Characterization of fused aromatic multirings in lignin via synchronous fluorescence spectroscopy and catalytic hydro-depolymerization

The structural analysis of lignin is highly significant in practical terms. We investigated the presence of fused aromatic multirings (FARs) distinct from conventional phenylpropane units in seven lignin types using synchronous fluorescence spectroscopy (SFS) and quantified their relative proportions. Our findings reveal that FARs are present in all lignin samples, primarily composed of 2-6 rings. The relative distribution of FARs in lignin is ranked as follows: 2 rings > 1 ring > 4 rings > 3 rings > 6 rings > 5 rings. The relative contents of these FARs (1-6 rings) exhibit slight variations across all lignin samples. Notably, FARs with 2 rings are the highest at 35 %-53 %, while those with 5 and 6 rings are the lowest, collectively accounting for less than 1.37 %. Moreover, the relative contents of FARs with 1 and 2-6 rings range approximately 24 %-32 % and 68 %-76 %, respectively. Furthermore, the catalytic hydro-depolymerization of lignin was performed and the experimental results demonstrate that the hydro-liquefied oil from lignin contains aromatic compounds with 1-7 rings, and the relative contents of aromatic compounds with 1 ring and 2-7 rings in the oil are 29 % and 71 %, respectively, which is consistent with that of lignin SFS. These findings are also consistent with the existing literature and experimental results. This study offers an explanation for the low yields observed in current depolymerization processes targeting small molecules within lignin and establishes a theoretical foundation for achieving high yields in the preparation of multiring aromatic hydrocarbons.