Modification of Coal Tar by Nitrous Oxide

ISSN -X, Coke and Chemistry,, Vol., No., pp. 9. Allerton Press, Inc.,. Original Russian Text S.A. Semenova, D.P. Ivanov, O.M. Gavrilyuk, N.I. Federova, Z.R. Ismagilov,, published in Koks i Khimiya,, No., pp.. CHEMISTRY Modification of Coal Tar by Nitrous Oxide S. A. Semenova a, D. P. Ivanov b, O. M. Gavrilyuk a, N. I. Federova a, and Z. R. Ismagilov a a Institute of Coal Chemistry and Chemistry of Materials, Siberian Branch, Russian Academy of Sciences, Kemerovo, Russia e-mail: semlight@mail.ru, icms.kemsc.ru b Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia e-mail: divan@catalysis.ru Received August, Abstract The modification of coal tar by nitrous oxide is considered. Preliminary N O of the tar increases the yield of light distillate fractions and reduces the content of unsaturated indene compounds and polyaromatic hydrocarbons in the oils. DOI:./SXX More stringent quality requirements on coal products call for research on the physicochemical modification of tar and pitch. Two basic types of physicochemical modification and inhibition methods may be distinguished: () methods of increasing the molecular interaction and polycondensation of tar and pitch hydrocarbons (thermooxidation []; and by mineral and organic acids [], Lewis acids [], and hydrolytic lignin []); () methods of reducing the molecular interaction of tar and pitch hydrocarbons and ensuring sharper separation of the components in tar distillation (cavitation []; and by a high-frequency electric field []). A promising method of oxidative modification of hydrocarbons is the use of nitrous oxide (N O). Nitrous oxide is active in the oxidative catalysis of hydrocarbons in the gas phase for example, the selective hydroxylation of paraffins and aromatic hydrocarbons []. Recently, nitrous oxide has been used in the noncatalytic liquid-phase oxidation of alkenes, cycloalkenes [], and polymers []. Noncatalytic oxidation by N O may be used for alkenes of different classes, including linear, cyclic, and heterocyclic alkenes and their derivatives []. In view of the reactivity of nitrous oxide and the presence of unsaturated compounds forming azeotropic mixtures in coal tar, researchers are interested in the change in composition and properties of coal tar on modification by nitrous oxide. In turn, the redistribution of unsaturated compounds may change the molecular interactions in the initial material and hence the separation of the tar components in thermochemical processing. We investigate coal tar from OAO Altai-Koks, with the following characteristics: density. kg/m ; moisture content.; and ash content.. Prior to modification, we remove benzene-insoluble components from the tar (free carbon). The tar is treated with nitrous oxide in a -cm stainless-steel reactor (Parr Instruments), equipped with a mixer. We introduce ml tar (dissolved in ml benzene) in the reactor. Helium is fed to the reactor to displace the air, and then medical-grade (99.) nitrous oxide is introduced (. mol), with mixing. At room temperature, the pressure in the reactor is atm. The reactor is heated at /min to and held for h. Analysis of the gas phase after the experiment indicates conversion of the nitrous oxide, with the formation of nitrogen and carbon dioxide. After the experiment, the reactor is cooled to room temperature, the reaction mixture is discharged, and the solvent is boiled off. Modification of coal tar by means of nitrous oxide reduces its content of carbon and hydrogen and increases the content of oxygen and nitrogen (Table ). Since there is little change in the identified forms of Table. Chemical composition of the initial and modified coal-tar samples Functional composition, Elementary composition, (daf) Atomic ratio mg-equ/g C H O N S COOH OH CO O/C N/C Before 9.9......... After N O.......... 9

SEMENOVA et al.. Wave number. Wave number Absorption.. Absorption.. Fig.. IR spectra of tar before () and after () with nitrous oxide. Fig.. IR spectra of neutral tar fraction before () and after () with nitrous oxide. oxygen, we may attribute the increase in oxygen content in the tar to the formation of inactive heterocyclic forms. Redistribution of the forms of oxygen and nitrogen in the tar is indicated by IR spectroscopy (on an Infralyum-FT- instrument). Thus, a tar sample treated by N O is characterized by increased optical density of the C=O-group bands of aromatic ketones and aldehydes ( and cm ) and lactones ( cm ), as we see in Fig.. The nitrogen-bearing structures are identified by the appearance of new IR absorption bands and the intensification of existing bands: and cm for N H; ~ cm for C N; ~ cm for N O; and cm for N=N; and 9 cm for C N=O. For other hydrocarbon groupings, the most prominent change (reduction in absorption) is seen in the region cm, which characterizes the degree of hydrogen substitution in the aromatic rings. We know that the reaction of unsaturated compounds with nitrous oxide is based on a nonradical mechanism, through,-addition of N O molecule to the C=C bond of the alkene; an oxodiazoline complex is formed as an intermediate product [9] R CH CH R + + N N O N N O R CH CH R R CH C R +N. Table. Group composition of coal tar Before After N O organic bases O Yield of group fractions, (rel.) acids and phenols asphaltenes neutral oil neutral tar..... 9.... 9. Probably, the appearance of bands corresponding to C N, N=N, and N O bonds in the IR spectrum of the tar, as well as its increased nitrogen content (Table ), indicates that some proportion of intermediate oxodiazoline structures may be retained in the tar. Separation of the tar into groups of components with common properties (organic bases, acids and phenols, asphaltenes, oil, neutral tars) indicates redistribution of the qualitative and quantitative composition of the fractions after modification. Thus, in the group composition of coal tar treated with nitrous oxide (Table ), there is a higher yield of products containing functional groups and heteroatoms based on nitrogen (organic bases) and oxygen (acids and phenols, neutral tars). The yield of the basic fractions (asphaltenes and neutral tars) is significantly reduced, evidently because of reaction with N O. All the fractions are studied by IR spectroscopy. The differences in the IR spectra of the initial and modified tar samples are more apparent in the IR spectra of the group fractions. The greatest changes are seen for oxygen- and nitrogen-based fractions: organic bases, the total phenol acid fraction, and the neutral tar. Besides the differences observed for the modified tar samples (Fig. ), we also observe for the neutral tar fraction, for example, increased intensity of the absorption bands of the CH ( cm ) and C=C ( cm ) aromatic groups, with change in the ring substitution (9 cm ), as we see in Fig.. The proportion of axial structures (9 and cm ) is reduced. The absorption of C O groups in the range cm is changed; the bands corresponding to simple cyclic esters ( and cm ) are more pronounced. Two intense bands appear at and cm, corresponding to lactones and anhydrides. Note that, in the absorption regions of the oxygen groups ( and cm ), strong bands corresponding to C N and N O bonds are usually recorded, as confirmed by the presence of intense absorption bands at cm (C N), cm (N O), and cm (N=N), and 9 cm (C N=O bonds). In other words, we may COKE AND CHEMISTRY Vol. No.

MODIFICATION OF COAL TAR BY NITROUS OXIDE Table. Composition of neutral oil Group Content in oil fraction, (rel.) before after N O Unsaturated hydrocarbons 9.. Biaromatic hydrocarbons 9.. Triaromatic hydrocarbons.. Tetraaromatic hydrocarbons.. Pentaaromatic.. hydrocarbons Hexaaromatic hydrocarbons. O,N,S-Heterocyclic.9 9. hydrocarbons Oxygen compounds (phenols, acids, ketones, aldehydes).. suppose that the neutral tar fraction consists of products of oxidative nitriding of the tar. The neutral tar fraction may be studied by chromatographic mass spectroscopy on an Agilent 99S- instrument (Table ). The chromatographic data reveal around compounds. The oil mainly consists of ( )-ring hydrocarbons: from naphthalene to benzo[g,h,i]perylene. The unsaturated compounds are indene, acenaphthylene, acepyrene, and indenopyrene. Heterocyclic compounds present in the oil include small quantities of benzofurans, benzothiophenes, carbazole, and acridine. In neutral oils, N O significantly reduces the content of unsaturated hydrocarbons in the indene series and polyaromatic compounds: pyrene, benzo[a]anthracene, benzo[a]pyrene perylene, and benzo[g,h,i]perylene. The proportion of bi- and triaromatic compounds is increased: specifically, alkyland cycloalkyl derivatives of naphthalene and phenanthrene. Aromatic ketones and aldehydes are identified among the oxygen compounds. The carbonyl groups in these compounds take the form of substituents in the aromatic ring (diphenyl carbaldehyde), in groups conjugate with the aromatic ring (tetrahydrophenanthrene), and in the crosslinking of benzene rings (diphenyl methanone). Among heterocyclic hydrocarbons, we observe compounds of complex structure with pyrazole rings, which have not previously been seen in oils. Since most industrial methods of coal-tar processing are based on high-temperature, we investigate the thermal stability of the tar in the present work by thermogravimetric analysis (on a Netzsch STA 9 instrument, with an Aeolos mass-spectrometric attachment, in helium). Tar pyrolysis is characterized by a wide gas-liberation range ( ), with a primary maximum of the mass-loss rate at and two secondary peaks at and (Table ), corresponding to the distillation of aromatic, alkyl aromatic, and heterocyclic hydrocarbons with similar boiling points: around (alkyl benzene, pyridine), (phenol, benzaldehyde, benzofuran, benzonitrile, indene, etc.), (benzothiophene, quinoline, indole, naphthalene, biphenyl, fluorene, acenaphthene, etc.) and also (tri- and tetracyclic hydrocarbons). The main stage in thermal destruction of the tar ends at, when of the product has been removed. The total mass loss of the tar samples is ; coke formation accounts for the remaining 9. The of coal tar with nitrous oxide leads to a more pronounced peak at on the differential thermogravimetric curve; the maximum gas liberation is shifted to significantly (by ) lower temperatures. The maximum gas-liberation rate V max and total mass loss on heating the product to are increased (Table ). The range of primary heat liberation T T is hardly changed. At higher temperatures (above ), the intensity of distillation and thermal destruction in the modified tar is reduced. The changes in tar pyrolysis are probably due to the formation of low-molecular neutral oxygen- and nitrogen compounds on tar modification, with greater volatilization at relatively low temperatures, as confirmed by mass-spectrometric analysis of the pyrolytic products (Fig. ). Thus, for tar after N O, in the temperature range where light fractions boil off (<), the liberation rate of volatile hydroaromatic, heterocyclic, Table. Thermogravimetric analysis of coal tar before and after with nitrous oxide* (T T ), T n, T max, V max, /min Δm ( ), Δm ( ), Δm ( ) Before After N O ; ; ;.. 9.. ; 9; 9.... * Here Δm is the mass loss in the corresponding interval; T T is the temperature interval of primary gas liberation; T n are the temperatures corresponding to the maximum gas liberation; T max is the temperature at the primary maximum of gas liberation; V max is the rate of maximum gas liberation. COKE AND CHEMISTRY Vol. No.

SEMENOVA et al. Ionic current, A Ionic current, A 9 m/z.... Temperature, Temperature, m/z 9 Ionic current, A Ionic current, A m/z m/z Temperature, Temperature, Fig.. Emission rate of volatile products with m/z = (nitro compounds), 9 (phenols, benzene homologs), (cycloalkanes), and (triarenes) in the pyrolysis of coal tar before () and after () with nitrous oxide. and slightly condensed aromatic products (m/z =,, 9,,, and ) is higher. No marked increase in liberation rate is observed for the oxygen compounds H O (m/z = ) and CO (m/z = ); that indicates that the compounds formed are neutral. For modified tar, the liberation rate of nitrogen-bearing structures (m/z = and ), phenol (m/z = and 9), cycloalkanes (m/z = 9 and ), benzene homologs (m/z =,,, and so on), and naphthalene (m/z =,,, and so on) is also greater. In the range corresponding to thermal destruction of the tar ( ), the peaks corresponding to the emission of volatile polyaromatic products (m/z =,,, and so on) are less intense and shifted to higher temperatures. That may be associated with the development of polymerization (Figs. and ). Thus, with nitrous oxide results in oxidative modification of the tar composition, with the formation of neutral oxygen compounds. Modification considerably increases the relative content of the neutral tar fraction in the tar s group composition, thanks to the reaction of neutral oils and asphaltenes with the nitrous oxide. Reaction products include alkylaromatic and naphthene-aromatic hydrocarbons, aromatic ketones and aldehydes, and heterocyclic compounds of pyrazole type, formed by the reaction of nitrous oxide with unsaturated indene structures. Thermogravimetric data show that the of coal tar by nitrous oxide stimulates the liberation of volatile products on heat, with increase in the yield of light volatile fractions (boiling point <). The conversion of unsaturated indene structures involved in the formation of molecular complexes and azeotropic mixtures should result in sharper separation of the products in tar distillation. ACKNOWLEDGMENTS We thank A.S. Kharitonov of the Institute of Catalysis for assistance in formulating the experiments; and the following specialists from the Institute of Coal Chemistry and Chemistry of Materials and the Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences, for assistance in deriving and interpreting the results of physicochemical analysis: COKE AND CHEMISTRY Vol. No.

MODIFICATION OF COAL TAR BY NITROUS OXIDE V.Yu. Mal sheva, L.M. Khitsova, and T.G. Vychikova (elementary analysis, IR spectroscopy); N.A. Korol (chemical analysis of O groups); A.A. Nefedov (thermogravimetric analysis); and A.A. Volgin (chromatographic mass spectroscopy). REFERENCES. Sidorov, O.F., Thermooxidation of Coal-Tar Pitch: Reaction of Oxygen with Hydrocarbon Pitch, Koks Khim.,, no. 9, pp... Sidorov, O.F. and Saul, O.P., Influence of Acids on the Yield and Quality of Coal-Tar Pitch, Koks Khim., 9, no., pp. 9.. Karpin, G.M. and Kondratov, V.K., Structure and Properties of Donor Acceptor Complexes of Aromatic Compounds Formed by γ, β, and α Fractions of Tar and Pitch with Lewis Acids: Development of Structural Models, Koks Khim.,, no., pp. 9.. Kovalev, E.T., Influence of Initiating Additives on the Processing of Coal Tar and Its Fractions, Koks Khim.,, no., pp. 9.. Baikenov, M.I., Omarbekov, T.B., Amerkhanova, Sh.K., et al., Influence of Cavitation on the Properties of Coal Tar: Gas Liquid Chromatographic Data, Khim. Tverd. Topl.,, no., p... Leont ev, A.V., Fomicheva, O.A., Proskurnina, M.V., and Zefirov, N.S., Chemistry of Nitrous Oxide, Usp. Khim.,, vol., no., p... Semikolenov, S.V., Dubkov, K.A., Starokon, E.V., et al., Liquid-Phase Noncatalytic Oxidation of Butenes by Nitrous Oxide, Izv. Akad. Nauk, Ser. Khim.,, no., p. 9.. Semikolenov, S.V., Dubkov, K.A., Echevskaya, L.G., et al., Modification of Polyethylene by Selective Oxidation of Double Bonds in Carbonyl Groups, Vysokomol. Soed., Ser. B,, vol., no., p. 9. SPELL:. ok COKE AND CHEMISTRY Vol. No.