Formation and Degradation of Biphenyl Structures During Alkaline Oxidation of Phenols with Oxygen

Mechanical pulps are bleached by treatment with oxygen gas and alkali in the presence of borohydrides. The combined treatment gives brightness gains of 15-20 points» comparable with those achieved using stabilized alkaline 4% hydrogen peroxide. By contrast the reaction of oxygen and alkali alone darkens high-yield pulps; and treatment with borohydride in the absence of added oxygen bleaches only partially, to give about half the brightness gained if oxygen is added. Evidence is presented that the bleaching occurs via the conversion of oxygen molecules (O 2 ) to hydroperoxide ions (HOO~) by reaction of the oxygen with ionized phenolic groups in the lignin. The hydroperoxide ions are thus generated in situ in the lignin where they bleach by reacting with coloured lignin quinone and coniferaldehyde chromophores. Without the addition of borohydride the lignin phenolic groups are irreversibly oxidized by oxygen to coloured products. The primary role of the borohydride is to regenerate the phenolic groups and maintain an oxygen -> hydroperoxide[(O 2 ) -> (HOO~)J cycle. In principle other reducing agents could be used as alternatives but in practice only sodium borohydrides and substituted borohydrides have been found to be sufficiently stable to the reaction conditions. In a secondary role, the borohydride also contributes to the bleaching by reducing radicals, organic peroxides and carbonyl compounds present in the pulps.

Section: The Mechanismmentioning

confidence: 98%

The Bleaching of Mechanical Pulps with Oxygen and Borohydride

Leary¹,

Wong²,

Giampaolo³

1997

“…Phenolic phenylcoumaran structures that are oxidized to stilbenes also give 8 together with the isophthalic acid derivatives (10 and 12) upon oxidation (Gellerstedt and Lindfors 1984). Both are also known to arise from molecular oxygen degradation of monophenolic biphenyl structures (Kratzl et al 1966). The production of the phthalic acid 11 likely results from the degradation of condensed β-6 lignin structures.…”

Section: Low Molecular Weight Reaction Productsmentioning

confidence: 99%

The Reactions of Lignins with High Temperature Hydrogen Peroxide Part 2. The Oxidation of Kraft Lignin

Kadla¹,

Chang²,

Jameel³

1999

A technical pine kraft lignin was subjected to alkaline hydrogen peroxide oxidation in the presence of DTMPA and molecular oxygen at various temperatures. In the presence of DTMPA the lignin was found to undergo increasing levels of oxidation and degradation with increasing temperature. At 110°C over 80 % of the kraft lignin was degraded. Analyses of the degraded lignins indicated that both phenolic and nonphenolic lignin moieties were degraded. At 90°C the addition of molecular oxygen resulted in further lignin demethoxylation, but did not decrease the amount of phenolic hydroxyl groups or hydrogen peroxide consumed. In the absence of DTMPA the hydrogen peroxide was rapidly degraded, and accompanied by only minimal lignin oxidation.

“…In any reaction system involving phenoxyl radicals, oxidative coupling, in particular dimerization, will be expected to occur and is evident by the presence of oc-5, 4-O-5 and 5-5' bicreosol. The nature of the dimerization process has been discussed by Kratzl et al (1966Kratzl et al ( , 1967 and consists of phenoxyl/benzyl radical coupling followed by aromatization. These dimeric compounds are known to react under alkaline conditions in the presence of hydrogen peroxide degradation products and molecular oxygen (Kratzl et al 1966).…”

Section: Reaction Of Cyanamide-peroxide With Creosolmentioning

confidence: 99%

Reactions of Lignin with Cyanamide Activated Hydrogen Peroxide. Part 2. The Degradation Mechanism of Phenolic Lignin Model Compounds

Kadla¹,

Chang²,

Chen³

et al. 1998

The reactions of monomeric phenolic lignin model compounds with cyanamide activated hydrogen peroxide were studied. Analyses of the various reaction products indicate four main reactions occur; aromatic hydroxylation, demethoxyiation, radical coupling and oxidative ring opening. The predominate products from apocynol and acetoguaiacohe were dicarboxylic acid compounds, maleic/fumaric, oxalic, and succinic acid derivatives, consistent with degradation by Superoxide. In the degradation of creosol, 3-hydroxy-4-methoxy-6-methyl-N-methylaniline was the major compound isolated, providing evidence of a cyanamide radical. Finally, plausible pathways to the formation of Superoxide anion and cyanamide oxidation products from alkaline hydrogen peroxide and cyanamide are proposed and their impact on delignification is discussed.