The effect of pO2 in the range 2.1-6.6 bar (continuous supply) was tested for lignin (softwood kraft lignin supplied by MeadWestvaco Corp.) at initial con­centration 60 g/l and initial temperature 393 K with a total pressure (N2, O2 plus water vapor) of about 9.3 bar [36]. The results demonstrate that the main effect of pO2 was on the rate of vanillin formation, shortening the time to maximum, with no influence on yield [118], as depicted in Fig. 12.9a. Other authors [110, 125] have studied the effect of O2 using two approaches: (1) continuous supply of

oxygen to the reactor, maintaining the initial pO2 in the course of reaction; (2) O2 introduced at the beginning and immediately interrupted. In the second case, it was observed a rapid decrease of pressure due to the O2 consumption in reaction. While Xiang and Lee [125] reported lower yields on vanillin and syringaldehyde for the first approach, Wu et al. [110] demonstrated that the yields of products did not change, but the rate of vanillin and syringaldehyde formation was higher for the reaction with continuous supply of O2, in accordance with Mathias’s results [36, 118]. Furthermore, after the maximum, an accentuated decrease of aldehydes concentration was noticed for constant supply of O2. This observation is in accordance with the higher rate of vanillin decline after the maximum depicted in Fig. 12.9a and in Fig. 12.10a for higher initial pO2. In the oxidation of a hardwood lignin [110], this effect was more evident for syringaldehyde probably due to its higher reactivity than guaiacyl counterparts in alkaline systems [156] and under conditions of O2 oxidation in alkaline medium [157] leading to its faster degradation.

The origin of lignin and, consequently, its structure and reactivity have influ­ence on kinetic parameters. The reaction order of vanillin production with respect to the oxygen concentration was 1.75 [116] for Westvaco Co. kraft lignin and 1.00
for an Eucalytus lignosulfonate (for both vanillin and syringaldehyde), showing that the oxidation of first lignin has a higher dependence of oxygen concentration in reaction medium.