BackgroundIn recent years, biorefining of lignocellulosic biomass to produce multi-products such as ethanol and other biomaterials has become a dynamic research area. Pretreatment technologies that fractionate sugarcane bagasse are essential for the successful use of this feedstock in ethanol production. In this paper, we investigate modifications in the morphology and chemical composition of sugarcane bagasse submitted to a two-step treatment, using diluted acid followed by a delignification process with increasing sodium hydroxide concentrations. Detailed chemical and morphological characterization of the samples after each pretreatment condition, studied by high performance liquid chromatography, solid-state nuclear magnetic resonance, diffuse reflectance Fourier transformed infrared spectroscopy and scanning electron microscopy, is reported, together with sample crystallinity and enzymatic digestibility.ResultsChemical composition analysis performed on samples obtained after different pretreatment conditions showed that up to 96% and 85% of hemicellulose and lignin fractions, respectively, were removed by this two-step method when sodium hydroxide concentrations of 1% (m/v) or higher were used. The efficient lignin removal resulted in an enhanced hydrolysis yield reaching values around 100%. Considering the cellulose loss due to the pretreatment (maximum of 30%, depending on the process), the total cellulose conversion increases significantly from 22.0% (value for the untreated bagasse) to 72.4%. The delignification process, with consequent increase in the cellulose to lignin ratio, is also clearly observed by nuclear magnetic resonance and diffuse reflectance Fourier transformed infrared spectroscopy experiments. We also demonstrated that the morphological changes contributing to this remarkable improvement occur as a consequence of lignin removal from the sample. Bagasse unstructuring is favored by the loss of cohesion between neighboring cell walls, as well as by changes in the inner cell wall structure, such as damaging, hole formation and loss of mechanical resistance, facilitating liquid and enzyme access to crystalline cellulose.ConclusionsThe results presented herewith show the efficiency of the proposed method for improving the enzymatic digestibility of sugarcane bagasse and provide understanding of the pretreatment action mechanism. Combining the different techniques applied in this work warranted thorough information about the undergoing morphological and chemical changes and was an efficient approach to understand the morphological effects resulting from sample delignification and its influence on the enhanced hydrolysis results.

This work investigates the evolution of cellulose crystals from sugarcane lignocellulose (bagasse and leaves) submitted to prehydrolysis (hydrothermal, dilute acid, or steam explosion) and soda delignifications. Raw and treated materials are characterized by X-ray diffraction with modeling of area-detector patterns. Three treatment effects are correlated: increase in cellulose content, quantified by strong acid hydrolysis; increase in average cellulose crystallite width, inferred from sharper 200 diffraction peaks; and decrease in crystallite distortion, evidenced by d 200 -spacing approaching reference values. Crystal contents measured according to recent developments (in Driemeier and Calligaris, J Appl Cryst 44:184-192, 2011) are compared to cellulose contents. Limitations for this comparison are discussed. Results are consistent with minimum non-crystalline cellulose in raw lignocellulose, and with partial cellulose decrystallization or more defective crystallites in treated materials.

A simplified kinetic model for autohydrolysis of hemicelluloses from sugarcane bagasse was evaluated in the context of a global process in a batch reactor. In this work, all sugars, oligomers, and decomposition products from hemicelluloses were taken into account in the determination of kinetic parameters. This approach has not been reported in previous kinetic studies which assume just the xylan, xylose, and furfural as compounds from hemicelluloses. Experimental results for hemicelluloses removal from residual solids and dissolved fractions in the liquor are reported at various temperatures and reaction times. Because of the decomposition of sugars in the pretreatment liquors, optimal temperatures and reaction times were maintained at 170°C and 90 min. Under these conditions, 61.7 % of hemicelluloses were converted to oligomeric and monomeric sugars. In addition, 90 % of cellulose was preserved in the residual solid fraction. Correlation parameters between the kinetic model and experimental data validate the proposed model, although deviations from an Arrhenius-type model were observed. The reaction steps for production of oligomers, monomers, and sugar derivatives/decomposition compounds present activation energies of 143.1, 158.9, and 138.3 kJ/mol, respectively.

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