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Energy & Fuels 1998, 12, 996-1000
Two-Dimensional EPR Spectroscopic Studies on the Radicals in Argonne Premium Coals Tadaaki Ikoma,* Osamu Ito, Shozo Tero-Kubota, and Kimio Akiyama Institute for Chemical Reaction Science, Tohoku University, Katahira 2-1-1, Aobaku, Sendai 980-8577, Japan Received March 17, 1998. Revised Manuscript Received May 18, 1998
Two-dimensional electron paramagnetic resonance (2D-EPR) spectra of Illinois No. 6 and Upper Freeport coals have been measured. From 2D-nutation spectroscopy, it is confirmed that the main EPR signal around g ) 2 is attributed to free radicals, but not to radical pairs. Both coals showed nuclear modulation effects due to1H and naturally abundant 13C nuclear spins. Spinecho correlation spectroscopy (SECSY) experiments elucidate the distribution of modulation frequencies included in a broad EPR spectrum. The observed dip in the 13C matrix spectrum obtained from SECSY is interpreted in terms of the decrease of vitrinite radicals, which are weakly interacting with 13C nuclei, in the central region of the EPR spectrum. The analysis of the broad hyperfine spectra due to 1H and 13C nuclei in the hyperfine sublevel correlation spectra suggests the existence of several kinds of radicals in coal. Most of unpaired electrons are considered to be trapped in aromatic rings. The ratio of 13C/1H of hyperfine spectra increases with coal rank, suggesting a new index of coal rank.
Introduction Electron paramagnetic resonance (EPR) spectroscopy has played an important role in coal characterization since the first EPR spectrum of coal was observed.1,2 The observed EPR signals are mainly associated with naturally occurring free radicals and/or electron donoracceptor complexes,3,4 while paramagnetic inorganic species such as transition metal ions are also present in coal. The characterization of the paramagnetic species in coal is a challenging problem. Therefore, many studies of this subject have been carried out by conventional continuous wave EPR (cw-EPR), multiple frequency EPR, pulsed EPR, and electron nuclear double resonance (ENDOR).5 Pulsed EPR spectroscopy has a significant potential for enhancing the frequency resolution of the spectrum, the time resolution in observing a short-lived species, and the sensitivity.6-8 The two-dimensional (2D-) EPR is one of the new pulsed EPR methods. This technique makes interpretation of spectra easier and enables one to obtain detailed information about the structure and dynamics of the paramagnetic species. To characterize the paramagnetic species in coal, we have applied 2D-EPR spectroscopic techniques such as (1) Uebersfeld, J.; Etienne, A.; Combrisson, J. Nature (London) 1954, 174, 615. (2) Ingram, D. J. E.; Tapley, J. G.; Jackson, R.; Bond, R. L.; Murnagham, A. R. Nature (London) 1954, 174, 797. (3) Duber, S.; Wieckowski, A. B. Fuel 1984, 63, 1474. (4) Doetschman, D. C.; Mustafi, D. Fuel 1986, 65, 684. (5) Botto, R., E., Sanada, Y., Ed. Magnetic Resonance of Carbonaceous Solids; Advances in Chemistry Series 229; American Chemical Society: Washington, DC, 1993. (6) Schweiger, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 265. (7) Keijzers, C. P., Reijerse, E. J., Schmidt, J., Ed. Pulsed EPR: a new field of applications; North-Holland: Amsterdam, 1989. (8) Dikanov, S. A.; Tsvetkov, Y. D. Electron Spin-Echo Envelope Modulation (ESEEM) Spectroscopy; CRC Press: Boca Raton, FL, 1992.
2D-nutation, spin-echo correlation spectroscopy (SECSY), and hyperfine sublevel correlation spectroscopy (HYSCORE) methods. The electron spin-echo (ESE)detected transient nutation method is useful for determining the effective spin multiplicity of the paramagnetic species in a disordered sample.9-11 It has been pointed out that charge-transfer complexes with triplet multiplicity contribute to the inhomogeneous EPR signals in coal.3,4 In the present study, we examined the effective spin multiplicity for the broad EPR signal around g ) 2 using the 2D-nutation technique. The nutation spectra were observed as a function of the magnetic field. The data for SECSY are obtained by recording the ESE shape (t2) as a function of the time interval (t1) between the two microwave (MW) pulses. The timedomain spectra are transformed into the frequencydomain spectra, giving the usual EPR spectrum along the f2-axis and the ENDOR spectrum along the f1-axis, respectively. This method provides the distribution of nuclear modulation frequencies included in an inhomogeneous EPR spectrum. HYSCORE is a 2D-ESE envelope modulation spectroscopy modifying a three-pulse stimulated echo method by insertion of a mixing pulse of 180° between the second and third pulses.12 It has been shown that this experiment offers an excellent resolution of the hyperfine (hf) splitting even in disordered systems.13,14 The (9) Isoya, J.; Kanda, H.; Norris, J. R.; Tang, J.; Bowman, M. K. Phys. Rev. B 1990, 41, 3905. (10) Isoya, J.; Kanda, H.; Uchida, Y. Phys. Rev. B 1990, 42, 9843. (11) Mizuochi, N.; Ohba, Y.; Yamauchi, S. J. Phys. Chem. A 1997, 101, 5966. (12) Ho¨fer, P.; Grupp, A.; Nebenfu¨r, H.; Mehring, M. Chem. Phys. Lett. 1986, 132, 279. (13) Shane, J. J.; Ho¨fer, P.; Reijerse, E. J.; De Boer, E. J. Magn. Reson. 1992, 99, 596.
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Figure 1. The pulse sequences for 2D-nutation (a), SECSY (b), and HYSCORE (c). In each of the three experiments, a MW power of 1 kW was adequately attenuated to create 90° pulses of the following durations: 32, 8, and 16 ns, respectively. The integration of ESE, recording of the waveform of ESE, and detection of the ESE height were carried out for the 2D-nutation, SECSY, and HYSCORE, respectively.
hf splittings due to 13C and 1H were clearly observed in the present coals at room temperature. It was found that the relative signal intensity of 13C and 1H reflects the character of the coal. Experimental Section The two types of coals, Illinois No. 6 (IL) and Upper Freeport (UF), among eight Argonne premium coals, were employed for the 2D-EPR measurements. These coals were donated from the Argonne National Laboratory through the Argonne Premium Coal Program. Approximately 5 mg of coal was carefully transferred to the quartz sample tubes under a nitrogen atmosphere for minimum exposure to air. The samples were degassed at room temperature overnight and were sealed in the quartz tubes. All measurements were carried out with an X-band pulsed EPR spectrometer (Bruker ESP-380E). A dielectric resonator with a low Q-factor of about 100 and a 1 kW TWT amplifier were employed. This system has a dead time of ca. 88 ns. Unwanted signals in a detection period were eliminated by suitable phase rotations of the MW pulses. All EPR spectra were recorded at room temperature. The MW pulse sequences for the 2D-EPR spectroscopy used in the present paper are shown in Figure 1. For the nutation experiment, the total intensity of the two-pulse ESE signals S(t1) was detected by incrementing the width (t1) of the nutation pulse. Relatively long pulse widths (32 and 64 ns) were used, and the MW power was adjusted to maximize the ESE intensity. These two pulses served as 90 and 180° pulses for the selective excitation of the spin packet in the inhomogeneously broadened spectrum.15 Instead, the signal S(t1, B0) was measured as a function of the external magnetic field B0. Therefore, the off-resonance effects due to the finite bandwidth of the MW pulse were eliminated. The S(t1, B0) is converted into a frequency domain S(f1, B0) spectrum by Fourier transform (FT) along the t1 direction. As shown in Figure 1b, the echo shape S(t1, t2) was measured as a function of the pulse separation of t1 in the SECSY experiment. Two MW pulses with a flip angle of 90° (8 ns) were applied to excite the entire EPR spectrum. Two times of FT for S(t1, t2) were carried out after removing the background envelope along the t2 time domain by subtraction using a polynomial function. The frequency domain spectrum S(f1, f2) is depicted in the form of contour plots in which the projections to the orthogonal axes give the EPR and ENDOR spectra. (14) Po¨ppl, A.; Kevan, L. J. Phys. Chem. 1996, 100, 3387. (15) Kevan, L., Bowman, M. K., Ed. Modern Pulsed and ContinuousWave Electron Spin Resonance; John Wiley & Sons: New York, 1990.
Figure 2. The two-pulse ESE-FT spectrum (a), the cw-EPR spectrum in derivative form (c), and the numerically integrated spectrum (b) of c for UF coal. The time width for two MW pulses is 8 ns, and the interpulse separation is 88 ns. In the HYSCORE experiment consisting of 4 pulses, the pulse widths of 16 and 32 ns were applied for the 90 and 180° pulses, respectively (Figure 1c). The 2D-time-domain signals S(t1, t2) were converted into 2D-frequency-axes by a double FT after the subtraction of the unmodulated components from the original data. The pulse interval (τ) between the first and second pulses was fixed at 120 ns, where we can simultaneously observe broad hf spectra due to 1H and 13C.
Results and Discussion ESE FT- and 2D-Nutation Spectra. Figure 2a shows the ESE-detected FT-EPR spectrum of UF coal. The spectrum was obtained from the FT of the later half of the ESE generated by two MW pulses of 8 ns. It is clear that the ESE-detected FT-EPR spectrum is in good agreement with the numerically integrated cw-EPR spectrum (Figure 2b). This result means that the 8 ns MW pulses effectively excite the entire spectrum of the radicals in coal. The observed spectra clearly indicate the overlap of the broad (∆ω ) 34 MHz) and narrow (∆ω ) 6 MHz) spectra, where ∆ω is the line width at half-height. These spectra have been assigned to the signals associated with vitrinite and inertinite macerals, respectively, from the cw-EPR experiments on separated maceral samples.16,17 The ratio between these macerals depends on the character of the coal. In the case of IL coal, the amount of the inertinite maceral is less than that for UF. The ESE-detected FT-EPR experiment on IL coal gave the same result as the cw-EPR measurements. On the other hand, the free induction decay (FID)-detected FT-EPR method did not show the broad EPR signal of the vitrinite maceral in coal. This is due to the fast decay of the broad EPR spectrum. (16) Silbernagel, B. G.; Gebhard, L. A.; Dyrkacz, G. R.; Bloomquist, C. A. A. Fuel 1986, 65, 558. (17) Thomann, H.; Silbernagel, B. G.; Jim, H.; Gebhard, L. A.; Tindall, P.; Dyrkacz, G. R. Energy Fuel 1988, 2, 333.
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Figure 3. The field swept nutation spectrum of IL coal. The time width for the nutation pulse was varied from 0 to 4080 ns. The arrows indicate the nutation frequencies for the doublet and triplet state species (7.08 and 10.01 MHz) estimated from a comparison with DPPH of S ) 1/2.
Figure 3 depicts the 2D-nutation spectrum of IL coal observed as a function of B0. The nutation frequency (f1) was obtained at 7.08 MHz over the full range of the magnetic field. The same nutation frequency was also observed for UF coal. The value of 7.08 MHz was the same as that for the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical of S ) 1/2. The results indicate that the EPR signals around g ) 2 of the coals arise from isolated radicals. We could not detect any trace of the radical pair or charge-transfer complexes with triplet multiplicity, for which the nutation frequency is predicted to be 7.08 × x2 MHz.11 These facts agree with the results reported by Doetschman et al.4 SECSY. The SECSY spectra of IL and UF coals are shown in Figure 4. In both coals, the nuclear modulation effects of about 5% of the ESE intensity were observed. The summed-up projection spectra to the axes of the offset frequency (f2) and the ENDOR frequency (f1) correspond to the conventional 1D-ESE-FT-EPR and ENDOR spectra, respectively. In these projected ENDOR spectra, the peaks appeared at 14.8 and 3.7 MHz due to the Larmor frequencies of the 1H and 13C nuclei, respectively. These signals can be assigned to the matrix peaks. The sum peak of the 1H matrix line was also detected at 29.6 MHz. The other broad band was recognized around 10 MHz. Though we cannot exactly identify the origin of the broad band, it may be the series of hf peaks due to some protons or their combination lines which are often detected as broad signals in the two-pulse ESEEM spectra for randomly oriented systems.8 The spectra projected to the offset frequency axis, which corresponds to the magnetic field in a conventional EPR spectrum, indicate that in IL coal the inertinite maceral content giving a narrow signal is smaller than that in UF coal. It is clear from the spectra projected to the ENDOR frequency axis that the relative
Figure 4. The contour maps of SECSY for IL (a) and UF (b) coals. The projection spectra on the f1- and f2-axes were calculated by summation over the entire range.
signal intensities of the 13C and 1H matrix peaks are very different between these coals. This fact implies that the content of naturally abundant 13C interacting with the electron spin of the radicals is smaller in IL coal than in UF coal. The distribution of the ENDOR frequency can be represented as a contour map. It is very interesting that the distribution depends on the character of coal. In IL coal, the 1H matrix line spreads over a wide range of the f2-axis, while the 13C matrix line is situated in a smaller region. In UF coal, on the other hand, the 13C matrix line is found over the entire range along the f2axis. This is the reason the 13C matrix peak in UF coal shows a strong integrated intensity in the projected spectrum. It is noted that the broad band around 10 MHz of the f1-axis exists only in the high-frequency region of f2 in both coals. The intensity of the matrix peak directly reflects the number of weakly interacting nuclei. Therefore, we carefully examined the spectra centered at the positions of the 1H and 13C matrix peaks. The 13C matrix spectrum of IL coal acquired at 3.7 MHz of f1 represents an interesting shape (Figure 5a); the spectrum shows a dip at the central part. The full-width at half-height of ca. 10 MHz for the dip is bigger than that of 6 MHz for the sharp EPR spectrum related to the inertinite maceral. Moreover, the depth of the dip is larger than the height of the sharp component. These facts indicate that the dip originates from the radicals in the vitrinite maceral rather than inertinite. This is supported by the observation of the sharp line due to the inertinite
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Figure 5. The 13C matrix spectra for IL (a) and UF (b) coals obtained from a slice of the SECSY spectra at f2 ) 3.7 MHz corresponding to the 13C Larmor frequency. The shadow peak in b is attributed to an inertinite radical.
radicals on the dip in the 13C matrix spectrum of UF coal (Figure 5b). The dip around the central part of the 13C matrix spectrum of coal is similar to the magnetic field dependency of the depth of 13C modulation in the three-pulse echo experiments.18 It has been reported that the 13C frequency signal is more intense on the edges of the EPR spectrum of radicals. Three possibilities for the origin of the dip are considered: a fast spin relaxation process, an alternation of the depth parameter, or a decrease in the number of interacting nuclei. The last factor is most likely in this case. If the spectrum of the 1H matrix peak showed a similar dip to that in the 13C spectrum, a certain fast relaxation process such as spectral or instantaneous diffusion only around the center of EPR spectrum would become the most probable candidate for the dip.19 However, there was no dip in the 1H matrix spectrum. The change in a modulation depth parameter in a specific spectral region seems to be unreasonable because a matrix peak is insensitive to the orientation of the external field. Therefore, we concluded that the weakly interacting vitrinite radicals with 13C decreased and the radicals with a small hf coupling constant increased around the center of the EPR spectrum. This consideration was corroborated by the well-separated 13C hf splitting of 2.2 MHz that appeared in the ENDOR spectrum obtained from the cross section at the offset frequency of the dip position (f2 ≈ 0 MHz). HYSCORE. The HYSCORE spectra of the IL and UF coals are shown in Figure 6. The broad hf signals due to the 1H and naturally abundant 13C clearly appeared in the spectra. The hf signals exist around 4, 12, and 18 MHz in the 1D-projection spectrum. These signals can be distinguished as two different series of peaks in the off-diagonal of the 2D-representation, which are approximately situated along the frequency diagonal BD. One ridge crosses the frequency diagonal AC around (3.7 MHz, 3.7 MHz), which is the position of the 13C Larmor frequency. Another ridge intersects the location corresponding to the 1H matrix (14.8 MHz, 14.8 MHz). From these cross positions, the former spectrum is regarded as the hf spectrum due to 13C, and (18) Snetsinger, P. A.; Cornelius, J. B.; Clarkson, R. B.; Bowman, M. K.; Belford, R. L. J. Phys. Chem. 1988, 92, 3696. (19) Doetschman, D. C.; Dwyer, D. W. Energy Fuel 1992, 6, 783.
Figure 6. The HYSCORE spectra of IL (a) and UF (b) coals. The projections on the f1- and f2-axes are the skyline spectra. Inset shows the contour map of the 13C hyperfine spectrum of IL coal. The short arrows in the maps indicate the crossing positions of two different types of spectra due to 13C.
the latter is attributed to the 1H hf spectrum. The imperfect excitation by the mixing pulse and artifacts from the spectrometer caused the diagonal peaks on the line AC, such as the intense peak at (15.8 MHz, 15.8 MHz). The 1H hf spectrum has a ridge that extends straight in the BD direction and shows a narrow width in the AC direction. The 1H hf spectra of the IL and UF coals are similar to each other. These characteristics can hardly be interpreted in terms of a large anisotropy of the hf interactions for a certain radical since the HYSCORE spectrum of a powder sample has a tendency to bend with increasing anisotropic hf interaction.14,20 The observed 1H hf spectrum can be explained by the existence of radicals with various sizes of isotropic hf interactions and small anisotropic interactions. The disappearance of the broad 1H hf spectrum near the 1H matrix position (14.8 MHz, 14.8 MHz) is caused by the blind spot effect at τ ) 120 ns. Hence, the suppression of the hf signal does not correspond to a decrease in the radical with a very small coupling constant. The width of the 1H hf spectrum reaches about 13 MHz. This value is close to the maximum of an isotropic hf coupling (20) Ho¨fer, P. J. Magn. Reson. 1994, A111, 77.
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constant for a radical compound with a few aromatic rings. Most of the coupling constants for R or β protons of aliphatic radicals, in which an unpaired electron localizes on a carbon nucleus, are larger than 30 MHz. Therefore, the majority of the radicals in these coals are expected to be aromatic molecules with a few aromatic rings. The 13C hf spectrum seems to consist of two types of ridges. There is a straight line component parallel to the line BD as in the 1H hf spectrum and a bent component behind the straight spectrum. The 13C hf spectrum showed intensive peaks at the crossing positions of the straight and bent components as indicated by arrows in Figure 6a. The separation between these peaks is close to the 13C hf splitting observed by the SECSY experiment. While the straight spectrum can be formed from a variety of radicals with distinct hf splittings of 13C, the bent component is explained by the existence of radicals such as a polycyclic aromatic compound which keeps many similar carbon atoms within a molecular framework. The small dipolar hf parameter A⊥ of 1.2 MHz is estimated from the maximum vertical distance of the bent component from the straight component.14 This hf value corresponds to an approximate 1% spin density, based on the assumption of the anisotropic hf interaction between the 13C nuclear spin and the electron spin in the 2p orbital of the same carbon. The apparent hf splitting for the 13C was estimated to be less than 8 MHz from the HYSCORE spectrum. These values for the 13C hf parameters also belong to the category of the smaller hf coupling constant even among the aromatic radicals. Hence, we regarded the observed radicals as aromatic radicals holding unpaired π electrons delocalized over a few condensed rings. The 13C hf spectrum of UF increases the intensity in comparison with the IL coal. The 13C bent component of the UF coal became more intense than the IL coal. These results strongly suggest the growth of a number of larger aromatic radicals in the UF coal, in which the unpaired electrons are able to delocalize even more effectively. It is consistent with the fact that the aromatization of carbons increases in the UF coal. The comparison between intensities of the 13C and 1H hf spectra is very interesting from the viewpoint of coal science because the hf spectrum exhibits interactions with the magnetic nuclei of a radical moiety rather than the surrounding molecules. However, a single 2DHYSCORE spectrum, which is obtained at a certain τ value, is unfortunately distorted by the blind spot effect
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so that the intensity of the hf spectrum does not immediately manifest the number of magnetic nuclei interacting with an unpaired electron. This spectroscopic problem can be, in principle, resolved by the three-dimensional HYSCORE method,20 although the measurement takes too long a time to be useful for the case of coal. The ratio of the 13C signal to the 1H signal (13C/1H) in a single 2D-HYSCORE spectrum, nevertheless, is practically useful for the investigation of the relative changes in the radical structure among various coals. The decrease in the HYSCORE signal due to the blind spot effect is roughly regarded as leaving some radicals out of consideration. In the case of τ ) 120 ns, the 1H with hf coupling constants smaller than about 5 MHz was neglected. In other words, the ratio of 13C/ 1H is slightly enhanced for 13C. The ratios of 13C/1H were calculated as 0.14 for IL coal and 0.28 for UF coal from the HYSCORE spectra in Figure 6. The ratio 13C/ 1H increases with coal rank, and the variation in the ratio indicates the difference in the mean structures for many radical molecules in coals. Summary We have studied the radicals in IL and UF coals using 2D-EPR spectroscopy. It is confirmed by 2D-nutation spectroscopy that the broad cw-EPR spectrum around g ) 2 arises from free radicals excluding triplet radical pairs. The two-pulse SECSY measurements of these coals established that the matrix spectra of 13C and 1H depend on the offset frequency. The depression of the 13C matrix spectrum at the central part is interpreted as the decrease in the number of weakly interacting 13C with unpaired electrons in the vitrinite macerals. The broad hf spectra of 13C and 1H were observed using HYSCORE measurements. The hf spectra are explained by the existence of various types of aromatic radicals and indicate that coal radicals are aromatic molecules with some condensed rings. In addition, it is found that the ratio of 13C/1H of the HYSCORE spectra depends on the coal rank, which can be used as a new index for coal characterization. Acknowledgment. This work was supported by Grants-in-Aid of Scientific Research on Priority Area “Carbon Alloys” (No. 09243201) and No. 07404040 from the Japanese Ministry of Education, Science, Sports and Culture. The authors thank Dr. P. Ho¨fer of Bruker Co., Ltd., for his helpful discussion. EF980053Q
