STRUCTURE OF LIGNINS IN PULPS .4. COMPARATIVE EVALUATION OF FIVE LIGNIN DEPOLYMERIZATION TECHNIQUES

Five potential lignin depolymerization techniques were evaluated with regard to their analytical value in combination with oxidation by permanganate and hydrogen peroxide. The methods included involved alkaline solutions of cupric oxide and nitrobenzene, acetolysis with thioacetic acid, derivatization with thioglycolic acid followed by alkaline hydrolysis, and reductive aminolysis with sodium in liquid ammonia. Gas and gel permeation chromatography served for the separation of product mixtures, and milled wood lignins of Loblolly pine and red alder were the lignin substrates. Whereas thioacetolysis proved most efficient in terms of lignin depolymerization, the side chains of its product mixtures were insufficiently oxygenated to warrant effective conversion into polycarboxy aromatic acids by permanganate. Cupric oxide and alkali were deemed most effective because of greatest overall yields of five prominent permanganate oxidation products combined with mildness (measured in terms of ratio of condensed to uncondensed degradation products). Suitable depolymerization conditions were found to be five hours reaction time at 174°C. The results were in general agreement with data reported by Pepper et al. and Miksche et al. © 1979 Springer-Verlag.