Oxidation of Organosolv Lignins in Acetic Acid

Abstract

The oxidation of four lignins obtained by organosolv pulping of eucalyptus wood (Acetosolv-eucalyptus Acetosolv lignin [EAL]), sugarcane bagasse (Acetosolv-bagasse Acetosolv lignin [BAL] and in acetone/water/FeCl₃-bagasse acetone/water lignin [BAWL]), and a softwood mixture (Organocell, Munich, Germany) was performed to obtain vanillin, vanillic acid, and oxidized lignin. Experiments were carried out in acetic acid under oxygen flow using HBr, cobalt(II), and manganese(II) acetates as catalysts. After 10 h the total vanillin and vanillic acid yields were BAL 0.05 mmol, EAL 0.38 mmol, BAWL 0.45 mmol, and Organocell 0.84 mmol. Acetosolv lignins are cross-linked, which explains the lower yields in mononuclear products. The reaction volume (ΔV) of this reaction is -817 cm³/mol, obtained in experiments performed under oxygen pressure, showing the high influence of pressure on the oxidation. The major part of the lignin stays in solution (oxidized lignin), which was analyzed by infrared spectroscopy, showing an increase in carbonyl and hydroxyl groups in comparison with the original lignin. The oxidized lignin can be used as chelating agent in the treatment of effluents containing heavy metals.