Mechanistic study on levoglucosan condensation and decomposition in the condensed phase during cellulose pyrolysis

In this study, the condensation, as the fundamental step of polymerization, and decomposition mechanisms of levoglucosan in the condensed phase during cellulose pyrolysis were investigated using density functional theory (DFT) calculations to investigate non-specific solvent effects. The condensed phase was simulated using an implicit solvent model, while the dielectric constants of levoglucosan at different temperatures were determined by theoretical calculations. This study considered levoglucosan condensation reactions to form anhydro-disaccharides with various α and β linkages (including 1,2-, 1,3- and 1,4-glycosidic bonds) and levoglucosan decomposition reactions including 1,2-dehydration, 1,3-dehydration, and ring-opening reactions. Our results show that the formation of β-anhydro-disaccharides prefers the direct condensation mechanism of two levoglucosan molecules through a four-member ring transition state, while kinetically more disadvantageous α-anhydro-disaccharides are generally formed by the condensation of levoglucosan molecule and α-d-glucose from levoglucosan hydrolysis. However, levoglucosan condensation without catalysis is unlikely to take place at typical temperatures of cellulose pyrolysis, due to the low reaction rate at low temperatures. At high temperatures above 625 K, levoglucosan condensation is also inhibited due to the higher ΔGθ and lower reaction rates, compared to those of levoglucosan decomposition. A high temperature above 800 K is needed for the obvious decomposition of levoglucosan, mainly through the ring-opening reaction to form 5-keto-6-deoxy-glucose, which is further promoted by the dielectric environment of the condensed phase during cellulose pyrolysis. Further calculations on acetic acid-catalyzed levoglucosan condensation and decomposition suggest that the consumption of levoglucosan at low temperatures is mainly due to catalysis of acetic acid on polymerization and further decomposition of polymers.