Figure 12.9b reveals the importance of the OH — on the yield of the process of lignin oxidation at the conditions of this study: a considerable increment of vanillin is achieved by increasing NaOH concentration. The reasons for this impact on yield were already stated in Sect. 4.4.1 (regarding postulate chemical mechanism of oxidation). However, other considerations should be noted: the concentration of vanillin at each moment is the result of formation and degra­dation in the reaction medium. Fargues et al. [149] studied the kinetics of vanillin oxidation and concluded that at pH <11.5 the vanillin oxidation become more significant being of second order in vanillin concentration and zero order in O2 concentration. At pH >11.5, the reaction rate of vanillin oxidation is first order for both vanillin and O2 concentration. Therefore, at least 2 M in NaOH is required to achieve the favorable condition to preserve the produced vanillin. Figure 12.10b demonstrates clearly the initial pH effect on vanillin oxidation. It should be pointed out that the pH is the parameter in discussion, since the same alkali concentration could lead to different values for aqueous solutions of different lignins, depending of the raw material composition. Considering this, the pH of the final solution should be measured at each case, confirming the required value. The operational problems (incrustation in the reactor) related with the solution NaOH 4 M had led Mathias et al. to avoid to such concen­tration and to adopt the 2 M.

For lignosulfonate (from E. globulus), the reaction order relative to OH — concentration is 1.9 for vanillin and 1.4 for syringaldehyde. The reason suggested for these high values was the step involving the desulfonation reaction in the case of lignosulfonates (carrying SO3- groups at Ca and Cy see Fig. 12.3) [113]. The removal of sulfonic groups leads to the formation of units with double bonds in propane lateral chain (as intermediate IIa in Fig. 12.7), species that are naturally more reactive with O2 than the saturated counterpart.