Cleavage of Covalent Bonds in the Pyrolysis of Lignin, Cellulose, and Hemicellulose

This study examines the cleavage of covalent bonds and the radical behavior in the pyrolysis of lignin, cellulose, and hemicellulose at 350, 400, and 440 degrees C. In the presence of a sufficient amount of a hydrogen donor solvent, i.e., 9,10-dihydrophenanthrene (DHP), the quantity of covalent bonds cleaved is determined, and the quantity of stable radicals formed is measured. It is found that more than 99.9% of the radicals generated in the pyrolysis are coupled to hydrogen donated by DHP. The pyrolysis of the biomass can be characterized as cleavage of two types of covalent bonds, weak and strong. The cleavage of the weak bonds dominates the pyrolysis in the early stage, less than 10 min at 440 degrees C, whereas that of the strong bonds becomes significant in the later stage, and its rate is promoted by DHP. The amounts of the weak bonds are 0.89 X 10(-2), 0.63 X 10(-2), and 0.68 X 10(-2) mol/g in lignin, cellulose, and hemicellulose, respectively. These values correspond to the sum of the C-al-O and C-al-C-al bonds linking the unit structure in lignin and the C-al-O bonds in the glucosidic linkage in cellulose and hemicellulose, respectively. Activation energies for the cleavage of these bonds in lignin, cellulose, and hemicellulose are 90.5, 129.7, and 126.5 kJ/mol, respectively.