A density functional theory study on the mechanism of levoglucosan pyrolysis

The pyrolysis mechanism of levoglucosan (one of the major product from cellulose pyrolysis) was investigated by using density functional theory at B3LYP/6-31++G(d,p) level. Four possible reaction pathways were proposed and the geometries of reactant, transition states, intermediates and products for each pathway were fully optimized; the standard thermodynamic and kinetic parameters of each reaction at different temperatures were calculated. The results showed that levoglucosan is converted to intermediate IM₁ via transition state TS₁ with an activation energy of 296.53 kJ/mol by breakage of C(1)-O(7) and C(6)-O(8) hemiacetal linkages and formation of C(5)-C(6)-O(7) circular structure, and then IM₁ is converted to intermediate IM₂ via transition state TS₂ with an activation energy of 234.09 kJ/mol. IM₂ can be further decomposed via four different pathways. Pathways 1 and 4 involve decarbonylation reactions with high energy barriers, and as a result, they are unlikely to occur; on the other side, the energy barriers for the rate-determining steps of pathways 2 and 3 are much lower, which are kinetically favorable and possible the major reaction channels for IM₂ pyrolysis.