Carbenes and nascent hydrogen for reductive derivatisation of technical lignins

Motivation: Traditional pulp and paper processes as well as emerging biorefineries generating cellulosic ethanol from lignocellulosic agricultural waste material lead to large amounts of kraft lignin and side-stream straw lignin. Both types of technical lignins, i.e., organosolv lignins and kraft lignins, offer various amounts of different functional groups, mainly aliphatic and phenolic hydroxyl groups and double bonds. Whereas the hydroxyl groups are normally used for lignin functionalisation, a more versatile type of functionalisation is obtained when leaving the hydroxyl groups intact and creating carbon-carbon bonds. Design of experiment: Lignin modification was thus performed using nascent hydrogen, as well as carbenes of varying reactivity. Exploiting the possibilities for in situ production of the reactive species, various technical lignins were subjected to standardised reactions conditions. Re-isolated modified lignin samples were subjected to GPC analysis, FTIR analysis, quantitative 31P NMR after phosphitylation, as well as quantitative {13C-1H}-HSQC analyses. Results: Molar mass distribution effects were noted in the modified lignins. In case of carbenes, detection of new, unexpected cross-peaks in the HSQC spectra area specific for newly emerging C=C bonds were observed, while other ‘typical’ lignin signals in the aromatic region underwent drastic changes. Together with unexpectedly decreased amounts of the respective free phenolic OH groups in the modified lignins, quinone-formation involving G- and H-type aromatic rings of lignins through a Reimer-Tiemann mechanism8 is postulated, among other reactions and rearrangements. In case of treatment of lignins with nascent hydrogen, structural changes lead to yet different, ‘atypical’ lignins.