Photoreactivity of lignin model compounds in the photobleaching of chemical pulps .2. Study of the degradation of 4-hydroxy-3-methoxy-benzaldehyde and two lignin fragments induced by singlet oxygen

The degradation induced by singlet oxygen (sensitized by methylene blue) of 4-hydroxy-3-methoxy-benzaldehyde (vanillin) and two lignin fragments (from rice husks and Eucalyptus grandis wood) in methanol solution was investigated, For vanillin, a kinetic study was performed using three different pH values. The rate of degradation of vanillin was highly dependent on the pH due to the ionization of phenol groups, In addition, the physical quenching of singlet oxygen by vanillin was more sensitive than the chemical reaction to an increase in pH. The degradation of lignin under the influence of singlet oxygen was influenced by the existence of free phenol groups. Conjugated carbonyl groups were formed and their quantity was higher than that expected from the consumption of phenols during irradiation. This was attributed to the participation of benzylarylether structural elements in the photodegradation process, (C) 1997 Elsevier Science S.A.