A theoretical study on the mechanism of levoglucosan formation in cellulose pyrolysis

To understand the mechanism of levoglucosan formation in cellulose pyrolysis, the pyrolysis of cellobiose as a model compound was investigated theoretically by using UB3LYP/6-31G(d) methods. Three kinds of pyrolysis reaction paths were designed; the equilibrium geometries of the reactants, intermediate, transition states, and products were optimized. The standard thermodynamic and kinetic parameters of pyrolysis reaction were calculated at different temperatures. The results showed that the free radicals IM₁a and IM₁b can be formed by homolysis of glycosidic bond and the reaction is endothermic with an energy of 321.26kJ/mol. Free radical IM₁a may react further via transition state TS₁a and lead to the formation of levoglucosan, with an energy barrier of 202.72kJ/mol. Compared to consecutive reactions, concerted reaction of the formation of levoglucosan P₁ and glucopyranose P₂ via the transition state TS₂ in pyrolysis of cellobiose has a lower energy barrier of 377.54kJ/mol. Addition of H⁺ would be in favor of breakage of glycosidic bond; the intermediate IM₃ formed can hardly transform to levoglucosan, which is in accord with the related analysis of experimental results.