The production of phenolic compounds, mainly vanillin, from several sources of lignin in alkaline medium with O2 has been the subject of many publications in the last decades [20, 36, 92, 110-113, 115-125]. Some researches have been devel­oped also in acidic medium [126-128] or in ionic liquids [129, 130]. Biotechno­logical routes [100, 101, 117], electro-oxidation of lignin [56] and the use of microwaves [131] have also been considered. Some studies reported also the direct oxidation of wood to produce vanillin and syringaldehyde [ 132]. The separation of product from the reaction medium was also a subject of intense research [133­138], as will be pointed out in the next section.

The chemical oxidation of the spent liquors or lignin, from several sources, is focused in operating conditions, apparatus, and catalyst. The general aim is to achieve the maximum conversion of lignin to vanillin (in the case of softwoods) or also to syringaldehyde (in the case of hardwoods or annual plants). Besides the process yields on products, also the kinetics laws, and the chemical mechanisms have been achieved. The impact of a particular lignin on its performance toward the oxidative process have been considered more recently [20]. A summary of some of the representative studies concerning raw material, conditions, and results are gathered in Table 12.3.

Insights on reaction mechanism of lignin oxidation have been consistently developed by Tarabanko et al. [19, 132, 139-142], although some are not so recent papers [143-146] constitute also a valuable survey of information. In this chapter, the mechanism of lignin oxidation will be briefly described.