Oxidation of hardwood kraft-lignin to phenolic derivatives with oxygen as oxidant

A hardwood kraft lignin was oxidized in alkaline medium to obtain phenolic compounds (syringaldehyde, vanillin and its acids). To avoid lignin condensation, the lignin was precipitated from a black liquor with a calcium salt dissolved in a water soluble alcohol. Oxygen was the oxidant employed, and copper (II) and cobalt (II) salts were used as catalysts. Effect of temperature, reaction time, oxygen pressure, alkali concentration and catalyst on yield and product distribution were studied. In all the range of variables lignin conversion and aldehyde yield remains low and the more important effect on aldehyde yield was due to the alkali concentration, which must be fixed at about 2 N. The precipitation method did not significantly increase the aldehyde yield, in contrast with the results of nitrobenzene and CuO oxidations. In catalyzed oxidations, no increase in phenolic aldehydes was observed and, with some catalyst, the conversion into phenolic derivatives was reduced. Lignin conversion into low molecular weight products is responsible for the low phenolic product yield and the type of catalyst could lead the oxidation into phenolic products or into low molecular weight acids.