Energy & Fuels 1989,3, 402-411
402
an estimate of gas evolution after dealkylation. When the distribution of aromatic ring classes in the feed was calculated, it had been assumed that the distributions of aromatic ring numbers in Fr-PP are approximately the same as those in the feed SRC I1 itself. The observed amounts of all compound classes diminished gradually with longer reaction times. Amounts of organic gases evolved from respective compound classes were calculated by ‘H NMR as described above. These values were added to the respective weight percentage of the corresponding compound classes at different reaction times and are shown with their cumulative values (shaded region in Figure 8 as organic gas). For two-ring naphthalene-type aromatics (Fr-Dl), the total weight percent of evolved gases calculated and upgraded oil were approximately constant, corresponding to the content of Fr-D1 in feed. For Fr-D1, dealkylation and deheterogenation of naphthalene derivatives are the main reactions, judging from the low value of “loss”, which means there was little or no serious hydrocracking of aromatic rings. Meanwhile, significant hydrocracking was presumed to occur in Fr-T1 and Fr-T2, and the amounts of oil products and gases do not coincide with the contents
of each compound classes. Some reactions cause slight rupture of aromatic rings, followed by their breakup into lighter, volatile molecular compounds (designated as loss in Figure 8). Larger aromatic ring compounds suffer some degradation of their ring structure.
Conclusion The difficulty in utilizing coal liquids as chemical feedstock stems from the complexity of their chemical components. The object of this study was to modify coal liquids to a limited number of more or less pure chemical components in large concentrations. Upgrading of coal liquids by dealkylation of aromatic compounds is an effective procedure because their constitution consists mwtly of large numbers of aromatic components with varying attached saturated structures. This indicates a potential for providing simple and fairly pure aromatics, such as naphthalene, fluorene, phenanthrene, and pyrene as chemical feedstock easily and in great quantity. Acknowledgment. We express our thanks to Prof. K. Tanabe and Associate Prof. H. Hattori for the preparation of catalysts.
‘HNMR Spin-Lattice Relaxation in Bituminous Coals CSIRO Division of
Wesley A. Barton* and Leo J. Lynch Coal Technology, P.O.Box 136, North Ryde, N S W 2113, Australia
Received January 20, 1989. Revised Manuscript Received March 21, 1989
Measurements of proton nuclear magnetic resonance (‘HNMR) spin-lattice relaxation in a wide selection of Australian bituminous coals varying in rank and petrographic composition have been carried out. In all cases the relaxation behavior can be closely fitted by the sum of two exponential components. The results are compared with those of previous studies and discussed in terms of the composition, molecular structure, and properties of bituminous coals. The two spin-lattice relaxation components, which occur in very different proportions in vitrinite and inertinite macerals, may arise from structurally different regions in the coal or, as is more likely, from a combination of direct and spin diffusion-limited relaxation of proton magnetization by unpaired electrons. The overall relaxation rate is appreciably greater for inertinite than for vitrinite macerals, a difference attributed to higher concentrations of organic free radicals and/or inorganic paramagnetic species in the inertinites. The intermediate relaxation rate found for liptinites is probably determined by differences in average molecular mobility as well as in unpaired electron concentration between the macerals. For coals of similar maceral composition, there is a minimum in relaxation rate near 87% C (daf). The variations with rank in the relaxation rate for the vitrinite and inertinite macerals below 87% C are attributed largely to the effects of changes in oxygen functionality. The sharp increase in relaxation rate above 87% C is associated with the concomitant increase in free-radical concentration and growth of graphite-like structures.
Introduction Proton Spin-Lattice Relaxation in Organic Solids. In a hydrogen-containing material placed in a static magnetic field, the net alignment of the proton magnetic moments gives rise to a macroscopic magnetization M , which has a value Mo when the proton moments and the surrounding molecular lattice are in thermal equilibrium. If this equilibrium is disrupted by an external perturbation (e.g. a radiofrequency pulse), proton spin-lattice relaxation is the process whereby the equilibrium magnetization Mo 0887-0624/89/2503-0402$01.50/0
is restored. In homogeneous solids the magnetization recovery is usually exponential (sometimes after an initial nonexponential behavior-see below) with a time constant known as the spin-lattice relaxation time T l . In heterogeneous materials (such as coals) the intrinsic proton relaxation behavior and therefore the time constant would be expected to vary between different parts of the structure. Proton spin-lattice relaxation is induced by modulation of the magnetic interactions of the protons at frequencies 0 1989 American Chemical Society
'HNMR Spin-Lattice Relaxation in Coals
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comparable to their resonance frequency, typically lo8 Hz. These fluctuations in the proton interactions arise from rapid molecular motions and also, when unpaired electrons are present, from spin relaxation processes of the unpaired electron population. The proton spin-lattice relaxation process is enhanced by increased interaction as may occur with greater hydrogen or unpaired electron concentration. For a given magnetic interaction strength, this relaxation process occurs most rapidly when the spectral density of the modulation near the resonance frequency is greatest. Although the static magnetic dipolar interaction between protons and unpaired electrons in organic solids such as coal is, on average, considerably weaker than the protonproton coupling,'J unpaired electrons can make an important contribution to proton spin-lattice relaxation via proton spin diffusion, which is often rapid compared to the intrinsic relaxation rates remote from the unpaired electron relaxation centers. Relaxation then is limited either by the rate at which unpaired electrons can relax magnetization associated with protons in their immediate neighborhood (the fast spin diffusion limit) or by the rate of transfer of proton magnetization to the vicinity of unpaired electron sites (the slow spin diffusion limit). The efficacy of other strong relaxation centers such as rotating methyl groups is likewise enhanced by the spin diffusion process. For a homogeneous material, whereas the rapid spin diffusion case is characterized by exponential relaxation: in the diffusion-limited regime the relaxation is nonexponential during the initial stages of the process because proton magnetization close to unpaired electrons is relaxed by the direct dipolar interaction at a rate that is rapid compared to spin diffusion. In this case it has been shown theoretically and confirmed by experiment4 that, after saturation of the nuclear resonance, the magnetization M, initially increases linearly with time t and then is proportional to t'/2 before approaching an exponential recovery to its equilibrium value. In a heterogeneous or multiphase system, the observed relaxation behavior of the magnetization will be a linear combination of the behavior of the separate phases only when there is negligible exchange of magnetization between the phases on the time scale of the measurement; Le., the phases are effectively uncoupled. Then the relaxation time constants of the observed magnetization recovery represent the intrinsic spin-lattice relaxation times of the separate phases. Usually, however, the observed relaxation behavior is modified by significant magnetization exchange between the phases. In the limit of rapid exchange (i.e. strong coupling) between all phases, a single exponential recovery of the magnetization (preceded by an initial nonexponential region in the diffusion-limited case) is measured, and the relaxation rate is a population-weighted average of the intrinsic relaxation rates of the individual phases. Thus the proton NMR spin-lattice relaxation measurement, like all other techniques, has limited resolution and can only reveal structural heterogeneities of a material accordingly. In organic solids structural features of dimensions m and/or processes characterized by times s are typically resolved by this measurement. The theoretical behavior described above for the homogeneous system (or the heterogeneous system with (1) Geretein, B. C.; Chow, C.; Pembleton, R. G.; Wilson, R. C. J.Phys. Chem. 1977,81, 565-570. (2) Barton, W. A.; Lynch, L. J. J. Magn. Reson. 1988, 77, 439-459. (3) Blumberg, W. E. Phys. Rev. 1960, 119, 79-84. (4) Haupt, J.; Muller-Warmuth, W. 2. Naturforsch. 1967, 22A, 643-650.
Energy & Fuels, Vol. 3, No. 3, 1989 403
strong coupling between all phases) in the spin diffusionlimited case is similar to that corresponding to the sum of two exponential functions and thus may be confused, in practice, with the relaxation behavior expected for a weakly coupled two-phase system. However, in principle, if fast spin diffusion occurs within each phase of the two-phase system, these two cases can be distinguished because theory predicts different dependences of the relaxation rate on the resonance frequency in the limits of fast and slow spin d i f f ~ s i o n . ~ Previous Studies. Retcofsky and f i d e l 6were the first to report measurement of 'H NMR spin-lattice relaxation in a bituminous coal. Gerstein et al.' observed this relaxation to be a single exponential process in five vitrains after the specimens were dried under vacuum. Yokono et a1.6 found that the dependence of T1 on the resonance wo1/2) for several bituminous coals frequency wo (i.e. 2'' was consistent with relaxation in the spin diffusion-limited regime and considered the relaxation centers to be free radicals. Lynch and W e b ~ t e detected r~~ nonexponential relaxation behavior at room temperature in a brown coal and a bituminous coal, which were dried and sealed under nitrogen. For the bituminous coal the recovery of the magnetization to its equilibrium value was satisfactorily fitted by the sum of two exponential components. Ripmeester et al.gfirst reported nonexponential magnetization recovery for evacuated specimens and attributed this behavior to the existence of spin diffusion-limited relaxation on the basis of the frequency dependence of the measured Tl, which was defined as the time constant of the recovery at long times. In a study of 60 coals varying widely in rank and origin, Wind et al.'O observed that the relaxation was exponential only for coals with the slowest relaxation. Nonexponential relaxation was attributed to spin diffusion not being sufficiently rapid, especially in the case of faster relaxation, to average out the differences in intrinsic relaxation rates between different (extended) regions of the coal structure. Relaxation rates T1-' were calculated according to the formula T1-'= (In 2 ) / W where iY2is the time required for the magnetization to recover by 50%. Kuriki et observed nonexponential relaxation for most of a set of 22 coals that ranged from lignites to bituminous coals. The data were fitted by up to three exponential components and an average T1 was defined as l/Tl = Cici/Tliwhere ci is the fraction of the 'H NMR equilibrium signal represented by the component with time constant Tli. Jurkiewicz et a1.12 considered the two distinct exponential Componentsinto which they resolved the relaxation of a Polish high-volatile bituminous coal measured at temperatures between 128 and 308 K as representing structurally different regions of the coal. They concluded that the exponential character of both components indicated the occurrence of fast spin diffusion to free-radical centers within each of these regions. In addition, a minimum in the time constant of the more rapidly relaxing Q
(5) Retcofsky, H. L.; Friedel, R. A. Fuel 1968, 47, 391-395. (6) Yokono, T.; Miyazawa, K.; Sanada, Y.; Marsh, H. Fuel 1979,58, 896-897. (7) Lynch, L. J.; Webster, D. S. J . Magn. Reson. 1980, 40, 259-272. (8) Webster, D. S.; Lynch, L. J. Fuel 1981, 60, 549-551. (9) Ripmeester, J. A.; Couture, C.; MacPhee, J. A.; Nandi, B. N. Fuel 1984,63,522-524.
(10) Wind, R. A.; Duijvestijn, M. J.; van der Lugt, C.; Smidt, J.; Vriend, H. Fuel 1987,66,876-885. (11) Kuriki, Y.; Hayamizu, K.; Yumura, M.; Ohshima, S.; Kawamura, M. In 1987 International Conference on Coal Science; Moulijn, J. A., Nater, K. A., Chermin, H. A. G., Eds.; Coal Science and Technology 11; Elsevier: Amsterdam, 1987; pp 399-402. (12) Jurkiewicz, A.; Idziak, S.; Pislewski, N. Fuel 1987,66,1066-1068.
404 Energy & Fuels, Vol. 3, No. 3, 1989
component was obtained a t -280 K. This feature and a similar T1 minimum recorded for a high-volatile bituminous coal by Ripmeester et al? were attributed to changes in thermally activated molecular mobility including that of methyl group rotation. Shorter apparent 2'' values for aromatic than for aliphatic protons in coals have been detected by means of 13C NMR with cross-polarization and magic-angle spinning.lg-15 Differences between the intrinsic 2'' values for protons in aliphatic and aromatic parts of the coal structure would be greater than those observed in these experiments because of the averaging effect of spin diffusion and probably because the measured specimens were not under vacuum. A difficulty with the interpretation of proton spin-lattice relaxation data for coals predominantly in terms of freeradical effects is that both the proton relaxation time13J4J6 and the unpaired electron c o n ~ e n t r a t i o n ~ ~generally J~J~ increase with rank up to -86% C. In fact, Sullivan et al.13 obtained a strong positive correlation between 2'' for both aliphatic and aromatic protons and unpaired electron concentration for nine coals with 7 0 4 2 % C (maf). However, Wind et al.1° found that, for a large set of coals varying widely in rank and origin, the relaxation rate Tl-' is not simply related to unpaired electron concentration but is remarkably similar for many coals spanning a range of radical concentrations. Both Sullivan et al.13 and Wind et al.1° therefore suggested that the ranking of proton spin-lattice relaxation in bituminous coals at room temperature is determined by differences in their molecular mobilities rather than radical concentrations. Sorbed oxygen molecules, which contain two unpaired electron centers, can greatly alter the proton spin-lattice relaxation behavior because of the spin diffusion process. Also adsorbed water molecules can enhance the proton relaxation of the host material, particularly if the water molecules interact strongly with the host and thereby modify its molecular properties. Indeed, it has been shown that oxygen1J0J3J6and moisture7J3J5sorbed by coals significantly enhance the spin-lattice relaxation. Therefore, it is important to desorb oxygen and water prior to NMR measurements that seek to characterize a material such as coal. It is somewhat difficult to properly compare the results of the different relaxation studies of coals referred to above because efforts to remove sorbed molecules from coal specimens prior to their NMR measurement have varied considerably and also because various definitions of a characteristic spin-lattice relaxation time have been adopted. In the discussion below 2' will refer to the relaxation time as defined in the particular study being considered. Scope of Present Investigation. 'HNMR spin-lattice relaxation data, obtained by careful measurement, for a broadly representative set of Australian bituminous coals are presented. The influence of maceral composition (to which little attention has been given in the past) and other (13) Sullivan, M. J.; Szeverenyi, N. M.; Maciel, G. E.; Petrakis, L.; Grandy, D. W. In Magnetic Resonunce. Introduction, Aduanced Topics and Applications t o Fossil Energy; Petrakis, L., Fraissard, J. P., Eds.; NATO AS1 Series C124; Reidel: Dordrecht, The Netherlands, 1984;pp 607416. (14) Stephens, J. F.; Burgar, M. I.; Corcoran, J. F.; K h a n , J. R.; Leow, H. M. Structural Characterisation of Australian Coals; National Energy Research, Development and Demonstration Program End of Grant Report, Project No. 401,1985. (15) Newman, R. H.; Davenport, S. J. Fuel 1987,66, 579-580. (16) Yokono, T.; Sanada, Y. Fuel 1978,57, 334-336. (17) Retcofsky, H. L. In Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1982; Vol. 1, pp 43-82.
Barton and Lynch
coal characteristics on the relaxation behavior is investigated. The present results and those of earlier studies are compared and further discussed in terms of the molecular structure and properties of bituminous coals. A preliminary account of the results presented here appeared in an earlier publication.l8
Experimental Section The 54 bituminous whole coals and maceral concentrates used in this study varied widely in their proportions of vitrinite and inertinite (mostly semifusinite and inertodetrinite) macerals, but their liptinite (mostly cutinite and sporinite) contents were all 119 vol % and 10 H % liptinite. Data for two torbanites,a subbituminous coal, and two semianthracites are also shown.
Tis= 0.211s
1 lp
300 0.oc
I
'
0.0
05
10
15
20
r Is1 Figure 1. (a, top) Two-component exponential fit to a typical set of data (0)for the magnetization function 1- Mz/Moversus pulse separation T: (i)Fit to the more slowly relaxing component. (ii) Fit to the more rapidly relaxing component after subtracting the fit in part i. (b,bottom) Recovery of the magnetization Mz/Mo with 7 for the same data ( 0 )as in part a, showing the curve obtained from the two-componentfit in part a after optimization. Open circles represent deviations (offset by +0.5 and magnified lox) between data points and the fitted curve.
Af + A, < 1 (most values lie in the range 0.95-0.98),which implies a more rapid initial increase in magnetization than that given by the two-component fit and hence the existence of a small, extremely rapidly relaxing (time constant 10 H% liptinite from the regression has no significant effect on the results obtained for pure vitrinite and inertinite. This association of the more slowly relaxing component predominantly with vitrinites and the larger proportion of the more rapidly relaxing component with inertinites reflects differences in
structure between these two maceral types. However, these Af values differ significantly from the expected values of -0 and -1 (even allowing for the complication of the
liptinite content) if the two material phases simply corresponded to the vitrinite and inertinite macerals. Furthermore, if this were the case, the distinction between the
’HNMR Spin-Lattice Relaxation in Coals two relaxation components would be expected to diminish with increasing rank as the vitrinite and inertinite maceral types become more similar in structure. However, the ratio of the relaxation rates of the two components as measured by Tl,/Tlf shows a slight increase with rank across the bituminous range. Therefore, in the context of a two-phase model, the further implication is that both maceral types would consist of structural regions of two different types but with proton populations in very different ratios. Single-phase Relaxation Center Model. For this model Af is associated with the proton population whose magnetization is directly relaxed by the relaxation centers. The fact that Af increases with increasing inertinite content (Figure 4) is consistent with the higher concentration of unpaired electrons expected in inertinites than in vitrini t e ~which , ~ ~ would result in a larger fraction of protons in inertinites being relaxed by the direct process. The observation that the relaxation rate Tltl of the more rapidly decaying component (but not T1c1) tends to decrease with greater hydrogen content for the bituminous coals indicates that this component does not represent spin diffusion-limited relaxation (which is expected to become faster with increasing hydrogen concentration for organic solids) and thus supports the relaxation center model. The deviations from exponential behavior observed in the faster decaying component especially for inertinite concentrates and the existence of a small, extremely rapidly relaxing fraction of the magnetization that tends to increase with greater inertinite and/or ash content (and thus is probably that of protons most strongly coupled to unpaired electrons associated with free radicals or paramagnetic ions) are consistent with this relaxation center model. Mechanisms of Spin-Lattice Relaxation in Coal Macerals. In the further discussion the relaxation center model will be favored because it is consistent with the data obtained and structural features of coals that can be simply related to the two-phase model have not been identified. The higher proportion of more rapidly relaxing proton magnetization in inertinites than in vitrinites is reflected in the general decrease in the average rate (T1-l)of spinlattice relaxation with increasing vitrinite content for the bituminous coals (Figure 2). The sensitivity of T1-l to maceral composition diminishes as vitrinite content increases. Coals with >10 H% liptinite do not appear to significantly alter the trend shown by the specimens with lower liptinite contents. The T1-l values for the two liptinite-rich torbanites lie between those for vitrinite and inertinite concentrates (Figure 2). The fact that the Ti1 values obtained for the liptinite-rich specimens are higher than those found for vitrinite concentrates is contrary to the result obtained by Stephens et aLl4 for macerals separated from a Chinese coal. The ranking indicated by Figure 2 that T,-’(inertinite) > T,-’(liptinite) > T,-l(vitrinite) is confirmed by statistical extrapolation of the data for the average relaxation time Tl for all bituminous coals studied plus the two torbanites to “pure average” macerals using a linear regression analogous to that in eq 3 (Figure 3). (Tl was used in preference to TIT1for the regression because the Tl data vary more linearly with maceral composition than those for Tldl(Figure 2).) This analysis yields T1-l values of -30, 10, and 4 s-l for “pure average” inertinite, liptinite, and vitrinite, respectively. However, the uncertainties in these results (particularly those for pure iner-
-
(24) Petrakis, L.; Grandy, D. W. Free Radicals in Coals and Synthetic Fuels; Coal Science and Technology 5; Elsevier: Amsterdam, 1983; Chapter 4.
Energy & Fuels, Vol. 3, No. 3, 1989 409 tinite and liptinite) are quite large, partly because of the variation of T1-l with rank in the bituminous range (see below). Exclusion of the torbanites from the linear regression does not significantly alter the results for pure vitrinite and inertinite, but the liptinite result is then meaningful only for the coals with 60 26in
(25) Lockhart, N. C. Personal communication. (26) Sakurovs, R.; Lynch, L. J.; Maher, T. P.; Banerjee, R. N. Energy Fuels 1987,1, 167-172. (27) Bakr, M.; Akiyama, M.; Sanada, Y.; Yokono, T. Fuel 1988, 67, 294-295.
Barton and Lynch
410 Energy &Fuels, Vol. 3, No. 3, 1989
* II
201
15 -
-
I
a
I
I
7
+-
10
-
a a
a
* *
a a
J
I-
a m a
a a mam
I 80
85 Carbon
90
95
(YOdmmf 1
Figure 10. Plot of relaxation rate TI-'against carbon content for the coals with >80% C (dmmf)studied by Wind et al.'O The data were extracted from Table I of ref 10.
H% vitrinite, the relaxation time clearly increases to a maximum at -87-88% C and falls sharply at higher rank (Figure 5b). This trend in Tlarises not only from similar trends in the component time constants Tlf and T,, but also from the variations with rank in the fractional amplitudes Af and A,. With increasing rank through the bituminous range, Af initially shows a general decrease, passes through a minimum at -86% C and then increases significantly above -88% C (Figure 7). The T1values for vitrinite-rich coals of lower rank appear to fall along two separate curves that converge above -88% C (Figure 5b). In most cases the longer Tl values for the specimens represented in the upper curve correspond to values of Tlf and Tl,, both of which are longer than those for other vitrinites of similar rank. Also Af + A, is closer to unity for the vitrinites with the longer TI values. Since the values of both Af + A , and Tlf tend to decrease with increasing ash content, it is possible that the observed dichotomy in T1 behavior of vitrinites is due to differences in the contribution to proton relaxation from paramagnetic species in the mineral matter. However, such a dichotomy in Tl values is not apparent for inertinite concentrates for which the relaxation time also increases with rank up to -87% C. The influence of inorganic species in the coals on the proton spin-lattice relaxation behavior is being investigated more thoroughly and will be discussed further in a subsequent publication. The short Tl values measured for a subbituminous coal and two semianthracites (all containing >60 H% vitrinite) are consistent with those expected from extrapolation of the rank dependence obtained for vitrinites within the bituminous range (Figure 5b). Previous room-temperature studies of both whole c0a1s'~J~ and vitrinite concentrates1J4 spanning a wider range of carbon contents than the coals investigated here indicate an increase in T1 with rank up to -86-88% C (daf) and a sharp decrease at higher rank. Even the relaxation rates determined by Wind et al.1° for coals of widely varying origin (and plotted against carbon content in Figure 10) show evidence of a broad minimum in the 86-90% C range. The Tl values reported by Stephens et al.I4 for vitrinite concentrates from a less representative selection of Australian coal seams than those considered here lie close to the lower curve in Figure 5b at lower rank, but at higher rank their values are somewhat shorter than those obtained in this study possibly because their spec-
imens were not degassed under vacuum prior to NMR measurement. No specimens from the seams represented in the upper group of data in Figure 5b were measured by Stephens et al.14 A distinct maximum is also shown by the time constant T1, of the more slowly relaxing component (Figure 6a) (but much less so by Tlf(Figure 6b)) when plotted against mean maximum vitrinite reflectance &max even when all coals are included. The more definite vitrinite rank dependence shown by T1, (Figure 6a) than by the average Tl(Figure 5a) or Tlf (Figure 6b) for all coals confirms the stronger association of the more slowly relaxing component with the vitrinite macerals. From the results presented above, it is clear that the relaxation behavior of bituminous coals is influenced systematically by both rank and maceral composition factors. This is also demonstrated in Figure 8, which shows the linear regression of the average relaxation rate T1-l with both a rank and a maceral parameter. The parameter ( & m a - 1.2)2has been used in an effort to "linearize" the rank dependence of T1-'. The relative significance of the two independent variables in the regression is comparable. Influence of Rank on Relaxation Mechanisms. The analytical data obtained for the coals used in this study (Table I) are consistent with the generally established relationships between the properties of coals of similar rank and maceral composition. Therefore, the minimum in relaxation rate obtained near 87% C particularly for vitrinite-rich coals can be identified as a further manifestation of the major change which occurs in the coalification process as the anthracitic coals evolve from the bituminous ones. Between 80 and 86% C coalification is characterized by the following changes in coal composition: (i) a considerable reduction in oxygen functionality, presumably mostly due to loss of phenolic and ether groups;28(ii) little change in total hydrogen content and therefore considerable reduction in its atomic association with oxygen; (iii) a slight trend, but with considerable variability, for reduction in aliphatic structures and increase in carbon aromaticityaB However, there is evidence to suggest that no significant change in the ring condensation index occurs over this rank range.30 Near 87% C, a rapid increase in carbon aromaticity and ring condensation index c~mmences,~*~O the extent to which vitrinite-rich coals are penetrated and swollen by organic solvents begins to fall sharply2a31and the onset of a more rapid increase in free-radical concentration in whole coaldo and vitrainsl' occurs. These changes reflect the fact that, above -87% C, rapid aromatization and formation of extended and more ordered aromatic structures dominate the coalification process. In the above context, factors that can be considered to influence the observed spin-lattice relaxation behavior are radical concentration (not forgetting the possible influence of inorganic unpaired electrons), radical type, hydrogen concentration, and molecular mobility/aliphatic-aromatic composition. (i) Radical Concentration. Because the proton relaxation rate is expected to increase with free-radical concentration, the general increase in the radical concentration with carbon content throughout the bituminous (28) Attar, A,; Hendrickson, G. G. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982; pp 131-198. (29) Miknis, F. P.; Sullivan, M.; Bartuska, V. J.; Maciel, G. E. Org. Geochem. 1981,3, 19-28. (30) Gerstein, B. C.; Murphy, P. D.; Ryan, L. M. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982; pp 87-129. (31) Sanada, Y.; Honda, H. Fuel 1966,45,451-456.
IH NMR Spin-Lattice Relaxation in Coals range10J7cannot by itself explain the minimum in relaxation rate but only the sharp rise with rank above -88% C. A more rapid increase in total radical concentration with carbon content above -87% C is consistent with greater stabilization of carbon radicals by delocalization over larger aromatic clusters as anthracitic coals evolve.30 (ii) Radical Type. With increasing rank there is a progressive change in the nature of the free radicals from predominantly those localized on heteroatoms such as oxygen in the lower rank coals to largely those delocalized over aromatic structures in high-rank Such a trend is consistent with the significant decrease in oxygen content with increasing rank in the bituminous range. This change in the nature of the radicals can be expected to affect the proton-unpaired electron interaction and hence the efficiency with which proton magnetization is relaxed by the radical centers. The observed trend in the proton relaxation rate between 80 and 86% C is consistent with the tendency for electron spin resonance spectral line widths and electron-nuclear hyperfine coupling constants to decrease with increasing rank in the bituminous range.32 These trends can arise from a decrease in the magnetic coupling between proton and unpaired electron moments and/or changes in the dynamics of the unpaired electron spins. The spin relaxation rates of the unpaired electrons are somewhat less than proton resonance freq~encies.~J* Thus, if changes in the electron spin dynamics are to contribute to the decrease in proton relaxation rate with increasing rank below 86% C, the electron spin relaxation rates must also decrease with rank. Published data on these trends are contradictory since Smidt and van Kreveled3 found the electron spin-lattice relaxation rate to decrease with rank up to -86% C for vitrains whereas Thomann et al.34measured an increase with rank in the bituminous range for both spin-lattice and spin-spin relaxation rates of unpaired electrons in vitrinite macerals. (iii) Hydrogen Concentration. There is a slight decrease in the atomic fraction of hydrogen as coal rank increases from 80 to 86% C (e.g. Table I) and a decrease in mass density between 80 and 85% C.s6 Both of these changes are likely to result in a reduction in proton-proton interaction and hence in the proton spin-lattice relaxation rate. The increase in mass density above -86% (2%may contribute to the observed rise in the relaxation rate above -87% C, but any effect would be offset by the significant fall in hydrogen content above -88% C. (iv) Molecular Mobility and Composition. Ripmeester et a1.9 and Jurkiewicz et a1.12 demonstrated that molecular dynamics is a significant factor in the proton spin-lattice relaxation of coals. Thermally activated molecular mobility associated mainly with the aliphatic-rich parts of coal structure26t21would be expected to enhance the spin-lattice relaxation in the structures involved. Thus the reduction in the proportion of these structures expected in the 80-86% C range should be a further factor contributing to the decrease in average relaxation rate of the coal protons. For the aromatic macerals (vitrinite and inertinite) this factor is not thought by the authors to be (32) Retcofsky, H. L.;Hough, M. R.; Maguire, M. M.; Clarkaon, R. B. In Coal Structure; Gorbaty, M. L., Ouchi, K., Eds.; Advances in Chemistry Seriea 192; American Chemical Society: Washington, DC, 1981; pp 31-58. (33) Smidt, J.; van Krevelen, D. W. Fuel 1959, 38, 355-368. (34) "homann, H.; Silbemegel, B. G.; Jin, H.; Gebhard, L. A.; Tmdall, P.; Dyrkacz, G. R. Energy Fuels 1988,2,333-339. (35) van Krevelen, D. W. Coal; Coal Science and Technology 3; Elsevler: Amsterdam, 1981; Chapter 16.
Energy & Fuels, Vol. 3, No. 3, 1989 411 dominant. Rather the major reduction in oxygen functionality, so that hydrogen is associated less with the localized oxygen radicals and more with the delocalized carbon radicals of small aromatic ring clusters, is expected to be the main reason for the decrease in relaxation rate as rank increases from 80 to 86% C. This is consistent with the observation of Wind et al.'O that the proton relaxation rate of coals containing >6.5 w t % oxygen increased markedly with oxygen content.
Conclusions The proton spin-lattice relaxation behavior in Australian bituminous coals can be approximated by the sum of two exponential components. The components cannot, in general, be related to a molecular/macromolecular model of coal based on the mobile and immobile structures detected in pyridine-swollen specimens. Nor can they be simply related to maceral types. However, the proportions of the two components do correlate with maceral composition: the more rapidly relaxing component is greater in inertinites, and the more slowly relaxing component predominates in vitrinites. These two components may be associated with structurally different regions whose relative abundance is related to maceral type, but it is considered more likely that they reflect a combination of direct and spin diffusion-limited relaxation of protons by unpaired electrons. Either interpretation is consistent with there being a significantly greater concentration of organic free radicals and/or inorganic paramagnetic species in inertinite than in vitrinite macerals. The average spin-lattice relaxation rate estimated for liptinite macerals lies between the rates obtained for vitrinites and inertinites. However, it is likely that differences in molecular mobility as well as in unpaired electron concentration determine this ranking. For the set of coals studied, there is an indication of a minimum in average spin-lattice relaxation rate near 87% C (daf). Vitrinite-rich specimens show a clearer minimum near this carbon rank in agreement with the results of previous studies. This rank dependence is also apparent in a plot of the relaxation time of the slower component (predominant in vitrinites) against vitrinite reflectance for all the coals. The extremum in relaxation behavior coincides with the onset of rapid aromatization and formation of extended and more ordered aromatic structures in the coalification process. The decrease in relaxation rate with rank up to -87% C probably is governed by the reduction in oxygen functionality. Above -87% C the relaxation rate rises sharply due to a rapid increase in carbon radical concentration. I t is clear from this study that as one seeks to more carefully measure and interpret 'H NMR spin-lattice relaxation data for coals, the task becomes more complex in that simple correlations of relaxation parameters with other objective data on coals are not realized and there are many highly variable factors that influence the measured relaxation behavior. Acknowledgment. We acknowledge the significant contribution made by D. S. Webster to the NMR measurements and to the development of the data analysis software. The coal specimens and their chemical and petrographic analyses were supplied by T. P. Maher of the Joint Coal Board. Technical assistance with specimen preparation was provided by N. Thomas. Software developed by R. Sakurovs was used for statistical analysis of the data.
