Structural Characterization of Kraft Lignin for Its Green Utilization

Laboratory-obtained kraft lignins have been studied via elemental analysis (EA), 1H-NMR, FTIR, GPC to analyse the chemical structure. The kraft lignins were subjected to oxidation with H2O2 using Co(salen) as a biomimetic catalyst at mild condition (80oC, 240 min). The kraft lignins isolated from eucalyptus (EKL) and bamboo (BKL) had a low hydroxyl (oxygen) contents, a low molecular weights (~3000 g·mol-1) and a uniform molar mass distribution with promising results for oxidative conversions. EKL shows a tendency to higher purity and higher functional group content in hydroxy group, methoxy group, β-O-4 linkage than BKL. Oxidations cause depolymerization of kraft lignin, in which kraft lignin was selectively oxidized by the catalyst. Oxidation products, 2-methoxy phenol and 3-methoxy-4-hydroxy benzaldehyde, can be observed on treatment of kraft lignin. Catalytic oxidation of EKL lignin shows a higher yield of products, most likely due to the high content of β-O-4 ether linkages, hydroxyl groups. Both structural and oxidative characteristics suggest that EKL would be a better feedstock in the production of aromatic products, as it presents higher amounts of hydroxyl functional groups, β-O-4 linkages, and as it presents higher molecular weight and higher purity.