Study On The Pyrolysis Mechanism Of Lignin Model Compounds With β-O-4 Linkages

The current predicament of exhausting fossil fuels and aggravating environmental problems can be expected to be solved by the utilization of biomass. Lignin is an abundant natural polymer. It has the potential to produce aromatic compounds, thus partially replacing the fossil fuels. During the fast pyrolysis of lignin, bio-oil rich in aromatic compounds can be produced. Study on the pyrolysis mechanism of lignin will provide theoretical basis for the selective control of the pyrolysis process, thus it is crucial for developing efficient selective pyrolysis technology. In this study, density functional theory (DFT) was employed to study the pyrolysis mechanisms of lignin model compounds with (3-0-4 linkages:(1) The initial decomposition mechanism of G type lignin model dimer with (3-0-4 linkage (Guaiacylglycerol-β-guaiacyl Ether) was studied. Six initial decomposition mechanisms, including Cβ-O homolytic decomposition, Ca-Cp homolytic decomposition, concerted decomposition, etc. were compared. The results indicate that the Cβ-O homolytic decomposition has the lowest energy barrier, and it is the most favorable reaction during the initial decomposition of the model dimer.(2) Bond Dissociation Energies (BDEs) of the Cβ-P bonds in 14 lignin model compounds with (3-0-4 linkages were calculated. The effects of the hydroxyl and methoxyl groups at different positions on the BDEs were discussed. The interactions between different groups were also studied. The results indicate that there is minor influence of the hydroxyl group at Ca. The hydroxyl group at Cγ hardly affects the BDE when there is no methoxyl group at R.2. However, when there is methoxyl group at R.2, hydrogen bond will be formed between the hydroxyl group at Cγ and the methoxyl group at R2, and thus, the spatial structure of the model compound will be strengthened and the BDE is increased. The methoxyl groups at R2 and R3 can lower the BDEs in different degrees because of the possible hydrogen bond between the methoxyl group at R2 and hydroxyl group at Cγ. The methoxyl group at R1 has minor effects on the BDE. Moreover, it does not participate in the interactions with other functional groups.(3) For the initial mechanisms of Guaiacylglycerol-(3-guaiacyl Ether with relative lower energy barriers, further detailed study on the subsequent pathways was carried out. Comparison over the Cβ-O homolytic decomposition and two concerted decomposition mechanisms indicates that the Cβ-O homolytic decomposition mechanism is the most favorable pyrolytic mechanism. The pyrolytic products of the mechanism have the lowest energy barriers, and the most favorable products are guaiacol,3-methoxy-4-hydroxy styrene and 3-methoxy-4-hydroxy benzaldehyde. Guaiacol can go through further secondary reactions and produce cathechol and phenol.(4) Detailed study on a lignin trimer with α-O-4 and β-O-4 linkages was carried out to analyze the pyrolytic reaction pathways and the influence of a-O-4 linkage on β-O-4 linkage during the pyrolytic process. It is found that the BDEs of the Cα-Oα and Cβ-Oβ bonds of the trimer are similar with that of dimers with the same linkages. It indicates that the α-O-4 and β-O-4 linkages of the trimer hardly interact with each other. Due to the lower BDE, Cα-Oα bond goes through decomposition in the first place. Further Cp-Op decomposition can take place with a much lower BDE than the direct decomposition of Cp-Op bond of the trimer and the Cβ-O bond of the dimer. In this way, the a-O-4 linkage can promote the decomposition of the β-O-4 linkage. The most favorable pyrolytic products of the trimer are phenol and p-coumaralcohol. Dimers with double bond, which are the potential precursors of tar, will also be produced.