Structural Parameters of Argonne Coal Samples - American Chemical

13 Structural Parameters of Argonne Coal Samples 13

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch013

Solid-State C NMR Spectroscopy Yoshio Adachi and Minora Nakamizo Government Industrial Research Institute, Kyushu, Shuku-machi Tosu-shi Saga-ken 841, Japan 13

Solid-state C NMR spectroscopy was applied to the structural characterization of the Argonne standard coal samples. The apparent ratio of quaternary to tertiary aromatic carbons was determined by using combined dipolar-dephasing and cross-polarization—magic-angle spinning (CP—MAS) NMR experiments. By using the apparent ratio and the value of the fraction of aromatic carbon, several parameters can be obtained on the average chemical structure of coals. Among the Argonne samples, a low-volatile bituminous coal consists of approximately four aromatic rings with a few ethyl groups as alkyl substituents. High-volatile bituminous coals are composed of two aromatic rings with approximately two propyl groups and one hydroxyl group as substituents. Both the subbituminous and lignite coals have approximately four aromaticringswith approximately three pentyl groups and two oxygen-containing substituent groups, such as hydroxyl or carboxyl groups.

SoLID-STATE HIGH-RESOLUTION NMR SPECTROSCOPY can be performed by using cross-polarization (CP) and magic-angle spinning (MAS) (1). On the CP—MAS N M R spectra of solid carbonaceous fuels, aromatic 0065-2393/93/0229-0269$06.00/0 © 1993 American Chemical Society

Botto and Sanada; Magnetic Resonance of Carbonaceous Solids 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

and aliphatic absorption bands can be observed separately (2-4). Two problems are associated with the reliability of the quantitative measurements of aromaticity of coal by CP—MAS spectroscopy. One problem is the difference in the rate of polarization transfer from H to C spins between aromatic and aliphatic carbons during CP (5—8). Therefore, the selection of contact time for CP is very important for a precise determination of aromatic- and aliphatic-carbon contents. Another problem arises from spinning side bands (SSB) of aromatic bands with high-field measurement of coals because SSB overlap with aliphatic bands. The TOSS (total suppression of side bands) pulse sequence is a powerful technique for reducing the intensity of SSB, but careful treatments of instrumental factors are needed when the signal intensity of a TOSS spectrum is discussed (9-13). In spite of these problems, detailed chemical structures of coals have been investigated by using the analysis of carbon functional groups (14-18) and the dipolar-dephasing (DD) technique (19-25).


X

1 3

In this chapter, the structural parameters of eight coals from the Argonne National Laboratory Premium Coal Sample Program were determined by using conventional C N M R spectroscopy and DD. 1 3

Experimental Details Eight of the coals from the Premium Coal Sample Program at Argonne National Laboratory were used in this study. The elemental analyses of the Argonne coals were published previously (26). Solid-state C NMR spectra were obtained on a Brucker AC-200 NMR spectrometer with a double air-bearing CP-MAS probe head and *H and C high-power amplifiers. A sample was packed in a ceramic capsule, and the capsule was spun at rates of 3 and 4 kHz. The acquisition parameters for all spectra were as follows: 2-ms CP contact time, 1024-2048 accumulations with a 4-s repetition time, 3.5-MS 90° *H pulse width, 31.25-kHz spectral width, 2048 data points, and 50-Hz line broadening. The contact time of 2 ms was chosen for an accurate determination of thefractionof aromatic carbon, / , in the solid-state measurements. The / value determined from the solution spectrum of coal-tar pitch in chloroform-d was nearly equal to that of the same sample measured in the solid state with the 2-ms contact time. Chemical shifts were calibrated with respect to tetramethylsilane using glycine as a secondary standard. The/ values of the coal samples were calculated from the integrated intensities of aromatic bands and associated SSB in conventional CP-MAS spectra that were obtained with a 3-kHz spinning rate. The contents of aliphatic carbons in C, CH, CH^, and C H groups were also calculated with a 3-kHz spinning rate. The contents of carboxyl, oxygenated, alkyl-substituted, and protonated and bridgehead aromatic carbons were calculatedfromthe integrated intensities of the 1 3

1 3

a

a

a

3


13.

ADACHI & NAKAMZO

Structural Parameters ofArgonne Coals

271

aromatic bands of conventional CP-MAS spectra obtained with a 4-kHz spinning rate. Fourier transform infrared (FUR) spectra of coal samples were measured on a Digilab FTS-60 system.

Results and Discussion


1 3

C NMR Spectra of Coals.

1 3

The C N M R spectra of the minois No. 6 coal used in this study are shown in Figure 1 with two different spinning rates, 3 and 4 kHz. Each spectrum is composed of aromatic (90-165 ppm) and aliphatic (0-50 ppm) bands. In the conventional C P - M A S spectra, several SSB appear, and one of the side bands overlaps completely with the aliphatic bands in the spectrum obtained with a 4-kHz spinning rate. The intensity ratios of aliphatic to aromatic bands obtained

3kHz spinning

4kHz spinning Ar-Ar & ArH

Aromatic Aliphatic 300

200

100 ppm

0

Aromatic Aliphatic ^

300

200

100 ppm

13

0

Figure 1. 50-MHz C NMR spectra obtained with different spinning rates of Illinois No. 6 coal



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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

with the TOSS sequence are always smaller than those obtained with the conventional CP—MAS measurements irrespective of the spinning rates used. Because the SSB cannot be completely eliminated even if the TOSS sequence is employed, the unextinguished SSB overlap partly with the absorption bands of the carboxyl (165-190 ppm) and ether (50-90 ppm) carbons at a spinning rate of 3 kHz and with those of the carbonyl (190-210 ppm) carbons at 4 kHz (16,18). In the conventional C P - M A S measurements with a 3-kHz spinning rate on Illinois No. 6 coal, the aliphatic bands around 30 ppm can be observed separately from the SSB. One shoulder band observed in the 3-25-ppm region of aliphatic band in Figure la is assigned to methyl carbons. In the spectrum obtained with a 4-kHz spinning rate, the carboxylcarbon ( - C O O ) signal is observed at 180 ppm as shown in Figure lb. In the aromatic band, there are two shoulders; the shoulder band at approximately 155 ppm is assigned to the oxygenated aromatic carbons, A r - O , such as phenol (17). The second shoulder band in the region of 135-148 ppm is assigned to the alkyl-substituted aromatic carbons, A r - R . Aromatic bands around 127 ppm are assigned to the aromatic protonated ( A r - H ) and bridgehead (Ar-Ar) carbons (18). In Table I, the carbon distributions of the Argonne coal samples are shown. These distributions were obtained from integrated intensities of the conventional C P - M A S spectra without curve-fitting techniques. Carbonyl and ether carbons, which may exist to a slight extent, were ignored because their absorption bands overlapped with the SSB in the C P - M A S spectra. Discounting the carbonyl and ether carbons does not seem to cause serious errors in the estimation of the aromatic-carbon contents.

Table I. Carbon Distributions of Argonne Coal Samples by Conventional CP-MAS Spectroscopy

Coal Pocahontas Upper Freeport Stockton Pittsburgh Illinois No. 6 Blind Canyon Wyodak Beulah-Zap

0.834 0.767 0.723 0.703 0.673 0.610 0.613 0.692

-COO

Ar-O

Ar-Al

Ar-H and Ar-Ar

0.8 0.1 1.4 0.9 1.1 1.6 3.8 4.7

2.3 2.4 4.7 5.3 4.4 5.9 7.0 6.2

14.0 12.8 14.0 14.9 13.6 10.9 10.1 11.3

66.2 61.4 52.2 49.2 48.3 42.6 40.4 47.0

C, CH, and CH

CH

7.4 13.0 19.3 19.2 23.7 28.2 29.9 22.9

9.2 10.2 8.4 10.5 9.0 10.8 8.8 7.9

2

N O T E : All values are given as percents.


3

13.

ADACHI & NAKAMIZO

Structural Parameters ofArgonne Coab

273

The contents of carboxyl and oxygen-substituted aromatic carbons de crease with increasing / (Table I). However, the content of methyl car bons does not vary greatly among the coal samples studied. The values listed in Table I were used in the calculation of structural parameters of coals.

Dipolar Dephasing. In Figure 2, D D spectra are shown for Illi nois No. 6 coal. The spinning rate was 3 kHz, and the delay time, t was

(3) *al The ratio of extinction coefficients of aromatic to aliphatic C - H absorp tion bands, e /e , is assumed to be 0.5 in most cases (28, 29). ar

al


13.


ADACHI & N A K A M Z O

277

The substitution coefficient, i? , was obtained from the aromatic bands of the C P - M A S spectra that were obtained with a 4-kHz spinning rate. If the ratio of the substituted aromatic carbons to the protonated and bridgehead aromatic carbons is defined as / , R can be deter mined by using the following equation. s

a r

s / P B

C

a s

+ C

a r s

s

i + /Wr

=

2 + /q/r

a r

a

+ /s/PB


(4) 3

where C Λ/ΡΒ

a r

3Γ

=

C p

ar

+

a r s

c

5

ir

( )

a r

where C and C are the number of bridgehead and substituted aromatic carbons, respectively. The ratio / / p , can be calculated from the intensity ratio of the aromatic shoulder band (135-165 ppm) to the aromatic band (95-135 ppm). Because the band of the carboxyl-substituted aromatic carbons over laps with those of the protonated and bridgehead aromatic carbons, the intensity of the aromatic carbons must be corrected for the overlapping of both bands. In this case, the contribution of the aliphatic carboxyl group to the intensity of the aromatic carbons was assumed to be negligible. In Table III, the structural parameters that were obtained from the lozenge model of Ollivier and Gerstein (27) are shown for the Argonne coal samples. The number of rings, N , for the Argonne coal samples is not large, ranging from 1.6 to 4.4. The number of substitution, n, is 2.3—6.3. The average chain length of aliphatic carbons, n , was obtained from the intensity ratio of all the aliphatic carbons to the methyl carbons and is 1.8-4.4. A low-volatile bituminous coal (Pocahontas) consists of approxi mately four aromatic rings with a few ethyl groups as alkyl substituents. A medium-volatile bituminous coal (Upper Freeport) consists of approxi mately three rings with a few substituent ethyl or propyl groups. Three high-volatile bituminous coals (Illinois No. 6, Pittsburgh, and Stockton) are composed of two aromatic rings with two propyl groups and one hydroxyl group. Wyodak subbituminous coal consists of approximately four aromatic rings and three pentyl, two hydroxyl, and one carboxyl groups. Beulah—Zap lignite has approximately three aromatic rings with two or three pentyl, hydroxyl, and carboxyl groups. B

s

a r

S

B

R

al


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

Table III. Structural Parameters of Argonne Coal Samples Coal


Pocahontas Upper Freeport Stockton Pittsburgh Illinois No. 6 Blind Canyon Wyodak Beulah—Zap

*

ar

1.27 1.04 0.73 1.24 0.88 1.46 3.17 2.10

f

ar S/PB

l

0.26 0.25 0.40 0.44 0.40 0.45 0.57 0.53

R

η

5

0.32 0.29 0.33 0.41 0.35 0.43 0.60 0.52

3.6 3.0 1.6 2.4 1.8 2.7 4.4 3.3

3.1 2.6 2.3 3.3 2.6 3.7 6.3 4.8

1.8 2.3 3.3 2.8 3.6 3.6 4.4 3.9

Conclusion Structural characterization of the Argonne standard coal samples was made by using a combined technique of conventional C P - M A S N M R spectroscopy and D D . Most of the Argonne coal samples consist of con densed aromatic compounds with two to four benzene rings and a few alkyl substituents ranging from ethyl to pentyl groups. Some bituminous coals have oxygen-containing substituents such as hydroxyl and carboxyl groups. These results are approximately in good agreement with those from Solum et al. (25).

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ADACHI & N A K A M Z O

13. 14. 15. 16. 17. 18.


19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.


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RECEIVED

for review June 7, 1990.

ACCEPTED

revised manuscript December 11,

1990.


Structural Parameters of Argonne Coal Samples - American Chemical

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