In general oxidative depolymerization of lignin is carried out to produce aromatics with an increase in oxygen-containing groups, mostly aldehydes. The pro­duction of vanillin (3-methoxy-4-hydroxybenzaldehyde) by oxidative depolymerization of lignin, mainly from black liquor of sulfite pulping is the most well-known pro­cess. This commercial process is typically performed at 160—175 ° C under alkaline conditions using a copper catalyst by Borregard in Norway. Especially softwood lignin is yielding relatively higher amounts of vanillin as compared to hardwood lignin where syringaldehyde may prevail (Evju, 1979).

Other researchers used hydrogen peroxide for oxida­tive depolymerization. Kraft lignin was treated at 90 °C by a biomimetic system, using hemin as a catalyst and hydrogen peroxide as an oxidizing agent, which mimics the catalytic mechanism of lignin peroxidase. Rela­tively high yields of vanillin 19%, vanillic acid 9%, 2-methoxyphenol 2% and 4-hydroxybenzaldehyde 2% were obtained (Suparno et al., 2005). Xiang and Lee (2000) found that alkaline peroxide treatment of lignin at 80—160 °C yields mainly low molecular weight organic acids (up to 50%) with only traces of aromatics, which are rapidly degraded by hydrogen peroxide.

Sales et al. (2004, 2007) studied the alkaline oxidation of sugarcane soda lignin with a continuous fluid bed with a palladium chloride PdCl3.3H2O/g-A^Os catalyst at 100—250 °C and 2—10 bar partial oxygen pressure. Total aldehyde yield on lignin was 12%. Zakzeski et al.

(2010) reported other predominantly catalytic lignin oxidation processes yielding aromatic aldehydes and acids, which do not exceed 10% on lignin basis. Howev­er, lignin model compounds show in some catalytic pro­cesses good conversions, which are promising to further develop catalytic strategies for lignin depolymerization in a biorefinery concept.

Voitl and Rudolf von Rohr (2010) studied a process for producing vanillin and methyl vanillate from kraft lignin by acidic oxidation in aqueous methanol with H3PMo12O40 as a homogeneous catalyst in the presence of 10 bar oxygen. A stable yield of 3.5 wt% vanillin and

3.5 wt% methyl vanillate can be obtained together with 60 wt% of oligomeric products in the extract. The mono­mers can be effectively separated using organic solvent nanofiltration (Werhan et al., 2012).