Lignin is found in large amounts in wood and lignocellulosic biomass. It can be a valuable source of biobased aromatics. However, due to a lack of efficient lignin biorefinery processes, it is mainly burnt for energy recovery.

Loredano Pollegioni, Elena Rosini, University of Insubria, Varese, Italy, Claudia Crestini, Ca’Foscari University of Venice, Italy, and colleagues have developed a green upcycling process for the valorization of industrial lignins. The team performed sequential enzymatic treatments using different laccases as well as β-etherases and glutathione lyases (Lig system).

The team treated fractionated Kraft and Organosolv lignin with these enzymes. Kraft lignin arises from harsh alkaline treatments leading to the almost complete cleavage of the β-O-4 aryl ether linkages. Organosolv lignins are more similar to native lignin with a significant amount of β-O-4 linkages. The researchers used laccases from Bacillus licheniformis and from Funalia trogii for Kraft and Organosolv lignin, respectively, followed by the Lig system from Sphingobium sp. SYK-6 (β-etherases Lig E and Lig F as well as the glutathione lyase Lig G).

Using this approach, monomeric and dimeric aromatics were generated through lignin depolymerization (pictured). The laccase-Lig system cleaves non-phenolic aryl glycerol β-O-4 aryl ether bonds. Thus, the efficacy of the β-O-4 aryl ether bonds cleavage is related to the aliphatic/phenolic OH group ratio of the treated lignin: Higher values result in a net depolymerization, significantly enhancing the concentration of extracted products, while for low values, oxidative coupling processes lead to lignin polymerization. The work highlights the importance of the structural characterization of the starting lignin.

• The Laccase‐Lig Multienzymatic Multistep System in Lignin Valorization,
  Elisa Vignali, Matteo Gigli, Simone Cailotto, Loredano Pollegioni, Elena Rosini, Claudia Crestini,
  ChemSusChem 2022.
  https://doi.org/10.1002/cssc.202201147