Alkaline aerobic oxidation of native softwood lignin in the presence of Na⁺-cyclic polyether complexes

Degradation mechanism of a lignin model compound during alkaline aerobic oxidation: formation of the vanillin precursor from the β-O-4 middle unit of softwood lignin

“…For instance, large size organic cations such as Bu 4 N + and complex cations between Na + and several polyethers stabilize the end group B in Scheme 1 and renders the overall degradation more vanillin-oriented. 16,37 These data clearly indicate the importance of the pathway in Scheme 1 in alkaline aerobic oxidation of lignin.…”

“…These reaction conditions were employed also in our previous studies. 16,37 A typical HPLC chromatogram obtained after the VGL degradation in Fig. 1A indicates good separation of the oxidation products and unreacted starting material: VGL (6.5 min), veratryl alcohol (1) (15.5 min), vanillin (2) (17.0 min), veratric acid (3) (19.5 min), veratraldehyde (4) (22.5 min), V2 (16.6 min) (see the experimental section for details of the analytical conditions).…”

“…Most of these products have been reported also in our previous studies as the major oxidation products from a nonphenolic β-O-4 type lignin model compound. 16,37 The products in Fig. 1A were classified into two types: compounds 1-4 as the products with C 1 side-chains and V2 as a product with a C 2 side-chain.…”

“…This process is the rate-determining in overall vanillin formation and surely one of the most important steps. 16,37 We therefore set our purpose in this study to the elucidation of the fundamental reactions underlying this oxidative process. Methods in tracking ongoing reactions in the lignin polymer with 2D NMR techniques have been rapidly developed in recent years.…”

“…The oxidation conditions in this study were selected according to our previous studies. 16,18,37 We adapted a concentrated alkali, almost atmospheric air (no pressurized O 2 ), and no additives to enhance the oxidation. On the other hand, many studies addressing lignin conversion through aerobic oxidation and also studies on oxygen beaching of pulps have employed reaction conditions considerably different from the above ones: in many respects, our reaction conditions are simpler than those in other studies.…”