Thermal degradation of microcrystalline cellulose in concentrated phosphoric acid

Abstract

The most common renewable fuel is ethanol. It is a liquid produced by fermentation of glucose which is very abundant in cellulose based materials. Bioethanol is achieved in a two-steps process: 1. hydrolysis of the cellulose included in the ligno-cellulosic materials to fermentable reducing sugars; 2. fermentation of such sugars to ethanol. While the second fermentation step, mediated by yeasts or bacteria, is very well established, the first one must be still assessed for process optimization. Many efforts have been made to identify solvents for cellulose in order to develop methods for the achievement of fermentable glucose. In the presence of mineral acid, cellulose undergoes hydrolysis whose extent depends on acid concentration, reaction temperature and duration of treatment. In the last years, dissolution in phosphoric acid (a weak mineral acid, non toxic and safer to use as compared to other inorganic acids) has been increasingly considered as a simple and economic method for cellulose pretreatment before its degradation to glucose. Up to now, it has not been reported yet cellulose conversion to glucose in H3PO4 solutions. Aim of the present study was to evaluate kinetic of glucose formation during cellulose degradation by using 85% phosphoric acid. It was investigated the role of residence time on the efficiency of glucose release during the hydrolysis of microcrystalline- cellulose in H3PO4 at the constant reaction temperature of 80°C. Moreover, High field NMR measurements showed that heating treatment is fundamental to have conversion of cellulose in glucose with formation of same degradation products, how supported by GC MS analysis results. In addition, relaxometry measurements with a fast field cycling setup have been carried out in order to observe how heating treatment change the relaxation time distribution of cellulose acid solutions, thereby suggesting that as cellulose degradation occurred , presence of smaller units determined the restoring of the T1 distribution of the complex mixture to a value close to that of pure phosphoric acid.