Routes of degradation of β- O-4 syringyl and guaiacyl lignin model compounds during photobleaching processes

Abstract

The photolysis of guaiacyl and syringyl β- O-4 lignin model compounds (I, II, III and IV) with a phenol or α- O-4 substituted in the phenacyl moiety was done by three different methods, designed to perform the photobleaching of lignocellulosics. The photolysis systems were carried out in alkaline methanolic solution (pH∼11), and consisted in: (a) H 2O 2, (b) methylene blue (MB)/O 2 and (c) TiO 2. The fragments produced in the reactions were analyzed by mass spectrometry–gas chromatography technique. The results show that the reaction is dependent on the substituent in the phenacyl moiety. The most important route for the degradation mechanism seems to be the incorporation of the hydroxyl radical in the aromatic rings. Semi-empirical calculations of the charge distribution over the atoms in the model compounds show that this parameter is important to decide the reaction mechanisms. For the unprotected phenolic model compound the production of phenolic fragments is higher than in the labile substituted α- O-4 model compound. On the other hand, the photodegradation of the model compound (IV) using method (a) yielded coniferyl alcohol as the principal product. The processes (a) and (c) showed to be the most aggressive for the studied compounds.