Advanced Magnetic Resonance Techniques Applied to Argonne

33

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch033

Advanced Magnetic Resonance Techniques Applied to Argonne Premium Coals 1

2

Bernard G. Silbernagel and Robert E. Botto 1

Corporate Research, Exxon Research and Engineering Company, Annandale, NJ 08801 Chemistry Division, Argonne National Laboratory, Argonne,IL60439 2

The application of a wide variety of magnetic resonance techniques to Argonne Premium coals provides a great deal of information about both the coal samples and the methodology being employed for their examination. Electron paramagnetic resonance (EPR) observations at high frequencies and as a function of temperature give a clearer picture of the different types of carbon radicals present in these coals. Electron spin-echo (ESE) measurements of the radical relaxationtimesreveal significant variations in coals of different ranks. Electron nuclear double resonance (ENDOR) techniques, both continuous wave and pulsed, reveal details of the interaction of the unpaired electron of the radical with the nuclei on its host molecule and adjacent molecules. Various NMR techniques have been applied to the quantitation of the percentage of aromatic carbon in the various coals. These advanced techniques, coupled with studies of coal changes with chemical (oxidation, pyrolysis, and reactions with donor and acceptor molecules) and physical (physisorption and solvent swelling) changes expand our understanding of these coals.

0065-2393/93/0229-0629$06.00/0 © 1993 American Chemical Society

In Magnetic Resonance of Carbonaceous Solids; Botto, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.


630

M A G N E T O RESONANCE OF CARBONACEOUS SOLIDS

I N T H E A R E A O F M A G N E T I C R E S O N A N C E great progress has been made, and recent advances have had consequences in the study of a common series of coal samples: the Argonne Premium coals (1). Many of the techniques discussed in this book have been developed over the past few years, but their consistent application to a common series of fossil-fuel materials has not heretofore occurred. A discussion of these new techniques and the associated theory was presented in Chapters 1-7. Here we will focus on the specific applications to the Argonne Premium coals and describe the additional knowledge that is available in principle from such applications. Magnetic resonance in coals is a rich scientific field with a long history (2). A variety of species can be observed: nuclei (particularly C and H), the unpaired electrons associated with carbon radicals in the system, and paramagnetic ions of transition metals that may be incorporated in minerals or occur as individual ions in the organic matrix of the coal. As indicated in Figure 1, N M R and E P R (electron paramagnetic resonance) studies have largely been mutually exclusive. Nuclei lying within ~10 Â of a carbon radical or paramagnetic impurity have been unobservable because of broadening and relaxation effects of the electron spin. Con1 3

X

Figure I. Schematic diagram of electron spin (large arrow) and nearby nuclear spins (small arrows) observed in coals. Nuclei lying within the exclusion radius (indicated by the circle) are not observable by NMR spectroscopy because of broadening effects from the electron spin.


33. SILBERNAGEL & Β ο ί τ ο Advanced Magnetic Resonance Techniques versely, interactions of the electron spin with nuclei (mostly protons) in their environment are responsible for the width of the E P R absorption, but the EPR spectra observed in coals are usually featureless, and a quan titative conclusion about the radical environment cannot be drawn from the result. New techniques, particularly electron spin-echo (ESE) spec troscopy, electron spin-echo modulation, and pulsed electron nuclear dou ble resonance (ENDOR) spectroscopy are providing new, quantitative information about these nearby nuclei. Electron magnetic resonance techniques differ from those of NMR in another important way. The protons and C nuclei are distributed throughout the organic matter of the coal sample, and NMR probes the molecules in the coal "democratically" when the experiments are properly performed. By contrast, only one unpaired electron is present for every 10 to 10 carbon atoms, and simple chemical considerations indicate that they will be associated with the aromatic molecules in the system.


1 3

3

4

Electron Magnetic Resonance Spectroscopy of Argonne Premium Coals In any attempt to relate coal chemistry to E P R observations, important considerations are (1) how homogeneously the radicals may be distributed in the coal and (2) how representative these radicals are of the total coal. Such questions will benefit from the availability of the widest possible amount of information. We will explore the following questions: 1.

What does electron magnetic resonance say about the molecular chemistry and the molecular structure of the coals?

2.

How do these chemical and structural properties vary with rank?

3.

How is the coal changed by the treatments to which it may be sub jected, such as oxidation, pyrolysis, chemical processing, swelling, or reactions with reagent molecules, and what role do the associated minerals play in this process?

Another implicit question concerns the effect that handling the samples before examining them may have on the observed properties.

Nature of Carbon Radicals in Coal. Regarding the nature of the paramagnetic centers in coal, are they molecules bearing a single unpaired electron, or does the E P R absorption arise from pairs of spins



632

MAGNETIC RESONANCE OF CARBONACEOUS SOLIDS

such as might be found in a donor—acceptor complex? In a donor—acceptor complex, a paramagnetic response would be expected only from complexes in which the spins were in a triplet state, because the singlet configuration is diamagnetic. As the temperature varies, the relative populations in the triplet and singlet states of the two spin systems would vary, and the variation of the intensity of the EPR signal would not follow Curie law behavior. Some workers have observed non-Curie law behavior and have postulated significant levels of donor-acceptor complexes in coals (3). However, such experiments must be done with great care because non-Curie law behavior can occur as an artifact of the experimental procedure in a number of ways. In Chapter 31, Rothenberger et al. report that the temperature dependence of the EPR intensity for the coals of all ranks, from lignite (Beulah—Zap) to low-volatile bituminous (Pocahontas No. 3) follows Curie law for temperatures from 100 to 300 K. A detailed analysis of lower temperature measurements (to ~10 K) suggests that the deviations from Curie law that do occur are associated with thermal effects such as heat transfer.

EPR Absorption Line Shapes. The E P R absorption has long been recognized to be complex. It often consists of a broad and a narrow component. The broad component is associated with macérais like vitrinite, the lignin-based component of the coal, and waxy materials like exinite or sporinite, which come from the leaves and spores of trees. The narrow component is attributed to highly metamorphosed, inertinitic materials in the coal. Because the E P R spectra accumulated at the conventional X band microwave frequency (~10 GHz) do not usually have well-defined features, line-shape analyses are difficult to do with much precision. Measurements at 35 GHz, also quite commonly available, indicate significantly more asymmetry to the line. High-frequency E P R spectrometers operating at 94 and 240 GHz, as reported by Clarkson et al. in Chapter 27, and 2 mm (~150 GHz), as reported by Tsvetkov et al. in Chapter 23, dramatically illustrate the extent of anisotropy in these lines. Such a variation can arise from two factors: (1) several lines of roughly similar shape but displaced in resonance frequency from one another (i.e., having different g values, where the g value is the E P R analog of a chemical shift of the absorption line) or (2) one or more lines that have a g-value anisotropy. One way of testing this factor is to do high-frequency measurements on individual maceral types of samples that have been isolated by density centrifugation techniques. For an Illinois No. 6 coal, examination of the isolated maceral samples demonstrates that both frequency shifts and g-value anisotropies are present for sporinite and vitrinite macérais. The data of both Chapters 23 and 27 indicate that


33. SILBERNAGEL & Βοττο Advanced Magnetic Resonance Techniques this anisotropy is most pronounced for low-rank coals (up to a carbon content of 81%), a finding that suggests a combination of heterocyclic and aromatic radical species. Pulsed E N D O R experiments, as reported by Thomann et al. in Chapter 30, also provide strong evidence for the exist ence of significant levels of heteroatom radicals in the low-rank coals.


Discriminating RadicaltypesUsing ESE Relaxation Tech niques. Differences in the relaxation properties of different radical types allow these species to be studied individually. In parallel E P R and E S E examinations, Silbernagel et al. (Chapter 29) made a systematic attempt to establish whether the properties inferred from the E S E measurements of T (the phase memory decay time) and T (the electron spin-lattice re laxation time) were consistent with the two component signals observed in the EPR. Five of the coal samples (Pocahontas No. 3, Upper Freeport, Pittsburgh No. 8, Lewiston-Stockton, and Illinois No. 6) showed both broad and narrow EPR absorptions, and spectral decomposition allows the determination of their widths and relative intensities. The broad signal was known to be inhomogeneously broadened, and spin echoes were easily observed, with the width of the spin echoes, 1/Γ * ( Γ * is the spin dephas ing time), in good agreement with the EPR line width. By contrast, the narrow line is homogeneously broadened, and no echo was formed. However the rate of decay of the free induction decay (FID) gave a value of 1/Γ * that was in excellent agreement with the observed E P R values. Detailed measurement of T for these systems indicates that the relaxation processes are complex because the results are not described by a single exponential. A portion of this nonexponentiality is associated with spectral diffusion of the magnetization during the course of the relaxation process, which is particularly significant for the higher rank coals. However, some of the effect probably is associated with a dis tribution of T values for different molecular types in the same coal. A quantitative evaluation of this effects remains to be done, with an appropriately demineralized series of samples. A direct attempt to associ ate relaxation properties with maceral types (4) is a useful exercise, but may encounter the problem that there is not a strict division in relaxation properties among radicals in macérais of different types. u

m

2

2

2

1E

1E

Electron-Nuclear Interactions Using ENDOR Spectroscopy. The broad signal seen in coals is the result of interactions between the electron spins and nuclei in the vicinity, but a profile of the magnitudes of the interactions encountered can be obtained only by using double-resonance techniques. These observations can be done either by con-


633


634

M A G N E T O RESONANCE OF CARBONACEOUS SOLIDS

ventional C W (continuous-wave) E N D O R techniques, as reported by Chen et al. in Chapter 24, or by pulsed E N D O R techniques, as reported by Thomann et al. in Chapter 30. The CW technique is dependent on the relative values of the electronic and nuclear relaxation processes and is difficult to observe in many cases. For each coal, a matrix signal, resulting from coupling of the electron spin to protons at considerable distance from the radical, was observed in CW E N D O R . By contrast, the pulsed E N D O R sees both a matrix signal and a broader signal associated with the local hyperfine interactions from protons residing on the radical molecule itself; the result is a spectrum that extends over a frequency range of ~20 MHz. This result is in contrast to the ~0.5-MHz width associated with the matrix E N D O R signals. The results of the pulsed E N D O R work are particularly revealing: The size of the matrix E N D O R contribution falls with increasing coal rank, a reflection of the decrease in proton density in these higher rank coals. Even more significant is the fact that the shape of the local E N D O R signal is nearly identical for all of the bituminous coals, a feature suggesting that major changes in the size of aromatic molecules (i.e., the number of aromatic rings) do not occur during this phase of the coalification process. This report is the first direct spectroscopic evidence of this important result. Another opportunity afforded by the pulsed double-resonance techniques is the chance to actually study the dynamics of the nuclei that reside within the exclusion radius and are therefore inaccessible to conventional N M R spectroscopy. In this case the dynamics of the nuclei are detected by their perturbation of the electron spin properties that occurs as a result of their mutual hyperfine coupling.

Coal Structure and Swelling. In addition to the individual molecules, the properties of coal and its reactivity are strongly influenced by the extent to which reagents can be introduced into the solid coal. As a first consideration, access is afforded by the network of pores in the coal. However, the coal solid can also swell to accommodate molecules within the organic structure itself. The ability of the organic matter to swell will be determined by the nature and the strength of the interactions among the individual molecules. Several factors can influence this macrostructure in the coal: covalent bonds between units, charge-transfer interactions and 7r-bonding between aromatic units, and hydrogen bonding resulting from polar interactions between heteroatomic components of the coal. These latter polar interactions can be perturbed by the introduction of polar swelling solvents, such as pyridine, in the coal. The extent of the local swelling then can be probed by introducing paramagnetic molecules of different geometries into the swollen coal and determining the number


33. SILBERNAGEL & Βοττο Advanced Magnetic Resonance Techniques of such molecules that can be incorporated. By using a variety of polar and nonpolar solvents and spin probes of different geometries, a rankdependent swelling behavior is documented, as reported by Spears et al. in Chapter 25. In low-rank coals, pores become much more cylindrical when swelled by polar solvents. In high-rank coals, where hydrogen bonding is less likely to play a role, the pore geometry is not changed by the swelling process.


Rank-Dependent Effects in Argonne Premium Coals. Be cause the Argonne Premium coal series spans a very wide range of coal ranks, it is important to determine if the E P R and E S E parameters observed are consistent with our fundamental understanding of coal chem istry. In a series of observations on isolated macérais from Pennsylvania State University (PSOC) coals of comparable range in rank, systematic variations of E P R parameters were observed (5). For increasing coal rank, that is, increasing weight percent carbon of the organic matter in the coal, g values fell (to values expected for aromatic radicals), the density of radicals generally increased, the line width of the carbon radical signal increased (the line-width increase reflects this radical density increase), and the radicals became progressively less easy to saturate by the application of higher microwave powers. E S E measurements revealed increasing values of 1 / Γ and VT as well as increased spectral diffusion with in creasing coal rank, all of which are consistent with a higher density of rad icals and an increased aromaticity in the higher rank coals (6). The present data do not always follow this trend: Wide variations are found in the measured radical density for a given coal sample, g values that are observed lie outside of the range expected for either aromatic or heterocyclic molecules, and a monotonie increase of the relaxation proper ties with increasing coal rank is not observed. Although some valid ques tions about the sample-handling history of some of the PSOC coal sam ples remain, these systematica suggest that another variable is at work in the Argonne samples that remains to be completely specified. Μ

m

Electron Magnetic Resonance and the Chemical Transfor mation of Coals. In the previous section, we restricted our discussion to the examination of the coals in the "native" state, assuming that the effects of handling are negligible and that handling the coal samples has not resulted in a change of their resonance properties. Actually, the very process of opening the sample ampules will change the samples, presum ably causing a loss of some of the water vapor that resides in the pores of the coals and by introducing molecular oxygen when the samples are exposed to air. Clarkson et al. report in Chapter 27 that the samples


635

636

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

were opened and immediately run. Tsvetkov et al. report in Chapter 23 that the samples were run immediately after opening and 1 h later. The narrow line attributed to inertinites vanished, and the broader line broad ened further after a 1-h exposure to air. These effects can be explained by the introduction of paramagnetic 0 molecules into the coal with resulting magnetic broadening (7). Silbernagel et al. (Chapter 29) prepared the samples in a dry box, which was then evacuated and back-filled with helium and sealed without any exposure to air. Rothenberger et al. (Chapter 31) transferred the sam ples in a nitrogen atmosphere and sealed them without evacuation; this approach provided the closest approximation to the "as-received" coals. Treatment can also be varied. Chen et al. (Chapter 24) immediately evacu ated and sealed a portion of the samples while another portion was exposed to air and then sealed. Buckmaster and Kudynska (Chapter 26) report that their sample was prepared as-received, dried in an inert atmos phere, and saturated with moisture. The E S E results for the samples that have been treated differently show different behavior. Parallel measurements on air-exposed and evacu ated samples (Chapter 24) in most cases do show an increase in Γ at room temperature with evacuation of the sample, but T actually falls with evacuation in two cases, from opposite ends of the rank spectrum (Pocahontas No. 3 low-volatile bituminous coal and Beulah-Zap lignite). The values of T and T (the electron spin-spin relaxation time) vary considerably from one series of measurements to the next. These ambient-temperature handling processes can have significant effects on the samples. The most detailed survey of these phenomena, reported in Chapter 26, is an E P R study where changes in the shape of the E P R line can be detected with great sensitivity by using an Argand diagram analysis. Even storing the samples in an argon atmosphere changes g values and radical densities because of water loss that occurred during storage. Oxidation occurs at temperatures as low as 50 °C, and its rate depends critically on the amount of water in the samples at the time. Some of these changes are the result of interactions with the organic matter, but some result from changes of the mineral matter and the organ ically coordinated transition metal ions present in the as-received coal. Pyrolysis of the coals, another form of chemical reaction, is briefly surveyed in Chapter 24. Treatment at 673 Κ in an evacuated tube led to a substantial increase in the radical densities of the samples and a corres ponding decrease in Γ . In several instances it also led to significant de creases in Γ , which again might be expected for increasingly aromatic, ordered hydrocarbons (6). Another series of experiments involves the reaction of the coal with donors (such as triethylphosphine oxide, TEPO) and acceptor molecules (such as iodine and tetracyanoquinone, TCNQ). A significant increase in


2

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M

2

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637 33. SILBERNAGEL & ΒοτίΌ Advanced Magnetic Resonance Techniques the carbon radical intensity occurs at higher ranks with the addition of an I acceptor. At low ranks, adding the T E P O donor decreases the radical intensity, but adding a T C N Q acceptor increases the radical intensity. These systematics suggest that the radical species in the coal are radical cations and that the electron-transfer chemistry depends on the chemical form (and perhaps the steric character) of the probe molecules.


2

Solid C NMR Spectroscopy of Argonne Premium Coals: An Interlaboratory Comparison 1 3

The subject of quantitation in solid C N M R analyses of coal and other insoluble carbonaceous materials has been the focus of intensive debate over the past decade. As in other disciplines of fuel science, the complex nature of carbonaceous fuels, together with the diversity of samples chosen for study, has made comparisons between published results diffi cult. With the inception of the Argonne Premium Coal Sample Program, it is now possible to conduct an interlaboratory study on a common suite of pristine coal samples spanning a diverse rank range. This interlabora tory study permits a valid comparison of NMR data measured with dif ferent experimental techniques and under a variety of different experi mental conditions. Table I is a compilation of carbon aromaticity values for the eight Argonne Premium coals reported in 13 independent studies. The N M R measurements were carried out at static field strengths ranging from 1.4 to 9.4 Τ (corresponding to carbon resonant frequencies of 15-100 MHz) and using a variety of methods, including cross-polarization-magic-angle spin ning (CP-MAS), single-pulse excitation (SPE) with MAS, MAS with total suppression of spinning sidebands (TOSS), and dynamic nuclear polariza tion-cross-polarization (DNP-CP) without MAS. Carbon aromaticities were determined from CP experiments employing contact times generally between 1 and 2 ms, and in certain instances, by mathematically fitting sig nal intensities derived from variable contact-time experiments. Proton decoupling field strengths were typically between 40 and 82 kHz; MAS fre quencies varied from 3 to 13 kHz. Recycle-delay times ranged from 1-3 s in CP experiments to 60 s in SPE experiments. The mean values derived for carbon aromaticities of the Argonne Premium coals and standard deviations obtained from the data in Table I are presented in Table II. The data have been analyzed according to groups that correspond to three experimental categories: mean aromatici ty values from all C P - M A S measurements, values determined by C P MAS at a static field of 2.3 Τ only, and those values derived from SPE (90° pulse-acquire-recycle delay) measurements.


638

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. Survey of Carbon Aromaticity Values for Argonne Premium Coals Laboratory BZ a

A

0.69

b

B

ILL

PITT

BC

LS

UF

0.61

0.67

0.70

0.61

0.72

0.77

c

0.65

-

-

0.59

—

d

O

_

—

0.67

C

-

0.67

—

0.67

-

-

-

0.63

POC 0.83

0.76

-

0.69

0.72

0.75

0.63

0.76

0.78

0.86

0.58

0.60

0.61

0.53

0.63

0.66

0.77

E

0.71

0.59

0.66

0.70

0.59

0.71

0.78

0.84

Ff

0.77

0.76

0.70

0.72

0.67

0.77

0.80

0.89

0.65

0.53

0.72

0.78

0.66

0.77

0.78

0.86

0.66

0.63

0.72

0.72

0.65

0.75

0.81

0.86

0.61

0.63

0.72

0.72

0.65

0.75

0.81

0.86

H

0.69

0.63

0.71

-

-

-

0.81

0.86

V

0.76

0.77

0.78

0.84

0.77

0.83

0.83

0.89

V

0.66

0.67

0.72

0.73

0.63

0.78

0.82

0.85

0.72

—

— —

— —

0.64 0.61

—

—

-

-

0.56

-

-

0.87

e


W

G * h

k

K

0.73 0.83

-

0.81 0.83

l

L M

M

0.74

0.66

0.72

0.75

0.68

0.71

0.83

0.89

0.70

0.65

0.72

0.74

0.67

0.75

0.82

0.86

0.76

0.75

0.78

0.77

0.65

0.79

0.81

—

0.65

0.63

0.70

0.72

0.63

0.69

0.77

-

ABBREVIATIONS: BZ, Beulah-Zap; W, Wyodak-Anderson; ILL, Illinois No. 6; ΡΠΤ, Pittsburgh No. 8; BC, Blind Canyon; LS, Lewiston-Stockton; UF, Upper Freeport; and POC, Pocahontas. a

Chapter 13: 4.7-T CP-MAS; 90° pulse, 3.5 μ&\ i , 2 ms; i/ , 3 kHz. ^Reference 11: 7.05-T CP-MAS (TOSS); 90° pulse, 5 /zs; * , 2-3 ms; i/ , 4 kHz. Chapter 14: 4.7-T CP-MAS; 90° pulse, 6 /*s; r^, 1 ms; z/ , 3.7-4.5 kHz. ^Chapter 16: 9.4-T CP-MAS (TOSS); 90° pulse, 4 μ&\ f , 1 ms; v 4 kHz. Chapter 17: 7.05-T CP-MAS; 90°x pulse, 5 /xs; 2 ms; 4 kHz. /Chapter 20: 7.05-T SP and CP-MAS (2.3-T CP-MAS); 90° pulse, 5 MS; t curve, i/., 14 kHz (4 Hz). c p

r

c p

r

c

r

cp

v

e

c?

^Reference 12: 2.3-T CP-MAS; 90°x pulse, 6 μ&; t

c?

^Reference 9: 4.7-T TOSS; 90°x pulse, 5 μ&; i

c p

curve, u , 4 kHz. T

, 2 ms;

4 kHz.

'Reference 13: 1.4-T DNP-CP-MAS; nonspinning. ^Reference 9: 2.3-T CP-MAS; 90°x pulse, 3.5 μ&; t ,

1.5 ms; ν , 4 kHz.

c?

τ

^Private communication (V. Pang and G. E. Maciel): 2.3-T SP and CP-MAS; t

cl

curve. 'Reference 14: 2.3-T SP and CP-MAS; 90° pulse, 4.5 ^s; t

Q?

m

Reference 15: 2.3-T SP and CP-MAS; t

cp

curve, i/., 4 kHz.

curve, i/ , 4.5 kHz. T


33. SILBERNAGEL & Βοττο Advanced Magnetic Resonance Techniques Table Π. Carbon Aromaticity for Argonne Premium Coals Coal Type


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

CP-MAS 0.85 0.80 0.75 0.63 0.72 0.71 0.62 0.69

± ± ± ± ± ± ± ±

0.01 0.02 0.02 0.03 0.03 0.02 0.04 0.05

(12) (10) (9) (11) (H) (10) (11) (12)

CP-MAS at 23 Τ 0.85 0.82 0.76 0.64 0.73 0.72 0.65 0.67

± ± ± ± ± ± ± ±

0.01 0.01 0.02 0.02 0.01 0.00 0.02 0.05

(5) (4) (4) (5) (4) (4) (4) (5)

SPE 0.86 0.81 0.76 0.66 0.75 0.73 0.74 0.76

± ± ± ± ± ± ± ±

0.05 0.02 0.04 0.02 0.03 0.04 0.03 0.03

(3) (3) (3) (4) (3) (3) (3) (4)

NOTE: All entries are mean values and standard deviations; the number of measurements is in parentheses.

For Illinois No. 6 and coals higher in rank, the mean carbon aroma ticity values that were determined for the three data sets can be con sidered the same within the limits of the standard deviations. This condi tion means that the variability in carbon aromaticities reported by dif ferent laboratories for these coals is greater than the differences observed between experimental techniques, that is, CP and SPE experiments. The mean values from SPE experiments, however, always tend to be on the high end of the range of CP values. Differences in mean carbon aromaticity values for CP and SPE exper iments are significant for the two lower rank coals, Wyodak-Anderson subbituminous and Beulah—Zap lignite samples. For these coals, the mean carbon aromaticities for SPE measurements are clearly higher, by approximately 10 and 20%, respectively. These differences may be accounted for, in part, by complications from paramagnetic transition metal ions that are intimately associated with the coal organic phase (8, 9). The presence of these paramagnetic metal species in Wyodak-Ander son and Beulah—2^ap coals adversely affects the spin—lattice relaxation properties of these coals (9). Shortening of proton Τ relaxation times via interactions with paramagnetic species clearly results in reduced crosspolarization efficiencies, and thus leads to lower derived C P - M A S aroma ticity values (see Chapter 18). Variances associated with the C P - M A S data of the two low-rank coals are the highest for the entire suite of Argonne coals. In fact, the variance in the C P - M A S data, for which there is a statistically relevant number of measurements, is generally seen to increase with decreasing coal rank. These larger variations in experimental data for the lower rank coals may be a result of different sample-handling procedures, which could affect the analytical outcome of the experiment because of the presence of different amounts of water in the samples or incidental oxidation during χ


639



640

transfer and analysis. Clearly, the lower rank coals should be most suscep tible to the effects of water removal and oxidation, because the low-rank coals contain the highest levels of both water and paramagnetic transition metal ions. Any changes in these properties due to different sample han dling would have a more profound influence on the C P - M A S measure ments for these samples. Comparing mean CP and SPE aromaticity values for Argonne coals with aromaticities obtained by other pulse techniques shows the expected trends. Carbon aromaticities obtained using the TOSS pulse sequence by laboratory H (Table II) are in good agreement, but those reported by laboratories Β and D are significantly lower than mean CP or SPE aroma ticity values. This disparity in TOSS results illustrates the severe demands that must be placed on proper implementation of the TOSS pulse sequence for quantitative analysis. On the other hand, aromaticity values obtained via D N P - C P experiments (laboratory I) tend to be high, a result that is entirely consistent with a methodology involving polarization transfer of magnetization from free radicals that reside predominantly on aromatic structures in coals. Finally, it is instructive to determine whether mean carbon aromatici ties derived from this interlaboratory exercise show rank dependencies for the Argonne coals that are consistent with our fundamental view of coal structure. Previous observations on coals spanning comparable ranges in rank consistently have shown systematic trends between N M R carbon aro maticities and coal rank as defined by carbon content or hydrogen-tocarbon ratio of the coal. The data in Table II, however, do not show definitive trends with rank. In fact, within the error limits of the analyses, CP- and SPE-derived aromaticities seem to be independent of rank through the (Beulah-Zap) lignite to (Lewiston-Stockton) high-volatile bituminous (HVB) coal range. Moreover, data for the Blind Canyon coal is anomalous and appears to correlate better with hydrogen-to-carbon ratio rather than carbon content. Only for the higher rank coals are trends between carbon aromaticities and coal rank obvious. Perhaps changes in aromaticity with rank for low-rank coals are less pronounced than had been previously thought. However, the number of samples in the Argonne suite may be insufficient statistically to allow for such an analysis of the data.

Discussion A wide variety of new magnetic resonance techniques can be successfully applied to coals. In some cases, like CW E N D O R , such a success is by no means a foregone conclusion. These experimental efforts are particularly important because they have been performed on the same series of coal


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samples, which furthermore have been treated in a very uniform manner up to the time when they were opened in the individual laboratories. Once the samples were opened, the variety of different handling procedures that were employed undoubtedly resulted in alterations of the samples from their pristine state because of the loss or addition of water or such incidental oxidation as may have occurred. Enough overlap exists in the experiments performed for comparisons to be made of what would nominally be the same measurements. Significant variations in the E S E relaxation properties are found for the same coals, some of which can be explained by the different sample-handling procedures. The same may be said of the variation in N M R carbon aromaticity values, particularly for the lower rank coals. Furthermore, variations in the EPR parameters (g value, line width, and radical density) are large enough to suggest that other factors may also be at play. As an example of this effect, the E P R radical densities reported by the various researchers are presented in Table III. Variations greater than an order of magnitude are observed, and no systematic trend is seen in the variation of the radical density in the results of a given researcher. We propose that the major reason for this variability is the presence of magnetic transition metal ions, principally iron, that are in intimate contact with the organic matter in the coal (9,10). The Argonne Premium coals are unique in the fact that they have not been subjected to any drying or oxidation processes during their collection and preparation. As a result, actual free transition metal ions, interacting with the functionalized surface of the coal pores, are still present in these samples. In particular, F e ions, which would be expected to be quite abundant in these materials (because the iron content in the coals is on the order of 0.5-5 wt% of the whole coal), should be present as single ions or multiple-ion complexes. In the presence of oxidants, these soluble complexes oligomerize and ultimately form hematite (Fe 0 ) (10), but subsequent EPR examinations demonstrate that the ions can occur at high density ( ^ l O per gram of coal) in the as-received coals (9). Citric acid washing is adequate to remove these signals from the coal and to lead to carbon radical values that are much less variable and other EPR properties that are consistent with the known variations in coal chemistry. 3 +

2

3

20

It is perhaps ironic that the variations of EPR and N M R properties that have been seen in these observations on the Argonne coals are most likely the result of the great care with which the samples have been handled in the course of their preparation.

Acknowledgments B. G . Silbernagel gratefully acknowledges stimulating discussions with the conference participants as well as with J. W. Larsen, R. A . Flowers, L . A . In Magnetic Resonance of Carbonaceous Solids; Botto, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

33. SILBERNAGEL & B o n o Advanced Magnetic Resonance Techniques

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Gebhard, G . N. George, H . Thomann, and M . L . Gorbaty. R. E . Botto gratefully acknowledges support from the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under Con tract No. W-31-109-ENG-38, and many helpful discussions with R. E . Winans, G . R. Dyrkacz, R. Hayatsu, and K. S. Vorres, as well as associa tions with several colleagues over the years: C.-Y. Choi, D. C. French, J. V. Muntean, L . M . Stock, A R. Thompson, and C.-J. Tsiao.


References 1. Vorres, K. S. EnergyFuels1990, 4, 420-426. 2. For a detailed review of the methodology and applications of EPT tech niques to coal, see Retcofsky, H. L. In Coal Structure; Gorbaty, M. L.; Ouchi, K., Eds.; Advances in Chemistry 192; American Chemical Society: Washington, DC, 1981; pp 37-58. For a detailed review on the application of NMR to coal aromaticity, see Gerstein, B. C.; Murphy, P. D.; Ryan, L. M. In Coal Structure; Meyers, R. A., Ed.; Academic: New York, 1982; pp 87-126. 3. Duber, S.; Wieckowski, A. B. Fuel 1984, 63, 1474-1475. 4. Doetschman, D. C.; Dwyer, D. W.; Mustafi, D. Presented at the Interna tional Chemical Congress of Pacific Basin Societies, Honolulu, HI, December 17-22, 1989. 5. Silbernagel, B. G.; Gebhard, L. Α.; Dyrkacz, G. R.; Bloomquist, C. A. A. Fuel 1986, 65, 558-65. 6. Thomann, H.; Silbernagel, B. G.; Jin, H.; Gebhard, L. A.; Tindall, P.; Dyr kacz, G. R. EnergyFuels1988, 2, 333-339. 7. Thomann, H.; Goldberg, I. B.; Chiu, C.; Dalton, L. R. In Magnetic Reso nance: Introduction, Advanced Topics, and Applications to Fossil Energy; Petrakis, L.; Fraissard, J. P., Eds.; NATO ASI Series C124; D. Reidel: Dor drecht, Netherlands, 1984. 8. Silbernagel, B. G.; Gebhard, L. Α.; Flowers, R. A.; Larsen, J. W. Energy Fuels 1991, 5, 561-568. 9. Axelson, D. E.; Botto, R. E. Prepr. Div. Fuel Chem. Am. Chem. Soc. 1988, 33, 50-57. 10. Cotton, F. A.; Wilkenson, G. In Advanced Organic Chemhtry: A Comprehensive Text; Interscience-Wiley: New York, 1972; pp 858-64. 11. Shi, J.-F.; Wang, Y.-S.; Qiu, L.-S.; Chen, S.-B.; Wu, X.-L.; Wu, X.-W. Presented at the International Chemical Congress of Pacific Basin Societies, Honolulu, HI, December 17-22, 1989. 12. Solum, M. S.; Pugmire, R. J.; Grant, D. M. EnergyFuels1990, 3, 187. 13. Jurkiewicz, Α.; Wind, R. Α.; Maciel, G. E. Fuel 1990, 69, 830. 14. Muntean, J. V. Ph.D. Thesis, University of Chicago, 1990. 15. Love, G. D.; Snape, C. E.; Carr, A. Proc. Int. Conf. Coal Sci. (Newcastle Upon Tyne, England), 1991; pp 64-67. RECEIVED for review September 29, 1990. November 4, 1991.

ACCEPTED revised manuscript


Advanced Magnetic Resonance Techniques Applied to Argonne

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