Energy & Fuels 1998, 12, 631-636
631
Interactions between a Bituminous Coal and Aromatic Hydrocarbons at Elevated Temperatures Richard Sakurovs† CSIRO Division of Coal and Energy Technology, P.O. Box 136, North Ryde 2113, Australia Received November 25, 1997. Revised Manuscript Received February 24, 1998
Proton NMR has been used to investigate interactions between a high-volatile bituminous coal and a number of aromatic hydrocarbon additives during heating at elevated temperatures. A coal-additive ratio of 4:1 was used. Coal tar pitch and aromatic hydrocarbons such as anthracene were observed to penetrate the coals at temperatures >200 °C and substantially increased the extent to which the coal was mobile at these temperatures. If, after the hydrocarbon had penetrated the coal, the mixture was allowed to cool to room temperature and then was reheated, the extent to which the mixture was mobile was substantially altered. The extent to which the reheated mixture was mobile at 100 °C decreased with both increasing molecular weight and increasing polarity of the additive but was not dependent on the melting point of the additive. Studies using deuterated anthracene showed that the coal in these reheated mixtures itself started to become significantly mobile at ∼150 °C, that is, at temperatures at which neither coal nor anthracene alone was mobile. These results are interpreted in terms of a general interaction between the coal and the hydrocarbons that is probably electrodynamic in nature.
Introduction Bituminous coals are held together by all of the attractive forces experienced by organic compounds: covalent bonds, electrostatic interactions (ionic and hydrogen bonds), and electrodynamic interactions.1,2 Electrodynamic interactions occur when there is no net charge difference between attracting species, but instantaneous variations in the distribution of electric charge on these species generate net attractive forces. These interactions include the van der Waals and π-π interactions. As the rank of the coal increases from high- to low-volatile bituminous, electrostatic interactions are believed to become less important, and π-π interactions increasingly dominate the noncovalent interactions.1,2 Much of the information regarding the importance of noncovalent bonding has been obtained from studies of the interactions between coal and solvents.3 NMR studies have shown that solvents such as pyridine and N-methylpyrrolidinone extensively mobilize many coals at ambient temperatures1,3 and below.4 Thus, these solvents must break many of the attractive forces that keep coal solid. These are thus considered to be “good” solvents for coal. Because hydrocarbons such as anthracene can extract coals in high yields,5,6 they can be considered to be good solvents as well. However, unlike pyridine, aromatic hydrocarbons interact physically
with coal only via electrodynamic interactions. Thus, studies of mixtures of coals and aromatic hydrocarbons should help to identify the importance of these interactions in stabilizing coal structure. However, there have been no NMR studies of interactions between coal and polycyclic aromatic hydrocarbons, mainly because most of these hydrocarbons are solid at the temperatures at which most NMR experiments are performed. Two effects complicate investigations of the interactions between coal and aromatic materials such as anthracene. First, hydrogen transfer reactions can occur at temperatures as low as 280 °C over several hours;7,8 thus, exposure of these mixtures to such temperatures should be kept brief. It is also known that the physical structure of coals is irreversibly altered (relaxed) if exposed to solvents;9 thus, it is implied that the interactions observed in the coal-additive mixture refer to the interactions between the additive and the physically relaxed coal. Proton magnetic resonance thermal analysis (PMRTA) measures the NMR signals of materials while they are being heated.10 Because PMRTA can be routinely performed at elevated temperatures, the effects of solvents that are solid at room temperature on coals can be studied. For example, PMRTA was used to establish that, at temperatures >300 °C, coal tar and petroleumbased pitches penetrate bituminous coals and substantially modify their thermoplastic behavior.11 This paper
†
Fax 61-2-94908909; e-mail
[email protected] (1) Nishioka, M.; Larsen, J. Energy Fuels 1990, 4, 100-106. (2) Gorbaty, M. L. Fuel 1994, 73, 1819-1828. (3) Given, P. H.; Marzec, A.; Barton, W. A.; Lynch, L. J.; Gerstein, B. C. Fuel 1986, 65, 155-163. (4) Yang, X.; Silbernagel, B. G.; Larsen, J. W. Energy Fuels 1994, 8, 266-275. (5) Nishioka, M. Fuel 1992, 71, 941-948. (6) Bendale, P. G.; Zeli, R. A.; Nishioka, M. Fuel 1994, 73, 251255.
(7) Pajak, J. Fuel Proc. Technol. 1989, 21, 245-252. (8) Pajak, J. Erdoel Erdgas Kohle 1996, 112, 518-519. (9) Larsen, J. W.; Flowers, R. A. II; Hall, P. J. Energy Fuels 1997, 11, 998-1002. (10) Sakurovs, R.; Lynch, L. J.; Barton, W. A. In Magnetic Resonance of Carbonaceous Fuels; Botto, R., Sanada, Y., Eds.; Advances in Chemistry Series 229; American Chemical Society: Washington, DC, 1993. (11) Sakurovs, R.; Lynch, L. J. Fuel 1993, 72, 743-749.
S0887-0624(97)00215-6 CCC: $15.00 Published 1998 by the American Chemical Society Published on Web 04/23/1998
632 Energy & Fuels, Vol. 12, No. 3, 1998
Sakurovs
Table 1. Extent of Fusion of Annealed 4:1 Coal-Additive Mixtures at Various Temperatures additive polyphenyls biphenyl fluorene dihydroanthracene m-terphenyl 1,3,5- triphenylbenzene m-quinquephenyl other nonpolar aromatics anthracene d10-anthracene phenanthrene pyrene 9-methylanthracene 9,10-diphenylanthracene polar aromatics 4,4′-dipyridyl carbazole dibenzofuran 7,8-benzoquinoline acridine coal run 1 coal run 2 pitch
MW
MP, °C
annealing temp, °C
F(50 °C), %
F(100 °C), %
F(150 °C), %
F(200 °C), %
154.2 166.2 180.3 230.3 306.4 382.5
71 116 110 87 174 172
200 200 200 250 350 300
18.7 16.2 15.2 10.6 2.9 0
25.9 24.0 24.6 21.3 11.2 8.2
29.0 31.0 32.7 28.7 19.8 19.6
31.3 35.6 36.5 33.3 28.5 29.8
178.2 188.3 178.2 202.3 192.3 330.4
217 220 101 151 79 248
250 250 250 250 250 350
7.3 1.7 9.8 9.4 9.4 0
16.0 4.0 18.7 16.5 19.0 0.4
21.9 7.3 26.7 23.7 27.9 9.6
33.5 14.4 34.2 30.3 35.0 18.8
156.2 167.2 168.7 179.2 179.2
111 246 85 51 110
200 270 200 250 250
5.0 2.9 14.8 3.6 0 2.7 2.1 0
12.1 10.2 21.7 13.3 8.8 4.4 2.9 0
21.5 18.1 27.5 23.3 19.2 2.5 1.2 10
31.7 26.9 31.1 32.7 30.0 2.9 1.0 19.6
250
describes a PMRTA study of the interactions that occur between a high-volatile bituminous coal and a number of aromatic hydrocarbons while their mixtures are being heated. It shows that materials such as anthracene, which do not participate in electrostatic attractions, can extensively disrupt the structure of a high-volatile bituminous coal at elevated temperatures, thereby indicating that electrodynamic interactions are important in holding this coal together. Experimental Section The coal used in this study was obtained from a commercial sample of Liddell seam coal from the Hunter Valley, New South Wales, Australia. It was first crushed to -4 mm, and the -0.125 mm fraction was discarded. The remaining sample was subject to a float/sink separation at 1.3 g/cm3, and the floats were collected. The product was crushed to -0.212 mm for the PMRTA experiments. It had 0.7% moisture (air-dried), 3.1% ash (db), 83.7% carbon (daf), 5.6% hydrogen (daf), mean maximum vitrinite reflectance, 0.82% and 89% vitrinite (mmf). The additives examined were a Koppers coal tar pitch (quinoline-insolubles free, softening point ∼ 113 °C), and the following model compounds, all of which are solids at room temperature: anthracene (ANT), 98% deuterated anthracene (DANT), 9-methylanthracene (9MA), phenanthrene (PHE), acridine (AC), 7,8-benzoquinoline (BQ), 4,4′-dipyridyl (DPY), biphenyl (BP), 1,3,5-triphenylbenzene (TPB), fluorene, dibenzofuran, carbazole, m-terphenyl, dihydroanthracene, m-quinquephenyl, pyrene, and 9,10-diphenylanthracene (Aldrich). All of these compounds were used as received. Their melting points and molecular weights are listed in Table 1. A coal-additive ratio of 4:1 by weight (air-dried) was used for the PMRTA studies of the mixtures. In the PMRTA experiments, a 0.4 g sample was heated at 4 °C/min under flowing nitrogen from room temperature to a temperature that allows these hydrocarbons to penetrate the coal. In many experiments, the mixture was then immediately allowed to cool to room temperature and then reheated at 4 °C/min. The mixture is said to be “annealed” after the first heating step. The maximum temperature reached during the annealing step is termed here the “annealing” temperature. For most measurements, the annealing temperature was set to 250 °C. Higher annealing temperatures were used when this temperature was not high enough to allow full penetration of the coal
by the additive or the additive was not molten at this temperature. Lower annealing temperatures were used for the lower molecular weight additives so that they did not evaporate significantly during annealing. Annealing temperatures are listed in Table 1. During both heating steps NMR “solid-echo” signals12 (with a pulse separation of 12 µs) were measured at 2 s intervals and summed over 30 s, using an automated PMRTA instrument based on a Bruker Minispec and operating at 20 MHz. The summed signals were recorded every 75 s during the pyrolysis, giving a temperature resolution of 5 °C. A 2 s interval was used between consecutive solid-echo pulse sequences, which was long enough to allow full recovery of the signal amplitude of the coal (maximum T1 ∼ 150 ms), and the pitch (maximum T1 ∼ 400 ms near room temperature). However, the pure model compounds, when solid, had T1 values that were longer than the repetition interval. For example, the T1 of biphenyl at room temperature is reported as 910 s.13 This meant that during the first heating step, the NMR signals produced by the model compound in a mixture were attenuated until the additive melted. However, the T1 values of the annealed mixtures were comparable to that of the untreated coal, and thus their signals were not attenuated because of T1 effects. The substantially reduced T1 value for the additives in the annealed mixtures confirms that they were intimately associated with the coal. Data Analysis. The NMR signals obtained from PMRTA measurements can be characterized in a number of ways. A particularly useful characterization is one that provides a measure of the extent to which the sample is mobile. The method used here to estimate this quantity was to Fourier transform the time-domain signal to get the frequency domain response g(ν) from which the M2T16 parameter10 was calculated using the formula
M2T16 )
∫
16kHz 2
0
ν g(ν) dν/I(0)
(1)
where I(0) is the NMR signal amplitude at zero time. Although there are a variety of ways of analyzing NMR signals to obtain estimates of the extent of mobility of materials, the (12) Powles, J. G.; Strange, J. H. Proc. Phys. Soc. (London) 1963, 82, 6-15. (13) Murray, D. P.; Dechter, J. J.; Kispert, L. D. J Polym. Sci., Polym Lett. 1984, 22, 519-522.
Interaction between Coal and Aromatic Hydrocarbons
Energy & Fuels, Vol. 12, No. 3, 1998 633
Figure 1. F pyrograms of the coal, the pitch, and a 4:1 coalpitch mixture before and after annealing at 250 °C. The dashed line indicates the F pyrogram expected if the coal and pitch did not interact.
Figure 2. F pyrograms of the coal and 4:1 mixtures of coal and anthracene (ANT), and coal and deuterated anthracene (DANT). The dashed lines indicate the F pyrogram of the coaladditive mixtures during the first heating step.
M2T16 parameter was preferred because (1) it is robust (not seriously affected by small signal distortions), (2) a value can be obtained for any signal shape (does not require an assumed model for curve-fitting) and, (3) unlike parameters based on line width, it is additive10 when weighted by the signal amplitude per gram of sample. M2T16 varies from 0 for fully mobile materials to ∼52 kHz2 for fully rigid materials.10 The M2T16 parameter is rescaled here to give a more intuitively understood fusion parameter (F) using the formula
However, when the annealed mixture was cooled and then reheated, the F values at temperatures below 200 °C were found to be much lower than during the first heating stage. Since the F values of neither the pitch nor the coal alone were affected by prior heat treatment even at 300 °C, this result demonstrates that after the pitch penetrated the coal, the pitch was partially immobilized by the coal at temperatures below 200 °C. The F value of the annealed coal-pitch mixture increased smoothly with temperature from values substantially less than those predicted if there was no interaction to values substantially greater than this prediction. There was no evidence of a sharp transition at any intermediate temperature. Coal-Anthracene Mixtures. The melting point of anthracene in the coal-anthracene mixture during the first heat treatment is clearly evident in the F pyrogram by the rapid increase in F value near 200 °C (Figure 2). No such sharp increase is evident during the second heating stage, which indicates that there was no crystalline anthracene in the annealed mixture: the anthracene was presumably completely absorbed by the coal. However, the pyrograms also show that, contrary to the findings for the coal-pitch mixture, the coalanthracene mixture was more fused during the second heat treatment than during the first in the temperature range from ambient to 200 °C. The annealed coalanthracene mixture was partially fused even at temperatures as low as 50 °C, that is, 170 °C below the melting point of anthracene. To check if supercooling effects were responsible for the observed fusion in the mixture on reheating, a sample of annealed mixture was stored overnight at -18 °C prior to reheating. This freezing had no effect on the subsequent fusion behavior of the mixture, indicating that supercooling effects are unlikely to explain the observed fusion of the mixture at low temperatures. The extent to which the annealed mixture was fused increased steadily with temperature up to ∼320 °C, above which the anthracene evaporated. A second experiment was performed using a 4:1 mixture of coal and deuterated anthracene. The deuterated anthracene did not give a proton NMR signal, and so the fusion behavior recorded when its mixtures with coal are heated was that of the coal only. Thus, any differences between the F pyrograms of the coal and
F ) 100(52 - M2T16)/52
(2)
thereby providing a value that is a measure of the percentage of the material that is mobile.14 F ranges from 0 for fully rigid materials, such as dry coal at room temperature, to 100 for a fully mobile material, such as molten pitch or water. Plots of F values against temperature, termed F pyrograms, were obtained by a cubic spline smoothing of the data. Because F is theoretically an additive parameter, its predicted value for any mixture having components that do not interact is the average of the F values of the components weighted by the signal amplitude per gram of each component. Significant differences between the measured value for the mixture and this calculated value indicate that the components of the mixture interact to affect the extent to which the mixture is fused. This calculation assumes that T1 attenuation effects, if present, are the same for the components and the mixture. Of the mixtures examined here, this was true only for coal and pitch, and thus comparisons between calculated and measured F values were confined to these mixtures.
Results Coal-Pitch Mixture. Figure 1 shows the plots of F versus temperature (the F pyrograms) for a 4:1 mixture of coal and pitch during the two heat treatment steps. The low F values of the coal below 300 °C indicate that it is essentially rigid at these temperatures. During the first heat treatment stage, up to ∼200 °C, the F values of the 4:1 coal-pitch mixture were the same as those expected if there was no interaction between the components. Above 200 °C the measured F value started to increase above the values predicted for no interaction. This agrees with previous findings11 and indicates that the pitch had started penetrating and mobilizing the coal. (14) Sakurovs, R. Fuel 1997, 76, 615-621.
634 Energy & Fuels, Vol. 12, No. 3, 1998
Figure 3. F pyrograms calculated for the anthracene in the 4:1 coal-anthracene mixtures before and after annealing.
Figure 4. F pyrograms of a 4:1 mixture of coal and 7,8benzoquinoline before and after annealing.
the coal with added deuterated anthracene could be attributed to the deuterated anthracene’s modifying the extent to which the coal becomes mobile. (No significant exchange of deuterium between anthracene and the coal was expected below 280 °C at the heating rate used.) Figure 2 shows that deuterated anthracene started to mobilize the coal as soon as it melted. When the annealed mixture was reheated, a significant fraction of the coal is seen to be fused, even at temperatures N-heterocyclic compounds, although at temperatures of ∼200 °C this distinction was lost. The stronger binding of the aromatic hydrocarbons compared to the polyphenyls of the same molecular weight can be attributed to the former compounds being able to form stronger π-π-type bonds. The N-heterocyclics were even more strongly held, which indicates that electrostatic interactions add to the stability of coaladditive interactions. Specific binding sites between pyridine and coal have been previously postulated.15,16 This simple picture is muddied by the fact that the oxygen-containing dibenzofuran appears to bind to the coal as weakly as the polyphenyls and that carbazole, which is a proton donor as well as acceptor, is similar in its behavior to N-heterocyclics. The extent of fusion of annealed coal-additive mixtures at a given temperature generally decreased with increasing molecular weight of the additive for the nonpolar hydrocarbons. This is intuitively reasonable since molecular motion of the mixture would be expected to decrease with increasing molecular weight of the additive. The presence of deuterated anthracene increased the extent of fusion of the coal by 35% at 300 °C, indicating that at least 35% of the coal structure must have become intimately associated with the anthracene. Penetration of coals by materials such as anthracene must be rapid at these temperatures. This suggests that there is some degree of molecular mobility in coal that allows molecules the size of anthracene to diffuse through it. Limited molecular mobility in coals even at relatively low temperatures has been observed.17,18 (This molec(15) Vassallo, A. M.; Wilson, M. A. Fuel 1984, 63, 571-573. (16) Ripmeester, J. A.; Hawkins, R. E.; MacPhee, J. A.; Nandi, B. N. Fuel 1986, 65, 740-742. (17) Wert, C. A.; Weller, M. J. Appl. Phys. 1982, 53, 6505-6512. (18) Weller, M.; Wert, C. A. Fuel 1984, 63, 891-896.
Sakurovs
ular mobility would not be detected by PMRTA if the characteristic frequency of its motion was 0.5 nm diameter are present throughout the coal structures at these temperatures but not at lower temperatures, which seems unlikely. When relatively small amounts of pyridine or benzene are added to coal, pyridine is more strongly bound than benzene (Barton and Cheng, unpublished data), which agrees with the trends observed here. However, pyridine is a more effective coal solvent than is benzene. Thus, it is not implied that strongly bound additives (such as dipyridyl or pitch) are poor coal solvents. Rather, the converse would be expected: compounds that bind strongly to coal at low concentrations would be expected to dissolve coal more readily when added at high concentrations. The results presented here were confined to one coaladditive ratio and one coal. Studies of the influence of coal rank on the magnitude of the effect of interactions of these additives with coal would demonstrate how the relative importance of the contribution of polar and nonpolar interactions to coal structure varies with coal rank. Conclusions High-temperature NMR measurements of mixtures of a bituminous coal and a variety of aromatic hydrocarbons have revealed the following: Aromatic hydrocarbons readily penetrated the coal at elevated temperatures. The rate of penetration of coal was slower for bulky molecules but occurred even for molecules as bulky as 1,3,5-triphenylbenzene at temperatures of ∼300 °C. The interaction between aromatic hydrocarbons and the coal affected the physical behavior of both materials. The magnitude of the interaction was independent of both the shape and the melting point of the hydrocarbon molecule. The extent of molecular mobility of the coal-additive mixtures at 100 °C decreased steadily with increasing molecular weight for molecules of the same class. For a given molecular weight, the extent of fusion decreased in the order polyphenyls > polycyclic aromatics > N-heterocyclics, suggesting that the strength of the attraction between the coal and additive increases in the reverse order. EF9702153 (19) Larsen, J. W.; Hall, P.; Wernett, P. C. Energy Fuels 1995, 9, 324-330.
