Revealing the pyrolysis behavior of 5-5′ biphenyl-type lignin fragment. Part I: A mechanistic study on fragmentation via experiments and theoretical calculation

Abstract

Biphenyl structure plays an important role in the thermal degradation of woody lignin due to its non-negligible proportion, and its high binding energy of Caryl-Caryl bond leads the lignin fragments not easy to be break into small-molecule products. The in-depth study on the pyrolysis mechanism of biphenyl-containing structures is conducive to solve the shortcoming and providing technical support for lignin depolymerization. In this works, we report a mechanistic study on the pyrolysis of biphenyl structure. The pyrolytic pathways had been proposed based on the thermochemical characteristics, the functional groups evolutions and the products distributions during the pyrolysis process. DFT calculation were following carried out to explain the reaction rules at the microscopic (molecular) level. The results showed that both endothermic and exothermic procedures were coexistent during biphenyl dimer pyrolysis, along with the releasing of small molecular such as CH4, CO, H2O, etc.; there are two mutual-promotive and competitive fragmented pathways during pyrolysis, i.e. 5-5′ cleavage and substituent breaking. The demethylation of the methoxy group is the most prior initiation, followed by the dehydrogenation of edge groups, instead of the inaccessible CAr-Cx bonds homolysis. The 5-5′ cleavage pathway can partially be realized with the assistance of the hydrogen radical, which need only overcoming a 7.10 kcal/mol energy barrier. On the contrary, the fragmentation mostly involves the substituent breaking, and the biphenyl products formation through a series of radical-mediated reactions.