Mechanism of acid-catalyzed pyrolysis of levoglucosan: Formation of anhydro-disaccharides

Levoglucosan is an important primary product in the pyrolysis of cellulose. Through density functional theory DFT calculation, this work theoretically investigates the possible reactions mechanisms of the polymerization of levoglucosan to generate various alpha and beta linkages (including 1,2-, 1,3-, and 1,4-glycosidic bond) anhydro-disaccharides and beta-d-glucose under the catalysis of H2SO4. Thermodynamic and kinetic analysis reveals that for the polymerization without catalysis, anhydro-disaccharides with beta linkages are most likely to be generated by direct polymerization of two molecules of levoglucosan, while different anhydro-disaccharides with alpha linkages are most likely to be generated by different reaction mechanisms. The Gibbs free energy barriers Delta G double dagger of the rate-determining steps for the generation of anhydro-disaccharides and beta-d-glucose increase with temperature, and the reactions are exothermic, suggesting that they prefer to occur at low pyrolysis temperatures. The addition of H2SO4 catalysis drastically reduces the Delta G double dagger of various reactions by 108.3-216.1 kJ/mol at 400 K and promotes the production of various anhydro-disaccharides and beta-d-glucose. The electron localization function and Mayer bond level analysis reveal that H2SO4 participates in the reaction mainly through the hydrogen protons in the hydroxyl group and the lone pair electrons in the oxygen atom. The strong proton transfer ability of H2SO4 makes it a strong catalyst. And due to the longer S-O bond in H2SO4 than the O-H bond in H2O, H2SO4 performs more capable of breaking spatial barriers, which can substantially reduce the Delta G double dagger for the formation of anhydro-disaccharides with alpha linkages.