ABSTRACT
In contrast to cellulose and hemicellulose, lignin is the only polymeric component of the plant ber that absorbs both visible and near-UV light. In Chapter 2, the absorption of light in the ultraviolet and visible regions of the spectrum was described as characteristic of di-and tri-substituted benzene conjugated to a small population of aromatic carbonyls, α,β-unsaturated carbonyls, quinones and catechols. The excited electronic states formed after light absorption can generate reactive radical species, which in turn react with oxygen to form chromophores. Alternatively, the excited states can sensitize the conversion of triplet oxygen to the very reactive singlet oxygen. The photooxidation of lignin causes depolymerization through cleavage of interunit bonds and yellowing through the oxidation of the aromatic groups. This chapter describes the results of research elucidating the
Introduction ............................................................................................................ 555 Photodegradation of Lignin ................................................................................... 556
Depolymerization .............................................................................................. 556 Lignin Degradation .......................................................................................... 558
Cleavage of the β-O-4 Aryl Ether Bond ..................................................... 558 Phenacyl Aryl Ether ...................................................................................... 558 β-O-4-arylether Group ................................................................................ 563 Coniferyl Alcohol Group ..............................................................................564 Stilbenes ....................................................................................................... 565 Phenyl Coumarone .......................................................................................566 Dibenzodioxocin .......................................................................................... 567
The Role of Singlet Oxygen .................................................................................. 569 Description of 1O2. ............................................................................................ 569 Reactions and Reactivity ................................................................................... 570
Reactions with Lignin Model Compounds ................................................... 571 Reactions with Isolated Lignin ..................................................................... 576 Reactions with Lignin-Containing Wood Fiber ........................................... 578
Summary ................................................................................................................ 579 References .............................................................................................................. 579
detailed reaction pathways leading to photodegradation and chromophore formation (yellowing).
