Pyrolysis mechanism law of β-O-4 lignin dimer model compounds: A density functional theory study

Lignin is the second abundant natural renewable after cellulose. Pyrolysis is usually transformed lignin into high value-added platform compounds. In order to fully utilized lignin, the pyrolysis mechanism needs an in-depth understanding. Herein we performed density function theory (DFT) calculation to unveil the initial mechanism of pyrolysis of 9 representative β − O− 4 lignin dimer model compounds with phenyl-1, 3-propanediol as the main body. Several cleavage pathways were investigated, including Cβ−O bond breaking（Cβ−O bond homolysis reaction, Maccoll elimination reaction and retro-ene fragmentation reaction） and Cα−Cβ bond breaking. Results show that the substituents on the B-ring have a greater effect on the pyrolysis pathway of lignin dimer than that on the A-ring. When no substituent on the ortho-position of B-ring, the retro-ene fragmentation reaction is the dominant pathway. And, its main reason is these structures have no steric hindrance in the process of hydrogen transfer. The Cβ−O bond homolysis is the dominant pathway when methoxy group exists on ortho-position of B-ring. Maccoll elimination reaction also has certain advantages due to the influence of electronic factors of methoxyl group on ortho-position of B-ring. The cleavage reaction of Cβ−O bond is always prior to that of Cα−Cβ bond. The effect of water molecules on the pyrolysis reaction has been also discussed. The results showed that H2O could promote the Maccoll elimination I reaction, but did not change the dominant pyrolysis mechanism.