Structural Determination in Carbonaceous Solids Using Advanced

Dec 15, 2000 - Theoretical predictions of the principal values of chemical shift tensors in model compounds are used in the interpretation of the expe...
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Energy & Fuels 2001, 15, 14-22

Structural Determination in Carbonaceous Solids Using Advanced Solid State NMR Techniques Jian Zhi Hu,§ Mark S. Solum,‡ Craig M. V. Taylor,+ Ronald J. Pugmire,*,‡ and David M. Grant§ Departments of Chemistry and Chemical and Fuels Engineering, University of Utah, Salt Lake City, Utah 84112, and Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received August 23, 2000. Revised Manuscript Received September 27, 2000

Detailed structural information in carbonaceous solids with complex molecular structures may be obtained with a combination of three advanced solid-state NMR techniques, i.e., dipolar dephasing with variable dephasing times, 13C CP/MAS spectral editing, and an isotropicanisotropic correlation experiment called PHORMAT. It is shown that the ratio of protonated to nonprotonated aromatic carbons may be determined with high accuracy by the conventional dipolar dephasing experiment with variable dephasing times. The fractions of aliphatic CH, CH2, CH3 and nonprotonated (C) carbons may be obtained by employing 13C CP/MAS spectral editing techniques. The 2D PHORMAT experiment is used to obtain the structural information in the aromatic region of coal. Theoretical predictions of the principal values of chemical shift tensors in model compounds are used in the interpretation of the experimental chemical shift tensors. The potential application of these methods is illustrated using a sub-bituminous coal DIETZ (PSOC-1488).

1. Introduction The combination of magic angle spinning and high power proton decoupling has been utilized extensively for obtaining structural information in fossil fuels such as coals.1-10 A number of studies were directed to quantitatively determining the aromaticity (fa), defined as the fraction of aromatic carbons in a coal.5,6 Information about aromatic cluster size may be obtained from the fa parameter along with the ratio of protonated to * To whom correspondence should be addressed. § Department of Chemistry. + Los Alamos National Laboratory, Los Alamos, NM 87545. ‡ Department of Fuels Engineering. (1) (a) Gerstein, B. C.; DuBois Murphy, P.; Ryan, L. M. Aromaticity in Coal. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982. (b) Gerstein, B. C.; Ryan, L. M.; DuBois Murphy, P. An Estimation of Average Polynuclear Aromatic Ring Size in an Iowa Vitrain and a Virginia Vitrain. In Coal Structure; Gorbaty, M. L., Ouchi, K., Eds.; Advances in Chemistry Series 192; American Chemical Society: Washington, DC, 1980. (2) DuBois Murphy, P.; Cassady, T. J. and Gerstein, B. C. Fuel 1982, 61, 1233. (3) Pruski, M.; De la Rose, L. and Gerstein, B. C. Energy Fuels 1990, 4, 160. (4) Gerstein, B. C. Fingerprinting Solid Coal Using Pulse and Multiple Pulse Nuclear Magnetic Resonance. In Analytical Methods for Coal and Coal Products; Kall, C., Jr., Ed.; Academic Press: New York, 1979; Vol. III, Chapter 51. (5) Snape, C. E.; Axelson, D. E.; Botto, R. E.; Delpuech, J. J.; Tekeley, P.; Gerstein, B. C.; Pruski, M.; Maciel, G. E.; Wilson, M. A. Fuel 1989, 68, 547. (6) Silbernagel, B. G.; Botto, R. E. Advanced Magnetic Resonance Techniques Applied to Argonne Premium Coals. In Magnetic Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Y., Eds.; Advances in Chemistry Series 229; American Chemical Society: Washington, DC, 1993; p 629. (7) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. (8) Pan, V. H.; Maciel, G. E. Fuel 1993, 72, 451. (9) Wind, R. A.; Duijvestijn, M. J.; van der Lugt, C.; Smidt, J.; Vriend, H. Fuel 1987, 66, 876. (10) Jurkiewicz, A.; Wind, R. A.; Maciel, G. E. Fuel 1990, 69, 830.

nonprotonated carbons in the aromatic region after subtracting out phenolic and substituted carbons.1,7 Detailed structural information regarding the fractions of different types of functional groups was obtained traditionally by using chemical shift ranges in a 13C spectrum.1,7 However, due to the complexity of the structures of coal of different ranks, no clean cutoff shifts exist between different types of functional groups. For example, in the aliphatic region the resonances from C, CH, CH2, and CH3 carbons may overlap with one another. In the aromatic region, protonated, bridgehead, and aliphatic substituted carbons may overlap in a chemical shift range from 90 to 135 ppm. Identification of these structural units has been a long-standing problem in coal research. This problem is examined utilizing several recently developed solid sate NMR techniques, i.e., the 13C CP/MAS spectral editing technique11-13 pioneered by the Zilm group11 and the isotropic-anisotropic chemical shift correlation experiment called PHORMAT developed at Utah.14,15 It has been reported11-13 that spectra containing only one of the basic functional groups (i.e., C, CH, CH2, and CH3) may be obtained by 13C CP/MAS spectral editing techniques. Using experimental parameters derived from a model compound, the fraction of each type of functional group relative to the total carbons in the (11) Wu, X.; Burns, S. T.; Zilm, K. W. J. Magn. Reson. 1994, A 111, 29. (12) Hu, J. Z.; Wu, X.; Yang, N.; Li, L.; Ye, C.; Qin, K. Solid State NMR 1996, 6, 187. (13) Hu, J. Z.; Harper, J. K.; Taylor, C.; Pugmire, R. J.; Grant, D. M. J. Magn. Reson. 2000, 142, 326. (14) Hu, J. Z.; Wang, W.; Liu, F.; Solum, M. S.; Alderman, D. W.; Pugmire, R. J.; Grant, D. M. J. Magn. Reson. 1995, A 113, 210. (15) McGeorge, G.; Alderman, D. W.; Grant, D. M. J. Magn. Reson. 1999, 137, 138.

10.1021/ef0001888 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/15/2000

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Table 1. The Percentage of Different Types of Carbons in PSOC-1488 Coal Obtained by Various Methodsa a:

13C

CP/MAS and Dipolar-Dephasing Experiments

fa (%) (>90 ppm)

fal (%) (0-90 ppm)

fraction of nonprotonated carbons (90-165 ppm) (%)

fraction of protonated carbons (90-165 ppm) (%)

64

36

66 ( 2

34 ( 2

b:

13C

CP/MAS spectra editing experiments

>90 ppm

shift range

0-90 ppm

carbon type

nonprotonated carbons

CH

C

CH

CH2

CH3

% of total

45b

19

2.2

11.2

14.4

8.2

c: Subassignments of the Nonprotonated and Methyl Carbons chemical shifts (ppm)

% of total

types of carbonsc

0-16 16-25 25-35 50-90 90-145 150-165 165-190 >190

2.7 2.6 2.1 4.1 31.1d 6.4d 4.6 2.9

-(CH2)n-C*H3 aro-C*H3 aro-(CO)-C*H3, -O-(CO)-C*H3, ali (C*)-O-ali (C*), ali (C*)-O-aro (C), ali (C*)-OH aromatic bridgehead and substituted aromatic carbons aro (C*)-OR, aro (C*)-OH -C*OOR, -C*OOH -(C*O)-

a To obtain the fraction (i.e.,19%) of protonated aromatic carbons in (b), the data in (a) and (c) for the fraction of nonprotonated carbons >165 ppm are used. b 45 ) 31.1 + 6.4 + 4.6 + 2.9 c “aro” denotes “aromatic”. “ali” denotes “aliphatic”. d 31.1 + 6.4 ≈ (fa - 4.6 - 2.9)66% ) (64 - 4.6 - 2.9)66% ) 37.3.

sample may be estimated. Hence, the technique is promising for obtaining qualitative structural information in the aliphatic region of a coal.11 In the PHORMAT experiment, which is a variation of Gan’s original magic angle turning experiment,17 the 13C chemical shift anisotropy (CSA) information is correlated with the isotropic chemical shift value in a two-dimensional (2D) experiment, where one dimension contains the isotropic chemical shift spectrum and the other displays chemical shift anisotropy (CSA). It is well-known that the 13C CSA is sensitive to the detailed electronic structure around a nucleus. As a result, the principal values of 13C CSA for protonated, bridgehead, and aliphatic substituted aromatic carbons are quite different. Hence, the tensor information may be used to assist in the interpretation of the 13C isotropic spectrum. The structural information provided by the spectral editing technique and the PHORMAT experiment is complementary. The dipolar dephasing experiment with variable dephasing times provides accurate data on the ratio of protonated to nonprotonated carbons in the aromatic region. A combination of these three types of experiments is used in this work to obtain structural details in fossil fuels such as coals. In addition, modifications of the PHORMAT experiment that produce spectra containing only protonated, or nonprotonated carbons are developed to simplify the measurement of the overlapping CSA in the aromatic region. Theoretical predictions of the principal values of the CSA in model compounds for which experimental data is not available are also used to aid the interpretation of the CSA data observed in the coals. As a typical application, a subbituminous coal (DIETZ, PSOC-1488) is used in this paper. (16) (a) Alemany, L. B.; Grant, D. M.; Alger, T. D.; Pugmire, R. J. J. Am. Chem. Soc. 1983, 105, 6697. (b) Newman, R.H. J. Magn. Reson. 1992, 96, 370. (17) Gan, Z. J. Am. Chem. Soc. 1992, 114, 8307.

2. Experimental Results The 13C CP/MAS dipolar dephasing experiments were performed on a Chemagnetics CMX-100 NMR spectrometer operating at 2.35 T using a standard MAS probe with a 7.5 mm Chemagnetics sample rotor. The pulse sequence used for the dipolar dephasing experiment was the type reported by Alemany et al.16a The sample spinning rate was 4.1 kHz. This spinning rate was sufficient to place the first spinning sideband (90 ppm outside the aliphatic region. The aromaticity (fa), i.e., the ratio of the carbons with shift values greater than 90 ppm, was determined to be 0.64. A combination of Gaussian and Lorentzian functions describe the decay of the integral intensity of the aromatic band as a function of the dephasing time.7 The Gaussian component, with a decay constant of about 23 µs, corresponds to the protonated aromatic carbons. The Lorentzian component typically exhibits a decay constant greater than 230 µs and is due to the nonprotonated aromatic carbons. The results obtained on PSOC-1488 are summarized in Table 1a. The high accuracy of the dipolar dephasing experiment16a has also been justified at 2.35 T at spinning rate of nearly 4 kHz.16b 13C CP/MAS spectral editing experiments were performed on a CMX-200 NMR spectrometer. A standard Chemagnetics MAS probe with a 7.5 mm sample rotor spinning at 4.0 kHz was used for the measurements. The pulse sequences used for the spectral editing experiments were the modified version13 of the original experiments reported by Zilm et al.11 Five experimental spectra, i.e., a spectrum containing all carbons, a CH3 + C only, a C only, a CH only, and a CH2 only spectra, were acquired. All of these spectra, except that of the C only, were semiquantitative. The C only spectrum was used to aid the interpretation of the CH3 + C only spectrum so that the CH3 and C resonances could be identified separately. As reported previously11-13 fumaric acid monoethyl ester (FAME) was used to setup the experimental conditions. The five experimental 13C

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Hu et al. Table 2. The Experimental Principal Values of 13C CSA Tensor in 2,6-DMN Obtained by Various Kinds of Methods carbon

Figure 1. The five experimental 13C CP/MAS spectral editing spectra obtained on PSOC-1488 (∼260 mg). (a) The spectrum containing all the carbons, (b) C + CH3 only spectrum, (c) C only spectrum, (d) CH only spectrum, and (e) CH2 only spectrum. The accumulation numbers are (a) 20,000, (b) 20,000, (c) 40,000, (d) 20,000 for both the (+) and the (-) sequences (d) and 20,000 for both the (+) and the (-) sequences (e). The so-called (+) and (-) sequences are detailed in ref 13.

CP/MAS spectral editing spectra obtained on PSOC1488 are given in Figure 1. The synthesized spectrum using the subspectra in Figure 1 is compared with the standard CP/MAS spectrum in Figure 2. The fraction of nonprotonated, CH, CH2, and CH3 extracted from these experiments are summarized in Table 1b. Subassignment of the nonprotonated carbons based on the evaluation of the (CH3 + C) only spectrum in Figure 1b is given in Table 1c. The PHORMAT data were acquired on a Varian VXR200 NMR spectrometer operating at a field of 4.7 T equipped with a homemade large-sample-volume magic angle turning (MAT) probe with a 20 mm sample rotor.14 Because of the large sample, it was difficult to achieve high RF field for both the 13C and 1H channels at the same time. Two power levels for the decoupling channel were used with 230 w for cross-polarization (CP) and 600 w for decoupling. In this way, a 30 kHz RF field was used for cross polarization while a decoupling field of about 48 kHz was achieved. The PHORMAT pulse sequence used to obtain the nonprotonated carbons was

δ22(ppm)

δ33(ppm)

Standard-PHORMAT 162 71 142 22 190 -2 130 23 122 17 72 12

C2,6 C4,8 C4a,8a C3,7 C1,5 OCH3

235 224 200 205 175 79

C2,6 C4,8 C4a,8a C3,7 C1,5 OCH3

The Dipolar-Dephased PHORMAT 235 161 71 - -a --200 190 -2 ----------

C2,6 C4,8 C4a,8a C3,7 C1,5 OCH3 a“-

Figure 2. (a) Reproduction of Figure 1a. (b) The synthesized spectrum by using the subspectra given in Figures 1b, d, and e. Since the attenuation factor for the nonprotonated C and methyl CH3 are different in Figure 1b, a cutting point of 35 ppm suggested in ref 11 are set to distinguish the methyl carbons (35 ppm). This cutting shift is justified by the nonprotonated C only spectrum in Figure 1c. (c) The difference spectrum between (a) and (b). The estimated confidence by comparing the residual signal in (c) to (a) is better than 90%.

δ11(ppm)

δMAS(ppm) 156.9 130.6 130.6 119.8 105.4 54.1 156.9 130.6 130.6 119.8 105.4 54.1

The PHORMAT for Pure Protonated Carbons ---156.9 223 142 24 130.6 ---130.6 204 130 22 119.8 174 121 16 105.4 79 71 11 54.1

-“ indicates unavailable.

obtained by inserting a dipolar dephasing segment in the 1H channel following the cross polarization with a dephasing time of 60 µs to suppress the signal from the protonated carbons. The PHORMAT spectrum containing only the protonated carbons was synthesized by using the data from the preceding pulse sequence and the data from a conventional PHORMAT with a short contact time of 40 µs. The concept of this experiment is demonstrated in Figures 3-6 using the model compound 2,6-dimethxoynaphthalene (2,6-DMN). The principal values of the 13C CSA for the various carbons in 2,6DMN obtained from the standard, dipolar dephased, and the pure protonated PHORMAT experiments are compared in Table 2. The experimental spectra obtained on PSOC-1488 using the techniques described are summarized in Figures 7 and 8. The principal values of the CSA for the dominant tensors that were obtained by measuring the shifts of the break points are given in Table 3. 3. Theoretical Predictions Chemical shift anisotropy (CSA) calculations and geometry optimizations were performed with the GAUSSIAN9818 suite of programs using the DFT approach and the D95** basis set.19 The chemical shift calculations employed the method proposed by Cheeseman et al.20 (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A. Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; HeadGordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998. (19) Dunning, J. H.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York. (20) Cheeseman, J. R.; Trucks, G. W.; Keith, T. A.; Frisch, M. J. J. Chem. Phys. 1996, 104, 5497.

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Energy & Fuels, Vol. 15, No. 1, 2001 17

Figure 3. Expanded contour plot of the standard 2D PHORMAT spectrum of 2,6-DMN (left) and sliced powder patterns for each carbon (right). This spectrum was acquired with the following experimental parameters. A sample spinning rate of 25 ( 0.2 Hz was used. The cross-polarization time, total echo time for each segment, recycle delay times were 2 ms, 100 µs, and 2s, respectively. Acquisition dimension (ta) FIDs with 360 complex points were transferred to spectra with 60 kHz spectral width. The 2D data were collected at 86 different values of the evolution time tb incremented by periods of 80 µs. This results in a maximum evolution time of 6.88 ms and an evolution spectral width of 12.5 kHz. The eight types of free-induction decays specified in ref 14 were acquired with the (+) and the (-) PHORMAT pulse sequences using a total of 256 scans at each tb value, resulting in a total measuring time of 11.4 h.

Figure 4. Expanded contour plot of the dipolar-dephased 2D PHORMAT spectrum of 2,6-DMN (left) and sliced powder patterns for each carbon (right). This spectrum was acquired using the same experimental parameters as those given in Figure 3 except that a dipolar dephasing time of 60 µs was used.

with the B3LYP hybrid functional,20 which uses Becke exchange functional21a and the LYP correlation functional21b as implemented in the GAUSSIAN98 program. The 13C shifts were converted to the TMS scale by subtracting the estimated absolute chemical shielding of TMS, 185.4 ppm, which was obtained from the calculated shielding in methane minus the 7 ppm reported as the difference between liquid TMS and gasphase methane,22 i.e., υTMS ) (185.4 ppm - σcalcd). Theoretical predictions on vanillin, 3,4-dimethoxybenzaldehyde, and syringaldehyde are summarized in Table 4 with experimental results given in parentheses for comparison. The linear correlation parameters between the theory and the experimental data given in Table 4 are summarized in Table 5. Theoretical predictions on (21) (a) Becke, A. D. Phys. Rev. 1988, A38, 3098. (b) Lee, C.; Yang, W.; Par, R. G. Phys. Rev. 1988, B37, 785. (22) Jameson, A. K.; Jameson, C. J. Chem. Phys. Lett. 1981, 134, 461.

a series of relevant molecules that are derived from vanillin and syringaldehyde are reported in Table 6. 4. Discussion 4.1. 13C CP/MAS Spectral Editing. The H/C ratio in PSOC-1488 can be obtained readily using the data given in Table 1b; 84 protons per 100 carbons (i.e., 45 × 0 + 19 × 1 + 2.2 × 0 + 11.2 × 1+ 14.4 × 2 + 8.2 × 3 ≈ 84) were found. This result agrees well with that (C100H81N1S0.3O17) obtained from elemental analysis,24 indicating that the fraction of each functional group (23) Zheng, G.; Hu, J. Z.; Zhang, X.; Sheng, L.; Ye, C.; Webb, G. A. Chem. Phys. Letts. 1997, 266, 533. (24) Smith, K. L.; Smoot, L. D.; Fletcher, T. H.; Pugmire, R. J. The Structure and Reaction Processes of Coal; Plenum Press: New York, 1994. (25) Pettersen, R. C. The Chemical Composition of Wood. The Chemistry of Solid Wood; Rowell, R. M., Ed.; Advances in Chemistry Series 207; American Chemical Society Washington, DC, 1984; p 57.

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Figure 5. Expanded contour plot of the short-contact-time 2D PHORMAT spectrum of 2,6-DMN (left) and sliced powder patterns for each carbon (right). This spectrum was acquired using the same experimental parameters as those given in Figure 3, except that a contact time of 40 µs was used and the number of tb increments was 50.

Figure 6. Expanded contour plot of the 2D PHORMAT spectrum of 2,6-DMN (left) and sliced powder patterns for each carbon (right) for pure protonated carbons. This spectrum was a difference spectrum between Figures 4 and 5. In the subtracting process, the data for the dipolar-dephased spectrum was truncated to the same length as that in Figure 5.

obtained from 13C CP/MAS spectral editing method is semiquantitative. An estimate of the oxygen content in this coal may be obtained using the fractions for different types of nonprotonated carbons given in Table 1c. Since the oxygen functional group may involve either one or two carbons (e.g., in some cases carbons may be bonded to either an -OH group or an -O-R group), only a lower and an upper limit can be defined. On the basis of these considerations, oxygen content per 100 carbons falls in the range 13 (2.9 + 4.6 + 6.4/2 + 4.1/2) to 23 (2.9 + 4.6 × 2 + 6.4 + 4.1). On the basis of elemental analysis (i.e., 17 oxygen atoms per 100 carbons), both -OH and -O-R groups are found in this coal. The oxygen functional groups appear to be weighted slightly in favor of ether linkages as opposed to phenolic groups. It should be pointed out that nitrogen and sulfur atoms are ignored in this analysis due to their low concentrations. 4.2. The PHORMAT Experiments. In the standard PHORMAT spectrum of 2,6-DMN (left-hand side in Figure 3), well resolved resonances, corresponding to (26) Wilson, M. A. N. M. R. Techniques and Applications in Geochemistry and Soil Chemistry; Pergamon Press: Elmsford, New York, 1987. (27) Schobert, H. H. The Chemistry of Hydrocarbon Fuels; Butterworth & Co (Publisher) Ltd: Markham, ON, 1990.

Table 3. The Dominant 13C CSA Principal Values Obtained from the Sliced Dipolar-Dephased PHORMAT Spectra in Figure 7 for the Nonprotonated Carbons and from the Pure Protonated PHORMAT Spectra in Figure 8 for the Protonated Carbons δ11 (ppm)

δ22 (ppm)

δ33 (ppm)

δav (ppm)

269 235 208(227) 213(227) 209 206 202 201 200 194 184 170 163

Nonprotonated Carbons 177 94 180 163 71 156 158 23(59) 130(148) 163 14(53) 130(148) 163 24 132 160 21 129 160 22 128 159 27 129 158 21 126 153 21 123 153 26 121 140 30 113 111 23 99

221 214 205 193 182

146 137 130 130 134

Protonated Carbons 15 127.3 17 122.7 20 118.3 17 113.3 16 110.7

δiso (ppm) 179 153 141a 136a 134 131 130 129 128 123 118 113 108 131.3 125.2 119.1 112.9 110.5

a Powder patterns for the -R substituted and oxygen substituted aromatic carbons are overlapped in this range.

protonated C1,5, C3,7, and the nonprotonated C2,6, are observed. There are also overlapped resonances, i.e., the

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Figure 7. Contour plot of the dipolar-dephased 2D PHORMAT spectrum of PSOC-1488 (left) and sliced powder patterns at selected isotropic chemical shift values (right). This spectrum was acquired using the same experimental parameters as those given in Figure 4, except 30 tb increments, 15360 scans at each tb increment, a recycle delay time of 1.2 s, and a contact time of 2 ms were used. The total measuring time was about 6.4 days.

Figure 8. Contour plot of the 2D PHORMAT spectrum of PSOC-1488 (left) and sliced powder patterns at selected isotropic chemical shift values (right) for pure protonated carbons. This spectrum was a difference spectrum between a short-contact time PHORMAT spectrum with a contact time of 40 µs and Figure 7.

isotropic chemical shift of protonated carbons C4,8 overlaps with that of the nonprotonated C4a,8a. Hence, this sample offers an ideal case for testing various kinds of analytical methods. It is clear in Figure 4 that the signals corresponding to the protonated aromatic carbons are suppressed by the dipolar dephased variation of the PHORMAT experiment, leaving a spectrum containing only the nonprotonated carbons together with some residual signal

from the methoxy carbons. The tensor pattern for C4a,8a is thus successfully isolated. This simplified spectrum permits extraction of the principal values of 13C CSA for the nonprotonated carbons. Unlike the dipolar dephased PHORMAT where a clean suppression of the protonated carbons is achieved, even a short contact time PHORMAT experiment (see Figure 5) exhibits some contribution from the nonprotonated carbons. The residual signal from the nonpr-

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Table 4. The Calculated and Experimental (inside the parentheses) 13C CSA Principal Values in Vanillin, 3,4-Dimethoxybenzaldehyde and Syringaldehydea

a The experimental data for Vanillin and 3,4-dimethoxy-benzaldehyd are from Ref (23). The data for syring-aldehyde were obtained at this laboratory using the standard PHORMAT experiment.

Table 5. Correlation Parameters between the Calculated and the Experimental 13C Chemical Shift Principal Values in Table 4 type of data

slope

intercept(ppm)

std. dev. (ppm)

R2

all the data in Table 1 C1 carbon only

1.020

-1.5

8.3

0.988

1.015

1.8

2.4

0.999

onated carbons in Figure 5 can be eliminated by subtracting a properly weighted fraction of the dipolar dephased PHORMAT (Figure 4) from the data in Figure 5. This process results in a PHORMAT spectrum containing only the pure protonated carbons and the results are presented in Figure 6. The tensor line shape for the protonated carbon C4,8, that was overlapped with C4a,8a in the standard PHORMAT, is now isolated and the corresponding principal values can be identified easily. Within experimental error the principal values of the nonprotonated carbons obtained from the dipolar dephased PHORMAT are consistent with those obtained with the standard PHORMAT experiment (Table 2). Such is also true of the principal values of the proto-

nated carbons obtained from the pure protonated PHORMAT experiment. From a visual inspection of the sliced spectra presented at the right-hand side in Figure 7, it is found that an isotropic chemical shift value of 153 ppm is observed for the well defined CSA powder pattern for the oxygen substituted aromatic carbons. Typical powder patterns for alkyl and aryl substituted aromatic tensors are observed in the chemical shift range from 130 to 141 ppm and the principal values are given in Table 3. The powder pattern slices, taken at isotropic shift values from 118 to 130 ppm, are also dominated by features that are similar to those of substituted carbons. Evidence of the presence of axially symmetric tensor from bridgehead carbons, though less prominent and not given in Table 3, is also visible in this isotropic chemical shift range (i.e., 118-130 ppm). A surprising finding from Table 3 is that the principal values of the tensors between an isotropic chemical shift value of 118 to 130 ppm are not dominated by the axially symmetric tensor found in the bridgehead carbons.

Structural Determination in Carbonaceous Solids Table 6. The Calculated

13C

Energy & Fuels, Vol. 15, No. 1, 2001 21

CSA Principal Values in a Series of Suggested Molecules That Are Derived from Vanillin and Syringaldehydea

a A and B are derived from vanillin and C, D, E are derived from syringaldehyde. These are basic lignin structural units.25-27 F is a possible structure unit in coal when pyrogallol is condensed.27 H and CH3 are used to simulate structures that consist of carbon and proton atoms.

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Rather, this region is dominated by tensor patterns of the type observed for the -(CO)H substituted aromatic carbons found in Vanillin, 3,4-dimethoxybenzaldehyde and syringaldehyde (see Table 4). These data suggest that the PSOC-1488 coal contains a substantial amount of these structural units. This suggestion is supported by the intensity in the chemical shift range from 50 to 60 ppm in the standard 1D 13C CP/MAS spectrum in Figure 1a; methoxy carbons fall in this range. However, their percentage would be small due to the rather low spectral intensity between 50 and 60 ppm in Figure 1b (i.e., the CH3 + C only spectrum). A variety of structures that may be considered as possible candidates can be derived from vanillin and syringaldehyde. These structural examples can be found in Table 6. The 13C CSA principal values predicted at position C1 from A-E in Table 6 agree well with the experimental values given in Table 3. The preponderance of OH groups relative to OCH3 groups in these derivatives accounts for the low spectral intensity between 50 and 60 ppm found in Figure 1a. There are two special cases, i.e., B and D in Table 6, where the C5 substituent is a CH3 group. The isotropic chemical shift value for these CH3 substituted aromatic carbons are predicted at 121 ppm, an isotropic shift that would normally be assigned to bridgehead carbons. Furthermore, it is known from Table 3 that nonprotonated carbon tensor patterns are found at isotropic chemical shift values from 108 to 113 ppm. These unusual principal values may be explained by structural units similar to those of F in Table 6, where the substituted position C1 is both ortho and para to positions substituted by oxygen functional groups. The predicted principal values for the C1 carbon in case F are 177, 124, and 18 ppm for the δ11, δ22, and δ33, respectively, in agreement with experimental results. Thus, when coal contains relatively large amounts of oxygen functional groups, substituent effects can move the isotropic chemical shift values for the substituted aromatic carbons into the shift range of bridgehead carbons. An over estimation of the aromatic cluster size is expected in such cases when the method given in ref 7 is used. Typical isotropic chemical shift values for the protonated aromatic carbons are found from 125.2 to 131.1 ppm in Table 3, indicating the presence of protonated carbons similar to those found at the C6 position in vanillin and 3,4-dimethoxy-benzaldehyde. The principal values found at isotropic chemical shift values from 110.5 to 119.1 ppm are consistent with those of the protonated carbons that are ortho to the oxygen substituted positions (see Tables 4 and 6). Hence, the data from protonated and the nonprotonated carbons provide complementary information. The suggested structural units given in Tables 4 and 6 are supported by the experimental data.

Hu et al.

Table 5 presents the correlation between the theory and experimental data with a standard deviation of about 8.3 ppm, a slope of 1.020 and a correlation coefficient of 0.988. Since the model compounds in Table 6 have structures similar to those in Table 4, it would be reasonable to conclude that the level of accuracy in Table 6 is similar to that in Table 4. 5. Conclusion The 13C CP/MAS dipolar dephasing experiment with variable dephasing times provides accurate information on the fraction of protonated versus the nonprotonated carbons for chemical shift values >90 ppm. The 13C CP/ MAS spectral editing methods can provide semiquantitative data on the aliphatic carbon fractions of nonprotonated (C), CH, CH2, and CH3 of a coal. The H/C ratio obtained from these data is comparable with that from elementary analysis. An estimation of oxygen content is also possible by investigating the C + CH3 only spectrum along with the C only spectrum. However, only a range of the carbon-oxygen substituent parameter may be estimated due to the diversity of the types of oxygen functional groups. The dipolar dephased PHORMAT experiment, along with the variation for pure protonated carbons can be used to simplify the measurement of the principal values of the 13C CSA in a system with complex molecular structure. The tensor parameters serve as fingerprints to identify possible structures that fall between a chemical shift range of 90-135 ppm, where the protonated, substituted aromatic carbons and bridgehead carbons overlap. In a low rank coal such as PSOC1488, the oxygen substituent effects from the high oxygen content may move the isotropic chemical shift values for the substituted aromatic carbons into the chemical shift range (i.e.,