High-Field NMR Studies of Argonne Premium Coals - American

16 High-Field NMR Studies of Argonne Premium Coals

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch016

Jiangzhi Hu, Liyun Li, and Chaohui Ye

1

Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics, Academia Sinica, Wuhan 430071, Peoples Republic of China Seven Argonne Premium coals were studied by cross-polarization (CP)—combined rotation and multiple-pulse spectroscopy (CRAMPS) in a 9.4-T magneticfield.The apparent carbon aromaticities of the coals obtained at high field via static CP spectra agreed well with those measured at low field, that is, 2.3 T. A detailed discussion of the spectral distortion in the total suppression of sidebands (TOSS) experiment at low spinning rates is given. The experimental evidence presented in this chapter shows that static CP measurements are promising for coal studies.

SoLID-STATE NMR TECHNIQUES such as cross polarization (CP) and magic-angle spinning (MAS) (1), combined rotation and multiple-pulse spectroscopy (CRAMPS) (2), and dynamic nuclear polarization (DNP) (3) have been useful tools for the study of solid fossil fuels. These studies have been continually carried out in a relatively low, generally below 4.7 T, magnetic field because N M R measurements of solid fossil-fuel samples in a high field do not gain much in resolution, as would be expected for liquids. Moreover, in a higher magnetic field, a faster sample-spinning rate would be required to meet the so-called rapid-rotation condition (4) in order to prevent spinning sidebands (SSB). Therefore, a minimum rate 1

Corresponding author 0065-2393/93/0229-0311$06.00/0 © 1993 American Chemical Society

Botto and Sanada; Magnetic Resonance of Carbonaceous Solids Advances in Chemistry; American Chemical Society: Washington, DC, 1992.


312

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

of 8 kHz at 4.7 Τ and 16 kHz at 9.4 Τ is required for MAS studies of coals in order to prevent the first SSB of the aromatic carbons from overlapping the resonances of the aliphatic carbons. These higher spinning rates have not been routinely accessible. Extensive efforts to obtain higher spinning rates have been made, and the fastest rate, about 23 kHz, was achieved recently (5). In addition, M A S probe suppliers (e.g., Doty Scientific, Columbia, SC and Chemagnetics, Fort Collins, CO) currently provide MAS probes that can spin at 10 kHz. A n attractive feature of the high-field experiment is its high sensi tivity. The N M R signal-to-noise ratio increases with the 7/4 power of the field strength (6). For example, the sensitivity at 9.4 Τ is nearly 17 times larger than that at 1.9 T; the latter case involves an 80-MHz proton resonant frequency. In other words, a time-saving factor of 280 would be gained by obtaining the measurements at 9.4 Τ rather than that at 1.9 T, because the signal-to-noise ratio is proportional to the square of the signal-accumulation numbers. The apparent C aromaticities obtained from cross polarization with magic-angle spinning (CP-MAS) spectroscopy are apparently smaller than those obtained from CP experiments (7, 8). This result is partially because MAS eliminates some weakly dipolar couplings so that the CP mechanism is partially broken in the process. The aromatic portions of coals contain more carbons that are remote from protons than do the aliphatic portions; therefore, M A S reduces the apparent aromaticity in comparison with static CP measurements. Conducting N M R studies of coal at high field with a static-CP approach would be expected to result in an increased detection sensitivity without spinning problems. We report here the measurements of seven Argonne Premium coals in a 9.4-T magnetic field via CP, C P - M A S , and CRAMPS techniques. Dipolar dephasing (DD) was also used to extract structural parameters. The C aromaticities that we obtained in the CP experiment agreed well with those obtained at 2.3 Τ by the Utah group (9) using C P - M A S . How ever, the aromaticities obtained with the CP—MAS experiment are signifi cantly smaller than those obtained with CP measurements at high field, and this difference is due to spectral distortion in the total suppression of sidebands (TOSS) technique (10). The distortion will be discussed in this chapter. 1 3

1 3

Experimental Details The seven coal samples were obtained from the Argonne Premium Coal Sample Program. All of the coal samples were packed into rotors in a nitrogen environ ment, and the experimental measurements were performed immediately. The CP, CP-MAS, and CRAMPS spectra were obtained with a Bruker MSL-400 spec trometer with a protonfrequencyof 400.13 MHz and a C frequency of 100.63 1 3


16. Hu ET AL. High-Field NMR Studies ofArgonne Premium Coals

313

MHz. The MAS rate was approximately 4 kHz, and the TOSS sequence was used to eliminate SSB. The 90° pulse width was 4 ^s, and the proton-decoupling strength was 64 kHz in both the CP and CP-MAS experiments. In the CRAMPS experiments, the 90° pulse width was 1.95 μ% and the MREV-8 homonucleardecoupling sequence described in ref. 11 was used. (MREV is an acronym created from the surnames of the originators of the method.) The contact time for the CP and CP-MAS experiments was 1 ms. We carried out the C CP-MAS experiments and extracted the structural parameters of the coals in a manner similar to that described by Solum et al. (9), and the same symbols that they used are also presented in this chapter for con venient comparison. Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch016

1 3

Results and Discussion 1 3

The static CP C spectra were obtained within 10 min with a reasonable signal-noise ratio at 9.4-T high-field measurements. The static CP aromaticities were obtained by digital subtraction of the spectrum from a stan dard sample. The standard sample had been carefully studied at a lower field, and its aromaticity was 0.86. The digital-subtraction procedure was first described by Wind et al. (7). In Figure 1, such a measurement is shown. The C spectrum of an Argonne Premium coal (top) was digitally added to the standard spectrum in an inverse-phase mode (middle), and the result is a difference spectrum (bottom). The apparent aromaticity (the fraction of aromatic carbons, / ) can then be easily calculated from the following equation: 1 3

a

Λ

= [fa(0)(S -

Β - A) + B]/S

(1)

where / (0) is the aromaticity of the standard sample, S is the spectral integration of the sample to be measured, Β is the spectral integration of the difference of the aromatic portions, and A is the integration of the difference of the aliphatic portions. Figure 2 shows the stack plot of the C CP spectra of the seven coals. The aromaticities of these coals as determined by C P - M A S - T O S S and CP measurements only are listed in Table I. The relevant values obtained by the Utah group (9) are also included in the table for com parison. The carbon structural parameters that were obtained with the heteronuclear-dephasing and variable-contact-time experiments are shown in Table II. The proton aromaticities of coals are generally obtained from their CRAMPS spectra, which are digitally decomposed into aromatic and ali phatic portions. Figure 3 shows a stack plot of the CRAMPS spectra of a

1 3



314

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

τ

1

1

ι

300

ι

1

200

1

1—

1

100

0

(ppm) Figure 1. The aromaticity-extraction process using the CP spectrum. The C static CP spectrum of a coal (top) was added to the inverse-phase spectrum of the standard sample (middle) to obtain a difference spectrum (bottom). Spectral integration was then used to determine aromaticity. 13

the seven coals. Their resulting proton aromaticities i f (CRAMPS) are listed in Table III. Determination of proton aromaticities of coals by digi tal decomposition of the CRAMPS spectrum is very tricky and can be fraught with errors because the proton spectra of coals are usually poorly resolved. However, the proton aromaticities, can be roughly estimated, as ff *, by using the carbon structural parameters in Table II and the follow ing equation: a

a

H**

= /

H a

/ |/

H a

+ 3/ * + 2 / al

H al

+ Kf °] zl

(2)

where Κ is a parameter between 1 and 3 depending on the structure of


16. Hu ET AL. High-Field NMR Studies of Argonne Premium Coals

315

501


101

701

401

301

601

202

0

200 PPM 13

Figure 2. C CP NMR spectra of the seven Argonne Premium coals. The spectra are stackedfromhigh aromaticity (top) to low aromaticity (bottom). The numbers are sample numbers; they are explained in Table I. H

coal, if * is the hydrogen aromaticity from carbon N M R spectra,/ is the fraction of protonated aromatic carbon, / is the fraction of methine or methylene carbon, / * is the fraction of methyl or nonprotonated carbon, a n d / ° is the fraction of carbon bonded to oxygen. We set the value of Κ at 1.5. The Η * values of the seven coals are also presented in Table III. C aromaticity is an important parameter for the characterization of coal structure. Many investigations have been undertaken to determine / via C N M R spectroscopy and to relate this parameter to the coal rank. The quantitative accuracy of the measured aromaticity value has long been a

a

H

a l

a l

a l

Λ

1 3

a

1 3


316

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 I. Apparent Aromaticities of the Argonne Premium Coals Sample No.


501 101 701 401 301 601 202 801

Coal

Rank

CP-MASTOSS

CP

CP-MAS, low field

Pocahontas No. 3 Upper Freeport Lewiston-Stockton Pittsburgh No. 8 Illinois No. 6 Blind Canyon Wyodak-Anderson North Dakota

LVB MVB HVB HVB HVB HVB SB L

0.77 0.66 0.63 0.61 0.60 0.53 0.58 0.60

0.86 0.80 0.74 0.72 0.70 0.67 0.65 0.65

0.86 0.81 0.75 0.72 0.72 0.65 0.63 0.61

0

N O T E : The error estimate for/ determination via CP was ±0.03. : LVB, low-volatile bituminous; MVB, mid-volatile bituminous; HVB, high-volatile bituminous; SB, subbituminous; and L, lignite, These/ values were obtainedfromreference 9. a

ABBREVIATIONS

fl

a

Table II. Carbon-Structure Distribution Parameters of the Argonne Premium Coals Sample

hi

501 101 701 401 301 601 202

0 0 0 0 0.01 0.01 0.04

0.35 0.33 0.28 0.27 0.21 0.23 0.18

0.51 0.47 0.46 0.45 0.48 0.43 0.43

0.03 0.05 0.07 0.07 0.08 0.09 0.10

0.12 0.15 0.13 0.14 0.12 0.13 0.18

0.14 0.20 0.26 0.28 0.30 0.33 0.35

0.36 0.27 0.26 0.24 0.28 0.21 0.15

f hi

H

0.08 0.08 0.14 0.14 0.15 0.18 0.22

s

f

l

al

0

l

al

0.10 0.10 0.11 0.11 0.12 0.13 0.10

0.0 0.02 0.01 0.03 0.03 0.02 0.02

H

N O T E : The error estimates are as follows: / / and/ , ± 0 . 0 1 ; / and/ *, 0.02; / a J a V , / , / , a n d / / , ±0.03. ABBREVIATIONS: / , fraction of aromatic carbon;/ ,fractionof nonprotonated, aromatic carbon; / ,fractionof phenolic or phenolic ether carbon; / , fraction of alkylated aromatic carbon; / ,fractionof aromatic bridgehead carbon, and/ ,fractionof aliphatic carbon. a

C

N

a

al

al

B

a

a l

c

N

a

a

p

s

a

a

B

a

H

a

a subject of debate (13). Significant errors can arise in the measurements of aromaticity by C P - M A S C N M R spectroscopy due to spin dynamics in coals in the rotating frame where the CP processes that occur at the aromatic and aliphatic portions are inhomogeneous. In addition, the paramagnetic centers in coals create unequal relaxation effects on the spins of 1 3



16. Hu ET AL. High-Field NMR Studies ofArgonne Premium Coals

j

10

1

f

0

317

r-

PP-H 1

Figure 3. H CRAMPS NMR spectra of the seven Argonne Premium coals. The series sequence of the stack plot is the same as that in Figure 2. the aromatic and aliphatic portions, and the concentration of the para magnetic centers varies with coal rank. As shown in Table I, the aromaticities obtained by CP only at 9.4 Τ are comparable to those obtained at 2.3 Τ (9). However, the C P - M A S TOSS values are much lower. As mentioned by Snape et al. (13), and Botto and Axelson (14), in some cases the values of aromaticity obtained with the TOSS sequence are almost identical to those obtained at a low magnetic field without TOSS. In our case, the significant differences be-


318



Table III. Proton Aromaticity of the Argonne Premium Coals Sample

(CRAMPS)

(CP-MAS)

501 101 701 401 301 601 202

0.55 0.45 0.42 0.40 0.33 0.27 0.26

0.48 0.39 0.30 0.28 0.22 0.22 0.23

± ± ± ± ± ± ±

0.04 0.03 0.04 0.04 0.06 0.03 0.03

± ± ± ± ± ± ±

0.08 0.05 0.05 0.04 0.03 0.03 0.03

tween CP and C P - M A S - T O S S aromaticities are mainly due to the spec tral distortion that is inherent in the TOSS technique when a low spinning rate is used. We will discuss some of our analytical results in the following paragraphs. The TOSS sequence (10) can, in principal, partially restore the spec tral intensity of centerbands. Olejniczak et al. (15) determined that the loss of the centerband intensity occurs when the spinning speed is slow with respect to the chemical-shift anisotropy. Recently, Raleigh et al. (16) discussed the sideband suppression experiment. We (17) discussed the intensities of spinning sidebands for inhomogeneous interactions in rotat ing solids via rotating echoes. Similarly, the centerband intensity can be calculated as a function of the ratio of chemical anisotropy w S to spinning rate ω . The calculations will not be discussed in detail here; however, the equation that describes the restoration of centerband intensity with the TOSS sequence by the rotating-echo intensity ratio I (0)/I (0) is as fol lows: Q

χ

T

2π π Σ Ζ Ί Ζ J-2i(A)Ji(B) exp [ί21(φ / ο ο ά

2

i

TW

_ 2 π

/ο(0)

0

- φ )] sin β άβ ι

(

π

U

J * ϊ J I Σ 4 (B)J-2k(A) exp [i2fc(fc - Φι)] I sin β άβ 0 0 k 2

In eq 3, Ι ( 0 ) is the intensity of the centerband under the TOSS experi ment, and 7 (0) is the centerband intensity under MAS only. The variable / is the Bessel function of the first kind, and all the parameters in equa tion 3 were defined in reference 17. The integrations represent powder averaging over the whole spin system, the powder molecules having ran dom orientation in space with an equal probability in each unit solid angle. τ

0



16. Hu E T AL. High-Field NMR Studies ofArgonne Premium Coals

τ

1

1

1

1

319

1

IT CO)

O0o5U / 0r Figure 4. The intensity of the centerband vs. the ratio between chem ical anisotropy and spinning rate. The values of η are as follows: 0, solid line; 0.5, dashed line; 1, dotted line.

In Figure 4, plots of I (0), 7 (0), and I (0)/I (0) versus ω^δΙω , with respect to various asymmetric parameters, rç, are shown. The relative intensity of the centerband / (0) drops rapidly when the relative inverse spinning rate, ω δ/ω , increases, that is, when the spinning rate decreases. However, J (0) is always positive when ω δ/ω changes. On the other hand, 7 (0) (the centerband intensity with TOSS) decreases more slowly than J (0) does when ω δ/ω is less than 4. This means that the TOSS sequence restores to some extent the centerband intensity in comparison with the MAS spectrum. The effect has been predicted in the original arT

0

T

0

Q

0

τ

0

0

τ

T

0

0

τ


γ

320


tide (10). The value of J (0) becomes negative when ω δ/ω reaches a point between 4 and 6 with respect to the value of η. This condition makes the use of the TOSS approach very critical, especially for coal stud ies when the static magnetic field is high and the spinning rate is relatively low. In order to visualize the complicated TOSS restoration function in regard to MAS, the curves of J (0)/I (0) versus ω δ/ω are shown in Fig ure 4. The ratio increases, a result indicating that the TOSS spectral res toration of the centerband occurs only at a relatively high spinning rate (i.e., the safest region for spectral restoration by TOSS is that in which ω ί / ω is less than 3). Beyond this region when the spinning rate is lower, the ratio exhibits a funny behavior. Therefore, severe spectral distortion will appear for those samples that cover a wide range of chemical shifts. The matter is complicated even more by the dependence of the chemical shift on the asymmetric parameter. In our study, ω δ/ω is less than 4 for the aromatic portions of coal when ω = 4 kHz at 9.4-T high field, and the aromatic portions of the coal do not meet the spectral-restoration condition, but the aliphatic ones do. As a result, the apparent aromaticities obtained from the TOSS measure ments appear substantially lower than those obtained from static CP. We conclude that the ratio of ω δ/ω equal to 3 represents a criterion of magnetic-field strength and spinning rate for the quantitative use of TOSS. The requirements of a high spinning rate to alleviate the TOSS spectral distortion may create a problem because a faster spinning rate eliminates the CP mechanism to a greater extent and hence also affects the measured aromaticity. In summary, from the evidence presented in this chapter, we suggest that a combination of a static CP experiment with a high magnetic-field strength be used for aromaticity measurements of coals. The carbon-structure distribution parameters listed in Table II were also determined in a manner similar to that described by Solum et al. (9), and the symbols that they used are also presented for convenient com parison. In our C P - M A S - T O S S data extraction, the intensity ratio of the aromatic portion to the aliphatic portion was corrected in accordance with the static CP experiments. Therefore, the intense distortion from the TOSS approach in a high magnetic field was eliminated. The results listed in Table II are comparable to those of Solum et al. (9), and this fact again supports our static CP experiments in a high magnetic field. The proton aromaticities, i î , obtained from the CRAMPS spectra are comparable to the if * values (estimated by the carbon parameters with eq 2, as shown in Table III). Hence, eq 2 may provide a means to indirectly measure proton aromaticity. However, more relevant comparisons should be done with the estimation approach because of the usually tricky decomposition of the CRAMPS spectrum, as mentioned previously. T

0

T


0

0

0

τ

τ

Γ

0

τ

Γ

0

τ

a

a


16. Hu ET AL. High-Field NMR Studies of Argonne Premium Coals 321 DNP (18) has been proven to be a powerful approach for the study of coal. DNP enhances the N M R signal by irradiating at or near the electron Larmor frequency, and this aspect of DNP makes the measurements in a low magnetic field much easier. DNP measurements also provide useful information on electronic structures in solid coals. A homemade 82 MHz-54 GHz DNP spectrometer was recently developed in our laboratory. In Figure 5, the C spectra of an anthracite with an aromaticity of 0.95 are shown. With a sample volume of 0.1 mL, the C CP spectrum (bottom trace) was obtained with 26,000 scans in approximately 7.3 h with a 1-s recycling time. Because the proton signal had been enhanced by DNP, the C D N P - C P spectrum (middle trace) required only 1000 scans and approximately 14 min, and a slightly better signal-to-noise ratio was obtained. The C signal can also be directly enhanced by DNP. The C 1 3

1 3


1 3

1 3

,

f

300

1 3

1

,

200

.

,

100

1

1

0

1

1

PPM 13

Figure 5. C spectra obtained with a 1.9-T magneticfieldof an anthracite. The aliphatic portion in the spectrum was largely missed because the paramagnetic centers were mainly situated near the aromatic portions in the solid coal.


322

M A G N E T O RESONANCE OF CARBONACEOUS SOLIDS

DNP spectrum (top trace) was obtained with 12 scans in 6 min with a recycling time of 30 s. The substantial difference between the DNP and D N P - C P spectra reveals that there are more unpaired electrons located near the aromatic portions of the coal because the enhancement of the aromatic portion was much greater than that of the aliphatic portion. We are currently studying the Argonne Premium Coals with DNP.


Acknowledgment This study was supported by the National Natural Science Foundation of China. Karl S. Vorres provided the coal samples. X . Zhang helped with the preparation of this manuscript.

References 1. Schaefer, J.; Stejskal, E. O. J. Am. Chem. Soc. 1976, 98, 1031. 2. Gerstein, B. C.; Chou, C.; Pembleton, R. G.; Wilson, R. C. J. Phys. Chem. 1977, 81, 565. 3. Wind, R. Α.; Trommel, J.; Smidt, J. Fuel 1979, 58, 900. 4. Maricq, M.; Waugh J. S. J. Chem. Phys. 1979, 70, 3300. 5. Dec, S. F.; Wind, R. A.; Maciel, G. E.; Anthonio, F. E. J. Magn. Reson. 1986, 70, 355. 6. Hoult, D. I.; Richards, R. E. J. Magn. Reson. 1976, 24, 71. 7. Wind, R. Α.; Duijvestijn, M. J.; van der Lugt, C.; Smidt, J.; Vriend, J. Fuel, 1987, 66, 876. 8. Ye, C.; Wind, R. Α.; Maciel, G. E. Sci. China,Ser.A 1988, 29, 968. 9. Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. 10. Dixon, W. T. J. Chem. Phys. 1982, 77, 1800. 11. Gerstein, B. C.; Dybowski, C. R. Transient Techniques in NMR in Solid; Academic: New York, 1985. 12. Bronnimann, C. E.; Zeigler, R. C.; Maciel, G. E. J. Am. Chem. Soc. 1988, 110, 2023. 13. Snape, C. E.; Axelson, D. E; Botto, R. E; Delpuech, J. J.; Tekeley, F.; Ger stein, B. C.; Fruski, M.; Maciel, G. Ε.; Wilson, M. A. Fuel 1989, 68, 547. 14. Botto, R. E.; Axelson, D. E. Prepr. Fuel Chem. Div. Am. Chem. Soc. 1988, 33(3), 50. 15. Olejniczak, Ε. T.; Vega, S.; Griffin, R. G. J. Chem. Phys. 1984, 81, 4804. 16. Raleigh, D. P.; Olejniczak, E. T.; Griffin, R. G. J. Chem. Phys. 1988, 89, 1333. 17. Ye, C.; Sun, B.; Maciel, G. E. J. Magn. Reson. 1986, 70, 241. 18. Wind, R. Α.; Duijvestijn, M. J.; van der Lugt, C.; Manenschijn, Α.; Vriend, J. Prog.Nucl.Magn. Reson. Spectrosc. 1985, 17, 33. RECEIVED for review June 8, 1990. ACCEPTED revised manuscript December 17, 1990.


High-Field NMR Studies of Argonne Premium Coals - American

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