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Energy & Fuels 2009, 23, 1778–1780
First High-Resolution 11B Nuclear Magnetic Resonance (NMR) Spectra of Coal Fly Ash by Satellite-Transition Magic Angle Spinning (STMAS) NMR Takafumi Takahashi,*,† Shunsuke Kashiwakura,‡ Koji Kanehashi,† and Tetsuya Nagasaka‡ AdVanced Technology Research Laboratories, Nippon Steel Corporation, 20-1 Shintomi, Futtsu 293-8511 Japan, and Graduate School of EnVironmental Studies, Tohoku UniVersity, Sendai 980-8579, Japan ReceiVed October 14, 2008. ReVised Manuscript ReceiVed January 30, 2009 Coal fly ash (CFA) is generated as a byproduct during coal combustion in thermal power plants. Most of this is used to make cements, mortars, and soil amendments1 under encouragement for suppressing the cost of disposal, although a part is still landfilled. CFA contains hazardous trace elements, such as boron (B), arsenic (As), and selenium (Se), in amounts that are condensed from raw coals.2,3 In particular, leaching of boron from CFA has become an important issue in modern energy industry and environmental science. It has been reported that the absolute amounts of boron leached in pure water increase with an increase in the concentration of boron in CFA and its leaching ratio changes depending upon the pH of the contacted solution.4 The latter suggests that the leaching behavior of boron is varied depending upon the structure into which boron is incorporated. Hence, in application and management of CFA, the local environment of boron as well as its absolute concentration should be an important factor. Although the concentrations of boron in CFA have been investigated by several researchers,4-6 there is no robust study on structural characterization of the species where boron is incorporated. It is difficult to specify the boron-containing phase by conventional analytical technique, such as X-ray diffraction (XRD), because its diffraction peaks are hidden in those of major phases, such as aluminosilicates. X-ray absorption fine structure spectroscopy (XAFS), although having an ability to focus on an individual element, is not also suitable to analyze local environments of boron because of its light mass. The coordination number of boron in raw coals and low-temperature ashes has been investigated by 11B magic angle spinning (MAS) solidstate nuclear magnetic resonance (NMR).7,8 However, for CFA, neither an 11B MAS spectrum nor a higher resolution one without the effect of second-order quadrupolar interaction has been reported. In this study, the first observation of high* To whom correspondence should be addressed. Fax: +81-439-80-2746. E-mail:
[email protected]. † Nippon Steel Corporation. ‡ Tohoku University. (1) Haynes, R. J. J. EnViron. Manage. 2009, 90, 43–53. (2) Asokan, P.; Saxena, M.; Asolekar, S. R. Resour. ConserV. Recycl. 2005, 43, 239–262. (3) Panel on the Trace Element Geochemistry of Coal Resource Development Related to Health (PECH). Trace-element geochemistry of coal resource development related to environmental quality of health. National Academy Press, Washington, D.C., 1980; pp 1-8. (4) Iwashita, A.; Sakaguchi, Y.; Nakajima, T.; Takanashi, H.; Ohki, A.; Kambara, S. Fuel 2005, 84, 479–485. (5) Cox, J. A.; Lundquist, G. L.; Przyazny, A.; Schmulbach, C. D. EnViron. Sci. Technol. 1978, 12, 722–723. (6) Jankowski, J.; Ward, C. R.; French, D.; Groves, S. Fuel 2006, 85, 243–256. (7) Kuwabara, T.; Kambara, S.; Moritomi, H. J. Jpn. Inst. Energy 2007, 86, 455–461. (8) Burchill, P.; Howarth, O. W.; Richards, D. G.; Sword, B. J. Fuel 1990, 69, 421–428.
resolution 11B NMR spectra for CFA was accomplished by satellite-transition magic angle spinning (STMAS),9,10 which is similar to multiple-quantum MAS (MQMAS) NMR in many ways but possesses higher sensitivity.11,12 Two kinds of CFA samples, N-CFA and W-CFA, provided by a coal-fired power plant in Japan were used for the solidstate NMR experiments. Their major chemical compositions (silicon, aluminum, iron, etc.), except for the amount of calcium, are very similar. The concentration of calcium in W-CFA (11.9 wt %) is much larger than that in N-CFA (3.99 wt %). The boron concentration by weight is found to be approximately 540 ppm for N-CFA and 580 ppm for W-CFA. The average particle size is found to be 5.04 µm for N-CFA and 8.31 µm for W-CFA by the laser diffraction technique.13 11B MAS and STMAS spectra were acquired using a 16.4 T narrow bore magnet (JEOL-ECA700) equipped with a homebuilt STMAS probe that can generate rf field strengths up to 200 kHz for 11B. A bottom part of 3.2 mm zirconia (ZrO2) rotor was filled with Na2SO4 for precise setting of the magic angle (54.736 ( 0.002°), and its center part was filled with CFA powders for the measurements. The STMAS probe equipped with a MAS controller achieved stable rotor spinning (20 ( 0.003 kHz) and long-term keeping of an accurate magic angle (54.736 ( 0.002°). A double quantum filter (DQF) was employed to eliminate the undesired signals arising from central transition (CT)-CT transfer and any outer satellite-transition-CT transfer. Furthermore, a soft-pulse added mixing (SPAM) technique14,15 (Figure 1) was implemented into the DQF-STMAS to improve its sensitivity. The 11B MAS spectra were collected using an inherent 18° pulse (0.68 µs), calibrated using saturated H3BO4 aqueous solution and with a recycle interval of 0.5 s for N-CFA and 2 s for W-CFA, which are 5 times the spin-lattice relaxation time (T1) to archive full longitudinal relaxation. The 11B MAS spectra of both N- and W-CFA (Figure 2) show a peak around 17 ppm that corresponds to a three-coordinate B oxide (trigonal BO3)16,17 and one around 0 ppm that corresponds to a four-coordinate (9) Gan, Z. J. Am. Chem. Soc. 2000, 122, 3242–3243. (10) Ashbrook, S. E.; Wimperis, S. Prog. Nucl. Magn. Reson. Spectrosc. 2004, 45, 53–108. (11) Ashbrook, S. E.; Wimperis, S. J. Magn. Reson. 2002, 156, 269– 281. (12) Dowell, N. G.; Ashbrook, S. E.; Wimperis, S. J. Phys. Chem. B 2004, 108, 13292–13299. (13) Kubo, Y. Elution behavior of boron in coal fly ash. Master Thesis, Tohoku University, Sendai, Japan, 2005; pp 15-16. (14) Amoureux, J.; Delevoye, L.; Fink, G.; Taulelle, F.; Flambard, A.; Montagne, L. J. Magn. Reson. 2005, 175, 285–299. (15) Takahashi, T.; Kanehashi, K.; Shimoikeda, Y.; Nemoto, T.; Saito, K. J. Magn. Reson. 2009, manuscript to be submitted.
10.1021/ef800886n CCC: $40.75 2009 American Chemical Society Published on Web 02/19/2009
Communications
Figure 1. DQF-SPAM pulse sequence used in the 11B STMAS experiments. HP1 and HP2 are hard pulses, while SP1 (π) and SP2 (π/2) are soft pulses. The duration between HP2 and SP2 was set as short as possible to obtain the signals of coherence pathways, which are discarded in the z-filter experiments.
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of spinning side bands to the central peak of trigonal BO3 is quite weak, which simply reflects an amorphous characteristic of the trigonal BO3. This result suggests that boron and iron in W-CFA are not found in the same structure. In the quantitative analysis of 11B MAS NMR spectrum for N-CFA, the area ratios of spinning side bands must be included as signals for the trigonal BO3. Nevertheless, the concentration of Fe2O3 (3-3.6%) in both samples is not a level to yield a critical effect on peak assignments of NMR spectra. To average out the second-order quadrupolar interaction, highresolution 11B STMAS spectra were acquired (Figure 3). It should be noted that the 11B 3QMAS spectra with sufficient signal intensity were not obtained in the same scan time (6 days) because of the low sensitivity caused by inefficient MQ coherence excitation.19 In the STMAS spectra, the projection on the F1 axis provides a spectrum that is free from the secondorder quadrupolar interaction, whereas the projection on the F2 axis provides one corresponding to the MAS spectrum.20 It is found that the broad peaks on the F2 projection for N-CFA are actually ascribed to one trigonal BO3 site. The 11B STMAS spectrum of N-CFA demonstrates that the structure of trigonal BO3 has a large degree of distribution in the quadrupolar interaction rather than in the chemical shift, which reflects an amorphous characteristic of the trigonal BO3. The quadrupolar coupling constant (PQ) and the isotopic chemical shift (δiso) were estimated using the definition below ν03 170 (4S(2S - 1))2 (δF1 - δF2) 80 (4S(4S + 1) - 3) 10 17 10 δiso) δF1 + δF2 27 27
PQ )
Figure 2. 11B MAS spectra for (a) N-CFA and (b) W-CFA acquired at 16.4 T. Total scans were 20 000 for N-CFA and 35 000 for W-CFA. An asterisk (*) indicates the spinning side bands.
oxide (tetrahedral BO4).17,18 The full widths at half-maximum (FWHM) for the tetrahedral BO4 peak (1.14 kHz) are consistent between N- and W-CFA, while the FWHM for the trigonal BO3 peak in W-CFA (2.41 kHz) is wider than that in N-CFA (1.57 kHz). The spinning side bands with strong intensity in the 11B MAS spectrum of N-CFA (Figure 2a) suggest that the trigonal boron in N-CFA is spatially distributed close to iron; iron and most of the boron in N-CFA are probably incorporated into the same structure. In the 11B MAS spectrum of W-CFA, although comparable in concentration of Fe2O3 to N-CFA, the intensity (16) Sen, S.; Xu, Z.; Stebbins, J. F. J. Non-Cryst. Solids 1998, 226, 29–40. (17) Youngman, R. E.; Zwanzigner, J. W. J. Phys. Chem. 1996, 100, 16720–16728. (18) Turner, G. L.; Smith, K. A.; Kirkpatrick, R. J.; Oldfield, E. J. Magn. Reson. 1986, 67, 544–550.
where δF1 and δF2 are the shifts for the center of gravity of the F1 and F2 projections, respectively. We found that δiso ) 20.2 ppm and PQ ) 2.7 MHz for the trigonal BO3 site of N-CFA. In contrast, the 11B STMAS spectrum for W-CFA shows a substantially wide line width on the F1 projection, indicating the large distribution in quadrapolar interactions and chemical shifts. This result provides evidence that the trigonal BO3 in W-CFA has more of an amorphous characteristic than that in N-CFA, as implied in Figure 2. Such a difference between N- and W-CFA may be due to the kind of raw coals, thermal history during the combustion, and so on. A complete assignment of the trigonal BO3 sites in these CFA samples is in progress, comparing the STMAS spectra of the CFA (Figure 3) and of several kinds of boron oxides and also considering the element mapping of boron and other elements obtained by FIB-TOF-SIMS. Thus far, compounds of nCaO-B2O3 (n ) 1, 2, and 3) are predominant candidates to explain the spectra of the trigonal BO3. The results will be reported in another paper. The tetrahedral BO4 site observed in the 11B MAS spectra (Figure 2) was not identified in the STMAS spectra because of its low proportion. Nevertheless, the 11B MAS spectra is enough to distinguish the tetrahedral BO4 adequately, because quadrupolar interaction for tetrahedral BO4, which has a highly symmetric structure, is generally too small (PQ ≈ 0)21 to generate broadening or splitting in a MAS spectrum. (19) Amoureux, J. P.; Fernandez, C.; Frydman, L. Chem. Phys. Lett. 1996, 259, 347–355. (20) Amoureux, J. P.; Huguenard, C.; Engelke, F.; Taulelle, F. Chem. Phys. Lett. 2002, 356, 497–504. (21) Mackenzie, K. J. D.; Smith, M. E. Multinuclear Solid-State NMR of Inorganic Materials, Materials Series; Pergamon: Oxford, U.K., 2002; Vol. 6, p 421.
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Figure 3. 11B STMAS spectra for (a) N-CFA and (b) W-CFA acquired with DQF-SPAM pulse sequence. The spectrum a was acquired using 16 t1 increments of 50 µs, 4800 transients, and a recycle delay of 0.5 s, while the spectrum b was acquired using 16 t1 increments of 50 µs, 18000 transients, and a recycle delay of 1 s. Quadrupolar interaction transfers the contour plots along the QIS line. When no quadrupolar interaction occurs (PQ ) 0 MHz), the center of gravity of the contour plots is located on the CS line.
In conclusion, we have accomplished a first observation of the satisfactory 11B MAS spectra and the high-resolution 11B STMAS spectra of N-CFA and W-CFA. From the 11B MAS spectra, the dominant local environment of boron in both N- and W-CFA is found to be trigonal BO3, which probably differs in the disposition of boron and iron atoms. Furthermore, from the 11B STMAS spectra, it is found that the trigonal BO3 sites differ in the degree of distribution of
chemical shifts and quadrupolar interactions. The differences in local environment might affect the solubility of trigonal BO3 and consequently produce the unique leaching ratios of boron in the CFA samples. Hence, this study opens up a new possibility that the leaching behavior of boron is understood at the molecular level. EF800886N
