First Application of 27Al Multiple Quantum Magic-Angle Spinning

Aug 17, 2004 - matter in natural coals, because of incomplete cancellation of the second-order quadrupolar effects, although 3QMAS spectra gave better...
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Energy & Fuels 2004, 18, 1732-1737

First Application of 27Al Multiple Quantum Magic-Angle Spinning Nuclear Magnetic Resonance at 16.4 T to Inorganic Matter in Natural Coals K. Kanehashi* and K. Saito Advanced Technology Research Laboratories, Nippon Steel Corporation, 20-1, Shintomi, Futtsu-city, Chiba 293-8511, Japan Received March 3, 2004. Revised Manuscript Received July 8, 2004

27Al multiple quantum magic-angle spinning (MQMAS) spectroscopy at high magnetic field (16.4 T) was applied for the first time to the structural analysis of inorganic matter in natural coals. Resolution of 3QMAS spectra at 7.0 T was insufficient for identification of the inorganic matter in natural coals, because of incomplete cancellation of the second-order quadrupolar effects, although 3QMAS spectra gave better resolution than conventional magic-angle spinning (MAS) spectra. On the other hand, an increased magnetic field (16.4 T) greatly improved 3QMAS spectral resolution, compared to that obtained at 7.0 T; hence, it facilitated the assignment of some minerals. Moreover, 3QMAS spectra recorded at 16.4 T led to signal enhancement by a factor of ∼4, compared to spectra taken at 7.0 T. Higher field strengths became useful for the analysis of inorganic matter in natural coals that have a low concentration of aluminum (∼2.0 mass %). 3QMAS and 5QMAS spectra at 16.4 T were also compared. As far as inorganic matter in natural coals is concerned, the spectral resolution was almost unchanged between 3QMAS and 5QMAS, although the excitation of 5 quantum coherences is less efficient than that for 3 quantum coherences. It is concluded that the combination of MQMAS techniques with high magnetic field is a very effective for characterization of inorganic matter in natural coals and is especially wellsuited to the analysis of clay minerals that have low crystallinity.

Introduction Detailed studies on the inorganic matter in coals are very important, from the viewpoint of both geology (coalification and diagenesis) and coal utilization.1-4 X-ray diffractometry (XRD) is a common and powerful tool in the analysis of inorganic matter in coals. Several characterizations of clay minerals and aluminosilicates in natural coals,5,6 low-temperature ashing residues,7-12 * Author to whom correspondence should be addressed. Telephone: +81-439-80-2264. FAX: +81-439-80-2746. E-mail address: kanehasi@ re.nsc.co.jp. (1) Watt, J. D. The Physical and Chemical Behaviour of the Mineral Matter in Coal under the Conditions Met in Combustion Plant, Part 1; British Coal Utilization Research Association Literature Survey; British Coal Utilization Research Association: Leatherhead, Surrey, U.K., 1968. (2) Watt, J. D. The Physical and Chemical Behaviour of the Mineral Matter in Coal under the Conditions Met in Combustion Plant, Part 2; British Coal Utilization Research Association Literature Survey; British Coal Utilization Research Association: Leatherhead, Surrey, U.K., 1969. (3) Stach, E.; Mackowsky, M. T.; Teichmuller, M.; Taylor, G. H.; Chandra, D.; Teichmuller, R. Stach’s Textbook of Coal Petrology; 3rd Edition; Gebruder Borntraeger: Berlin and Stuttgart, Germany, 1982. (4) Cutler, A. J. B.; Flatley, T.; Hay, K. A. CEGB Res. 1978, 8 (October), 13. (5) Thompson, A. R.; Botto, R. E. Energy Fuels 2001, 15, 176. (6) Daniels, E. J.; Altaner, S. P. Am. Mineral. 1990, 75, 825. (7) Martinez-Tarazona, M. R.; Spears, D. A.; Palacios, J. M.; Martinez-Alonso, A.; Tascon, J. M. D. Fuel 1992, 71, 367. (8) Adolphi, P.; Sto¨rr, M. Fuel 1985, 64, 151. (9) Pike, S.; Dewison, M. G.; Spears, D. A. Fuel 1989, 68, 664. (10) Miller, R. N.; Yarzab, R. F.; Given, P. H. Fuel 1979, 58, 4. (11) Allen, R. M.; Carling, R. W.; VanderSande, J. B. Fuel 1986, 65, 321. (12) Barnes, J. R.; Clague, A. D. H.; Clayden, N. J.; Dobson, C. M.; Jones, R. B. Fuel 1986, 65, 437.

and combustion coals13 have been performed using XRD. However, it is often difficult to assign inorganic matter through the use of XRD when samples are noncrystalline, as are several clay minerals that give the poorly resolved patterns. In contrast to XRD, the solid-state nuclear magnetic resonance (NMR) technique is wellsuited for the analysis of noncrystalline compounds, as well as crystalline compounds, and is independent of particle size. Therefore, solid-state NMR has become important and effective in the studies on the chemical structures of materials of low crystallinity, such as minerals14-25 and glassy compounds.22,26-33 Most of (13) Wilson, M. A.; Young, B. C.; Scott, K. M. Fuel 1986, 65, 1584. (14) Kinsey, R. A.; Kirkpatrick, R. J.; Hower, J.; Smith, K. A.; Oldfield, E. Am. Mineral. 1985, 70, 537. (15) Axelson, D. E. Fuel Sci., Technol. Int. 1987, 5, 561. (16) Stebbins, J. F. In Mineral Physics and Crystallography: A Handbook of Physical Constants; American Geophysical Union: Washington, DC, 1995. (17) Lippmaa, E. T.; Ma¨gi, M.; Samoson, A.; Engelhardt, G.; Grimmer, A. R. J. Am. Chem. Soc. 1980, 102, 4889. (18) Ma¨gi, M.; Lippmaa, E. T.; Samoson, A.; Engelhardt, G.; Grimmer, A. R. J. Phys. Chem. 1984, 88, 1518. (19) Hansen, M. R.; Jakobsen, H. J.; Skibsted, J. Inorg. Chem. 2003, 42, 2368. (20) Lippmaa, E.; Samoson, A.; Ma¨gi, M. J. Am. Chem. Soc. 1986, 108, 1730. (21) Woessner, D. E. Am. Mineral. 1989, 74, 203. (22) Baltisberger, J. H.; Xu, Z.; Stebbins, J. F.; Wang, S. H.; Pines, A. J. Am. Chem. Soc. 1996, 118, 7209. (23) Mozgawa, W.; Fojud, Z.; Handke, M.; Jurga, S. J. Mol. Struct. (THEOCHEM) 2002, 614, 281. (24) Cheng, X.; Zhao, P.; Stebbins, J. F. Am. Mineral. 2000, 85, 1030. (25) Stebbins, J. F.; Du, L.-S.; Kroeker, S.; Neuhoff, P.; Rice, D.; Frye, J.; Jakobsen, H. J. Solid State NMR 2002, 21, 105.

10.1021/ef040029e CCC: $27.50 © 2004 American Chemical Society Published on Web 08/17/2004

27Al

MQMAS NMR of Inorganics in Natural Coal

these compounds include Si and Al atoms; hence, 29Si and 27Al solid-state NMR techniques, especially magicangle spinning (MAS), have been very widely used for their chemical characterizations. Because 29Si has a nuclear spin number of I ) 1/2, it is sufficient to apply MAS for line narrowing, although the spin-lattice relaxation time (T1) is generally long. 29Si MAS NMR gives the isotropic spectrum because of a removal or reduction of both the dipole interaction and the chemical-shift anisotropy. Therefore, 29Si MAS NMR provides precise structural information and quantification on the silicon environments, such as the number of the nearest-neighbor Si and Al atoms that are connected tetrahedrally via O atom bridges denoted as Qn (where n is the number of shared O atoms with other silicate or aluminate tetrahedra). For natural coals5,34 and coal ashes,12,13,15,35-37 characterizations of inorganic matter have been reported using 29Si NMR. However, most of the research gives only the distinction between quartz and other clay minerals. It is only possible to estimate the content of clay minerals, such as kaolin (91.5 ppm), montmorillonite (93.7 ppm), and illite (91.0 ppm), through the use of spectral simulation techniques, because of the similar chemical shifts and relatively broad signals that cause the overlap of these signals. 27Al NMR has been also applied to investigate inorganic matter in natural coals5,12,15,34,36,38-40 and coal ashes.12,13,15,35,37,39,40 Because the natural abundance of 27Al is 100%, 27Al NMR has sufficient sensitivity. The applications of 27Al MAS NMR provide information on the local molecular environment of aluminum, especially for the determination of the coordination around the Al nucleus. The electron densities surrounding aluminum with octahedral and tetrahedral coordination are different; therefore, well-separated chemical shifts of ∼0 and 70 ppm are obtained, respectively.41 Estimates of the octahedral Al/tetrahedral Al ratios have been used to provide a general description of the inorganic content in coals. However, solid-state 27Al MAS spectra often encounter a considerable decrease in resolution. For an 27Al nucleus with a spin number of I ) 5/2, extensive signal broadening, quadrupolar splittings, and shifts caused by the second-order quadrupolar interaction make the spectral analysis complicated. Because of these broad(26) Murdoch, J. B.; Stebbins, J. F. Am. Mineral. 1985, 70, 332. (27) Maekawa, H.; Maekawa, T.; Kawamura, K.; Yokokawa, T. J. Non-Cryst. Solids 1991, 127, 53. (28) McManus, J.; Ashbrook, S. E.; MacKenzie, K. J. D.; Wimperis, S. J. Non-Cryst. Solids 2001, 282, 278. (29) Padro, D.; Schmidt, B. C.; Dupree, R. Geochim. Cosmochim. Acta 2003, 67, 1543. (30) Merzbacher, C. I.; Sherriff, B. L.; Hartman, J. S.; White, W. B. J. Non-Cryst. Solids 1990, 124, 194. (31) Stebbins, J. F.; Xu, Z. Nature 1997, 390, 60. (32) Kanehashi, K.; Saito, K. Chem. Lett. 2002, 7, 668. (33) Stebbins, J. F.; Oglesby, J. V.; Kroeker, S. Am. Mineral. 2001, 86, 1307. (34) Thompson, A. R.; Botto, R. E. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1987, 32, 280. (35) Pang, L. S. K.; Vassallo, A. M.; Wilson, M. A.; Phong-anant, D.; Salehi, M.; Baker, J. W. Proc. Coal Res. Conf. 1989, 2, 393. (36) Burchill, P.; Howarth, O. W.; Sword, B. J. Fuel 1991, 70, 361. (37) Ramesh, A.; Kozinˇski, J. A. Fuel 2001, 80, 1603. (38) Howarth, O. W.; Ratcliffe, G. S.; Burchill, P. Fuel 1987, 66, 34. (39) Burchill, P.; Howarth, O. W.; Sword, B. J. Fuel Process. Technol. 1990, 24, 375. (40) Burchill, P.; Richards, D. G.; Warrington, S. B. Fuel 1990, 69, 950. (41) Fyfe, C. A.; Feng, Y.; Grondey, H.; Kokotailo, G. T.; Gies, H. Chem. Rev. 1991, 91, 1525.

Energy & Fuels, Vol. 18, No. 6, 2004 1733 Table 1. Chemical Composition of Coal Samples Compositiona coal

Cb

H

Oc

Al

Si

A B C D

91.0 90.2 88.0 75.7

3.7 3.9 4.5 4.8

2.6 3.1 4.3 16.6

1.8 1.3 2.0 0.4

1.5 1.2 1.9 0.7

a Values given as percentages, unless noted otherwise. b Dryash-free (daf) percentage. c Organic oxygen percentage.

enings, Al signals may be severely overlapped. Spectral fitting can be applied for assignment of 27Al NMR spectra; however, it is more difficult to obtain accurate deconvolution peaks, because of the quadrupolar splittings. As a result of these problems, the application of 27Al NMR has almost been limited to only estimation of the tetrahedral Al/octahedral Al ratio in raw coals and coal ashes. In 1995, two-dimensional (2D) multiple quantum magic-angle spinning (MQMAS) NMR was introduced for half-integer quadrupolar nuclei.42 MQMAS techniques are capable of averaging the second-order quadrupolar interaction, by means of the correlation of the multiple quantum and single quantum coherence; hence, severe broadening problems are canceled and spectral analysis is facilitated over traditional MAS methods. Moreover, a standard MAS probe is able to be used for the MQMAS experiment, whereas other techniques such as double-rotation (DOR)43 and dynamic-angle-spinning (DAS)44 involve a complex mechanical rotation of the sample with two different angles. Recently, 27Al MQMAS spectra have come into widespread use for studies on aluminosilicates28,29,45 and minerals.22,46 To the best of our knowledge, however, few reports on MQMAS investigations of natural coals that contain several aluminosilicates and clay minerals have been reported so far. In the present study, we have conducted an 27Al MQMAS experiment on natural coal at high magnetic field (16.4 T) for the first time. We show that it is possible to assign the type of minerals and obtain the quantitative information from MQMAS data. Moreover, the detailed information about NMR parameters of the isotropic chemical shift and the quadrupolar parameter were calculated. The results in high-field MQMAS analyses are compared with those from lower-magneticfield (7.0 T) MQMAS spectra. Experimental Section Samples. Four coal samples of widely differing rank (carbon content of 75.7-91.0 mass %) were selected for 27Al NMR measurements: coal A, Copabella; coal B, South Walker; coal C, Baran; and coal D, Shinka. Natural coals (not coal ashes) were used for NMR experiments in the present studies. The results of the chemical composition, as determined by chemical analysis and fluorescent X-ray analysis, are given in Table 1. Kaolin and R-alumina (produced by Aldrich Chemical Co.), (42) Frydman, L.; Harwood: J. S. J. Am. Chem. Soc. 1995, 117, 5367. (43) Samoson, A.; Lippmaa, E.; Pines, A. Mol. Phys. 1988, 65, 1013. (44) Mueller, K. T.; Sun, B. Q.; Chingas, G. C.; Zwanziger, J. W.; Terao, T.; Pines, A. J. Magn. Reson. 1990, 86, 470. (45) Stebbins, J. F.; Kroeker, S.; Lee, S. K.; Kiczenski, T. J. J. NonCryst. Solids 2000, 275, 1. (46) Ashbrook, S. E.; MacKenzie, K. J. D.; Wimperis, S. Solid State NMR 2001, 20, 87.

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Figure 1. Pulse sequence and coherence transfer pathway diagram of 3QMAS. montmorillonite (JCSS-3101, supplied by the Japan Clay Science Society), and muscovite (from Mt. Ishikawa in Fukushima, Japan) were also measured for the assignment of a type of inorganic matter in natural coals. Nuclear Magnetic Resonance Experiments. 27Al NMR experiments were performed at ω0/(2π) ) 182.4 MHz with a JEOL model ECA-700 spectrometer (with a magnetic field of 16.4 T) and at ω0/(2π) ) 78.3 MHz with a Varian/Chemagnetics model CMX-300 instrument (with a magnetic field of 7.0 T). (Note: ω0 is the Zeeman frequency.) At 16.4 T, a 4-mm NB single-resonance MAS probe (from JEOL) was used, with a sample spinning rate of ωr/(2π) ≈ 18 kHz; at 7.0 T, a 4-mm WB MAS probe (from Varian/Chemagnetics) was used, with a sample spinning rate of ωr/(2π) ≈ 16 kHz. For 27Al MAS with or without proton decoupling spectra, the single-pulse excitation sequence with a radio-frequency (rf) flip angle of ∼18° was used in 27Al MAS spectra to avoid the quadrupolar nutation effects. A proton-decoupling rf power of ω1/(2π) ≈ 40 kHz was applied in proton-decoupled MAS spectra. For odd half-integer quadrupolar nuclei such as 27Al, the frequency shifts due to the second-order quadrupolar interaction, whereas the multiple quantum transition (-m T +m) is not affected by the first-order quadrupolar interaction. The (+m T -m) transition frequency affects the second-order quadrupolar interaction: 2QI ω-mT+m )

ω2Q [A CI (m) + A2(θ,φ)CI2(m)P2(cos β) + ωL 0 0 A4(θ,φ)CI4(m)P4(cos β)] (1)

where ωQ is the quadrupolar coupling constant and ωL is the Larmor frequency; P2 and P4 are the second- and fourth-order Legendre polynominals, respectively; A0 is a constant that is proportional to the isotropic quadrupolar shift; A2(θ,φ) and A4(θ,φ) are orientation-dependent coefficients; and CI0(m), CI2(m), and CI4(m) are zero-, second-, and fourth-rank coefficients, respectively, that are dependent on the -m T +m transition and the spin number I. During the MQMAS experiment, multiple quantum coherence (-m T +m) is excited and evolves for a period of t1. It is then converted to single quantum coherence (-1/2 T +1/2) and isotropic echoes are formed at t2. When samples spin at the magic angle, the P2(cos β) term in eq 1 becomes zero. Isotropic echoes then are detected at the expected acquisition times:

|

CI4(m)

|

Figure 2. 27Al MAS NMR spectra of coal A without proton decoupling, for magnetic fields of (a) 7.0 T and (b) 16.4 T. Asterisk (*) denotes spinning sidebands. and 5 quantum coherences were used. No proton decoupling was applied in the MQMAS experiment. A typical rf-field strength of ω1/(2π) ) 192 kHz was used for both the multiple quantum excitation pulse and conversion pulse, whereas a weaker rf-field strength of ω1/(2π) ) 18 kHz was used for the z-filter selective pulse. Typical pulse durations for 3QMAS were 2.8 µs (excitation), 1.0 µs (conversion), and 14 µs (z-filter); typical pulse durations for 5QMAS were 4.0 µs (excitation), 1.5 µs (conversion), and 14 µs (z-filter). A hypercomplex sequence was used to collect pure absorption-mode line shapes.48 Because skewed 2D data were obtained in this MQMAS sequence, 2D spectra were paralleled to the F2dimension by the sharing transformation. For MQMAS spectra in this study, scaling along the F1-axis was performed, so that the slope of the chemical shift (CS) axis in the 2D spectra was equal to 1. The 27Al chemical shift was referenced to a 1 mol/L AlCl3 aqueous solution at -0.1 ppm. Pulse repetition rates of 1-3 s were used in 27Al MAS and MQMAS experiments. Under a magnetic field of 16.4 T, 1 day was needed to collect the 3QMAS spectra of natural coals, whereas the same analysis under a magnetic field of 7.0 T required 2-3 days.

Results and Discussion

For the 27Al nucleus (I ) 5/2), |CI4(m)/CI4(1/2)| ) 19/12 and 25/12 when 3Q and 5Q excitation is applied, respectively. In this study of MQMAS, the three-pulse sequence with a z-filter, which is a selective 90° pulse at low rf power, was applied, because of the symmetric coherence transfer, yielding a nondispersion line shape (Figure 1).47 The excitation of 3

First, the dependence of conventional MAS spectra on external magnetic fields is discussed. Figure 2 shows 27Al MAS spectra of coal A recorded at different magnetic fields. At 7.0 T, octahedral and tetrahedral signals are broad and considerably asymmetrical in coal A. The use of a much faster spinning rate of 18 kHz, compared with that observed in previous reports, avoids the overlap of the octahedral/tetrahedral peaks and spinning sidebands. A spinning rate of 18 kHz effectively

(47) Amoureux, J. P.; Fernandez, C.; Steuenagel, S. J. Magn. Reson. 1996, A123, 116.

(48) Massiot, D.; Touzo, B.; Trumeau, D.; Coutures, J. P.; Virlet, J.; Florian, P.; Grandinetti, P. J. Solid State NMR 1996, 6, 73.

t2 )

CI4(1/2)

t1

(2)

27Al

MQMAS NMR of Inorganics in Natural Coal

Energy & Fuels, Vol. 18, No. 6, 2004 1735

Figure 4. Correlation between the percentage of tetrahedral Al species and the percentage of carbon in organic matter. Table 2. Distribution of Tetrahedral, Pentacoordinated, and Octahedral Al Species in Coals, as Determined by 27Al MAS NMR Distribution of Al Species (%)

Figure 3. 27Al MAS NMR spectra of natural coals without proton decoupling, for (a) coal B, (b) coal C, and (c) coal D. Asterisk (*) denotes spinning sidebands.

averages the dipole interaction and the chemical-shift anisotropy; therefore, the broad line widths observed in these spectra are caused by the quadrupolar interaction and/or the dispersion of chemical sites. Consistent with this interpretation, line widths decreased drastically, which permitted much better spectral resolution at 16.4 T. Moreover, it is found that at least two octahedral Al sites exist, at 4.5 and 12.3 ppm. Also note that both octahedral and tetrahedral signals shift to higher frequency: from 1.7 ppm to 4.5 and 12.3 ppm (octahedral sites) and from 64.6 ppm to 70.4 ppm (tetrahedral site). From these results, it is concluded that the Al line widths in coals are greatly influenced by the secondorder quadrupolar interaction, because of the improvement in spectral resolution and higher frequency shift observed in the higher-field experiment. 27Al MAS spectra of other coals at 16.4 T are shown in Figure 3. Octahedral Al signals at 3-6 ppm and tetrahedrally coordinated Al peaks at 70-72 ppm of other coals were also much better resolved, compared with the results from earlier work at lower external fields. In previous 27Al MAS studies of natural coals and coal ashes, the second-order quadrupolar broadening that was caused by using a lower magnetic field (4.7-9.4 T) and slower spinning rates (3-5 kHz) complicated detailed characterization of the minerals.12,13,15,34-40 By contrast, 27Al MAS spectra recorded at 16.4 T with a much faster spinning rate (18 kHz) are completely devoid of spectral overlap of the octahedral/tetrahedral peaks with spinning sidebands. These results are especially advantageous to the analysis of coal D. There are no spinning sidebands between the octahedral and tetrahedral resonances; hence, it is possible to obtain the quantita-

coal

tetrahedral

A B C D

21 16 6 7

pentacoordinated

octahedral

38

79 84 94 55

tive information of each Al species for coal D, including very broad signals in the range of 20-70 ppm. As a result, the distribution of each Al atom with a different coordination number can be obtained precisely. We also measured 27Al MAS spectra with proton decoupling (not shown here); however, the spectral features hardly changed in any of the samples. These results indicate that the Al-H dipole interaction is almost averaged at a spinning rate of 18 kHz. Thompson et al. reported that there were two types of octahedral Al species at ca. 0 ppm: one was characterized as being a broad signal and the other was characterized as being a narrow one.5 The broad species caused by amorphous compounds were found in lowrank coals (carbon content of ∼75 mass %) and were thought to be organic Al species bound to the oxygen functional groups. Our result on low-rank coal D (carbon content of 75.7 mass %) does not show broad signals near 0 ppm that would correspond to octahedral sites. Instead, signals in the region of 20-70 ppm are observed; the peaks found in this region are consistent with pentacoordinated Al sites. Moreover, because high O/C ratios are characteristic of coal D, these pentacoordinated Al atoms may be assigned to the organic Al species bound to the oxygen functional groups.5 The percentages of the tetrahedral, pentacoordinated, and octahedral Al ratios in the coals, as obtained by 27Al MAS NMR, are shown in Table 2. It was reported that the degree of diagenesis of the mineral matter correlates with illitization of the clay minerals, as reflected in an increase in the tetrahedral Al ratios;5 furthermore, this increase is correlated with the maturity of organic matter in the coals.49 Figure 4 shows the relationship between the percentage of tetrahedral Al species and the percentage of carbon in the organic matter of the coals. Although these two parameters do not plot in a linear relationship, as found previously,5 the trend in Figure 4 is almost the same as that observed in the former work: the ratios of the tetrahedral Al increase as the carbon content of the coals increases. Our 27Al MAS results that were conducted at 16.4 T show that information on the aluminum coordination (49) Jakobsen, H. J.; Jacobsen, H.; Lindgreen, H. Fuel 1988, 67, 727.

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Figure 5. 27Al 3QMAS NMR spectra of coal A for a magnetic field of (a) 7.0 T and (b) 16.4 T. Asterisk (*) denotes spinning sidebands.

could be obtained more accurately at high magnetic field, compared to lower-field experiments, because of reduction of the second-order quadrupolar effect. However, 27Al MAS NMR is not capable of completely averaging the second-order quadrupolar interaction, in principle. Therefore, it is difficult to infer the exact identity of inorganic matter in natural coals and determine the actual number of Al sites from 27Al MAS NMR, even at 16.4 T. To further identify inorganic matter in natural coals, 27Al 3QMAS NMR was performed. Figure 5 shows 27Al 3QMAS spectra of coal A at 7.0 and 16.4 T. In the presented MQMAS spectra, the horizontal axis is defined as the MAS dimension, whereas the vertical axis corresponds to the MQ isotropic dimension. The projection along the MAS dimension from 27Al MQMAS spectra generally resembles the line shape of 27Al MAS spectra obtained under the same field conditions. At 7.0 T (Figure 5a), only broad dispersions of octahedral cross sections, drawn as contour lines, are observed, suggesting the existence of multiple Al sites; however, the cross sections are not completely separated. We also attempted to perform 5QMAS experiments at 7.0 T; however, 5QMAS spectra with extremely poor signalto-noise ratios were obtained in runs with a duration of 1 week, because of the lower excitation efficiency of 5 quantum coherences. In contrast, 27Al 3QMAS that was performed at 16.4 T yielded a very high-resolution spectrum that distinguishes two main octahedral Al

Figure 6. 27Al 3QMAS NMR spectra of natural coals for (a) coal B, (b) coal C, and (c) coal D. Asterisk (*) denotes spinning sidebands.

sites in coal A. Moreover, signal enhancement was achieved by a factor of ∼4, compared to the 3QMAS results at 7.0 T. Because of the increase in sensitivity at 16.4 T, 27Al 5QMAS spectra with good signal-to-noise ratios could be obtained. However, the spectral resolution of the 5QMAS spectrum was comparable to that of 3QMAS, although it takes a considerably longer time to measure the 5QMAS spectrum. Considering the limitations in regard to measuring times, the combination of 3QMAS and a magnetic field of 16.4 T provides

27Al

MQMAS NMR of Inorganics in Natural Coal

Energy & Fuels, Vol. 18, No. 6, 2004 1737

Table 3. Tentative Assignment and Quantification of Aluminates in Natural Coals, as Determined by 27Al NMR Component Content (mol %) coal A B C D

kaolin

montmorillonite

trace trace 69 17

muscovite

23 65 31 trace

71 32 trace trace

alumina 6 3 trace trace

others

PQ )

[

17 10 δ + δ 27 1 27 2

]

1 17 I (2I - 1) 2π 10 2I + 3

1/2

(δ1 - δ2)

(3) ω0 450

δCS (ppm)

(4)

where ω0 is the Zeeman frequency. The δCS and PQ values of octahedral peaks in the coals are summarized

PQ (MHz) Coal A

trace trace trace 83

very suitable results for structural analysis of aluminosilicates in natural coals, with respect to both spectral resolution and sensitivity. 3QMAS spectra of the other coals are shown in Figure 6, for comparison. The spectral features among the coals are quite different. Two main signals associated with octahedral sites were observed in coal B (Figure 6a); these signals resemble those in coal A. In coal C (Figure 6b), the octahedral Al resonance can be characterized by a single peak, and its distribution is smaller than that in coals A and B. The 3QMAS spectrum of coal D (Figure 6c) is considerably different; the main octahedral Al region is characterized by several octahedral species, as well as a pentacoordinated Al site. Next, an attempt was made to make an assignment for each Al peak, by means of comparison of the octahedral peak position (the MQ isotropic shift positions (δ1) along the F1-dimension and the center-ofgravity positions (δ2) along the F2-dimension) in 3QMAS spectra between the coals and reference samples (kaolin, R-alumina, montmorillonite, and muscovite). The 2D peak positions (δ2, δ1) and the degrees of peak dispersion in kaolin, R-alumina, montmorillonite, and muscovite are different from each other (27Al 3QMAS spectra of reference samples are not shown here); therefore, it is possible to characterize inorganic matter in the coals. We tentatively identified and quantified the distribution of aluminates in coals A-D, as shown in Table 3. Each Al species was quantified by deconvolution of the 27Al MAS spectra. It is considered that other minerals also exist in natural coals; therefore, it can make more precise assignment through the measurement of many reference samples. There is also a possibility of the presence of mixed-layer minerals. Moreover, the value of the isotropic chemical shift (δCS) and the quadrupolar parameter (PQ) can be derived from δ1 and δ2 in 2D MQMAS spectra, according to the following equations if the isotropic shift along the F1-axis is scaled so that the slope of the CS-axis in the 2D spectra is equal to 1:50

δCS )

Table 4. Isotropic Chemical Shift (δCS) and Quadrupolar Parameter (PQ) of Each Cross Section in Natural Coals, Calculated Using 27Al 3QMAS NMR Spectra

6.1 14.3

2.4 2.4 Coal B

7.4 14.9

2.5 2.6 Coal C

7.5 -7.0 -3.2 8.2 14.5

2.3 Coal D 2.9 3.4 2.6 3.4

in Table 4. This table shows that the PQ values in the coals are almost equal to each other, except for some species in coal D. Because the PQ values generally show the degree of the structural symmetry (smaller PQ values indicate higher symmetry and larger PQ values indicate lower symmetry), some octahedral Al species with larger PQ values (2.9-3.4 MHz) in coal D are considered to have complex and low-symmetry structures. Some research groups have quantified the inorganic matter in coals from the tetrahedral Al ratio: 0% for kaolin or biedilite, 0%-10% for montmorillonite, and 24%-29% for illite or mica-montmorillonite.5,14,21 Its assignment is very easy to implement; however, it is sometimes difficult to quantify minerals in natural coals using the tetrahedral ratio, because of the existence of mixtures of several mineral species. For example, although there is 7% tetrahedral Al in coal D, which may have been assigned to montmorillonite in earlier works, it is found that the Al species in coal D do not correspond to montmorillonite but rather to kaolin and other types of minerals in the octahedral regions, judging from the 2D peak positions (δ2, δ1) in kaolin and montmorillonite. In contrast to the octahedral Al species, the cross section of tetrahedral Al was observed at almost the same position in the 2D spectra and was independent of a variety of coals. Therefore, the assignments of aluminosilicates by the octahedral Al positions may be more precise than those by the tetrahedral Al positions. From these results, 27Al MQMAS NMR recorded at high magnetic field is considered to be a very effective method for the characterization of inorganic matter in natural coals, although 29Si NMR techniques have been dominantly used for the identification of natural coals and coal ashes. Application of both 27Al MQMAS NMR and 29Si MAS NMR provides more-detailed discussions of the inorganic matter in coal. EF040029E (50) Fernandez, C.; Pruski, M. In 10th Annual Varian/Chemagnetics Solid-State NMR Workshop; Varian: Palo Alto, CA, 1999; p 31.