The oxidation of lignin to produce vanillin has been demonstrated at high pH (almost 14), and high temperatures (higher than 100°C) with molecular oxygen (oxygen pressure equal or higher than 3 bar). The main advantages of this oxidant are its environmental friendliness, the high efficiency per weight of oxidant, and comparatively low price (for example, air can often be used). The limitation in this process is the low solubility of oxygen in the reaction medium of NaOH (and lignin) in the high operational temperatures [147, 148]. Nevertheless, the oxygen partial pressure should be controlled to avoid further oxidation of vanillin [149, 150]. The high pH is required for the total ionization of phenolic groups and conversion to reactive quinonemethide as presented in Fig. 12.7 (initial step I-II). The pKa values of lignin-related phenolics are in the range of 10-11.5 at 25 °C [25] decreasing as the temperature raise [151]. However, it is expected that the phenolic groups are even less acidic in the lignin macromolecule with the concomitant higher pKa than the reported. A minimum of 2 M in NaOH is referred in some

Fig. 12.7 Proposed mechanism for lignin oxidation by Tarabanko et al. [140, 141] here represented for a typical guaiacyl type unit patents [148, 152]. Therefore, the initial pH value in the range 13-14 has been used in several works to maintain a high alkalinity in the entire reaction time. One other side, in the proposed mechanism by Tarabanko et al. [140, 141] (Fig. 12.7) the strong alkaline medium is required also for the proton detachment (step III-IV) and nucleophilic addition of OH — to the intermediary quinonemethide (step V-VI) and also for the final retroaldol cleavage (final step in Fig. 12.7). In fact, this step is the main difference between the mechanism proposed by Tarabanko for vanillin production and the mechanism via dioxetane formation established by Gierer et al. [146].

The energy barrier for electron transfer from the organic substrate to the oxidant is usually high. Considering this, the temperature should be one of the most significant factors to consider in lignin oxidation. Catalyst has also been consid­ered to improve the yields and selectivity.

Lignin oxidation with O2 in alkaline medium has been intensively studied in Laboratory of Engineering of Separation and Reaction (LSRE, Porto) [20, 36, 116-122, 153, 154]. The batch experiments have been performed in a jacketed reactor Buchi with a capacity to 1 l with control and register temperature, pressure and gas flow. The reaction mixture composed by NaOH and lignin is kept at high stirring and the reactor is purged and pressurized with N2. At steady state tem­perature, the oxygen is introduced at controlled pressure and the reaction is con­sidered to start at this point. During the reaction, the total pressure is maintained constant thought feed of O2. Samples are collected at controlled time intervals and, after acidification, the compounds are extracted with organic solvent and GC-FID analysis or recovered by solid-phase extraction and analyzed by HPLC-UV using external calibration [154].

Fig. 12.8 Products concentration and temperature evolution during the reaction time for lignin oxidation with O2 in alkaline medium (Tj = 393 K, pO2 = 3 bar, Pt = 9.7 bar, CL = 60 g/l, CNaOH = 80 g/l) for two different lignins [20]. a Kraft lignin from softwood isolated by Lignoboost process (supplied by Innventia AB) referred as LKBoostS in Table 12.2 and Table 12.3. b Organosolv beech wood lignin (supplied by Fraunhofer, Germany) referred as LOrgsB in Table 12.2