Reference. Stilbenes Negligible ~3% of linkages 114

b-O-4 48 107 85% lower 52

b–5 7–8 108, 109 Slightly greater 49, 96

9–12 110

b–1<2 107

3.5 111

5–5 10–11 110 49, 96

(16)b 109 Slightly greater

(24–26)b 112

4-O-5 4–5 109, 113 Slightly greater 49,96

b-b

Stilbenes Negligible ~3% of linkages 114

Vinyl ethers Negligible 0.5–1% of linkages 52

DPMc Negligible Debatable amountsd 106, 113, 115

a. Approximate differences for a residual kraft lignin from a 30 kappa pulp.

b. Values represent % of phenyl propane units containing 5,5′ linkages.

c. Diphenylmethane structures of various linkages.

d. Amounts ranging from ~5% to >60% have been reported.

Gellerstedt and Zhang [61] summarized some of the residual kraft lignin features,

as follows:

_ A low remaining amount of b-O-4 structures [52].

_ Linkages between lignin and polysaccharides.

_ The presence of reduced structures such as methylene and methyl

groups [48].

_ A high degree in discoloration [116].

638 7Pulp Bleaching

_ A successive increase of “condensed” structures with high degree

of delignification [49].

_ An uneven distribution of lignin across the fiber wall.

As noted, a successive cleavage of b-O-4 structure takes place in the kraft cook

(Tab. 7.8) ([61]). Moreover, the data in Tab. 7.8 highlight changes in the number of

substructures during pulping and bleaching. An increasing number of b-O-4 linkages

during oxygen delignification (Tab. 7.8) was also observed by Balakshin et al.

[79].

Tab. 7.8 Number of substructures per 100 C9 in some isolated

lignin samples (adapted from Ref. [61]) prepared by acid

hydrolysis [117].

Lignin sample b-O-4 b–5 b-b

MWL 39a 11 2

Residual lignin, kappa = 30 9–10 5 2

Residual lignin, kappa = 18 5–7 3 1

Dissolved kraft lignin 5 2 2

Residual lignin, from a commercial pulp, kappa = 26 11 6 2

Residual lignin, after an oxygen stage, kappa = 9.3 18 8 2

a. Includes a-hydroxy-b-O-4 and dibenzodioxocin structures.

Comparison of the relative stability and susceptibility of different structures of

dimeric model compounds to degradation by oxygen is difficult due to the different

conditions used, because the oxidation rate in these reactions is highly

affected by system parameters such as pH, temperature, oxygen charge, and reactant

charge [90,118]. Stilbene structures (Tab. 7.9) are rapidly degraded under oxygen-

alkali conditions [118]. Phenolic stilbenes and vinyl ethers degrade across the

double bond, whereas the stilbenes oxidize over one hundred times faster [90].

Under oxygen-alkali conditions, the model compounds in the second row of Tab.

7.9 react an order of magnitude slower than the vinyl ethers, to form phenolic

aldehydes, alcohols, ketones, and carboxylic acids along with aliphatic acids as the

main degradation products [90].

7.3 Oxygen Delignification 639

Tab. 7.9 Relative susceptibility of model compounds to oxygen (from Ref. [90]).

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