The typical profiles of phenolic products and temperature as function of time is shown in Fig. 12.8 for the oxidation of a softwood kraft lignin (a) and hardwood organosolv lignin (b) [20]. The yields of vanillin and syringaldehyde clearly predominate over the vanillic acid and syringic acid. The concentration of the phenolic aldehydes and their respective acids increases continuously until a maximum value which is coincident with the maximum temperature (Tmax). In fact the reaction is exothermic, increasing the initial temperature (Ti) of reaction (393 K): the values of AT (Tmax-Ti) reported in these reactions were 13 and 9 K for softwood and hardwood lignins, respectively, (Fig. 12.8). For a commercial kraft lignin, Indulin AT, increases at the same order (10-15 K) were found [121], although the rate of oxidation as well as the heat of reaction differs between lignins. After that maximum, the concentration of products decreases continuously due to the dominance of degradation over the production reactions.

The phenolic acids are formed by the cleavage of Ca-Cb in the propane chain of ppu (as shown in Fig. 12.7), as for aldehydes, but they undergo further oxidation. The profiles of vanillic acid and syringic acid are very close to the corresponding alde­hydes, with a maximum at the same reaction time, followed by an analogous decline. The ratio between these two products is a measure of process selectivity for the aldehydes. For the lignins and conditions corresponding to data presented in Fig. 12.8, it is noteworthy that, at the maximum yield for softwood (40 min) and hardwood (25 min), the calculated vanillin/vanillic acid ratio is 1.7 and 1.6, respectively. This is a rather similar value, considering that the individual yields are quite different. One other side, the ratio syringaldehyde/syringic acid for the hard­wood lignin (values taken at the maximum—12 min) is much higher: 13.1. This is an
indication of higher selectivity of the process for the syringyl units of lignin. The oxidation of vanillin is pointed out as the main route for vanillic acid production (as well as for other secondary products) [155]. However, it is interesting to notice the similar and parallel behavior of formation and degradation of the aldehydes and respective acids, and also the strong decrease of vanillic acid for long reaction time in comparison to vanillin. Gierer et al. [146] reported different routes for vanillin and vanillic acid production from lignin oxidation, and not as subsequent reactions, which, at least partially, would be the reason for the behavior observed.

The increase on the production of syringaldehyde in the first 10 min of reaction, and the pronounced decrease after the maximum (Fig. 12.8b) were remarkably high compared to the behavior of vanillin. These facts are related with the different oxidation rates of guaiacyl and syringyl units of lignin. The syringyl units have higher reactivity than guaiacyl counterparts in alkaline systems [156] and under conditions of O2 oxidation in alkaline medium [110, 157]. Thus, the oxidation of syringyl units is faster than guaiacyl units for both production and degradation of aldehydes.

Besides vanillin and syringaldehyde, and their respective acids, acetovanillone was also found as secondary product. Acetosyringone was also found in the case of hardwood LOrgsB, as well as p-hydroxybenzaldehyde in both lignins, but in rather low concentration (<0.05 wt% lignin, not shown in Fig. 12.8). Vanillin, vanillic acid, and acetovanillone have been reported as the three principal compounds found in the reaction mixture. For example, yields of 2-8% of these three major compounds were obtained from alkaline oxidation of lignosulfonates from Nor­wegian spruce [155].

The mechanism for the lignin oxidation that expresses the formation of aceto derivatives as acetovanillone and acetosyringone [140] is based on the competing addition of OH — to a-position and to у-position of quinonemethide: the first leads to the acetoderivatives and the second leads to vanillin and syringaldehyde. Acetovanillone (also known as acetoguaiacone, or apocynin, with an odor similar to vanillin) is an interesting precursor of veratric acid (3,4-dimethoxybenzoic acid), a building block for synthesis of pharmaceuticals. Acetovanillone is already isolated after the lignosulfonate oxidation at Borregaard [158].

Other compounds have been also referred as secondary products from lignin oxidation with O2 in alkaline medium as for example, guaiacol, dehydrovanillin, 5-carboxyvanillin, 5-carboxyvanillic acid, homovanillin, and the syringaldehyde counterpart [113, 155].