**2. Alkaline aerobic oxidation of lignin in the presence of organic cations**

In vanillin production via alkaline aerobic oxidation of lignin, simple alkaline solution, such as aqueous NaOH solution, is generally employed as a reaction medium. Alternatively, our research group developed a new alkaline medium, that is, tetrabutylammonium hydroxide (TBAH) for lignin degradation. The chemical structure of tetrabutylammonium ion, Bu4N+ , is presented in **Figure 1**. TBAH is a salt of Bu4N+ and OH<sup>−</sup> and its aqueous solution exhibits strongly alkaline nature, as aqueous NaOH solution does. In this respect, TBAH is similar to NaOH, but a distinguishing characteristic of TBAH is that it possesses the bulky organic cation.

*Depolymerization of Native Lignin into Vanillin, Vanillic Acid, and Other Related Compounds… DOI: http://dx.doi.org/10.5772/intechopen.112090*

**Figure 1.**

*Organic cations employed in our studies [32, 33]. a) In the case of* **12C4***-Na+ , a complex cation in which Na+ is bound by two molecules of* **12C4** *is also reported [34].*

#### **Figure 2.**

*Formation behavior of vanillin (*⬧*) and vanillic acid (*◾*) from Japanese cedar wood flour (A) and sodium lignosulfonate (B), during their alkaline aerobic oxidation under air in a sealed system (volume of the gas phase: 8 mL). Reaction conditions: sample 14 mg/ reaction solution (1.25 mol/L aqueous TBAH solution or aqueous NaOH solution 2.0 mL)/ 120°C. a) Yield from the wood flour is based on its Klason lignin amount [33].*

**Figure 2** shows the changes with time in yields of vanillin and its related substance, vanillic acid (4-hydroxy-3-methoxybenzoic acid); when sodium lignosulfonate or Japanese cedar (*Cryptomeria japonica*) wood flour is oxidatively degraded at 120°C under air in the aqueous NaOH solution or aqueous TBAH solution with their concentration being 1.25 mol/L. In the vanillin production from the sodium lignosulfonate, the raw material for current lignin-based vanillin production [33], it has been indicated that the -SO3 − moieties in the raw material play important roles in the vanillin formation [35, 36]. This point can be confirmed by comparing the yield of vanillin in the NaOH medium in **Figure 2** between wood flour and sodium lignosulfonate. In other words, in the case of the NaOH medium, the yields of vanillin and vanillic acid are comparable between sodium lignosulfonate and the wood flour (vanillin yield: 3–5 wt%; vanillic acid yield: 1–3 wt%). Considering significant degradation of the original lignin structures during its isolation process, it can be concluded that sodium lignosulfonate gives the target compounds in relatively high yields. This result would be attributed to the presence and the positive effect of the -SO3 − moiety.

On the other hand, higher yields of vanillin and vanillic acid are obtained in the TBAH medium, regardless of the presence of the -SO3 − moiety, as shown in **Figure 2**. Also, the wood flour exhibited much higher vanillin yield in TBAH than sodium lignosulfonate did in the same medium. This result is interesting from the perspective of reducing sulfurcontaining waste during the vanillin production; efficient vanillin production from the wood flour, an originally sulfur-free material, was achieved in TBAH, which leads to the establishment of processes that are not bothered by the sulfur-containing waste. It should be also emphasized that the vanillin production was carried out without any transition metal additives. The employed reaction temperature, 120°C, was much lower than those generally adapted in current vanillin production processes.

The vanillin production shown in **Figure 2** takes a long reaction time, over 43 h, to reach the maximum product yield. The current process solves this problem by using pressurized air and high temperatures to promote the oxidation. **Table 1** summarizes the maximum yields of vanillin and vanillic acid and the time to reach them for the oxidation of Japanese cedar wood flour in various reaction media. It becomes clear that the addition of NaOH(s) to the reaction system and replacing the air in the reaction system with pure O2 significantly shortens the reaction time and increases the yields of the products [16, 20]. Finally, the yields of vanillin and vanillic acid under optimum conditions are 23.2, 1.2 wt%, respectively, and the time to reach of this yield is 4.0 h. Thus, in the TBAH media, it is possible to improve the reaction efficiency at low temperature of 120°C without pressurizing the reaction system, which is advantageous compared to the current process with harsh reaction conditions. For vanillic acid, on the other hand, the compound tends to form in relatively low yields under the conditions in which vanillin production is facilitated. This may suggest that the production pathways of vanillin and vanillic acid are in trade-off relationship.

The above yields of vanillin and vanillic acid are remarkable in the history of lignin chemistry. One of the most important reactions in lignin chemistry is alkaline nitrobenzene oxidation (AN oxidation). This is an oxidative degradation of a lignin-containing sample such as wood and isolated lignin by nitrobenzene to give *p*-hydroxybenzaldehydes reflecting the substitution pattern of the lignin sample, mostly vanillin in the case of softwood lignin. Despite its more than 60-year history, AN oxidation has been one of the most efficient lignin degradation methods in use today and the product yield in the AN oxidation has been employed as a benchmark to measure the efficiency of other oxidation methods. **Table 1** shows that the yields of vanillin and vanillic acid in the TBAH+NaOH/O2 system are comparable to those of

*Depolymerization of Native Lignin into Vanillin, Vanillic Acid, and Other Related Compounds… DOI: http://dx.doi.org/10.5772/intechopen.112090*


*a The yield is based on the Klason lignin content of the wood flour.*

*b The reaction time is the one in which the yield of vanillin becomes maximum.*

*c The alkaline nitrobenzene oxidation (AN oxidation) was carried out under the established conditions.*

*d In the TBAH+NaOH medium, [Bu4N+ ] is set to 1.25 mol/L with [OH<sup>−</sup>] being adjusted to the presented value by adding NaOH(s) to the TBAH solution.*

*e "Oxygen" means the experiment is carried out under pure O2. <sup>f</sup>*

*The reaction medium is prepared by adding the ether compounds to the NaOH solution, where* **18C6***,* **15C5***, and* **12C4** *are the polycyclic ethers with different cavity sizes (see* **Figure 1***), and* **TEG** *and* **TRG** *are tetraglyme and triglyme (non-cyclic polyethers with different chain lengths), respectively.*

#### **Table 1.**

*The yield of vanillin and vanillic acid from Japanese cedar wood flour after its oxidation in several oxidation systems [16, 32, 33].*

AN oxidation. These results indicate that the TBAH-based alkaline aerobic oxidation is a highly efficient oxidation method and exploit the potential of the raw material to produce the target compounds.

The high yield of vanillin achieved in the aforementioned TBAH is clearly due to the presence of the bulky tetrabutylammonium ion, compared to Na<sup>+</sup> . This led us to the idea of producing vanillin under the presence of various bulky organic cations. Namely, we added polycyclic ethers, that is, 18-Crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane, **18C6**), 15-Crown-5 (1,4,7,10,13-pentaoxacyclopentadecane, **15C5**), and 12-Crown-4 (1,4,7,10-tetraoxacyclododecane, **12C4**) to an aqueous NaOH solution and studied the alkaline aerobic oxidation behaviors of Japanese cedar wood flour in these reaction media. These crown ethers capture Na<sup>+</sup> in the aqueous NaOH solution, forming three types of complex cations **18C6**- Na<sup>+</sup> , **15C5**-Na<sup>+</sup> , and **12C4**-Na<sup>+</sup> as shown in **Figure 1** [37]. It is expected that these organic cations would exhibit enhancing effects on the vanillin production, which is similar to those observed for Bu4N<sup>+</sup> .

The aerobic oxidation of Japanese cedar wood flour was conducted in a 4.0 mol/L aqueous NaOH solution in the presence of 2.0 mol/L of the three crown ethers. As shown in **Table 1**, the vanillin yield under the three crown ethers (**18C6**, **15C5**, **12C4**) is 15.2–16.1 wt%, which is clearly higher than that in the pure NaOH system. In contrast, in the aerobic oxidation using equimolar amounts of tetraglyme (**TEG**) and triglyme (**TRG**) and non-cyclic analogs of **15C5** and **12C4** respectively, the vanillin

yields are 8.9 and 5.4 wt%, nearly the same as that obtained in the pure NaOH system. This can be explained by the fact that cyclic polyethers generally exhibit much stronger complexing ability toward metal cations than their corresponding non-cyclic polyethers [38], suggesting that the increased vanillin yield due to the addition of the crown ethers results from the formation of the complex cations in **Figure 1**. Moreover, the increase in vanillin yield by cyclic polyethers seems to depend highly on their Na+ accommodation ability. For instance, the addition of 1,4-dioxane, which has a considerably smaller ring size than the three crown ethers, results in a vanillin yield of 5.5 wt% (**Table 1**). This is likely because the cavity size of 1,4-dioxane is too small to accommodate Na+ .

For vanillic acid, another major product, the yield from Japanese cedar wood flour in the presence of complex cations is 2.3 wt%, regardless of the types of the crown ethers. This value is nearly identical to that obtained in the absence of the complex cations (2.4 wt%). This phenomenon of minimal influence of the complex cations on the production of vanillic acid is also observed in the aforementioned aerobic oxidation of Japanese cedar wood flour in the presence of Bu4N<sup>+</sup> . Thus, it can be stated that the complex cations formed from Na<sup>+</sup> and the crown ethers and Bu4N<sup>+</sup> exert quite similar effects, significantly increasing the yield of vanillin while having little impact on the formation of vanillic acid. In the following section, we will elaborate on this issue from the perspective of reaction mechanisms. Moreover, the addition of **TEG** and **TRG**, which are non-cyclic polyethers, does not substantially increase the vanillin yield, but it increases the yield of vanillic acid. For instance, the addition of **TRG** significantly increases the yield of vanillic acid from 2.4 to 9.2 wt% (**Table 1**). Although there are few clues to explain these results rationally at this point, they represent intriguing phenomena when considering the potential for further reaction control by complex cations.
