Formation mechanism of CH4 during lignin pyrolysis: A theoretical study

In order to understand the formation mechanism of CH4 during the lignin pyrolysis process, 4-(3-hydroxypropyl)-2-methoxyphenol was selected as the G-type lignin monomer model compound. Theoretical studies on its pyrolysis process were conducted by using the density functional theory (DFT) method. The results show that the key of CH4 formation in the process of lignin pyrolysis is the homolytic cleavage of the O–CH3 bond of the methoxyl functional group to generate methyl radical. Then the methyl radical will abstract H to form CH4 through the unimolecular reaction or bimolecular reaction. The former reaction is difficult to occur due to the limited number of free H radicals in the pyrolysis process of lignin. The latter reaction is more likely to take place because of the lower reaction energy barriers. Both the alkyl side chain and the characteristic functional groups of the benzene ring can undergo bimolecular reactions to donate H for methyl radicals to form CH4.