A density functional theory study on formation mechanism of CO, CO2 and CH4 in pyrolysis of lignin

In order to understand formation mechanisms of CO, CO2 and CH4 in lignin pyrolysis, the decarbonylation, decarboxylation and demethylation reactions of various phenylic lignin model compounds were theoretically investigated by using density functional theory methods at B3LYP/6-31G++(d,p) level. Eight decarbonylation reaction pathways (1)–(8), eight decarboxylation reaction pathways (9)–(16) and a demethylation reaction pathways (17) were designed. The standard kinetic and thermodynamic parameters of every reaction pathway at different temperatures were calculated. The calculation results show that energy barriers of decarbonylation reaction pathways (1)–(4) are apparently higher than those of decarbonylation reaction pathways (5)–(8), and then CO is formed more easily through decarbonylation of phenyl (p-hydroxyphenyl, guaiacyl and syringyl) acetaldehyde; energy barriers of decarboxylation reaction pathways (9)–(12) are lower than those of decarboxylation reaction pathways (13)–(16), and then CO2 is formed more easily through decarboxylation of phenyl (p-hydroxyphenyl, guaiacyl and syringyl) formic acid. Compared with step-by-step radical reaction, the energy barrier of concerted reaction is lower, and CH4 is formed more easily through concerted reaction.