A computational study on thermal decomposition mechanism of β-1 linkage lignin dimer

In order to gain a better understanding of the detailed mechanism of lignin pyrolysis and the formation mechanism of its main products, the thermal decomposition processes of a β-1 linkage lignin dimer model compound 1 (1,2-p-hydroxyphenyl-1,3-propanediol) were theoretically investigated by employing density functional theory methods at B3LYP/6-31G(d, p) level. Three possible pyrolytic pathways: the homolytic cleavage of Cα–Cβ bond, the homolytic cleavage of Cβ–Cγ bond and the concerted reactions, were proposed. The activation energies of each reaction step in the pyrolysis processes were calculated and the temperature effect on the pyrolysis processes was analyzed. The calculation results indicate that the homolytic cleavage reaction of Cα–Cβ bond and concerted reaction pathway (3) could be the major reaction channels, and the homolytic cleavage reaction of Cβ–Cγ bond and concerted reaction pathway (1) could be the competitive reaction channels in the pyrolysis processes of β-1 linkage lignin dimer. The concerted reactions would dominate over the free-radical homolytic reactions at lower temperatures, while at high temperatures the free-radical reaction would dominate over the concerted reactions in the β-1 linkage lignin dimer pyrolysis processes.