**1. Introduction**

Currently, environmental issues, such as the depletion of fossil resources and global warming, are of worldwide concern, which has raised growing interest in renewable and environmentally friendly biomass resources as an alternative to fossil resources. Among the various types of biomasses, wood is a promising resource because of its huge stocks and inedibility. Wood is composed mainly of three components: cellulose, hemicellulose, and lignin. Among these, lignin constitutes 20–35% of the wood [1]. Because of its aromatic nature, lignin is expected as biobased source of low molecular weight (MW) aromatics that are indispensable in the chemical industry [2–8].

The production of low MW aromatics from lignin requires conversion methods that facilitate effective cleavage of interunit linkages in lignin polymer. Among many methods currently suggested, alkaline aerobic oxidation is a promising way from the viewpoint that lignin is effectively degraded by nontoxic molecular oxygen [9–21]. Aerobic oxidation of lignosulfonate in waste liquor from sulfite pulping of softwood has been put into practical use as industrial vanillin (4-hydroxy-3-methoxybenzaldehyde) production [22–27].

Vanillin is a versatile compound that is useful as a source of medicines and polymer materials, as well as a traditional fragrance [24–30]. The vanillin production from lignosulfonate is usually carried out in aqueous alkali, that is, aqueous NaOH solution, under compressed air in the presence of a Cu2+-based catalyst. This method was a major industrial vanillin production method until 1990s. However, the yield of vanillin is usually less than 10% based on the original sodium lignosulfonate [25, 31]. In addition to this relatively low yield of the target compound, high temperature and air pressure, typically 160–200°C/0.5–2.0 MPa, required for the process and management of wastewater containing sulfur compounds are drawbacks of this biomassbased vanillin production [25]. Recent vanillin production is, therefore, substituted almost with petrol-based ones, that is, formylation of guaiacol [27].

In light of recent environmental concerns, however, the production method of vanillin derived from biomass should be continuously studied and improved as it could be a breakthrough in establishing a method to produce low MW aromatic compounds from wood. To promote the adoption of the lignin-based vanillin production method in the chemical industry, it is necessary to address some of the shortcomings of this method described above. One solution to the sulfur-containing waste problem is to use native lignin in wood, as well as lignosulfonate as raw material for vanillin. Developing methods that provide the target product in good yield without the use of transition metals or high-temperature and high-pressure conditions would be another issue to tackle. Furthermore, mechanistic knowledge of alkaline aerobic oxidation as the foundation of the process development is essential for realizing these goals.

This chapter presents the results of fundamental and applied studies on vanillin production from lignin, conducted mainly in the authors' research group, from the above perspective. In addition to vanillin, other related oxidation products, for example, vanillic acid, are also produced in the aerobic oxidation of lignin. The formation of these compounds will also be briefly summarized.
