Progress of oxidation of non-phenolic lignin moiety in an oxygen bleaching process via the conversion of non-phenolic into phenolic lignin moiety

Abstract

To examine whether non-phenolic lignin moiety is converted into the new phenolic counterpart by the participation of active oxygen species (AOS) under oxygen bleaching conditions, a b-O-4 type dimeric lignin modelwas treated under conditions similar to those for oxygen bleaching. A phenolic compound, 4-hydroxy-3-methoxybenzyl alcohol, was added together to generate AOS in the reaction of compound I or IV. The results obtained suggested that AOS attack the side chains of compounds I, III, and IV accompanied by the b-O-4 bond cleavages and the liberations of the corresponding phenolic compounds, 2-methoxyphenol (II), 3,5-difluorophenol (V), and compound V, respectively. However, the rapid liberation of compound V in the oxygenalkali treatment of compound III may mainly be caused by alkaline-induced reactions and/or some oxidation reactions of the primary degradation product of compound III, a muconic acid derivative.

Predominant formation of aromatic aldehyde and acid from a dimeric β-O-4-type lignin model compound under hydrogen peroxide bleaching conditions with high pH levels

“…Compound 1 is quite stable without oxygen under otherwise the same conditions [35]. A non-phenolic lignin model compound such as compound 1 is not directly attacked by molecular oxygen but by AOS generated in either system accompanying the reaction between molecular oxygen and a co-existing phenolic compound, TMPh or Valc, which is the generator of AOS [32,35]. A major AOS is negatively charged oxyl anion radical (O⋅), the conjugate base of hydroxyl radical (HO⋅), in either of the O 2 -TMPh or O 2 -Valc system before TMPh or Valc, respectively, disappears from the system owing to the high pH levels [32].…”

“…Although we previously confirmed that the β-O-4 bond of compound 1 cleaves and a small amount of guaiacol is liberated in the O 2 -Valc system [35], almost no guaiacol was detected in the O 2 -TMPh system. Guaiacol is commonly oxidized by molecular oxygen.…”

“…Compound 1 is quite stable without oxygen under otherwise the same conditions [35]. A non-phenolic lignin model compound such as compound 1 is not directly attacked by molecular oxygen but by AOS generated in either system accompanying the reaction between molecular oxygen and a co-existing phenolic compound, TMPh or Valc, which is the generator of AOS [32,35].…”

“…As basic knowledge about the O 2 -TMPh or O 2 -Valc system, the followings were indicated in our previously published papers [31][32][33][34][35][36]. Compound 1 is quite stable without oxygen under otherwise the same conditions [35].…”

“…After the disappearance of TMPh or Valc, AOS are produced as radicals generated in chain-type reactions propagating in either system. O⋅ preferentially attacks the side-chain of a non-phenolic lignin model compound analogous to compound 1 rather than the aromatic nucleus [29,32,35].…”