Energy & Fuels 1994,8, 266-275
266
Phase Behavior and Macromolecular Structure of Swollen Coals: A Low-Temperature lH and 2H NMR Study? Xiaolin Yang,r Bernard G. Silbernagel*l§and John W. Larsenl Exxon Research and Engineering Company, Annandale, New Jersey 08801, and Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 Received July 12, 1993. Revised Manuscript Received November 2, 199P
'H and 2H NMR line-shape studies have been used to investigate the low-temperature phase behavior of pyridine-ds and N-methylpyrrolidinone-dg (NMP) swollen-PittsburghNo. 8 coal, Illinois No. 6 coal, and Zap lignite. Good solvents like pyridine and NMP greatly reduce secondary interactions within coals, transforming them from the glassy to the rubbery state at room temperature when the solvent-to-coal mass ratio ( Ws/W,) is over about 1. Using deuterated swelling solvents permits complementary studies of the coal structure (with lH NMR) and dynamics of the deuterated solvent molecules (with 2H NMR). For solvent swollen coals with W,! W , 2, the 'H NMR line width is reduced by more than an order of magnitude, indicating the onset of a high level of molecular motion which averages the dipolar interactions among protons. The increase of line width with decreasing temperature is reversible, independent of solvent employed, and similar for all three coals examined (lignites to high-volatile type A bituminous coals). The line-width increase extending from 280 to 170 K and centered at 210 K is attributed to a glass-to-rubber transition. The 2H NMR studies of the deuterated solvent molecules suggests that they become immobile at significantly lower temperatures than that observed for the transition of the coals. Analyses of the NMR data suggest isolation of the individual components of the coal as a consequence of the swelling process.
-
-
~~
Introduction Understanding the phase behavior of macromolecular systems is an important aspect of their characterization, not only for the structural information it provides but also because the structural phase directly and significantly affects the behavior of the material.2 Although coal is one of the most complicated organic materials known,3 its phase behavior exhibits many characteristics which are common to typical cross-linked polymer system^.^ In fact, the phase behavior of swollen coal is being studied by several groups because of its important role in coal reactivity and behavior."" One characteristic phase behavior of a macromolecular system is its glass-to-rubber transition. At room temperature, untreated coal is a glassy material.12 Several
* Author to whom correspondence
should be addressed. A preliminary account of some of this work has appeared in Energy & Fuels (ref 1). $Engelhard Corporation. IExxon Research & Engineering Co. I Lehigh University. @Abstractpublished in Aduance ACS Abstracts, December 1,1993. (1) Larsen, J.; Silbernagel, B.; Yang, X. Energy Fuels 1993, 7, 1146. (2)Treloar, L. R. G. The Physics of Rubber Elasticity; Clarendon Press: Oxford, U.K., 1975. (3)van Krevelan, D. W. Coal, Coal Science and Technology;Elvesier: London, 1981 Vol. 3.Sanada, Y.; Honda, H. Fuel 1956,9,516. (4)Green, T.; Kovac, J.;Brenner, D.; Larsen, J. W. In Coal Structures; Mayers, R. A., Ed.; Academic: New York, 1982.Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985,50,4729-35. (5)Lucht, L. M.; Larsen, J. M.; Peppas, N. A. Energy Fuels 1987,1, 56-8. (6) Yun, Y.; Suuberg, E. Energy Fuels 1992, 6,328-30. (7)Mackinnon, A. J.; Hall, P. J. Fuel 1992,71,974-5. (8)Barton, W. A.; Lynch, L. J.; Webster, D. S. Fuel 1984,63,1262-8. (9)Kamienski, B.; Pruski, M.; Gerstein, B. C.; Given, P. Energy & Fuel 1987, 1, 45-50. (10) Jurkiewicz, A,; Bronnimann, C. E.; Maciel, C. E. Fuel 1990, 69, 804-9. (11)Yang, X.;Larsen, J. M.; Silbernagel, B. G. Energy Fuels, 1993,7, 439-445. t
~
different glass-to-rubber transition temperatures have been reported for untreated coals and there is disagreement as to the identity of Tg.Solvent swelling dramatically reduces Tgin many systems, including coals. Recently, we reported the results of a quantitative 'H NMR spinecho relaxation study of the phase behavior of pyridine-ds and NMP-dg swollen coals at room temperature.ll Like earlier workers,a10 we found that both Illinois No. 6 and Pittsburgh No. 8 coals undergo a significant phase change from a glassy material to a much more mobile, presumably rubbery, form when swollen with either pyridine or NMP. This is in good agreement with the mechanical measurements of Brenner.12 This work raised some obvious questions: What is the effect of temperature on swollen coals? Does cooling induce any phase transitions such as a rubber-to-glass transition? Does the balance between rigid and mobile coal components depend on temperature? A second class of questions addressesthe swelling process itself. How effective are the solvent molecules at removing the secondary interactions (polar,van der Waals, aromatic K--a, etc.) among the components of the coal structure? How similar are the physical properties of the organic structures in coals of different ranks after the secondary interactions are removed? To provide a wider range of secondary interactions, we have added Beulah-Zap lignite to the coals of the previous study (Illinois No. 6, a highvolatile bituminous coal of type C, and Pittsburgh No. 8, a high-volatile bituminous coal of type A). We have also chosen two swelling solvents, pyridine and NMP, of comparable polarity but different molecular structure. To probe the process of removing secondary interactions, we have examined coals with a significant range of solventto-coal ratios ( W J Wc). As we will demonstrate in the (12)Brenner, D. Fuel 1984,63, 1324-8. Brenner, D. Fuel 1985, 64, 167-73.
0SS7-0624/94/250S-0266~0~.5QIQ0 1994 American Chemical Society
Structure of Swollen Coals following sections, the secondary interactions between coal structure components can be removed at sufficiently high W$ W,values. The temperature response of these isolated components is very similar for all three coals, i.e., from the lignite to the upper end of the high-volatile bituminous rank range. Parallel examination of the coal and solvent molecule variations with temperature confirm that the response we see is representative of changes of properties of the coal structure and not solvent freezing effects. We have chosen to use lH wideline NMR as the principal tool in these phase behavior studies because it is very sensitive to structural mobility in the sample.13 The NMR properties observed depend not only on its chemical environment of the coal but also on its physical state. For example, in a glassy solid, the NMR properties are dominated by the static dipolar-dipolar interactions of protons in the lattice. The spin-spin relaxation decay is fast and can be approximated by a Gaussian functi0n.l' Fourier transformation of the free induction decay (fid) signal gives a broad NMR peak of roughly Gaussian shape. On the other hand, a rubbery macromolecular structure allows more freedom of motion. This mobility leads to an exponential decay of the nuclear magnetization which, after Fourier transformation, yields a greatly narrowed NMR peak of Lorentzian shape. The dramatic difference in the NMR line width for glassy and rubbery materials provides a convenient and sensitive way toprobe the phase changes in swollen coals. A key element in our experimental strategy has been to use perdeuterated swelling solvents. Since there is no hydrogen atom exchange between the solvent and the coal organic structure, we can use 1H NMR to study variations of the coal structure and 2H NMR to study changes in the physical properties of the solvent. This is particularly important since both the coal structure and the imbibed solvent molecules are likely to experience changes in physical properties between 200 and 300 K. Motion also narrows the 2H NMR spectrum and partial motional averaging of the 2H quadrupole interaction reflects the degrees of motional freedom accessible to the perdeuterated m01ecules.l~ Spin-lattice relaxation has also been used to study properties of various materials. Although it is not directly related to local nuclear interactions, it is sensitive to the phase changes, e.g., the glass-to-rubber transition, of a material.15 Since spin-lattice relaxation is influenced only by high-frequency fluctuations in the structure (i.e., motions at the Larmor frequency of the protons-lo9 s-l for our experimental conditions), it is complementary to the line-width variation experiments, which are sensitive to fluctuations at frequencies 105 s-l. High temperature NMR has been used by Lynch and coworkers to probe the plasticity of coals.16 Little work has been done at low temperature, except that Jurkiewicz and co-workers studied the 1H spin-lattice relaxation of swollen coals as a function of temperature.17 In the present
Energy & Fuels, Vol. 8, No. 1, 1994 267 work, we studied the phase behavior of swollen coals using NMR techniques to probe different parts of the swollen coals: lH line width for coal protons, 2H NMR for sorbed deuterated molecules, and spin-echo relaxation for the whole swollen system. Experimental Section The procedures for sample preparation and NMR measurements are given here briefly because detailed descriptions have been provided previously.llJ* The coals used in this work were obtained from the Argonne Premium Coal Program.l9 Native coals were dried under a vacuum of about 1od Torr at room temperature for about 48 h (these conditions remove all water detectable by 1H NMR).ls Swollen coal samples were prepared in 5-mm NMR tubes under a dry Nz atmosphere. The coalsolvent mixtures were warmed in an oven at 60 "C for more than 2 weeks and then kept at room temperature for more than 2 months before the measurements to ensure equilibrium. The lH line width was monitored as a function of contact time with the solvent to establish that equilibrium had been reached. The preextracted swollen Illinois No. 6 coal samples were prepared by exhaustively Soxhlet extracting the coal with NMP-dg for 5 days, and then removing a portion of the solvent by evacuation. The swelling ratios of the samples were determined by two methods: comparingthe integrated solvent NMR signal against standard samples, and weighing the swollen and dried samples. Both methods give the same results within experimental error. Deuterated pyridine (99.96% D) was obtained from Aldrich Chemical Co. NMP-d9 was obtained from Cambridge Isotope Lab (99%D). The NMRmeasurements were carriedout usinga Bruker MSL360 pulsed NMR spectrometer, operating at 360.0 MHz for 'H nucleiand 55.8MHz for 2H. The signalwas recorded by a Bruker BC-131transient digitizer with a sampling speed of 0.2 ps. The data were stored and analyzed by an Aspect-3000 computer. The NMR probe is proton-free and was shimmed to give a line width of less than 100 Hz for a liquid cyclohexane sample. For 'H, a solid-echopulse sequence was used. Typical values for the 90" pulse width and pulse spacing are 1 and 3 ps, respectively. For 2H, a quad-echo pulse sequence was used, and the typical parameter settings are 4 and 3ps for pulsewidthand pulse spacing. For a typical NMR experiment,the repetition time, scannumbers, data points for FID, and data points for Fourier transformation are 6 s, 16, 4 K and 32 K, respectively. Temperature control is crucial in variable temperature experiments. In this work, the temperature was controlled and measured by a Bruker variable-temperature unit. Using the methanol chemical shift to calibrate the temperature of both proton and deuterium probes,20we found that the fluctuation of the temperature is less than f l "C, and the error in absolute temperature reading is less than f 3 "C.
Results
In this section, we will begin with a discussion of the swelling process at room temperature, with our principal focus on the variation of the proton line widths of the coal structure with solvent loading ( W$Wc). We will then describe the variation of proton line width with temperature and demonstrate its reversibility. We will then examine solvent- and rank-dependent effects on the (13) See, for example: Abragam, A. Principles ofNuclear Magnetism; temperature dependence of the proton line width. After Clarendon Press: Oxford, U.K., 1961. this focus on the coal structure provided by the 'H NMR (14) Silbernagel, B. G.; Garcia, A. R.; Newsam, J. M.; Hulme, R. J. observations, we will study the solvent behavior by 2H Phys. Chem. 1989,93,6506-11. (15) Miller, J.B.;McGrath,K.J.;Roland,C.M.;Trask,C.A.;Garroway, NMR. Finally, we will probe the high frequency response A. N. Macromolecules 1990,23,4543-7. (16) Lynch, L. J.; Webster, D. S. Fuel 1982,61, 271-5. Lynch, L. J.; Webster, D. S.; Sakurovs, R.; Barton, W. A.; Maher, T. P. Fuel 1989, 579-83. (17) Pislewski, N.; Jurkiewicz, A.; Tritt-Goc, J. Exp. Tech. Phys. 1988, 36, 387-95.13.
(18) Yang, X.; Garcia, A. R.; Larsen, J. W.; and Silbernagel, B. G. Energy Fuels 1992,6, 651-5. (19) Vorres, K. S. Energy Fuels 1990,4, 420-30. (20) Bruker MSL-360 Operation Manual.
Yang et al.
268 Energy &Fuels, Vol. 8, No. 1, 1994
I '
,
I
I
I
I
'1
I PyridinedS/lllinoisNo 6 Coal 1
i-
I
35x10'
I
I
I
No. 6 Coal ws/wc=1.5
Pyrldine-dl/lilinois 3c
-
25
N
5
p5 0
.--I
20
15
to
5
0
-WslWc= -
I
180
1
ZOO
220
240
260
280
Temperature (K)
0.0 0.2 0.5 1.o
Pyridine-d5, Zap Llgnite WslWc=1.5
30
I .3 1.5 2.0 23
-
, 160
25
\
1
I-
2.6
0
60000
40000
0
20000
Herb
Figure 1. Effect of solvent loading on lH NMR line shape for pyridine-d5 swollen Illinois No. 6 coal at room temperature. 40x10'
35
b,
-t+-
5 1 0 . J .
0.0
I
I Pyrldlne-d,Mls/Wc=l.5 1
1
0.4
.
0.8
.
, 1.2 1.6 WSIWC
.
PlltsburgliNo. 8
, 2.0
.
, 2.4
.
I 2.8
Figure 2. Effect of solvent loading on lH NMR line width for pyridine-& swollen IllinoisNo. 6 coal,Pittsburgh No. 8 coal, and Zar, lienite at room temDerature. of the coal structure by examining the spin-lattice relaxation of solvent extracted coals. 1. Effect of Solvent Loading at RoomTemperature. Figure 1 shows 1H spectra as a function of solvent loading for pyridine-ds swollen Illinois No. 6 coal. The line shapes of the spectra change from Gaussian to Lorentzian as solvent loading increases. Figure 2 gives a more quantitative measure of the full width at half-maximum peak height (which will be referred to subsequently as the line width (AH)in this paper) as a function of solvent loading for pyridine-ds swollen Pittsburgh No. 8 coal, Illinois No. 6 coal, and Zap lignite. All the three coals experience a similar and significant line narrowing as solvent loading increases. The line narrowing is nearly complete at a solvent-to-coal mass ratio (W$W,) of about 1. The line
I
I
I
160
180
2w
I 220
I
I
240
260
280
Temperature (K)
Figure 3. (a, top) Effect of heating-coolingorder on lH NMR line width for pyridine-dsswollenIllinoisNo. 6 coal as a function of temperature at W$ W, = 1.5. (b, bottom) Same as (a) except for Zap lignite. width of Zap lignite is significantly broader than the two bituminous coals for W$W,> 1.0,suggestingthat complete mobility of the lignite is not achieved at the highest W,/ W , ratios achievable for that coal (-2.0). It has been reported that the line broadening caused by electrons and free radicals in coals is negligible.10*21-22 In order to verify this for the swollen coals studied in this work, we measured the proton NMR intensity of pyridine ds swollen Illinois No. 6 coal as a function of solvent loading. The proton intensity, properly normalized for volume of swelling solvent, was constant to within & l o % ,independent of solvent loading. The line-width variations observed are therefore due to the mobility of the coal structure. 2. Effect of Temperature. The lH line width of the swollen coals broadens dramatically as the temperature is lowered. Figure 3a shows the line width of a typical swollen coal sample (Illinois No. 6lpyridine-d~,Wd W, = 1.5) as a function of temperature. At room temperature, the line is narrow and Lorentzian. As the temperature decreases, the line broadens dramatically and its shape changesfrom Lorentzian to Gaussian. The line broadening levels off in the vicinity of 170 K. The temperature at which the maximum variation in line broadening occurs is about 210 K. (This temperature will be called the transition temperature in this paper.) The differences between the cooling and heating curves are within experimental error, indicating that the transition is reversible. Similar reversible transitions were found for swollen (21)Jurkiewicz, A. Fuel 1986,65,1022-4. (22)Lynch, L.J., Sakurovs, R.;Barton, W.A. Fuel 1986,65,110&11.
Energy & Fuels, Vol.8, No. 1, 1994 269
Structure of Swollen Coals I
I
I
I
I
I
I
(Illlnols No. 6 , Ws/Wc=2.0
k
1
Pyrldlne-d5/lllinols No 6 Coal 30
z.
'z.-
25
-/
20
-
15-
c
-I 10
0 4 160
I
I
I
I
I
I
180
200
220
240
260
280
I-
-
0
I
I
I
160
160
2W
I
I
Effect of different solvents on 1HNMR line width for pyridine-dsand NMP-dsswollen IllinoisNo. 6 coal as a function of temperature at W J W, = 2.0. Pittsburgh No. 8and Zap lignite samples. Figure 3b shows the reversible line-width change as a function of temperature for a swollen Zap lignite sample (W$ W,= 1-51,and shows perhaps a small additional change in line width after 5 months of storage. 3. Effect of Solvent and Solvent Loading. The melting points of pyridine and NMP are 231 and 249 K, respectively,23 about 20 deg and 40 deg higher than the transition temperature of the swollen coals. Because embedding solvent in coal can decrease its melting temperature,%it is necessary to establish if the observed transition is caused by solvent freezing. Three experiments were designed to answer this question. In the first experiment, the lH line width of a NMP-ds W,= 2.0) was measured swollen Illinois No. 6 sample (W$ as a function of temperature. Those results are compared in Figure 4 with data from a pyridine-ds swollen sample withsame levelof solvent loading. If the low-temperature transition is due to the solvent freezing, the transition temperature for the NMP swollen coal should occur at a higher temperature. As shown in Figure 4, this is not the case. The two swollen coals have essentially the same transition temperature. This indicates that the transition is due to the coal structure, rather than the solvent freezing. In the second experiment, the 'Hline width of pyridinedg swollen coals was studied as a function of both temperature and solvent loading, as shown in Figure 5a where the lH line width of pyridine-ds swollen Illinois No. 6 coal is plotted as a function of temperature for three different values of W$W,at 1.5., 2.0, and 2.6. The NMR line width for the W8/W,= 1.5 sample is broader at high temperatures than for the other two samples. No significant difference is seen between the W$W,= 2.0 and 2.6 samples, indicating that the behavior of the coal organic matter is independent of coal swelling for WB/ W,1 2.0. Similar line-width responses were seen with Pittsburgh No. 8 and Zap lignite samples. Figure 5b shows the lH line width of pyridine-ds swollen Zap lignite as a function of temperature for three different solvent loadings: W,/ W,= 1.5., 1.7, and 2.0. All of these results indicate that this transition is a property of the coal rather than the solvent. ~
(23) Handbook of ChemistryandPhysics, 73thed., CRC: BocaRaton, FL, 1992. (24) Hall,P. J.; Larsen, J. W. Energy & Fuel 1991,6228-9. Hall, P.; Larsen, J. Energy FueZs 1993, 7,47-51.
I 240
I
260
280
I
I
Temperature (K)
Temperature (K)
F i g u r e 4.
I 220
35x10'
1
I
I
I
I-
IPyrldlne-d5, Zap Lignite] 25
5-
04 i6p
I
I
I
I
I
I
180
2W
220
240
260
280
k
Temperature (k)
(a, top) Effect of solvent loading on 'H NMR line width for pyridine-ds swollen Illinois No. 6 coal as a function of temperature. (b, bottom) Same as (a) except for Zap lignite. F i g u r e 5.
In the third series of experiments, 2HNMR of the solvent was also studied as a function of temperature. As we will discuss below (section 51, the solvent molecules are largely mobile at temperatures in this transition region, again supporting the observation that changes in line width are associated with changes in motion of the coal protons. 4. Effect of Coal Rank. Figure 6a shows the IH line width as a function of temperature for three coals swollen with pyridine-ds at W$W,= 1.5. All three swollen coals show a similar change in NMR behavior as the temperature decreases: a narrow Lorentzian line shape at room temperature and a broad Gaussian at low temperature. There is a well-defined trend the higher the coal rank, the sharper the transition. When W JW,equals 2.0, as shown in Figure 6b, the difference in the transition temperature for the three coals is significantly smaller. These data suggest that, for a sufficiently high degree of swelling (Ws/Wc 1 2.0 for the examples shown here), the mobility of the organic components in the swollencoal is independent of rank for coals which range from lignites to the high-rank end of high-volatile bituminous coals. While this behavior might be different in coals of higher rank, and may exhibit some dependence on organic matter type in other coals, the similarity of response for these three coals is striking. The systematics of the experiment suggest that only when W$W,is above a critical value (about 2.0) is the coal network fully mobile. 5. Effect of Preextraction. It is believed that extraction of coal with agood solvent can cause the organic coal matrix to relax to a more stable c ~ n f o r m a t i o n . ~ ~ Extraction can also remove a significant amount of organic
Yang et al.
270 Energy &Fuels, Vol. 8, No. 1, 1994 35x10’ J
I
I
I
I
I
I
1
1 Pyrldlne-d,/Ws/Wc=l.5 I
-
30
I
4
35x10’
L
I
I
l
l
1
Pre-extracted Illinois No. 6 Coa NMP-d9, Ws/Wc=1.5
30-
-
-
I 25
I
d
IO
5-
5
0 160
180
2W
I
I
220
240
260
280
I
I
I 160
I
I
I
I
180
200
220
240
I
260
Temperature (K)
Temperature (K) I
I 260
35x10’
t
WslWc=I .5
I c‘5 .-t
-
25
‘E .-
15
-I
-I
L
0 160
180
2W
220
240
260
280
Temperature (K)
I
I
I
I
160
180
200
220
‘-----+I I
I
240
260
,
c J
280
Temperalure (K)
Figure 6. (a, top) Effect of temperature on lH NMR line width for pyridined6 swollen Illinois No. 6 coal, Pittsburgh No. 8 coal, and Zap lignite at W,/ W, = 1.5. (b, bottom) Same as (a) except W$WC= 2.0.
Figure 7. (a, top) Effect of heating-cooling order on 1H NMR line width for NMP-ds swollen preextracted Illinois No. 6 coal as a function of temperature at WJ W, = 1.5. (b, bottom) Same as (a) except for nonextracted swollen coal.
matter from coals.’l There is a great deal of speculation about the kinds of molecules removed. Do they represent a specific mobile phase? Are the physical properties of the residue significantly more rigid? It is, therefore, important to study the effect of extraction on the phase behavior of swollen coals. Figure 7a shows the lH line width of a NMP-dg swollen preextracted Illinois No. 6 coal sample (WJW, = 1.5) as a function of temperature. Possible hysteresis in the organic response was examined by line-width observations conducted upon cooling and heating. In order to illustrate the extraction effect, the 1H line width of NMP-d9 swollen nonextracted Illinois coal (WE/W , = 1.5) is plotted as a function of temperature in Figure 7b. Comparing parts a and b of Figure 7, the cooling curvesfor both samples are similar with a transition temperature at about 210 K. The heating curves are different. For the nonextracted sample, only one reversible transition was found at about 210 K. Two transitions were observed for the preextracted sample: (a) one at about 210 K and another at about 250 K. We believe that the 210 K transition in Figure 7a is same as observed for the nonextracted swollen coals. The 250 K transition is close to the melting point of NMP (249 K). However, more evidence is needed to establish whether this onset of greater mobility in the coal structure (as reflected in the narrowing of the proton NMR) is related to melting of the solvent-presumably in pores of the extracted coal structure. Several additional experiments can help to clarify this reversibility problem. In unertracted Illinois No. coal, swollen with NMP to W$W , = 2.0, a very small irreversible
response is observed, again near 250 K (Figure 8a). There was no observed irreversibility of Illinois No. 6 coal swollen with pyridine at either WE/W, = 2.0 or 2.6. Also in extracted Illinois No. 6 coal, swollen to W$W, = 2.0, an even more pronounced hysteresis is observed (Figure 8b) than in the W$ W, = 1.5 case. These data suggest that, particularly in the extracted coals, the NMR molecules are in phase regions of significant size for freezing to occur. The absence of a transition upon cooling of the sample is presumably the result of supercooling of the NMP. The most strikingaspect of these results is that all of the protons in the coal structure appear to be affected by the solvent, since there is no obvious departure from the usual line shape in this temperature region. 6. 2HNMR of the Solvent. 2HNMR provides details about the solvent behavior in coals at different temperatures. As shown in Figure 9a, at room temperature 2H NMR of NMP-ds in extracted Illinois No. 6 coal shows only a very narrow peak. The line width is almost independent of the solvent loading when W$ W , is over about 1.0. The horned peak patterns expected for deuterated solids do not appear until temperature is lowered to about 200 K. There is a sharp transition between 200 and 180 K (Figure 9a-3). In Figure 9a, the “horned” peaks with a splitting constant of about 83 kHz arise from the rotating methyl group of NMP-ds, while the horned peaks with a splitting constant of about 171 kHz are due to the rest of the deuterium atoms.14 This “horned” peak pattern remains the same as the temperature is decreased to about 140 K. When the sample is warmed up, as shown in Figure 9a, similar 2H NMR
Structure of Swollen Coals 35x10'
I
4
Energy &Fuels, Vol. 8, No. 1, 1994 271
I
[NMP-d9/lllinois No. 6, Ws/Wc=2.0
20 -
15
-
10
-
I
i
5-
0
I 160
I 180
I
I
I
I
I
200
220
240
260
280
I-
Temperature (K) 1 oJ 5000
I
-
I
I
I
2H NMR Llnewldth (Center Peak)
30
Swollen llllnols No. 6 Coal
25
-
r
20
6
15
3ow
-
2000
-
'5
t
3
10
5
0 160
180
200
220
240
260
280
I
I
I
1
2w
220
240
280
Temperature (K)
1
280
Temperature (K)
Figure 8. (a, top) Effect of heating-cooling order on lH NMR line width for NMP-de swollen preextracted Illinois No. 6 coal as a function of temperature at WJ W, = 2.0. (b, bottom) Same as (a) except for nonextracted swollen coal. patterns were found at 180 K irrespective of whether the sample was being warmed or cooled. At 200 K, the isotropic motion again becomes conspicuous,though the solid signal persists to higher temperature for the heating run than the cooling run. The "horned" structure representing residual solids at these higher temperatures is consistent with the hysteresis observed with lH line width. This experiment supports two earlier hypotheses which were based on the lH NMR results: there is only one phase transition for the cooling cycle (190 K), and two phase transitions for the heating cycle (190 and 250 K); the 190 K transition is the same for both heating and cooling cycles and is the dominant one. The line broadening of the 2HNMR center peak (Figure 9a) provides additional information about the behavior of solvent in the temperature range between 298 and 200 K, i.e., before the 190 K phase transition. In Figure 9b the line width of the center peak was plotted as a function of temperature for three samples with same level of solvent loading: N M P - d ~and pyridine-& swollen nonextracted coal and NMP-ds swollen preextracted coal. All three samples show a similar temperature dependence: a continuous line broadening. As shown in Figure 9c, a linear relationship was found between In ( A H 1 1 2 , ~- AH1/2~rland UT, where m 1 / 2 , T and A H ~ Irepresent ~,~ the line width at temperature T and the high-temperature limit. From the slopes of the plots in Figure lOc, activation energies of 4.2, 4.2, and 4.8 kcal/mol were derived for the solvent motion in the three samples. 7. Spin-Lattice Relaxation of Preextracted Swollen Coals. The lH line width is determined by the spin-
-
2H NMR (Center Peak) Illinois No. 6, Ws/Wc=1.5
3.4
3.6
3.6
I
I
I
I
1.0
4.2
4.4
4.6
,
4.8
I
5.0
IOOOm (K)
Figure 9. (a, top) Effect of temperature on 2H NMR for NMPde swollen preextracted Illinois No. 6 coal at WJW, = 1.5. (b, middle) Effect of temperature on the line width of 2H NMR center peak for NMP-de and pyridine-& swollen Illinois No. 6 coal and NMP-de swollen preextracted Illinois No. 6 coal at WJ W, = 1.5. (c, bottom) In (AZ-Z~,~,TA Z - Z ~ /as Z ~a function of 1000/T for the same samples used in (b). spin relaxation which, in turn, reflects the mobility of the structure. The spin-lattice relaxation probes high-frequency vibrations and conformational changesof the "solid lattice".13 We have chosen to examine the extracted samples, since the homogeneity of the organic matter is likely to be higher. 21' relaxation measurements were conducted using a standard saturation recovery techn i q ~ with e ~ a~ pulse train consisting of 20 pulses. The 21' is defined by the following equation: (25) Fukushima, E.; Roeder, S. B. W. Experimental Pulse NMR, Addison-Wesley Publishing Co.: Reading, MA, 1981.
p
p
A
272 Energy & Fuels, Vol. 8, NO. 1, 1994
4x10'
S In-Lattlce Relaxation
Yang e t al.
Table 1. Chemical Composition of Host CoaldB coal Zap
Illinois No. 6
PittsburghNo.8 a
t 160
180
2W
220
240
260
260
Temperature (K)
Figure 10. Effect of temperatureon spin-lattice relaxation time for dried and NMP-d9swollen ( W$ W, = 1.5)preextracted Illinois No. 6 coal.
M ( 0 ) - M(T)= M ( 0 ) exp(-T/T,)
(1)
where M ( 0 ) and M(T)are the magnetization at time zero as a function and time. The plot of In ([M(O)-M(T)]/M(O) of T for the swollen preextracted coal samplez6gives a straight line with a linear correlation coefficient ( r ) in the range of 0.995-0.999. TI was obtained from the slope. For the dried preextracted coal sample, deviations from the straight line were observed at long T values (correlation coefficient 0.985-0.990). Since we are mainly interested in a low-temperature phase change, this linear treatment is accurate enough for our purpose. Figure 10 demonstrates the temperature dependence of UT1 for both dried and NMP-dg swollen preextracted coal samples. The observed relaxation rate (UTI)is roughly proportional to temperature for both the dry and swollen coals, with no obviousfeatures in the temperature region where the linewidth variations occur. The relaxation rate is reduced by approximately a factor of 4upon swelling-a very dramatic effect. If paramagnetic centers are responsible for the proton relaxation, this large reduction in UT1 could arise from a combination of the dilution of the paramagnetic centers upon swelling and the motional decoupling of the proton dipole interaction which is usually responsible for the transfer of energy to the paramagnetic centers.
Discussion These experimental results can briefly be summarized as follows: Solvent swelling of coals leads to a dramatic reduction in proton line width and a change in proton line shape which indicates a significant amount of motion of the coal organic structure on the time scale of lo4 s in the swollen coal (Figures 1 and 2). This transition is largely complete for W./ W , 1,but residual broadening remains for the lower rank samples, particularly the Zap lignite(Figure 2). The temperature dependence of the proton line width below 300 K is remarkably similar for all completely swollen coals (W,/W, 2 2). The increase in proton line width with decreasingtemperature is reversible upon temperature cycling (Figure 3a), independent of solvent used (Figure 4) and independent of WE/W , at sufficiently high values of that parameter (usually WE/W, 1 2.0; Figure 5a,b). Pronounced hysteresis in line-width behavior is seen in preextracted Illinois No. 6 coal with
-
(26)Simon, G.; Baumann, K.; Gronski, L. Macromolecules 1992,25, 3624-8.
rank
C
lignite
73 78 83
hvCb hvAb
w t %'
(OK)
(H/C)
H
0
atomic
atomic
4.8
20
0.206
5.0
14 9
0.135 0.081
0.789 0.769
5.3
0.766
Moisture- and ash-free basis.
high loadings of NMP (Figures 7a,b and 8a,b). Parallel zH NMR studies of the solvent molecules indicate that they become immobile at temperatures below the transition region for the coal organic structure; spin-lattice relaxation measurements of high-frequency fluctuations in the dry and swollen extracted Illinois No. 6 indicate no marked change of 1/T1behavior in the transition region (Figure 10). However 1/T1is roughly 4 times smaller in the swollen coal due presumably to dilution of the spatial distribution of paramagnetic impurities and decoupling of intermolecular proton dipole interactions. In considering these well-defined systematics for the motion of the organic components in swollen coals, three questions arise. What can we deduce about the "rubbery" behavior of swollen coals from the current data? What is the nature of the rubber-to-glass transition of the coal organic matter which is observedupon cooling the sample? What do the quantitative systematics of these NMR results tell us about molecular interactions in the swollen coals? These will form the topics for the present section. Before a detailed discussion of the physical properties, a brief examination of the chemistry of the organic matter of the host coalslg is in order, as shown in Table 1. The changes in chemistry with rank are best reflected in the variation of the atomic O/C and H/C ratios. The (O/Qat varies by a factor of 2.5 from the low-rank lignite to the high-rank Pittsburgh No. 8 coal. A significant amount of this oxygen loss is associated with removal of polar groups (like carboxyls) from the organic structure during the process of coal maturation. Such functionalities will play a major role in establishing secondary interactions among the organic components of the coal. By contrast the (H/ Qat is nearly identical for all three coals, suggesting that the global hydrocarbon composition is similar throughout the rank range. 1. Rubbery Behavior of Swollen Coals. Coals contain a three-dimensionally cross-linked macromolecular network? Recent work strongly indicates that the structural character of this network is due to both covalent and weak non-covalentinteractions.Sl0 Peppas and co-workers showed that swelling significantly reduced the glass-torubber transition temperature and found an equilibrium swelling ratio (WE/W,) of about 0.8.5 Brenner reported that pyridine-swollen coals were rubbery at room temperature,12which is evidencedby the fact that thin sections of swollen coals demonstrate high swelling and mechanical reversibility. The results obtained in this work demonstrate that solvent swelling transforms coal from a glassy material to a more rubberlike material when W$ W , is over about 1 (Figures 1and 2). The equilibrium swelling ratio deduced from this work is about 2.0 for the coals and solvents studied, a value that is higher than those obtained by Peppas and co-workers.5 Furthermore, this result is in good agreement with our recent 'H spin-echo relaxation study of swollen coa1s:'l only when W6/W , is over 2.0 does the deconvolution of the fid signal give a constant ratio
Structure of Swollen Coals of intensities for the various decay components observed. More than 70% of the protons in the swollen coals show a highly mobile behavior. The high molecular mobility of swollen coals, reflected in the 10-folddecrease in line width from that observed in dried coals, indicates that swelling greatlyreduces the modulus of coal. It has been suggested that the swelling creates sufficient free volume for reorientation of the macromolecularchain segments which were locked in place in dried ~0als.24 From the viewpoint of molecular interactions in coals, swelling greatly reduces the cross-link density of coals by breaking the secondary interactions (such as hydrogen bonding, van der Waals interactions, and weak complexes). A more quantitative discussion about the effect of swelling on the weak molecular interactions is given in section 3. 2. The Glass-to-Rubber Transition of Swollen Coals. One of the characteristic properties of a macromolecular structure is its glass-to-rubber transition. The glass-to-rubber transition temperature (T,)for a swollen macromolecular gel is usually controlled by the crosslink density and the extent of swelling.26~27 The transition observed in the present experiment is centered at about 210 K and extends over a broad temperature range. The transition is reversible and independent of solvent and solvent loading for the two solvents and three coals studied. It is reasonable to assume that the transition is due to the glass-to-rubber transition of swollen coal gel. 2H NMR results provide more detailed information about the behavior of the solvent, particularly the two transitions centered at about 190and 250 K. The splitting constants of NMP at low temperature (Figure 9a) indicate that the solvent molecules are essentially motionless, except for spinning of the methyl group about its threefold symmetry axis. The 190K transition observed by2HNMR thus involves a change of solvent from an isotropic liquidlike state to a motionless solid state. The transition temperature for the solvent is about 20 deg lower than the glass-to-rubber transition of the coal network revealed by 1H NMR. This suggests that the solvent molecules stop their isotropic motion only after the coal-solvent gel system has adopted a rigid, glassy state. Using a DSC technique, Hall and Larsen observed a second-order transition for NMP swollen preextracted Illinois No. 6, which they tentatively assigned to a glass-to-rubber t r a n ~ i t i o n .The ~~ transition occurs over a narrow temperature range (about 20 deg) centered at 170 K. It is clear from the present work that this observation is not due to a glass-to-rubber transition of coal because no transition of coal protons was observed near 170 K. The 170 K transition observed by DSC is close to the 190 K transition revealed by 2H NMR, i.e., the stopping of the isotropic motion of solvent molecules. Both transitions occur over a narrow temperature range. The 20-deg difference between DSC and 2H NMR is most likely due to the sensitivity of NMR to higher frequency motions.14 The last transition observed was the hysteresis of the proton line width observed upon warming in the preextracted samples of Illinois No. 6 coal (Figures 7a and 8b). In those cases, the broadening of the proton NMR line width upon cooling followed the usual behavior for the immobilization of the organic structure but the complete narrowing of the proton line width upon heating was impeded until temperatures on the order of 250 K were (27)Errede, L.Molecular Interpretations of Sorption in Polymers; Springer-Verlag: Heidelberg, 1991.
Energy & Fuels, Vol. 8, No. I, 1994 273 reached, which corresponds to the melting point of NMP. The 2H NMR of the NMP in these samples reveals the presence of solid NMP up to -250 K in these samples. The striking feature of these data is that the presence of some solid solvent in the sample influences all of the protons in the organic structure. Comparison of Figures 7a and 8b suggests that the hysteresis is more pronounced if there is more solvent in the sample. One possible explanation of these effects could be that the frozen NMP produces strain fields on the organic which inhibit the onset of certain kinds of motion in the coal as the sample is warmed. This is particularly likely if the NMP crystals were to adopt a preferred orientation in the coal matrix upon nucleation, which would lead to the generation of significant strain fields in the coal organic. 3. Molecular Interactions in Swollen Coals. A quantitative analysis of the present NMR data provides some insights about the molecular interactions in coals and how they relate to the coal structure and the effect of swelling and temperature variations. Effect of Swelling on the Weak Molecular Interactions. It is well recognized that the glassy behavior of untreated coals results from bath intra- (covalent bonding) and inter(weak molecular bonding) molecular interactions.s7 It is believed that a good solvent like pyridine can significantly break the intermolecular interactions, causing swelling of the coal structure.u However, this general picture is largely based on macroscopic measurements such as mechanical studies and is qualitative. NMR is a microscopic technique, which provides structural information at the molecular level. The NMR line widths reflect the molecular structure by the strength of the dipolar interactions between the protons, quantitatively represented by the second moment (M2) of the NMR spectrum.28For a powder made of crystallites of random orientations, the second moment is related to the lattice structure through the equation i#j
where y is the magnetogyric ratio, Z is the spin quantum number, rij is the internuclear distance, and N is the number of protons occupying inequivalent positions in the repeating structural unit. Since the contribution of each individual proton interaction is additive, M2 can be divided into contributions from inter- and intramolecular interactions: M2,bt = M2,kkr + M2,ktra.e In an earlier discussion of NMR properties and coal structure,29an estimate of 10.8 G2was proposed for the second moment contributed from intramolecular interactions, M2,intra which is consistent with an average moment for protons in the coal structure. For a Gaussian line shape, the 'H NMR line width, AHlp, is related to the total second ~ equation28 moment, M Z ,by~the AHll2 = 2.35(M2,bd)112
(3) Thus, the contribution of the intermolecular interactions to the second moment, M2,inter can be deduced by the equation M2,inter
M2,total- M2,intra
(4)
In the present work, the 'H NMR spectrum of dried coals (28)van Vleck, J. H. Phys. Rev. 1948,74, 1168-83. (29)Gerstein, B.C.;Chow,C.; Pembleton, R. G.; Wilson, R. C. J. Phys. Chem. 1977,81, 565.
274
Yang et al.
Energy &Fuels, Vol. 8, No. 1, 1994
Table 2. Second Moment of Dried and Swollen Illinois No. 6 Coal at 150 K
dried coal swollen coal (nonextr.) swollen coal (preextr.)
44600 30 871 30 822
19.8 9.5 9.5
10.8 10.8 10.8
9.0 -0 -0
and swollen coals a t low temperature is of Gaussian line shape. From the line-width measurements and the use of eqs 3 and 4, one can estimate the contributions from interand intramolecularinteractions, respectively, and establish how much the intermolecular interaction is diminished by the swelling. Table 2 lists the values of second moments for untreated, NMP-d9 swollen ( W d W , = 2.01, and NMPdg swollen (W$ W , = 2.0) preextracted Illinois No. 6 coal at T = 150 K. The values listed in Table 2 reveal some interesting features of the structure of untreated and swollen coals: 1. For the untreated coal, intra- and intermolecular interactions have comparable contributions to the line broadening. 2. For the swollen coals, the Mz,t,,Mis due almost entirely to the intramolecular interactions. In other words, most of the secondary interactions in coal are removed by the solvent swelling. 3. Preextraction has no significant effect on the swollen coal structure. After swelling, a swollen macromolecular framework is established regardless whether the small molecules were extracted or not. At 150 K, the extractable molecules are quite motionless. We must point out here that this second moment analysis is only semiquantitative. While the second moment can be determined quite accurately (*5%). The line shape is not strictly Gaussian and the value of M Zfor protons of different chemical types (aliphatic vs aromatic) will be significantly different. What is observed is a statistical average. However, the basic features of the above arguments discussed above are correct. Energetics of Line Broadening. Another important parameter that connects the NMR line-width measurement and molecular dynamics of coal structure is the correlation time, T,, defined as the lifetime of a molecule or a structural domain in an equilibrium position.l3 For a simple, first-order estimate, the correlation time can be deduced from the line width through the following equation:s0
UT, = w(AH1/2- AHlIz,JP tan [0.5((AHIlz~l/Z,r)~(~1/ 2-l/2,r))211 ,,
(5) where a is a constant250nthe order of 1, and r and 1in the subscripts denote the residual line width at high temperature and the asymptoticline width at low temperature. For thermally activated processes, the correlation time would depend exponentially on the absolute temperature: 7,
= 7, exp(EJRT)
(6)
where E, is the activation energy. Using eqs 5 and 6, a linear relationship was found ~ ) 1 / T for the swollen coals studied in between in ( 1 / ~and this work. Figure 11 demonstrates such a plot for a pyridine-& swollen Illinois coal sample, which gives an E, value of about 5 kcal/mol. Similar values were obtained (30) Solid Electrolites, Hagenmuller and van Gool, Eds.; Academic Press: New York, 1978; Chapter 7.
,
,
I
I
I
1
3.5
4.0
4.5
5.0
5.5
6.0
1000/T(K)
Figure 11. Dependence of the correlation time on temperature for pyridine-& swollen Illinois No. 6 coal at WJ W, = 2.0.
for Pittsburgh No. 8 coal and Zap lignite. These results are very close to the activation energies obtained for the line broadening of 2H NMR center peak (Figure 9c). The existence of the linear relationship and the small activation energy values indicate that the line broadening of swollen coal gels at low temperature is a thermal and, most likely, a diffusion-controlled process.31 Similar conclusionshave been reached by others.22 The above correlation time analysis is only semiquantitativebecause AH112,1and hH1, 27 have not fully leveled off at 150 K and room temperature, and eq 5 assumes Lorentzian relaxation decay. Nevertheless, we believe that the qualitative picture suggested by the above analysis is approximately correct. Weak Molecular Interactions in the Zap Lignite. Among the three coals studied in this work, the Zap lignite behavesdifferently than the other two. As shown in Figure 2, its high-temperature line width is significantly broader when swollen than the other two coals. The change in line width as a function of swelling ratio is smaller for Zap lignite. The different behavior of the lignite can be also seen in Figure 6: its line width is significantly broader over the entire temperature range and its glass-to-rubber transition is not as sharp as the other two coals. This is not surprising if one considers the fact that Zap lignite is not really a U ~ ~ a l nIts. 3oxygen and hydrogen content are significantly higher than a bituminous coal. It is believed that more polar noncovalent interactions exist in the lignite, such as those involvingthe carboxyl gr0ups.3~This may reduce the motion of protons or proton-containing groups, causing the line broadening behavior and decreasing the sharpness of the glass-to-rubber transition. The Nature of Solvent-Coal Interactions. I t is interesting to know the strength of the coal-solvent interactions responsible for the 190 K transition. One of the unique features of 2H NMR is that its spectral shape and width can provide details about the interactions between sorbed molecules and the substrate. This is because that the quadruple interaction Hamiltonian for the 2HNMR has the form:33 H = (e2qQ/h)[3cos2 /3 - 11/2[3 cos2 0 - 11[31,* I ( I + 111 (7) where eq is the strength of the electric field gradient, eQ (31) Lamen, J. W.;Lee, D. Fuel 1983,62, 1351-4. Shawer, S.,EnergyFuels, 1990,4,74, and references (32) Larsen, J.W.; therein. (33) See, for example: Cohen, M. H.; Reif, F. Solid State Chemistry 5; Academic Press: New York, 1957.
Structure of Swollen Coals is the magnitude of the 2H nuclear quadruple moment, /3 is the angle between the axis of molecular rotation and the direction of electric field gradient, and fl is the angle between the axis of rotation and the applied magnetic field. The direction of the electric field gradient is along a C-D bond in a deuterated organic molecule. The factor [3 cos2 /3 - 1]/2 in eq 7 is the narrowing factor of the quadrupole splitting due to the motion of molecules. For a static molecule, the factor is unity, and for a completely isotropic motion the factor is zero (the horned peaks are replaced by a narrow single peak). When the motion is partial, the narrowing factor between 1and 0 is observed. Thus, for a deuterated-solvent-swollenmaterial, the shape and the quadruple splitting constant give valuable information about the detailed motion of the deuterated molecules and its interaction with the solid lattice. As shown from Figure 9, three types of deuterium motions were observed: completely isotropic liquidlike free motion, methylgroup rotation of solid deuterium with a splitting constant of about 83 kHz, and completely “frozen” solid deuterium with a splitting constant of about 171 kHz. Importantly, the splitting constant of the “hornedn peaks remained almost constant over its observational temperature range (Figure 9a). Similar results were obtained for pyridine-d5 swollen coals, which give a splitting constant of 172 kHz for solid deuterium at low temperature. These microscopic spectroscopic results point to the fact that the interaction between the solvent and coal matrix which is responsible for the 190 K transition is actually quite weak because an anisotropic narrowing would result if the motion of NMP-d9 or pyridine-d, were partially restricted. These results are in good agreement with those of Vassallo and Wilson on 13C CP/MAS NMR studies of pyridine swollen Liddle coals (W$ W , = 1.0).34 They concluded from their study that, although pyridine bonds to coal, it does so in a manner which allows motion sufficient to average most of the chemical shift anisotropy. A similar conclusion was also reported by Ripmeester and co-workers, based on a 15N NMR study of pyridine embedded in fresh and oxidized (34)Vassallo, A. W.;Wilson, M. A. Fuel 1984,63,571-3.
Energy & Fuels, Vol. 8, No. 1, 1994 275 coals:35the solvent weakly interacts with coal by physical sorption and hydrogen bond formation. Maciel and coworkers studied 2H NMR of pyridine-d5 swollen Utah Blind Canyon coal.% The splitting constant they obtained is 168 kHz (WJW, = 0.03). The splitting constant measured for the low-temperature pyridine-d5swollen coal (W$ W, 2 1)in this work is identical to that for the almostdried coal (W,/Wc = 0.03) of Maciel and co-workers,% providing further evidence for the existence of the glassto-rubber transition of the swollen coals.
Conclusion The study of lH and 2H NMR line width as a function of temperature and solvent loading in the present work has proved to be a simple and sensitive method to investigate the phase behavior of coals and swollen coals. From this study, we found that (1) coal is transformed from a glassy material to a mainly rubbery material after swelling in deuterated pyridine and NMP solvents. This is evidenced by the transition from a broad Gaussian ‘H line shape to a narrow Lorentzian one as solvent loading increases. (2) Swollen coals undergo a significant phase change, probably a glass-to-rubber transition, in a temperature range centered at about 210 K, evidenced by the line width transition of lH and 2H NMR as a function of temperature. (3) Both inter- and intramolecular dipole interactions contribute to the line broadening of coals. The solvent swelling destroyed almost all the intermolecular interactions with the possible exception of some residual interactions in Zap lignite. (4) The interaction between solvent and coal in the swollen coal gel is weak. From 2HNMR spectra, the sorbed solvent moleculesshow mainly two types of motion in coal: completely isotropic motion at high temperature and static at low temperature. (5) The swollen coal behaves like a cross-linked polymer gel system. Acknowledgment. We thank A. Garcia and Q. Zhang of Exxon for their technical assistance. (35)Ripmeester, J. A.;Hawkins, R. E.; MacPhee, J. A.; Nandi, B. N. Fuel 1986,65,740. (36)Jurkiewicz, A.;Frye, J. S.; Maciel. G.E. Fuel 1990,69,1507-IO.
