An investigation of coal by means of e.s.r., 1H n.m.r., 13C n.m.r. and dynamic nuclear polarization

An investigation of coal by means of e.s.r., ‘H n.m.r., 13C n.m.r. and dynamic nuclear polarization” Robert A. Windt, Michael J. DuijvestijnS, Jaap SmidtS and Han Vriend

Cees van der Lugt,

tDepartment of Chemistry, Colorado State University, Fort Collins, CO 80523, USA Department of Applied Physics, Delft University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands (Received 18 October 7986)

Sixty coal samples of different rank and origin have been investigated by means of e.s.r., ‘H n.m.r. and ’ 3C n.m.r., the last two in combination with dynamic nuclear polarization (DNP). The following parameters have been determined: the number of free radicals, the e.s.r. linewidth, the ‘H Zeeman relaxation rate, the ‘H relaxation rate in the rotating frame, the ‘H DNP enhancement, the ’ 3C DNP enhancement, the ’ 3C Zeeman relaxation rate and the r % aromaticity, observed via ‘H-’ ‘C cross-polarization (CP), both with and without magic-angle spinning (MAS). The relations between these parameters and coal rank have been investigated. Moreover, with DNP special experiments have been performed which provide information about the localization and the mobility of the unpaired electrons present in these coals. Finally, DNP has been used to investigate various features of the quantitative analysis of coal via 13C n.m.r. MAS was found to reduce the measured 13C aromaticity, and for three coals it was shown that even without MAS only = 50% of the aromatic ‘?C nuclei are detected by the CP technique. (Keywords: analysis of coal; e.s.r. spectroscopy; n.m.r. spectroscopy; dynamic nuclear polarization)

In this paper results are given of an investigation on coal by means of magnetic resonance studies performed at the Delft University of Technology over the period 1979985. The goals of the research were: 1, to investigate the relations between the parameters obtained via magnetic resonance and the coal rank; 2, to obtain information about the molecular structure of coal; and 3, to investigate the quantitative analysis of coal using magnetic resonance techniques. From more than three decades of study in this area it was well known that e.s.r., ‘H n.m.r. and i3C n.m.r. provide very useful information about the properties of coal. (For a survey ofthe results readers are referred to the many excellent reviews on magnetic resonance in coal’ -’ and the references cited therein.) However, in many investigations conclusions were drawn about such issues as the ranking of coal via magnetic resonance using a relatively small series of coal samples, often of similar ranks and origins. Therefore it was decided to carry out a broad-based investigation of the general character of magnetic resonance results on coals, and e.s.r., ‘H n.m.r. and 13C n.m.r. experiments were performed on a series of 60 coal samples, varying in rank from soft brown coal to meta-anthracite and originating from different continents. Moreover, it was also known that in solids, like coal, that contain unpaired electrons, an n.m.r. signal can be enhanced by irradiating at or near the electron Larmor frequency, an effect referred to as dynamic _ * This paper was presented at the American Chemical Society Symposium ‘New Applications of Analytical Techniques to Fossil Fuels’, held at New York City, USA, April 1986 $ Present addresses: M.J.D.: Philips Medical Systems Div., PO Box 10000, 5680 DA Best, The Netherlands; J.S.: Institut Teknologi Bandung, Bandung, Indonesia OO&2361/87/07087~10$3.00 0 1987 Butter-worth & Co. (Publishers)

876

FUEL, 1987, Vol 66, July

Ltd

nuclear polarization (DNP)‘-lo. Thus a major part of this investigation involved ‘H and i3C n.m.r. experiments in combination with DNP. The experimental e.s.r. and n.m.r. techniques used in this study will be described only briefly in this paper, as detailed descriptions have already been given elsewhere’ -6. Also, as both the theory of DNP in combination with ‘H and i3C n.m.r. in solids and the experimental set-up used in these works have been described in other reports11-20, only a brief review of these matters will be given here. In this paper the emphasis will be placed on the experimental results and their use to achieve the scientific goals mentioned above. EXPERIMENTAL** Sample preparation

The experiments were performed on 60 coal samples provided by various commercial and non-commercial coal banks, including the Dutch coal bank SBN and the Exxon coal bank in Baytown, USA. Table 1 gives results from the proximate and ultimate analyses presented for coal samples numerically ordered according to increasing volatile matter content (and to decreasing %C for cases in which the %VM was the same). The coals originate from Germany, Belgium, England, Australia, South Africa, Japan and the USA. The samples were prepared by first grinding the coal in a nitrogen atmosphere to a grain size < 0.21 mm. For all measurements except magic-angle spinning the powders were put into Pyrex tubes of 4mm i.d. to a height of z 1 cm. The tubes were then evacuated to 10W5torr for **Experimental

work was carried

out at the Delft University

E.s.r., ‘H now., Table 1 The coal samples studied with their corresponding ’ T n.m.r.

i3C n,m.r. and DNP studies of coaf: R. A. Wind et al.

parameters following from proximate

and

ultimate

analyses,

e.s.r.,

‘H

n.m.r.

and

--__ pe

50

1x0 200 I50 I90 160 I I

FRCi Aelg ALIst. FRG

f I.. t: _. --._-

19.X 20 2 22. I 222 22.3

90.’ XX.? 90.6 x9 3 RX (I

USA FRG Brig. SA

23 9 260 26.5 27.0

SA

2X.6

89.6 8X.7 86.X 852 X5.5

SA

2x 9

834

44

BeI%%

29.9 3O.b 30.X 31.0

X6.‘) Mh X7.1 X1.‘)

5.4 5.5 5.2 4.6

Beig.

II 1. Ia Ii 2( __ __21 2: 2.3 24 25

~~

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Be& PRG AlIst

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31.9 33.5 34.0 34 3

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__ ___~__ _ -..USA 4t.6 USA 41.7 USA 42. I USA 43.1 USA 43.7

it 52 5.1 54 $5

USA USA USA USA USA

43.7 45.2 45.2 45 6 45 7

X3.5 74.4 71.7 80.0 77 7

USA FRG FRG USA Aust

so.2

73.4 6Y.f 66 3 74.0 X5 5

__~

IO.1 99 io.! to.6 9.9

4.9 5 0 5.2 41

5.1 s.5 5.4 52

2.4 34 34 3 3 24

II 0 10.1 IO.0 7.4 78

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96

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5.4 5.2 5.7 6.1 5.4

54 t3.3 71 6.1 93

5.6 5.0 5.1 51 5.8

II 9 82 IS 7 89 6.6

__-

il f%smined

50.6 53.5 58.6 91 0

51 4.6 4.4 5.9 IO.4

2.? 2.Y J:! 4.0 3.0

19.7 25.5 28.1 I X.4 2.7

2x 26 17

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76

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21 I5 I6 6.2

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20

lb

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4.3 10.3 2.9 4.5 3.4

IO.8 If.2 10.7 Il.7 Y.3

I Y 2.0 1.i f .o 2.0

13.3 Il.5 2.4 37 5.7

I f .3

f.4

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11.8 31 f I.9 3.1 X.7

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2.0 I.0 f.5 l.b I.6

20.0 I I .o 43.5 53 3 I

1-f 0 7x 064 1.1 I3

3.6 12s 140 17.2 45 5 __. ._,I._

11.X 93 9.2 IO.9 8.9

064 0.31 0.18 O.?Y O.IH

I41 IfJ ‘?(I 67 45

KW 1700 Xc% X(x) 500 _ _... NO X00 400 few, 9(K)

0. I2 0040

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110 90

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1 .(K) 098 _fO.Ol” 0.97’~#.01 O.YhS +o.ozs 094 *o.lv

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0.87” 0.88 0.87’ 0.82 0.81

+ 0.02 40.02 50.02 *ct.02 co.03

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0.87 0.86 0.805 0.86 0.83

to.01 5 kO.03 1: 0.02 +O.O2 ItO. ---

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I

0. I I 0.45

0.83 0.80 0.73 0.80 0.84 _ .~_~____

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2.3 4.1 II I .3 70

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0.04’1 0.0x7 0043

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24 29 37 20 :o

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2.4 39 3 0 I I. 10.5 ___~_~~

9.5

10.8

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zs 57 58 59 60 __~

3.0 4.9 2.2 3.x 5.2

2.8 40 5.6 X.2 75 4.7 _____~__.______~___.__

X4.2 79 I 87 4 83.X

85 5 79.1 77 5 X5.8 XI Y ~_ 80 6 84.3 7X 0 82. I x4 5

_

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34 x 35.1 3s 6 3h.I 37.4 ~~ ~~ 3X,.5 38.6 39.4 39.4 40 I

USA USA USA Japan USA

4 6 4.9 4.7 4.7

tiakx

.~~__.__ 0.77 0.77 078 0.77 0.76

+0.02 f0.03 +O.O? kO.01 ItO.

0.7 f J + 0.02~ 0 70 _tO.Q2 0.73 to.01 0 75’ _fO.O2J 0.72 +0.02’

0.85 + 0.03 0.7X’ * 0.0’5 085 +003 0.78’k0.01s 0.80 20.01 _~~

0.73 0.72 0.735 072 0.67

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07?‘+O.o1* 0.825 2 0.03~ 0 76 +0.02’ 0.76: IO.02 0.80’ + 0.0 I

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basis

2 2 h at room temperature and sealed off. It was found that evacuation decreased the proton Zeeman relaxation time by a factor 2-4, which means that without evacuation the relaxation is mainly governed by paramagnetic oxygen in air. For the MAS experiments no vacuum-sealed rotors were available, so the samples were put into the rotors (3.3mm i.d., filling height 13 mm) without evacuation. Experimentat set-up All magnetic resonance measurements were performed at room temperature. The ‘H and r3C experiments were carried out in an external magnetic field of 1.4 T,

corresponding to a proton frequency of 60 MHz, a ’ 3C frequency of I_5MHz and an e.s.r. frequency of 40 CHz. The probe consisted of an n.m.r. coil, double-tuned to 60 and 15 MHz. The microwave irradiation used in the DNP experiments was introduced into the sample using a combination of a horn antenna and a (movable) reflector. A Varian klystron was used as a microwave source. The maximum microwave power was 13 W, corresponding to a maxims amplitude of the microwave field of 0.06 mT. During the experiments without MAS the samples were cooled with air to minimize heating by the microwaves. The MAS experiments were performed in the same probe, with a maximum frequency of z 6 kHz. A detailed

FUEL, 1987, Val 66, July

877

E.s.r., ‘H n.m.r., 13C n.m.r. and DNP studies of coal: R. A. Wind et al. description of the probe has been published elsewherei4. Finally, the e.s.r. measurements were performed using a home-built e.s.r. spectrometer operating at 9.07 GHz.

RESULTS

20 PH--I

AND DISCUSSION

In solids containing both a nuclear spin system and unpaired electrons, DNP can be used to transfer the polarization of the electron spin system to that of the nuclei. As the polarization of the electrons is a factor /Ye/,& larger than that of the nuclei (ye and Y,,are the magnetogyric ratios of the electrons and nuclei, respectively), the result is a nuclear polarization that is enlarged by a magnitude up to the same factor Iye[/y,,(e.g., a factor 660 for ‘H nuclei and a factor 2600 for i3C nuclei). DNP is obtained by irradiating with a frequency w at or near the electron Larmor frequency, w,, during a period that is several times the nuclear Zeeman relaxation time, before observing the n.m.r. signal. Four types of DNP mechanisms can be distinguished, depending on the type and time-dependence of the electron-nucleus interaction term Hen (Refs. 16, 18): Overhauser, solid-state, and direct and indirect thermal mixing effects. An Overhauser effect occurs when H,, contains a term that is time-dependent on a time scale comparable with o,- ’ and is observed in solids containing mobile electrons. The nuclear polarization enhancement, P,, can be positive or negative, depending on the scalar or dipolar character of H,, . The value lPn/ becomes maximal when w = w,, and P,(w) is symmetrical around 0,. When H,, is dipolar in character and contains a timeindependent term, P,(o) - 1 is antisymmetrical around w,. The enhancement can be caused by the solid-state effect, for which jPni becomes maximal when w = w, + w, (w, is the nuclear Larmor frequency), and/or by the socalled direct and indirect thermal mixing effects, for which [P,l becomes maximal when o ‘v w, + w 1,2 and w = o, _t w,, respectively (0 112 is the e.s.r. linewidth and wi is a frequency which lies between Q..+~and w, ). Figure f shows the ‘H and i3C DNP enhancements, P, and Pc, respectively, of the low-volatile bituminous coal no. 11, given as a function of w-w,. It follows from Figure la that the ‘H enhancement curve contains a part that is symmetrical around w,, caused by a (positive) Overhauser effect. This can occur when unpaired electrons are delocalized over some aromatic regions in the coal, such that protons close to these regions may experience time-dependent interactions with the electrons. The antisymmetrical part of the enhancement curve results from the other three DNP mechanisms mentioned above16-‘8*20, and is caused by immobile free-radical sites or by delocalized electrons interacting with protons remote from the regions of delocalization. It also follows from Figure la that IP,-- l/ becomes maximal at an off-set frequency of 60 MHz, i.e., when o = 0&f WH (I& iS the proton bmnOr freqUenCy). It can be shown’6 that for thesevalues ofw, lPH- 11is proportional to B:N,(W;)-‘(AB,,,)-‘, where B, denotes the amplitude of the microwave irradiation field, N, is the concentration of fixed paramagnetic centres, Wg is the proton Zeeman relaxation rate and AB,,, is the e.s.r. linewidth (full width at half height). The proportionality factor depends upon various factors, one of which is the strength of the time-independent electron-proton

878

a

FUEL, 1987, Vol 66, July

b-w

--L 50

)

zn

(MHZ]

1KJ

-1C

-2c

~

Figure 1 The ‘H and 13C DNP enhancement of coal no. 11 as a function of w--o,. B, =5 x lows T. (a) The ‘H enhancement: curve A, Overhauser effect; curve B, solid-state + thermal mixing effects; curve C, total enhancement, l =experimental. (b) The 13C enhancement: l = experimental; curvereflects mainly the enhancement due to the thermal mixing effect

interactions. It was found that for a B, field of 6 x lo-‘T, PH can be predicted rather well by the formula:

w=wiw,

(1)

if N, is expressed in cm - 3, W,” in s - ’ and AB 1,2 in T. Equation (1) will be used in this paper to investigate the enhancements observed in all coals except coal no. 1 (see below), The proton enhancement curves of the coals 1,2,6,18, 24 and 34 were also measured. With the exception of coal no. 1, the curves obtained were similar to those shown in Figare la, which means that in these coals also all four DNP mechanisms contribute to the enhancement. For the meta-anthracite (coal no. 1) only an Overhauser effect was observed, probably indicating that in this high-rank coal, which contains large aromatic clusters”, the unpaired electrons are delocalized to such an extent that the time-independent electron-proton interactions are averaged out. It is seen in Figure Zb, where PC-- 1 is given as a function of o-w,, that the 13C enhancement is antisymmetrical around w, within measuring error. This means that a possible Overhauser enhancement is overshadows by the large enhancement due to timeindependent electron-carbon interactions. The enhancement is mainly caused by the direct thermal mixing effect, and becomes optimal for a microwave off-set of 17 MHz. It follows that, for the specific value of B, employed, the maximal 13C DNP enhancement is about an order of magnitude larger than the maximal ‘H enhancement; this fact is mainly due to the much smaller carbon Zeeman relaxation rate’ 6- ’ 8*20.A similar result is found for other coals (see Tuble 1, which also summarizes the magnetic resonance data of this study). The DNP effect can be used for the following purposes: (i) The ‘H or 13C enhancement factor can be used as a parameter to characterize coal. (ii) The proton enhancement curve provides information about the presence of fixed and mobile unpaired electrons. Moreover, from Equation (I) the amount of fixed paramagnetic centres can be calculated if, besides PHI W,”and AB,,, are also known. Also, the e.s.r. lineshapes corresponding to the fixed and mobile

E.s.r., ‘H n.m.r., 13C n.m.r. and DPJP studies of coal: R. A. Wind et al.

electrons can be determined separately from the enhancement curve, as PH(o) - 1 for (w- w,l> oH reflects the e.s.r. lineshape corresponding to the fixed electrons, whereas the Overhauser curve reflects the (saturated) e.s.r. lineshape corresponding to the mobile electrons. (iii) The n.m.r. signal enhancement due to DNP will increase the signal-to-noise ratio ofan n.m.r. spectrum, or reduce the measuring time of an n.m.r. experiment necessary to obtain a certain signal-to-noise ratio. This is especially important for 13C n.m.r. where, because of the low natural abundance of the 13C isotope, the measuring I:mes are usually long, even if a lH-13C crosspolarization experiment” is employed. This DNP advantage is illustrated in Figure 2, where ’ 3C spectra of the low-volatile bituminous coal no. 11 obtained via different experiments, both without (a<) and with (d-f) magic-angle spinning, are shown. The spectra (a) and (d) were obtained via CP only, whereas the spectra (b) and (e) were obtained via the so-called DNP-CP experiment, in which before the actual cross-polarization the proton polarization is enhanced via DNP. It follows that for this coal the use of DNP reduces the measuring time from hours to minutes. Within the limitations imposed by the slgnal-to-noise ratio, it can be seen that the lineshapes of the spectra obtained via CP(MAS) and DNP-CP(MAS) are the same, indicating that the DNP enhancement ofthe aromatic and aliphatic carbons are the same. This means that the spin diffusion within the abundant proton system is capable of averaging out possible differences in DNP enhancements. The same result has been found in all other coals. (iv) DNP can provide information about the localization of the free electrons. This is illustrated in Figures 2c and 2f: In these cases the 13C signal was obtained with a standard (single-pulse) FT experiment, but before the 90” observing pulse the 13C polarization was enhanced directly via DNP (henceforth referred to as the DNP-FID experiment). Unlike the situation for the proton system, spin diffusion within the rare ’ 3C system is so weak that it cannot average out possible polarization differences. This means that the polarization of ’ 3C spins

close to the unpaired electrons is enhanced more than that of remote 13C spins. It follows from the Figures 2c and 2f that in DNP-FID experiments almost no aliphatic carbons are observed, indicating that the radicals are localized mainly in the aromatic regions of the coal. This result was found to be true for all coals for which the 13C DNP-FID spectra were determined (i.e., for all the coals of Tuble 1 for which PC has been measured). (v) DNP opens the possibility of performing special experiments that provide information about the quantitative analysis of coal via 13C n.m.r. This matter will be dealt with later in this report. Churacterization of coal

In this section the issue of how the different parameters obtained via e.s.r., ‘H n.m.r. and 13C n.m.r. are related to coal rank is addressed. The values of the different magnetic resonance parameters determined in this study are given in Table I. E.s.r. The e.s.r. linewidth, AB,!,, and the concentration of unpaired electrons, N,, have been determined. AB,,, (full width at half height) has been obtained from the firstderivative spectrum, assuming a Lorentzian lineshapez3. In about half of the (degassed) coal samples the e.s.r. spectrum consists of a broad and a narrow component. The narrow line, which is probably due to the fusinite in the coalz4, represents only a small fraction of the total amount of unpaired electrons (typically a few, up to a maximum of 4 10 %), and in this paper has been neglected in analysis of the data. Hence AB,,, represents the broad component. It follows from Table 1 that there does not exist a clear relation between AB, ,2 and the coal rank, although AB,!, increases slightly for decreasing coal rank. The small linewidth of coal no. 1 probably results from exchange narrowing’j, and is another indication of the large mobility of the electrons in this coal. N, has been calculated on a dmmf (dry, mineral-matter free) basis, and is expressed in Table 1 in cme3 instead of the usual g- l. To this end the weight percentage has been multiplied by the specific density given by Chichez5. lrsually in the literature N, (expressed in g-l) is plotted against the carbon content, and it is found1~21~23.26-28 that for %C < 95, N, decreases with decreasing XC. The results ofthis study agree with this observation. However, the spread in possible N, values for a specific value of %C is large, and a regression formula given by Retcofsky” resulted in a standard deviation of 7.0 x 10’ 8 in the free radical contents. N, (dmmf, cm - 3, was also analysed as a function of the I)/,VM (dmmf). The result of this correlation is given in Figure 3. Surprisingly, two dependences are observed, which can be described by the empirical relations:

Curve A: 4&l

360 260

160

Hwm)

b -lb0

260

nbo

1;o lb0

N,,=j0.15+7.6[1+(9~10-“VM6)]-’ x exp(-4.2

fjbpm)

Figure 2 13C spectra of coal no. 11 obtained via different experiments. (a) Via CP: match time=0.9ms, no. scans=90000, recycle delay= 0.6 s; (b) via DNPCP: no. scans = 200, other parameters as in (a): (c) via DNP-FID: no. scans = 8, recycle delay = 60s; (d) via CP(MAS): no. scans = 72 000, other parameters as in (a); (e) via DNPCP(MAS): no. scans= 500, other parameters as in (a); (f) via DNP-FID(MAS): no. scans = 4, recycle delay = 60 s

x lO-21/M)) x 1019 (2)

Curve B: N,,={O.l5+88[exp(-O.lVM)])

x 1019

(3)

The standard deviation of both curves is z 3.0 x lo”, which is considerably smaller than the value found by Retcofsky. This indicates that there exists a clear

FUEL, 1987, Vol 66, July

879

E.s.r., ‘H n.m.r., 13C n.m.r. and DNP studies of coal: R. A. Wind et al. -

Flc -

7 5

7

:

3-

presumably, the higher Wg value is caused by the increasing amount of free radicals. A clear relation is seen between W,” and the oxygen percentage for larger values of %O. This is illustrated in Figure 4, which shows a plot of W,” as a function of %O. For %O> 10, WF is caused completely by the oxygen. The most plausible explanation is that a small fraction of the total amount of oxygen is present as paramagnetic 02, as it is known that this species is very efficient in reducing the proton Zeeman relaxation time. For larger values of %O (> 6.5 %) the following empirical relation can be determined between WF and x0:

- 6+E E 5P o‘i 4-

zQ 21 10

20

30

40

50

60

W;=

% VM (dmmf)

The number of unpaired electrons, N,, as a function of the volatile matter, %VM. l =Experimental; curve A: N,,, given by Equation (2); curve B: Ne2, given by Equation (3) Figure 3

relationship between N, and V/M (the weight per cent volatile matter), and suggests that the volatile matter consists of molecular groups positioned at sites that become free radicals if I/M decreases. The behaviour of N, described by curve B does not appear to have been reported before: the N, values in the literature agree with those of curve A. In this respect it is worth noting that a large number of coals with N, values following curve B originate from South Africa and Australia. This might mean that the number of unpaired electrons depends on the evolution of the coal, as it is known that the evolution is different for coals in the northern and southern hemispheres29.

.5+0.47(x0-6.5)‘,

%0>6.5

(6)

This means that the measurement of W,” provides an easy way to determine directly %O (which is usually determined by difference) for high-oxygen coals. The proton Zeeman relaxation in the rotating frame, WF, has been measured in a (spin) lock field, Bin, of 1 mT. For many samples the relaxation behaviour was found to be strongly non-exponential, which is again the result of a distribution of relaxation rates and a proton-proton spin diffusion incapable of averaging out the relaxation differences. Therefore the values of I$ given in Table 1 were determined by using a formula similar to Equation (5). In a number of samples Wp” is determined by the organic free radicals in the coal. This is because the Zeeman relaxation time of these radicals is of the order of ~LS(Ref. 34) hence of the order of (ynB,,)- ‘, which makes these radicals an efficient relaxation source for the relaxation in the rotating frame3’. No clear relation has been found, however, between Wz and coal rank.

iH n.m .r . With ‘H n.m.r. the Zeeman relaxation rate, IV!, the relaxation rate in the rotating frame, W:, and the DNP polarization enhancement, PH, have been determined. The relaxation rates have been measured in experiments in which the ‘H n.m.r. signal is enhanced via DNP. For Zeeman relaxation it was found in many samples that the magnetization, h4z, did not relax exponentially. The time behaviour of MZ could approximately be described by Mz(oo)-M,(t)=[Mz(co)-M,(0)]exp[-(At)Xl (4) where K varied between 1 for the smaller relaxation rates and 0.5 for the largest relaxation rates. This behaviour is probably due to the fact that there exists a distribution of relaxation rates, and that for large relaxation rates proton-proton spin diffusion is not fast enough to average out the differences in relaxation rates30m3j, resulting in a non-exponential relaxation behaviour. The values of Wg given in Table 1 were calculated using the formula WY = In 2/t 1,*

(5)

where triz is the time for which M,(w) - M,(t) = O.S[M,(co)- M,(O)]. It follows from Table 1 that in general no simple relation exists between IVYand N,, so that WF is not determined by the organic radicals. This is in accord with the results of Sullivan et al.28. In fact, for many coals WY is remarkably constant and = 3.5 + 1 s- ‘. For these coals the relaxation is probably governed by proton-proton dipolar interactions rendered timedependent by molecular motions. For the high-rank coals 1-4, Wg increases with increasing coal rank. Here,

880

FUEL, 1987, Vol 66, July

%0 [dmmf)

The ‘H Zeeman relaxation rate, I@, as a function of the oxygen content, ‘LO. 0 =Experimental; solid curve: the empirical relation between Wp and %O, given by Equation (6) Figure 4

E.s.r.. ‘H n.m.r.. 13C n.m.r. and DNP studies of coal: R. A. Wind et al.

The proton polarization enhancement due to DNP, Pu, has been determined under the conditions of a microwave irradiation field with an amplitude of 0.06 mT at a frequency w = o, - o,. This has been done for all coal samples with the exception of coal no. 1, for which w was set equal to w,. The reason is that in this coal, as has been mentioned above, only an Overhauser effect is observed. From Table 1 it can be seen that PH becomes maximal for xC2.92, and decreases for increasing and decreasing carbon contents. This is illustrated in Figure 5, where the solid curve represents an empirical relation between PH and XC, given by P,=1+27{exp[-2.3~10-~(92-%C)~]J, XC<92

(7)

In order to explain this variation in PH, the proton enhancement was calculated using Equation (1) by substituting the measured values of N,, WF and AB,,,. The calculated enhancement, A, is given in Table 1. In general, the agreement between A and PH is satisfactory, which means that the variation in PH is mainly due to changes in N, and W,“, as the variation in AB,,, is only small. It is remarkable, however, that, for the coals with N, values close to curve B of Figure 3, A is considerably larger than PH. Two explanations for this are possible: (i) the numerical constant in Equation (1) is smaller for these coals than the given value of 2.4x 1O-21 (which can happen, for instance, if the aromatic clusters are larger in these coals, resulting in a larger delocalization of the unpaired electrons and a reduced time-independent electron-nucleus interaction); and (ii) only a part of the measured radicals are present in the organic matter of these coals. In this respect it is worth noting that, if we assume that for all coals N, is given by the value N,, predicted by Equation (Z), the agreement between the calculated enhancement, A,, obtained from Equation (1) with Nel instead of N,, and the observed enhancement PH is much better (see Table I). This suggests that in thecoals with N, values close to those of curve B in Figure 3 only a part, Nelr is present in the organic matter, with the remainder, N,, - N,i, located in the inorganic matter. Which of these explanations is correct is still to be decided. 13C n.m.r. With i3C n.m.r. the r3C polarization due to DNP, PC, the 13C Zeeman relaxation rate, @, and the aromaticity, fa, have been determined. PC and c have

%C (dmmfj

Figure 5 The maximum ‘H polarization enhancement due to DNP, PH. as a function of the carbon content, XC. l = Experimental; solid curve: the empirical relation between PH and XC, given by Equation (7)

been measured by means of r3C DNP-FID measurements, i.e., for the aromatic 13C nuclei (see above). The aromaticity has been measured using ‘H-l 3C cross-polarization experiments. More or less linear relations are found between PC and PH, and between @ and W,“. This means that the variation in PC with coal rank is due to variations in N, and the Zeeman relaxation rate again (but now 6 instead of Wg), and that lI$ is determined by the same interactions that determine WF. On average, PCis about 8 times larger than PH and fi is about 90 times smaller than WF. As PC and W‘j do not provide more information for coal characterization, the results will not be treated further in this paper. The aromaticity (the ratio of the number of aromatic carbons to the total amount ofcarbon, f,)is considered to be one of the key parameters that characterize the coal structure’. Therefore many investigations have been dedicated to the determination of f, via 13C n.m.r. and relating this parameter to the coal rank (see Refs. 2-6 and the literature cited therein). Here the aromaticity is defined as the ratio of the number of carbons with sp- and sp2-hybridization to the total number of carbons, including those with sp3-hybridization35. ,f, is usually measured via i 3C CP(MAS) experimentsj5 - 39; here it was determined via both ‘jC CP and 13C CP(MAS) methods, using ‘H DNP to enhance the signals. The CP match time used was 0.9 ms for all coals. An example of 13C spectra obtained with these two methods has already been given in Figures 2b and 2e. The aromaticity obtained from the DNP-CP(MAS) spectra, (fa)CP,MAsj, has been calculated by defining the aliphatic carbons as those with chemical shifts between 0 and 70ppm (from TMS) and the aromatic carbons as those with chemical shift values > 70ppm. This procedure could not be applied directly to the DNP-CP spectra, as the aromatic and aliphatic chemical shift anisotropies overlap. Therefore in these cases the aromaticities, (fa)c-, were calculated by digital subtraction of the spectrum of the highly aromatic anthracite sample, coal no. 2, from the measured spectrum of the coal of interest. This eliminated the aromatic envelope and the remaining aliphatic signal was integrated, yielding the ‘aliphaticity’; (fa)cr is then calculated by difference. (In spirit, this method resembles that of Wemmer et al.40, where the aromatic and aliphatic signals were separated by comparing the lineshapes with those of model compounds.) The results are given in Table I, where the listed uncertainties reflect the 95 “, probability intervals. In typical literature correlations between fa and coal rank, f, is plotted as a function of the carbon content35-39. It is found that f,decreases with decreasing carbon content, although the spread in possible fa values for a given carbon content is generally large. In fact, almost no correlation is observed between fa and carbon content for S/,C < 86. The same kind of pattern was found here, again with a disappointingly large spread of aromaticity values for a given %C [compare, for values of coals 19, 20, 38, 41, 44 example, the (S, kPcMAS, and 60: for all these coals y/,C values are 85.5 f 0.3 whereas varies from 0.83 to 0.19!3. A much better Mk PcMASj relationship was found between f, and the percentage volatile matter. VM (as is illustrated in Fiaure 6). It follows that both dkP and (f, kPcMASl hiby correlated to %VM, both decreasing with increasing % VM. This probably means that the aliphatic groups,

are

FUEL, 1987, Vol 66, July

881

E.s.r., ‘H n.m.r.,

0.51

13C n.m.r.

1

10

I

I

20

and DNP

I

30

40

studies

1

1

50

60

of coal: R. A. Wind

I\ 70

, 80

% VM (dmmf)

et al.

the aromatic portions of coals contain more nonprotonated carbons than the aliphatic portion4*, it is possible that MAS reduces the fraction of aromatic carbons that are observable, thus lowering the apparent aromaticity. This issue is illustrated in Figure 7, where the DNP-CP(MAS) spectra are given for the low-volatile coal no. 11, obtained for two spinning frequencies. It is seen that by increasing the spinning frequency from 3.1 to 5.2 kHz the aromatic signal is reduced considerably, resulting in a decreased apparent aromaticity. Therefore it can be concluded that the (f,)cp values in Table 1 are a better representation of the ‘true’ aromaticity than the values. [In this respect it should be noted that (f,k p(MAs) values given in the literature3s-39 agree the (f, kPtMASj with those (f, kP(MAs)values obtained here: hence these values are probably also too low.]

0.9 -

Quantitative

0.61 0

0.5

I

1

I

I

,

10

20

30

40

50

cl* 60

I

1

70

80

%VM (dmmf)

(a) (J&-p as a function of the volatile matter, %VM. l = Experimental; solid curve: the empirical relation between (.fa)cp and FM, given by Equation (8). (b) (j&WAS) as a function of VM. 0 = Experimental; solid curve: the empirical relation given by Equation (9); 0 = (J&p(MAs) results obtained from Ref. 38; 0 = (j&p(MAs) results obtained from Ref. 35 Figure 6

analysis

by 13C n.m.r.

Coal consists of many different molecular groups, and this often manifests itself in the r3C spectra obtained with MAS, in the form of some fine structure (peaks and shoulders) in the aromatic and aliphatic regions. This is especially true in low-rank coals owing to the presence of large amounts of heteroatoms35’38, but even for highrank coals structural information in addition to the aromaticity can be obtained. As an example Figure 7a is considered. Thanks to the excellent signal-to-noise ratio obtained as a result of the DNP signal enhancement, the aliphatic line is clearly separated into two peaks, corresponding to the methyl (6 = 20 ppm) and methylene (35 ppm) carbons. Similarly, the aromatic line consists of three peaks, with maxima at approximately 128, 140 and 153 ppm, originating from 13C nuclei bonded to a hydrogen, a carbon and an oxygen atom, respectively.

which increase in number for increasing %VM, are closely related to the volatile matter. The following empirical relations were found between (,f,)cp and respectively, and %VM : (fa)CP(MAS, (f,kp=l-4.7x

1O-3 VM+ 1.5 x lo-‘sin(27rT/M/26), I’M6 40%

(f,k,=O.808-9.2x (f,kP(MAsj=I-6.7X

0-3(1/M-40), 0K31/M+2x

(8a)

I’M>40%(8b) lo-* sin(2nI/M/28), I/M640%

(f,),-P(MASJ=0.741- 1.08 x lo-*(I/M-40),

(9a)

VM>40% (9b)

The standard deviation in fa is 0.021 for (f,)cp and 0.027 values for (L kPcMASj.Figure 6b alSO shows (fa)CP(MASj obtained from Refs. 35 and 38, which are in agreement (within the limits of experimental error) with the results reported here. (The data reported in Refs. 36, 37 and 39 could not be incorporated as ‘ZVM values were not given in these publications.) It follows from Figure 6 and Equations (8) and (9) that (fa kP(MAsJis systematically lower than (fa)cp. This can be caused by a systematic error arising from the different procedures used to determine but it is considered more likely that (f, kpand(S,kP~MAS~y the difference is caused by the MAS technique itself. In order to eliminate spinning sidebands in the DNPCP(MAS) spectra spinning frequencies of z 3 kHz have been applied. This can significantly decrease the 13C CP(MAS) intensity of carbons remote from protons4r. As

882

FUEL, 1987, Vol 66, July

i a

b

r”~~l’~~~l”“r”‘~l”“r”“l 250 200 150

100

50

0

-50

6 (ppm) The 13C DNPCP(MAS) spectrum ofcoal no. 11 obtained at two spinning frequencies: (a) f, = 3.1 kHz, (j&p(MAs) = 0.87; (b) fr = 5.2 kHz, (f&p(MM)= 0.82. From the non-spinning DNP-CP experiment a value (f&.=0.89’ was obtained Figure 7

E.s.f.,

‘14nmf.,

Also, special experiments can be performed, such as variable CP match time and interrupted decoupling experiments. In these techniques the i3C signal due to protonated and non-protonated carbons can be separated, which can assist in assigning the different peaks. For an elaborate treatment of this matter readers are referred to Refs. 4-6 and the literature cited therein. A point of major concern, however, is how quantitative the results are. The possibility that the application of MAS decreases the intensity of the aromatic line in a CP(MAS) experiment has already been mentioned, and the fact that the proton rotating frame relaxation rate, Wi, is very large in many coals can cause problems. This latter Issue has a serious consequence, namely that the match time in a CP experiment has to be made rather short in order to avoid signi~cant losses in the ’ 3C signal, so that carbons with a proton-carbon contact time comparable with or longer than (WF)- ’ will be observed only weakly, if at a1143,44.This is even more serious if MAS is applied, as MAS will decrease weak proton~arbon couplings, resuiting in the need for an increased CP contact time. Finally, ’ 3C nuclei in the vicinity of paramagnetic centres might have spectral lines broadened to such an extent that they are not observed. As the unpaired electrons are mainly located in the aromatic region of the coal, which also contains the most non-protonated carbons, it is conceivable that not all the aromatic i3C nuclei are detected, resulting in an incorrect, reduced value for the aromaticity. The problem of quantitativeness in the ’ 3C CP(MAS) analysis of coal has been the subject of many in investigations 38-40.43.45-50 which the values reported for the percentage’of carbons observed varied from 40 to 100%. Hence this matter is still unresolved. Two alternative experiments are reported here which provide information about the percentage of carbons detected via CPn.m.r. In both experiments DNPplayed a crucial role. As already stated above, DNP can be used to enhance the ‘“C signal directly (the DNP-.FID experiment) or indirectly (the DNP-CP experiment). From results shown in Figure 2it was concluded that with DNP-FTD mainly aromatic carbons are observed. Now it can also be seen from Figures 2e and 2f that the width of the aromatic peak obtained via DNP-FlD(MAS) is larger than that obtained via DNP-CP(MAS). This means that with DNP-FID some carbons are observed that are not detected via DNP-CP, presumably because the carbons are too remote from the protons. The larger width indicates that these carbons are in the vicinity of the unpaired electrons. In order to investigate this possibility further the following two experiments have been performed : (i) Vuriable CP mutc~-time experiments. By varying the match time in a CP experiment, information is obtained about the magnitudes of the carbon-proton interactions, which are related to the proton-carbon distances occurring in a coal”. This experiment has been performed on the low-volatile coal no. 11, both after ‘H DNP and lH-+13 C cross polarization, and after 13C DNP and 13C--+‘H cross polarization. Figure 8 shows the results for the aromatic r3C nuclei. It follows that of the carbons observed via DNP-CP, 24 % have a protoncarbon CP time constant of 20 ps (direct C-H bonds) and 76’)/ have a time constant of 250~s (indirect CC-H bonds}. For the carbons detected via DNP-FID a much

13C mm.

and DNP studies of coal: R. A. Wind et al.

0.011

1

1

‘

‘

1 0.5

t

1

1

1

I 1.0

rM(ms) Figure 8 The magnitude of the aromatic ‘ 3C signal of coal no. 11asa function of the match time, tM. Curve A: after ‘H DNP and iH-+‘3C CP; 0 =experimental; solid curve: M&cII)-M&)= [M&co)Mc(0)][0.24 exp( - 5 x 109) + 0.76 exp( - 4 x 1O’t)]. Curve B: after ’ 3C DNP and 13C--+lH CP; @=experimental; solid curve: MC(W)Mc(t)=[MC(“)-Mc(O)][O.I4exp(-5 x 109)+0.44exp(-4x 103t)+ 0.42 exp( - 0.77 x 103r)]. The experimentaf points have been corrected for the ‘H and 13C rotating frame relaxation

slower decay is observed during the 13C-‘H cross polarization: about 42% of the carbons have a time constant of 1300~s (carbons two or more bonds away from protons). It is therefore concluded that in this coal only 58% of the aromatic carbons are detected via the ‘H --+ ’ 3C CP experiment. (ii) 13C rotating frame relaxation. It has been mentioned above that the organic free radicals in coal provide an efficient m~hanism for nuclear rotating frame relaxation. It can be shown33 that the rotating frame relaxation due to paramagnetic centres of a rare nuclide system like 13C is described by an equation similar to Equation (4) with K=O.5: Mc(t)/Mc(O)5=f(t)=exp[

- (H$t)“.s]

(10)

Equation (10) is derived using the assumption of a homogeneous distribution of both the unpaired electrons and the nuclides. If a portion of the nuclei that are located in the vicinity of the electrons is not detected (e.g., because of a severe broadening of the spectral lines), Equation (10) is valid only for larger values oft, and the value 1 -f(O) is equal to the percentage of carbons not detected. The rotating frame relaxation of the aromatic carbons of coal no. 11, observed via DNP-CP and DNP-FID, was measured, and the results are given in Figure 9. For the case of DNP-FID it was determined that f(O)-0.9, which means that in this experiment 10% of the aromatic 13C nuclei are not detected, presumably because they are too close to the electrons to be observed. For the relaxation observed via DNP-CP it was determined that f(O)-0.56 (i.e., 447: of the aromatic carbons are not detected via CP). This is in accord with the results of the variable match-time experiment. These results probably mean that 34% of the carbons that are located in the vicinity of the electrons are too remote from the protons to be observed via CP [which is in agreement with the observation, made above, that the aromatic line in a DNP-FID(MAS) spectrum is broader than that in a

FUEL, 1987, Vol 66, July

883

E.s.r., ‘H n.m.r., ‘3C n.m.r. and DNP studies of coal: R. A. Wind et al. Table 2

The percentage of detected aromatic carbons via ‘%Z DNPFID and ’ jC DNPCP experiments for the coals 2, I1 and 24. The ‘true’ aromaticity is obtained by using (f&-p and correcting the aromatic intensity for the percentage of aromatic carbons not observed Detected aromatic carbons (“/,)

Figure 9 The rotating frame relaxation of the aromatic 13C signal of coal no. 11 observed via DNP-FID (0) and DNPCP (0). The lock field used was 4mT. WF=93s-’

DNP-CP(MAS) spectrum]. Also measured were the 13C rotating frame relaxations of coals 2 and 24, again both via DNP-FID and DNP-CP. The results are summarized in Table 2. It can be seen that in the anthracite (coal no. 2) less aromatic carbons are observed via DNP-FID than in the other two coals, in agreement with the larger amount of radicals in this coal (see Table I). It was also found that the percentage of aromatic carbons detected by DNP-CP decreases with increasing coal rank, which is in agreement with the generally accepted picture of aromatic clusters in coal, increasing in size with increasing coal rank. Finally, in Table2 the ‘true’ aromaticities in these three coals have also been calculated, using the (f,)cp values and the percentages of detected aromatic carbons in the DNP-CP experiments, and assuming that lOOa%of the aliphatic carbons are detected. As expected, the ‘true’ aromaticities are considerably larger than (fa)--, especially for the lowerrank coals. CONCLUSIONS It has been found that several parameters obtained via magnetic resonance techniques can be used to characterize a coal and to obtain information about its molecular structure. Both the number of unpaired electrons and the 13C aromaticity are strongly correlated to the volatile matter content, %VM, and decrease for increasing %VM. This suggests that the volatile matter consists mainly of aliphatic groups positioned at sites that become organic free radicals if the amount of aliphatic groups is decreased by some chemical transformation. Furthermore, for oxygen contents larger than % 10% the proton Zeeman relaxation rate, W,“, is completely determined by a small fraction of paramagnetic oxygen, which is proportional to x0. Hence, in coals with “/do > 10, the measurement of Wg is an easy alternative way of determining oxygen content. It should be kept in mind that WF has to be measured on coal samples that have been degassed for at least a couple of minutes, as for all the coals considered WF is increased by a factor 24 if the coals have not been evacuated. This is due to the paramagnetic oxygen in air. The origin of the paramagnetic oxygen remaining in the coal after degassing is unknown. It is possible that these radicals

884

FUEL, 1987, Vol 66, July

Coal no.

Via DNP-FID

Via DNPCP

(/gkp

‘True’ aromaticity

2 11 24

80 90 90

46 56 65

0.98 0.895 0.85

0.99 0.94 0.90

also originate from air, and have been trapped permanently in the coal. (Support for this hypothesis comes from the fact that an increase in the W,”value of coal no. 11 of % 30% was observed when this coal was exposed to air for a couple of months before degassing.) Measurements of Wz on premium coals, which have not been exposed to air, could provide more information on this matter. The use of DNP in coal research is manifold. For instance, the proton DNP enhancement factor can be used to determine coal rank, and especially for higherrank coals the signal enhancements are so large that the *3C aromaticity can be measured in minutes or less. This, together with the fact that the other parameters mentioned above can also be determined very quickly, opens the possibility for an on-line characterization of coal via magnetic resonance which can be used, for instance, to investigate the heterogeneity of a coal load, or to optimize coal combustion processes. Moreover, the DNP effect can be used to obtain information about the localization of the unpaired electrons and the aliphatic groups. The 13C spectra that are obtained by directly enhancing the 13C polarization reveal that the radicals are mainly located in the aromatic region of the coal, whereas the 13C spectra obtained by indirect enhancement via ‘H DNP show that the aromatic and aliphatic protons are enhanced by the same (average) factor. This means that the aliphatic groups in the coal are sufficiently close to the aromatic groups that protonproton spin diffusion is capable of averaging out polarization differences of the two groups. From the measurement of the DNP enhancement as a function of the microwave frequency, it has been found that both fixed and mobile electrons are present in coal (with the exception of the meta-anthracite coal no. 1 in which only mobile electrons have been observed). In the present situation the use of DNP is restricted to higher-rank coals: for coals with a carbon content ~75 ‘4 no enhancements have been found. It should be kept in mind, however, that, as the enhancement is proportional to the square of the microwave amplitude, B:, the applicability of DNP will be extended to lower-rank coals if B, can be increased. Much attention has been given to the quantitative analysis of coal via ’ 3C n.m.r. It has been found that MAS lowers the apparent aromaticity of a coal, and that even the aromaticity value obtained via CP without MAS is too low, because not all the aromatic carbons are detected via CP. Special experiments utilizing DNP were designed to investigate the percentage of aromatic carbons detected, and for the three coals for which this has been carried out values of 46, 56 and 65% were found. This

E.s.r., ‘H n.m.r., 13C n.m.r. and DNP studies of coal: R. A. Wind et al.

means that at the present stage of investigations the variation of the aromaticity as a function of coal rank at best displays the quantitative behaviour of this parameter. The main reason for not all the aromatic carbons being detected in a CP experiment is the large proton rotating frame relaxation rate, WF. Therefore it would seem to be very worthwhile to explore the strategy of obtaining 1‘C spectra via an alternative means of polarization transfer, the nuclear solid-state effect52, in which the results do not depend on Wi. Investigations in this direction will be undertaken in the near future at Colorado State University. Moreover, it should be noted that the investigations reported in this paper have been carried out on untreated whole coals, and that similar experiments can be performed to obtain information iibout the coal macerals’ 9 or coal combustion processes. In this respect it should be noted that various processes (e.g., heating of the coal) produce an increase in the number of free radicals27*53, so that it is possible that DNP can be applied successfully even in those coals Mhere N, for the untreated coal is too small to provide a noticeable DNP enhancement. This, together with the possibility of using DNP for the detection of other rare spin species like I 7O or 33S in coal, will also be investigated in the near future.

ACKNOWLEDGEMENTS These investigations were supported by a grant from the Project Office for Energy Research of the Netherlands Energy Research Foundation ECN, within the framework of the Dutch National Coal Research Program. One of us (M.J.D.) was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO). The authors with to thank Dr G. E. Maciel for valuable comments on the manuscript.

13 14

I5 16 17

18

19

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

REFERENCES

5 6 7 8 9 10 11 12

van Krevelen, D. W. ‘Coal’, Elsevier, 1969 Karr, C., Jr., rd. ‘Analytical Methods for Coal’, Academic Press, 1978 Gorbaty, M. L., Larsen. J. W. and Wender, I.. eds. ‘Coal Science’, Academic Press, 1982 Petrakis, L. and Fraissard, J. P., eds. ‘Magnetic Resonance. Introduction, Advanced Topics and Applications to Fossil Energy’, D. Reidel, 1984 Axelson, D. E. ‘Solid State Nuclear Magnetic Resonance of Fossil Fuels. An Experimental Approach’, Multiscience, 1985 Davidson, R. M. ‘Nuclear Magnetic Resonance Studies ofcoal’, Report No. ICTIS/TR 32, IEA Coal Research, 1986 Abragam, A. ‘The Principles of Nuclear Magnetism’, Oxford University Press, 1961 Goldman, M. ‘Spin Temperature and Nuclear Magnetic Resonance in Solids’, Oxford University Press, 1970 Abragam, A. and Goldman, M. Rep. Progr. Phys. 1978,41,395 Abragam, A. and Goldman, M. ‘Nuclear Magnetism, Order and Disorder’, Oxford University Press, 1982 Wind, R. A., Trammel, J. and Smidt, J. Fuel 1979, 58, 900 Wind. R. A., Duijvestijn, M. J., Smidt, J. and Trammel, J. Proc. Int. Conf. on Coal Science, Verlag Glueckauf GmbH, 1981, p. 812

39 40 41 42 43 44 45 46 47 48 49

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