Density functional theory studies on pyrolysis mechanism of β-O-4 type lignin dimer model compound

Lignin is the main component of biomass with a complex, heterogeneous, three-dimensional polymeric structure of three main monolignols (p-coumaryl, coniferyl, and sinapyl alcohol). In order to understand the pyrolysis mechanism of lignin and identify the chemical pathways for the formations of key products during pyrolysis, the pyrolysis processes of β-O-4 type lignin dimer model compound 1 (1-phenyl-2-phenoxy-1,3-propanediol) were theoretically investigated by employing density functional theory (DFT) methods at the B3LYP/6-31G(d,p) level. Based on related experimental and calculation results of bond dissociation energies of β-O-4 type lignin dimer, three possible pyrolytic pathways (the homolytic cleavage of CβO bond, the homolytic cleavage of CαCβ bond and the concerted reactions) were proposed, the activation energies of each reaction step were calculated, and the temperature effect on pyrolysis processes was analyzed. The calculation results indicate that the homolytic cleavage reaction of CβO bond and concerted reaction pathways (3) could be the major reaction channels, and the homolytic cleavage reaction of CαCβ bond and concerted reaction pathways (1) and (2) could be the competitive reaction channels in pyrolysis processes. The concerted reactions would dominate over free-radical homolytic reactions at lower temperatures, while at high temperatures the free-radical reaction (CO homolysis) would dominate over the concerted reactions.