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Energy & Fuels 2002, 16, 40-47
Exploring Radicals in Carbonaceous Solids by Means of Pulsed EPR Spectroscopy Tadaaki Ikoma,* Osamu Ito, and Shozo Tero-Kubota Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Received July 3, 2001. Revised Manuscript Received September 10, 2001
Pulsed electron paramagnetic resonance (EPR) spectroscopy has been applied to elucidate the radical structures of Argonne premium coals and the reaction processes in coal tar pitches (CTP). Using two-dimensional nutation spectroscopy, the main EPR signals around g ) 2.00 for the coals were confirmed to be attributed to free radicals rather than spin clusters with higher spin multiplicities. The broad 1H- and 13C-hyperfine spectra of the coals were detected separately by hyperfine sublevel correlation (HYSCORE) spectroscopy. It is concluded that the main coal radicals are π radicals delocalized in pericondensed aromatic hydrocarbons with more than seven aromatic rings, which are larger than those of the most probable aromatic molecules in coals. HYSCORE spectroscopy, furthermore, helped us to understand the chemical change in the cation radicals in the charge-transfer complexes that accelerate the dehydrogenative polymerization of I2-treated CTP.
Introduction Electron paramagnetic resonance (EPR) spectroscopy is one of the useful tools to characterize various carbonaceous materials such as coal, coal tar pitch (CTP), and so on, because most of them show intense EPR signals.1,2 The observed signals are mainly connected with naturally occurring organic radicals, while the signals due to paramagnetic inorganic species such as transition metal ions are also present in some cases. The characterization of the paramagnetic organic compounds in the complex carbonaceous solids is still a challenging assignment.3 For instance, the spin multiplicity of the paramagnetic species has been the subject of controversy. There are numerous studies in the literature supporting free radicals with doublet spin multiplicity, but several groups have recently reported the presence of weak magnetic interaction between two or more unpaired electrons, which suggests thermally excited triplet radical pairs or spin clusters having the higher multiplicities.4-7 As another subject, the molecular size of the radicals is not yet sufficiently clear, * Author to whom correspondence should be addressed. Fax: +8122-217-5612. E-mail:
[email protected]. (1) Uebersfeld, J.; E Ä tienne, A.; Combrisson, J. Nature (London) 1954, 174, 614. (2) Ingram, D. J. E.; Tapley, J. G.; Jackson, R.; Bond, R. L.; Murnagham, A. R. Nature (London) 1954, 174, 797-798. (3) Magnetic Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Y., Eds.; Advances in Chemistry; American Chemical Society: Washington, DC, 1993; No. 229. (4) Rothenberger, K. S.; Sprecher, R. F.; Gastellano, S. M.; Retcofsky, H. L. In Magnetic Resonance of Carbonaceous Solids; Botto, R. B., Sanada, Y., Eds.; Advances in Chemistry; American Chemical Society: Washington, DC, 1993; No. 229, pp 581-603. (5) Smirnov, T. I.; Smirnov, A. I.; Clarkson, R. B.; Belford, R. L. J. Phys. Chem. 1994, 98, 2464-2468. (6) Wie¸ ckowski, A. B.; Pilawa, B.; Lewandowski, M.; Wojtowicz, W.; Słowik, G. P. Appl. Magn. Reson. 1998, 15, 489-501. (7) Wie¸ ckowski, A. B.; Pilawa, B.; Wojtowicz, W.; Słowik, G. P.; Wachowska, H. Fuel 2001, 80, 451-453.
although the aromatic radicals, in which unpaired electrons are able to delocalize substantially, have been accepted as stable radicals in many carbonaceous solids.3,8 What is more, little is known about the change in the chemical structure of radical compounds during the aromatization reactions, though the radicals are an important reactive intermediate.9,10 The heterogeneity of carbonaceous solids and the single smoothing structure of EPR signals have mainly made it difficult to solve the unsettled questions given above. Pulsed EPR has a significant potential for enhancing the sensitivity and the frequency resolution of the spectrum.11-13 These spectroscopic improvements allow us to obtain detailed information about the structure and dynamics of paramagnetic species. In this article, we provide an overview of our recent studies that have been carried out using a pulsed EPR method in order to clarify the chemical structure and spin multiplicity of radicals in coal.14,15 The reaction process for the polymerization of CTP has also been studied by the pulsed EPR.16 We will begin with a short primer on the pulsed EPR spectroscopy and then discuss the results including newly obtained data in comparison with suitable model molecules. (8) Lewis, I. C.; Singer, L. S. In Chemistry and Physics of Carbon; Walker, P. L., Thrower, P. A., Eds.; Marcel Dekker: New York, 1981; Vol. 17, p 1. (9) Singer, L. S. Carbon 1978, 16, 409-415. (10) Singer, L. S.; Lewis, I. C. Carbon 1978, 16, 417-423. (11) Schweiger, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 265-292. (12) Keijzers, C. P.; Reijerse, E. J.; Schmidt, J. Pulsed EPR: A new field of applications; North-Holland: Amsterdam, 1989. (13) Dikanov, S. A.; Tsvetkov, Y. D. Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy; CRC Press: Boca Raton, 1992. (14) Ikoma, T.; Ito, O.; Tero-Kubota, S.; Akiyama, K. Energy Fuels 1998, 12, 996-1000. (15) Ikoma, T.; Ito, O.; Tero-Kubota, S.; Akiyama, K. Energy Fuels 1998, 12, 1363-1368. (16) Miyajima, N.; Akatsu, T.; Ikoma, T.; Ito, O.; Rand, B.; Tanabe, Y.; Yasuda, E. Carbon 2000, 38, 1831-1838.
10.1021/ef010148j CCC: $22.00 © 2002 American Chemical Society Published on Web 11/27/2001
Exploring Radicals in Carbonaceous Solids
Figure 1. Cw- and pulsed EPR spectra of Illinois No. 6 coal observed at room temperature. (a) Numerically integrated cwspectrum, (b) FID signal and its FT-spectrum, and (c) ESE signal and its FT-spectrum.
Pulsed EPR In pulsed EPR experiments, microwave (MW) pulses are utilized instead of continuous MW. PIN-diode switches usually form the MW pulses. In contrast to a continuous wave (cw) EPR spectrometer, a standard machine using a pulsed MW of the X-band is equipped with a resonator of low Q factor and a traveling wave tube amplifier for MW. Application of short intense MW pulses results in response signals such as free induction decay (FID) and electron spin-echo (ESE) from samples. The art of the pulsed EPR method is illustrated in Figure 1 with an example of the spectrum for Illinois No.6 (IL) coal, which is comprised of narrow and broad signals. Because of the dead time (τD) of the pulsed EPR instrument used, it was impossible to measure any FID and ESE signals within the τD time after the MW pulses. Therefore, the fast FID corresponding to the broad EPR spectrum due to vitrinite maceral cannot be detected. However, the Fourier transformed (FT) spectrum of the ESE that consists of two FIDs back to back17 generally agrees with the cw-EPR spectrum.14,18 One of the advantages of the pulse method is the capability of direct observation of the spin relaxation processes using various pulse trains. Although the time domain parameters concerned with the spin relaxation reflect the mobility of the radical species in coal,19,20 the concen(17) Mims, W. B. In Electron Paramagnetic Resonance; Geschwind, S., Ed.; Plenum: New York, 1972; pp 263-351. (18) Doetschman, D. C.; Dwyer, D. W. Energy Fuels 1992, 6, 783-792. (19) Ito, O.; Seki, H.; Iino, M. Bull. Chem. Soc. Jpn. 1987, 60, 2967-2978. (20) Ito, O.; Kakuta, T.; Iino, M. Carbon 1989, 27, 869-875.
Energy & Fuels, Vol. 16, No. 1, 2002 41
Figure 2. Schematic illustration of the motion of a magnetization vector corresponding to an electron spin in a rotating axis system (a) before and (b) after applying the MW pulse; the z-axis is parallel to the external magnetic field. The other axes are taken in the framework rotating around the z-axis at the same frequency as the MW.
tration of the radicals,18,21-24 and the adsorption of O2,25,26 we may leave the details of these topics to the original articles. As shown in Figure 2, the nutation behavior is a rotational motion of a magnetization vector caused by the interaction with the MW field (B1) in a rotating axis system. In the case of the weak limit of B1 compared with the zero-field splittings (ZFS), the nutation frequency (νn) for the EPR allowed transitions is analytically written as follows:27,28
νn ) γeB1xS(S + 1) - MS(MS - 1)
(1)
Here, γe is the magnetogyric ratio for electron spin; νn depends on the spin multiplicity (S), and the spin (21) Singer, L. S.; Lewis, I. C.; Riffle, D. M.; Doetschman, D. C. J. Phys. Chem. 1987, 91, 2408-2415. (22) Doetschman, D. C.; Mustafi, D.; Singer, L. S. J. Phys. Chem. 1988, 92, 3663-3669. (23) Silbernagel, B. G.; Gebhard, L. A.; Bernardo, M,; Thomann, H. In Magnetic Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Y., Eds.; American Chemical Society: Washington, DC, 1993; No. 229, pp 539-559. (24) Bowman, M. K. In Magnetic Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Y., Eds.; American Chemical Society: Washington, DC, 1993; No. 229, pp 605-626. (25) Kuppusamy, P.; Wang, P.; Zweier, J. L. Magn. Reson. Med. 1995, 34, 99-105. (26) Liu, K. J.; Miyake, M.; James, P. E.; Swartz, H. M. J. Magn. Reson. 1998, 133, 291-298. (27) Isoya, J.; Kanda, H.; Norris, J. R.; Tang, J.; Bowman, M. K. Phys. Rev. B 1990, 41, 3905-3913. (28) Astashkin, A. V.; Schweiger, A. Chem. Phys. Lett. 1990, 174, 595-602.
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Figure 4. MW pulse sequences for (a) 2D-nutation and (b) HYSCORE experiments.
Spin Multiplicity
Figure 3. Vector model of the electron (e) and nuclear (n) spins (a) in thermal equilibrium and (b) after applying the MW pulse. An adiabatic inversion of the e-spin results in a precessional motion of the n-spin and induces a periodic local field (Blocal) at the position of the e-spin.
To determine the spin multiplicity of the paramagnetic species in coals, as a standard sample, we employed an ethanol solution including several stable nitroxide radicals (S ) 1/2) and a biradical (S ) 1) of 2,6-bis(1′-oxyl-4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1′Himidazol-2′-yl)pyridine (abbreviated as bisimpy) in which
sublevels (MS) resonating with the applied MW. Hence, the nutation spectrum can identify S.29,30 In the disordered sample with a small anisotropy of ZFS, the magnetic inhomogenity such as the g, ZFS, or hyperfine strains may make the resonance lines too broad to observe obvious spectral structures. Because a similar situation is often encountered in heterogeneous carbonaceous solids, the application of the nutation technique is useful to separate the spectra of coexisting radicals with different spin multiplicities. The coupling between the electron and the nuclear spins can cause a modulation of the ESE intensity. The spectrum in the frequency domain obtained by FT of ESE envelope modulation (ESEEM) can be attributed to an alternative of the electron-nuclear double resonance (ENDOR) that enhances spectral resolution substantially. Figure 3a illustrates an electron spin (e) and a nuclear spin (n) under an external magnetic field (B0). The e-spin is almost quantized along B0, but the n-spin orients to the effective field (Beff) composed of B0 and the magnetic dipole field (Bhf) due to the e-spin. Bhf reverses its direction, which leads to a sudden change in Beff, when the resonance with the e-spin takes place due to a strong MW pulse (Figure 3b). As a consequence, the n-spin starts to precess along the new Beff. This periodic motion of the n-spin induces an oscillating local field (Blocal(t)) that modulates the intensity of ESE. This coherent interaction is a basic mechanism for the ESEEM phenomenon and is called a nuclear modulation effect. Hyperfine sublevel correlation (HYSCORE) spectroscopy used mainly in our study is one of 2D-ESEEM methods.31-33
a pyridine spacer links two imino nitroxide radicals. Because the nitroxide radicals were diluted to 10-4 mol/ dm3 in the ethanol solvent, the magnetic interaction between the nitroxide radicals can be negligibly small because of the long interradical distance. The magnetic interaction only within the bisimpy biradical becomes effective. The ground and excited states of bisimpy are the triplet (S ) 1) and the singlet (S ) 0), respectively, because the interradical interaction of the bisimpy biradical is ferromagnetic. Therefore, both the doublet and triplet species coexist in the mixed sample. The cwEPR signals of the nitroxide radicals and bisimpy biradical are overlapped around 345 mT corresponding to g ) 2.00 and apparently become a broad spectrum like that of coal. To demonstrate the usefulness of the nutation spectroscopy, we carried out two-dimensional (2D) nutation experiments for the model system mixed with the doublet and triplet species. The nutation oscillation was obtained by recording the intensity of the ESE as a function of the duration (tn) of the MW pulse that was applied before the last two pulses for readout (Figure 4a). The 2D spectrum can be obtained as a consequence of collecting the nutation data at individual external
(29) Mizuochi, N.; Ohba, Y.; Yamauchi, S. J. Phys. Chem. A 1997, 101, 5966-5968. (30) Sato, K.; Yano, M.; Furuichi, M.; Shiomi, D.; Takui, T.; Abe, K.; Itoh, K.; Higuchi, A.; Katsuma, K.; Shirota, Y. J. Am. Chem. Soc. 1997, 119, 6607-6613.
(31) Ho¨fer, P.; Grupp, A.; Nebenfu¨r, H.; Mehring, M. Chem. Phys. Lett. 1986, 132, 279-282. (32) Shane, J. J.; Ho¨fer, P.; Reijerse, E. J.; De Boer, E. J. Magn. Reson. 1992, 99, 596-604. (33) Ho¨fer, P. J. Magn. Reson. A 1994, 111, 77-86.
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Energy & Fuels, Vol. 16, No. 1, 2002 43
Figure 6. Molecular structure and spin density distribution of cation radical of naphthacene.
Figure 5. 2D-nutation spectra of (a) a mixed sample including several nitroxide radicals and a bisimpy biradical diluted in an ethanol glassy matrix and (b) Beulah Zap coal. The measurement temperature is 6 K.
fields.14
magnetic Figure 5a shows the 2D-nutation spectrum of the model sample observed in the ethanol rigid matrix at 13 K. The broad EPR spectrum was observed as shown in the sideview on the magnetic field, which is the same as the cw-EPR spectrum. However the spectrum is well resolved along the direction of the nutation frequency. Equation 1 indicates that νn for the triplet biradical is x2 times that of the doublet radical. According to this frequency ratio, it is understood that the main peak at νn ) 22.6 MHz is attributed to the nitroxide radicals and the weak signal at νn ) 32.0 MHz arises from the bisimpy biradical. Besides those nutation signals, an intense signal was also detected at the Zeeman frequency for 1H of νn ) 14.8 MHz, but this is irrelevant to the subject of concern here. The sharp line is not due to the nutation of electron spin. This line originates from the nuclear modulation effect of many
protons in the ethanol matrix surrounding the radical and biradical molecules, which is namely a spin-locked ESEEM evolved during the long MW pulse for nutation.34,35 We should note here that the 2D-nutation spectroscopy can separate the EPR signals having different spin multiplicities. Figure 5b depicts the 2D-nutation spectrum for Beulah Zap (BZ) coal measured at low temperature. The clear signals at νn ) 14.8 and 23.6 MHz were detected over the full range of the magnetic field. From the comparison with the model system, it is obvious that the observed lower frequency of 14.8 MHz is the 1Hmatrix peak and that the higher frequency of 23.6 MHz corresponds to the doublet state. No signal is detected at 33.4 MHz ()23.6 MHz × x2), which is the nutation frequency for the triplet state in this case. Furthermore, we could not observe any clear signals due to the higher multiplet states in the temperature range from 4 to 300 K. Similar 2D-nutation spectra were also observed for the other seven premium coals. The 2D-nutation experiments indicate that the EPR signals around g ) 2.00 obtained below room-temperature arise from isolated radicals rather than radical pairs or spin clusters. This conclusion is in good agreement with the results reported by Doetschman and Mustafi.36 Molecular Size As shown in Figure 6, naphthacene consists of four benzene rings that are linearly condensed; it has been reported that its molecular size is comparable to the most probable aromatics in coal.37,38 We therefore adopted the cation radical of naphthacene prepared in a concentrated H2SO4 solution. In analogy with coal radicals, the naphthacene radical also gave a broad EPR spectrum under low temperatures, because of the anisotropic hyperfine interactions of many R-protons, which make it difficult to resolve hyperfine splittings. On the other hand, the HYSCORE measurements were able to (34) Zhong, Y. C.; Pilbrow, J. R. Chem. Phys. Lett. 1994, 222, 592596. (35) Zhong, Y. C.; Pilbrow, J. R. J. Magn. Reson. A 1994, 110, 245247. (36) Doetchman, D. C.; Mustafi, D. Fuel 1986, 65, 684-693. (37) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187-193. (38) Kidena, K.; Murata, S.; Nomura, M.; Artok, L. J. Jpn. Inst. Energy 1999, 78, 869-876.
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Figure 7. HYSCORE spectra for (a and b) naphthacene cation radical in concentrated H2SO4 at 30 K and (c and d) Pocahontas coal at room temperature. The τ values between the first two pulses of a, b, c, and d are 32, 128, 32, and 128 ns, respectively.
extract the hyperfine signals from the broad EPR spectra by virtue of the nuclear modulation effect, as shown in Figures 7a and 7b. The HYSCORE measurements were basically performed by a modified threepulse ESEEM sequence (PI-τ-PII-t1-Pmix-t2-PIII) where a mixing MW pulse (Pmix) was inserted between the second and third pulses (Figure 4b).15 The nuclear modulation effect can be observed by recording the ESE intensity as a function of both the t1 and t2 times. However, the nuclear modulation effect in the threepulse ESEEM method also depends on the τ time, which is the pulse interval between the first two pulses.13 This unavoidable τ dependence in the HYSCORE spectroscopy results in suppression of the hyperfine spectrum and is called a blind spot effect. At τ ) 32 ns in which the blind spot effect causes little distortion of the 1H-hyperfine signals,33 the broad spectrum of 1H was observed along the ν2 ) 2νH - ν1 line (Figure 7a). νH is the proton Zeeman frequency of 14.8 MHz. The sharp signal at (νH, νH) is attributed to
a 1H-matrix peak mainly due to the protons of H2SO4 aqueous solution as solvent. On both sides of the 1Hmatrix peak, there are two pairs of signals split by small and large hyperfine interactions of protons. We have estimated the hyperfine splittings of the ring protons in the naphthacene cation radical, based on the spin density distribution by a semiempirical molecular orbital (MO) calculation using the INDO-UHF method. The inner signals in the HYSCORE spectrum come from the eight protons bound at the two end rings in naphthacene. The outer hyperfine signals arise from the four protons at the two inner rings having a high spin density. On the other hand, the 13C-signal was not observed for the naphthacene cation even on employing τ ) 128 ns, which is an optimized time for the 13Chyperfine coupling (Figure 7b). In the case of τ ) 128 ns, the 1H-hyperfine spectrum is distorted by the blind spot effect and the signals due to the protons at the inner rings are enlarged. Many carbons having a π spin density lower than 0.04 are necessary to indicate the
Exploring Radicals in Carbonaceous Solids
Energy & Fuels, Vol. 16, No. 1, 2002 45
13C-signal
in the HYSCORE spectrum. Hence no detection of the 13C-signal indicates that the molecular size of naphthacene is not large enough to possess many carbons with such a low spin density. Figure 7c shows the typical 1H-hyperfine spectrum obtained by employing τ ) 32 ns for the radicals in Pocahontas (PH) coal. The other seven premium coals also presented similar 1H-spectra measured with τ ) 32 ns.15 A sharp 1H-matrix peak is observed at (14.7 MHz, 14.7 MHz). The other peaks on the frequency diagonal of ν2 ) ν1 result from the coherent electric noise of the spectrometer. The broad 1H-hyperfine signal perpendicular to the diagonal ν2 ) ν1 line was clearly detected from 9 to 20 MHz through the matrix peak. As the projection spectra indicate clearly, the 1Hhyperfine spectrum of coal radicals does not possess an obvious structure. This spectroscopic feature is different from those of homogeneous model radicals such as naphthacene. The intensity of the hyperfine signal decreases gradually on going out from the central matrix position to both the lower and higher frequency sides. This finding suggests that various radicals with 1Hhyperfine coupling constants smaller than ca. 10 MHz (0.36 mT) coexist in coal. The contour plot of the hyperfine signals shows an almost linear narrow ridge situated along the line of ν2 ) 2νH - ν1 in the twodimensional spectrum, which cuts the frequency diagonal ν2 ) ν1 at the 1H-Zeeman frequency of (νH, νH). The HYSCORE spectrum tends to shift toward the higher frequency from the ν2 ) 2νH - ν1 line if the dipolar hyperfine interaction is large.39-42 Hence, the observed shape of the 1H-hyperfine spectrum giving a straight thin ridge indicates that coal radicals should have different types of 1H-hyperfine coupling constants with relatively small anisotropy. In contrast to the naphthacene radical, the 13C-signal of coal radicals appeared together with a part of the 1Hsignals in the HYSCORE spectrum measured with τ ) 128 ns (Figure 7d). The series of correlation peaks in the off diagonal around (3.7 MHz, 3.7 MHz) are identified as the 13C-hyperfine spectrum, because the Zeeman frequency of the 13C nucleus (νC) is 3.7 MHz at the magnetic field for measurement of this spectrum. The contour map for the 13C-hyperfine signals shows both the bending and straight spectral ridges. The straight ridge on the ν2 ) 2νC - ν1 line centered at (νC, νC) is realized to result from a variety of radicals with distinct hyperfine splittings. On the other hand, the deviation of the hyperfine signal from the ν2 ) 2νC - ν1 line is interpreted in terms of the anisotropic hyperfine interaction with 13C nuclei. The hyperfine parameter |A|| of 13C is estimated to be 3 MHz from the maximum splitting of the bent spectrum. This value is a reasonable magnitude as a principal value of the parallel hyperfine interaction of 1% spin density in an atomic p orbital of a 13C atom,43 assuming that the unpaired electrons are delocalized in the π-conjugated molecular framework of coal radicals. (39) Reijerse, E. J.; Dikanov, S. A. Pure Appl. Chem. 1992, 64, 789-797. (40) Dikanov, S. A.; Bowman, M. J. Magn. Reson. A 1995, 116, 125-128. (41) Po¨ppl, A.; Kevan, L. J. Phys. Chem. 1996, 100, 3387-3394. (42) Szosenfogel, R.; Goldfarb, D. Mol. Phys. 1998, 95, 1295-1308. (43) Morton, J. R.; Preston, K. F. J. Magn. Reson. 1978, 30, 577-582.
Figure 8. Correlation diagram between the elemental analytical data (13C/1H (Ratom)) and the intensity ratios of the HYSCORE spectra observed with τ ) 128 ns (13C/1H (RHYS)). BZ, Beulah Zap; WA, Wyodak Anderson; IL, Illinois No. 6; BC, Blind Canyon; LS, Lewis Stockton; PB, Pittsburgh; UF, Upper Freeport; PH, Pocahontas.
It is noteworthy that the simultaneous observation of the hyperfine spectra for 1H as well as 13C at τ ) 128 ns enables us to compare the amounts of 13C and 1H included in coal radicals, although the 1H-hyperfine signal is significantly suppressed around the 1H-matrix frequency of (νH, νH) by the blind spot effect. The main features of the 1H- and 13C-hyperfine spectra for the other seven premium coals resembled those of PH coal.15 However, the relative intensity (13C/1H) of the 13Chyperfine signals to the 1H-hyperfine signals varied with the coals. As shown in Figure 8, the 13C/1H ratio of HYSCORE (RHYS) increases with the 13C/1H atom ratio of the coal (Ratom). The connection between them can be regressed by an almost linear relationship of RHYS ) 40Ratom - 0.35. The good correlation between the RHYS and Ratom ratios can be explained by the fact that the carbons having a π spin density lower than 0.04 increase in number with the rank of the coal. This result leads to the conclusion that the molecular size for the statistically major coal radicals becomes large in high-rank coal. The signal intensity of carbon is very sensitive to the molecular geometry because of the small natural abundance of 13C (1.11%). The probability of detecting the 13C signal becomes high as the number of equivalent carbons in a molecule increases. The high molecular symmetry of pericondensed aromatics such as coronene and perylene is superior to the catacondensed polyacene in regard to equivalent carbons. Figure 8 shows that the RHYS values for the coals are either comparable to or greater than those of perylene and coronene radicals. This fact supports that two-dimensionally condensed molecules are major in coal radicals. With respect to the coals indicating the very high RHYS values, a specific molecular association may take place around the radicals in the macromolecular structure of the coal. The unpaired electron can be distributed between aromatic molecules connected by a noncovalent interaction like
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Figure 9. 13C-hyperfine spectra obtained by projecting the HYSCORE signals of 13C for (a) as-received CTP and CTP iodine-treated for (b) 12 h, (c) 24 h, and (d) 72 h.
a π-π complex; therefore, the increase in 13C-hyperfine signals is expected for the strongly associated large molecules. A closer discussion on the correlation between RHYS and Ratom is difficult, because only several model samples exhibit the 13C-signal because of their relatively small molecular size. However, the increase in the RHYS is interpreted at least by the increase in the carbon content of radical molecules in the coal. Reactive Center Most of the useful carbon materials are synthesized from CTP, which is mainly a mixture of polycyclic aromatic hydrocarbons with a wide molecular weight distribution. Introduction of iodine into CTP at a temperature near the softening point of CTP is especially effective in increasing the carbon yield and also changes the optical texture of the carbonized substance from a flow type to isotropic.44 The interaction between halogen molecules and aromatic hydrocarbons in CTP can form a charge-transfer (CT) complex that plays an important role in the carbonization of CTP. In fact, the EPR signals due to the organic cation radicals produced from the electron transfer reaction with iodine drastically increased in intensity with the treatment time.16 Although the cation radicals gave a single cw-EPR spectrum without structure, we succeeded in detecting the 1H- and 13C-hyperfine spectra separately by the HYSCORE spectroscopy. Figure 9 depicts the 13C-hyperfine spectra obtained by the HYSCORE method measured with τ ) 128 ns. In the I2-treated CTP, the signals in the range of 2.85.1 MHz increase in intensity relatively. The additional 13C-hyperfine signals near ν are assigned to the cation C radicals in the CT complex. The signals close to νC originate from the radicals with smaller hyperfine coupling constants, indicating the efficient delocalization (44) Kajiura, H.; Tanabe, Y.; Yasuda, E. Carbon 1997, 35, 169-174.
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of unpaired electrons in the cation radicals. The MO calculation suggested that the aromatic components giving the intense 13C-signals with the splitting below 2 MHz should have 10 or more benzene rings. These aspects imply that the cation radicals of CT complexes consist of aromatic compounds having molecular sizes larger than the average radicals in the as-received CTP. Although iodine molecules are able to penetrate the CTP uniformly, the CT-complexation seems to take place preferentially with the larger aromatic components of CTP. This is probably due to the difference in oxidation potential in the various constituent molecules. In fact, recently, Sasaki et al. have found selective complexation in which doping with iodine preferentially forms the CT complexes with basic nitrogen compounds and five- or six-ring aromatic hydrocarbons included in coal or CTP.45-47 The effective polymerization of the CTP was also observed during the iodine treatment at 373 K. It has been therefore concluded that the neutral radicals were formed due to deprotonation of the cation radicals with relatively large aromatics in the CT complexes, which promoted the polymerization of the CTP.16 Summary and Outlook We have studied the structures of the radicals in eight Argonne premium coals and the reactive species in the iodine-treated CTP using pulsed EPR spectroscopy. The 2D-nutation spectroscopy elucidated that the broad cwEPR spectra of the coals observed below room-temperature arose from free radicals with doublet spin multiplicity, excluding triplet radical pairs. The broad hyperfine spectra of 13C and 1H could be observed separately by employing HYSCORE methods. It is found that the intensity ratio of 13C/1H in the HYSCORE spectrum with τ ) 128 ns shows a good correlation with the atomic percentage of coal, which can be used as a new index for characterization of radicals in carbonaceous solids. From the comparison with model polycondensed aromatic radicals and the MO calculations, we reached the conclusion that two-dimensionally π conjugated molecules such as coronene and/or even larger aromatic molecules constitute the majority of coal radicals. The average molecular size of the coal radicals is larger than that for the most probable aromatic compounds in coal. The peculiarity of the molecular size for radicals might be associated with the instability of radicals with small size and/or a specific structure such as a π-π stacking between aromatic compounds. Further research on the molecular association would elucidate the reason for the high 13C/1H ratio of radical molecules. In addition, the HYSCORE method has clarified the chemical structure of the CT complex that functions as an important reactive center for dehydrogenative polymerization of the I2-treated CTP. The HYSCORE method suggests the great possibility of the pulsed EPR spectroscopies in investigating the solid-state reactions of carbonaceous materials such as mesophase formation, pyrolysis and so on. (45) Sasaki, M.; Sanada, Y. J. Jpn. Pet. Inst. 1991, 34, 218-233. (46) Keneko, T.; Sasaki, M.; Yokono, T.; Sanada, Y. In Magnetic Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Y., Eds.; American Chemical Society: Washington, DC, 1993; No. 229, pp 530-538. (47) Sanada, Y.; Kumagai, H.; Sasaki, M. Fuel 1994, 73, 840-842.
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Acknowledgment. The authors thank the collaborators who have contributed to the series of studies and whose names are given in the list of references. The authors are also grateful to Professor M. Iino (Tohoku University), Dr. H. Kumagai (Hokkaidou University), and Dr. K. Norinaga (Tohoku University) for their encouragement and valuable comments in writing this article. Special thanks are due to Professor H. Oshio
Energy & Fuels, Vol. 16, No. 1, 2002 47
(University of Tsukuba) and Mr. M. Yamamoto (Tohoku University) for providing us with the nitroxide radicals and the biradical. This work was partially supported by a Grant-in-Aid of Scientific Research (No. 12740310) from the Japan Ministry of Education, Science, Sports, and Culture. EF010148J
