Energy & Fuels 1993,7,90-96
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Three-Dimensional NMR Microscopic Imaging of Coal Swelling in Pyridine David C. French, Stephen L. Dieckman? and Robert E. Botto* Chemistry Division and Materials and Components Technology Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439 Received September 8,1992. Revised Manuscript Received November 2,1992
Results from two different NMRI techniques for spatially mapping the mobile proton distribution in coals swollen with pyridine-d5 or pyridine are reported. Three-dimensional (3-D) back-projection methods using a reconstruction algorithm based on a Radon transform inversion were employed to image a specimen of Utah Blind Canyon coal swollen with pyridine-d5. Images of the mobile macromolecular structure of Utah coal resulting from solvent swelling were obtained using 2'1 contrasting. Solvent penetration was also monitored using 2'2 weighting in a 3-Dgradient-recalled echopulse sequence and image reconstruction by 3-D Fourier transform methods. Using this technique, images of Utah Blind Canyon and West Virginia Lewiston-Stocktoncoals swollen with pyridine were obtained which show the distribution of the ingressed solvent. The presence of macerals, microfractures, and mineral regions which differ with regard to pyridine accessibilitywere contrasted in the images with a resolution of 40-80 pm over three spatial dimensions.
Introduction Since the early work of Sanada and Honda, solvent swelling has been employed to probe the physical structure of coal.' The swelling behavior of bituminous coals in solventssuch as pyridine has been used to assess different strengths or types of secondary interactions which determine the macromolecular structure of coal.'-* These include weak interactions of less than -15 kcal mol-', such as hydrogen bonding, van der Waals interactions, weak complexes, and a-electron intermolecular interactions.+ll The secondary interactions which can be broken by one solvent may be unaffected by a different solvent. A high degree of secondary interactions imparts rigidity to the macromolecular structure in coals, whereas disruption of these secondary interactions by a good swellingsolvent mobilizesthe structure. Pyridine has been found to effectively relax all of the secondary interactions It is known in coal, leaving only covalent bonds inta~t.~JO that the chemical heterogeneity of coal leads to anisotropic swelling.12 Coal swelling has been found to be greater perpendicular to the bedding plane than parallel to it.l3 Cracking at mineral-organic interfaces is also expected due to differential swelling of the organic matrix relative to mineral matter.
* Author to whom correspondence should be addressed.
t Materials
and Componenta Technology Division. (1) Sanada, Y.; Honda, H. Fuel 1966,45, 295. (2) Green, T.; Kovac, J.; Brenner, D.; Laraen, J. W. Coal Structure; Meyers, R. A., Ed.;Academic Preas: New York, 1982; Chapter 6. (3) Nelson, J. R.; Mahajan, 0.P.; Walker, Jr., P. L. Fuel 1980,59,831. (4) Peppas, N. A.; Lucht, L. M. Chem. Eng. Commun. 1984,30,291. ( 5 ) Suuberg, E. M.; Lee, D.; Larsen, J. W. Fuel 1985,64, 1668. ( 6 )Green, T. K.; West, T. A. Fuel 1986,65,298. (7) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985,50,4729. (8) Larsen, J. W.; Shawver, S. Energy Fuels 1990,4,74. (9) Brenner, D. Fuel 1985,64, 167. (10) Quinga, E. M. A.; Larsen, J. W. In New Trends in Coal Science; Yurum, Y., Ed.;HRT NATO AS1 Series 244; Kluwer Academic Publishers: Boston, 1988. (11) Quinga, E. M.; h n , J. W. Energy Fuels 1987,1,300. (12) Brenner, D. Fuel 1984,63, 1324. (13) Cody,Jr., G. D.; Larsen, J. W.; Siskin, M. Energy Fuels 1988,2, 340.
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Figure 1. (a, upper) The 3-Dback-projectionand (b, lower)3-D
GRE pulse sequences.
Earlier, we reported the first results of lH NMRI experiments for spatially resolving the individualmacerals within a solid ~ 0 a l . lTwo-dimensional ~ NMRI proved to be a promising tool for spatial mapping resinite and vitrinite macerals in a dried specimen of Utah Blind Canyon coal. Important information concerning solvent accessibility to the pore structure and maceral domains in coals also may be obtained by imaging mobile proton distributions resulting from solvent swelling. Images of coals swollen with perdeuterated solvents can be used to map mobile (14) Dieckman, S. L.; Gopalsami, N.; Botto, R. E.Energy Fuels 1990, 4,417.
1993 American Chemical Society
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Imaging of Coal Swelling in Pyridine
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Figure2. The IHsingle-pulse NMR spectrum of the Utah Blind Canyon coal swollen with pyridine-d5.
Figure 4. An 80-pm slice near the center of the Utah specimen. The in-plane resolution is 80 pm.
Fourier transform (FT). Both techniques display proton distributions of mobile phases within the solid preferentially. Images with resolutions of 40-80 pm show the presence of different macerals, mineral cleats, microfractures, and the distribution of mobile macromolecular phases in solvent-swollencoals. Regionswhich differ with regard to pyridine accessibility are also contrasted in the images.
Experimental Section
Figure 3. A 3-D surface-renderingof the Utah coal swollen to 7 x 7 X 5 mm3 with pyridine-&.
phases in the coal macromolecular structure, while images obtained with protic solvents map distributions of the ingressed solvent. Here, we report the results from two different threedimensional(3-D)NMRI techniques for spatiallymapping mobile proton distributionsin coalsthat have been swollen with pyridine-& or pyridine. Three-dimensional backprojection techniques employing a 3-D Radon transform inversion reconstruction algorithm were used to map mobile proton distributions on the basis of 2'1 (spin-lattice relaxationtime) differences. Contrastbased on differences in T2 (spin-spin relaxation time) was achieved by employing 3-D gradient recalled echo (GRE)pulse sequence for data acquisition, and image reconstruction by 3-D
Samples. The sample used for NMRI by the 3-Dbackprojection technique was a specimen of Utah Blind Canyon (APCSNo. 6) high-volatile bituminous coal, obtained from the Argonne Premium Coal Sample Program. The coal was dried in vacuo overnightat 40 "C, and then swollen to 7 X 7 X 5 mm3 with pyridine-ds (100.0% D,Stohler Isotope Chemicals) vapor in a saturated atmosphere for 2 weeks. Drying the coal produced no discernible structural changes in the specimen, whereas solvent swellingproduced a profound volume change and visible cracking. The specimen was placed in a 20 mm 0.d. sample tube and tightly capped. It was not necessary to place a solvent source in the tube due to the large sample size and since imaging was initiated immediately. The samples used for imaging by the 3-D GRE technique were solid specimens of Utah Blind Canyon (APCS No. 6) and West Virginia Lewiston-Stockton (APCS No. 7) high-volatile bituminous coals, obtained from the Argonne Premium Coal Sample Program. The coals were allowed to imbibe pyridine vapor in a saturated atmosphere for a t least 3 weeks. The specimens were approximately 1 mm3 in size and nearly cubic in shape. These were placed in 5 mm 0.d. NMR sample tubes and tightly capped. Loss of imbibed pyridine was minimized by placing a paper wick moistened with solvent, above the specimen situated well outside the NMR coil. NMR Imaging. The NMR imaging system used in this study consisted of a Bruker CXP-100 NMR spectrometer fitted with a home-built imaging accessory which is described in detail
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Figure 6. An optical photograph of the Utah coal used in the GRE experiment.
Figure 5. 3-D surface-renderings of the pyridine-swollen (a, top) Utah (1 X 1X 1 mm3)and (b, bottom) Lewiston-Stockton (1 X 1 X 0.5 mm3) coals. e1se~here.l~ The accessory, designed specifically for examining solid materials, consists of a versatile home-built IBM-PC based pulse programmer, three (X, Y, Z)Techron audio range l-kW gradient amplifiers, a radio frequency pulse-shaping unit, and a home-built, singly-tuned imaging probe capable of operating at l-kW rf levels. The probe also contains forced-air-cooledgradient coils capable of operating with duty cycles in excess of 20% while producing a maximum linear magnetic field gradient of 58 G/cm over a spherical space of 30 mm in diameter. In the 3-D back-projection experiment, linear combinations of the X,Y,and 2 gradient strengths are used to vary the total gradient vector strength in order to sample the entire 3-D space. Thus, a 3-D Radon transform of the proton density is obtained. The reconstruction algorithm of the 3-D Radon transform inversion was implemented as proposed by Grangeat and coworkers. l8 The 3-D back-projection NMRI data were acquired using 128 complex data points, and a total of 3600 projections (30 8 angles over n/2 radians X 120 4 angles over 2 r radians). The pulse sequence is shown in Figure la. A gradient strength of 25 G/cm, a 20 mm i.d. Helmholtz coil, and a spectral width of 200 kHz were used. A total of 64 signals were accumulated per projection by using a hard, approximately 90° pulse and a recycle delay time (15) Gopalsami, N.; Foster, G.A.; Dieckman, S. L.; Ellingson, W.A.; Both,R. E. In Review of Progress in QNDE; Thompson, D. 0.;Climenti, D. E., Us.; Plenum Press: New York, 1990. (16) Grangeat, P.; Hatchadourian, G.; Le Masson, P.; Sire, P. Logical Radon. Notice Descrbtive Des Alaorithmes Ef Des Proarammes 1990. Version2.1 du 13-04-1h, LETI/DSYS/SETI/90-180PS Grenoble, 1990.
Figure 7. An optical photomaph of the Lewiston-Stocktoncoal used in the GRE experiment. -
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Figure 8. Four 40-pm xy-slices (1 X 1 mm2) near the center of the Utah coal swollen with pyridine (a-d clockwise from upper left). The in-plane resolution is 40 pm. (TR) of 0.25 s. A voxel resolution of 80, X 80, X SO, pm3 was achieved. A total of 22 h was required for data acquisition using these parameters. For the GRE experiments, a hard, approximately 90° pulse and a 5 mm i.d. solenoid coil were used. The pulse sequence is shown in Figure lb. A gradient strength of 29 G/cm and a spectral width of 63 kHz were used. The data matrix size was 128 X 128 X 128, the time to echo (TE) was 450 ps, and a 60 ms TR was employed. The number of signals accumulated per projection was 128 for the Utah Blind Canyon and 512 for the LewistonStockton coals (although 128afforded good signal-to-noise);4096 projections were acquired. The voxel resolution was 40, X 40, X 40, pm3. Data acquisition required 16 hours for the Utah Blind Canyon coal and 42 hours for the Lewiston-Stockton coal. Image reconstruction was accomplished by 3-D FT.
Results and Discussion The proton NMFt spectrum of a specimen of Utah Blind Canyon coal swollen with pyridine-d5 is shown in Figure 2. Three distinct proton resonances are evident. There is a broad resonance (ca.27 kHz) arisingfrom rigid protons, and there are two considerably narrower resonance lines discernible as aromatic and aliphatic signals arising from protons in environmentsof high molecular mobility (inset, Figure 2). Line simulation subspectrawere calculated from
the experimental data, using a Pascal routine supplied by Bruker Instruments, as shown in Figure 2. Analysis of the broad and narrow components revealed that 14% of the protons are found in regions of high mobility. The fraction aromaticity (faH) of mobile phase protons calculated from line simulation was 0.45 compared to a value of 0.24, obtained from proton CRAMPS analysis of the dried coal. The estimated contribution to the totalproton signal from residual protons in the pyridine-da was less than 0.196,while the observed faH increased nearly 2-fold as a result of swelling. Clearly, there is a substantial enhancement in the amount of motionally narrowed aromatic proton signal due to swelling. The 3-D surface-rendered NMR image of the Utah coal specimen swollen with pyridine-ds is shown in Figure 3. Here, a short TR value was employedto suppressthe broad signal from the more rigid componentpossessingthe longer TI. The proton distributionof mobile phases in the swollen coal was mapped exclusivelyin this way. A user-adjusted threshold was applied to define the minimum intensity used to calculate the contiguous surface in the 3-D rendering. This threshold was chosen to afford the most accurate representation of the sample topology while suppressing low signal-to-noisefeatures near the sample surface.
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Figure 9. Two 50-pm xy-slices (1 X 0.5 mm2) near the center I f the Lewiston-Stockton coal swollen with pyridine (a, top; b, 3ottom). The in-plane resolution is 50-pm.
The 3-D surface-rendering is a good representation of the general topology of the Utah coal specimen. For example, the dark feature on the front left of the object (arrow, Figure 3) is known to be a macroscopic crack from solvent swellingthe coal. Therefore, image contrast results here from a region devoid of protons. However, it should be emphasized that image contrast is a function of mobile proton density, as well as total proton density. Other dark features on the surface arise from regions where rigid macromolecular proton signal is suppressed by employing a short T R the magnetization of the rigid phase is saturated due to the longer TIof these protons. An 80-pm thin section (slice) through the 3-D radon reconstruction shown in Figure 4 is an image of a horizontal plane near the center of the Utah coal specimen. Image contrast arises from the same sources affecting the 3-D surface-rendering. Mobile proton phases resulting from relaxation of the secondary interactions in the coal by swelling with pyridine-d5 appear bright. Void spaces or mineral deposts which lack protons appear dark. The crack, clearly visible at the lower right of the image (arrow, Figure 4), was produced by solvent swelling the coal. In addition to regions of low proton density, regions of the rigid macromolecular network with long 2'1's in the coal appear dark due to the 2'1 saturation effects. The lH single-pulse NMR spectrum of the LewistonStockton coal exhibited a single resonance from the
French et aZ.
pyridine protons exclusively, whose line width was 424 Hz. The 'H line width of pyridine in the Utah coal was 340 Hz. The T1 of pyridine protons in both samples was measured by progressive saturation, and a value of approximately 1.3 ms was obtained. The baseline-tobaseline y-projection width of the Lewiston-Stockton coal recorded with the GRE pulse sequence and the phaseencoding gradients off was 11.2 kHz and that of the Utah coal was 13.5 kHz. The pyridine natural line widths and baseline-to-baseline y-projection widths, along with the l-mm3 specimen sizes, afforded an in-plane resolution of 50 and 40 pm, respectively, for the two coals. The 3-D surface-rendered GRE images of the Utah and Lewiston-Stockton pyridine swollen coals are shown in Figure 5. Here, image contrast results from employing a TE value sufficiently long to acquire only the echo from mobile protons with long spin-spin relaxation times, T2. Therefore, bright features result from regions in the coals which contain a high density of mobile pyridine, and the dark areas result from regions which are inaccesible to the solvent. The artifacts visible in the images arise from a local commercial FM broadcast near the spectrometer operating frequency. For visual comparison with the surface renderings, a photograph of the Utah coal is shown in Figure 6, and one of the Lewiston-Stockton coal is shown in Figure 7. There is a close correspondence between the shape of the Utah coal specimen and the surface-rendered GRE image geometry. Also, features devoid of pyridine such as pits, cracks, and gouges, e.g., the triangular depression at the bottom center of the Utah specimen (Figure 6),appear magnified in the image. The same is true in comparing the photograph of the Lewiston-Stocktonspecimen (Figure 7) with the 3-D surface-rendering(Figure 5b). Notice that the bevelled comer appears in both the photograph and the NMR image. The vertical feature on the front face of the 3-D surface-rendering shows a microscopic structure contiguous with the surface of the specimen which is inaccessible to pyridine. The photograph shows that this is a solid region in the coal rather than a void space. The slices of the interior xy-planes of the Utah and Lewiston-Stockton coals obtained with the GRE pulse sequence are shown in Figures 8 and 9. The geometry of these slices shows a good correspondence with the 3-D surface-renderings and the photographs. Figure 8 shows sequential transaxial 40-pm slices of the Utah coal. The dark feature at the top center of Figure 8d is a microscopic fracture produced by solvent swelling. Bright features in the slices show areas of high pyridine density. Dark features result from areas of low pyridine penetration, microfractures, and void spaces. Figure 9 shows sequential transaxial 50-pm slices of the interior of the Lewiston-Stockton coal. The images show the presence of contrasting horizontal bands in the xyplane. These bands correspond to the lamellar structures visible in the photograph (Figure 7). The dark vertical feature (Figure 9b, right-hand side) corresponds to the same which appears as a void in the 3-D surface-rendering and shows that this interior structure runs through the object to its surface. Since the photograph shows that this area of the specimen is solid, the dark structure in the imagesis clearly a region of low pyridine accessibilityrather than a void space. When the coal was swollen with pyridine, the secondary interactions binding strata within the specimen were relaxed. Contrasting bands in the images show that differential swellingin the bedding plane
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Figure 10. X-ray radiograph of the Utah coal wed in the GRE experiment. Dense structures appear bright, while cracks and voids appear dark.
has produced microfractures and regions with greater or lesser pyridine accessibility. The heterogeneity of the coalmatrix producesvariations in the magnetic susceptibility throughout the specimen. Diffusion of pyridine through areas with susceptibility differences,particularlyat organic-mineral interfacesand at the surface of microfractures, substantially shortens the proton 2'2. Image contrastresultingfrom susceptibility effects is enhanced by utilizing the GRE pulse sequence, because magnetizationwhich is dephased by susceptibility differencesis not refocused by the read gradient. Mineral deposits may contain metal atoms with unpaired electrons which also enhance 2'1 and 2'2 contrast due to the paramagneticsusceptibilityof these environments. These dark features appear disproportionately large compared to the physical size of the correspondingstructures due to the susceptibility difference at the interface, and concomitant increase in the proton relaxation rates. X-ray radiographswere taken of the Utah and LewistonStockton coals (Figures 10 and 11). In general, contrast in the radiographs is produced by variation in the density of the specimens. Dense structures such as minerals appear brighter, while cracksand voids appear darker than the organic matter. The front faces of the radiographs are the same as in the photographs (see Figures 6 and 7) and lie in planes parallel to the 40-pm slices of the interior xy-projections(Figures8and 9). Clearly,the NMR images of the pyridine ingressed in the coal specimensshow much greater heterogeneity than the X-ray radiographs. This is particularly true for the Lewiston-Stockton specimen where the radiograph is nearly homogeneouscompared to the xy-projections which show strata of differential pyridine uptake parallel to the bedding plane of the coal. Once again, the dark vertical features in the NMR images must be structureswhich are inaccessibleto pyridine rather
than voids because of the lack of such features in the radiograph (Figure 11). On the other hand, the X-ray radiograph of the Utah specimen (Figure 10) shows two distinct bright features. These two structures, a nodule near the center of the specimen and a vertical vein to the left, are composed of much denser material than the rest of the coal matrix. These bright features are most likely mineral deposits. There is alsoa dark feature, inaccessibleto pyridine, toward the center of the xy-projections of the Utah specimen (Figure 8). Since these 40-pm thin sections are images near the center of the object, it is clear that the nodule is buried in the interior of the specimen, while the vein (absent in these interior NMR images) is located near the surface. Also, the slices display a NMR image contrast which is richly heterogeneouscomparedto the radiograph, showinghighly variable pyridine accessibilitythroughout the interior of the Utah coal due to mineral deposits, microfractures, voids, and variations in maceral composition.
Summary and Conclusions The focus of this initial study was the development of two different 3-DNMRI techniques for spatially mapping the distribution of mobile protons in solvent swollen coals. Back-projection methods were used to produce 2'1 (i.e., spin-lattice relaxation time) contrasted images of a Utah coal swollen with pyridine-&. The relatively large size of the specimen (7 mm X 5 mm) required to attain an adequate signal-to-noise ratio of individual projections limited image resolution to 80 pm over three dimensions. More than 14 OOO projections would have been necessary to achieve40-pm resolution in the image, making the total acquisition time (4 days) prohibitively long.
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Figure 11. X-ray radiograph of the Lewiston-Stockton coal used in the GRE experiment.
On the other hand, the GRE technique offered greater image resolution and contrast utilizing 2' (i.e., spin-spin relaxation time) weighting. Here again, there is a tradeoff between resolution and the total experimental time required to achieve adequate signal to noise in the image. The total experimental time was favorable in this instance owing to the intense signal from ingressed pyridine in the samples. Specimen size, however, was limited to 1mm3 in order to achieve a resolution of 40 pm, given the natural proton line widths observed and the maximum allowable gradient strengths to produce sufficient echo intensity. It should be pointed out that, when GRE imagingwas applied to a 1-mmpyridine-& swollen coal sample, the echo signal was extremely weak, making the total accumulation time prohibitively long. This preliminary investigation has shown that 3-D NMRI is a promising tool for studying solvent swelling of coals. This nondestructive technique reveals rich microscopic detail concerning the penetration and accessibility of a solvent into the interior of coal specimens. Further
development of this technique also presents the possibility for visualization and analysis of the dynamics of the coal swelling process which can be used to elucidate coal physical structure. In addition, several preparatory pulse sequences prior to the imaging experiment may be employed for the quantitative characterization of NMR relaxation phenomena and solvent diffusion in coals. Image contrast produced by differences in the mobility of the macromolecularnetwork in the coal will be compared with image contrast produced by differences in solvent accessibility in the same specimen. Coal specimens with known petrography will be utilized and correlated with NMR images obtained using multiple-pulse proton decoupling methods prior to the solvent-swelling measurements.
Acknowledgment. This work was performed under the auspicesof the Officeof Basic Energy Sciences,Division of Chemical Sciences, U.S.Department of Energy, under Contract No. W-31-109-ENG-38.
