Hydrothermal conversion of alkali lignin showed that considerable high yields of formic acid and acetic acid were attained [49].
The element components of alkali lignin are shown in Table3.2:
3.3.1 Production of Formic Acid and Acetic Acid by the Usual Hydrothermal Oxidation
Experiments were performed over a wide range of conditions with temperature varying from 260 to 320C, oxygen supply varying from 60 to 120 %, and reaction time varying from 30 to 150 s to test whether formic or acetic acid can be pro- duced in large quantity.
First, experiments were performed by varying reaction temperature from 260 to 320C with the additive ratio of oxygen supply 100 % for reaction time 120 s. As shown in Fig.3.10a, the yield value of acetic acid increased with the reaction temperature from 260 to 300 C, and the value decreased when the temperature changed to 320C. The biggest yield of acetic acid was 12.3 %. In contrast, the higher yield of formic acid was acquired at 280C, which was 4.9 %. It suggested that higher temperature was a plus factor for the production of formic acid and acetic acid, but the yields decreased when the temperature was too high because of the decomposition process.
Figure3.10b presents the effects of oxygen supply on the yields of formic acid and acetic acid in the hydrothermal reaction. The additive ratio of oxygen supply varied from 60 to 120 % at 300 C for 120 s. For the four H2O2 supplies, the highest yield of formic acid was 4.1 % with the additive ratio of H2O260 %. That of acetic acid was 12.3 % with the additive ratio of oxygen supply 100 %.
Compared with formic acid, the acetic acid was the major product. These results further indicate that the oxidative decomposition of formic acid is easier than that of acetic acid. The change regularity of the yield of acetic acid was not in agreement with that of formic acid. Probably it is hard to acquire the highest yields of formic and acetic under the same reaction condition.
Figure3.10c shows the effects of reaction time on the yields of formic acid and acetic acid at 300C with the additive ratio of oxygen supply 100 %. As seen in the figure, the yield of acetic acid increased gradually with prolonging the reacting time while the yield value of formic acid decreased at the same progress.
Table 3.2 Element
composition of alkali lignin Element C H N O
Value (%) 61.26 5.90 0.87 31.25
The highest yield of acetic acid was 12.3 % with the reaction time 120 s. For formic acid, the value is 4.5 % with 30 s. This may imply that higher yield of formic acid can be acquired with a short reaction time. For acetic acid, high yield needs longer reaction time.
3.3.2 Improvement of the Yields of Formic Acid and Acetic Acid with the Addition of Alkali
In this section, the alkaline hydrothermal conversion of real lignin into formic acid and acetic acid was introduced. The addition of alkali may be an effective way to prevent oxidative decomposition of formic acid. Though many reports claim that
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Reaction time (s)
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Formic acid Acetic acid Formic and acetic acid
(a)
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(c) Fig. 3.10 Effects of reaction temperature on the yields of formic acid and acetic acid (a120 s, oxygen supply 100 %), oxygen supply (b300C, 120 s), and the reaction time (c300C, oxygen supply 100 %)
alkali can accelerate the decomposition of organic compounds, our previous studies have demonstrated that alkali can prevent organic compounds from being oxidized [50]. In our study, we have performed the experiments over a wide range of conditions with temperature varying from 260 to 340C, oxygen supply from 40 to 120 %, NaOH addition from 0 to 2.0 M, and reaction time varying from 30 to 150 s [51]. In this section, the additional amount of alkali was introduced, and then the effects of reaction conditions on the formic acid and acetic acid yield under alkaline hydrothermal conditions were discussed.
The effects of alkali amount on the yields of formic acid and acetic acid could be seen in Fig.3.11. These results suggested that a higher yield of formic acid could be realized when the oxidative decomposition of formic acid was inhibited by additional alkali. However, the function of alkali was unclear now. Some researchers thought that alkaline may change the reaction mechanism, and the metal ions also have some effects on the oxidation reaction [52]. However, experimental results showed that the production of formic acid and acetic acid was accelerated significantly.
The effects of reaction conditions under alkaline hydrothermal conditions are introduced in the next section. First, experiments were performed by varying reaction temperature from 260 to 340C with the oxygen supply 100 % for 90 s.
As shown in Fig.3.12a, the change trends of yields of formic acid and acetic acid with temperature are similar. As the temperature increases from 260 to 340C, the yields first increase and then decrease. However, for the different small molecule acids, the highest yield is obtained at different temperatures. At 280C, the highest yield of 9.4 % for formic acid can be achieved, while the maximum yield of 23.8 % is achieved at 300 C for acetic acid. The results suggested that there existed the different optimal temperatures in the alkaline hydrothermal conversion of lignin to different small molecule acids.
The effects o oxygen supply, varied from 40 to 120 %, at 300C for 90 s are shown in Fig.3.12b. The highest yield of formic acid was 8.3 % with the oxygen supply 80 %. That of acetic acid was 23.8 % with the oxygen supply 100 %. The yields of formic and acetic acid were much lower with the oxygen supply 40 %.
The reason for that was that the reaction was mainly the oxidation decomposition of lignin molecule in initial stage of the reaction and there were not much formic acid or acetic acid produced. With the increase of the oxygen supply, the inter- mediates, namely a lot of small molecule organic chemicals, were further oxidized, and so more formic and acetic acid were produced. Meanwhile, the excessive oxygen supply may also cause the oxidation of formic and acetic acid, so the yields decreased.
Figure3.12c shows the effects of reaction time on the acid yields at 300C with the oxygen supply 100 %. As seen in Fig.3.12c, the yield of acetic acid increased gradually with prolonging the reaction time, and then decreased slightly from 90 to 150 s while the yield of formic acid decreased at the same progress. The highest yield of acetic acid was 23.8 % with the reaction time 90 s. For formic acid, the biggest value is 10.3 % for 30 s. This may imply that higher yield of formic acid
can be acquired with a short reaction time. For acetic acid, the situation is just the opposite. The reason for that may be the different pathways of the production of formic and acetic acid.
3.3.3 Possible Explanation for the Production of Formic Acid and Acetic Acid from the Oxidation of Alkaline Lignin
Oxidation pathways of lignin for formic acid and acetic acid formation were illustrated. As reported by Jin et al., formic acid was probably formed in the oxidation of lignin compound by the rupture of benzene ring, and the products were further oxidized. The key step in the process was the formation of small molecular organic acid. Some products, such as maleic acid, malonic aicd, pro- pionic acid, oxalic acid, etc., were formed. Some experimental studies thought the oxidation of maleic acid largely prevailed over thermal decomposition [53]. There is a lack of information to doubtless affirm that this is a true pathway for phenolic compound oxidation under the current experimental conditions [54].
Proposed pathways of hydrothermal conversion of lignin into formic acid are showed in Fig.3.13. Figure3.13shows the GC/MS chromatogram of a sample.
Some intermediate products were labeled in the figure. The main process of production of formic acid and acetic acid formed unsaturated dicarboxylic acids, such as maleic acid, fumaric acid, levulinic acid and butanedioic acid, as showed in Fig.3.8. There were some other saturated dicarboxylic acids produced as intermediate products.
Suzuki [55] has done some research of lignin model compounds. The results were similar to the hydrothermal oxidation of alkali lignin in this study. The main
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Yields of acids (%)
NaOH additional amount ( M) Formic acid
Acetic acid
Formic acid + Acetic acid Fig. 3.11 Effects of NaOH
on the yields of formic acid and acetic acid (with oxygen supply 100 % at 300C for 90 s)
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(c) Fig. 3.12 Effects of reaction time on the yields of formic acid and acetic acid (aNaOH 1.0 M, 300C, oxygen supply 100 %), reaction temperature (bNaOH 1.0 M, 90 s, oxygen supply 100 %) and oxygen supply (cNaOH 1.0 M, 300 C, 90 s)
process of production of formic acid and acetic acid formed unsaturated dicar- boxylic acids, such as maleic acid, fumaric acid, levulinic acid, and butanedioic acid, as shown in Fig.3.13. During the hydrothermal oxidation reaction of alkali lignin, acetic acid had the highest yield among all carboxylic acid because acetic acid was the hardest to be oxidized. Knowing that saturated dicarboxylic acids and unsaturated dicarboxylic acid of glutaconic acid with 5 carbon atoms can produce a higher yield of acetic acid, so it is possible to increase the yield of acetic acid by controlling reaction pathways to enhance the formation of saturated dicarboxylic acids and unsaturated dicarboxylic acid of glutaconic acid with 5 carbon atoms.
There were some other saturated dicarboxylic acids produced as intermediate products.