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Issue 3, 2005
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Kinetics and mechanism of the sensitized photodegradation of lignin model compounds

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Abstract

The kinetics of the sensitized photodegradation of a variety of well-defined lignin model compounds was studied to determine the mechanisms responsible for lignin's photochemically-mediated oxidation. Monomeric and dimeric models representing lignin's phenolic end groups and nonphenolic dimers representing its inner core were studied. It was determined that the rate constants for the reaction of the deprotonated phenolic models with singlet oxygen (1O2) range from 0.96 to 7.2 × 107 M−1 s−1. The models were substituted with zero, one, or two electron-donating methoxy groups on both aryl rings and, while the rate constants showed little dependence on the substitution of the nonphenolic ring, the rate constants increased dramatically with increasing methoxy substitution of the phenol. Reaction between these deprotonated models and 1O2 is thus proposed to occur at the phenolate ring. Under neutral conditions, it was observed that the phenolic models react with excited state sensitizer, with this reaction also occuring at the phenol ring. The sum of the rate constants for quenching of and reaction with excited state sensitizer by lignin model compound ranges from 5.4 to 75 × 107 M−1 s−1. This study corrects previous reports that attribute the sensitized degradation of neutral lignin model compounds to reaction with 1O2. A nonphenolic aromatic ketone inner-core model was observed to undergo direct photolysis, and its reduced analog was not degraded by direct photolysis or reaction with 1O2 or excited state sensitizer. The oxidized inner-core model was also shown to be able to act as a sensitizer for the degradation of a phenolic lignin model compound.

Graphical abstract: Kinetics and mechanism of the sensitized photodegradation of lignin model compounds

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Publication details

The article was received on 12 Nov 2004, accepted on 20 Jan 2005 and first published on 04 Feb 2005


Article type: Paper
DOI: 10.1039/B416956E
Citation: Photochem. Photobiol. Sci., 2005,4, 268-274
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    Kinetics and mechanism of the sensitized photodegradation of lignin model compounds

    A. M. McNally, E. C. Moody and K. McNeill, Photochem. Photobiol. Sci., 2005, 4, 268
    DOI: 10.1039/B416956E

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