Energy & Fuels 1997, 11, 245-246
245
Communications 129Xe
NMR Two-Dimensional Exchange Spectroscopy of Diffusion and Transport in Coal† Xu Zhu,‡,§ Igor L. Moudrakovski,‡ and John A. Ripmeester*,‡,§
Steacie Institute for Molecular Science, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6, and Ottawa-Carleton Chemistry Institute, Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6 Received June 10, 1996. Revised Manuscript Received August 22, 1996 Xe is a highly polarizable, chemically inert atom with a diameter of ∼4.3-4.4 Å. 129Xe has spin 1/2, its natural abundance is 26.4%, and the detection of its resonance absorption is quite straightforward.1 When trapped inside the coal micropores, a Xe atom will sample all of the different environments of the system. The resulting 129Xe chemical shift will be a function of the sorption energy, the available void space for the Xe atom, and the temperature.4 Once exchange of Xe between distinct trapping sites becomes slow on an NMR time scale, 129Xe NMR spectroscopy can be a very useful probe for resolving structural heterogeneity of coal micropore system. Two studies applying this technique to coal micropores have been reported.8,9 In this work we demonstrate the utility of 129Xe twodimensional exchange spectroscopy (2D EXSY) to study gas diffusional properties in coal micropores. 129Xe 2D EXSY NMR spectroscopy has been used to study the heterogeneity of amorphous polymers.5 More recently, this technique has been used to explore the diffusion process between the different domains in zeolites with very different length scales between the domains.6 Therefore, in the slow exchange limit, 2D EXSY NMR can provide useful information on gas diffusional pathways within the coal micropores. In addition to the diffusional pathway information, the intensities of the cross-peaks can also provide insight into the rate of the diffusion process.6 Two samples were considered for this study: sample A was obtained from Alberta Power and sample B from the Argonne Premium Coal Sample Program (ACPS).8 The properties of the two samples are summarized in Table 1. Experimental details are given in ref 7.
The 1D spectrum of sample A (Figure 1, inset) shows two lines. The narrow line near 0 ppm arises from free Xe gas interacting with the external surface of coal particles. The other line (peak maximum at 154 ppm) is quite broad and arises from Xe trapped in coal micropores. For sample B, an additional line (peak maximum at 111 ppm) was found compared to sample A and can be explained by the fact that there are two distinct pore size regions in sample B. This observation is consistent with a previous report.3 As a first step in the study we used 1D 129Xe NMR spectroscopy to monitor the process of Xe uptake by the coal micropores. This was done by measuring the ratio of the integrated peak intensities for adsorbed and free gaseous Xe as a function of time after Xe gas was sealed into the sample tube (Figure 1). For sample A, the uptake data can be described quite well with an
* Author to whom correspondence should be addressed. † Published as NRCC No. 39127. ‡ Steacie Institute for Molecular Science. § Ottawa-Carleton Chemistry Institute. (1) Barrie, P. J.; Klinowski, J. Prog. Nucl. Magn. Reson. Spectrosc. 1992, 24, 91-108. Raftery, D.; Chmelka, B. F. NMR Basic Principles and Progress; Springer-Verlag: New York, 1994; pp 111-158. Dybowski, C.; Bansal, N.; Duncan, T. M. Annu. Rev. Phys. Chem. 1991, 42, 433-464. (2) Ripmeester, J. A.; Ratcliffe, C. I. J. Phys. Chem. 1990, 94, 76527656. (3) Wernett, P. C.; Larsen, J. W.; Yamada, O.; Yue, H. J. Energy Fuels 1990, 4, 411-413. (4) Tsiao, C.; Botto, R. Energy Fuels 1991, 5, 87-92. (5) Kentgens, A. P. M.; van Boxtel, H. A.; Verweel, R. J.; Veeman, W. S. Macromolecules 1991, 4, 3712-3713. (6) Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A. Appl. Magn. Reson. 1995, 8, 385-399.
(7) To prepare the Xe-coal samples, each sample was weighed and packed into 10 mm diameter Pyrex glass tube. The tube was then attached to a vacuum manifold for out-gassing at 10-5 Torr. Before introduction of Xe gas, the coal sample was heated slowly to 105 °C and then maintained at this temperature under vacuum for several hours to completely remove the water present in the pores. After the sample was cooled to ambient temperature (21 °C), a measured amount of Xe gas (99.995%) was condensed into the tube with liquid nitrogen and the tube was flame-sealed. 129Xe NMR spectra were recorded at frequencies of 55.3 and 83.0 MHz on Bruker MSL-200 and Bruker AMX-300 spectrometers, respectively. A solid-state probe with horizontal solenoid geometry (10 mm diameter samples) was used. To circumvent problems with phasing of the very broad spectra, a twopulse spin-echo sequence (π/2-τ-π-τ-echo) was used together with a phase list (Turner, G.; Smith, K.; Kirkpatrick, R.; Oldfield, E. J. Magn. Reson. 1986, 67, 544-550). 2D EXSY data were collected by using the following pulse sequence: (π/2-t1-π/2-tm-π/2-t2)n in TPPI mode (Drobny, G.; Pines, A.; Sinton, S.; Weitkamp, D.; Wemmer, D.
S0887-0624(96)00084-9 CCC: $14.00
Table 1. Chemical and Elemental Properties of Coal Samples A and Ba property source rank ARc H2O (%) AR ash (%) MAFd C (%) MAF H (%) MAF N (%) MAF S (%) MAF O (%) median particle sizee (µm)
sample A Alberta Power subbituminous 18.59 7.30 75.51 5.11 1.62 0.59 17.25 43.8
sample B ACPSb No.
3 high volatile bituminous 7.97 14.25 77.67 5.00 1.37 2.38 13.51 33.6
a Composition unit is wt %. b APCS, Argonne Premium Coal Sample Program. c AR, as received. d MAF, moisture and ash-free. e Standard deviations of analysis are 1.3 and 1.2, respectively.
Published 1997 by the American Chemical Society
246 Energy & Fuels, Vol. 11, No. 1, 1997
Communications
Figure 1. Ratio of absorbed to free Xe as a function of time for samples A (bottom) and B (top). (Inset) 1D 55.3 MHz 129Xe NMR spectra of Xe adsorbed in coal samples A (bottom) and B (top).
exponential function with a time constant of 7.6 h. In the case of sample B, there is very rapid initial uptake, followed by a period of slower uptake which can again be described by an exponential function, this time with a time constant of 3.2 h. Samples A and B reach 90% gas uptake at 4 and 16 h, respectively. Since the average particle sizes are not very different (Table 1), there must be an intrinsic difference between diffusion processes in the two coals. It can be postulated that the diffusion into the coal pore system involves the larger pores, which were identified from the spectral line at 111 ppm for sample B and which are absent in sample A. If the larger and smaller pore systems in sample B are intimately connected, the larger pore system would certainly provide a lower barrier to diffusion into the coal particles. The general correctness of this postulate was checked by performing 2D EXSY experiments on samples A and B. Figure 2 shows the contour plots of 129Xe 2D EXSY spectra of coal sample B with different mixing times. The contour plot of the spectra clearly shows the two distinct pore size regions coexisting in sample B. The Xe atoms in these two pore size regions do not exchange in the mixing time of 0.5 ms. However, when the mixing time increases to 15 ms, exchange does occurs between these two pore size regions. However, it is pertinent to point out that with this mixing time there is no evidence of Xe exchange between the gas phase and adsorbed phase for either coal. This result satisfactorily confirms the explanation postulated for the faster uptake process of Xe gas by sample B; that is, the Xe transport into the micropores involves the larger pore size region. The length scale over which the two pore types are connected cannot be determined as the xenon diffusion constant is not known. A rough comparison can be made with xenon sorbed in polymer blends considering that crossFaraday Symp. Chem. Soc. 1979, 13, 49). The acquisition time t2 is the same as in the 1D experiment and is one of the two 2D NMR variables. The labeling time t1 is incremented equally for each successive pulse sequence and acts as the second variable. The tm in the pulse sequence is the mixing time, during which Xe is allowed to exchange between different trapping sites. If during the mixing time exchange occurs between different sites, off-diagonal cross-peaks will be present in the 2D EXSY spectrum. Typically 64 and 128 points were acquired in the t1 and t2 dimensions, respectively. Before Fourier transformation, the t1 dimension was zero-filled to 256 points. (8) Vorres, K. S. User’s Handbook for the Argonne Premium Coal Sample Program; Argone National Laboratory, Argonne, IL, 1993.
Figure 2. 129Xe 2D EXSY spectra of Xe adsorbed in coal sample B with different mixing times: (a) tm ) 0.5 ms; (b) tm ) 15 ms.
peaks become visible for mixing times of the order of 50-500 ms for domain sizes of 1-5 µm.9,10 Fick’s law for diffusion relates the mean square diffusion length, 〈r(t)2〉, to t, so that a cross-peak first appearing at ∼15 ms suggests that the two pore types are connected on a length scale the order of 0.5 µm if the diffusion constants are not too dissimilar. In conclusion, 129Xe 2D EXSY NMR spectroscopy yields valuable information of gas diffusion and transport in coal micropores. This technique can be further used to extract information of diffusion constants and diffusion path lengths of Xe in coal and also the activation energy for diffusion between Xe atoms in different pore regions. EF960084Q
(9) Tomaselli, M.; Meier, B. H.; Robyr, P.; Suter, U. W.; Ernst, R. R. Chem. Phys. Lett.. 1993, 205, 145. (10) Mansfeld, M.; Flohr, A.; Veeman, W. S. Appl. Magn. Reson. 1995, 8, 573.
