loading with CTec3 cellulases provided optimum conditions with 22% of the pulp hydrolyzed to monosaccharides and about one-third of the original substrate remaining as lignin-containing cellulose nanofibrils (LCNFs). Prolonged hydrolysis (72 h) resulted in 42% hydrolysis of the original substrate with only 16% of the original S-BCTMP recovered as LCNFs. Although the LCNFs contained high levels of lignin (26.8%−38.5%), they were successfully used to prepare transparent films showing a high contact angle (82.8°) and strong UV-blocking properties. It was apparent that enzyme-mediated modification of CTMP has the potential to produce both fermentable sugars and higher-value LCNFs.

Although selective enzyme treatments have been used to successfully fibrillate chemical pulps, high lignin-containing mechanical pulps have proven to be more recalcitrant. When a bleached chemi-thermomechanical pulp (BCTMP) was sulfonated prior to enzymatic treatment, relatively good fibrillation was achieved, although some pulp hydrolysis occurred after 6 h hydrolysis when using a commercial cellulase enzyme preparation (Cellic CTec 3). To try to minimize pulp losses, various enzyme cocktails, including endoglucanase (EG), xylanase, mannanase, and lytic polysaccharide monooxygenase (LPMO), were assessed for their ability to enhance fibrillation while minimizing cellulose hydrolysis. It was apparent that the yield as well as the zeta potential of the lignin-containing cellulose nanofibrils increased with enzyme treatment. This was likely due to an increase in surface charge and a decrease in particle size after LPMO and hemicellulase treatments, respectively. When carbohydrate-binding modules (CBMs) were used to quantify fiber changes, it was apparent that sulfonation had increased the accessibility of enzymes, while the combined action of the hemicellulases and LPMO increased EG accessibility to the less-ordered regions of the mechanical pulp, resulting in enhanced fibrillation. This work described, for the first time, the synergistic action of EG and various accessory enzymes enhancing mechanical pulp nanofibrillation.

To be effective, steam pretreatment is typically carried out at temperatures/pressures above the glass transition point (Tg) of biomass lignin so that it can partly fluidize and relocate. The relocation of Douglas‐fir and corn stover derived lignin was compared with the expectation that, with the corn stover lignin's lower hydrophobicity and molecular weight, it would be more readily fluidized. It was apparent that the Tg of lignin decreased as the moisture increased, with the easier access of steam to the corn stover lignin promoting its plasticization. Although the softwood lignin was more recalcitrant, when it was incorporated onto filter paper, it too could be plasticized, with its relocation enhancing enzymatic hydrolysis. When lignin recondensation was minimized, the increased hydrophobicity suppressed lignin relocation. It was apparent that differences in the accessibility of the lignin present in Douglas‐fir and corn stover to steam significantly impacted lignin fluidization, relocation, and subsequent cellulose hydrolysis.

No abstract