Energy & Fuels Preliminary theoretical calculations applying extended Huckel molecular orbital calculations (EHMO) performed by Subbaswamy" indicate that cation radical species are plausible intermediates in the selective cleavage of bond a in I under carbon black catalyzed reaction conditions. EHMO calculations for compound I indicate that, as expected, bond d has the lowest strength.14 In a thermal process without an initiator, breaking of this bond should, most probably, initiate further reactions. Using EHMO, it is possible to mimic the oxidation described above and to remove "mathematically" one electron from I. In keeping with the known values for oxidation potentials of naphthalenic and benzene compounds,12removal of the electron would be done preferentially from the naphthyl unit. Relative bond strengths of the ion radical of I with the charge centered on the naphthyl moiety can be computed. When the calculations are performed in this way, it is seen that the bond between the naphthyl unit and the methylene linkage is the weakest in the ion radical and so (14) Subbaswamy, K. R. University of Kentucky, personal communication.
1991,5, 87-92
87
the most susceptible to breaking. Work is in progress aimed at elucidating the mechanism of action of this carbon black as well as other carbon blacks or carbon-derived materiah6 We are also studying the reactivity of other related model compounds within the same family of I.
Acknowledgment. We thank Prof. Paul Dowd and his group at the University of Pittsburgh for the synthesis and characterization of compound I, Louise Douglas for GC/ MS work, Prof. K. R. Subbaswamy for EHMO calculations, Prof. F. Freund for CDA measurements, and Prof. Dan Farcasiu for helpful discussions. This work was supported in part by an appointment to the Postgraduate Research Training Program under Contract NO. DEAC05-760R00033 between the US. Department of Energy and Oak Ridge Associated Universities. Reference in the paper to any specific commercial project, process, or service is to facilitate understanding and does not necessarily imply ita endorsement or favoring by the United States Department of Energy. Registry No. I, 127833-53-8.
129XeNMR Investigation of Coal Micropores Chihji Tsiao and Robert E. Botto* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 Received June 22, 1990. Revised Manuscript Received July 30, 1990
The microporous structures of three Argonne Premium coals, and a weathered sample and three oxidized samples of Illinois No. 6 coal (APCS No. 31, have been investigated by "Xe NMR spectroscopy. An analytical model has been developed which approximates the average pore sizes and pore swelling characteristics for the coals according to changes in lBXe chemical shifts as a function of xenon pressure. Pore regions can be described which differ in size (-6 and -10 A in diameter) or chemical composition. The effects of weathering and oxidation at elevated temperatures on the pore size and swelling of Illinois No. 6 coal have also been examined.
Introduction Coals are generally thought to be cross-linked, threedimensional macromolecular networks whose main building blocks are aromatic and hydroaromatic units connected and by predominately methylene (-CHz-), oxygen (-0-), sulfur (-S-) cross-links.' The irregular arrangement of these building units in three dimensions is responsible for the extensive pore structures. In general, three pore-size regimes exist in coals: macropores with diameters of more than 500 A, mesopores with diameters in the range of 20-500 A, and micropores with diameters less than 20 A. Porosity in coal is of great significance because of its influence on coal behavior during mining, preparation, and utilization pr0cesses.V For these reasons, many analytical means have been employed during the past few decades in an attempt to characterize coal porosity, pore volume, surface area, and pore-size distribution. The subject has been reviewed extensively by Mahajan4 and Marsh.6 *Author to whom correspondence should be addressed.
Recently, Larsen and Wernett6 have studied the adsorption (BET) of COz and a series of aliphatic hydrocarbons on Illinois No. 6 coal. These authors suggested that many pores are closed to the external surface and to reach them an adsorbate must diffuse through solid coal, rather than through the pore network. Bartholomew et al.' determined surface areas of three Argonne Premium (1) Larsen, J. W.; Kovac, J. In Organic Chemistry of Coal; Larsen, J. W., Ed.; ACS Symposium Series No. 71; American Chemical Society: Washington, DC, 1978; pp 34-49. (2) Mahajan, 0. P.; Walker, P. L., Jr. In Analytical Methods for Coal and Coal Products; Karr, Jr., C., Ed.:Academic Press: New York. 1978 Vol. 2, pp 125-162. (3) Mahajan, 0. P.; Walker, P. L., Jr. In Analytical Methods for Coal and Coal Products; Karr, Jr., C., Ed.; Academic Prees: New York, 1978; Vol. 2, pp 465-494. (4) Mahaian. 0. P. In Coal Structure: Mevers. R. A... Ed.: . Academic Press: New-York, 1982; pp 51-87. (5) Marsh, H. Carbon 1987, 25, 49-68. (6)Larsen, J. W.; Wernett, P. Energy Fuels 1988, 2, 719-720. (7) Bartholomew, C. H.; White, W. E.; Thornock, D.; Wells, W. F.; Hecker, W. C.; Smwt, L. D.; Smith, D. M.; Williams, F. L. Prepr. Pap. Am. Chem. Soc., Diu. Fuel Chem. 1988, 33, 24-31.
0 1991 American Chemical Society Q88~-Q62~/91/25Q5-QQ87$02.5Q~Q
Tsiao and Botto
88 Energy & Fuels, Vol. 5,No. 1, 1991 Table I. Argonne Premium Coal Samples and Compositions wt%
no. coal rank C H O S APCS Upper med. vol. bit. 85.50 4.70 7.51 0.74 No. 1 Freeport APCS Wyodaksubbituminous 75.01 5.35 18.02 0.47 No. 2 Anderson APCS Illinois No. 6 high vol. bit. 77.67 5.00 13.51 2.38 No.3
coals, two PETC coals, and a Utah Scofield coal from nitrogen and carbon dioxide adsorption measurements. Pore volumes were determined by nitrogen adsorption and mercury porosimetry and from NMR spin-lattice relaxation measurements of samples saturated with water. Solid densities were determined by helium displacement. Their experimental results indicated that, while large fractions of the internal surfaces of the coals are penetrated by carbon dioxide, significantly less of the internal surfaces are penetrated by nitrogen molecules. The data suggested that a major portion of the pore structure is accessible only through openings which are smaller than 1 nm (< 10 A). Cooray, Kispert, and W U Ustudied ~ ~ ~ the pore-size distributions of several high-volatile bituminous coals by EPR spectroscopy. They monitored the mobility of nitroxide spin probes varying in molecular size that were diffused into the swellable pores of coal samples. In addition, they were able to estimate the ratio of basic/acidic sites, a distribution for hydrogen-bonding sites, and the effect of different solvents on coal swelling behavior. Coal porosity has been studied by small-angle neutron scattering (SANS) for the several dried coals and samples that were exposed to two swelling solvents.lOJ1The SANS data demonstrated that the pore structure of a coal swollen in pyridine changed dramatically from its original state. Stephens and Gethner12also investigated the light transmission of solvent saturated thin sections of Illinois No. 6 bituminous coal. From the optical data, these authors were able to obtain information on changes in the geometry and volume of pores in the coal resulting from solvent immersion. lBXe NMR spectroscopy has been proved an ideal probe to investigate the structural properties of microporous materials such as ze01ites.l~ Xenon is a monatomic gas having a large van der Waals radius (2.2 A) and is chemically inert. Any small perturbations of its large, polarizable electron cloud are transmitted directly to the nuclear environment, and this is responsible for the large range of '%e chemical shifts. In addition, l%e has nuclear spin of 1/2, its NMR receptivity is 32 times greater than 13C,14 and this means that '%e is substantially easier to observe than 13C under similar conditions. The spin-lattice reof xenon in the gas phase is extremely laxation time (T,) long ( lo6 s).15J6 However, when xenon atoms are adsorbed in the pores or channel systems of microporous materials, TI is greatly reduced owing to enhanced xenon-xenon collisions and to collisions between xenon and the "walln. Consequently, one is able to detect those xenon N
(8) Cooray, L. S.; Kispert, L. D.; Wuu, S. K. Prepr. Pap. Am. Chem. SOC.,Diu. Fuel. Chem. 1988, 33, 32-37. (9) Kispert, L. D.; Cooray, L. S.; Wuu, S. K. Prepr. Pap. Am. Chem. Soc., Diu. Fuel. Chem. 1987, 32, 286. (IO) Gethner, J. S. J. Appl. Phys. 1986,59, 1069. (11) Winans, R. E., Thiyagarajan, P. Energy Fuels 1988,2,356-358.
(12) Stephens, R. B.; Cethner, J. S., submitted to Energy Fuel. (13) Fraissard, J.; Ito, T. Zeolites 1988, 8,350. (14) Reisse, J. Nouu. J. Chim. 1986, 10, 665. (15) Hunt, E. R.; Carr, H. Y. Phys. Reo. 1963,130, 2302. (16) Wernett, P. C.; Larsen, J. W.; Yamada, 0.; Yue, H. J., private communication.
Table 11. Oxidation Conditions Used for Illinois No.6 Coal Samdes sample conditions temp, "C time, h weathered ambient several months APCS No. 3 (WOAC3) heated in oxygen 100 2 APCS No. 3 (01AC3) (400 Torr) heated in oxygen 200 2 APCS No. 3 (02AC3) (400 Torr) heated in oxygen 300 2 APCS No. 3 (03AC3) (400 Torr) n N
e X 0,
\
E
0
c
-3 x c u) C (u
n c 0 C
z
0
,
200
I
I
400
600
I 10
Xenon Pressure (torr) Figure 1. Adsorption isotherms for xenon in three Argonne Premium Coals: (m) APCS No. 1; (0) APCS No. 2; ( 0 )APCS No. 3.
atoms in micropores selectively when sufficiently fast pulse repetition rates are employed. In the present work, we examine the use of "Xe NMR spectroscopy to characterize pore structures in three Argonne Premium coals, and to monitor changes in pore structure that occur for Illinois No. 6 coal upon weathering and oxidation at elevated temperatures in the presence of oxygen. We also discuss the swelling of coal pores with the introduction of xenon gas.
Experimental Section The coal samples were obtained from the Argonne Premium Coal Sample Program." Table I summarizes the origin, rank, and composition (wt %) of the coals used in this work. Oxidized samples were prepared by heating the coals under a partial pressure of oxygen (400 Torr) at a given temperature for 2 h. After each measurement, the sample was successively oxidized at a given temperature for further use. The preparation of oxidized Illinois No. 6 coal (APCS No. 3) samples is summarized in Table 11. Approximately 0.4 g of coal is loaded in a Nh4R tube fitted with a sealable J. Young stopcock in a nitrogen-filled glovebox. A grease-free glass manifold is used for outgassing the sample at lo6 Torr. Prior to '%e adsorption, the sample was slowly heated to 100 O C over 8-10 h following the procedure recommended by Mahajan and Walker? It was then maintained at this temperature overnight under vacuum to remove the water present in the pores completely. After cooling to ambient temperature (21 "C), xenon gas (99.995%) was introduced into the sample and maintained until equilibrium of xenon adsorption was obtained. The uptake of xenon gas sorbed was recorded with a digital vacuum instrument, Telvac I1 (Fredericks Co.). Xenon adsorption isotherms were determined at room temperature and exhibited a BET Type I1 dependence on xenon pressure. The weight of samples outgassed at 21 and 100 "C are corrected for residual water content by subtracting the weight loss on heating similar samples from 21 to 100 "C. (17) Vorres, K. S. Users Handbook for the Argonne Premium Coal Sample Program, ANL, Oct. 1, 1989,11. MAF-based data.
Micropores in Argonne Premium Coals
400
PPM
0
Energy & Fuels, Vol. 5, No. 1, 1991 89
- 200
0
200
400
600
800
Xenon Pressure (torr)
Figure 3. Plots of xenon chemical shifts versus xenon pressure for APCS No. 1: (m) pore region I; ( 0 )pore region 11.
-200
->>/ v
400
\k-&J
'&q&"q#*
''I
200
PPM
0
r
- 200
Figure 2. "%e NMR spectra of xenon adsorbed in three Argonne Premium Coals. Values in parentheses indicate xenon pressure; pore regions 1-111 appear in ascending order to higher field.
All lSXe NMR spectra are recorded with a Bruker AM 300 spectrometer operating at 83.02 MHz (7.0 T), employing a pulse width of 9.8 pa (90° pulse angle) and a recycle delay of 0.5 s. Each spectrum is the accumulation of MOO0transients. The chemical shifts are reported with reference to the extrapolated shift of bulk xenon at P = 0. Deshielded (downfield) xenon chemical shifts are considered positive. Line simulation subspectra were calculated from the original data by using a Pascal line simulation program LINESIM (Bruker version 880101)18on the Aspect 3000 computer. Results and Discussion Figure 1 shows absorption isotherms of xenon gas for three Argonne Premium coal samples: Upper Freeport medium-volatile bituminous coal (APCS No. l), Wyodak-Anderson subbituminous coal (APCS No. 2), and 11linois No. 6 high-volatile bituminous coal (APCS No. 3). The curves reflect a BET Type I1 dependence on xenon pressure. One sees that the xenon uptake is rather low for all three coals even at xenon pressures up to 800 Torr. The higher xenon uptake observed for Illinois No. 6 coal indicates that its free micropore volume is more than double those of Upper Freeport and Wyodak-Anderson coals. Figure 2 is a compilation of NMR spectra of xenon adsorbed in the three coals. The lBXe resonance at 0.0 ppm (18) Barron, P. Instruction Manual for Linesim; Bruker (Australia) Pty. Ltd., 1988.
arises from xenon adsorbed on the surface of the sample and/or on the outer surface of meso- and macropores, and this resonance has been used as a secondary chemical shift reference. The main resonance bands in the spectra appear broad and asymmetrical. The major contribution to the overall line widths is presumed to be a distribution of micropore sizes in the coal samples, although other contributions such as magnetic susceptibility broadening may contribute to the line widths as well. Given the line widths found for the coals, pore-size distributions on the order of 3-5 A can be expected. Spectra for Upper Freeport and Wyodak-Anderson coals display two resonances that appear to be caused by xenon in two different environments (pore regions I and 11). For the Illinois coal, the situation is more complicated and its lBXe NMR spectrum reveals three distinct chemical shift regions (pore regions I, 11, and 111). To specify lBXe NMR parameters such as chemical shift and resonance intensity as exactly as possible, each spectrum is decomposed into the sum of several Lorentzian subspectra by using a Pascal line simulation program as shown in Figure 2 (dashed lines). Generally, the NMR chemical shift of xenon adsorbed in a microporous solid can be expressed by Fraissard's which has several terms, each characterizing an interaction that a xenon atom experiences where p is the xenon density in atoms per gram of dry sample; 6 ( p ) is the chemical shift of xenon gas relative to the bulk gas at p = 0; 6, is a term characteristic of interaction between xenon and the pore "wall"; 6, and a2 are coefficients describing the effect of two-body and threebody xenon-xenon collisions, respectively; and bE is the electrostatic field term. In the case of coal, one knows the ouerall sorption in a given sample from the isotherm but cannot calculate the specific xenon density in each pore region. However, the xenon chemical shifts are plotted as a function of xenon pressure. This is demonstrated for APCS No. 1 and APCS No. 2 in Figures 3 and 4, respectively. From Figure 3, one sees that the chemical shifts increase initially with the xenon pressure to a maximum value of about 560 Torr, and then decrease with xenon pressure thereafter. We attribute this reversal in chemical shift at higher xenon pressures to swelling of the coal pores. Previous work by Reucroft and Pate122has clearly dem(19) Ito, T.;Fraissard, J. Proc. 5th Int. Conf. Zeolites 1980, 510. (20) Ito, T.;Fraissard, J. J. Chem. Phys. 1982, 76, 5225. (21) Ito, T.;Fraissard, J. J. Chem. Phys. Lett. 1984, 111, 271.
Tsiao and Botto
90 Energy & Fuels, Vol. 5, No. 1, 1991
sample APCS No. 1 APCS No. 2 APCS No. 3 WOAC3 01AC3 02AC3 03AC3
Table 111. *"Xe NMR Chemical-Shift Parameters Calculated from Ea 2 for Coal Samdes 60, PPm 6, (ppm/Torr) 6, x lo4, ppm/Torr-* I I1 I11 I I1 I11 I I1 I11 127 f 35 118 f 51 143 f 38 225 f 2 178 f 3 129 f 6 130f 6
128 f 19 115 f 4 148 f 7 124 f 9 119 f 28 137 f 2 139 f 2
71 f 63 f 74 f 109 f
26 17 26 1
0.33 f 0.13 0.31 f 0.19 0.28 f 0.14 0.00 f 0.09 0.09 f 0.01 0.15 f 0.01 0.15 f 0.01
0.18 f 0.07 0.15 f 0.02 0.13 f 0.02 0.18 f 0.03 0.22 f 0.10 0.08 f 0.01 0.07 f 0.00
0.16 f 0.09 0.25 f 0.06 0.16 f 0.09 0.01 f 0.00
2.9 f 1.1 2.6 f 1.6 1.9 f 1.2 0 0
0 0
1.6 f 0.6 1.1 f 0.1 1.0 f 0.2 1.5 f 0.2 1.7 f 0.9 0 0
2.5 f 0.8 2.4 f 0.5 1.4 f 0.8
0
Table IV. Pore Diameters (A) and Relative Concentrations (7%) for Different Pore Regions in Coal Samples sample I I1 I11 6.3 f 0.7 (49.8 f 7.1) 6.3 f 0.6 (50.2 f 7.0) APCS No. 1 6.6 f 1.3 (43.6 f 1.8) APCS No. 2 6.7 f 0.2 (56.4 f 1.8) APCS No. 3 5.8 f 0.4 (41.8 f 4.4) 5.7 f 0.2 (33.7 f 2.6) 9.4 f 1.9 (24.5 f 3.8) 4.6 f 0.1 (34.9 f 4.8) 5.2 f 0.1 (48.2 f 6.0) 6.2 f 0.3 (38.9 f 6.4) 6.2 f 0.3 (39.9 f 6.4)
WOAC3 OlAC3 02AC3 03AC3 220
200
I
-I
0
6.4 6.5 6.0 5.9
200
400
600
800
Xenon Pressure (torr) Figure 4. Plots of xenon chemical shifts versus xenon pressure for APCS No. 2: (m) pore region I; ( 0 )pore region 11.
onstrated increases in coal surface areas approaching 9% due to swelling by soluble gases such as C02. Analogous to Fraissard's approach, a mathematical model has been formulated to approximate both the micropore size and swelling characteristics in coals according to the following expression 6(P) = 6, + a g - 6,P (2) where P is the xenon pressure in Torr; 6(P)is the observed chemical shift of adsorbed xenon relative to bulk gas at pressure P; a0 is the extrapolated chemical shift at P = 0 and is analogous to the same value in Fraissard's equation; 6, is a coefficient describing xenon-xenon collisions (assuming negligible contributions from three-body collisions); and 6, is a term to describe the swelling characteristics of coal micropores. The parameters 60, 6,, and 6, that were obtained from fitting data to eq 2 using linear regression for pore regions I, 11, and I11 are reported in Table 111, along with standard deviations for the parameters. Calculated fits to the data for APCS No. l and APCS No. 2 are represented by the solid lines in Figures 3 and 4. It is interesting to note the a0 values corresponding to pore regions I and I1 are similar, e.g., for APCS No. 1, 6o is 127 f 35 ppm for region I and 128 f 19 ppm for region 11. One possible explanation for these results is that there are two chemically different pore structures having approximately the same pore size. (22) Reucroft, P. J.; Patel, H. Fuel 1986, 65, 816.
f f f f
0.2 (35.6 f 3.4) 0.8 (35.2 f 4.0) 0.1 (47.1 f 2.4) 0.1 (60.1 f 5.2)
11.6 f 2.2 (29.5 f 5.4) 9.1 f 1.8 (16.0 f 3.8) 6.9 0.1 (13.9 f 3.2)
*
In general, the pore network in coal can either be considered aromatic or aliphatic in nature. Several investigators have reported laXe chemical shifts of xenon gas dissolved in a variety of organic solvents,23and it appears that the major contribution to the xenon chemical shift arises from van der Waals interactions, more specifically on the London dispersion forces, between solvent and xenon molecules.u With ' W e solvent shifts in mind, we tentatively assign the resonance at approximately 200 ppm (pore region I) to xenon adsorbed within voids of aromatic clusters and the resonance around 170 ppm (pore region 11) to xenon adsorbed within aliphatic regions of the coals. For APCS No. 3, the resonance at 110 ppm (pore region 111) may be assigned to a larger pore structure that constitutes about 25% of the total micropore volume in relative concentration (see Table IV). Generally, 6, is composed of three terms arising from interactions that are independent of xenon pressure25 60
= 6,
+ 6A + ,6
(3)
where 6, the xenon-wall collision term, is characteristic of pore structure; ,6 gives the effect of the magnetic susceptibility of the sample and 6A is a term accounting for the interaction of xenon with additional adsorbent species. For a pure sample, 6A is equal to zero. It is assumed that magnetic susceptibility (6,) effects are negligible with respect to the '%e chemical shift range. This assumption is valid provided the levels of paramagnetic species in the coal samples are comparable. EPR measurements indicate that the concentrations of free radicals in APCS No. 1-No. 3 are similar (within f10%).26927 Consequently, the chemical shift parameter, a0, obtained by extrapolation of the chemical shift to zero xenon pressure, may be considered characteristic of the pore structure. Demarquay and FraissardZ8were able to obtain the relationship between 6, and the dimension of the micropores and channels by calculating the mean free path, L, of the sorbed xenon by using the following equation 1/60 = (1 + L/2.054)/6, (4) where 6, = 243 ppm. Three different models have been (23) Reisse, J. Nouu. J. Chim. 1986, 20, 665. (24) Luhmer, M.; Dejaegere, A.; Reisse, J. Magn. Reson. Chem., in
press.
(25) Tsiao, C.-j.; Corbin, D. R.; Dybowski, C. R. J. Phys. Chem. 1990,
94, 867.
(26) Jurkiewicz, A.; Wind, R. A,; Maciel, C. E. Fuel 1990, 69, 830. (27) Muntean, J. V.; Ph.D. The&, University of Chicago. (28) Demarquay, J.; Fraissard, J. Chem. Phys. Lett. 1987, 136(3-4), 314.
Micropores in Argonne Premium Coals
Energy &Fuels, Vol. 5, No. 1, 1991 91
uptake in samples which underwent mild oxidative treatment (weathering or heating in oxygen at 100 "C) is somewhat lower than that for the fresh coal. Upon more severe oxidation of the coal at 200 and 300 "C, xenon uptakes become progressively smaller. This suggests that the free pore volume collapses at the elevated oxidation temperatures. Figure 6 shows the "Xe NMR spectra of xenon sorbed in Illinois No. 6 coal under different oxidation conditions. The resonance at about 110 ppm (pore region 111)becomes broader with weathering, and upon further oxidation at 100 and 200 "C, this resonance decreases in intensity and then completely disappears when the sample is heated to 300 OC. From Table IV, one sees that pore region I11 undergoes a reduction in average pore size from 9.4 A (APCS No. 3) to 6.9 A (02AC3). With continued heating at 300 "C, this pore structure collapses further and merges with pore region 11. Concomitant with this observation is the sudden increase in relative concentration of pore region I1 (60.1%). Figure 7 shows the plots of '%e chemical shifts of pore regions I, 11, and I11 versus xenon pressure for the fresh and oxidized Illinois No. 6 coal samples. The chemical shifts of pore region I (top) in the fresh coal sample increase initially with xenon pressure and then decrease due to swelling of the pores. The chemical shifts become independent of xenon pressure in the weathered sample (WOAC3), where cross-linking may increase as a result of weathering. The average diameter of the pores in region I decreases correspondingly from 5.8 to 4.6 A (see Table IV), such that only one xenon atom may be accommodated by each pore, and collisions between xenon atoms become virtually nonexistent (6, = 0). As the average diameter of the pores approaches the van der Waals diameter of xenon, the xenon atoms possess significantly fewer degrees of freedom. Hence, they behave as if dissolved in the coal matrix. Accordingly, the lBXe shifts become independent of xenon pressure. Upon further oxidation at elevated temperatures, the average pore diameter (6.2 f 0.3 A)
0
0 0
0
t
0
c
4 >. + .lA
0
2A
C
0,
0
1.5-
8
0 C
al
X 1 1
300
I
I
I
I
I
400
500
600
700
800
Xenon Pressure (Torr)
Figure 5. Xenon adsorption isotherms for pristine and oxidized Illinois No. 6 coal samples: ( 0 )fresh coal;).( weathered coal; (D) coal treated at 100 O C ; (0) coal treated at 200 "C;(A)coal treated at 300 "C.
used to describe the pore structure in amorphous materials: these include a sphere, cylinder, and layered plate. Perhaps the most realistic model for coal assumes that a majority of its micropores are represented by cylinder-like channels. Accordingly, the diameter of pores can be estimated from the following expression28 L = D - Dxe (5) where Dxe is the diameter of xenon atom (4.4 A). The calculated values of pore diameters and the relative concentrations of the different pore regions for the coal samples are reported in Table IV. The effects of weathering and oxidation on the pore structure and swelling ability of Illinois No. 6 coal (APCS No. 3) have been examined. Figure 5 shows the adsorption isotherms for APCS No. 3 coal samples that have been treated under different oxidation conditions. The xenon
A
WOAC3(568)
,
.--400
0I AC3(566)
I
2oo
400
2oo
I
0
PPM
A
I
A
02AC3(584)
I
-200
400
200
,
I
I
--I
-200
PPM
I
1--r-----l-r--
PPM
..
\-
- 200
,
I
400
T-----r---
2oo
PPM
I
0
- 200
1
Figure 6. lBXe NMR spectra of xenon gas adsorbed in different oxidized Illinois No. 6 coal samples; pore regions 1-111 appear in ascending order to higher field.
Tsiao and Botto
92 Energy & Fuels, Vol. 5, No. 1, 1991 260 250 n
E,
For pore region 11 (Figure 7, middle), lBXe chemical Lshifts exhibit the same negative dependence at high xenon
I
8
240
Q
4
230
'c
5, 0
.-
f 6
220 210
200 ,$
A-
190 180
1
A I
I
1
I
220
I1
210 n
E
-
200
cn
180
.-
170
Q Q
+ YI :
0 E
190
a,
6
160 150 140
I
I
I
I
160
n
E
-
I
140
'1 Q Q
6
*E
if
130 120 110.
100
90
!
80 300
I
I
I
400
500
600
,
700
I
800
Xenon Pressure (torr)
Figure 7. Plots of xenon chemical shifts versus xenon pressure for pore regions I (top), 11(middle), and I11 (bottom) of pristine weathered and oxidized Illinois No. 6 coals: (m) fresh coal; (0) coal; ( 0 )coal treated at 100 "C; (0) coal treated at 200 "C; (A) coal treated a t 300 O C .
returns to about its original size. This is consistent with the pores opening up as a result of the expulsion of volatiles from the coal. The "%e shifts increase linearly with xenon pressure, as would be expected for a rigid pore network created by cross-linking from oxidation.
pressures for APCS No. 3, WOAC3, and 01AC3. When the sample is heated at 200 or 300 "C, the chemical shifts increase linearly with xenon pressure, again indicating the loss of swelling ability after intense oxidation. No obvious changes in the pore size of region I1 can be discerned either upon weathering or on oxidation (see Table IV). Comparing the experimental data, one finds that the aliphatic pore structure (11) is less affected by oxidation than the aromatic pore structure (I). These findings are consistent with previous carbon spin-lattice relaxation evidencea that suggest the most prominent weathering effects are confined to coal aromatic structures, for which large decreases in molecular mobility have been observed. For pore region 111, plots of laXe shifts versus pressure show similar trends to those of region 11. The average pore diameters of AF'CS No. 3, WOAC3, and 01AC3 remain the same. However, when the sample is heated to high temperatures, the shiftsare again seen to increase linearly with xenon pressure and the pore diameters decrease as a result of pore collapse. The calculated values of 6, for the Illinois No. 6 coal samples are reported in the last columns of Table 111. The marked decrease in 6, as a function of oxidation temperature illustrates the profound influence of oxidation on the swelling ability of coal micropores.
Conclusions This work has demonstrated the utility of "Xe NMR spectroscopy to investigate the microporous structures of three Argonne Premium coals. Using this technique, we have been able to estimate the average size of coal pores with diameters that are less than 20 A. The relative concentrations of the different pore structures found, as well as a measure of the swelling of pores by xenon gas, can be estimated. The experimental data suggest that there are two pore structures of similar size (about 6.0 A in diameter) having different chemical compositions. These tentatively have been assigned to aromatic and aliphatic regions within the coals. For Illinois No. 6 coal, an additional pore structure can be discerned having a larger pore diameter of approximately 10 A. The variation in pore size and reduction in swelling ability seen upon weathering or oxidation at elevated temperatures have provided direct insights into changes in coal microporous structure and properties during oxidative processes. Future studies will focus on providing better estimates of the pore-size distributions and developing a more accurate theoretical model to describe the swelling characteristics in these systems. Acknowledgment. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, US. Department of Energy, under contract no. W-31-109-ENG-38. We thank Dr. Karl S. Vorres for providing Argonne Premium Coal Samples and Mr. Art Kostka for technical assistance. (29) Tsiao, C.-j.; Botto, R. E. Paper presented at the 3rd Chemical Congress of Pacific Basin Societies,Honolulu, HI, December 17-22,1989.
