Energy & Fuels 2000, 14, 1245-1251
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Characterization of Micro-Domain Structure of Solvent-Swollen Coal by Proton Spin Diffusion and Small Angle Neutron Scattering Koyo Norinaga* and Masashi Iino Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai, 980-8577, Japan
George D. Cody Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, Washington, DC 20015
Pappannan Thiyagarajan Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, Illinois 60439 Received May 18, 2000. Revised Manuscript Received September 26, 2000
The macromolecular structure of a high volatile bituminous coal (the Blind Canyon) is characterized by osmotic dilation experiments using deuterated solvents (pyridine or benzene/ pyridine mixed solvent). Nano scale structural changes were quantitatively revealed using proton spin diffusion experiments and qualitatively verified with small-angle neutron scattering (SANS) measurements. The SANS data reveal systematic changes in the coherent scattering behavior of osmotically dilated coal particles in a binary solvent system of benzene + pyridine. These results show that volumetric strain derived from osmotic stresses results in the development of phaseseparated domains at nanoscales. Quantitatively, the effect of solvent-to-coal mass ratio (S/C) on the proton relaxation characteristics was examined. It is shown that the fractional amount of mobile hydrogen yielding the exponential decay, fMH, increased to a maximum of 0.5 with increasing solvent concentration. Above S/C ) 2.24, fMH remained nearly constant, indicating that solvent impenetrable regions exist in the swollen coal even at S/C ratios as high as 4.72. Although there exist at least two distinct structural regions in the swollen coals based on the transverse relaxation characteristics, longitudinal relaxation experiments are fit best by a single component revealing that spin diffusion is rapid in the swollen coals. The dynamics of spin diffusion were determined using a partially modified Goldman-Shen pulse sequence and analyzed by simple geometric models of two-phase systems. The average distance between the adjacent solvent impenetrable domains, di, was estimated on the basis of the diffusive path length for each spatial dimension by employing simple one-, two-, and three-dimensional models. Estimates of di derived from the SANS data lie between those derived for the one- and two-dimensional diffusion models suggesting that the actual structure may be roughly laminar, but with sufficient curvature or other irregularity to yield diffusion behavior lying between the two simple models considered. The results of this study clearly reveal that volumetrically measured osmotic strain is not affine down to the macromolecular level.
Introduction The phenomenon of solvent-induced swelling of coal has long been used to characterize the macromolecular structure of coal. In particular, the Flory-Rehner theory and variants thereof that have been used to estimate the molecular weight between cross-link points1-6 have * Author to whom correspondence should be addressed. Fax: +8-22-217-5655. E-mail:
[email protected]. (1) Sanada, Y.; Honda, H. Fuel 1966, 45, 295. (2) Kirov, N. Y.; O’Shea, J. M.; Sergeant, G. D. Fuel 1968, 47, 415. (3) Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structures; Mayers, R. A., Ed.; Academic Press: New York, 1982. (4) Nelson, J. R. Fuel 1983, 62, 112. (5) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729.
been frequently employed to relate the macromolecular network parameters to the degree of swelling in good solvents. The Flory-Rehner7 theory tacitly assumes that the deformation of the elementary chains of the network is affine, down to the molecular level, thus any observed macroscopic deformation (e.g., osmotic dilation) of a given sample is assumed to correspond linearly with a change in the statistical distribution of chain lengths of the coal macromolecule. In other words, the coal macromolecule is assumed to dilate uniformly at the segmental scale when we relate the macroscopic swell(6) Lucht, L. M.; Peppas, N. A. Fuel 1987, 66, 803. (7) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953.
10.1021/ef000102a CCC: $19.00 © 2000 American Chemical Society Published on Web 10/26/2000
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ing to molecular characteristics such as the cross-link density. Recent experiments exploring 1H NMR transverse relaxation characteristics (proton spin diffusion), reveal that the coal hydrogen in the pyridine-swollen state must be divided into two groups: hydrogen with relaxation characteristic of solids, and hydrogen with relaxation characteristic of liquids.8-15 For example, Barton et al.10 have shown that up to 60% of coal hydrogen becomes mobile when immersed in deuterated pyridine, while the remaining 40% remains in rigid structures and is characterized by an NMR signal component that is similar to the signal for the corresponding dry coal. These results appear to suggest that the osmotically dilated coal is phase separated into domains that are solvent-rich and solvent-depleted. Recently, Norinaga et al.16 used proton diffusion measurements to determine the scale of the heterogeneity in the phase-separated structure of swollen coals. Five coals of different ranks were osmotically dilated via saturation with deuterated pyridine and were analyzed via 1H NMR spectroscopy using a partially modified Goldman-Shen pulse sequence and modeled using simple geometric models of a two-domain (phase) system. These calculations indicated that the solvenrich phase domains in the swollen coals exist as discrete regions in the size range of 20-200 Å, requiring that coal does not swell affinely at the molecular scale but rather dilates nonuniformally leading to nanoscale structural heterogeneities. These results place our understanding of the macromolecular structure of coal in some jeopardy and certainly raise questions regarding the applicability of any affine statistical thermodynamic model to describe macroscopic, osmotic, strain of coal measured by bulk solvent swelling. The former studies focused on comparing dry vs fully osmotically dilated coals. In the present study the stepwise modification of coal structure accompanying sequential osmotic dilation experiments is explored. Macromolecular rearrangement is determined using combined NMR and small-angle neutron scattering (SANS) methods to record and quantify changes accompanying progressive osmotic dilation of coal using a per-deuterated binary solvent system. The use of SANS provides an independent verification of the progressive development a two-phase system with increased osmotic dilation. Proton diffusion experiments are used to determine the fraction of solvent-rich vs solvent-poor domain sizes at a given dilation interval via the solution of the spin diffusion equation. Optimization of the morphology of the domains is approached iteratively using estimates derived from both SANS and (8) Jurkiewicz, A.; Marzec, A.; Idziak, S. Fuel 1981, 60, 1167. (9) Jurkiewicz, A.; Marzec, A.; Pislewski, N. Fuel 1982, 61, 647. (10) Barton, W. A.; Lynch, L. J.; Webster, D. S. Fuel 1984, 63, 1262. (11) Kamienski, B.; Pruski, M.; Gerstein, B. C.; Given, P. H. Energy Fuels 1987, 1, 45. (12) Jurkiewicz, A.; Bronnimann, C. E.; Maciel, G. E. Fuel 1990, 69, 804. (13) Jurkiewicz, A.; Bronnimann, C. E.; Maciel, G. E. High-Resolution 1H NMR studies of Argonne premium coals. In Magnetic Resonance in Carbonaceous Solids; Botto, C. E., Sanada, Y., Eds.; Advances in Chemistry; American Chemical Society: Washington, DC, 1993; 229, 401. (14) Yang, X.; Larsen, J. W.; Silbernagel, B. G. Energy Fuels 1993, 7, 439. (15) Yang, X.; Silbernagel, B. G.; Larsen, J. W. Energy Fuels 1994, 8, 266. (16) Norinaga, K.; Hayashi, J. i.; Chiba, T.; Cody, G. D. Energy Fuels 1999, 12, 1239.
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NMR. Finally, the effect of the extent of solvent loading on the proton relaxation and spin diffusion properties is addressed. Experimental Section Samples. The sample of Blind Canyon coal used in these studies was supplied from Argonne Premium Coal Sample suite. The reported elemental composition of the dried Blind Canyon coal (hereafter referred to as BL) was C ) 80.7 wt %, H ) 5.8 wt %, N ) 0.6 wt %, S ) 0.2 wt %, and O ) 12.0 wt % on a dry-ash-free basis.17 For the NMR experiments mean particle size was as received, i.e., finer than 150 µm. The general protocol involved charging 0.3 g of BL into a 10 mm o.d. NMR tube. To this tube an excess of per-deutero pyridine (Aldrich, 99.99% atom D), py-d5, was added and sealed under a pressure of less than 2 Pa while frozen in liquid nitrogen. Solvent-to-coal mass ratio (S/C) ranged from 0.36 to 4.72. For the SANS experiments a sample of vitrain from the Blind Canyon was ground in a mortar and pestle and sieved to pass 500 µm but hold at 250 µm. These samples were then exhaustively extracted in pyridine (19 wt % extraction), washed in methanol, and then dried in vacuo at 333 K. Approximately 0.15 g of this sample was loaded into a suprasil cylindrical cell with a 2 mm path length (vol ) 0.7 mL). To the solid, 0.6 mL of deuterated solvent (benzene/pyridine mixtures) was added. Care was taken to ensure that the coal particles were able to fully dilate without particle-particle confinement. SANS. SANS data were measured at the Intense Pulsed Neutron Source of Argonne National Laboratory, using the Small Angle Diffractometer (SAD). This instrument uses pulsed neutrons derived from spallation with wavelengths in the range of 0.1-1.4 nm and a fixed sample-to-detector distance of 1.54 m. The scattered neutrons are measured using a 64 × 64 array of position-sensitive, gas-filled, 20 × 20 cm2, proportional counters with the wavelengths measured by timeof-flight by binning the pulse to 67 constant ∆t/t ) 0.05 time channels. The size range in a SANS experiment is constrained by both the geometry of the instrument and the wavelength of the neutrons which determine the working range of momentum transfer Q:
Q ) 4πλ-1 sinθ
(1)
where θ is half the Bragg scattering angle and λ is the wavelength of the neutrons. Given the characteristics of the SAD18 at the Intense Pulsed Neutron Source (IPNS), useful SANS data in the Q range of 0.005-0.25 Å-1 can be obtained in a single measurement. The reduced data for each sample is corrected for the backgrounds from the instrument, the suprasil cell, and the solvent, as well as for detector nonlinearity. Data are presented on an absolute scale by using the known scattering cross-section of a silica gel sample. 1H NMR. NMR measurements were carried out at 303 K using a JEOL Mu-25 NMR spectrometer equipped with a spin locking unit operating at a proton resonance frequency of 25 MHz. The solid-echo pulse sequence,19 90°x-t-90°y (90° phase shift) provided an approximation to the complete free induction decay (FID). Typical values for the pulse width, pulse spacing, repetition time, and number of scans were 2.0 µs, 8.0 µs, 6 s, and 32 s, respectively. The saturation recovery pulse sequence, 90°x-t-90°x, was used to monitor the recovery of the magnetization with the pulse separation time, t, and provided T1. (17) Vorres, K. S. User’s Handbook for the Argonne Premium Coal Sample Program; Argonne National Laboratory: Argonne, IL, 1993. (18) Thiyagarajan, P.; Epperson, J. E.; Crawford, R. K.; Carpenter, J. M.; Hjelm, R. P. In Proceedings of the International Seminar on Structural Investigation on Pulsed Neutron Sources; Dubna, 1993; p 194. (19) Powles, J. G.; Mansfield, P. Phys. Lett. 1962, 2, 58.
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Figure 1. Change in the volumetric swelling ratio with increasing volume fraction of pyridine in a solution with benzene. T1F was measured using a spin-locking pulse sequence,20 that includes a 90°x pulse followed by a reduced-amplitude pulse, phase-shifted 90°, and sustained for a variable time, t. The magnetization remaining at time t is monitored by observation of the free induction decay signal. The rotating frame measurements were made in a 6 G radio frequency field. The spin diffusion was monitored with the Goldman-Shen pulse sequence.21 To avoid the dead-time effect after the pulse, the original pulse sequence was modified as 90°x-t0-0°-x-t-90°xt1-90°y, according to Tanaka and Nishi.22 The analytical methods of the spin diffusion were described elsewhere in detail.16
Results and Discussion SANS Results. The relationship between the macroscopic volumetric swelling ratio and the volume fraction of pyridine in a solution with benzene is shown in Figure 1 for pyridine-extracted BL residue. The volumetric swelling ratio was determined following the method proposed by Green et al.23 As might be expected, this system exhibits a smooth increase in volumetric (osmotic) dilation with the concentration (hence activity) of pyridine. The slight departure from linearity is consistent with the strong interaction between pyridine and acidic sites in coal. Neutron scattering data obtained for the system of coal particles immersed in solutions of mixed perdeuterated benzene and pyridine are presented in Figure 2. Previous studies of the scattering behavior of dry coal particles24 exhibit grossly similar scattering behavior to that shown in Figure 2. Specifically, apparently pure incoherent scattering at high momentum transfer vector, Q, gives way to an exponential growth in coherent scattering intensity at low Q. The coherent scattering in the low Q region is well described by a power law where I(Q) ≈ Q-d, where d in the present case ranges from 2.2 to 2.6. Such power law scattering behavior is commonly observed from dry coal particles25,26 however, these previously reported power law exponents usually fall into the rough surface range of -3 < d < -4. Power (20) Hartmann, S. R.; Hahn, E. L. Phys. Rev. 1962, 128, 2042. (21) Goldman, M.; Shen, L. Phys. Rev. 1966, 144, 321. (22) Tanaka, H.; Nishi, T. Phys. Rev. B 1986, 33, 32. (23) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 935. (24) Cody, G. D.; Obeng, M.; Thiyagarajan, P. Energy Fuels 1997, 11, 495.
Figure 2. Coherent scattering intensity vs momentum vector for variably swollen BL residues in binary solvent of benzenepyridine. For clarity, only the scattering curves for the dry particles and osmotically dilated particles corresponding to 20, 40, and 100% pyridine solutions are shown. The dashed line at low Q highlights the power law region of coherent scattering that dominates the dry particle scattering and is present in all of the scattering curves independent of the extent of osmotic dilation. Growth of scattering intensity in the intermediate Q range correlates with the degree of osmotic dilation and indicates the formation of dense regions of scattering domains.
law exponents below -3, are not physically consistent with scattering off topologically rough surfaces. Although in the present case such power law behavior reveals a complex and disordered structure for the BL dry coal particles, it is not currently possible to be more specific regarding precisely what structure(s) are responsible for this complex scattering behavior. With the addition of solvent, resulting in the osmotic dilation of the coal particles, the coherent scattering behavior changes significantly and systematically. First, there are subtle changes in the power law region, i.e., the magnitude of d with the benzene:pyridine ratio. More interesting, however, is the obvious growth of a scattering peak or shoulder evident in the intermediate Q range, 0.01-0.3 Å-1 (Figure 2). Such scattering behavior is qualitatively consistent with the development of a dense system of scattering regions that formed as a direct consequence of osmotic dilation. A likely physical explanation for this new scattering component is that it results from phase separation accompanying osmotic dilation leading to the formation of solvent-rich and solvent-poor domains as has been suggested to occur in solvent-dilated coals.16 The scattering behavior of the dry and osmotically dilated coals could be simulated using a number of different geometric models. Unfortunately, the SANS data presented above do not provide a unique solution for a structure through direct inversion of the data. Efforts in this direction are being explored and will be presented separately. For the present purposes, however, these
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Figure 4. Transverse relaxation signals for pyridine-swollen BL coal.
Figure 3. Residual scattering intensity vs momentum vector for variably swollen BL residues in binary solvent of benzenepyridine.
data are sufficient to show that the previous interpretation derived from the proton spin diffusion measurements is reasonable. Notwithstanding the difficulties of directly simulating the scattering curves, a crude estimate of the mean domain interspacing length scale can be obtained by the following approach. First, we assume that the power law scattering exhibited in the dry coal particles and persisting over the range of osmotic dilation constitutes a background reflecting the intrinsic structural and chemical complexity of coal. No attempt (at this point) is made to ascribe a specific structure to this background. The development of nanoscale dense scattering domains and the observed scattering “peak” associated with osmotic dilation (Figure 2) is, therefore, considered to be a phenomena independent of the source the power law background. Whether this assumption is correct remains to be determined, however, crude estimation of the mean domain interparticle spacing will not be affected significantly by these assumptions. Treating the low Q power law scattering as being essentially a background, we can subtract the dry coal powder data from each of the osmotically dilated scattering curves highlighting the progressive development of the osmotic swelling-derived scattering peak (Figure 3). It has long been recognized that a coarse approximation of the mean interparticle scattering dimension can be derived by application of the Bragg relation whereby Qmax ≈ 2πr-1 (where r is interdomain distance), although it is acknowledged that the Bragg relationship cannot be rigorously applied to such features in the low Q region.27 The peaks revealed in Figure 3 are highly asymmetric with a peak at ∼0.015 Å-1 and a central moment closer to 0.03 Å-1. This would suggest a mean interdomain distance on the order of 200-400 Å. With (25) Bale, H. D.; Schmidt, P. W. Phys. Rev. Lett. 1984, 53, 596. (26) McMahon, P.; Snook, I. J. Chem. Phys. 1996, 105, 2223. (27) Guinier, A. X-ray Diffraction in Crystals, Imperfect Crystals, and Amorphous bodies; W. H. Freeman: San Francisco, 1963.
Figure 5. Change in fMH with S/C.
a more sophisticated analysis of the data (dependent, of course, on a clear picture of the physical structure responsible for the power law scattering in the low Q region) this estimate will be refined; however, it is unlikely that the such an estimate will deviate appreciably from the coarse estimate presented above. 1H NMR Results. The FID curves for the swollen BL coal are drawn as a function of decay time in Figure 4. Although the solvent swelling enhances the fraction of slowly decaying components, a portion of the coal hydrogens remains rigid. For a dipole-coupled rigid system such as dry coal, the time decay of the nuclear magnetization can be characterized by a Gaussian function. On the other hand, in a liquid or a liquidlike environment, the magnetization decay is approximately an exponential function. Therefore the observed FID was assumed to be expressed by the following equation and was analyzed numerically by the nonlinear leastsquares method:
I(t) ) IG(0) exp [-t2/2T2G2] + IL1 (0) exp [-t/T2L1] + IL2 (0) exp [-t/T2L2] (2) where I(t) and Ii(t) are the observed intensity at time t, and that attributed to component i, respectively, and T2i is the transverse relaxation time of the ith component. The fractions of hydrogen producing exponential decays, fMH, are plotted against S/C in Figure 5. fMH was increased up to 0.5 with an increase in S/C. However, fMH kept almost constant value above S/C ) 2.24,
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Table 1. Results of Proton Longitudinal Relaxation Measurements for Blind Canyon Coal Swollen in Deuterated Pyridine T1F [ms] S/Ca
T1Ff b
T1Fs c
T1 [ms]
0 0.36 0.68 1.03 1.33 1.67 2.24 2.57 3.52 4.72
0.7(0.52)
4.8(0.48)
1.0(0.52) 1.3(0.59) 0.8(0.67) 1.7(0.62) 1.7(0.54) 1.8(0.55) 1.7(0.54) 1.8(0.52)
5.7(0.48) 7.3(0.41) 5.3(0.33) 12.1(0.38) 12.9(0.46) 15.8(0.45) 16.1(0.46) 20.2(0.48)
66(1.00) 65(1.00) 108(1.00) 104(1.00) 112(1.00) 128(1.00) 141(1.00) 145(1.00) 130(1.00) 144(1.00)
Values in parentheses; fraction of each component, amass ratio of solvent to coal, bfast, cslow.
indicating that there exist the solvent-impenetrable regions in the swollen coal even at S/C ) 4.72. For the swollen coal samples, it is clear that there are domains that do not swell and are not penetrated by solvent as reported previously. The phase structures of the swollen coal are separated into at least two phases, i.e., a solvent-rich (SR) and a solvent-impervious phase (SI). To examine whether the spin diffusion process is active or not in the swollen coal samples, proton longitudinal relaxation was measured both in the laboratory and rotating frame. Table 1 lists the result of T1 and T1F measurement for the swollen BL coal. T1 is composed of one component while T1F can be analyzed by the sum of two exponential functions. From these results, one can clearly resolve the effects of spin diffusion. T2 signals are composed of three components without the effect of spin diffusion while T1F and T1 measurements are affected strongly by spin diffusion and the number of the components decreases from T1F to T1. The existence of at least two time constants for a rotating frame longitudinal relaxation process, i.e., T1F, in a system means that spin diffusion processes cannot effectively average the different dynamical properties of protons in different spatial domains on the relevant time scale of the specific relaxation process. On the other hand, in the time scale of T1 measurements, the distinctly separated spin systems were sufficiently averaged by the spin diffusion. The scale of spatial heterogeneities of the swollen coals can be estimated by evaluating the diffusive path length, i.e., the maximum linear scale over which diffusion is effective. The Goldman-Shen pulse sequence was thus employed to monitor the spin diffusion process. The advantage of the Goldman-Shen experiment is that the time for spin diffusion can be arbitrarily varied, and if this time is much less than T1, the analysis is straightforward. The Goldman-Shen experiment is a technique to put the separate spin systems at different spin temperatures and then sample them as a function of time so that their approach to equilibrium can be followed. In Figure 6, the recovery factor of the magnetization of SI phase, R(t), is plotted versus the square root of time, t1/2 for the solvent-swollen BL coals. It should be noted that S/C has almost no effects on the observed R(t). The time evolution of R(t) is analyzed by the diffusion equation solved by Cheung and Gerstein28 to get information on (28) Cheung, T. T. P.; Gerstain, B. C. J. Appl. Phys. 1981, 52 (9), 5517.
Figure 6. Recovery of proton magnetization in SI phase as a function of t1/2 for the solvent-swollen BL coal. Solid lines represent the best fit to the data using a diffusion model.
Figure 7. Domain shapes for each spatial dimension. Degree of freedom for the spin diffusion corresponds to 3, 2, and 1 for sphere, cylinder, and lamellar, respectively.
the diffusive path length, l. Since spin diffusion is similar to the spin-spin relaxation process, it is expected to occur much faster in SI phase than in SR phase. The rate-determining step of the spin diffusion would be the diffusion in SR phase. Thus l would correspond to the size of SR phase. The solid curve in Figure 6 represents the nonlinear least-squares fits to the data by using the diffusion equation. The analytical fits give l to be 70, 160, and 250 Å for one, two, and three dimensions, respectively. As shown in Figure 7, the morphology of the domain, i.e., the spatial dimension that is assumed to solve the diffusion equation, affects the estimated l values. Hence the information regarding the morphology of the domains is required for the precise evaluation of the domain size. Comparison of SANS and 1H NMR Results. To compare the NMR results with SANS, we must convert the spin diffusion path length into an interdomain spacing. The sphere of SI phase is assumed to be covered with a uniform layer of SR phase of thickness l as shown in Figure 8. This model allowed to produce the following two equations:
l ) rC - rSI
(3)
ΦSI ) ΦC(1 - fMH) ) (rSI/rC)d
(4)
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Figure 8. A 3-D domain model used to calculate the interdomain spacing from the spin diffusion distance. The sphere of SI phase is assumed to be covered with a uniform layer of SR phase of thickness l.
some curvature leading to proton spin diffusive behavior that would lie between the two simple geometric models. It is noted that although exhaustively extracted BL coals were used in the SANS experiments, nonextracted BL coals were used in the 1H NMR measurements. Therefore, care must be taken when we estimate the domain shape on the basis of the comparison of these results. Yang et al.14 examined the effect of Soxhlet extraction with pyridine on the fraction of mobile hydrogen of a bituminous coal. They found that the fraction of mobile hydrogens decreases 2-fold in the coal residues after extraction despite the fact only about less than 20% of the organic matter is removed during the extraction. In the present study it was observed that the fraction of mobile hydrogen, i.e., fMH, is not a sensitive parameter for the calculation of di. For example, if we employ fMH value of 0.25 instead of 0.5, the resulting di is 170 Å, indicating that the current measurements can safely discard the possibility of spherical domains. Conclusions
Figure 9. Changes in rC with S/C.
where ΦSI, ΦC, and d are volume fractions of SI domains, volume fraction of coal in the swollen coal gel, and spatial dimension of the domain, respectively. ΦC can be calculated by
ΦC )1/(1 + (S/C)Fc/Fs)
(5)
where Fc and Fs are helium density29 of BL coal and density of deuterated pyridine. In eq 4, we assumed that the hydrogen ratio is identical to the volume ratio. Changes in rC with S/C are shown in Figure 9. rC decreases with increase in S/C and keeps almost constant above S/C ) 2.24. The SI domain diminishes in size with solvent loading. Twice rC is taken to be equal to the interdomain spacing, di. di was estimated for each spatial dimension. The SANS measurements indicate a domain spacing with an average of between 200 and 400 Å. The di evaluated under one-dimension at around S/C ) 4 was approximately 160 Å, for the twodimensional model the computed di is closer to 450 Å. The SANS data clearly does not support spherical domain structure. Distinction between simple laminar or cylindrical domain structure is more tenuous. In reality, the actual domain structure is likely to be somewhat irregular, perhaps roughly laminar but with (29) Huang, H.; Wang, K.; Bodily, D. M.; Hucka, V. J. Energy Fuels 1995, 9, 20.
1H NMR spin diffusion and SANS measurements were carried out for BL coals swollen by deuterated solvent (pyridine or and benzene/pyridine mixed solvent). The effect of S/C on the 1H NMR relaxation characteristics was examined. fMH was increased up to 0.5 with increase in S/C. However, fMH kept almost constant value above S/C ) 2.24, indicating that there exist the solvent impenetrable regions in the swollen coal even at S/C ) 4.72. Whereas the transverse relaxation characteristics revealed the existence of at least two distinct structural regions in the swollen coals, the measured longitudinal relaxation was best characterized by a single component as spin diffusion is rapid in the swollen coals. The dynamics of spin diffusion were revealed using a partially modified Goldman-Shen pulse sequence and analyzed by a simple mathematical model of a two-phase system. Given a model of the domain structure, the diffusive path length was converted to the interdomain spacing, di. The resultant di evaluated for the one- and two-dimension models are near to that derived from SANS. These results suggest that the domain structure developed from osmotic dilation might reflect a roughly laminar structure with sufficient irregularity, e.g., curvature, to yield proton diffusion behavior that would lie between our simple one- and two-dimensional models. More significantly, both the SANS and NMR data show that a strictly affine treatment of macroscopic osmotic dilation experiments is not supported at the nanoscale level. At least part of the volume expansion of coals immersed in excellent swelling solvents is related to the formation of phaseseparated domains. This makes the interpretation of the statistical distribution of elastic chain lengths derived from macroscopic swelling measurements and the application of even modified statistical mechanical theories of swelling dubious.
Acknowledgment. The authors are grateful to Drs. Tadashi Yoshida and Masahide Sasaki of the Hokkaido National Industrial Research Institute for their useful advice on the NMR measurements. The authors are also grateful to Mr. Norihide Kudo of Hokkaido University
Micro-Domain Structure of Solvent-Swollen Coal
for his assistance of NMR measurements. This work was supported in part by a “Research for the Future Project” grant from the Japan Society for the Promotion of Science (JSPS), through the 148th Committee on Coal Utilization Technology. The neutron-scattering experiments were performed at the Intense Pulsed Neutron
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Source at Argonne National Laboratory. We gratefully acknowledge the help of Denis Wozniak of IPNS, Argonne National Laboratory. The IPNS is a Department of Energy-supported Facility. EF000102A
