Integrated Biorefinery Strategies for Enhanced Production of Cellulose Nanocrystals and Fermentable Sugars from Wood
Creators: Haddis, Dagem Zekaryas
Publisher: name: University of Alberta Library
Year: 2024

<p>The industrial process to produce bioethanol from lignocellulosic biomass has focused on complete or nearly complete hydrolysis of cellulose to fermentable sugars. This entails utilizing different pretreatments and high amount of advanced cellulase cocktail to deconstruct the lignin and hemicellulose and depolymerize the recalcitrant crystalline region of cellulose to fermentable sugars. Thus, these enzymatic approaches are often associated with high costs due to the cost of cellulase cocktails. Therefore, to offset such economic challenges, biorefinery strategies need to be designed that provide high fermentable sugars recovery with additional high value-added products from lignocellulosic biomass. In this context, the crystalline region of cellulose serves as a precursor for producing a nanostructured material called cellulose nanocrystals (CNCs), while the amorphous chains can be hydrolyzed to sugars and subsequently fermented to ethanol. In this thesis a biorefinery strategy was designed to enhance the yield of CNCs and fermentable sugars through an integrated process beginning with either hydrothermal or steam explosion pretreatment followed by enzymatic hydrolysis and then acid hydrolysis.<br>
The first study investigated the production of CNCs and fermentable sugars from wood pulp using a hybrid process that combined hydrothermal treatment, enzymatic hydrolysis, and acid hydrolysis. Hydrothermal treatment at 200°C improved cellulose crystallinity, forming CNC precursors through molecular reorientation. Subsequent enzymatic hydrolysis, employing a cellulase cocktail, released fermentable sugars from amorphous cellulose. Glucose and xylose yields plateaued at 24 h (32.8 ± 0.3 wt%) and (3.3 ± 0.2 wt%), respectively, during the 6-24 h enzymatic treatment. The hydrothermal and enzymatic treatment process significantly increased CNC yield by 2.1-fold and 1.4-fold compared to untreated wood pulp and hydrothermally treated<br>
pulp alone, using acid hydrolyzed feedstock respectively. Consequently, the hybrid treatment improved overall CNC yield by 1.3-fold compared to untreated wood pulp, and it showed no significant difference compared to hydrothermally treated pulp alone. Furthermore, CNC yield started decreasing after 18 hours of enzyme treatment, likely due to the enzyme depolymerizing the crystalline region of the cellulose. Further analysis confirmed comparable CNC quality in the hybrid treatment through crystallinity, zeta potential, and thermal stability analysis.<br>
The second study explored the impact of steam explosion pretreatment on the yield of CNCs. Conducting steam explosion prior to acid hydrolysis enhanced the crystallization of semi-crystalline/non-crystalline cellulose, generating additional CNC precursors from poplar wood as a feedstock. The crystallinity of steam-exploded poplar wood increased by 1.3-fold compared to untreated poplar wood, resulting in a 2.5-fold increase in the overall CNC yield. Importantly, the steam explosion pretreatment did not compromise CNC quality in terms of crystallinity and colloidal stability. However, the thermal stability of the CNCs improved due to an increase in crystal size caused by steam explosion. This study showcases a straightforward and scalable pretreatment approach that significantly enhances CNC yield during the acid hydrolysis step, thereby improving overall economic viability and commercial potential.<br>
The third study extended the work from the second study by introducing an enzymatic hydrolysis step between steam explosion pretreatment and acid hydrolysis to produce CNCs and fermentable sugars from poplar wood. The objective was to achieve efficient saccharification of amorphous cellulose and enhancing CNC yield from acid hydrolysis reactions. The 24 h enzyme treatment showed increasing glucose yield over time for both untreated and steam-exploded poplar wood, with optimal improvement observed within 12 h for untreated wood (6.9 ± 0.1 wt %) and 18 h for steam-exploded poplar wood (29 ± 1 wt %). The xylose yield in untreated poplar wood did not<br>
significantly increase with prolonged hydrolysis time, while in steam-exploded poplar wood, it increased until 18 h. Results indicated that steam explosion pretreatment and enzyme hydrolysis significantly enhanced the crystallinity of poplar wood, supporting the hypothesis increased accumulation of CNC precursors. This resulted in higher CNC yield for steam-exploded poplar wood (68.1 ± 0.4 wt % acid hydrolysis feedstock) compared to untreated poplar wood (23 ± 2% acid hydrolysis feedstock). Further characterization confirmed the combined steam explosion and enzymatic treatment of poplar wood resulted in CNCs of comparable quality, as evidenced by the stability of CNC suspensions, degree of crystallinity, and thermal stability analysis. The combined steam explosion and enzymatic treatment process proved effective in significantly enhancing CNC yield from poplar wood.</p>