Rotas verdes para obtenção de nanocelulose em biorrefinarias: desenvolvimento de estratégias via hidrólise enzimática e com ácidos orgânicos

Abstract

The growing demand for materials from sustainable sources with potential to replace petroleum-based polymers in technological applications projects lignocellulosic biomass as a promising raw material. In this context, nanocellulose has attracted interest due to its numerous properties and functionalities. In this work, different green routes associated with the concept of biorefinery to obtain cellulose nanocrystals (NCC) and cellulose nanofibrils (NFC) in biorefineries were studied. Initially, the use of different enzymatic cocktails was investigated obtain nanocellulose from eucalyptus cellulose pulp (model material) were evaluated. The results showed that cocktails with greater activity of the enzymes xylanases and endoglucanases favored the achievement of more uniform nanometric structures. In contrast, higher glucose release was reached when the enzymatic hydrolysis was performed by complexes containing the class of polysaccharide lytic monoxygenase enzymes (LPMO), which also resulted in nanostructures with higher crystallinity levels. Another method evaluated for nanocellulose extraction was the hydrolysis with citric acid, an organic acid that can be obtained in a biorefinery concept. This approach resulted in a single-step extraction and esterification of the nanocelluloses. Also, the charged surface groups of these nanomaterials improved the colloidal stability of the suspensions in aqueous medium. Another advantage of this method was the higher thermal stability of these nanomaterials compared to those obtained by using sulfuric acid. Based on these results, the use of the enzymatic route and citric acid for sugarcane bagasse was evaluated. Initially, study involved hydrothermal treatment, organosolv followed by the sugarcane bagasse bleaching by a mixture of hydrogen peroxide and sodium hydroxide. After these treatments, the bagasse was hydrolyzed by a commercial cellulolytic cocktail in order to obtain the glucose release integrated with the production of nanostructures. The subsequent study involved the enzymatic hydrolysis of the pre-treated bagasse by the hydrothermal process, which after the release of fermentable sugars was treated by the organosolve method and successively bleached submitted. The bleached residual material was then submitted to three different acid treatments: sulfuric acid, citric acid and a mixture of both acids. In both studies, after the organosolv treatment, 55.8% of the residual lignin was recovered with the potential to be used in other applications. The enzymatic route allowed the obtention of nanostructures with high thermal stability associated with a high glucose release, which can be used in the production of ethanol, organic acids and other bioproducts. The extraction by citric acid and by using a mixture of acids resulted in a nanocellulose with high thermal stability and good colloidal stability, attributed to the presence of charged functional groups on the material surface. In addition, all methods resulted in nanocellulosic materials with a high crystallinity index. Finally, the results here obtained by using different green routes for nanocellulose production may contribute to the selection of the method according to the desired application, in order to add value to the biorefineries residues.