OXIDATION OF HYDROLYSIS LIGNIN

Communication I.

OXIDATION BY OXIDES AND HYDRATES OF OXIDES OF METALS OF VARIABLE VALENCE

L. A. Pershina, V. P. Vasil’eva

(Presented by the Department of Organic Chemistry)

As is known, oxidation is one of the most widespread methods for elucidating the structure of organic compounds. As a result of oxidative cleavage, fragments of large molecules are formed, from which it is sometimes possible to establish the structure of the entire molecule.

Using the example of the oxidation of hydrolysis lignin to benzenepolycarboxylic acids and the establishment of the structure of the main core of hydrolysis lignin from the results of this oxidation, the application of the oxidative process to the solution of theoretical questions has been shown [1, 2, 3, 4]. But these same benzenepolycarboxylic acids are strong aromatic acids, from which esters are obtained that are used in the production of adhesives and plastics. In oxidation, many other valuable products are also obtained.

Consequently, the processes of lignin oxidation are studied with the aim of establishing its structure and obtaining valuable products.

Oxidation in the presence of catalysts—oxygen carriers—should be considered especially promising.

Catalysts make it possible to carry out oxidation at a lower temperature, i.e., under milder conditions, and thus reduce the development of processes of deep oxidation.

The benzene ring is very resistant to oxidizing agents; however, the possibility of its destruction during oxidation was established long ago.

The introduction of substituents of the first kind into the benzene ring increases its tendency toward destructive oxidation. But in the literature there are indications of the high stability of phenols toward oxidation by atmospheric oxygen. Prolonged heating of phenol, hydroquinone, and pyrogallol in a stream of oxygen at \(t = 150—180^\circ\) is not accompanied by chemical interaction of these compounds with oxygen [5].

The work of J. Pearl (a) on the oxidation of lignin model substances shows that in an alkaline medium, in the presence of CuO, HgO, and other oxide hydrates, oxidation of the side chain proceeds with preservation of the benzene ring containing \(OH'\) and \(OCH_3\) [6].

According to the ideas available in the literature on the structure of lignin [7], during its processing it is possible to obtain up to 60% aromatic compounds; this is the aim of the present work. In addition, the results of the work may also somewhat supplement the concepts

on the structure of lignin, namely on the amount of aromatic groups in lignin and on the nature of their linkage.

We carried out oxidation of sulfuric-acid lignin from the Krasnoyarsk hydrolysis plant in an alkaline medium, in the presence of oxides and hydrates of oxides of metals of variable valence, at \(t = 180^\circ\), for four hours.

Table 1

Experimental Part

Technical hydrolysis lignin from the Krasnoyarsk plant was extracted with hot water in a flask with a reflux condenser for 3 hours. For 10 g of lignin, 500 ml of water were taken. The lignin was then extracted with a hot alcohol–benzene mixture (1:1) for 5 hours. After filtration and drying, the lignin was treated with 72% H₂SO₄ and heated for 1 hour with water. The so-called “pure lignin” thus obtained contained 64.67% C; 6.0% H; 10.6% OCH₃; 0.5% COOH; 5.8% OH; moisture—8.7%.

A weighed portion of lignin, alkali, and catalyst (0.0007 mole) was placed in ampoules made of molybdenum glass. The hydromodule was 1:10. The ampoules were kept for 4 hours in a rotating autoclave at \(t = 180^\circ\).

The reaction mixture was subjected to the following treatment: the precipitate insoluble in alkali was filtered off and washed with warm water to a neutral reaction; the alkaline solution was acidified to an acid reaction with sulfuric acid 1:5 according to Congo red; for better coagulation of the precipitated lignin acids (or the precipitate soluble in alkali), the acidic filtrate was frozen. From the precipitate insoluble in alkali, from the lignin acids, and from the acidic filtrate, oxidation products were extracted 10–15 times with diethyl ether.

The amounts of the starting substances, reaction products, and their analysis are given in Table 1.

The lignin acids were separated into ether-soluble and ether-insoluble fractions (Table 2).

By paper chromatography with 2,4-dinitrophenylhydrazine, vanillin was detected in the ether-soluble products.

Table 2

Conclusions

1. Hydrates of oxides and oxides of metals with lower valency increase the yield of lignin acids. At the same time, a process of alkaline activation takes place, with slight cleavage of OCH₃, amounting to 1.5–2%, and an increase in carboxyl groups by 3–4%. These additives reduce the yield of ether-soluble substances.

2. Oxides of metals with higher valency are sources of oxygen. The oxidation process proceeds intensively and deeply.

The yield of lignin acids is low, and they contain a large amount of carboxyl groups.

The amount of ether-soluble substances increases, especially in the presence of vanadium pentoxide (13%).

References

1. M. I. Chudakov, S. I. Sukhanovskii, M. P. Akimova. On the benzenoid structure of hydrolysis lignin, ZhPKh, vol. XXXI, issue 3, 1959.

2. M. I. Chudakov. On the secondary aromatic polynuclear structure of lignin, Dokl. AN SSSR, vol. 137, 6, 1961.

3. M. I. Chudakov, A. P. Krasnova. Preparation of benzenepolycarboxylic acids from hydrolysis lignin by oxidation with HNO₃, ZhPKh, vol. 34, issue 12, 1961.

4. M. I. Chudakov, A. P. Krasnova. Preparation of benzenepolycarboxylic acids from hydrolysis lignin by alkaline oxidation, ZhPKh, vol. 34, issue 12, 1961.

5. S. R. Rafikov. On the mechanism of processes of self-oxidation and spontaneous combustion of coals, Izv. AN Kazakh SSR, issue 9, 1956.

6. I. A. Pearl and Donald L. Beyer, Studies of lignin and related products IX. J. Am. Chem. Soc. 76, 2224, 1954.

7. D. V. Tishchenko. Ozonation of lignin, ZhPKh, No. 3, 1959.