Energy & Fuels 1988,2, 333-339
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Carbon Radical Spin Dynamics in Isolated Vitrinite Coal Macerals H. Thomann,* B. G. Silbernagel, H. Jin, L. A. Gebhard, and P. Tindall Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801
G. R. Dyrkacz Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 Received October 6, 1986. Revised Manuscript Received December 23, 1987 Carbon radical phase coherence and spin-lattice relaxation rates have been measured at room temperature by electron spin echo (ESE) techniques for a series of vitrinite macerals isolated from whole coals by density gradient centrifugation. The rank range subbituminous C (63.6% carbon) to low-volatile b (88.3% carbon) has been studied. Phase coherence relaxation is dominated by electron dipolar couplings between radicals for all the samples studied. The limiting phasecoherence relaxation increase approximately linearly with carbon radical density, consistent with a lattice of rates, Td-’, randomly distributed spins. Instantaneous diffusion is observed for all samples, but the magnitude decreases as the vitrinite rank increases. Spin-lattice relaxation rates, are weakly dependent on rank for the low-rank macerals but become a strong function of rank above 80% carbon. The mass density versus total carbon decreases with increasing rank for low-rank macerals but increases with,increasing rank for the higher rank macerals. The minimum in mass density occurs at 80% carbon where the T1;l rates start to increase rapidly. The T1;l and mass density data are interpreted as arising from the formation of mesophase-like aromatic structures associated with an aromatic condensation, which has an onset near 80% carbon. Enhanced intermolecular interactions among aromatic corea provides new relaxation mechanisms enhancing T1;l rates and quenching instantaneous diffusion. The present results indicate that, over the rank range studied in the present experiments, vitrinite maceral carbon radical relaxation rates are a more sensitive probe of the microscopic intermolecular coal structure than of the carbon radical molecular structure.
Introduction Since the fiist ob~ervationl-~ of an electron spin resonance (ESR) signal from charred materials in 1954,numerous ESR studies of coals, chars, pitches, cokes, and other condensed hydrocarbons have been conducted to investigate the nature of the naturally occurring free radicals found in these carbonaceous materials and the role these radicals may play in hydrocarbon conversion processes. Additional impetus to study these indigenous radicals was provided by the similarity of the native radicals to the free radicals presumed to be involved in the early stagea of molecular polymerization and condensation reactions that occur during hydrocarbon pyrolysis.4 From initial ESR studies, Ingram suggesteds that the relatively high concentrations of stable free radicals found in these cabonaceous materials were produced from the free-radical cleavage of substituents on trivalent carbon atoms that were stabilized on proximate conjugated aromatic ring structures. However, more detailed investigations of the radical structure attempting to determine molecular properties such as the aromatic ring size, ring geometry, ring heteroatom, and substituent content and to correlate these variables to chemical reactivity met with only limited success. This has been especially true for studies of coals, where both chemical and physical heterogeneity have (1) Ingram,D. J. B.; Bennett, J. B. Philos. Mag. 1954, 45, 545. (2) Uebersfeld, J.; Filenne, A.; Combeleaon, J. Nature (London) 1954, 147, 614. (3) Winslow, R. A.; Baker, W. 0.; Yager, W. A. J . Am. Chem. SOC. 1955, 77,4751. (4) Fitzer, E.; Mueller, K.; Schaefer, W. In Chemistry and Physics of Carbon; Walker, P. L., Ed.;Marcel Dekker: New York, 1971; Volume 7. (5) Ingram, D. J. E. Free Radicals as Studied by Electron Spin Resonance; Academic: New York, 1958.
precluded the development of a detailed microscopic picture of the nature of the free radicals. A typical coal is comprised of inequivalent organic matter types, minerals, and an elaborate pore structure. While conventional ESR studies of whole coals have been extensively reported: the information content of a typical ESR spectrum is limited because of this heterogeneity. The ESR signals from the various organic matter constituents of the coal are typically overlapped to form a broadened, featureless absorption line. Associated minerals can also enhance spin relaxation and may contribute to line broadening. The ESR signature from the individual constituents of the coal in such cases are obscured. One way to recover some of the information lost in such an “averaged” spectrum is to constrain experimental conditions in order to selectively enhance the response of a particular spectral component. Such experimental techniques take advantage of additional spectroscopic transition selection rules to reduce the number of observed transitions and therby reduce the level of complexity of the spectrum. For example, electron nuclear double resonance (ENDOR) has been employed’ to observe separate signals from vitrain and fusion macerals where these signals severely overlapped in the ESR spectrum. Unfortunately, the strong dispersion of hyperfine interactions has limited the utility of ENDOR studies on most coals except for some exceptional cases.8 However, these more elaborate resonance techniques serve to demonstrate the (6) Retcofsky, H. L. In Coal Science and Technology; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic: New York, 1982; Vol. 1. (7)Miyagawa, I.; Alexander, C. Nature (London) 1979, 278, 40. (8) Retcofsky, H.L.; Hough, M. R.; Maquire, M. M.; Clarkson, R. B. In Coal Structure; Gorbaty, M. L., Ouchi, K., Eds.; Advances in Chemistry 192; American Chemical Society: Washington, DC, 1981.
0 1988 American Chemical Society
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Table I. Chemical, Physical, and ESR Properties of Isolated Vitrinite Coal Macerals radical density, PSOC 1019 TI:', Tml-', ID no./rank p, g/cm3 % C H/C O/C g AHpp,G spins/cms P1/*, mW MHz MHz 6.46 1.82 66.0 0.268 4.0 1.317 88.3 0.615 0.048 2.00288 317/ lvb 6.92 1.99 37.0 0.232 3.1 1.302 86.2 0.602 0.066 2.00278 409/mvb 2.00289 7.12 1.69 22.0 0.200 3.0 1.278 84.2 0.610 236/mvb 6.99 0.89 8.3 0.073 1.6 1.272 82.0 0.733 0.081 2.00304 2681hvAb 0.022 1.0 6.33 9.6 1.264 79.5 0.801 0.126 2.00290 2971hvAb 1.0 6.57 7.3 0.022 1.271 79.2 0.776 0.126 2.00293 297/hvAb 1.5 1.18 4.7 6.32 0.021 1.264 79.5 0.801 0.118 2.00292 297/hvAb 1.7 5.98 1.11 2.8 0.027 2971hvAb 1.305 78.0 0.723 0.143 2.00296 1.1 7.41 0.50 4.2 0.027 285/hvAb 1.286 76.9 0.791 0.160 2.00308 0.8 0.035 6.45 0.30 5.8 1.314 73.5 0.761 0.204 2.00321 8521hvAb 0.9 6.69 0.37 4.3 0.019 1.332 72.3 0.792 0.224 2.00319 5941hvCb 1.0 5.00 0.21 1.7 0.017 1.407 63.3 0.940 0.364 2.00350 240/subB 0.6 0.014 5.62 0.21 1.3 1.420 63.6 0.908 0.363 2.00333 1005/subC
utility of reducing the "level of heterogeneity" in the experimental results. Additional great strides in our understanding of coals (as well as other carbonaceous materials) could be made if the chemical constituents of the coal could be separated and individually studied by these powerful spectroscopic techniques. Electron spin echo (ESE) studies on whole coals have been previously reported."14 The recent utilization of density gradient centrifugation (DGC) techniquedSprovide the opportunity to systematically investigate the isolated organic constituents of a single coal. The use of the DGC technique reduces the level of chemical and physical heterogeneity of the sample studied while the ESE technique provides a direct and quantitative measure of the radical's nonequilibrium spin-relaxation properties. The importance of spin-relaxation effects as parameters for probing hydrocarbon radicals has been recognized for many years.I6 ESE techniques offer the opportunity to directly measure the phase coherence, T,, and spin-lattice, TI,, relaxation time of the coal radicals. Previous ESE studies have confirmed that the ESR line is inhomogeneously broadened.12 The overlapping vitrain and fusain ESR lines have been separated1' (spectrally titrated) by using a pulse-induced ESR, technique, suggesting that maceral types may be correlated with relaxation rates. Here we report an ESE study of carbon radical relaxation for a series of isolated vitrinite macerals over the rank range from subbituminous C to low volatile b. ESR and related techniques probe properties associated with resonance stabilized polynuclear aromatics (PNA), which are believed to play important roles in hydrocarbon conversion. Previous ESR studies of the maceral sample set used in the present study have established that the ESR properties (spin density, g values, line widths, line shapes, and mi(9) Thomann, H.; Goldberg,I. B.; Chiu, C.; Dalton, L. R. In Magnetic Resononce: Introduction, Advanced Topics and Applications to Fossil Energy; Petrakis, L., Fraissard, J. P., Eds.; NATO AS1 Series C124 D.
Reidel: Dordrecht, The Netherlands, 1984. (10) Doetachman, D. C.; Must.&, D. In Magnetic Resonance: Zntroduction, Advanced Topics and Applications to Fossil Energy; Petrakis, L., Fraiesard, J. P., Eda.;NATO M I Series C124; D. Reidel: Dordrecht, The Netherlands, 1984. (11)Schlick, S.;Kevan, L. In Magnetic Resonunce: Introduction, Advanced Topics and Applications to Fossil Energy: Petrakis, L., Fraissard,J. P., Ede.;NATO AS1 Series C124, D. Reidel: Dordrecht,T h e Netherlands 1984. (12) Schlick, S.; Narayana, P. A.; Kevan, L. J . Am. Chem. SOC.1978, 100,3322. (13) Doetachman, D. C.; Muatafi, D. Fuel 1986,65,873. (14) Schlick, S.; Narayana, M.; Kevan, L. Fuel 1986, 65, 873. (15) Dyrkacz, G. R.; Horwitz, E. P. Fuel 1982,61, 3. (16) Smidt, J. Nature (London) 1968,181, 176. (17) Dae, U.; Doetachman,D. C.; Jones,P. M.; Lee,M. Fuel 1983,62, 286.
Aodd,
TM-'?
MHz 3.9 3.9 3.8 3.1 2.6 3.4 2.7 3.2 3.3 1.8 3.1
MHz 7.7 6.9 6.8 4.8 3.7 4.7 4.2 4.0 4.1 2.6 4.0 1.4 1.5
1.2
0.9
crowave saturation power) correlate with many of the chemical and physical properties (optical density, chemical unsaturation, aromaticity, average molecular weight, H/C ratio, O/C ratio, and total carbon content). While ESR studies of separated macerals have proven to be a powerful combination of techniques, it is clear that a more quantitative, microscopic characterization requires more sophisticated spectroscopic probes. The relaxation data measured by ESE techniques can, in principle, be interpreted in terms of microscopic models, which is generally not possible by using ESR line-width and saturation data for an inhomogeneously broadened line shape. In this initial study, we contrast the ESE and ESR data on the same maceral sample set and examine the rank variation of the ESE-derived relaxation parameters. We were particularly interested in establishing the extent to which the chemical identity of the PNA and the intermolecular interactions contribute to the relaxation rates. The interplay of these effects determines whether changes in local structure or differences in the radical type dominate the paramagnetic relaxation properties as a function of coal rank.
Experimental Section A. Sample Preparation. Whole coal samples can be separated aa a function of density by using centrifugation techniques?6 The coal is fiist demineralized,ground to -3 pm dimensions and placed in a water slurry with surfactant added to prevent particle agglomeration. The slurry is then added to a density gradient that is formed in the centrifuge by mixing various ratios of a CsC1/H20solution. This technique produces a density resolution of -0.01 g/cm3 in the separated coal particles. T h e major maceral classes have different densities: exinites, the paraffinic organics from spores, algae and plant cuticles, have p 1.0-1.3 g/cm3; uitrinites, vascular, lignin-baaed organics, have p 1.25-1.45 g/cm3; inertinites, the partially transformed organic material sometimes referred to as mineral charcoal, have p > 1.4 g/cm3. By extracting samples of the appropriate density, it is possible to achieve maceral "purities" of >95%. The samples to be discussed here fall into this "purity" range and contain
