E.p.r. studies of thermal decomposition of vitrinite

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Fuel Vol. 74 No. 11, pp. 1654-1657, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/95/$9.50 + 0.00

E.p.r. studies of thermal decomposition of vitrinite Barbara Pilawa, Andrzej B. Wigckowski* and Marek Lewandowski t Department of Biochemistry and Biophysics, Faculty of Pharmacy, Silesian Medical Academy, Narcyz6w 1, PL-41200 Sosnowiec, Poland *Institute of Physics, Faculty of Mathematics, Physics and Technology, Pedagogical University, Plac Stowia6ski 6, PL-65069 Zielona Gdra, Poland and Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17/19, L-60179 Pozna6, Poland Institute of Coal Chemistry, Polish Academy of Sciences, Sowi~skiego 5, PL-44101 Gliwice, Poland (Received 27 October 1994; revised 31 March 1995)

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Vitrinite from Polish medium-rank coal (85.6 wt% C) heated in an inert atmosphere at 300-650°C was investigated by electron paramagnetic resonance in the X-band (9.3 GHz). The e.p.r, spectra of the vitrinite, recorded using different microwave powers, were approximated by superposition of a Gauss line and two Lorentz lines. The g factor, linewidth of the components and their fraction in the total spectrum were determined. A sharp increase in total concentration of paramagnetic centres after heating above 550°C was observed. Strong quenching and production of paramagnetic centres with broad Gauss and broad Lorentz lines above 550°C were detected. The heating of the vitrinite only slightly influenced the amount of paramagnetic centres with narrow Lorentz lines. The saturation behaviour of the all measured e.p.r, lines depended on heat-treatment temperature. (Keywords: vitrinite; thermal decomposition; electron paramagnetic resonance)

Electron paramagnetic resonance is a useful technique to study the evolution of the free-radical system in heated coals and macerals. Fowler et al. 1-5 investigated the pyrolysis of bituminous coal and extracted coals below 500°C by in situ e.p.r, spectroscopy. The total concentration of paramagnetic centres in the samples increased up to ~180°C and decreased between 180 and 300°C. At >300°C the creation of stable free radicals accounted for the rise in spin population 1. The interactions between evolved volatiles and solid char 2, correlation of tar yields from a fixed-bed pyrolysis reactor with concentration of paramagnetic centres in the char 3 and the influences of hydrogen donor solvent 4 and ZnC12-impregnation on the pyrolysis of coals 5 were discussed. Seehra et al. 6 measured the sharp increase in the amount of paramagnetic centres in coal, residue and extracts at temperatures from 400 to 600°C. Smidt and van Krevelen 7 noted that the number of free radicals in medium-rank vitrain (85.8wt%C) remained constant with increasing temperature up to ,,~300°C, above which a slight decrease set in; beyond ~400°C a sharp increase and between 500 and 600°C a decrease were recorded. For vitrains of very high (95.1 wt%) and very low (71.7 wt%) carbon contents the number of free radicals as a function of temperature was almost constant. The linewidth increased steadily with temperature for the vitrains of very high and low carbon contents, whereas for

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the medium-rank vitrain it remained constant at first, then decreased and finally rose again. The sharp decrease in spin-lattice relaxation time at >400°C confirmed that during carbonization the aromatic rings became pericondensed. Petrakis and Grandy 8 measured the variations in linewidth, g factor and concentration of paramagnetic centres in macerals with temperature up to 600°C. Very weak changes in concentration of paramagnetic centres in heated exinite and vitrinite were measured, whereas a strong increase above 400°C was noted for vitrinite. Linewidth and g factor were unchanged for intertinite even at 550°C, but both decreased for exinite and vitrinite. All the above e.p.r, measurements 1-8 refer to changes in the total spin population of heated coal and macerals. Several groups of paramagnetic centres belonging to different chemical structures, and which give e.p.r, lines with different values of linewidth and g factor, exist in both coal 8-2° and macerals 2°-24. Unpaired electrons of multi-ring structures, mainly delocalized 7r-electrons with strong exchange interactions, are responsible for the narrow components of the resonance absorption spectra 10- 16' 19- 24 . Paramagnetic centres of aliphatic structures or structures composed of a few aromatic rings are responsible for the broad lines 1°-16'19-24. The dipole interactions of unpaired electrons and the unresolved hyperfine structure of interactions of

E.p.r. studies of thermal decomposition of vitrinite. B. Pilawa et al. unpaired electrons with neighbouring protons are the reasons for the large linewidths of these signals. In the authors' previous work 24 using the microwave saturation technique, the e.p.r, spectrum of vitrinite was described as a combination of three signals: a broad Gauss line and two (broad and narrow) Lorentz lines. The aim of the present study was to determine the changes in the amount and behaviour of the three different groups of paramagnetic centres with these signals during thermal decomposition ofvitrinite. The multicomponent structure of the e.p.r, spectra of macerals has been demonstrated relatively recently2°-24. Thus the changes in behaviour of individual populations of paramagnetic centres caused by thermal decomposition of vitrinite at 300-650°C have hitherto been unknown. EXPERIMENTAL Vitrain was separated from Polish coal with a carbon content of 85.6 wt%. The coal contained 9 vol.% exinite, 77 vol.% vitrinite and 14 vol.% inertinite. The vitrain was demineralized using an aqueous solution of HC1 and HF at 50°C. Vitrinite (density 1.28-1.30gcm -3) was separated by centrifugation of the deminineralized vitrain in toluene-CCl4 mixtures; it contained 97vo1.% vitrinite, 1 vol. % exinite and 2 vol. % inertinite. The vitrinite was heated for 40 min in an argon flow at temperatures ranging from 300 to 650°C at 50 K intervals. The heated samples were mixed with SiO2 (1:5) and placed in thin-walled glass tubes. No e.p.r, signal from impurities was observed in the empty tubes. The measurements were performed using an X-band e.p.r, spectrometer, type SEX (9.3GHz, magnetic modulation 100kHz). The microwave frequency was recorded. E.p.r. spectra were obtained with attenuation of microwave power from 20 to 0.5 dB. The line-shape of the e.p.r, spectrum was analysed using a numerical algorithm25. The following parameters of the best-fit (smallest r.m.s, deviation) lines were calculated: g factor, linewidths and fraction of each component in the total spectrum. Double integration of the first-derivative e.p.r. spectra was performed to determine the total concentration of paramagnetic centres in the samples. The concentrations of paramagnetic centres responsible for the component e.p.r, lines were calculated from their percentages in the total spectrum. Ultramarine was used as the reference for the concentration of paramagnetic centres. RESULTS AND DISCUSSION

Figure 1 shows the increase in total concentration of paramagnetic centres in the vitrinite with increasing temperature of heating. This effect is extremely strong at >550°C. A similar relation between concentration of paramagnetic centres in vitrinite and temperature of heating was observed by Petrakis and Grandy s. Three different groups of paramagnetic centres exist in the vitrinite samples. The e.p.r, spectra of the vitrinite heated at 300-650°C, as well as at room temperature 24, are the superposition of a Gauss line and two Lorentz lines. The g values of the broad Gauss and the broad and narrow Lorentz lines are 2.0031, 2.0028 and 2.0027 respectively. Figure 2 shows the influence of pyrolysis temperature on the concentrations of the three types of paramagnetic centres. Heating at temperatures between

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Figure 3 Changes in linewidths of the three e.p.r, components of vitrinite with temperatureof thermaltreatment 500 and 650°C leads to strong quenching and production of paramagnetic centres responsible for the Gauss and broad Lorentz (1) lines. Heating at 300-650°C only slightly influences the amount of paramagnetic centres responsible for narrow Lorentz (2) line.

Fuel 1995 Volume 74 Number 11

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E.p.r. studies of thermal decomposition of vitrinite: B. Pilawa et al.

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Figure 3 shows the influence of thermal treatment of the sample on the linewidths of the three components. The width of the Gauss line at 650°C is greater than at 300°C, because of the increase in dipole interactions of unpaired electrons. The widths of the two Lorentz lines at 650°C are smaller than at 300°C. The relation is explained by an

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Fuel 1995 Volume 74 Number 11

increase in the narrowing effect of exchange interactions of unpaired electrons delocalized on larger aromatic structures. Figures 4-9 show the influence of microwave power on the intensities and linewidths of the three components of the e.p.r, spectra of vitrinite heated at 300, 500 and 650°C. The power of microwave saturation of all the e.p.r. lines detected increases with temperature of thermal

E.p.r. studies of thermal decomposition of vitrinite: B. Pilawa et al. decomposition o f the vitrinite. The correlation indicates that the spin-lattice relaxation time o f unpaired electrons in the vitrinite decreases at higher temperatures. CONCLUSIONS Three different groups o f paramagnetic centres exist in all the heated (300-650°C) vitrinite samples. The spin-spin and spin-lattice interactions o f the unpaired electrons change during thermal decomposition o f the vitrinite, because their e.p.r, linewidths and saturation behaviour depend on the temperature o f heating o f the sample. Thermal treatment above 550°C has a marked influence only on the concentrations o f paramagnetic centres responsible for the b r o a d Gauss and b r o a d Lorentz e.p.r, lines. ACKNOWLEDGEMENTS The authors are very grateful to Professor T. Wilczok, H e a d o f D e p a r t m e n t o f Biochemistry and Biophysics, Silesian Medical A c a d e m y , for support o f this research, which was sponsored by K o m i t e t Badafi N a u k o w y c h (Scientific Research Committee), Warsaw, Poland, Project no. 0732/P3/94/06.

6 7 8 9 10 11 12 13 14 15 16 17 18 19

20 21 22

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Smidt,J. and van Krevelen, D.W. Fuel 1959, 37, 355 Petrakis,L. and Grandy, D.W. In 'Free Radicals in Coals and Synthetic Fuels', Coal Science and Technology 5, Elsevier, Amsterdam, 1983. Retcofsky, H.L., Stark, J.M. and Friedel, R.A. Anal. Chem. 1968, 40, 1699 Schlick,S., Narayana, M. and Kevan, L. Fuel 1983, 62, 1250 Doetschman, D.C. and Mustafi, D. Fuel 1984, 63, 39 Doetschman, D.C. and Mustafi, D. Fuel 1986, 65, 684 Ito, O., Seki, H. and Iino, M. Bull. Chem. Soc. Japan 1987, 60, 2967 Wi~ckowski,A.B. Exp. Tech. Phys. 1988, 36, 299 Jeunet, A., Nickel, B. and Rassat, A. Fuel 1989, 68, 883 Nickel-Pepin-Donat, B., Jeunet, A. and Rassat, A. In 'Advanced Methodologies in Coal Characterization', Coal Science and Technology 15, Elsevier, Amsterdam, 1990, p. 149. Lebedev, Ya.S. Extended Abstracts, 'Congress Ampere on Magnetic Resonance and Related Phenomena', Stuttgart 1990, p. 508 Bresgunov,A.Yu., Dubinsky, A.A., Poluektov, O.G., Vorob'eva, G.A. and Lebedev, Ya.S.J. Chem. Soc. Faraday Trans. 1990, 86, 3185 Sczaniecki, P.B., Wi~ckowski, A.B., Wachowska, H. and Kozlowski, M. In 'Radio- and Microwave Spectroscopy', Proceedings of the Conference RAMIS-91, Ser. Phys. 67, A. Mickiewicz University, Poznafi, 1991, p. 267 Pilawa,B., Wi~ckowski, A.B. and Trzebicka, B. Mol. Phys. Rep. 1994, 5, 245 Pilawa,B., Wigckowski, A.B. and Trzebicka, B. Erd~l Kohle, Erdgas, Petrochem. 1990, 43, 187 Pilawa, B., Trzebicka, B., Wi~ckowski, A.B., Hanak, B., Komorek, J. and Pusz, S. Erdi~l Kohle, Erdgas, Petrochem. 1991, 44, 421 Pilawa, B., Trzebicka, B. and Wi~ckowski, A.B. Erd6l Kohle, Erdgas. Petrochem. 1992, 45, 308 Pilawa,B., Trzebicka, B. and Wi~ckowski, A.B. Fuel 1991, 70, 1109. Opfermann, J. 'Programmpaket Nichtlineare Ausgleichsrechnung', Friedrich-Schiller-Universitat, Jena, 1984, 42

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