Copper catalyzed alkaline aerobic lignin depolymerization: effect of botanical origin and industrial extraction process on reactivity supported through characterization†

Different lignins, differing in their botanical origin or the extraction method, were subjected to aerobic catalytic depolymerisation in basic aqueous media using a copper-based catalyst (CuO/TiO₂) towards aromatic compounds. Lignins were obtained from different kinds of biomass, including hardwood, softwood and wheat straw, and using different extraction methods such as kraft, soda or organosolv. Extensive characterization (elemental analysis; ¹H, ¹³C, ³¹P and HSQC NMR; FTIR-ATR; DLS and SEC) revealed structural differences between lignin samples. For example, organosolv softwood lignin shows a higher degree of β-O-4 linkages, while organosolv wheat straw lignin presents higher structural degradation. These changes resulted in differences during the catalytic aerobic depolymerisation. The structural properties were correlated with the reaction results. Thus, the average molecular weight is directly related to the observed degree of conversion, and the β-O-4 content is correlated with the yields of aromatic compounds obtained. It was also observed that the catalytic effect of the CuO/TiO₂ catalyst is more pronounced when the non-catalytic reaction shows low yields of aromatics. Finally, despite higher reactivity, hardwood lignin did not produce high yields of aromatic compounds due to the rapid degradation of the products.