Thermal pretreatment for fractionation and solubilization studies of lignocellulosic materials have shown the efficiency to improve the yields of extraction of hemi — celluloses. Boussarsar [20] have evaluated the SCB conversion by hydrothermal treatment. Optimal conditions were 170°C for 2 h, reaching higher solubilization of hemicellulose than that at 150°C and lower degradation of sugar monomers than at 190°C. However, analysis of thermal hydrolysates shows the presence of xylan oligomers and polymers with large chains. On the other hand, Sendelius [166] has evaluated the steam pretreatment conditions with respect to final ethanol yield, using SCB as feedstock. The variables considered were temperature (180, 190, and 205°C), time (5 and 10 min), and impregnating agents (water, 2% SO2 by weight of water in the bagasse and 0.25 g H2SO4 per 100 g dry matter). The most prominent tested pretreatment condition was: SO2-impregnation at a temperature of 180°C during 5 min, which gave a glucose yields in average 86.3% and xylose yields in average 72.0%. The fermentation of these hydrolyzed materials gave an overall ethanol yield of 80%, based on theoretical value.

Wet Oxidation

Wet oxidation (WO) is the process of treating material with water and either air or oxygen at temperatures above 120°C. Two types of reactions occur during WO: a low temperature hydrolytic reaction and a high temperature oxidative reaction. It has been demonstrated that a combination of alkali and WO reduces the formation of toxic furfuraldehydes and phenol aldehydes [97]. Martin [118] have investi­gated different conditions pH, temperature, and reaction time of WO pretreatment on fractionation and enzymatic convertibility of SCB, while pressure (12 bar) was kept constant. The highest cellulose content, nearly 70%, was obtained in the pretreatment at 195°C, 15 min and alkaline pH. The highest sugar yield in the liquid fraction, 16.1 g/100 g, was obtained at 185°C; 5 min and acidic pH. Although the analysis of the solid fraction in most of the pretreatments showed high degrees of hemicelluloses solubilization, the content of free sugars in the liquid fraction was very low. It is known that WO mainly catalyzes the transfer of hemicelluloses from the solid phase to the liquid phase, but it does not catalyze the hydrolysis of the liberated hemicelluloses molecules. The products of hemi — celluloses hydrolysis during WO are not monosaccharides, but sugar oligomers.