Cellulose nanofibers from eucalyptus pulp and their coproduction with biobutanol through enzyme-mediated treatment
Nanocellulose has potential applications in various sectors ranging from biomedical to environmental fields. The possibility of developing a wide range of potential applications could be attributed to the abundance and sustainability of its resources. Even though this emerging biomaterial has many o...
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| Format: | doctoralThesis |
| Language: | English |
| Published: |
2023
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| Subjects: | |
| Online Access: | https://hdl.handle.net/20.500.12008/37054 |
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| Summary: | Nanocellulose has potential applications in various sectors ranging from biomedical to environmental fields. The possibility of developing a wide range of potential applications could be attributed to the abundance and sustainability of its resources. Even though this emerging biomaterial has many outstanding properties and high availability in nature, the extraction of them from renewable feedstocks such as lignocellulosic biomass or cellulosic materials is still a main challenge. Cellulose is a structural material and, due to its association with lignin and hemicellulose, it is rather resistant to decomposition. To achieve the extraction of nanocellulose, energy-intensive methods or the consumption of chemicals or costly enzymes are required for its processing, which limits the implementation of its large-scale processing. In this work, enzymatic pretreatment combined with mechanical fibrillation was proposed as an alternative strategy for the isolation of cellulose nanofibers (CNF) from eucalyptus cellulose pulp. The enzyme-mediated pretreatment allowed to relieve intrinsic recalcitrance of fibers by size reduction and partial degradation, promoting cellulose accessibility for subsequent CNF extraction. Depending on the type of enzyme used and the working conditions, the release of soluble sugars from the carbohydrate fractions (cellulose and xylan) results inevitable. When cellulase and/or xylanase complexes were used, extensive cellulose (up to 46%) and xylan (up to 85%) hydrolysis occurred, even for short reaction times (4 h). Thus, fermentable sugars such as glucose (20-40 g/L) and xylose (6-13 g/L) were available as coproducts. These fermentable sugars were recovered and completely converted to biobutanol and other solvents (up to 15 g/L) by microbial fermentation employing Clostridium strains for an integral exploitation of the raw material. Biobutanol is considered an attractive product since it has superior properties as a biofuel compared to bioethanol, as well as being an important chemical in different industries. When endoglucanase monocomponent was used, almost negligible carbohydrate degradation occurred (<5%), which limited fiber fragmentation and depolymerization for further mechanical processing. Two mechanical methods were evaluated for the extraction of CNF from enzymatically pretreated eucalyptus pulp. Ultrasonication demonstrated to be an effective mechanical method for CNF isolation from local eucalyptus pulp, allowing to achieve CNF extraction yields of up to 98% from cellulase-treated pulp. Physicochemical and microscopic characterization of extracted CNF revealed significant uniformity of sizes in the nanometer scale (531-992 nm) with diameters in the range of 3 to 10 nm. Highly transparent and flexible nanocellulosic films were produced, which results attractive for potential applications. However, considering that the industrial application of ultrasonication is still limited, ball milling treatment was investigated as an alternative mechanical method for CNF extraction. Ball milling allowed to obtain CNF suspensions at higher consistency, but the high solid concentrations limited the effectiveness of the CNF extraction process and lower extraction yields (up to 35%) were obtained compared to ultrasonication. A sequential ball milling treatment configuration (up to 5 stages) allowed to improve up to 48% the CNF extraction yield from cellulase-treated pulp. CNFs with diameters in the range of 4 to 14 nm and aspect ratios of 220-230 were obtained by ball milling treatment, with enhanced thermal properties and crystallinity. Flexible nanocellulosic films were produced, even though they were not highly transparent compared to ultrasonicated CNFs. In summary, this work highlights the feasibility of coproducing CNF and biobutanol as high value-added products using eucalyptus cellulose Kraft pulp from a local pulp mill. This work provided insights into the coproduction of biomaterials and value-added chemicals in a forestry biorefinery concept, which could increase the overall revenue of the process. In addition, it intends to contribute to the knowledge of new techniques for obtaining nanocellulose derived from renewable sources, and its potential application. |
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