| Environmental concerns are leading to a heightened interest in chlorine-free bleaching processes. Alkali-oxygen systems are of particular interest since the by-products are considered to be environmentally benign. A disadvantage of oxygen processes, however, is that the selectivity is decreased and carbohydrates are degraded in addition to lignin. The objective of this work was to investigate the potential mechanisms during oxygen delignification.; Molecular oxygen is used in the alkali-oxygen system and can be reduced to other oxygen species predominately superoxide anions, hydrogen peroxide, hydroperoxy anions, hydroxyl radicals, and oxy radical anions. These specific oxygen species were generated and reacted with various lignin model compounds and the products were identified using high performance liquid chromatography and gas chromatography/mass spectrometry. Computational methods were used to help understand the chemistry involved by calculating the heats of reaction for each proposed mechanism. These calculations provide a powerful tool for supporting experimental results and probing the mechanistic details of reactions that are difficult to examine experimentally.; Experimental results from reactions of model lignin compounds (guaiacol, veratrole, vanillin and veratraldehyde), with these various oxygen species, suggest that hydroxyl radicals are responsible for hydroxylation, demethylation, and side chain oxidization/decarboxylation reactions. Computational results suggest it is unlikely that a one electron transfer from a phenoxy anion forms phenoxy radicals. Phenoxy hydrogen atom and methoxy methyl radical abstractions were calculated to be thermodynamically favorable with other oxygen species in addition to hydroxyl radicals. This is significant because it implies that phenoxy radicals can be formed in the absence of hydroxyl radicals. Once phenoxy radicals are formed, superoxide anions can degrade the aromatic ring into low molecular weight diacids; other oxygen species are less likely to facilitate ring degradation.; Additional work in our laboratory indicates that only hydroxyl radicals degrade carbohydrates. Superoxide anions are responsible for lignin degradation by cleaving the aromatic ring, but do not attack carbohydrates. Consequently, in order to make oxygen delignification more efficient, the concentrations of superoxide anions need to be maximized and hydroxyl radicals need to be minimized. There are still a lot of questions to answer in order to optimize these conditions. |
