OXIDATION OF HYDROLYSIS LIGNIN

III. Study of the products of lignin oxidation

L. A. PERSHINA, G. D. MAKAROVA, V. P. SHVACHEVA

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

In the study of products of lignin cleavage, paper chromatography is widely used [1].

The aim of the present investigation was to study, by means of paper and thin-layer chromatography, the oxidation products of hydrolysis lignin from the Krasnoyarsk Hydrolysis Plant in an alkaline medium in the presence of vanadium pentoxide.

Experimental part

The reaction products from the oxidation of hydrolysis lignin, previously extracted with hot water and an alcohol–benzene mixture, were separated into fractions soluble in alkali, ether, and water.

From the ether-soluble reaction products, the aldehyde, acidic, and phenolic fractions were isolated by conventional methods [2].

Table 1 gives the total yield of ether-soluble products and of individual fractions upon oxidation of lignin in the presence of 5% vanadium pentoxide relative to the lignin charge, 30% caustic soda, and a hydromodule of 1:10 for 4 hours at various temperatures.

Table 1

Elemental analysis and molecular weight of the fractions are given in Table 2.

Table 2

Study of the Aldehyde Fraction

The method of ascending chromatography was used. Samples of filter paper from the Leningrad Paper Mill under index “No. 2” were used.

The solvent system employed was benzene—n-butyl alcohol—water (6 : 1 : 1). To develop the chromatogram, a 0.1% solution of 2,4-dinitrophenylhydrazine in 2 N hydrochloric acid was used. As markers, a mixture of pure substances was taken: vanillin, p-hydroxybenzaldehyde, and protocatechualdehyde.

In all aldehyde fractions obtained at different oxidation temperatures, only vanillin was detected, in the form of a peach-colored spot with \(R_f = 0.55\).

Study of the Acidic Fraction

Chromatography was carried out by the generally accepted method [3]. As solvent, the system n-butanol—acetic acid—water (4 : 1 : 5) was used. The reference substances were 0.1% solutions of protocatechuic and vanillic acids.

The chromatographic paper was preliminarily freed from iron ions by successive treatment with 0.3 N hydrochloric acid solution, 0.5 N sodium hydroxide solution, distilled water, 0.1% phosphate buffer with pH 7.0–7.5, and then dried.

A solution of diazotized sulfanilic acid was used as the developer.

In the acidic fractions obtained by oxidation of lignin at different temperatures, vanillic acid with \(R_f = 0.95\) was detected. Protocatechuic acid was not detected.

Vanillic acid was isolated from the acidic fraction obtained by oxidation of lignin at 220°C. For this purpose the acidic fraction was dissolved in alcohol. A saturated solution of lead acetate was added to the alcoholic solution. The lead salts of vanillic acid are soluble in water. The precipitate was filtered off and washed several times with water. Hydrogen sulfide was passed through the filtrate and wash waters to remove the excess lead acetate; the filtrate was acidified with hydrochloric acid, saturated with sodium chloride, and extracted several times with ether. The ether extracts were dried over ignited \(Na_2SO_4\), and the ether was distilled off. The residue was recrystallized from water. Vanillic acid with m.p. = 197°C was obtained. Yield: 9% of the lignin sample.

Study of the Phenolic Fraction

For the study of the phenolic fraction, the method of thin-layer chromatography with an unfixed adsorbent layer [4] was used. As a

as the adsorbent, KSK-grade silica gel was used, previously washed free of iron impurities, ground, and sieved through a sieve with an aperture size of 0.1 mm.

Two solvent systems were tested: benzene–dioxane–acetic acid (95 : 25 : 4) and benzene–methanol–acetic acid (45 : 8 : 4). Diazotized sulfanilic acid was used as the developer; the standards were 0.1% solutions of pyrocatechol and guaiacol in alcohol.

In all phenolic fractions obtained by oxidation of lignin at different temperatures, pyrocatechol with \(R_f = 0.26\) and guaiacol with \(R_f = 0.82\) were detected.

From the phenolic fraction obtained by oxidation of lignin at 220°C, guaiacol and pyrocatechol were isolated by means of thin-layer chromatography. For this purpose, the alcoholic solution of the phenolic fraction was applied as a continuous line to a plate 12.5 cm wide and 17.5 cm long, with a silica-gel layer thickness of 1.5–3 mm. The plate was then placed in a chamber with the solvent system benzene–dioxane–acetic acid (95 : 25 : 4).

The bands of the separated substances were detected in ultraviolet light, marked, scraped off, collected in a funnel with a porous filter (No. 3 or No. 4), and eluted with ether.

In ultraviolet light, luminescence was observed: yellow for pyrocatechol with \(R_f = 0.62\), and pink for guaiacol with \(R_f = 0.78\). The yield of guaiacol was 2% of the lignin charge; the yield of pyrocatechol was 5% of the lignin charge.

For comparison, guaiacol and pyrocatechol were isolated from the phenolic fraction obtained under the same conditions by steam distillation [5]. Guaiacol is more volatile with steam than pyrocatechol. Guaiacol from the distillate and pyrocatechol from the residue after steam distillation were extracted with ether. The yield of pyrocatechol was 3.5% of the lignin charge. Guaiacol was obtained in the form of a brown oil, yield 1.5% of the lignin charge. The oil was treated with benzoyl chloride and alkali with shaking. The guaiacol benzoate obtained was recrystallized from ethyl alcohol, m.p. = 57°. Found, %: C 78.91; H 5.25. Calculated, %: C 79.23; H 5.66.

Conclusions

1. By paper and thin-layer chromatography, the following were detected in the acidic fraction of the oxidation products of hydrolysis lignin with vanadium pentoxide: vanillic acid; in the aldehyde fraction, vanillin; in the phenolic fraction, guaiacol and pyrocatechol.

2. Oxidation of hydrolysis lignin in the presence of 5% vanadium pentoxide, based on the lignin charge, 30% caustic soda, and a hydromodule of 1 : 10 for 4 hours at 220°C afforded: vanillic acid in a yield of 9%, guaiacol 1.5%, and pyrocatechol 3.5%, based on the lignin charge.

References

1. A. A. Sokolova, E. V. Nazar'eva, L. A. Semakova. ZhPKh, vol. XXXIV, no. 9, 1961.

2. L. A. Pershina, V. N. Kuksina, V. P. Vasil'eva. Izv. TPI, vol. 136, 36, 1965.

3. Chromatography on Paper. Edited by I. M. Hais and K. Macek, IL, Moscow, 1962.

4. A. A. Akhrem. Thin-Layer Chromatography. Nauka Publishing House, 1964.

5. I. Pearl, D. Beyer. J. Am. Chem. Soc. 76, 6106, 1954.