Kinetic Study of the Acid Post-hydrolysis of Xylooligosaccharides from Hydrothermal Pretreatment

Hydrothermal pretreatment of sugarcane bagasse is a water-based and environment-friendly process that results in almost complete hemicellulose solubilization in oligomeric form as xylooligossacharides (XOs). However, the soluble XOs cannot be utilized by microorganisms such as yeasts, and therefore, a further break down is necessary to generate pentose (C5) monomers that can be then biotransformed into ethanol or other metabolites. The kinetics of XOs post-hydrolysis with sulfuric, maleic, and oxalic acids (the latter two being dicarboxylic acids) in a sugarcane bagasse hemicellulosic hydrolysate was assessed in a bench-scale reactor (2 L). By means of a 2 2 full factorial design with center point triplicate, acid mass loading and temperature were varied from 0.5 and 2.0% and from 120 to 150 °C, respectively. An irreversible first-order consecutive reaction model of the hydrolysis of XOs in liquid medium was employed. Based on an Arrhenius-type equation, a kinetic parameter estimation was performed with genetic algorithms and the Runge-Kutta methods. For the three acids, the calculated exponential factors, A 0 n ( n = 1, 2, and 3), ranged from 10 12 to 10 15 min −1 ; the dimensionless parameters, m n ( n = 1, 2, and 3), ranged from 0.86 to 1.97; and the activation energies ranged from 89 to 129.8 kJ·mol −1 . The model—developed at microscale—correctly described the observed XOs, C5, and furfural post-hydrolysis profiles in bench scale and proved the dicarboxylic acids were more selective towards post-hydrolysis by having slower kinetics than sulfuric acid.