Oxidation of Lignin from Eucalyptus globulus Pulping Liquors to Produce Syringaldehyde and Vanillin

Lignin is one of the main components of the pulping liquors and a potential source of high-added-value chemicals. The aim of this work is to evaluate the potential of industrial Eucalyptus globulus sulfite liquor and kraft liquors (collected at different stages of processing before the recovery boiler) for the production of syringaldehyde (Sy) and vanillin (V) by oxidation with O-2 in alkaline medium. Oxidations were performed in a jacketed reactor with controlled temperature and pressure by (1) direct reaction of pulping liquors and (2) reaction of kraft lignins isolated from liquors. Lignins produced by LignoBoost technology were also evaluated. The products profiles were established, as well as the yields, temperature and O-2 uptake during the reaction. Sulfite liquor is the best raw material leading to the highest yield by direct oxidation (33 kg of Sy and 15 kg of V per lignin ton), followed by thin kraft liquor (KL) which, compared to evaporated and heat treated liquors, also led to lower proportion of byproducts such as syringic and vanillic acids. Lignin isolation benefits yields and selectivity but just in the case of KL, leading to 28 kg of Sy and 12 kg of V per ton of lignin. The performance of the materials studied is primarily affected by the lignin characteristics but also by the inorganic content of the pulping liquors, whereas no effect of the carbohydrates was noticed.