Investigation of Coal and a Naphthalene Pitch by ... - ACS Publications

Energy & Fuels 1996,9, 717-726

717

Investigation of Coal and a Naphthalene Pitch by Means of the Triple-EchoMagic Angle Turning Experiment Jian Zhi Hu,?$§M. S. Solum,?R. J. Pugmire,*$$Chaohui Ye,§ and D. M. Grant? Departments of Chemistry and Chemical and Fuels Engineering, University of Utah, Salt Lake City, Utah 84112, and Wuhan Institute of Physics, Chinese Academy of Sciences, Wuhan 430071, P.O. Box 71010, P. R. China Received March 6, 1995. Revised Manuscript Received April 6, 1995@

The triple-echo magic angle turning 2D experiment together with short contact time and dipolar dephasing variations is used to investigate the structure of a series of coals and a naphthalenederived pitch. The aromaticities obtained with the standard triple-echo MAT experiment a t 4.7 T with a single contact time are compared with those obtained from the standard 13C CP/MAS variable contact time experiment a t a magnetic field of 2.35 T and at a sample spinning rate of about 4.1 kHz. A high correlation (R2= 0.987) is found between the aromaticities for these two totally different experimental approaches. By introducing the chemical shift anisotropy into the experiment, structural information which is hidden in the standard 13C CP/MAS spectrum due to the severe overlapping of the resonance lines can now be extracted by the triple-echo MAT 2D experiments. It is found that the overlapping powder patterns for the protonated and nonprotonated aromatic carbons can be successfully separated by the short contact time and dipolar dephasing variations of the triple-echo MAT, thereby simplifying the interpretation of the isotropic shift dimension as well as simplifying the measurement of the 13C CSA principal values in such complicated systems. The measurements of 13CCSA principal values of protonated, bridgehead, and alkyl-substituted carbons in a naphthalene-derived pitch, a semianthracite, and an anthracite coal are presented as typical examples. Because of the sideband-free nature of the triple-echoMAT data in the isotropic chemical shift dimension, the quality of the 2D baseplane, and the technical simplicity in implementation of the experiment, the standard triple-echo MAT experiments appear t o be useful for qualitative and perhaps quantitative measurement of coals at any magnetic field strength.

Introduction One of the important contributions of high-resolution 13C CP/MAS to coal science is the characterization of coal structure and measurement of the structural parameters of It has been demonstrated8-'" that the structural parameters directly derived from 13C CP/MAS experiments can be utilized to predict the

* Author to whom correspondence should be addressed. Department of Chemistry, University of Utah.

* Department of Chemical and Fuels Engineering, University of

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Utah. 0 Wuhan Institute of Physics. @Abstractpublished in Advance ACS Abstracts, May 15, 1995. (1)Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187-193. (2) Snape, C. E.;Axelson, D. E.; Botto, R. E.; Delpuech, J. J.; Tekely, P.; Gerstein, B. C.; Pruski, M.; Maciel, G. E.; Wilson, M. A. Fuel 1989, 68,547-560. (3) Fletcher,T. H.; Solum, M. S.; Grant, D. M.; Pugmire, R. J. Energy Fuels. 1992. 6. 643-650. (4)'Fletcheg T. H.; Bai, S.; Pugmire, R. J.; Solum, M. S.;Wood, S.; Grant, D. M. Energy &Fuels 1993, 7 , 734-742. (5) Orendt,A. M.; Solum, M. S.; Sethi, N. K.; Pugmire, R. J.; Grant, D. M. Advances in Coal Spectroscopy; Meuzelaar, Henk L. C., Ed.; Plenum Press: New York, 1992; pp 215-223. (6) Pugmire, R. J.; Solum, M. S.; Grant, D. M.; Critchfield, S.; Fletcher, T. H. Fuel 1991, 70,414-423. (7)Magnetic Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Yuzo, Eds.; Advances in Chemistry Series 229; American Chemical Society: Washington, DC, 1993. (8) Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Solum, M. S.; Grant, D. M. Energy Fuels 1992,6, 414-431. (9) Fletcher, T. H.;Solum, M. S.; Grant, D. M.; Pugmire, R. J. Energy Fuels 1992, 6, 643-650. (10)Grant, D. M.; Pugmire, R. J.; Kerstein, A. R. Energy Fuels 1989, 3,175-186.

details of coal devolatilization and char formation processes. The validity of such simulations is dependent on the ability of the investigator to obtain a reliable spectral response from which to derive the required structural parameters. Due to the low natural abundance of the 13Cisotope (1.1%)and the range of 13C TI values generally encountered in coal, sensitivity enhancement via the standard cross-polarization (CP) method is widely used. Since coal is composed primarily of polyaromatic species with various types of substituents and cross-linking units, the 13CCP/MAS spectrum of the coal generally consists of two banded regions, one of which is dominated by the aromatic carbons with chemical shift values greater than 90 ppm while the other consists of aliphatic carbons with chemical shift values less than 90 ppm. One of the advantages of spinning a solid sample at the magic angle is the reduction of the line broadening contributions due to the chemical shift anisotropy (CSA). The chemical shifi anisotropy is proportional t o the strength of the external magnetic field and the CSA of aromatic carbons (200-240 ppm) is greater than that of the aliphatic carbons (20-100 ppm). In order to obtain a 13C C P W spectrum in which the aromatic carbon sidebands do not overlap the aliphatic carbon signals, it is necessary t o spin the sample at approximately 4 kHz at a magnetic field of 2.35 T. At a higher magnetic field strength a greater spinning rate is required. However, with the increase of sample spinning rate, one encounters inhomogeneous cross

0887-062495/2509-0717$09.00/00 1995 American Chemical Society

H u et al.

718 Energy & Fuels, Vol. 9, No. 4, 1995

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aY

I3c d : 0

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Y d :

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X e : 27'13

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Figure 1. Pulse sequences for the 2D MAT experiments. 90" pulses are represented by single rectangles; two adjacent rectangles denote a 180" pulse. The cross-polarization pulses are shaded. The time T is an integral number of rotor periods (excluding a multiple of three rotor periods). The magnetization precesses in the transverse plane during the periods labeled Q>1 and Q>2, and Q>1is along the longitudinal axis during the periods labeled L = L1 + L2, where L1 = T/3 - L2 - t1/3 and L2 is the fixed interrupted decoupling period. Normally, L1 t1/3 is less than 1.5 ms for coals. (a) Normal triple-echo MAT pulse sequence. A is an echo delay time determined by the probe ring-down and receiver recovery time. The character above each pulse indicates the phase of the pulse which is given in ref 18. (b) Dipolar dephasing triple-echo MAT pulse sequence. The spin-lock pulse is darkened. The character above each pulse indicates the phase of the pulse which is also given in ref 18.

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polarization among carbons with different carbonhydrogen dipolar interactions. This inhomogeneity creates difficulties in the quantitative measurement of the signal. On the other hand, the CSA exhibits three frequency components for each carbon in a powdered sample, the principal values of the chemical shift tensor that provide valuable information about the local electronic environment of the nucleus. Spinning the sample at the magic angle averages these three values to the isotropic chemical shift of each nuclues. Hence, elimination of the CSA increases spectral resolution but also throws away two-thirds of the information available for each carbon. One of the approachesll used to circumvent the sideband problem associated with high magnetic fields and the concomitant high spinning rates is the incorporation of the total sideband suppression (TOSS)12J3 technique into the standard CP/MAS experiment. This technique provides a sideband-free 13C CP/MAS spectrum at a sample spinning frequency less than the chemical shift anisotropy. However, in order to satisfy the minimal requirements for quantitative spectral restoration in the center band, the lower limit of the required sample spinning rate must be at least as great as V3 of the chemical shift anisotropy of the carbons under investigation.ll Spinning rates greater than 4 kHz have been shown to interfere with the efficiency through modulation of the carbon-hydrogen dipolar interaction. Recently, better sideband suppression techniques have been developed.14-16 Hence, spectral distortion can be reduced but not totally eliminated. (11) Hu, J. Z.; Li, L.; Ye, C. High-Field NMR studies of Argonne Premium Coals: In Magnetic Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Yuzo, Eds.; Advances in Chemistry Series 229; American Chemical Society: Washington, DC, 1993; pp 311-322. (12) Dixon, W. T. J . Chem. Phys. 1982, 77, 1800. (13),0lejniczak, E. T.; Vega, S.; Griffin, R. G. J . Chem. Phys. 1984, 81, 4804.

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F2 ("1 Figure 2. Standard triple-echo MAT 2D contour plot of a methylnaphthalene mesophase pitch, which was obtained using the following experimental conditions: sample spinning rate 44 f 0.1 Hz, an echo delay time A of 60 ps, a contact time of 0.8 ms, a recycle delay of 2 s, an acquisition dwell time of 31.25 ,us, and a tl increment of 93.75 ps. A 2D spectrum is obtained with the isotropic chemical shift axis inclined a t an angle 45" to the F2 axis. The data set contains 27 tl values. Data was acquired for 320 scans in both the real and imaginary data sets for a total experimental time of about 10 h.

Hence, quantitative experiments for coal are best achieved a t a magnetic field of 2.35 T in order to avoid the requirement for a high spinning rate and its associated inhomogeneous cross polarization problems. It has been reported17 that quantitative data can be obtained from anthracite coals by fitting the chemical shielding tensors in the static powder patterns. Experiments that provide the principal values of the chemical shift tensor are very useful in characterizing molecular structure since the data provide 3 times the information available from the isotropic chemical shifts obtained from MAS experiments. However, accurate CSA data can be obtained only in relatively simple systems that contain no more than 3-4 different chemical shift tensors because of severe overlapping of the powder patterns. Consequently, a method which is field independent and has the advantages of both static sample measurement and the high-resolution feature of the standard MAS experiment is highly desirable. Fortunately, the recently developed two-dimensional (2D) magic angle turning (MAT)experiments (the tripleecho MAT18 and PHORMATl9), when applied a t an extremely slow rotation rate, meet these criteria. This method is based on the experiment pioneered by Gan20 which is an analog of the magic angle hopping (MAH) (14) Song, Z.; Antzutkin, 0. N.; Feng, X.; Levitt, M. H. Solid State NMR 1993,2,143. (15) Antzutkin, 0. N.; Song, Z.; Feng, X.; Levitt, M. H. J . Chem. Phys. 1994,100,130. (16) Hong, J.; Harbison, G. J . Magn. Reson. Ser. A 1993,105, 128. (17) Sethi, N. K.; Pugmire, R. J.; Facelli, J. C.; Grant, D. M. Anal. Chem. 1988,60, 1574-1579. (18) Hu, J. Z.; Orendt, A. M.; Alderman, D. W.; Pugmire, R. J.; Ye, C.; Grant, D. M. Solid State NMR 1994,3, 298. (19)Hu, J. Z.; Wang, W.; Liu, F.; Solum, M. S.; Alderman, D. W.; Pugmire, R. J.; Grant, D. M. J . Magn. Reson. in press. (20) Gan, Z. J . Am. Chem. SOC.1992,114, 8307.

Investigation of Coal and a Naphthalene Pitch

Energy & Fuels, Vol. 9, No. 4, 1995 719

0

0

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. 0

ro 0

m

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a

rl n

a r.

a c

a c

Figure 3. A 45" sheared spectrum of of the data presented in Figure 2. The projections on the Fa and Fb axis are illustrated outside the contour box.

experiment reported by Bax et al.21and Hu et a1.22 In the triple-echo MAT experiment, a sideband-free isotropic chemical shift spectrum which has resolution in the isotropic shift dimension comparable to the standard CP/MAS spectrum is obtained by projecting the 2D data onto the evolution frequency axis. Concurrently, an essentially static powder pattern is projected onto the acquisition frequency axis and the powder pattern for each individual isotropic chemical shift is obtained by simply taking a spectral slice at the specified isotropic chemical shift value in the acquisition dimension. The details of the MAT experiments are described by Hu et al.19 One advantage of employing an extremely slow rotation rate is that a spinning sideband powder pat-

tern, which resembles a static powder pattern, is obtained since individual sidebands are no longer distinguishable from each other. Because of the very slow sample rotation involved in the experiment, apparent quantitative results have been obtained in coals from a properly selected single contact time. Furthermore, the sideband-free features of the experiment in the isotropic chemical shift dimension, the high quality of the 2D baseplane, and the technical simplicity of implementation render this experiment feasible for the investigation of coils even at high magnetic fields. In this paper, the power of the triple-echo MAT experiment is further explored in a study of coals and a naphthalenederived pitch at a magnetic field strength of 4.7 T.

(21)Bax,A,; Szeverenyi,N. M.; Maciel, G. E. J.Mugn. Reson. 1983, 52, 147. (22) Hu, J. Z.; Orendt, A. M.; Alderman, D. W.; Ye, C.; Pugmire, R. J.; Grant, D. M. Solid State NMR 1993,2 , 235.

The samples investigated in this paper are of three kinds: (1)a pitch sample derived from naphthalene and supplied by

Experimental Section

Hu et al.


Table 1. 19C CSA Principal Values Obtained from Selected Spectral Slices in the Powdered Naphthalene Pitch Obtained by the Triple-Echo Magic Angle Turning Experiments carbon type

611 622 833 8averaaea Standard Triple-Echo MAT

substituted

228 233

159 165

27 30

137.7 142.7

Short Contact Time Triple-Echo MAT protonatedc 206 130 5 114 206 207 208 209 209 209 210 211 214 215 220

222

132 133 134 134 134 134 134 135 135 138 136 134

8 10 10 9 10 14 13 15 17 15 17 16

115.4 116.4 117.5 117.2 117.5 118.4 119.3 120.3 122.0 122.7 124.3 124.0

Pitch: Dipolar Dephasing Triple-Echo MAT bridgehead 196 183 -17 120

180

140

100

80

60

40

20

ppm

196 196 198 198 199 202 202 208 202 210 203 212 205 213

184 184 185 185 186 185 186 165 186 167

188 168 188 166

-19 -15 -14 -12 -10 -10 -11 1 -12 1 -11 2 -10 2

120.4 121.9 123.0 124 125 125.7 125.7 124.5 125.3 126.1 129.1 127.7 130.5 127.3

&sob

137.8 141.7 114.6 115.4 116.2 117.0 117.8 118.6 119.4 120.2 121.0 121.8 122.6 123.4 124.2 121.0 121.8 122.6 123.4 124.2 125.0 125.8 126.6 126.6 127.5 127.5 128.3 128.3 129.1 129.1

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a Baverage = '/3(811 622 633). 6iso = the chemical shift value of the spectral slice. Tensor values for the protonated carbons with the isotropic chemical shift values larger than 124.2 ppm are not available by measuring the frequencies of the peak and break points at half-height positions because of severe overlapping with the powder patterns from bridgehead carbons.

180

140

100 80

60

40

20

pgn

Figure 4. (A) Isotropic chemical shift projections for the pitch sample obtained from the MAT and CP/MAS experiments. Top trace: dipolar dephasing triple-echo MAT with a dephasing time of 60 ps; middle trace: short contact time of 50 ps tripleecho MAT; bottom trace: standard triple-echo MAT. (B) The standard 13CCP/MAS spectra of the pitch sample, which were obtained at a sample spinning rate of about 4 k H z and at a magnetic field of 2.35 T. Top trace: dipolar dephasing CP/ MAS with a dephasing time of 60 p s ; middle trace: short contact time CP/MAS with a contact time of 50 p s ; bottom trace: standard CP/MAS with a contact time of 2.5 ms.

I. M o ~ h i d a(2) ; ~two ~ high-rank coals consisting of an anthracite (PSOC-867) and a semianthracite (PSOC-628); and (3) eight Argonne Premium Coals (APC).Approximately 6 g of each sample was loaded in the large rotor. The anthracite detuned the decoupler channel of the probe when a full rotor of the sample was used, apparently due to the conductivity of the sample arising from the delocalized electrons in this highly (23) Mochida, I.; Shimizu, K.; Korai, Y.; Otsuka, H.; Fujiyama, S. Carbon 1988,26(61, 843-852.

condensed aromatic ring system. In order t o overcome this problem, only approximately 1.5 g of this sample was used. The anthracite sample was placed in a 10 mm glass sample tube which was centered inside the sample rotor (18 mm i d . ) which then permitted tuning of the 'H channel. Sealed vials of the APC samples were opened before each measurement, and 5-6 g samples of each coal was placed in the rotor. The pristine Blind Canyon, Wyodak, and Zap samples taken directly from the sample vials were also found to detune the decoupler channel of the MAT probe. These three coals were then dried in a nitrogen environment by spreading each sample on a plate below a 100 W light bulb with the sample approximately 10 cm from the bulb. The temperature of the surface never exceeded 60 "C. After 3 h the samples were removed from the drying environment and placed in the NMR probe which was then sucessfully tuned. The pulse sequence for the triple-echo MAT sequence and the dipolar dephasing variation are presented in Figure 1.A detailed description of these pulse sequences is reported of the broad isotropic resonances e l ~ e w h e r e . ' ~ JBecause ~ generally found in coals, the maximum value of the phase evolution time t l required is only a few milliseconds. A minor modification is thus made in the pulse sequence used in this work in which a fixed length of interrupted decoupling period, L , is applied during each of the two store periods in order to reduce the total decoupling period, i.e., reducing the duty cycle of decoupling, so that a much shorter recycle delay time can be used. This modification is significant because of the short lH TIvalues generally encountered in coals. The 2D spectrum obtained from the triple-echo MAT pulse sequences produces



C 115.4

117.8

134.7

144.4

Figure 5. Selected spectral slices taken at the indicated isotropic chemical shift for the naphthalene pitch. Slices were obtained from (a) standard triple-echo MAT experiment; (b) short contact time triple-echo MAT experiment; (c) dipolar dephasing tripleecho MAT experiment. Outer Bridgehead

Figure 6. Model compound cartoon illustrating the types of aromatic carbons described in this paper; e.g., outer bridgehead, inner bridgehead, substituted, and protonated. a powder pattern for each specified isotropic chemical shift which is inclined relative to the acquisition dimension axis at an angle arctan(wd(3wd),where w1 and w2 are the evolution and acquisition spectral widths, respectively. In order t o obtain a 2D spectrum whose projection along the evolution axis gives the isotropic shift spectrum, these data must be sheared through the inclined angle. For all experiments reported in this paper, the evolution-time increments are chosen to be 3 times the acquisition dwell times, so that the spectral widths for the acquisition dimension are 3 times those for the evolution dimension. This results in 2D spectra with the bands inclined at an angle of arctan(1) = 45". After a 45" shearing operation, the isotropic shift spectrum is obtained from the projection on the evolution axis, while the powder patterns are taken as the perpendicular slices at the isotropic shift positions. The experiments were performed on a Varian VXR-200 spectrometer with a I3C frequency of 50.3 MHz. A home-made large sample volume probe18was used for the experiments.

Nitrogen gas is used for the driving and bearing supply of the MAT probe. Two power levels were used in the proton channel with 300 W employed for cross polarization and 600 W during the decoupling period. With this procedure, a 30 kHz field is obtained during the cross polarization period, while about 48 kHz is achieved during the decoupling period. The aromaticity(fa) of each sample was obtained from a standard tripleecho MAT experiment in which an optimized contact time was used. The contact time is determined by means of a series of 1D variable contact time experiments and then the same experimental conditions are used for the 2D measurements. In order to increase the amount of structural information available, two additional experiments were performed. A short contact time triple-echo MAT experiment with a contact time of 50-60 ps produces a spectrum dominated by the protonated carbons with minor contributions from nonprotonated carbons. A spectrum consisting primarily of the nonprotonated carbons as well as the methyl carbons is obtained with a dipolar dephasing sequence in which a 60 ps decoupling delay is inserted in the pulse sequence as shown in Figure lb. With these two experiments, the overlapping powder patterns for the protonated and nonprotonated carbons can be visually identified.

Results and Discussion Typical experimental results are demonstrated in Figures 2-5 for the naphthalene-derived pitch. The standard triple-echo MAT 2 D contour plot is given in

Figure 2. By shearing the data in Figure 2 by 45", one obtains a 2 D contour plot (Figure 3) w i t h the isotropic chemical shift axis perpendicular to the acquisition dimension. The aromaticity (fa>c a n be obtained by a simple volume integration of both the aromatic region with the isotropic chemical shift values larger than 90 ppm a n d the aliphatic region with the isotropic chemical

Hu et al.


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P2 ( m m )

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Figure 8. A 2D contour plot spectrum of the semianthracite PSOC-628 obtained with the standard triple-echo MAT experiment. The experimental conditions are the same as those for the standard triple-echo MAT of the naphthalene pitch except a contact time of 0.95 ms and a recycle delay time of 1.2 s were used. Data was acquired with 640 scans in both the real and imaginary data sets while 30 tl values are acquired. Total experimental time was approximately 13 h.

133.9

' I

is0

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a50

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o

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Figure 7. Condensed stack plot of spectral slices from 126.6 t o 133.9 ppm obtained from the standard triple-echo MAT experiment on the naphthalene pitch.

shifts values less than 90 ppm. The aromaticity is thus calculated as fa = IaromatidUaromatic 4- laliphatic), where I represents the volume integration value. The measured fa value for this pitch sample is 0.86. Using a variable contact times CP/MAS experiment, a value of 0.84 was obtained on a CMX-100 spectrometer operating at 25.1 MHz. A similar value was obtained from a Bloch decay experiment using a pulse delay of 120 s. The isotropic shift spectrum is obtained in Figure 3 by projecting the contour plot onto the evolution axis, F b . The traditional one-dimensional powder pattern spectrum is obtained by projecting the summed data along the acquisition axis, Fa,while the powder pattern at a specific isotropic shift value is obtained by taking a spectral slice at the specified isotropic chemical shifi position. The isotropic chemical shift projections obtained from the normal, short contact time, and dipolar dephasing experiments are given in Figure 4A,while the corresponding standard, short contact time, and dipolar dephasing 13C CP/ MAS spectra are given in Figure 4B for comparison. It is clear that similar general spectral features are contained in the isotropic chemical shift data sets derived from both the MAT and MAS experiments. In addition, the chemical shift anisotropy information, which is lost in the MAS experiment, is simultaneously obtained with the MAT experiment. The resulting powder patterns derived from Figure 4A (selected at the indicated isotropic chemical shift values) are shown in Figure 5 for the standard (Figure 5a), short contact time

Table 2. 13C CSA Principal Values Obtained from Selected Spectral Slices in the Semianthracite PSOC-628 Obtained by the Triple-Echo Magic Angle Turning Exueriments ~~

~

carbon type 611 6 2 2 633 Standard Triple-Echo MAT substituted 223 160 26 227 229 228

159 162 162

29 31 33

daverage

136.3 138.3 140.7 141.0

hiso -

137.3 138.8 140.6 142.4

Short Contact Time Triple-Echo MAT protonated"

198 208 209 210 217

126 136 135 137 141

10 8 10

14 16

111.3 118.1 118 120.3 124.6

Dipolar Dephasing Triple-Echo MAT nonprotonated (bridgehead) 193 -180 -19 118.0 196 197 203 207

178 179 184 185

-12 -11 -1 5

120.7 121.7 128.7 132.3

110.8 117.5 117.1 120.1 122.6

117.5 121.1 123.3 127.2 130.9

a Tensor values for the protonated carbons with the isotropic chemical shifi values larger than 122.6 ppm are not available by measuring the frequencies of the peak and break points at halfheight positions because of severe overlap with the powder patterns from bridgehead carbons.

(Figure 5b), and the dipolar dephasing triple-echo MAT experiments (Figure 5c). From an inspection of the spectral slices in Figure 5a, it is clear that significant differences exist in the shapes of the powder patterns taken at different isotropic shift values. The spectral slices taken at 115.4 and 117.8 ppm display a tensor powder pattern characteristic of a protonated aromatic carbon. The data obtained from the short contact time experiment (which does not fully polarize nonprotonated carbons) shown in Figure 5b exhibit the protonated



C

Figure 9. Spectral slices obtained with the triple-echo MAT experiment from PSOC-628 at the indicated isotropic chemical shift values: (A) the standard triple-echo MAT experiment with spectral slices taken at the indicated isotropic chemical shift values; (B) short contact time triple-echo MAT experiment; (C) the dipolar dephased triple-echo MAT experiment. Table 3. 13C CSA Principal Values Obtained from Selected Spectral Slices in the Anthracite PSOC-867 Obtained by the Triple-Echo Magic Angle Turning Experiments carbon type

611

622

633

daveraw

diea

bridgehead

193 194 196 199 204

178 178 182 183 187

-32 -27 -20 -12 -6

113.1 115 119.5 123.4 128.6

114.1 116.5 119.0 123.9 128.8

Table 4. Aromaticities Obtained from Both the Standard Triple-EchoMAT Experiment and the Standard 13C CP/MAS Variable Contact Time Experiment sample fa(MAT) fa(MAS) PSOC-867 1.0 1.0 PSOC-628 0.96 0.96O pitch 0.86 0.84 Pocahontas 0.86 0.86b Upper Freeport 0.80 0.81b Stockton 0.73 O.7ijb Pittsburgh No. 8 0.74 0.72b Illinois No. 6 0.72 0.72b Blind Canyon 0.64 0.65b Wyodak 0.62 0.63b 0.67 0.6P Zap a From ref 14. From ref 1. Average of data obtained from variable contact time experiments of five different vials.

*

carbon characteristic features in the spectral slices displayed between 115.4 and 125.1 ppm. From the dipolar dephasing data (which suppresses contributions from protonated carbons) in Figure 5c, the powder patterns in the 122.6-129.9 ppm range are dominated by the features associated with axially symmetric bridgehead or inner carbons in polycondensed aromatic hydrocarbons. The features of substituted aromatic

hydrocarbons begin to appear in the spectral slice at 132.3 ppm and dominate the slices in the 134.7-144.4 ppm region. The characteristic shape of the powder patterns of different types of carbons thus provides a unique means for interpretation of the types of carbons in materials that have severely overlapping isotropic chemical shifts. Hsu has utilized chemical shift tensor principal values in a careful study of this sample.242D chemical shift-chemical shift correlation data obtained by means of a flipper experiment was used t o estimate the average ring size and orientation in an extruded form of this sample. When the dipolar dephasing pulse sequence is use to remove the contributions to the powder patterns from protonated carbons, it is noted that minor differences seem to exist in the axially symmetric powder patterns in the chemical shift range where one would expect to find bridgehead carbons. A condensed stack plot of the powder patterns selected over the isotropic chemical shift range from 126.6 to 133.9 ppm is shown in Figure 7. In the isotropic chemical shift slices between 126.6 and 129.9 ppm (see Table 11, two break points are visible for 8 3 3 with values that fall in the range of approximately 5 t o -20 ppm. These values are consistent with t h o s e reported by Carter et al.25 and Hughes et aLZ6who observed that the d33 components of the inner and bridging carbons were separated by 11 ppm in both (24)Hsu, M. L.;Alderman, D.W.; Grant, D.M.; Pugmire, R. J.; Korai, Y.; Mochida, I. A Two-dimensional 13C NMR Study of Powdered and Oriented Mesophase Pitches, submitted for publication. (25) Carter, C. M.; Alderman, D.W.; Facelli, J. C.; Grant, D.M. J . Am. Chem. SOC.1987,109, 2639-2644. (26) Hughes, C. D.;Sherwood, M. H.; Alderman, D.W.; Grant, D. M. J . Magn. Reson. 1993,A102, 58-72.


Hu et al. ppa

ppn

in v%*

122.3

Figure 10. Spectral slices taken at the indicated istopropic chemical shift from the triple-echo MAT data of PSOC-867. (A) The standard triple-echo MAT data was obtained under the same experimental conditions as those for the standard triple-echo MAT of naphthalene pitch except a contact time of 4 ms and a recycle delay time of 2.5 s were used. Data were acquired with 480 scans in both the real and imaginary data sets and 22 tl values were acquired. Total experimental time was approximately 15 h. (B) Dipolar dephased triple-echo MAT data taken with a dephasing time of 60 ,us. The experimental conditions are the same as those for (A)exceDt data were acauired with 560 scans in both the real and imaginary data sets and 30 tl values were acquired. Total experimental time was appro&nately 24 h.

coronene and pyrene. A peak is observed a t approximately 185-190 ppm, and another shoulder can be seen in the 190-210 ppm range. These data suggest that two different types of brigehead carbons may exist in this sample. The break points observed in these nonprotonated carbons are contained in Table 1which also contains the break points estimated for the protonated carbons from the short contact time experiments. Above 129 ppm another apparent break point appears at approximately 30 ppm which is the frequency region where one would expect the 8 3 3 component of a substituted aromatic carbon tensor. Clear break points for the 8-22 components are not discernible in Figure 7 but a component in the 150-165 ppm region begins t o appear in the spectral slices a t approximately 129 ppm and higher, suggesting the appearance of the 822 com-

ponent of a substituent aromatic carbon. These features, together with the observation that the low-field side of the powder pattern extends down into the 220 ppm region in the lower traces, are a clear indication that an additional nonprotonated tensor is contained in the powder patterns. Hsu et al.24observed the presence of tensors from both substituted and bridgehead carbons in an oriented sample of this pitch sample and the MAT data confirm their data. Similar MAT experiments were performed on semianthracite (PSOC-628)and anthracite (PSOC-867)coals. A 2D contour plot of the standard triple-echo MAT data for the semianthracite is presented in Figure 8,where the aliphatic signal, containing information in the methyl and CHdCH regions, is clearly observed. The measured value of fa is 0.96 which agrees well with the


value reported previ0us1y.l~ The powder patterns obtained from individual spectral slices of the semianthracite data are shown in Figure 9. The low atomic WC value (0.427) for this coal is manifest in the appearance of significant contributions from axially symmetric tensors in the spectral slices in the 119.9132.9 ppm range in the normal MAT data (Figure 9A). In the spectral slices taken in the 135.3-141.1 ppm range the characteristic shapes of substituted aromatic carbons begin t o dominate the powder patterns. The powder patterns obtained from the dipolar dephasing experiment (Figure 9C) clearly show that a highly axially symmetric powder pattern is found as far upfield as 117.5 ppm while contributions from substituted aromatic carbons can be seen in the slices a t 135.3 and 137.8 ppm. The short contact time data (Figure 9B) demonstrate that powder patterns characterizatic of protonated carbons appear a t 112.6 ppm (and higher) while above 127.2 ppm a complex powder pattern is observed which, when compared with the powder patterns in Figure 9C, is composed of both axially symmetric and substituted carbon tensors. The principal values estimated from the powder patterns obtained from these MAT data are given in Table 2. It is interesting t o note that the principal values observed for the axially symmetric nonprotonated carbons seem to fall into two categories characterized by 833 values in the -10 to -20 ppm range and -1 t o 5 ppm range. The results suggest the presence of two different types of bridgehead or inner carbons as noted in the pitch data. The CP/MAS spectrum for the anthracite coal (PSOC867) shows no evidence of the presence of aliphatic carbon which is consistent with the triple-echo MAT data. Spectral slices taken from the 2D MAT data are given in Figure 10. The atomic WC value for this coal (i.e., the fraction of protonated carbons) is 0.13, indicating that the average ring structure is large, estimated to be approximately 300-350 carbons per ~ 1 u s t e r .By l~ removing the contributions from the small number of protonated carbons in the anthracite by means of the dipolar dephasing experiment, the spectral slices between 115 and 130 ppm in Figure 10B can be more readily analyzed t o determine break points for the bridgehead carbons. A particularly well-defined axially symmetric line shape is noted in Figure 10B at isotropic chemical shift values from 114.9 to 119.8 ppm. It is also noted that the peak that contains both the 611 and the 622 components in the spectral slices at isotropic chemical shifts from 122.3 to 129.6 ppm becomes broadened with increasing isotropic shift values. These data are consistent with the principal values of the chemical shift tensors reported for pyreneZ5wherein 811-822 = 6 ppm for the inner carbons and 811-822 = 26 ppm for the peripheral bridgehead carbons. The CSA principal values obtained from these data are given in Table 3. It is interesting to note the range of values observed for the 833 component. It appears that two different sets of values are again exhibited, one in the range -20 to -32 ppm with another apparent set at -6 at -12 ppm. Hughes et a1.26have reported 833 values of -26 and -16 ppm for the coronene inner and outer bridgehead carbons, respectively. It also is apparent in the spectral slices taken from the normal and the dipolar dephasing data that the powder patterns begin to take on the


a

A

-I 20

m w m

120

20

PPm

Figure 11. (a) Stacked plot of isotropic chemical shift projections of the eight Argonne Premium Coals obtained from the standard triple-echo MAT experiment with a single contact time (ct). The sequence of the plot from top t o bottom are: Pocahontas (ct = 2.0 ms), Upper Freeport (ct = 2.0 ms), Lewiston Stockton (ct = 1.0 ms), Pittsburgh No. 8 (ct = 2.0 ms), Illinois No. 6 (ct = 2.0 ms), Blind Canyon (ct = 2.0 ms), Wyodak (ct = 0.45 ms), and North Dakota (ct = 0.8 ms). (b) The standard I3C CP/MAS spectra of eight Argonne Premium Coals obtained at a magnetic field of 2.35 T and at a sample spinning rate of 4.1kHz. While a variable contact time was used to acquire the coal spectra, only the 2.0 ms contact time data is displayed. The sequence of plotting from top to bottom is the same as those in Figure 9a.

characteristic shape of substituted carbons at isotropic shifts greater than 132 ppm. Since no aliphatic carbons are observed in the CP/MAS spectrum of this coal, these data suggest that these tensor principal values would arise from biaryl linkages, which have chemical shifts in the same general shift range as aryl-substituted aromatic carbons.27 The signal to noise ratio becomes quite poor for spectral slices above 132 ppm and precise line shapes are difficult to determine due to the low population of substituted carbons. However, isotropic chemical shift data in polycondensed aromatic compounds containing single bond bridges within the aromatic structure are found in the 135-140 ppm range and, hence, the powder patterns observed at 134.5 and 137.0 ppm are most likely due to the presence of biaryl linkages in a polycondensed aromatic structure. No attempt was made to extract principal values for either the substituted or protonated carbons in this sample due to the low S/N for the former and the low atomic WC ratio for the latter. The standard triple-echo MAT experiment with a single contact time was performed on the Argonne Premium Coals. The resulting isotropic chemical shift projection spectrum for each coal is presented in Figure 11A. The standard 13CCP/MAS spectra, obtained at a magnetic field strength of 2.35 T and at a sample spinning rate of 4.1 kHz, are presented in Figure 11B for comparison. Spectral features similar t o those observed in the standard CP/MAS spectra are observed in the isotropic chemical shift projection of the tripleecho MAT data. Noticeable differences are also observed between these two sets of spectra. In the (27)Breitmaier, E.; Voelter, W. Curbon-13 NMR Spectroscopy,3rd ed.; VCH: Berlin, 1989; Chapter 4.

-


Carbon Aromaticity

'.OO

0.95 0.90

1 1

/

I

. . . . . . . . . . . . . . . . . . . .

0.60

0.65

0 . 7 0 0.75 0.80 0.85

0.90

0.95

.. 1.00

fa( C PIMAS) Figure 12. Correlation of the carbon aromaticities,fa, between the data obtained from the standard triple-echoMAT experiment and those from the standard 13C CP/MAS variable contact time experiment. A correlation factor R2 = 0.987 is

obtained. aliphatic region the relative contribution of the NMR signal between 0 and 25 ppm (presumably arising primarily from methyl groups) appears to be slightly higher in the MAT data (compared to the signal in the 25-50 ppm) than that observed in the standard MAS data. A similar feature is present in the pitch spectra shown in Figure 4. In the large sample volume probe used for the MAT experiments a maximum decoupling field of approximately 48 kHz is achieved. This level of decoupling power may be insufficient t o simultaneously fully decouple the strong dipolar interactions between the carbons and protons in aromatic CH and aliphatic CH and CH2 spin systems whereas the dipolar interaction is significantly reduced for the rapidly spinning methyl groups that require less heteronuclear decoupling field strengths to achieve complete decoupling. With inadequate decoupling field strength, resonances for the CH2 carbons would be expected t o be somewhat broadened and of lower apparent intensity. Such was recently observed in this laboratory for the CH2 carbon in methyl a-D-glucopyran~side.~~ While the volume integration for the MAT experiment is within 3-5% of the correct values for methyl, CH, and nonprotonated carbons of samples studied so far in this laboratory, the CH2 carbon in methyl a-D-glucopyranoside exhibited an 18%reduction in peak volume relative to the other carbons in this compound.lg A careful investigation is now underway in our laboratory to study the effects of y(H2) on line widths and analytical response functions for different types of carbons in the MAT experiments. Significant losses in volume area are not apparent in the coals studied. The aromaticity values of each coal was obtained by a volume integration of both the

Hu et al.

aromatic and aliphatic regions of the 2D data as previously described. The results are given in Table 4. The aromaticities obtained by the standard 13CC P W variable contact time experiment at a sample spinning rate of 4.1 kHz are also included in Table 4 for comparison. The results from these two totally different experiments are excellent with a correlation factor of R2 = 0.987 as shown in Figure 12. Apparently, whatever loss in intensity experienced by the CH2 groups is within the experimental error of CP/MAS and MAT experiments. These data indicate that reliable aromaticity values can be obtained for coals over the entire rank range by employing the triple-echo MAT experiment with a properly selected contact time.

Conclusions The data presented in this paper demonstrate that the triple-echo MAT experiment combined with the short contact time and the dipolar dephasing variations can be used t o examine the structural features of coals and related compounds which exhibit broad overlapping resonance lines in the isotropic chemical shift dimension. Because of the sideband-free feature in the isotropic chemical shift dimension, these triple-echo MAT experiments are applicable a t any magnetic field strength and should be especially useful at high field strengths where one can take advantage of the inherent increased sensitivity. In fact, high-field (9.4 T) MAT experiments have now been conducted in this laboratory which appear to confirm the advantages of using higher magnetic fields for these experiments. These experimental results are now being prepared for publication. The major problems associated with obtaining MAS spectra at high fields (the very high spinning speeds required to suppress or separate sidebands and the consequent difficulty in uniformly polarizing all spins) should be eliminated with the MAT experiments. Apparent quantitative results on coals can be obtained from a single contact time standard triple-echo MAT pulse sequence. By introducing the chemical shift anisotropy dimension, structural information which is impossible t o extract in the standard MAS experiments is now revealed with the MAT experiment. By separating the powder patterns of nonprotonated from those of the protonated aromatic carbons, the short contact time and the dipolar dephasing variations of the tripleecho MAT experiment greatly simplifythe measurement of the I3C CSA principal values in such complex systems. Acknowledgment. This work was supported by the Pittsburgh Energy Technology Center through the Consortium for Fossil Fuel Liquefaction Science (contract no. DE-FC22-89PC89852) and by Basic Energy Sciences, Office of Energy Research, U.S.Department of Energy (contract no. DE-FG03-94ER14452). EF9500447

Investigation of Coal and a Naphthalene Pitch by ... - ACS Publications

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