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

Abstract

Chemical modification of eucalypt lignin was investigated during kraft pulping and chlorine-free bleaching by comparing milled-wood lignin, kraft lignin, and 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 much higher in the kraft lignin. Semiquantitative HSQC NMR showed that the relative amount of β-O-4' (around 80% side-chains) and resinol-type substructures (around 15%) was only slightly modified during pulping and oxygen delignification. Other minor inter-unit linkages and terminal units were also detected. 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 being predominant. Oxygen delignification did not change inter-unit linkages, but an increase of oxidized units was found in the HSQC aromatic region, in agreement with the higher amount of pyrolysis markers with oxidized side-chains. HMBC NMR correlations showed that the oxidized units after oxygen delignification bore conjugated ketone groups, and conjugated carboxyls after peroxide bleaching. Presence of xylan and lignin-carbohydrate linkage signals was shown by the HSQC spectra of pulp lignins