Yellowing of lignin. II. Participation of oxygen in the photodehydrogenation of lignin model compounds.

Abstract

The oxidn. of phenolic lignin [9005-53-2] model compds., such as 1-(4-hydroxy-3-methoxyphenyl)propanol [6997-34-8], etc., in the presence of uv light having the wavelengths between 300 and 400 mm was investigated kinetically using α-carbonyl compds., such as propiophenone [93-55-0], etc., as sensitizers. Comparison of the reaction rates under N with those in air showed that O quenched the excited carbonyl species and that the PhOH was dehydrogenated by excited O. In the presence of air the rate of PhOH disappearance became independent of carbonyl concn. >15 mole%. The yellowing of high-yield pulp and wood was initiated by the excited carbonyl groups which in turn lead to the formation of excited O mols. which reacted with phenolic groups in lignin and the formed phenoxy radicals reacted further forming yellow products. [on SciFinder(R)]