Theoretical studies on formation mechanisms of CO and CO2 in cellulose pyrolysis

In order to understand formation mechanisms of CO and CO2 in cellulose pyrolysis, the pyrolysis of 2,3,4-hydroxyl-butyraldehyde and 2,3,4-hydroxyl-butyricacid as model compounds were investigated by using density functional theory methods at B3LYP/6-31++G(d,p) level. Three possible pyrolytic pathways of each model compound were proposed and geometries of reactants, transition states, intermediates and products were fully optimized. The standard thermodynamic and kinetic parameters of every reaction path in different temperature ranges were calculated. The calculation results show that the releases of CO and CO2 during cellulose pyrolysis are related to the decarbonylation and decarboxylation respectively, and decarbonylation and decarboxylation reactions are concerted processes via intramolecular hydrogen transfer. Decarbonylation are endothermic and decarboxylation are exothermic. The activation energy of decarbonylation of 2,3,4-hydroxyl-butyraldehyde is 294.0 kJ/mol, while the activation energy of decarbonylation of unsaturated olefine aldehyde formed via dehydration of 2,3,4-hydroxyl-butyraldehyde gets increased. The activation energy of decarboxylation of 2,3,4-hydroxyl-butyricacid is 311.7 kJ/mol, while the activation energy of decarboxylation of unsaturated olefine acid formed via dehydration of 2,3,4-hydroxyl-butyricacid gets decreased obviously, which indicates the dehydration favors the release of CO2.