Measurement of SpinâLattice Relaxation in Argonne Premium Coal

18

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch018

Measurement of Spin—Lattice Relaxation in Argonne Premium Coal Samples Chihji Tsiao and Robert E . Botto

1

Chemistry Division, Argonne National Laboratory, Argonne, IL 60439 Eight Argonne Premium coals and three weathered Argonne coal samples were investigated by using Ccross-polariza tion—magic-angle spinning (CP—MAS) NMR spectroscopy. Proton and carbon spin—lattice relaxation measurements were performed on each coal. The carbon spin-lattice relaxation time, T C; the carbon spin—lattice relaxation time in the rotating frame, T C; and the proton spin—lattice relaxation time in the rotating frame, T H, are reported for aromatic and aliphatic carbons of the 11 coal samples. In general, the proton and carbon spin—lattice relaxation data can be evaluated as the sum of two exponential decays. The longer components of carbon relaxation times in the labora tory and rotating reference frames vary in a systematic way with coal rank as expressed by percent carbon. The trends can be explained in terms of motional properties of the coals and the presence of paramagnetic species. Marked changes in the relaxation parameters have been observed between pristine and weathered coals. Reduction in proton T values upon weathering is shown to have an adverse effect on quantitation with CP. 13

1

1ρ

1ρ

1ρ

1

Corresponding author 0065-2393/93/0229-0341$06.00/0 © 1993 American Chemical Society

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


342

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

THE COMPLEX MOLECULAR DYNAMICS in synthetic polymers has been studied extensively with N M R spectroscopy during the past decade (1—5). However, such investigations of coals have been limited. Work in this area has been hampered by the complex nature of coal. Interpreta tion of nuclear relaxation, which underlies the study of molecular motion, is complicated by the existence of discrete heterogeneous domains in coals and by relatively large quantities of paramagnetic centers that are present either in the form of paramagnetic inorganic ions or as organic free radi cals. To date, an overwhelming majority of papers dealing with relaxation measurements on coals has focused on the study of * H N M R spin-lattice relaxation times both in laboratory ( Γ ) and rotating (T ) reference frames. Early * H N M R measurements (6-9) indicated that proton 7ys (7\ ) in coals can display either single exponential or nonexponential relaxation behavior. The observation by Yokono et al. (8) that Γ varied linearly with the square root of resonance frequency for several bitumi nous coals was consistent with diffusion-limited relaxation to paramagnetic centers. Webster and Lynch (10), Ripmeester et al. (11), and Wind et al. (12) later demonstrated similar behavior for a wide variety of evacuated coal samples. The work indicated that spin—lattice relaxation may be a fundamental property of coals themselves and independent of either their oxygen or moisture content. In two specific papers (12, 13) the authors independently proposed that proton relaxation in bituminous coals was influenced by differences in molecular mobilities rather than by the rela tive concentrations of paramagnetic species. A difficulty with the interpretation of spin—lattice relaxation of abun dant proton spins in coals rests with the ability to separate molecularmobility contributions from magnetization spin diffusion to paramagnetic centers within different phase (maceral) boundaries. Two thorough inves tigations of H N M R spin-lattice relaxation in coals by Barton and Lynch (14) and Wind et al. (15) attempted to address this issue. The authors independently concluded that simple correlations of proton relaxation with other coal parameters are not easily realized. The presence of resid ual amounts of molecular oxygen in evacuated samples and the effects of different concentrations of unpaired electrons within different domains in coals were thought to have a profound, yet undeterminable, influence on the relaxation times. Solum et al. (16) reported proton relaxation data for eight Argonne Premium coals and three oxidized coals and showed that oxidized samples have shorter Γ values. The problems are not confined to Γ measurements. Dudley and Fyfe (17) discussed proton T values for a pitch and three Canadian coals, and they emphasized the effects of paramagnetics, including oxygen, on their experimental results. Earlier studies performed in our laboratory indicated a general trend, but a lack of any definitive correlation, between χ

lp

H

Η

χ

a

Η

χ

Η

χ

lp


18.

343

TSIAO & BOTTO Measurement of Spin-Lattice Relaxation H

T values and radical concentrations for a series of "more homogene ous" maceral concentrates (IS). We concluded that Γ ^ values in macérais are clearly complicated by having two competing mechanisms for relaxation, molecular motion and heterogeneous spin diffusion to paramagnetic centers, and without any knowledge of the relative contributions of each to the overall relaxation times in the individual samples, any meaningful interpretation of the data would be impossible. For a homogeneous resinite sample doped with increasing amounts of a stable organic radical, however, an excellent correlation between Γ and radical con centration could be established (19). In contrast with proton relaxation, relatively little work has been done on the measurement of Ç relaxation times in coals. Sullivan and Maciel (20, 21) studied C spin—lattice relaxation in a bituminous coal from the Powhatan No. 5 mine. They found that the aromatic resonances in this sample decayed with a single time constant, and the aliphatic resonances could be separated into two distinct regions that had markedly different time constants for decay. Differences observed in the J values for different spectral regions were interpreted in terms of molecular motion, although the contribution of free radicals to spin relaxation is important as well. Botto and Axelson (22) investigated the effect of static-field strength on the T relaxation parameters of five Argonne Premium coals and five Canadian coals and correlated the relaxation times with various coal properties. In this chapter, we present the first comprehensive study of laboratory- and rotating-frame C and H spin-lattice relaxation times (Tfi T and Γ ) in coals. This study emphasized the suite of Argonne Premium coals in their pristine state and three intentionally weathered samples. Details of the investigation were directed to the issue of establishing a relationship between the relaxation data and structural and motional properties of the various coals. lp

Η

Η

1 / ?


1 3

1 3

c

1

c

1

1 3

c

X

Η

lp y

1 ρ

Experimental Details All of the coal samples were obtained from the Argonne Premium Coal Sample (APCS) Program. Table I presents the origin, rank, and elemental composition (weight percent) of the coals. In order to avoid exposure to oxygen, the coals were dried and transferred into sealed NMR rotors in a nitrogen-filled glove box. Weathered samples of APCS Nos. 2, 3, and 8 were prepared by exposing the coals to the atmosphere at ambient temperature for several months. Solid-state C cross-polarization-magic-angle spinning (CP-MAS) spectra were recorded at 2.3 Τ (25.18 MHz for C) with a Bruker CXP-100 spectrometer in the pulse Fourier transform mode with quadrature phase detection. The ceramic sample spinners had an internal volume of 250 #L and were spun at a 1 3

13


344

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

Table I. Argonne Premium Coal Samples and Their Compositions APCS No.

1


2 3 4 5 6 7 8

Coal

Rank

C

H

Ο

Upper Freeport Wyodak-Anderson Illinois No. 6 Pittsburgh No. 8 Pocahontas No. 3 Blind Canyon Lewiston—Stockton Beulah-Zap

MVB SB HVB HVB LVB HVB HVB L

85.50 75.01 77.67 83.20 91.05 80.69 82.58 72.94

4.70 5.35 5.00 5.32 4.44 5.76 5.25 4.83

7.51 18.02 13.51 8.83 2.47 11.58 9.83 20.34

^org 0.74 0.47 2.38 0.89 0.50 0.37 0.65 0.70

NOTE: The values for C, H, O, and organic S are given as weight percents. ABBREVIATIONS : LVB, low-volatile bituminous; MVB, medium-volatile bituminous; HVB, high-volatile bituminous; SB, subbituminous; and L, lignite. SOURCE: Reproducedfromreference 23. Copyright 1990. rate of 4 kHz. Each spectrum used for Τ measurement was a total accumulation of 400-1000 transients with a recycle delay of 2 s. The C chemical shifts were referenced to tetramethylsilane (TMS) by using tetrakis(trimethylsilyl)silane (TKS) as the secondary reference (24). The C spin-lattice relaxation times were measured with CP-MAS by using the Ύ pulse sequence described previously by Torchia (25). The pulse sequences used for 7 ^ and Γ measurements are illustrated in Figure 1 (26-28). In both cases, the protons were allowed to come to thermal equilibrium with the lattice prior to spin-locking along the y axis in the rotatingframeby using a 90° pulse from the radio frequency (rf) field (56 kHz). In the J experi ment shown in Figure la, the proton rffieldwas turned off immediately following a matched Hartmann-Hahn generation of a carbon polarization. During the variable delay period, free induction decays (FIDs) representing carbon magneti zation held in the rotatingframewere acquired and Fourier transformed. In the Γ experiment shown in Figure lb, the proton magnetization was spin-locked during a variable delay period prior to the contact period during which carbon polarization was established. In this case, variation in carbon magnetization was used to monitor the decay of proton magnetization held in the rotatingframeas a function of delay time. χ

1 3

1 3

χ

c

Η

1 ρ

c

l p

Η

1 ρ

Results and Discussion Carbon Γ- Relaxation. For carbonaceous solids placed in a static magnetic field, the alignment of carbon magnetic moments gives rise to a net macroscopic magnetization M whose value is M when the nut

Q


18.

T s i A O & Β OTTO

1 Η

Measurement of Spin-Lattice Relaxation

345

(90)x Decouple Spin Lock

Contact


13

Time Acquire FID

IVariable I Delay J

(b)

Η

(90)x

Variable Delay

Acquire FID

(a) c

Figure 1. Pulse sequences for (a) T

Jp

1

and (b) Tjf measurements.

clear moments and the molecular lattice are in thermal equilibrium. A characteristic time constant, or carbon spin-lattice relaxation time ( J ^ ) , governs the restoration of magnetization M for individual carbon isochromats within the sample when the equilibrium condition is perturbed by an external rf field. 0

Q


346


Recovery of magnetization Μ with time r for the carbons in coal samples appears to be more complex. Integrated signal intensities for aromatic and aliphatic resonances were fitted with a Simplix algorithm (29) to the sum of two exponential decays by the following equation: χ

M /M t

0

= A

exp ( - r / T

L

1>L

) + A

s

exp ( - r / r

) + A

(1)

c

where A and A are the fractional amplitudes for long (T ) and short ( J ) relaxation time constants, respectively, and A is the amplitudecorrection factor. These parameters are extracted from the data by means of a nonlinear least-squares fitting procedure, which is applied in the fol lowing manner. Initially, an exponential fit is made to the more slowly decaying part of the data. With T held constant in eq 1, a second exponential decay is fit by using the entire data set. Parameters obtained from the individual fits are then optimized by allowing the initial values obtained for Τ , Τ , A and A to vary such that the sum of the squared deviations between data points and the fitted curve is minimized. Relaxation times determined in this manner are estimated to be accurate to within ±15%. The C spin-lattice relaxation times and fractional amplitudes for the aromatic and aliphatic resonances in Argonne coals and weathered Argonne coal samples are summarized in Table II. The weighted-average spin-lattice relaxation time, r , where L

s

tL

1 S


lfS

c

1L

χL

χs

u

s

1 3

w

x

w

(r ) "

1

1

= A^T^y

1

l

+ A (T r s

(2)

liS

is defined to characterize the average relaxation behavior. The calculated T values are also presented in Table II. The relaxation times of the aromatic carbons are consistently longer than the relaxation times of the corresponding aliphatic carbons in a given sample, with the exception of Beulah-Zap lignite (APCS No. 8). Values for the aromatic carbons fall within two distinct ranges: 1.8-20.9 s for the long T components and w

x

c

t

0.3—8.9 s for the short r

c x

components. c

Ranges for the corresponding two components of aliphatic T

t

values

are 1.0-4.6 s and 0.1-0.6 s, respectively. The shorter relaxation times of the aliphatic carbons compared with aromatic carbons may reflect their greater molecular mobility. The T values of the three weathered coals are shorter as a result of paramagnetic molecular oxygen that is intro duced. The values rejported here are similar to those reported previously (22). The higher T * values observed for some coals, in particular the Upper Freeport sample, may be the result of careful handling of the fresh coal samples. c

t

t


18.


TSIAO & B O T T O

347

w

Table II. Carbon T and 7 \ Relaxation Times of Aromatic and Aliphatic Resonances in Coals t

Aromatic Resonances


APCS No.

Long Tj Short T Component Component W 2

T

Aliphatic Resonances Long Ύ Short Tj Component Component τ i w 2

A

1

20.9 (49.1%)

2.8 (50.9%)

4.9

2.7 (52.1%)

0.5 (47.9%)

0.9

2

2.9 (44.7%) 2.0 (62.5%)

0.6 (53.3%) 0.5 (37.5%)

1.0

1.3 (34.6%) 0.8 (45.9%)

0.3 (65.4%) 0.3 (54.1%)

0.4

3

8.9 (66.0%) 4.6 (65.9%)

3.0 (34.0%) 1.0 (34.1%)

5.3

2.6 (100%) 1.4

2.6

4

20.0 (17.2%)

8.9 (82.8%)

9.9

4.2 (100%)

— —

5

8.9 (45.3%)

1.7 (54.7%)

2.7

1.0 (48.6%)

0.6 (51.4%)

0.7

6

20.5 (38.3%)

1.8 (61.7%)

2.7

4.6 (35.8%)

0.3 (64.2%)

0.4

7

11.4 (80.3%)

1.6 (19.7%)

5.2

1.3 (68.9%)

0.2 (31.1%)

0.5

8

1.8 (41.6%) 1.1 (35.9%)

0.3 (58.4%) 0.7 (64.1%)

0.4

1.8 (51,4%) 1.8 (41.0%)

0.1 (48.6%) 0.1 (59.0%)

0.2

4.2

NOTE: Relaxation times are given in seconds, and the fractional amplitudes are in parentheses. Where two values are listed, the second value is that of the weathered coal sample.

c

Plots of long T components of aromatic and aliphatic carbons versus coal rank as indicated by percent carbon (see Figures 2 and 3) show an initial increase in J up to a maximum value of approximately 81-85% carbon, followed by a decrease. A regular trend is observed for the suite of coals, with the exception of the Lewiston-Stockton sample, whose T values typically lie off the correlation lines by a wide margin. The deviation may be due, at least in part, to the high contents of inertin ite and liptinite macérais present in the Lewiston-Stockton coal compared with other Argonne coals, which have high vitrinite contents (>85%). Figure 4 shows the plots of the weighted-average carbon spin—lattice relaxation times ( ^ ) as a function of coal rank. Although significantly greater scatter is observed in these plots, the trends with rank are similar to those found for long J components. The observation of multicomponent magnetization decays in macromolecular systems has generally been attributed to a distribution of relaxation times. Such T distributions are common in the C spectra of solids (27, 30). The distributions arise because of different spatial orientations of carbon-hydrogen dipoles in a solid relative to the static magnetic field, and they are observable as a result of the inherent isolation of the rare carbon spins. In the case of coals, two additional factors may influ1

c

1

c

t

w

c

1

c

1 3

1

American Chemical Society Library 1155 16th St. N.W. Botto and Sanada; Magnetic Resonance of Carbonaceous Solids Advances in Chemistry; American Chemical Society: Washington, DC, 1992. Washington. DC 20036

348

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


25-,

0+70

1

1

1

1

1

75

SO

85

90

95

C o a l Rank (2C) c

Figure 2. Plots of the long T components vs. coal rank for aromatic resonances of coals: ·, pristine samples; Π, weathered samples. i

ence this distribution as well. Variations in the levels of paramagnetic species and maceral compositions may affect the 7^° distributions in an unpredictable manner. No obvious trends are observed for the variation in relative amplitudes of the long and short T components with rank, contrary to earlier observations (22). Furthermore, the shorter T com ponents of both aromatic and aliphatic carbons generally show a poorer correlation with rank, leading to the large scatter found when the values of T are plotted against carbon content for the coals. This poor corre lation would suggest that factors such as unpaired electrons are manifest in shorter Γ components to a greater degree and that these factors could mask a real influence of molecular mobility. Conversely, the long Τ com ponents are less affected by paramagnetic species, and hence provide better estimates of molecular structure. Previous variable-field carbon relaxation experiments (22) on coals spanning a wider range of carbon contents than those investigated here support the foregoing conclusions. The ratios of the long 7^ values at c

1

c

1

W

2

€

χ

χ


18.

TSIAO & B O T T O


349


Si

y-i 70

1

1

1

1

1

75

80

85

90

95

C o a l Rank CAO C

Figure 3. Plots of the long T components vs. coal rank for ali phatic resonances of coals: ·, pristine samples; Π, weathered sam ples. 2

4.7 and 2.3 Τ for subbituminous through low-volatile bituminous coals are on the order expected (1.7-5.1) for a square dependence on field, as in the case of motional effects contributing to the relaxation processes. No such dependence, however, could be discerned for the short components of the decays or for T values of two lignite coal samples whose relaxation times were clearly dominated by the presence of high concentra tions of paramagnetic metal ions chelated to oxygen functionalities. Distinct maxima seen in the plots of the more slowly relaxing T components and T values against rank show a parallel with several other methods of analysis. Proton spin-lattice relaxation times measured independently by Barton and Lynch (14), Wind et al. (15), and Yokono and Sanada (31) for coals varying widely in origin show evidence of a broad maximum in the 85-90% carbon range. The carbon and proton relaxation data are consistent with established relationships between coal structural properties and the degree of maturation and reflect the major changes that occur during coal evolution. c

1

c

1

w

x



Coal is a cross-linked, three-dimensional macromolecular network. Data from solvent swelling of coal (32, 33) and stress-strain measurements (34, 35) have been used to obtain information about the elastic modulus of the macromolecular network. This modulus can then be used to calculate the average molecular weight between cross-links, M , a parameter that can be directly correlated with molecular mobility of the network. The shape of the curves of elastic modulus (34) or solvent swelling (32) plotted against coal rank resembles those plots obtained from data. The elastic constants show similar behavior to T as a function of rank with a relative maximum at 85% carbon and a relative minimum around 90% carbon. At greater values than 95% carbon the modulus rises sharply, a result that is consistent with the development of more graphitelike structures. A higher value of the modulus thus corresponds to a maximum in the cross-link density of the coals, and the results are in good agreement with the T results, which indicate that the lowest molecular mobility occurs at the highest cross-link density. Apparently, the c

c

t

c

1


18. TsiAO &

BOTTO


351


cross-link density increases with increasing coal rank up to the mediumvolatile bituminous (MVB) stage and subsequently decreases through the low-volatile bituminous (LVB) stage until rapid graphitization occurs at higher rank.

Carbon T~ Relaxation. In their pioneering work on glassy poly mers, Schaefer et al. (27) pointed out the utility of Τλ measurements for probing molecular dynamics as a potential source of information on the mechanical and physical properties of polymers on the molecular level. Later, VanderHart and Garroway (36) outlined the complications that are involved in extracting molecular-motion information from T experi ments. Both spin-spin and spin-lattice relaxation processes are known to contribute to carbon T . For highly crystalline materials, spin-spin processes (mutual spin flips between spin-locked protons and carbons) are usually dominant, and the T values do not adequately describe the molecular dynamics of the system. For most glassy and amorphous materials, however, in which strong H - H static dipolar interactions are negligible because of molecular motion, large interproton distances, or some combination of these, T is largely spin-lattice in character and can be interpreted in terms of localized segmental motions in the tens of kilohertzes frequency range (37). The T values of aromatic and aliphatic carbons in Argonne Premi um coals and three weathered Argonne coals are presented in Table III. The data were evaluated by using the two-component fitting procedure that was described previously. For aromatic carbons, T values range from 75 to 155 ms for the longer components and from 15 to 105 ms for the shorter components. The corresponding values for the aliphatic car bons are considerably shorter: 22-52 ms for the long components and 9-34 ms for the short components. T values of weathered versus pris tine samples show some interesting trends: relaxation times of the aromatic carbons in weathered coals are longer by a factor generally greater than 2, and those of the aliphatic carbons are approximately the same. Clear deviations from ideal exponential behavior of the magnetiza tion decay are observed in most cases. Nonexponential behavior may be interpreted as distributions of relaxation times due to the heterogeneity of the local environment. Plots of the aromatic and aliphatic T values (long components) against coal rank are shown in Figures 5 and 6. The general shapes of the curves resemble those obtained previously for the J values, with one notable difference. The T values increase with increasing rank until 80% carbon content and then remain constant thereafter; no decrease is seen for the higher rank coals and hence no definitive maxima are observed. lp

lp

c

lp

1

1

lp

lp

c

lp

c

lp

c

lp

C

1

c

lp


352


Table III. Carbon Τχ Relaxation Times of Aromatic and Aliphatic Resonances in Coals


Aromatic Resonances

Aliphatic Resonances

APCS No.

LongT Component

Short Ί Component

1

151 (55.3%)

69 (44.7%)

48

34

2

76 (75.2%) 172 (69.9%)

19 (24.8%) 24 (30.1%)

25 24

17

3

100 (64.3%) 187 (84.2%)

88 (35.7%) 99 (15.8%)

33 30

28

4

150 (60.8%)

106 (39.2%)

44

28

5

141 (50.7%)

66 (49.3%)

52

20

6

156 (25.0%)

61 (75.0%)

50 (19.7%)

20 (80.3%)

7

125 (61.7%)

46 (38.3%)

50 (52.7%)

16 (47.3%)

8

80 (64.1%) 150 (61.9%)

15 (35.9%) 51 (38.1%)

22 (31.2%) 23

9 (68.8%)

lp

Short T Component

Long T Component

1ρ

lp

lp

— —

—

N O T E : Relaxation times are given in milliseconds, and fractional amplitudes are in parentheses. Where two values are listed, the second value is that of the weathered coal sample. Values of aliphatic resonances reported for APCS Nos. 1-5 correspond to single exponential decay constants for methylene (~30 ppm) and methyl (~20 ppm) resonances, respectively.

c

As discussed previously, the interpretation of T data can be com plicated by the fact that spin-spin cross-relaxation processes as well as rotating-frame spin—lattice processes may contribute to the relaxation. Evidence to support the dominance of spin—lattice contributions to T is given in Table IV, which shows the dependence of T as a function of the rotating-frame field, B for a weathered Illinois No. 6 coal (APCS No. 3). A square dependence of T on the Β field occurs for the long com ponents of aromatic carbons and the aliphatic carbons in the coal, as would be expected in the case of the domination of T processes by molecular motion. If in fact spin-spin interactions were to dominate the relaxation, then an exponential dependence on the B field would be expected (38). The foregoing arguments reinforce the notion that changes observed in T for the coals provide insight into differences in macromolecular chain dynamics within the local environment of individual carbons. Inlp

c

lp

c

lp

v

c

lp

χ

lp

1

c

lp


18.


TSIAO & B O T T O

353


200 π

^ 1

70

1

1

1

1

!

75

80

85

90

95

C o a l Rank CdO c

Figure 5. Plots of the long T components vs. coal rank for aromatic resonances of coals: ·, pristine samples; weathered samples. lfi

creases in the relaxation times as a function of rank reflect decreases in molecular mobilities of the coal macromolecular networks. Changes observed upon coal weathering suggest decreases in molecular mobility as well. Moreover, weathering effects seem to be confined to the aromatic structures in coal, where large decreases in molecular mobility are observed. Oxidation of aromatic structures enhances their electron donor-acceptor properties. The increase in strength of the noncovalent interactions within the macromolecular network might, in fact, explain loss of mobility of aromatic structures with weathering. Negligible changes in T of the aliphatic carbons suggest that chain motions of the aliphatic structures are largely unaffected by weathering. Differences in J and Ί ~ values and their trends with rank and weathering are informative. The magnitudes of these two characterizations of mobility reflect, among other things, differences in the time scales for segmental motion within the macromolecular network. We propose that the decreases in T values observed with weathering and in the higher c

lfi

c

1

χ

c

1



c

Table IV. Dependence of T on B Field for Weathered Illinois No. 6 Coal (APCS No. 3) %p

%

Aromatic Carbon Component Long

Short

Aliphatic Carbon

67

264

101

41

56

187

99

29

45

133

86

22

B

7 (kHz)


355

18. TsiAO & Βοττο Measurement of Spin-Lattice Relaxation

rank coals can be related to the presence of paramagnetic species. In weathered coals, introduction of molecular oxygen and its rapid translational diffusion throughout the solid matrix provides an effective pathway for relaxation. In coals of higher rank, the development of free radicals in large polycyclic aromatic arrays would extend the effects of free radicals over larger distances and thus facilitate rapid relaxation of a greater frac tion of carbons. Dissimilar behavior of the two relaxation time constants emphasizes that 7^° is complicated by having two competing mechanisms for relaxation, and in the absence of a priori knowledge of the relative contributions from motional and paramagnetic processes, T is apparently less reliable as a parameter for molecular structure. c


t

Proton Relaxation. Because proton T relaxation times were measured indirecfly via the decay of C magnetization, only those protons affecting the observable carbon magnetization are actually measured. The values of aromatic and aliphatic carbons in the Argonne Premium coals and three weathered Argonne coal samples are presented in Table V. The decay of magnetization of the aromatic carbons is nonexponential, x

1 3

H

Table V. Proton T Relaxation Times of Aromatic and Aliphatic Resonances in Coals t

Methylene Resonances (~30ppm)

Methyl Resonances (~20ppm)

APCS No.

LongT Component

Short T Component

1

15 (37.3%)

4.4 (62.3%)

5.8

7.2

2

19 (27.9%) 5.0

2.3 (72.1%)

6.1 6.2

— —

3

19 (44.7%) 4.1

2.7 (55.3%)

5.5 5.1

4.9 —

4

15 (34.4%)

4.3 (65.6%)

6.4

5.7

5

16 (29.0%)

3.4 (71.0%)

3.3

4.5

6

22 (19.0%)

2.9 (81.0%)

5.6

2.9

7

20 (38.3%)

1.4 (61.7%)

2.0

8

3.7 4.0

lp

lf)

3.0 2.8

—

— —

N O T E : Relaxation times are given in milliseconds, and fractional amplitudes are in parentheses. Where two values are listed, the second value is that of the weathered coal sample.



356


I

1

1

1

1

250

I

1

1

1

I

1

200

1

1

1

150

'

I

1

1

1

1

100

I

1

1

1

1

I

50

1

1

1

1

0

I

-50

Chemical Shift, ppm 13

Figure 7. C CP—MAS spectra of pristine (top) and weathered (bottom) samples of Illinois No. 6 coal (APCS No. 3).

and the proton Τ values fall into two distinct ranges: 15-22 ms for the long components and 1.4-4.4 ms for the short components, with the exception of the Beulah-Zap lignite, which has an extremely short 7^ value of 3.7 ms. The fact that the lignite coal exhibits a significantly shorter 7 ^ than the other coals implies that paramagnetic species play a major role. Previous studies (22) have indicated that significant amounts (>10 spins/g) of F e species are chelated to oxygen-rich aromatic struc tures in this coal. Relaxation of the aliphatic resonances appears to follow exponential behavior. Aliphatic 7^ values are markedly shorter than those of the aromatic carbons: 2.0—6.1 ms for the methylene region and 2.9-7.2 for the methyl region. Apparently there is no correlation between either the aromatic or aliphatic Τ values and rank. Weathering of the Wyodak-Anderson subbituminous and Illinois No. 6 bituminous coals causes substantial reduction of aromatic 7^ values. Presumably the effect is due to the introduction of paramagnetic oxygen or to the creation of paramagnetic sites in the sample during the weather ing process. However, the aliphatic 7^ values are largely unaffected by weathering. χ

H

H

19

3 +

H

χ

H

H


18. TSIAO & Β OTTO Measurement of Spin—Lattice Relaxation

357

More importantly, weathering of the samples appears to have an adverse effect on the analytical outcome of the CP experiment, as indi cated by the spectra shown in Figure 7. A reduction in carbon aromaticity (jf ) of 0.05 units is observed in the spectrum of the weathered coal sam ple, corresponding to a decrease in aromaticity of about 7%. Closer inspection of the two spectra reveals a narrowing of the aromatic reso nance band for the weathered coal with a concomitant loss of signal inten sity in the shoulders appearing at 145 and 155 ppm. This change corresponds to the specific loss of carbon resonances for the more slowly cross-polarizing C-substituted and O-substituted aromatic carbons in the weathered sample brought about as a result of the large decrease in aromatic 7^ values. The foregoing results point to the need for careful handling of coal samples prior to N M R analysis. Reduction in 7 ^ values as a result of weathering may cause intensity distortions in C P - M A S spectra, a result leading to an underestimation of carbon aromaticity values. Indeed, weathering effects as a result of poor sample handling may be responsible for the discrepancies in the literature regarding carbon aromaticities of similar coals. a


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Acknowledgments This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under Contract No. W-31-109-ENG-38. The authors express apprecia tion to Karl S. Vorres for providing the Argonne Premium Coal Samples.

References 1. Slichter, W. P. NMR: Basic Princ. Prog. 1971, 4, 209. 2. McCall, D. W. Acc. Chem. Res. 1971, 4, 223. 3. Schaefer, J. Topics in Carbon-13 NMR Spectroscopy; Levy, G. C., Ed.; Wiley: New York, 1974; Vol. 1, p 149. 4. Schaefer, J.; Stejskal, E. O. Topics in Carbon-13 NMR Spectroscopy; Levey, G. C., Ed.; Wiley: New York, 1979; Vol. 3, p 284. 5. Bovey, F. Α.; Jelinski, L. W. J. Phys. Chem. 1985, 89, 571. 6. Retcofsky, H. L.; Friedel, R. A. Fuel 1968, 47, 391. 7. Gerstein, B. C.; Chow, C.; Pembleton, R. G.; Wilson, R. C. J. Phys. Chem. 1977, 81, 565. 8. Yokono, T.; Miyazawa, K.; Sanada, Y.; Marsh, H. Fuel 1979, 58, 896. 9. Lynch, L. J.; Webster, D. S. J. Magn. Reson. 1980, 40, 259. 10. Webster, D. S.; Lynch, L. J. Fuel 1981, 60, 549.


358 11. 12.


13.

14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

29. 30. 31. 32. 33. 34. 35. 36. 37. 38.

M A G N E T O RESONANCE OF CARBONACEOUS SOLIDS

Ripmeester, J. Α.; Couture, C.; MacPhee, J. Α., Nandi, Β. N. Fuel 1984, 63, 522. Wind, R. Α.; Duijvestijn, M. J.; van der Lugt, C.; Smidt, J.; Vriend, H. Fuel 1987, 66, 876. Sullivan, M. J.; Szeverenyi, Ν. M.; Maciel, G. E.; Petrakis, L.; Grandy, D. W. In Magnetic Resonance. Introduction, Advanced Topics and Applications to Fossil Energy; Petrakis, L., Fraissard, J. P., Eds.; NATO ASI Series C124; Reidel: Dordrecht, Netherlands, 1984;p607. Barton, W. Α.; Lynch, L. J. Energy Fuels 1989, 3, 402. Wind, R. Α.; Jurkiewicz, A.; Maciel, G. E. Fuel 1989, 68, 1189. Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1983, 3, 187. Dudley, R. L.; Fyfe, C. A. Fuel, 1982, 61, 651. Botto, R. E.; Wilson, R.; Winans, R. E. Energy Fuels 1987, 7, 173. Snape, C. E.; Axelson, D. E.; Botto, R. E.; Delpuech, J. J.; Tekely, P.; Ger stein, B. C.; Pruski, M.; Maciel, G. E.; Wilson, M. E. Fuel 1989, 68, 547. Sullivan, M. J.; Maciel, G. E. Anal. Chem. 1982, 54, 1606. Sullivan, M. J.; Maciel, G. E. Anal. Chem. 1982, 54, 1615. Botto, R. E.; Axelson, D. E. Prepr. Am. Chem. Soc. Div. Fuel Chem. 1988, 33(3), pp 50-57. Vorres, K. S. Energy Fuels 1990, 4, 420. Muntean, J. V.; Stock, L. M.; Botto, R. E. J. Magn. Reson. 1988, 76, 540. Torchia, D. A. J. Magn. Reson. 1978, 30, 613. Alla, M.; Lippmaa, E. Chem. Phys. Lett. 1976, 37(2), 260. Schaefer, J.; Stejskal, E . O.; Steger, T. R.; Sefcik, M. D.; McKay, R. A. Macromolecules 1980, 13, 1121. Jelinski, L. M.; Melchior, M. T. In NMR Spectroscopy Techniques; Dybowski, C. R.; Licher, R. L., Eds.; Marcel Dekker: New York and Basel, 1987; pp 311-334. Noggle, J. H. Physical Chemistry on a Microcomputer; Little Brown: Boston, MA, 1985. Gibby, M. G.; Pines, A.; Waugh, J. S. Chem. Phys. Lett. 1972, 16, 296. Yokono, T.; Sanada, Y. Fuel 1978, 57, 334. Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structure; Meyers, R. Α., Ed.; Academic: New York, 1982; pp 199-282. Sanada, Y.; Honda, H. Fuel 1966, 45, 295. Schuyer, J.; Djkstra, H.; van Krevelen, D. W. Fuel 1954, 33, 409. van Krevelen, D. W.; Chermin, H. A. G.; Schuyer, J. Fuel 1959, 38, 438. VanderHart, D. L.; Garroway, A. N. J. Chem. Phys. 1979, 71, 2773. Hester, R. K.; Ackerman, J. L.; Neff, B. L.; Waugh, J. S. Phys. Rev. Lett. 1976, 36, 1081. Fleming, W. W.; Lyerla, J. R.; Yannoni, C. S. In NMR and Macromolecules: Sequence, Dynamic, and Domain Structure; Randall, J. C., Jr., Ed.; ACS Symposium Series 247; American Chemical Society: Washington, DC, 1984; pp 83-94.

RECEIVED for review June 8, 1990. ACCEPTED revised manuscript January 30, 1991.


Measurement of SpinâLattice Relaxation in Argonne Premium Coal

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Measurement of SpinâLattice Relaxation in Argonne Premium Coal