**4. Conclusions**

This chapter presented high-yield vanillin production from lignin (mainly native softwood lignin) by the alkaline aerobic oxidation in the presence of organic cations. When conducting alkaline aerobic oxidation of lignin in the presence of bulky organic cations such as Bu4N+ and the complex cations formed between the crown ethers and Na<sup>+</sup> , a significant increase in the yield of vanillin from the lignin samples is observed. In this organic cation-containing system, the alkaline aerobic oxidation allows for the efficient production of vanillin at a temperature of 120°C, employing native lignin in the wood flour as the starting material. This vanillin production process is more advantageous than the current process using lignosulfonate in that it can achieve a high vanillin yield comparable to that of AN oxidation and does not produce sulfurcontaining effluent.

One of the vanillin production pathways in the alkaline aerobic oxidation of lignin involves the elimination of vanillin from the Cα-aldehyde structure, which is produced by the oxidative degradation of non-phenolic intermediate units in lignin. It has been found that the aforementioned organic cations improve the selectivity of this vanillin elimination reaction. Research on such reaction mechanisms and further mechanistic insights into the function of the organic cations are expected to lead to the development of more rational reaction control methods to improve the efficiency of the vanillin production processes.
