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Studies of reaction products of hydrolytic lignin with nitric acid Yu. G. Khabarov, D. E. Lakhmanov, D. S. Kosyakov, and N. V. Ul´yanovskii M. V. Lomonosov Northern (Arctic) Federal University, 17 nab. Severnoi Dviny, 163002 Arkhangelsk, Russian Federation. Fax: +7 (818 2) 20 1742. Email: [email protected] The properties of the reaction products of hydrolytic lignin with nitric acid in water— dioxane medium were studied. The products are a mixture of nitrogencontaining oligomeric compounds with high degree of polydispersity. Key words: hydrolytic lignin, nitric acid, depolymerization, dioxane.

Lignins belong to polymers with a complex structural organization. Because of the specifiсities of biochemical transformations leading to the formation of lignins in plant tissue, the primary structure of lignins does not possess a certainty characteristic of many other natural polymers. The basic structural elements of lignins are phenyl propane units: guaiacylpropane (R1 = OMe, R2 = H, R3 = OH), syringylpropane (R1 = R2 = OMe, R3 = OH) and phydroxyphenyl (R1 = R2 = H, R3 = OH), which in the real lignin macromolecule are bonded to each other with different bonds. A propane chain contains hydroxy and other functional groups. The phe nylpropane units in lignins are bonded with each other with ordinary ether and carboncarbon bonds. Apart from that, they are bound through more complex fragments, which simultaneously in clude both the ether and carboncar bon bonds (pinoresinol and phenyl coumarane structures). Phenylcoumar ane fragments are formed by the pro pane chain of one of the phenylpropane unit and the aro matic ring of the other. The pinoresinol fragments are formed involving only propane chains of two phenyl propane units. The structure of softwood lignin macromolecule sug gested by K. Freudenberg1,2 is shown below. In the course of chemical processing of wood, techni cal lignins are formed as the side products, whose chemi cal nature is determined by the process leading to their formation. Hydrolytic lignin (HL) belongs to technical lignins altered the most as compared to the natural com pound.3—5 Under conditions of acid hydrolysis of polysac charides in plant raw materials, lignin undergoes deep transformations. New naphthalene, anthracene, phenan threne, and benzofuran structures uncharacteristic of oth er kinds of lignins emerge in the hydrolytic lignin macro molecules through the condensation processes.6

Macromolecules of HL acquire threedimensional crosslinked structure. Its reactivity could not remain un affected by such changes. Modification of technical lignins is necessary for the directed alternation of properties and broadening the area of their practical application. Depolymerization7 is an im portant direction of modification of technical lignins, which leads to lowmolecularweight products: aromatic aldehydes (vanillin, syringaldehyde),8 carboxylic acids, phenols,9—12 quinonepolycarboxylic acids,13 and different kinds of hydrocarbons.14 One of the directions of modification of lignins, in cluding HL, is their conversion to nitrogencontaining derivatives. Lignins can react with ammonia, amines, ni tric and nitrous acids, hydrazine and its derivatives, hy droxylamine, isocyanates, urea. However, only nitric acid, ammonia, and urea are largescaled, commercially profit able agents.15 Depolymerization of lignin can be affected by nitric acid, the reaction with which leads to lignin derivatives with nitrogencontaining groups, a decrease in the molec ular weight of lignin, and an increase in its solubility. By now, there is known a large amount of methods of treat ment of lignins with nitric acid. Nitric acid is used in the reactions of electrophilic ni tration and at the same time exhibits properties of an oxi dant.16,17 The results of the study of the processes taking place in the reactions of lignins with nitric acid under different conditions are summarized in the monograph.18 Some time was devoted to the studies of the develop ment of nitric acid pulping of cellulose, the results of which were tested on an industrial scale. This pulping method was eventually refused for industrial use because of con comitant environmental problems. However, the interest to this sort of pulping process is not completely lost at the present time.19 Apart from that, treatment of wood with nitric acid can be used for the preparation of nitrogen containing agents stimulating the plant growth.20

Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 0237—0244, January, 2016. 10665285/16/65010237 © 2016 Springer Science+Business Media, Inc.

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Macromolecule of softwood lignin

Nitric acid is used for production of nitrocellulose, a basis for gunpowders, varnish, enamels, and plastics.21 There are known different ways of the use of nitric acid in the analytical practice of wood chemistry.22,23 An aque ous alcoholic solution of nitric acid is used in the analysis of wood raw material for determination of the cellulose content by the Kurshner and Hoffer method.22 One of the known methods for depolymerization of HL is its treatment with aqueous nitric acid.24,25 In this

method, HL is allowed to stand for 4 h at 83±2 °C in 5—10% aqueous nitric acid. After this, the product is washed until neutral pH and dried. Considerable part of HL remains insoluble even after fourhour treatment. Earlier,26,27 we have found that nitric acid in aqueous dioxane can serve as an exclusively efficient depolymeriz ing agent. 1,4Dioxane is a specific and versatile solvent, which, due to its high basicity, can form stable associates with different inorganic and organic compounds,28 includ

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Macromolecule of hydrolytic lignin

ing phenols, already when the content of dioxane in the organoaqueous solvent is above 20 mol.%.29 Under con ditions of the reaction of HL with nitric acid in the system water—dioxane, a complete depolymerization and partial dissolution of HL occur within 15—20 min. Only 15—20% of the compound remain undissolved. This residue almost completely is composed of carbohydrates. In this connection, it is very interesting to study the mechanism of depolymerization of HL in organoaque ous medium, a key moment in which is establishing the scope of the products formed. The present work is directed on the solution of this problem. Experimental Technical HL from Kirov Biochemical Plant has been chosen as an object of our studies. Before experiments, lignin was pre liminary purified from water and ethersoluble compounds and fractionated. The study was carried out for the fraction with the particle size of 0.2—1 mm. The content of Klason lignin in HL was 77.8%. The elemental analysis data (%): C, 65.9; H, 5.9; N, 0.2; O, 28.0. Depolymerization reaction. Depolymerization was carried out in the installation with a reflux condenser in a thermostat for certain periods of time (5, 30, 60, 90, and 120 min) at a certain temperature. For this, lignin (1 g) and a required amount of the reagent were placed in a flask. The reagent was prepared by mixing 65% nitric acid (reagent grade, Nevareaktiv) with 1,4di oxane (analytical grade, Komponentreaktiv) in the ratio of 1 : 4 (v/v). After the reaction reached completion, the undissolved

part was separated from the solution by vacuum filtration. Then, the filtrates were concentrated without heating in a vacuum desiccator over potassium hydroxide (reagent grade, Nevareaktiv). Electronic spectroscopy. Absorption spectra were recorded on a UV1650PC double beam spectrophotometer (Shimadzu, Japan) in quartz cells with a 1cm optical path in the 200—500 nm wavelength range. The working cell contained an alkaline solu tion of the reaction products, reference cell contained a neutral solution of the same concentration. IR spectroscopy. IR spectra were recorded on a FTIR8400S Fouriertransform spectrometer (Shimadzu, Japan) by the at tenuated total internal reflection method, using a MIRacle sin gle reflection accessory (Pike, USA). The spectral resolution was 4 cm–1, the distance between the points was 2 cm–1. Mass spectrometry. Mass spectrometric studies of lignin de polymerization products were carried out using a TripleTOF 5600+ high resolution hybrid timeofflight mass spectrometer (ABSciex, Canada) equipped with an electrospray ionization sys tem. Solutions of the samples in acetonitrile with a concentra tion of 10 mg L–1 were injected directly into the source of ions, using a syringe pump (7 μL min–1). Conditions of the mass spec tra recording: a negative ion mode, resolution >35000 FWHM, capillary voltage –4.5 kV, the m/z range 100—5000 Da. The scale of masses was calibrated using taurocholic acid as a stan dard. The m/z measurement error was below 5 ppm. Processing of experimental data and determination of elemental composi tion based on the accurate masses were performed using the PeakView software package (ABSciex, Canada). Size exclusion chromatography. Molecular weight charac teristics of the products of depolymerization of lignin by size exclusion chromatography were determined on a LC20 Pro minence HPLCsystem (Shimadzu, Japan), which includ

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ed a SECcurity doublechannel degasser (PSS, Germany), a LC20AD pump, a SIL20A autosampler, a CTO20A column thermostat, a SPD20A spectrophotometric detector, and a UDC 810 controller (PSS, Germany). Chromatographic data were col lected and processed using the WinGPC software package (PSS, Germany). Separation was carried out on a linear column with the Styragel HR 4E DMF styrenedivinylbenzene gel (Waters, USA), 4.6×300 mm, 5 μm at 50 °C. An eluent was N,Ndimeth ylformamide with added 0.01 M solution of lithium bromide to suppress the polyelectrolyte effects. The flow rate of the eluent was 0.25 mL min–1, the amount of the injected sample was 20 μL. The time of the analysis was 30 min. Detection was carried out at 280 nm wavelength, using a spectrophotometric detector. The system was calibrated using monodispersed polystyrene standards (PSS, Germany) with known Mw values in the range of masses of 800—100000 Da. The analyzed and standard samples were dis solved in the eluent, the concentration was 1 mg mL–1. Elemental analysis. Elemental composition was determined by catalytic burning in oxygen with subsequent gas chromato graphy of the products formed, using an EA 3000 CNHSana lyzer (EuroVector, Italy). Content of acid groups. The content of the acid groups (mmol g–1) in the studied samples was determined by potentio metric titration of a solution containing a known amount of the analyte with a solution of sodium hydroxide. The solution рH was measured using an Ekspert001 pHmeter (EkoniksEkspert, Russia) with an ESK10603 combined glass electrode (Iz meritel´naya tekhnika, Russia).

Results and Discussion Nitric acid has properties of both a nitrating and an oxidizing agent. The outcome of the reactions of nitric acid with aromatic compounds is affected by many differ ent factors: the composition of the agent, amount of nitric acid, solvent, etc. The following principal features of ni tration were found as a result of a multitude of studies of nitration of aromatic compounds modeling the lignin structure30—36 (model compounds 1—7): 1) electrophilic substitution of hydrogen atoms in the benzene ring takes

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place at position 5 or 6, depending on the presence of a free or an esterified phenol hydroxy group; 2) elimination of propanoic chain of the aromatic compound proceeds via the step of the formation of oxonium intermediate; 3) demethylation takes place; 4) oxidation proceeds with the formation of quinonoid structures and carboxy groups. The reaction of lignins with nitric acid is even more complex process. This is especially true for HL. Due to the presence in its macromolecules of annulated aromatic frag ments, the reactions with nitric acid lead to the formation of quinonepolycarboxylic acids.37 Thus, the reaction of HL with nitric acid results in the formation of new functional groups in the products, cleav age of carboncarbon and ether bonds. The preference of these processes is determined by different factors: steric accessibility of reaction centers, diffusion processes, sol vation effects, energies of different types of bonds in lig nin, etc. The large yield of the HL fraction isolated by concen tration of the dissolved part after the reaction is substanti ated by accumulation in the products of nitro and other oxygencontaining groups. Taking into account the known data on the reaction of model compounds of lignin with nitric acid, the reactions given in Scheme 1 can be sug gested as taking place when HL is treated with nitric acid. Naturally, the transfer of HL into the soluble forms involves depolymerization. Molecular mass of the starting HL virtually cannot be determined because of the three dimensional crosslinked structure. Since in the course of the reaction HL is transformed into the dissolved state, a possibility to evaluate molecular masses of the reaction products appears. For this, we car ried out the reaction of HL with nitric acid for 5, 30, 60, 90, and 120 min. Reaction products were isolated from the filtrates by solvent evaporation. Molecular masses of the reaction products were deter mined by HPLC. The results are given in Table 1.

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Scheme 1

Lig is the lignin residue.

that within first 5 min the most easily eliminated frag ments of the HL macromolecule from the outer layers of the threedimensional crosslinked structure are trans ferred into the solution. After 60 min, the molecular mass decreases because of the secondary processes in solution, since by this time all the HL is transferred into the solu tion, which is indicated by the yield of the undissolved part, remaining virtually unchanged after 30 min. Elemental composition also undergoes large changes in the course of the reaction. In the first moment, the content of nitrogen rapidly increases, reaching 5.35%,

The data on the changes of properties of the products in the course of the reaction of HL with nitric acid are also given in Table 1. The results of chromatographic analysis indicate that the products possess small molecular masses. The large values of the ratio Mw/Mn, reflecting polydispersity de gree, indicate that the reaction products are mixtures of compounds with considerable range of molecular masses. The molecular mass of the reaction products decreases with time, passing a maximum at 30—60 min. Such a pattern of the dependence can be explained by the fact

Table 1. Characteristics of reaction products of HL with nitric acid τ/min

0 (HL) 5 30 60 90 120

Mn/Da

— 285 347 350 303 292

Mw/Mn

— 8.3 9.6 9.5 7.3 5.9

Yield (%)

Content (%)

UDP

DP

N

C

H

O

— 68 22 17 17 17

— 141 101 110 122 130

0.20 5.35 4.57 3.74 3.19 2.57

63.87 41.16 43.00 43.43 41.41 38.62

5.87 4.72 4.48 4.77 4.86 5.12

28.06 48.77 47.95 48.06 50.54 53.69

AG /mmol g–1 — 3.50 3.74 2.85 3.07 3.28

Note. τ is the reaction time, Mw and Mn are the weight average and number average molecular masses, respectively, UDP and DP are the undissolved and dissolved parts of the products, respectively, AG is the content of acid groups.

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whereas then its gradual decrease is observed with time. This can be explained by the fact that nitration, which is an electrophilic substitution reaction of hydrogen atoms of the benzene ring of the phenylpropanoic unit proceeded rapidly due to the presence of activating electrondonat ing groups. The nitration is accompanied by oxidation, leading to the increase in the content of oxygen. The oxy gencontaining groups in the products, besides nitro and methoxy groups, can be hydroxy, carbonyl, and carboxy groups. The dissolution of the aromatic polymeric com ponent of HL occurs within 15—20 min. Using potentio metric titration, the presence of a considerable amount of acid groups was detected. The dissolution of HL in the course of the reaction with nitric acid also affected spectral characteristics of the reaction products, both electronic and vibrational. Lignins are aromatic polymers containing free phenol hydroxy groups, therefore, their ionization spectra usually have the absorption bands at 250 and 300 nm; the absorption in the region of 350 nm is caused by the presence of carbonyl groups, which are conjugated with benzene rings.38 The ionization spectra of the reaction products of HL with nitric acid are shown in Fig. 1. They strongly differ from the usual ionization spectra of lignins (see Fig. 1, curve 6). The displacement of the absorption band at 350 nm with time is, apparently due to disintegration of benzene rings. The reaction of HL with nitric acid is accompanied by oxidation not only of propanoic chains, but also the aromatic rings. This is indicated by the patterns of the IR spectra of the reaction products, in which, as compared to the spectrum of the starting lignin, the absorption bands at 1520 and 1590 cm–1 characteristic of benzene rings are virtually absent (Fig. 2). The mass spectra of depolymerization products are in good agreement with the data of the analysis of molecular weight distribution (MWD) and demonstrate a predomi nance of the compounds in the range of molecular masses from 100 to 400 Da. A 0.14 0.12

1 2

0.10

3

0.08

4

0.06 5 6

0.04 250

300

350

400

λ/nm

Fig. 1. Electronic ionization spectra of the products of the reac tion of HL with nitric acid after treatment for 5 (1), 30 (2), 60 (3), 90 (4), and 120 min (5) and the starting lignosulfonates (6).

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A 6 5 4 3 2 1 1800

1600

1400

1200

1000

ν/cm–1

Fig. 2. IR spectra of the products of the reaction of HL with nitric acid in aqueous dioxane after treatment for 5 (1), 30 (2), 60 (3), 90 (4), and 120 min (5) and the starting HL (6).

The mass spectrum (Fig. 3) exhibits about 20 peaks of the main products of lignin depolymerization with m/z 183.0033 (C6H3N2O5), 189.0199 (C10H5O4), 210.9982 217.0877 (C 13H13O 3), 227.9889 (C7H 3N 2O 6), (C6 H 2N 3O 7), 254.9884 (C 8H3N 2O 8), 270.9830 (C8H 3N 2O 9), 275.0933 (C 15H 15O 5), 287.0913 (C16H 15O 5), 299.0149 (C 10H7N 2O 9 ), 307.0292 (C11H7N4O7), and 325.0393 (C11H9N4O8). The reaction products can be separated into two groups, which contain or do not contain nitrogen. 2,4Dinitro phenol, picric acid, and nitrophenol were identified among the nitrogencontaining products. Compound containing no nitrogen are, apparently, aromatic and aliphatic carb oxylic acids formed in the course of the oxidative destruc tion of lignin. The identification was carried out using the data of the tandem mass spectrometry by comparison of the obtained mass spectra of the second generation ions with the corresponding spectra of the standard samples of individual compounds. The individual products of degra dation of lignin were not isolated from the mixture. Be cause of limitations of mass spectrometry in the identifi cation of isomers and the low intensity of the nitrophenol peak, the position of the nitro group in nitrophenol was not established. In the mass spectrum of the samples, attention should be paid to the predominance of the ion with m/z 217.0877 with the molecular formula C13H13O3, which was identi fied by tandem mass spectrometry with the collision in duced dissociation of the target ion. The MS2 spectrum of this ion contains four peaks of the product ions with m/z 173.0254 (C10H5O3), 145.1039 (C11H13), 115.0201 (C8H3O), and 103.0572 (C8H7). This suggests that the starting compound contains an aromatic ring and a carboxy group, as well as one more oxygencontaining group. The peak with m/z 217.0882 is transformed to the peak with m/z 145.1039 by decarboxyl ation and decarbonylation. The presence of the propanoic chain in the reaction products is confirmed by the pres ence of a product ion with molecular formula C10H5O3, which is formed by elimination of propane C3H8. The

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I•10–4 217.0875

4.5 4.0 227.9887 226.9933

3.5 3.0 2.5 188.0233 189.0199

2.0

242.9834

1.5

133.0131 178.9819

243.9833

216.0910

1.0 172.9886

0.5

201.9729

244.0450 247.0251

153.9879

287.0909 287.9737 270.9830

325.0393

286.9780

0 140

160

180

200

220

240

260

280

300

320

m/z

Fig. 3. Mass spectrum of the products of the reaction of HL with nitric acid at the reaction time of 60 min.

MS2 spectrum the most exactly corresponds to fragmen tation of the benzofuran structure, which can be formed from the phenylcoumarane fragments of lignins.

The large content of acid groups in the depolymerization products allowed us to suggest that they can be good complex ation agents (complexones) toward biogenic metal cations. Thus, the reaction of HL with nitric acid in the initial step proceeds as a rapid nitration, then within next 20—25 min all the HL is transferred into solution, and the depolymerization is accompanied by oxidative trans formations. The products of the reaction of HL with nitric acid in aqueous dioxane is a mixture of compounds of different nature and molecular masses. Nitrophenols were identi fied among the nitrogencontaining compounds. Com pounds with oxygencontaining functional groups of benzofuran nature constitute another group of products. This work was financially supported by the Ministry of Education and Science of the Russian Federation (Project No. 1321), using facilities of the Core Facility Center Ark tika (Northern (Arctic) Federal University) (unique iden tifier of the work RFMEFI59414X0004). References 1. K. Freudenberg, Science, 1965, 148, 595. 2. K. Freudenberg, Holzforschung, 1964, 18, 3.

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