Valorization of lignin into phenolic compounds via fast pyrolysis: Impact of lignin structure

Understanding the structure activity relationships between lignin and its pyrolysis products is of great significance toward thermochemical conversion of lignin. Herein, mild acidolysis lignins (MALs) isolated from softwood (pine), hardwood (eucalyptus), and herbaceous feedstocks (corn stalk and bamboo) were characterized and their activities toward fast pyrolysis were comparably evaluated. A detailed characterization of lignin structure demonstrates that eucalyptus MAL contains both guaiacyl (G) and syringyl (S) units, whereas pine MAL is enriched in G units. In addition to G, S, and p-hydroxyphenyl (H) units, corn stalk and bamboo MALs consist of tricin and hydroxycinnamic acids (ferulic and p-coumaric acids). Moreover, lignins from those plants are extensively acylated at the Cγ of the lignin side chain with p-coumarate groups. The results of fast pyrolysis of different lignin sources reveal a diverse range of aromatic compounds due to varying selective fractures on the linkages of lignin. Notably, herbaceous MALs afford higher amounts of phenolic compounds, among which 4-vinylphenol is the main pyrolysis product, suggesting the efficient cleavage of C–O linkages coupling with decarboxylation reaction in lignin. This work demonstrates the essential role for the insights gained in the characterization of lignin structure, which will guide for the rational design of thermochemical process toward lignin valorization into phenol compounds via fast pyrolysis.