Lignin modification during Eucalyptus globulus kraft pulping followed by totally chlorine-free bleaching: a two-dimensional nuclear magnetic resonance, Fourier transform infrared, and pyrolysis-gas chromatography/mass spectrometry study

Chemical modification of eucalypt lignin was investigated during kraft pulping and chlorine-free bleaching by comparing milled wood lignin, kraft lignin, and pulp enzymatic residual lignins. The syringyl-to-guaiacyl ratio (S/G) from analytical pyrolysis slightly changed during pulping and bleaching (S/G, 3-4) but was higher in the kraft lignin. Semiquantitative heteronuclear single quantum correlation (HSQC) nuclear magnetic resonance (NMR) showed that the relative amount of beta-O-4' (around 80% side chains) and resinol type substructures (15%) was slightly modified during pulping and oxygen delignification. However, a decrease of resinol substructures (to only 6%) was found after alkaline peroxide bleaching. The relative amount of surviving linkages in the highly phenolic kraft lignin was dramatically modified; resinols were predominant. Oxygen delignification did not change interunit linkages, but a relative increase of oxidized units was found in the HSQC aromatic region, in agreement with the small increase of pyrolysis markers with oxidized side chains. NMR heteronuclear multiple bond correlations showed that the oxidized units after oxygen delignification bore conjugated ketone groups.