STUDY OF CHEMICAL TRANSFORMATIONS OF LIGNIN AND ITS USE

L. A. PERSHINA

(Presented by the scientific-methodological seminar of the CTF)

Over the last decade, both abroad and in the USSR, the industry for processing plant raw material—the pulp-and-paper and hydrolysis industries—has developed with extraordinary intensity. The further development of these branches of industry very sharply raises the question of using wastes—various kinds of technical lignins—since in the chemical processing of raw material only about 60% of the organic matter is used rationally. At present, in the pulp-and-paper and hydrolysis industries of the USSR, up to 4 million tons of lignin accumulate annually [1]. A large part of the lignin has no rational use and is discharged into bodies of water. Thus, the problem of using enormous quantities of lignin is not only the rational utilization of plant resources, but also the preservation of major natural bodies of water from pollution. In addition, natural lignin, the most widespread natural polymer after cellulose and one that is constantly renewed, is of independent interest to organic chemists, like other substances whose structure and properties have not yet been fully elucidated.

In all the specialized literature concerning the chemistry of lignin, its poor degree of study is noted in comparison with a whole series of other, seemingly no less complex substances, such as, for example, proteins, hormones, enzymes, etc. This is explained by the complexity of its nature, the multivariant character of its structural units and the bonds between them, as well as by the extreme lability of this natural polymer, which irreversibly changes its properties as a result of chemical or thermal action.

The problem of using lignin, obviously, can be solved only by chemical means. A study of the properties, structure, and chemical transformations of isolated and technical lignins will make it possible in the future to expand the fields of application of lignin and to turn it into a valuable raw material for the needs of our country.

At the Department of Organic Chemistry of the Tomsk Polytechnic Institute, since 1958, studies have been carried out on the chemical transformations of isolated and technical lignins with the aim of using reaction products as flotation reagents, substitutes for pulverbakelite, fillers, and also as preparations possessing pesticidal, bactericidal, and ion-exchange properties.

The phenylpropane structural elements of lignin contain sufficiently reactive functional groups. The reactive capa-

the reactivity of the functional groups of lignin is determined primarily by the electronic nature and position of the substituents, as well as by steric hindrance and the morphology of lignin. We have theoretically demonstrated and experimentally proved the possibility of interaction of the hydroxyl and carbonyl groups of lignin with many organic compounds [2, 3, 4]. In the present review we shall dwell on the results of studies of chemical transformations of lignins that are of practical importance.

Phosphorylation of Lignins

Modification of natural and synthetic polymers by means of organophosphorus compounds is attracting increasing attention from chemists, since the result is materials possessing valuable properties. We have studied the interaction of lignin, its model substances, and products of chemical transformations with acid chlorides and ester chlorides of phosphoric and thiophosphoric acids, and with partial and complete esters of phosphorous acid.

Interaction of lignins with acid chlorides and ester chlorides of phosphoric and thiophosphoric acids.

Ester chlorides of phosphoric and thiophosphoric acids readily interact with many nucleophilic reagents. The reaction rate depends both on the degree of nucleophilicity of the reagent and on the structure of the phosphorylating substrate. As a hydroxyl-containing high-molecular-weight compound, lignin should interact with ester chlorides of thiophosphoric acids.

The complexity of the structure of the lignin polymolecule, with its great diversity of structural elements, compels one to resort to indirect approaches when studying the direction of the reaction, the conditions, and the reactivity of functional groups; that is, to study reactions on model compounds and then transfer the results to lignin.

On low-molecular-weight compounds modeling individual structural elements of lignin and containing phenolic and aliphatic hydroxyl groups, we have shown the regularities of the reaction with O,O-dialkyl chlorothiophosphates. To measure the reaction rate, graduate student A. I. Galochkina constructed an apparatus that permits automatic recording of changes in the concentration of the reacting substances.

Fig. 1. Ionization curves of substituted phenols in methyl ethyl ketone in the presence of potassium carbonate, at 50°C

The principle of operation of the apparatus is based on the use of the high-frequency conductometry method. Depending on the change in the concentration of the solution contained in the reaction flask, and consequently the change in high-frequency electrical conductivity, the voltage drop across the electrolytic cell changes, which is transmitted to

potentiometer, which, by recording the change in the voltage drop in the reaction flask, automatically records the course of the process as a function of time. Figure 1 presents curves characterizing the ionization process of phenols of different acidity, occurring in methyl ethyl ketone at 50°C in the presence of anhydrous potassium carbonate. When phenol, \(K_2CO_3\), and \(O,O\)-dimethyl chlorothiophosphate are added to the reaction flask containing the solvent, interaction occurs according to the scheme

\[ \mathrm{C_6H_5OH + (CH_3O)_2\overset{S}{\underset{\|}{P}}-Cl \to C_6H_5O-\overset{S}{\underset{\|}{P}}-(OCH_3)_2 + Cl'} \]

The instrument, recording this change in concentration, traces a kinetic curve. Figure 2 shows the curves characterizing the rate of interaction of \(p\)-nitrophenol with \(O,O\)-dimethyl chlorothiophosphate.

Fig. 2. Kinetic curves of the interaction of \(p\)-nitrophenol with \(O,O\)-dimethyl chlorothiophosphate in the presence of potassium carbonate: 1 — in acetonitrile, 35°C; 2 — in acetonitrile, 50°C; 3 — in methyl ethyl ketone, 70°C

The segment of the curve \(AB\) corresponds to the ionization process of \(p\)-nitrophenol; \(BD\), to the stage of interaction of the phenoxide ion with \(O,O\)-dimethyl chlorothiophosphate. For various phenols, the kinetic curves of the rate of interaction with \(O,O\)-dimethyl chlorothiophosphate, second stage, are presented in Fig. 3. Conditions: temperature 50°C, time 2 h, solvent—methyl ethyl ketone; for 0.01 mole of phenol, 0.01 mole of potassium carbonate and 0.01 mole of \(O,O\)-dimethyl chlorothiophosphate were taken. The rate constant, calculated by the graphical method, is equal to:

The correlation between the rate constant and the pKa of \(m\)- and \(p\)-substituted phenols has a linear dependence (Fig. 3).

The reactivity of phenols in the reaction with O,O-dimethyl chlorothiophosphate is determined both by the degree of ionization of the phenol and by the nucleophilicity of the substituted phenoxide ions formed. The rate of interaction of substituted phenoxide ions with O,O-dimethyl chlorothiophosphate decreases in the series
\(m\text{-}\mathrm{CH}_3\text{-}\mathrm{C}_6\mathrm{H}_4\mathrm{O}^{-} > \mathrm{C}_6\mathrm{H}_5\mathrm{O}^{-} > n\text{-}\mathrm{Cl}\text{-}\mathrm{C}_6\mathrm{H}_4\mathrm{O}^{-} > n\text{-}\mathrm{I}\text{-}\mathrm{C}_6\mathrm{H}_4\mathrm{O}^{-} > m\text{-}\mathrm{NO}_2\text{-}\mathrm{C}_6\mathrm{H}_4\mathrm{O}^{-} > n\text{-}\mathrm{NO}_2\text{-}\mathrm{C}_6\mathrm{H}_4\mathrm{O}^{-}\).

Since, under these conditions, the degree of ionization of phenols with high pKa values is small, the rate of interaction of phenols with O,O-dimethyl chlorothiophosphate increases in the series: \(m\)-cresol \(<\) phenol \(<\) \(n\)-chlorophenol \(<\) \(n\)-iodophenol \(<\) \(m\)-nitrophenol \(<\) \(n\)-nitrophenol.

Fig. 3. Kinetic curves for the interaction of substituted phenols with O,O-dimethyl chlorothiophosphate

The reactivity of compounds of the guaiacyl series decreases in the series: vanillin \(>\) isoeugenol \(>\) guaiacol \(>\) eugenol. On the basis of kinetic studies of the reaction of lignin model compounds with O,O-dimethyl chlorothiophosphate, we conclude that the phenolic hydroxyl groups of lignin with pKa above 9 do not interact under the above-described conditions with O,O-dialkyl chlorothiophosphates, and the reaction must be carried out in the presence of alkali or via the preliminarily prepared Na salt. The phenolic hydroxyl groups of lignin model compounds do not interact with O,O-dimethyl chlorothiophosphate in the presence of pyridine: the potentiometer records no change in voltage in the reaction flask. This has also been demonstrated by synthetic experiments: vanillin, guaiacol, isoeugenol, and eugenol were isolated unchanged from the reaction mixture in 100% yield. Thus, in phos-

Fig. 4. Dependence of the rate constant of the reaction of substituted phenols with O,O-dimethyl chlorothiophosphate on the degree of acidity of the phenols

In the phosphorylation of phenolic hydroxyl groups of lignin model compounds, the role of bases is reduced to activation of the phenol, i.e., to the formation of phenoxy anions; therefore, strong inorganic bases must be used for this purpose. In the phosphorylation of alcohols, the role of the base is reduced to activation of the phosphorylating reagent and to prevention of the formation of alkyl halides. In this case inorganic bases can perform only the latter role, but they cannot activate the alcohol. We have shown that aliphatic hydroxyl groups of lignin model compounds in anhydrous solvents, in the presence of potassium carbonate, do not react with O,O-dimethyl chlorothiophosphate. As might be expected, a positive result in this case is given by the nucleophilic catalysis of tertiary amines. The catalytic properties of amines are determined by their ability to react with the acylating agent and depend above all on stereochemical factors [5]. Toy [6] indicates the formation of a reactive complex of O,O-diethyl chlorothiophosphate with tertiary amines, although he was unable to isolate it. In the presence of pyridine in anhydrous organic solvents we succeeded in obtaining thiophosphates at the aliphatic hydroxyl groups of phenol alcohols and of alcohols of the guaiacyl and veratryl series.

The results of studying the conditions for phosphorylation of the phenolic and alcoholic hydroxyl groups of low-molecular-weight lignin models were used by us in the phosphorylation of lignins. The direction of the reaction of O,O-dialkyl chlorothiophosphates with dioxane lignin, copper-ammonia lignin, ultrasonic lignin, and lignin acids was investigated. The experimental data are given in Tables 1 and 2. Phosphorylation was carried out in an anhydrous methyl ethyl ketone medium, in the presence of pyridine or potassium carbonate, for 4 hours at the boiling temperature of the solvent; the amount of acid chloride was calculated from the content of total hydroxyl groups. In lignin thiophosphates the amounts of phosphorus and phenolic hydroxyl groups were determined. The amount of reacted hydroxyl groups was calculated by the formula

\[ \%\,OH=\frac{P_{\mathrm{det.}}}{a_{P(S)}}\cdot \left(\frac{1700}{OH_{\mathrm{det.}}}+M+1\right), \]

where \(a\) is the atomic weight of phosphorus or sulfur; \(OH_{\mathrm{det.}}\) is the content of the total amount of hydroxyl groups in the initial lignin preparation (%); \(M\) is the molecular weight of the radical that has been added. The formula was checked on model compounds. IR spectra of the initial and phosphorylated lignins were recorded.

On the basis of the data given in Tables 1 and 2, as well as the results of comparison of the IR spectra of the initial and phosphorylated lignins, it follows that the lignins studied enter into chemical interaction with O,O-dialkyl chlorothiophosphates, forming mixed esters of thiophosphoric acid:

\[ \mathrm{Lignin{-}OH} + \mathrm{Cl{-}P(=S)(OR^1)(OR^2)} \longrightarrow \mathrm{lignin{-}O{-}P(=S)(OR^1)(OR^2)} +\mathrm{HCl}. \]

No more than 50% of all hydroxyl groups are phosphorylated. In the IR spectrum of lignin thiophosphates the integral intensity of the band of stretching vibrations of hydroxyl groups in the region \(3400\text{–}3500\ \mathrm{cm}^{-1}\) decreased sharply; absorption bands of the \(P=S\) bond appeared at \(612\ \mathrm{cm}^{-1}\), \(580\ \mathrm{cm}^{-1}\). The interaction of lignins with O,O-dialkyl chlorothiophosphates in the presence of pyridine proceeds at aliphatic hydrox-

Table 1

Phosphorylation of dioxane lignin with O-methyl-O-ethyl chlorothiophosphate

Note: * the starting dioxane lignin contains 1,98% OH\(_{\text{phen.}}\).
** lignin taken in the form of the Na salt.

Table 2

Phosphorylation of lignins with O,O-dimethyl chlorothiophosphate

Continuation of Table 2

Note. * The starting lignins contain OH\(_{\text{phen.}}\) (%): ultrasonic, 1,8; copper-ammonia, 1,3; lignin acids, 4,2.

strong groups, since the amount of phenolic hydroxyls remains almost unchanged. In the IR spectra of phosphorylated lignins, absorption bands were retained in the region of deformation vibrations of phenolic hydroxyl groups at \(1375\ \mathrm{cm}^{-1}\) and in the region of stretching vibrations at \(1224\ \mathrm{cm}^{-1}\); absorption bands appeared that arose from vibrations of the \(P—O—C_{\mathrm{arph}}\) bond at \(1050\ \mathrm{cm}^{-1}\); bands in the region of \(1072\ \mathrm{cm}^{-1}\) and \(1033\ \mathrm{cm}^{-1}\), associated with \(C—O\) vibrations in secondary and primary alcoholic groups, disappeared. More than half of the phenolic hydroxyl groups of lignins interact with \(O,O\)-dialkyl chlorothiophosphates if the reaction is carried out in the presence of potassium carbonate. When the reaction is carried out in the presence of potassium carbonate and equimolecular amounts of pyridine, phosphorylation proceeds both at the alcoholic and at the phenolic hydroxyl groups, but only 40–50% of all hydroxyl groups react; \(\mathrm{OH}_{\mathrm{phen}}\) with pK above 9 do not participate in the reaction. But if the initial lignin is taken in the form of a Na salt prepared from lignin and an excess of sodium methylate in absolute methyl alcohol, then all phenolic hydroxyls react with \(O,O\)-dialkyl chlorothiophosphates.

Organophosphorus compounds are widely used for the destruction of plant pests, especially aliphatic-aromatic esters of thiophosphoric acid. But they have rather substantial disadvantages: high toxicity for humans and warm-blooded animals, as well as the high cost and scarcity of the starting raw material—phenol.

Taking into account the foregoing, and also the fact that synthesized \(O,O\)-dialkylthiophosphates of lignins exhibit insecticidal action, we used technical hydrolysis lignin and its derivatives instead of phenol in the synthesis of insecticides.

From technical hydrolysis lignin, chlorolignin, nitrolignin, and chloronitrolignin, phosphorus-containing lignins were obtained by the action of phosphorus trichloroxide in carbon tetrachloride. Of these, only the preparation obtained by phosphorylation of nitrolignin exhibited weak insecticidal action. Therefore, from nitrolignin a phosphorus-containing lignin of the phosphacol type [7] was obtained, which exhibited strong insecticidal action but proved to be extremely toxic to warm-blooded animals. As is known, mixed esters of thiophosphoric acid are less toxic than the corresponding esters of phosphoric acid. Therefore, our further investigations proceeded along the route of obtaining thiophosphoric-acid derivatives from technical hydrolysis lignin by the action of thiophosphorus trichloride and \(O,O\)-dialkyl chlorothiophosphates. \(O,O\)-Dialkylthiophosphates were obtained from hydrolysis sulfuric-acid and hydrochloric-acid lignins, chlorolignin, nitrolignin, cyanlignin, chloronitrolignin, and from alkali-activated hydrolysis lignin [8, 9, 10]. In tests for insecticidal activity, the product of the interaction of nitrolignin and \(O,O\)-diethyl chlorothiophosphate showed a strong effect. The optimal conditions for obtaining this preparation, which we named “lignothiophos,” were investigated [11]. The strongest insecticidal action is possessed by the preparation containing (%): 47.8 C; 5.7 H; 5.8 S; 5.2 P; 2.13 N. The insecticidal action of lignothiophos was studied by Assoc. Prof. A. V. Kovalenok [12, 13].

An obstacle to the production of an experimental industrial batch of lignothiophos is that nitrolignin of wood species is not produced as a commercial product at any enterprise of the hydrolysis industry. Since 1962, production has been established of cotton-husk nitrolignin, which can serve as a raw material for obtaining lignothiophos, since it contains hydroxyl groups in the benzene ring. We investigated the optimal conditions for obtaining the preparation from cotton-husk nitrolignin and \(O\)-methyl-\(O\)-ethyl chlorothiophosphate—“methyllignothio-”

phos.” The consumption rates of the starting products per 1 kg of nitrolignin are: 0.5 kg of dry alkali, 120 ml of pyridine, and 1.25 kg of O-methyl-O-ethyl chlorothiophosphate; the reaction temperature is 60°C, the time is 1 hour, and the pH of the reaction mass is 7.5–8. Methyllignothiophos can be used to control pests of agricultural crops, as well as ticks. During 1964, on the Krasnovodsk Plateau, a twofold treatment of camels with a 2% dust of methyllignothiophos was carried out. According to the conclusion of E. B. Kerbabaev, methyllignothiophos is a strong acaricide, possessing acute and prolonged action [14]. The toxic dose of the preparation for mice is 130 mg per kg of body weight. A basic technological scheme for obtaining methyllignothiophos has been developed.

Oximes have been obtained from hydrolysis lignin, as well as from its chloro- and nitro derivatives; by their interaction with O,O-diethyl chlorothiophosphate, O-iminyl thiophosphates were synthesized [15]. The O-iminyl thiophosphates of hydrolysis lignin and its derivatives exhibited a stronger insecticidal action than the corresponding thiophosphates.

Interaction of Lignins with Esters of Phosphorous Acid

Esters of phosphorous acid, possessing high reactivity and simplicity of synthesis, are widely used for obtaining various organophosphorus compounds. The introduction of phosphorus into the lignin polymer molecule is of great interest from the standpoint of obtaining new fire-resistant fillers, ion-exchange materials, and bactericidal preparations.

Proceeding from the presence in lignin of reactive hydroxyl and carbonyl groups, the following directions of the reaction of lignin with dimethyl phosphite should be anticipated:

a) transesterification of dimethyl phosphite by lignin with replacement of one or two methoxyl groups;

b) addition of dimethyl phosphite to carbonyl groups;

c) formation of esters of phosphorous acid with quinone groups.

We have investigated the interaction of dimethyl phosphite with ultrasonic lignin, copper–ammonia, dioxane, and lignin acids, as well as with 23 model compounds containing carbonyl groups and phenolic and aliphatic hydroxyl groups [16].

Carbonyl groups were not detected in the phosphorylated lignins. Consequently, dimethyl phosphite interacts with the carbonyl groups of lignin with the formation of α-oxyphosphine structures. This is confirmed by comparison of the IR spectra of the initial and phosphorylated lignins: the band at 1667 cm\(^{-1}\), convincingly interpreted by many authors as an absorption band of carbonyl groups located in the α-position, completely disappears in phosphorylated lignins. The total quantity of hydroxyl groups in the phosphorylated preparations decreased sharply, but the content of phenolic groups changed hardly at all. The intensity of the absorption bands in the region of stretching vibrations of hydroxyl groups decreased strongly, but absorption bands corresponding to vibrations of phenolic hydroxyl groups were retained. Thus, only aliphatic hydroxyl groups enter into the transesterification reaction of dimethyl phosphite, which is confirmed in experiments with model compounds.

From the results of studying the interaction of isolated lignins and model compounds with dimethyl phosphite it follows that phosphorus-containing fragments enter the lignin polymer molecule.

\[ \begin{array}{c} \mathrm{O}\\[-2mm] \|\\[-1mm] -\mathrm{O}-\mathrm{P}-\mathrm{H}\\[-1mm] |\\[-1mm] \mathrm{O}\\[-1mm] | \end{array} \qquad \text{and} \qquad \begin{array}{c} \mathrm{OH}\ \mathrm{O}\\[-2mm] |\ \ \|\\[-1mm] -\mathrm{C}-\mathrm{P}-\mathrm{OCH}_{3},\\[-1mm] |\ \ |\\[-1mm] \ \ \mathrm{OCH}_{3} \end{array} \]

which may be responsible for pesticidal and ion-exchange properties, and may also impart noncombustibility and nonflammability, especially compounds with a C–P bond. From lignins containing a phosphite group it is possible to obtain phosphates, amidophosphates, and to replace active hydrogen with any grouping of atoms. We have shown the possibility of obtaining, from technical hydrolysis lignin and dimethyl phosphite, preparations with sorption properties. The interaction of hydrolysis lignin with dimethyl phosphite proceeds through transesterification of the latter by the hydroxyl groups of lignin. In the IR spectra of phosphorylated hydrolysis lignin, the integrated intensity in the region of stretching vibrations of hydroxyl groups decreased greatly, and a continuous broad band appeared in the region \(1266—1150\ \mathrm{cm}^{-1}\), corresponding to the \(\mathrm{P}=\mathrm{O}\) bond.

The phosphite of hydrolysis lignin was obtained both in a medium of inert solvents [17] and in an excess of dimethyl phosphite in the presence of catalysts (metallic sodium, sodium ethylate). When the reaction is carried out in an o-xylene medium, up to 10% phosphorus is introduced into the lignin macromolecule. The dependence of the phosphorus content in the phosphorylation products on temperature and time was studied. The maximum amount of phosphorus—12.6%—is introduced into the lignin macromolecule at \(170^\circ\mathrm{C}\); increasing the temperature leads to destruction, since the amount of phosphorus in lignin decreases. The reaction proceeds over 4 hours; a further increase in the duration of the reaction is not reflected in the phosphorus content.

The sorption capacity of the phosphite and amidophosphate of hydrolysis lignin was studied under the direction of Assoc. Prof. S. A. Babenko with respect to certain metals, as well as to oils and phenols in waste waters of coke-chemical plants. Preliminary studies on wastewater purification gave the following results: 2.5 g of lignin phosphite absorbs 61 ml of oils and 18.13 mg of phenols. Studies are being carried out on the regeneration of lignin phosphite, and the technology for its production is also being developed.

OXIDATION OF HYDROLYSIS LIGNIN

Hydrolysis lignin is extremely heterogeneous in composition, which makes it difficult to use as a chemical raw material. It is expedient, by means of profound chemical changes, to obtain from lignin phenols, fatty and aromatic acids, and carbonyl compounds.

We have chosen oxidation processes for this purpose. Oxidation in the presence of catalysts 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 deep oxidation processes.

Oxidation of hydrolysis lignin of wood species and cotton husks was carried out in the presence of oxides and hydrates of oxides of metals of variable valence. Sulfuric-acid lignin was oxidized in an alkaline medium, in the presence of oxides and hydrates of oxides of metals of variable valence (0.0007 M), at \(t = 180^\circ\), for 4 hours [18].

Hydrates of oxides and oxides of metals with lower valence increase the yield of lignin acids. In this case, the process of alkaline acti-

with slight cleavage of methoxyl groups in an amount of 1.5–2%, and an increase in carboxyl groups by 3–4%. These additives reduce the yield of ether-soluble substances.

Metal oxides of higher valence are sources of oxygen. The oxidation process proceeds intensively and deeply. The yield of lignin acids is small, and they contain a large number of carboxyl groups. The amount of ether-soluble substances increases, especially in the presence of vanadium pentoxide (13%). A series of experiments was carried out with different amounts of vanadium pentoxide [19]. The reaction products were separated into alkali-, ether-, and water-soluble fractions. With an increase in the amount of vanadium pentoxide, the yield of alkali-soluble lignin acids decreases. The highest yield of ether-soluble substances (37%) is obtained with 5% vanadium pentoxide. The influence of temperature on the yield of ether-soluble substances during oxidation in the presence of 5% vanadium pentoxide was also studied. The highest yield of ether-soluble substances is obtained at a temperature of 230°C.

The ether-soluble substances are low-molecular-weight compounds, which we separated into aldehydes, phenols, and acids. During the oxidation of hydrolysis lignin in the presence of 5% vanadium pentoxide by weight of lignin, 30% NaOH, a hydromodulus of 1 : 10, at \(t = 230^\circ\), and a time of 4 hours, 37% ether-soluble substances were obtained, of which 53% consisted of phenols. Guaiacol was isolated from the phenolic fraction in an amount of 8.9% of the lignin taken. By paper chromatography, pyrocatechin was detected in the phenolic fraction; vanillic and protocatechuic acids in the acid fraction; and vanillin and acetovanillone in the carbonyl fraction.

The conditions for oxidation of hydrolysis lignin from cottonseed hulls and some of its derivatives in the presence of vanadium pentoxide were also studied, with the aim of obtaining low-molecular-weight ether-soluble substances.

Oxidation of hydrolysis lignin from cottonseed hulls was carried out in the presence of various amounts of vanadium pentoxide and alkali and at different temperatures. The yield of ether-soluble substances does not depend on the amount of alkali or the amount of vanadium pentoxide, but depends strongly on temperature. The maximum yield of ether-soluble substances (34.3%) is obtained under the following optimum conditions: 30% NaOH based on the lignin charge, 5% vanadium pentoxide, hydromodulus 1 : 10; temperature—260°, time—4 hours.

Methylated with dimethyl sulfate, hydrolysis lignin from cottonseed hulls (14.9% \(OCH_3\)) is oxidized under these conditions to give 50.2% ether-soluble substances. Oxidation of nitrolignin (3.92% nitrogen) gives 32% ether-soluble substances; hydrolysis lignin from coniferous species—22%; and hydrolysis lignin from cottonseed hulls activated by brief heating to 205° with alkali—31%.

Thus, in order to preserve aromatic units during the oxidation of lignin and, consequently, to increase the yield of ether-soluble substances, it is necessary to protect the “weak unit,” which is most readily oxidized, i.e., the hydroxyl groups.

Individual substances were isolated from the ether-soluble oxidation products. During oxidation of hydrolysis lignin from cottonseed hulls under optimum conditions, 12% of a mixture of guaiacol and pyrocatechin and 17% of vanillic and protocatechuic acids are obtained, based on the initial lignin charge.

The oxidation products of hydrolysis lignin may serve as raw material for obtaining pesticides. From guaiacol, pyrocatechin, and the products of their nitration, esters of O-methyl-O-ethylthiophosphoric acid were obtained. The preparations obtained were studied for insecticidal activity. Strong-

esters with a nitro group in the benzene ring possess insecticidal properties.

Electrochlorination of Hydrolysis Lignin

Hydrolysis lignin is chemically rather inactive. By using the process of chlorinating lignin, it is possible, on the one hand, to increase in it the number of reactive functional groups and, on the other hand, to make it considerably more active owing to the presence of mobile chlorine atoms.

The developed methods for chlorinating lignin involve the use of chlorine, which is inconvenient to handle, and the preparation of large volumes of chlorine water.

A. N. Novikov [20] demonstrated the possibility of electrochemical chlorination of lignin. At a low current density on an anode made of graphite or platinum, technical sulfuric-acid and hydrochloric-acid lignins are chlorinated with 5–15% hydrochloric acid or sodium chloride. In two hours, up to 25% chlorine can be introduced into lignin.

In electrochemical chlorination, the amount of chlorine introduced depends almost not at all on the concentration of hydrochloric acid, other conditions being equal. The amount of carboxyl groups increases sharply to 15%; the number of methoxyl groups decreases to 1.9%; the amount of hydroxyl groups remains practically unchanged.

Electrochemical chlorination of lignin has a number of advantages: the surrounding environment is improved; no special exhaust or purification system is required; for chlorination one can use a sodium chloride solution, which is cheaper than using chlorine; the volume of reaction vessels is small.

From chlorinated hydrolysis lignin, A. M. Petrova [21] obtained cyanide derivatives and lignin acids.

Use of Hydrolysis Lignin for Obtaining Pulverbakelite-Type Resins

Pulverbakelite is prepared on the basis of phenol-formaldehyde resins, whose disadvantages include the use of comparatively expensive and scarce phenol, as well as the evolution of a considerable amount of harmful gaseous products when hot metal is poured into molds. We obtained lignin–phenol–formaldehyde resins from sulfuric-acid hydrolysis lignin from the Krasnoyarsk plant and from hydrolysis lignin activated with caustic soda at \(t = 200\text{–}210^\circ\mathrm{C}\) for 6 hours [21, 22, 23].

Tests showed that the strength of sand specimens prepared with a binder made from lignin–phenol–formaldehyde resin, obtained at a phenol-to-lignin ratio of 3:1, is not inferior to specimens based on pulverbakelite.

In the production of pulverbakelite, 55% of the phenol and 80% of the formalin can be replaced by activated hydrolysis lignin; 25–30% of the phenol can be replaced by hydrolysis lignin. The influence of different batches of lignin and of its moisture content on the quality of lignin–phenol–formaldehyde resins was investigated. It was established that the optimum amounts of formalin depend on the batch of lignins. Moisture within the range from 4 to 30% does not impair the quality of the resins. Before use, the lignin must be passed through a sieve with openings 1 cm in diameter. Control of the cooking of lignin–phenol–formaldehyde resin has been proposed. A technological scheme for its production has been developed.

Use of Ultrasound for Purifying Hydrolysis Lignin and Isolating Native Lignin

To study the chemical transformations of hydrolysis lignin, it is necessary first to thoroughly purify the original lignin from impurities, especially those of a carbohydrate nature. Purification was carried out both by chemical methods and with the aid of ultrasound. The initial lignin contained: sulfuric acid—1.4%; readily hydrolyzable substances—1.3%; difficultly hydrolyzable substances—23.9%; lignin—67.3%. After ordinary purification (heating with water for 10 hours, with ether until decoloration for 8–10 hours), hydrolysis lignin was obtained containing 10.5% methoxyl groups and 85.7% lignin.

Technical hydrolysis lignin was purified of carbohydrates and resinous substances under the following conditions: hydromodule 1:15, time—30 min, at an acoustic-oscillation frequency of 21.8 kHz in an ordinary standard ultrasonic bath. After ultrasonic treatment the lignin was filtered off, washed with cold water, dried, and then treated with ether. Purification of technical hydrolysis lignin by ultrasound takes 30 min. The lignin purified with the aid of ultrasound contains 9.2% methoxyl groups and 88.0% lignin.

The numerous known methods for isolating lignin require much time and the performance of a whole series of operations. In most cases, during lignin isolation its partial oxidation and condensation occur, and the resulting preparation is poorly suited for study. To isolate native lignin from pine sawdust, we used ultrasound [25]. When ultrasound acts on wood sawdust, longitudinal splitting of the cellulose fibers into individual fibrils occurs, which leads to destruction of the biostructure of the wood and, apparently, to rupture of the lignin–carbohydrate bond [26].

The proposed method differs from the previously known one in that the wood sawdust is not ground in a vibratory mill, and the lignin is extracted from the wood with dioxane while being irradiated simultaneously with ultrasound.

The use of ultrasound makes it possible to combine the operations of “grinding” and “extraction.” To isolate lignin, mixtures of dioxane with water and acetone with water were used. The yield of the lignin preparation depends on the duration of ultrasonic irradiation: with increasing irradiation time the yield of lignin increases and, after 8 hours, reaches 15% relative to the initial lignin. A further increase in time does not increase the lignin yield. The physical and chemical properties of ultrasonic lignin were investigated [27, 28, 29]. The weight-average molecular weight was determined at the Arkhangelsk Forestry Engineering Institute by Assoc. Prof. V. D. Bogomolov by the method of non-established equilibrium in an ultracentrifuge in pyridine; its value is 1900–2000. The diffusion coefficient is \(1.462 \cdot 10^{-6}\ \text{cm}^2/\text{s}\), determined on a centrifuge.

The ultraviolet adsorption spectrum of lignin was obtained on an SF-4 spectrophotometer. The character of the UV spectrum is identical to the spectrum of Björkman lignin. Three bands were found: one with a clearly expressed maximum at \(281\ \mu\), and two in the form of bends in the curve—\(230\) and \(338\ \mu\). The molar absorption coefficient was calculated per phenylpropane unit. The IR spectrum of ultrasonic lignin is identical with the spectrum of Björkman lignin, but the absorption intensity in the region \(1710\text{–}1730\ \text{cm}^{-1}\) is higher for ultrasonic lignin. The concentration of free radicals, determined by the EPR method on an RE-1301 instrument, is \(1.1 \cdot 10^{16}\) per g.

A detailed study of the properties of ultrasonic lignin gives grounds to believe that the lignin isolated by this method is very close

to Bjorkman lignin and may be recommended for study as one of the suitable monodisperse models of protolignin.

REFERENCES

1. M. I. Chudakov. Collected volume Wood Chemistry, No. 1, 311, 1968.

2. B. V. Tronov, L. A. Pershina, V. M. Morozova, A. V. Kovalenok, A. I. Galochkin. Hydrolysis and Wood-Chemical Industry, 5, 1961.

3. L. A. Pershina. Study of the Interaction of Lignins with Organophosphorus Compounds. Doctoral dissertation, Riga, 1968.

4. L. A. Pershina, A. I. Galochkin, G. G. Lykhina. Collected volume Wood Chemistry, No. 1, 325, 1968.

5. L. M. Litvinenko, A. I. Kirichenko. Proceedings of the Conference on Problems in the Application of Correlation Equations in Organic Chemistry. Tartu, 1, 151, 1962.

6. Todt A., Beck T., J. Am. Chem. Soc., 72, 3191 (1950).

7. L. A. Pershina. Results of Research in Chemistry over 50 Years. Proceedings of the Interuniversity Conference, Tomsk, 79, 1968.

8. L. A. Pershina, S. N. Zagrebelnyi. Proceedings of TPI, 111, 45, 1961.

9. L. A. Pershina, A. I. Galochkin, V. P. Zhuchkov, V. V. Vagin. Author’s certificate No. 178235, October 30, 1965.

10. B. V. Tronov, L. A. Pershina. Abstracts of Reports and Communications of the IX Mendeleev Congress, No. 4, 133, 1965.

11. B. V. Tronov, L. A. Pershina, A. V. Kovalenok. Proceedings of TPI, 136, 39 (1965).

12. A. V. Kovalenok, L. A. Pershina. Reports of the Interuniversity Conference “Universities for Agriculture.” Moscow, issue 3, 1961.

13. A. V. Kovalenok, L. A. Pershina, K. S. Trubacheva. Collected volume Problems of Chemization of Agriculture in Tomsk Oblast, 148, 1965.

14. E. B. Kerbabaev, L. A. Pershina, V. I. Vashkev. Proceedings of TsNIDI, issue 18, 121, 1968.

15. B. V. Tronov, L. A. Pershina, A. I. Galochkin. Author’s certificate No. 164276, June 18, 1964.

16. L. A. Pershina, Yu. S. Pilipchuk, V. I. Shufledovich. Collected volume Wood Chemistry, No. 4, 1969.

17. L. A. Pershina, V. P. Zhuchkov. Decision on the issuance of an author’s certificate for application No. 1216224/23-4, October 10, 1968.

18. L. A. Pershina, V. P. Vasilieva. Proceedings of TPI, 136, 33 (1965).

19. L. A. Pershina, V. N. Kuskina, V. P. Vasilieva. Proceedings of TPI, 136, 36 (1965).

20. A. N. Novikov. Hydrolysis and Wood-Chemical Industry, No. 1, 1961.

21. A. M. Petrova. Proceedings of TSU, 170, 15 (1964).

22. L. A. Pershina, A. N. Novikov, M. V. Moiseenko, V. I. Komarova. Proceedings of TPI, 112, 64 (1963).

23. A. N. Novikov, L. A. Pershina, M. V. Moiseenko, V. I. Komarova. Proceedings of TPI, 112, 67 (1963).

24. L. A. Pershina, A. N. Novikov, A. I. Galochkin. Proceedings of TPI, 126, 46 (1964).

25. L. A. Pershina, B. A. Yankovskii. Proceedings of the First Scientific Conference of the Tomsk Branch of the All-Union Chemical Society named after D. I. Mendeleev, 96, 97, 1968.

26. B. A. Yankovskii, L. A. Pershina. Materials of the Conference of the Siberian Branch of the Academy of Sciences of the USSR. Irkutsk Institute of Organic Chemistry, 294, 1967.

27. L. A. Pershina, B. A. Yankovskii. Abstracts of Reports and Communications of the IX Mendeleev Congress, No. 4, 128, 1965.

28. B. A. Yankovskii. Collected volume Wood Chemistry, No. 1, 147, 1968.

29. B. A. Yankovskii, L. A. Pershina. Collected volume Wood Chemistry, No. 3, 57, 1968.