PHOTOREACTIVITY OF LIGNIN MODEL COMPOUNDS IN THE PHOTOBLEACHING OF CHEMICAL PULPS .2. STUDY OF THE DEGRADATION OF 4-HYDROXY-3-METHOXY-BENZALDEHYDE AND 2 LIGNIN FRAGMENTS INDUCED BY SINGLET OXYGEN

The degradation induced by singlet oxygen (sensitized by methylene blu e) of 4-hydroxy-3-methoxy-benzaldehyde (vanillin) and two lignin fragm ents (from rice husks and Eucalyptus grandis wood) in methanol solutio n was investigated, For vanillin, a kinetic study was performed using three different pH values. The rate of degradation of vanillin was hig hly dependent on the pH due to the ionization of phenol groups, In add ition, the physical quenching of singlet oxygen by vanillin was more s ensitive than the chemical reaction to an increase in pH. The degradat ion of lignin under the influence of singlet oxygen was influenced by the existence of free phenol groups. Conjugated carbonyl groups were f ormed and their quantity was higher than that expected from the consum ption of phenols during irradiation. This was attributed to the partic ipation of benzylarylether structural elements in the photodegradation process, (C) 1997 Elsevier Science S.A.