STUDIES ON THE DEGRADATION OF LIGNINS BY OXYGEN IN ACIDIC MEDIA

The degradation of native and residual (kraft) lignins by oxygen in acidic media has been studied using appropriate model compounds. A mechanism is proposed involving acid-promoted conversion of lignin units into ring-conjugated structures, followed by electron transfer from these structures to oxygen to yield hydroperoxyl radical and the cation radical. The latter, after addition of water, reacts with oxygen affording the corresponding peroxyl radical intermediate. Electron transfer from additional substrate to this peroxyl radical (in a chain reaction) and protonation give an ol-hydroxy hydroperoxide intermediate which undergoes heterolytic fragmentation to yield the corresponding carbonyl-containing products. Experimental support for the validity of this mechanism is provided. Acidic oxygenation of Lignin is compared with acidic degradation in the absence of oxygen (''acidolysis''), with degradation during chlorine dioxide bleaching, with acidic sulfonation, and with alkaline oxidative degradation under conventional oxygen bleaching conditions. The possibility of using oxygen in acidic media (''acidic oxygenation'') as an alternative delignification method is briefly discussed.