C NMR Studies on Coal and Coal Oxidation - American Chemical

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Solid-State C NMR Studies on Coal and Coal Oxidation 1

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J. Anthony MacPhee , Hiroyuki Kawashima , Yasumasa Yamashita , and Yoshio Yamada Downloaded by AUBURN UNIV on February 29, 2016 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch017

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CANMET Energy Research Laboratories, c/o 555 Booth Street, Ottawa, Ontario, Κ1Α 0G1, Canada National Institute for Resources and Environment, Onogawa 16—3, Tsukuba, Ibaraki 305, Japan 2

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Conventional and dipolar-dephasing C NMR spectra with cross-polarization and magic-angle spinning (CP—MAS) were recorded for the eight Argonne Premium coal samples at 300 MHz using a Bruker AC 300 spectrometer with 4 -kHz sample spinning Measured apparent aromaticities correlate well with the atomic H/C ratio (r = 0.96). Cer tain of these coals were subjected to oxidation in air at 100 °C for 168 h. For the fresh coals, the aromatic carbons exhibit a two-component (Gaussian—Lorentzian) decay in the dipolar dephasing experiment. The oxidized coals, in the range of dipolar dephasing delays investigated, exhibit only a Gaussian decay. Aliphatic carbons of bothfreshand oxi dized coals exhibit only a single Gaussian component. This behavior is explained in terms of the increased radical con centration in the oxidized coals. Studies of the dipolar dephasing behavior of model aromatic compounds (e.g., anth racene and naphthalene) were carried out, and the results of these help to interpret the results obtained for oxidized coals. JLHE SYSTEMATIC STUDY OF COAL STRUCTURE and chemistry has, in the past, been thwarted by the difficulty of obtaining representative and This chapter not subject to U.S. copyright Published 1993 American Chemical Society

In Magnetic Resonance of Carbonaceous Solids; Botto, Robert E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

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M A G N E T O R E S O N A N C E O F C A R B O N A C E O U S SOLIDS

pristine samples upon which to perform experiments. Results obtained in one laboratory often prove difficult to verify in another because of the unavailability of identical samples. The laudable effort of the Argonne Premium Sample Bank has changed this situation and will undoubtedly allow a qualitative step forward in coal science in the coming decade. We studied the eight Argonne Premium coal samples by means of C cross-polarization—magic-angle spinning (CP-MAS) solid-state N M R spectroscopy with several objectives in mind. The first was to assess the aromaticity data obtained at high field (300 MHz) and moderate sample spinning rate (4 kHz) to allow comparisons with data from other laboratories both under rigorously identical conditions and at higher spinning rates. Our second objective was to examine, within the limits imposed by these conditions, the dipolar-dephasing behavior of these coals; and the third objective was to examine oxidized samples of some of these coals in order to gain some information on the mechanism of coal oxidation. There remain many caveats concerning the interpretation of C CP—MAS solid-state N M R spectra of coals (2). A consensus will undoubtedly be long in coming.

Downloaded by AUBURN UNIV on February 29, 2016 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch017

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Experimental Details Coal Samples. The coal samples used were the eight samples provided by the Argonne Premium Coal Sample Program. These samples were provided as 100-mesh powders and were examined without further crushing. Care was taken with the fresh samples to avoid undue exposure to air by running the spectra as quickly as possible after opening the sample vials. Four samples were subjected to oxidation at 100 °C for 168 h. The analytical data for the oxidized samples as well as data relevant to thefreshcoals are presented in Table I. Complete analytical data are available for the Argonne Premium coal samples elsewhere (2) and will not be included here. Model Compounds. In addition to the coal samples, the dipolar-dephasing behavior of two model compounds, anthracene and naphthalene, was investigated in order to aid in the interpretation of the results for the coals. Because impurities seem to play a role in the relaxation behavior of such compounds, ultrapure samples of zone-refined compounds, obtained from the Tokyo Kasei Kogyo Co. Ltd., were used. Oxidized samples of anthracene and naphthalene were prepared by heating the samples in a sealed tube with air at 100 °C for 168 h. Spectra were obtained with a contact time of 2 ms, a repeat time of 120 s, and an accumulation of 100 scans.


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MACPHEE ET A L

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Solid-State C NMR Studies

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Table I. Analytical Data for Coal Samples


Coal No.

Coal Name

a

C

b

H

O

a

Spins/g (χ 10 ) 19

101 fr 101 ox

Upper Freeport

85.50 82.33

4.70 4.43

5.70 8.57

2.6 4.1

202 fr 202 ox

Wyodak-Anderson

75.01 66.22

5.35 3.67

19.28 28.09

3.8 1.0

301 fr 301 ox

Illinois No. 6

77.67 69.26

5.00 4.03

11.27 18.92

2.9 1.3

501 fr 501 ox

Pocahontas No. 3

91.05 87.18

4.44 3.98

3.44 7.04

3.9 8.5

N O T E : All values are given as moisture- and ash-free (MAF) weight percents. A B B R E V I A T I O N S : fr,fresh;and ox, oxidized. C and H analyses for the fresh eoals were obtained from the Argonne Premium Coal Sample Program User's Handbook. A l l Ο analyses were determined directly. a

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Electron Spin Resonance Experiments. Electron spin resonance (ESR) experiments were carried out at room temperature with a JEOL JESFE1X spectrometer. Spin concentrations were measured in vacuo, and 1,1-diphenyl-2-picrylhydrazyl (DPPH) was used as a calibrant. NMR Experiments.

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The C CP-MAS solid-state NMR spectra were measured on a Bruker AC 300 spectrometer at 75.46 MHz. A Bruker double airbearing-magic-angle-spinning solid-state probe was used. Ceramic spinners with an internal volume of ~250 mL were used at a spinning rate of ~4 kHz. For the CP experiments, the following operating parameters were used: a spectral width of 30 kHz, a 90° proton pulse of 5 #s, an acquisition time of 30 ms, a pulse repetition time of 4 s, and an accumulation of 1000 scans. The interferogram was multiplied by a sinebell window function, supplied by Bruker software, before Fourier transformation of the data. (The sinebell window function is an exponen tial function to remove the noise of NMR signals.) Apparent aromaticities, / ' , were determined by using integrated signal intensities for aromatic and aliphatic carbons. For purposes of integration, the base line was corrected by using stan dard software provided by Bruker. The treatment of spinning side bands (SSB) is a bit more involved and is discussed in detail in the following section. Variablecontact-time experiments were carried out with contact times between 0.5 and 5 ms. Hartmann-Hahn conditions and the magic angle were adjusted with a gly cine sample. a


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Dipolar-dephasing experiments were carried out by using the pulse sequence of Alia and Lipmaa, with a 180° pulse on each nucleus in the middle of the dipolar-dephasing period in order to remove linear-phase distortion ( J ) . This dipolar-dephasing period is of particular importance at the low spinning rates used in this study. Dipolar-dephasing times up to 80 μ$ were used with a contact time of 2 ms and a repeat time of 3 s. We considered it unreliable, under the present circumstances, to use dipolar-dephasing times longer than ~80 μ$ because of the difficulty of establishing a base line for integration.


Results and Discussion Aromaticity Determination. Calculating the aromaticity of a coal from a C C P - M A S spectrum in the presence of significant side bands is not a trivial matter, as is shown in Figure la, where the spectrum of coal 501 is given. Slow spinning (4 kHz) and high field (300 MHz) pro duce large SSBs, and the result is a loss of intensity of the peak corresponding to sp carbons. The integrated intensity of these side bands must be determined and added to the intensity of the sp carbon band. To obtain the intensity of the sp band, any intensity arising from the overlap ping sp SSB must be subtracted. These values are then used to obtain the apparent aromaticity, / \ The manner in which this is done depends on the how the side bands are distributed with respect to the main peak from which they are derived, but a priori, this is not known; therefore model compounds must be observed. The C C P - M A S NMR spectrum of anthracene is shown in Figure lb. The anthracene bands are labeled a, b, c, d, and e. In the coal spectrum (Figure la) all of these peaks can be integrated except that corresponding to sideband e. Because the intensi ties of peaks a and e are approximately equal in the anthracene spectrum, we have chosen to calculate the apparent aromaticity, / ' , assuming this to be the case for coal. In another study (4), the authors opted to correct for SSB in a slightly different manner based on the side-band distribution of coronene. Their method gives slightly higher aromaticities because they assume that SSBs d and e are equivalent in the coal spectrum. To check the validity of our approach we have obtained a single spectrum at 100 MHz with a 4-kHz spinning rate in which the SSBs were not a problem. For coal 301 at a cross-polarization time of 5 ms, we obtained a n / ' value of 0.69 (this value is consistent with the data shown in Figure 3a). T h e / ' values obtained in this way are reproducible to

C NMR Studies on Coal and Coal Oxidation - American Chemical

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