Lateral dimensions of cellulose crystallites have been widely reported to increase as plant biomasses are submitted to a variety of high-temperature chemical treatments. Cellulose co-crystallization is often indicated to be the phenomenon underlying crystallite dimensional change, but precise mechanisms and relationships with concurrent phenomena are not well understood. This work investigates cellulose structural evolution in sugarcane bagasse submitted to wide range (160-190°C, up to &50 % solubilization) of hydrothermal treatments, performed in pressurized liquid hot water. Cellulose structural characterization combines fiber and powder X-ray diffraction with analysis of two-dimensional diffraction patterns employing the Cellulose Rietveld Analysis for Fine Structure (CRAFS) model. We observe that increases in lateral dimensions of cellulose crystallites closely follow changes in bagasse chemical composition. For a given composition, treatment temperature per se seems to be secondary for changes in crystallites. Partial cellulose de-crystallization and decreasing distortion of crystallite unit cell are found to co-occur with increasing crystallite lateral dimensions. Our interpretation of results emphasizes the importance of removing hemicelluloses acting as intercrystallite spacers, which seems to be the limiting factor for cellulose cocrystallization.

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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