Kraft lignin chromophores

Determining chromophores and auxochromic groups in kraft lignin. 

A series of composite chromophoric systems is assumed to be present in kraft lignin. Some simpler chromophores are only absorbing within the ultraviolet (UV) region, and some of those which have a color are only hypothetical. Nonetheless, we considered it necessary to include a num-... 

Thus, we can assume that two types of stilbene structures contribute to the chromophoric systems of kraft lignin. If further double bonds or carbonyl groups are conjugated with the stilbene as in XIII, the resulting structure is likely to have absorption within the visible region of the spectrum. 

Estimating the contributions of the different potential chromophores to the color of kraft lignin will be the subject of further investigations. 

Complexes between quinones and phenols have also been proposed as important lignin chromophores in pulping liquors. Furman and Lonsky [75,76] attributed two-thirds of the light absorption of kraft lignin to quinone-phenol charge-transfer complexes. 

SI EaUcehag, J Marton, and E Adler. Chromophores in Kraft Lignin. Advances in Chemistry Lignin Structure and Reactions, 59. Washington, DC American Chemical Society, 1966, pp. 75-89. 

Condensation reactions of lignin have been mentioned 22) in connection with sulfite pulping as being responsible for color formation. However, most of the likely condensation reactions occurring during a kraft cook do not lead directly to chromophore formation. One potential condensation reaction which would give a chromophore is the formation of a chalcone (XIX) from vanillin and acetoguaiacone, both of which have been isolated by Enkvist 12) from black liquor. 

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