Computational study on pyrolysis mechanism of an α,β-diether-type lignin trimer model compound

In order to understand the pyrolysis mechanism of lignin trimer, 4-(1,2-diphenoxy)-propylphenol was selected as lignin trimer model compound containing both β-O-4 and α-O-4 linkages, and its pyrolysis processes were theoretically investigated by employing density functional theory method at M06-2X level with 6-31++G(d,p) basis set. The equilibrium geometries of the reactant, intermediates, transition states and products in the pyrolysis processes were fully optimized. The activation energy in each pyrolysis pathway was calculated. Analyses were performed to reveal the interaction of the β-O-4 and α-O-4 linkages in their cleavages, as well as the overall pyrolytic mechanisms and the pathways of product formation. The calculation results indicated for the 6 primary homolytic ways of the lignin trimer model compound, and the bond dissociation energies were in the order of Cα-O < Cβ-O < Cα-Cβ < Cα-C4 < C4″-O < C4′-O, with the lowest energy of Cα-O (250.6 kJ/mol) and the second lowest energy of Cβ-O (286.9 kJ/mol), followed by that of Cα-Cβ (300.5 kJ/mol). Hence, the homolytic cleavage of Cα-O bond should be the major reaction of primary pyrolysis for the trimer model compound, while the homolytic cleavages of Cβ-O and Cα-Cβ bonds were competitive pyrolysis reactions. The β-O-4 and α-O-4 linkages had little interaction effects in their primary cleavage reactions, because the bond dissociation energies of Cα-O and Cβ-O in the lignin trimer model compound were similar to those of the corresponding lignin dimer model compounds. However, after the initial cleavage of the α-O-4 (β-O-4) linkage, the formed intermediate had significantly reduced bond dissociation energy for the further cleavage of the β-O-4 (α-O-4) linkage. Based on the energy barrier of each reaction pathway for the primary homolytic cleavage of Cα-O bond, the major pyrolytic products of the model compound included phenol, 4-propenylphenol, 4-allyphenol, and dimer compound with double bonds (the precursor of large molecular products). Based on the primary homolysis of Cβ-O bond, the major pyrolytic products included phenol, 4-propenylphenol and 4-propylphenol. From the primary homolysis of Cα-Cβ bond, the major pyrolytic products included phenetole, 4-hydroxy benzaldehyde, benzene, p-cresol and phenol. In conclusion, based on the 3 homolytic mechanisms, the pyrolysis of the lignin trimer model compound will produce phenol, 4-propenylphenol, 4-allyphenol, and dimer compound with double bonds as the major products, as well as 4-propylphenol, phenetole, 4-hydroxy benzaldehyde, benzene and p-cresol as the competitive products. The research results will provide theoretical basis for the deep understanding of the pyrolysis mechanism of lignin.