PROCEEDINGS

of the Tomsk Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov

Vol. 196
1969

PROPERTIES OF LIGNIN ISOLATED FROM PINE WOOD BY MEANS OF ULTRASONIC VIBRATIONS

B. A. Yankovskii, Yu. S. Pilipchuk, L. A. Pershina

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

For the isolation of lignin from wood we used ultrasound [1]. In the present work the properties were studied of lignin isolated with the aid of ultrasound from the wood of Scots pine (Pinus silvestris) of 108 years of age.

Pine sawdust, preliminarily dried for 6 days at a temperature of 35°C in a vacuum drying cabinet, was ground in an M-10 vibratory mill for 30 min. The specific surface area of the sawdust was determined with a PSKh-2 instrument and amounted to \(12 \cdot 10^3\ \text{cm}^2/\text{g}\).

To remove resins, waxes, and other substances, the sawdust was extracted first with diethyl ether for 16 hours, and then with an alcohol-benzene mixture (1:1) for 48 hours in a Soxhlet apparatus.

The sawdust was dried in a vacuum drying cabinet at 35°C to 1–2% moisture content and analyzed for lignin content by the Komarov method [2], for methoxyl groups by the Zeisel method as modified by Fiebek and Schwappach [3], and for ash.

Analysis of pine sawdust:

Moisture — 1–2%.
Ash content — 0.82%.
Lignin content — 27.0%.
\( \mathrm{OCH}_3 \) content — 4.62%.

The source of electrical oscillations of ultrasonic frequency was a generator of the UZG-2.5A type. The principal operating parameters of the generator were:

anode current — 1.1 A,
grid current — 220 mA,
output voltage — 380 V,
magnetizing current — 28 A.

As the source of ultrasonic vibrations, a magnetostrictive transducer of the PMS-6 type was used, with an exponentially cylindrical concentrator soldered to it.

The initial diameter of the concentrator was 90 mm and was equal to the diagonal of the magnetostrictive stack. The transformation coefficient was 2.25; the total length of the concentrator was 242 mm.

Figure 1 shows the arrangement of the device for treating wood sawdust with ultrasound. The magnetostrictive stack and vessel 1, and the irradiation vessel 3, were cooled with running water.

Lignin isolated by means of ultrasonic vibrations differs basically in its properties from Björkman lignin.

Experimental Part

Isolation of lignin:
Twenty g of extractive-free and dried sawdust were placed in a vessel for ultrasonic irradiation (3), covered with 300 ml of aqueous acetone (15% water), and treated with ultrasound for 8–10 min. The temperature of the suspension did not exceed 40°C.

The sawdust treated with ultrasound was separated on a Büchner funnel with a paper filter and washed with a small amount of acetone. The acetone solution of lignin was evaporated to dryness at a temperature of 30°C and a residual pressure of 10 mm Hg. The brown precipitate formed was dissolved in a small amount of purified dioxane, and the resulting solution was centrifuged. The solution of lignin in dioxane was poured dropwise into dry diethyl ether with constant stirring. The white precipitate that separated was washed several times with fresh ether and dried. Further purification was carried out by the Björkman method [4].

Fig. 1. Diagram of the ultrasonic emitter: 1—magnetostrictive package with winding, 2—exponential-cylindrical concentrator, 3—irradiation vessel, 4—porous-rubber ring, 5—porous-rubber gasket, 6—housing.

One g of lignin was dissolved in 25 ml of 90% acetic acid and poured into a tenfold amount of water. The precipitate was washed free of traces of acid and dried. The lignin was then dissolved in an ethanol–dichloroethane mixture (1:1 by volume), 25 ml per 1 g of lignin. The solution was precipitated in ether; the lignin was separated in a centrifuge, washed with ether and n-hexane, and dried over phosphorus anhydride in a vacuum desiccator.

Chemical Composition

The carbohydrate content in the lignin, determined by the Bertrand method [3] after hydrolysis with 5% sulfuric acid, was 0.4–0.5% calculated as glucose.

The elemental composition of the isolated lignin was C—62.55%, H—6.35%, O—31.1%. The amount of methoxyl groups was 13.9–14.3%. Total hydroxyl groups were determined by acetylation according to Verley [2] and amounted to 10.4%. The content of phenolic groups in lignin was determined

by the method of high-frequency conductometry [6]. This method makes it possible simultaneously to determine also the presence of carboxyl groups, which was of particular interest to us.

Into the titration beaker were placed 60–70 mg of lignin; 2 ml of 0.2 N caustic soda and 90 ml of 60% aqueous acetone were added. The solution was kept for 20 min without access of air. Titration was carried out with 0.1 N hydrochloric acid. As the high-frequency instrument an oscillotitrator of the “Pungor” system, type OK-302, was used. Figure 2 gives titration curves for lignin in water—1, in 50% aqueous acetone—2, and vanillin in water—3. As the titration curve of lignin in acetone shows, the lignin preparation isolated by us does not contain carboxyl groups. The content of phenolic hydroxyls was 2.78%; the discrepancy between the analytical results did not exceed 0.1%.

Fig. 2. Titration curves: 1—lignin in water, 2—lignin in 50% acetone, 3—vanillin in water.

Spectra

The UV spectrum of ultrasonic lignin (Fig. 3, curve 1) was recorded on an SF-4 spectrophotometer in dioxane solution. Measurements were carried out in the wavelength region 200–400 mμ at a lignin concentration in the solution of 0.828% and a layer thickness of 0.0109 cm.

Fig. 3. Ultraviolet spectrum of ultrasonic lignin.

The molar absorption coefficient was calculated per phenylpropane unit with molecular weight 186.

The absorption in the region of 230 and 280 mμ in the UV spectra of lignin is customarily attributed to the guaiacyl grouping in systems I and II, with the quantitative ratio in favor of II.

![structural formulas labeled I and II]

The origin of the K band at 310–330 mμ is attributed to conjugation of the guaiacyl grouping with the carboxyl group, of type III or IV [7].

![structural formulas labeled III and IV]

The ultraviolet spectrum of lignin isolated with the aid of ultrasound, in the region 200–400 mμ, coincides completely with the UV spectrum of Björkman lignin (curve 2).

Recently, studies of infrared absorption spectra have acquired great importance in lignin chemistry. It has been established that the IR spectra of lignins obtained from one species of wood, but by different routes, are basically identical and differ only in the intensity of the absorption bands of the functional groups. Fig. 4 shows the IR spectrum of ultrasonic lignin (curve 2) and of Björkman lignin (curve 1). The spectra were recorded on an IKS-14 spectrophotometer with LiF and NaCl prisms. Spectrography was carried out in potassium bromide pellets; the lignin concentration was 5% [8].

![graph: infrared absorption spectra]

Fig. 4. Infrared absorption spectrum of ultrasonic lignin.

The IR spectrum of ultrasonic lignin is identical with the spectrum of Björkman lignin. However, the absorption intensity in the region 1710–1730 cm\(^{-1}\) for ultrasonic lignin (curve 1) is higher than for Björkman lignin (curve 2, Fig. 5). Absorption in the region 1720 cm\(^{-1}\) is usually assigned to the stretching vibrations of the carboxyl group and of the isolated ketone group.

The presence of carboxyl groups in Björkman lignin was established with the aid of infrared spectra [9]; however, they cannot be detected by chemical methods.

It was found by a number of researchers that lignin preparations possess paramagnetic properties [10—13], and the concentration of free radicals in lignin preparations depends on the method of preparation.

According to the views of [14, 15, 16], during the mechanical destruction of polymers, as a result of the rupture of chemical bonds, the formation of free radicals is possible.

Elpiner [16] believes that similar phenomena also occur in the field of ultrasonic waves. We were interested in how ultrasonic treatment of wood flour would affect the content of free radicals in the lignin preparation.

The concentration of free radicals was measured by the EPR method on an RE 1301 instrument. A lignin sample of about 40 mg was placed in a glass ampoule 5 mm in diameter and 150 mm long. All measurements were carried out in ampoules of identical geometric shape in the presence of air.

Fig. 5. Infrared absorption spectra of ultrasonic lignin and Björkman lignin.

As comparison standards, solid preparations containing $\alpha,\alpha$-diphenyl-$\beta$-picrylhydrazyl were used. The table gives the concentration of free radicals in various lignin preparations.

Table

Conclusions

1. From Pinus silvestris wood, lignin was isolated by means of ultrasonic vibrations.

2. In elemental composition, the quantity of functional groups, and the UV spectrum, the isolated lignin is quite close to Björkman lignin.

3. A difference was found between the IR spectra of ultrasonic lignin and Björkman lignin in the region 1710—1730 cm$^{-1}$.

LITERATURE

1. L. A. Pershina; B. A. Yankovskii. Investigations of lignin isolated by means of ultrasound. Abstracts of reports and communications of the IX Mendeleev Congress, No. 4, 1965.

1. A. V. Obolenskaya et al. Practical Work in the Chemistry of Wood and Cellulose. 58, 85, 137. Moscow, 1965.

2. F. Viebok, A. Schwappach. Ber., 63, 2818, 1930.

3. A. Björkman, Svensk papperstidn., 59, 477, 1956; 60, 158, 243, 285, 329, 1957.

4. T. M. Vasil’eva, G. P. Grigor’ev, K. P. Mishchenko. ZhPKh, issue 12, 2757—2761, 1965.

5. V. M. Reznikov, G. V. Sviderik, V. L. Levdikova, and G. D. Ponurov. ZhPKh, issue 6, 1314—1322, 1963.

6. V. M. Reznikov, Yu. S. Pilipchuk, L. S. Solov’ev. Proceedings of the First Scientific Conference of the Problem Laboratory. Sib. TI, 36, 1961.

7. K. N. Ekmann, I. I. Linberg. Paperi ja puu, 42, 1, 21, 1960.

8. G. Steeling, T. Reid, G. Tollin. J. Am. Chem. Soc., 85, 4048, 1963.

9. R. W. Pex. Nature, 188, 1185, 1960.

10. T. N. Kleinert, I. R. Morton. Nature, 196, 334, 1962.

11. M. I. Chudakov, V. I. Mal’tsev, V. E. Bronovitskii. Hydrolysis and Wood-Chemical Industry, issue 4, 13, 1966.

12. V. A. Kargin, G. M. Slonimskii. Doklady Akademii Nauk SSSR, 105, 4, 751—754, 1955.

13. V. A. Kargin, G. I. Sokolova. Problems of Physical Chemistry, 1, 18—21, 1958.

14. N. K. Baramboim, Proceedings of MTILP, collection 6, 45—50, 1956.

15. I. E. El’piner. Acoustical Journal, 5, 133—145, 1959.