The use of magnetic resonance parameters in the characterization of premium coals and other coals of various rank

The use of magnetic resonance parameters in the characterization of premium coals and other coals of various rank Antoni Department (Received

Jurkiewicz*, of Chemistry, 29 November

Robert

A. Wind

and Gary E. Maciel

Colorado State University, 1989; revised 76 February

Fort Collins, 7990)

CO 80523,

USA

Eight premium coals and twelve Polish coals were investigated by means of e.s.r., ‘H n.m.r., 13C CP-n.m.r. and dynamic nuclear polarization (DNP). The following parameters were studied: the concentration of free radicals, N,; the ‘H Zeeman relaxation rate, W’; the ‘H rotating-frame relaxation rate, W,“; the ‘H DNP enhancement factor, P,; and the CP-determined carbon aromaticity, If,),,. The results generally agree with the patterns established in an earlier study of the same parameters of 60 other coals. The following relationships have been found between the above parameters and parameters obtained via proximate and ultimate analyses: N, versus percentage volatile matter, W,” versus oxygen content, W,” versus N,, P, versus carbon content and (f,)cP versus percentage volatile matter. For the premium coals it was observed that exposure to air prior to degassing resulted in irreversible changes in N,, W,” and PH. (Keywords: electron spin resonance; n.m.r.; nuclear polarization)

The potential use of magnetic resonance for chemical structure elucidation in coal has been well recognized’ph. As coals are complex and heterogeneous3, to find generally valid relationships between properties and magnetic resonance parameters, it is advisable to study large sample sets, spanning differences of rank and origin, and to examine a few different types of magnetic ‘resonance parameters. Wind et ~1.~ studied 60 coals, differing in rank from lignite to meta-anthracite, by e.s.r., ‘H n.m.r. and 13C n.m.r., the latter two techniques being combined with dynamic nuclear polarization (DNP)8. Relationships were found between the ‘H DNP enhancement and the carbon content, between the concentration of unpaired spins and the percentage of volatile matter (%VM), between the carbon aromaticity and %VM, and between the ‘H Zeeman relaxation and the oxygen content*. This paper reports the results of similar experiments carried out on an additional 20 coals, twelve originating in Poland and eight originating in the USA (Premium Coal Bank of the Argonne National Laboratory). No Polish coals and no coals that had been protected from direct air exposure were included in the previous study7. In contrast, the Argonne Premium coals had never been exposed to air prior to study. Therefore, it is not obvious that for these coals the same relationships should be applicable. EXPERIMENTAL Table I gives results from the proximate and ultimate analyses of the 20 coals. For the magnetic resonance studies, the coals were ground in a nitrogen (gas) atmosphere and placed in 5 mm Pyrex n.m.r. tubes. The tubes were then evacuated at lop3 torr for 24 h at room *On leave from the Institute of Petroleum and Coal Chemistry, Polish Academy of Science, Gliwice ul. 1 Maja 62, Poland Presented at ‘Coal Structure 89’, 1618 October 1989, Jadwisin. Poland 0016-2361/90/07083GO4 I:(‘~) 1990 Butterworth-Heinemann

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temperature and sealed off. The e.s.r. measurements were performed at 9.5 GHz. All ‘H and i3C n.m.r. experiments were carried out using a home-built spectrometer with a static magnetic field of 1.4 T, corresponding to a ‘H Larmor frequency of 60 MHz and a 13C Larmor frequency of 15 MHz. The DNP measurements were carried out using the same spectrometer, and employing microwave irradiation at 40 GHz applied by a 13 W klystron, as described previously’. RESULTS

AND DISCUSSION

The following parameters were determined for the 20 coals: the concentration of unpaired electrons, N,; the ‘H Zeeman relaxation rate, W,“; the ‘H rotating-frame factor, relaxation rate, W,“; the ‘H DNP enhancement obtained via cross P,; and the 13C aromaticity polarization, Vg),,. The results are collected in Table 1. Concentration

of unpaired

electrons,

N,

Figure 1 shows a plot of the unpaired electron concentration, N,, of the 20 coals as a function of the percentage volatile matter, %VM (where VM is the corresponding fraction). Also given in Figure 1 are the results for the 60 coals investigated previously7. Different relationships were found for coals originating from the northern and southern hemispheres, presumably because of different geochemical conditions. For the northern hemisphere coals, which are of primary interest for the this relation is summarized present investigation, empirically by the following expression

N,=j0.15+7.6[1+(9~ x exp( -4.2

lo-“VM6)]-’ x 1O-2 VM)} x 1019

(1)

where N, is expressed in cme3. Examination of the results summarized in Figure I, shows that the Polish coals follow Equation (1) rather well, but that for some premium coals (especially coals

Magnetic resonance parameters to characterize coals: A. Jurkiewicz Table

1

Proximate

and ultimate

Coal no.

Country of origin

analysis

results and magnetic

VM (wt%)

C

(wtX)

fit%)

resonance

0

(wt%)

parameters

measured

on the coals of this study

zioI9

N, x lo-l9

Wj'

Wl’

-3

W’)

(s-l)*

1

b

(cm 1

et al.

101

USA

30.2

87.8

4.8

6.4

1.2

1.1

202

USA

48.5

75.9

5.4

17.7

2.6

1.15

301

USA

45.7

81.5

5.2

11.8

1.2

1.3

5.3

401

USA

40.8

85.0

5.4

8.2

1.3

1.1

2.9

?‘I)

I .6

1.3 45

P,,

170

12.6

550

I .4

0.77

io.01

8.7

170

3.0

0.78

F0.015

4.7

150

6.2

0.84 kO.01

2.5

630

43.5

0.89’ iO.01

510

3.8

0.77 kO.02

920

3.2

0.835k0.015

990

1.5

0.76

*0.04 kO.01

62

0.83s *O.Ol

501

USA

19.0

91.8

4.5

2.0

1.5

2.3

1.2

601

USA

47.8

81.3

5.8

11.3

1.7

1.o

8.1

701

USA

36.2

84.7

5.4

8.8

1.7

2.4

4.8

801

USA

49.2

73.9

4.9

20.0

2.3

1.7

WK9

Poland

21.3

87.6

4.9

7.8

2.8

5.7

1510

16.3

0.89

WK13

Poland

17.8

88.6

4.7

4.9

2.8

3.7

1200

19.6

0.885 kO.02

WK14

Poland

36.9

77.8

5.0

15.9

1.5

36.5

1070

2.1

0.82’ kO.02

WK22

Poland

38.1

78.9

5.1

14.3

1.5

43.3

830

2.3

o.73s kO.02

WK2512

Poland

35.8

77.4

4.5

16.4

1.6

38.8

890

2.3

0.84 kO.01

12 5.9

75

116

WK26

Poland

34.3

77.0

4.6

14.8

2.2

28.8

1100

2.2

0.78

kO.02

WK30

Poland

28.8

84.7

4.6

8.9

1.9

7.7

1070

4.6

0.83

iO.02

WK31

Poland

27.2

86.2

4.8

7.3

2.3

3.4

950

16.8

WK32

Poland

28.7

78.8

4.8

14.6

2.2

38.5

950

2.5

WK35

Poland

33.7

80.3

4.9

13.0

2.6

9.4

1070

5.0

WK36

Poland

33.1

81.6

5.1

11.8

2.8

7.1

990

6.2

0.83 kO.02

WK37

Poland

32.8

80.8

4.7

12.5

2.3

4.4

780

8.7

0.835 kO.015

0.845 iO.01

0.84’ k 0.02

a Determined on dmmf basis b Values after air exposure prior to degassing

1

x1019

20(

t ‘w r f 100

20

40

60

1

% VM CdmmD 10

Figure 1 Plot of the number of unpaired electrons, N,, as a function of the per cent volatile matter, %VM. Large dots, Polish coals; open circles, premium coals; squares, premium coals after air exposure; small dots, the sixty coals studied by Wind et a/.‘. A and B are empirical relationships obtained for coals originating from the northern and southern hemispheres, respectively

202, 501 and 801) rather large deviations are observed. This is due to the fact that these coals had never been exposed to air. Values of N, obtained after exposing these coals to air for a few months prior to evacuation are also given in Table 1 and Figure 1; a much better agreement between these values and the empirical relationship is observed, indicating that air exposure leads to irreversible changes in the amount of organic radicals.

20

30

%O(dmmfl rate, WF, as a function Figure 2 Plot of the ‘H Zeeman relaxation of the oxygen content, %O. Symbols as in Figure 1. The solid line represents the empirical relation given by Equation (3)

’ H spin-lattice relaxation Figure 2 shows a plot of the ‘H Zeeman relaxation rate, Wz”, as a function of the oxygen content, %O. In many coals the Zeeman relaxation is non-exponential’.“, and this has been ascribed to the presence of different domain structures”. Therefore, values of WF were calculated using the equation WF=ln2/t,,,

(2)

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Magnetic

resonance

parameters

to characterize

et al.

coals: A. Jurkiewicz

discussed in detail elsewhere7~9~‘4. DNP manifests itself as an enhancement of the nuclear spin polarization, when irradiation is applied at or near the electron Larmor frequency. The ‘H polarization enhancement, P,, has been determined under the solid-state condition for many coals’. This condition is obtained when the microwave irradiation frequency, o, is given by o =w, fo,, where o, and wn are the electron and proton Larmor frequencies, respectively. Figure 4 shows the relationship between P, (measured under the solid-state condition) and the carbon content, %C. The solid line represents the empirical relation, which for %C < 92% is given by the following equation7 P,=

i 4

2 N, x 10-19

6

(cm’?

Figure 3 Plot of the ‘H rotating-frame relaxation rate, WF, observed in a lock held of 1 mT, as a function of the unpaired electron concentration, N,. Symbols as in Figure I. The solid line represents the empirical relation given by Equation (4)

where tijz is the time for which M,(a)-M,(t)=0.5 [M,(coIM,(O)]. The solid curve in Figure 2 represents an empirical relationship between Wp and %O, described by the simple equation” WF= 10-2[%0]3

(3)

Figure 2 shows that for %O> 10% a clear trend exists between WF and %O. This has been explained7,” by the presence of paramagnetic O,, trapped permanently in the coal in increasing amounts for increasing %O. In fact, this relation can be used as a fast method of estimating the percentage of organic oxygen in coal, especially for %O > 10%. Also included in Figure 2 are the WF values of the premium coals that have been exposed to air prior to degassing. It can be seen, for each of the premium coals, that considerably larger Wp values are obtained after air exposure/evacuation, indicating that a part of the paramagnetic 0, is trapped irreversibly in the premium coals during air exposure. Figure 3 shows the relationship between the ‘H spin-lattice relaxation rate in the rotating frame, Wp”, observed in a lock-field of 1 mT, and N,. The rotating-frame relaxation is also non-exponential for many coals7. Therefore, W$’ was calculated using an equation similar to Equation (2). It can be seen in Figure 3 that W,” increases more or less linearly with N,, indicating that organic radicals are substantially responsible for the rotating-frame relaxation. This is in accordance with the fact that (W:)-’ is very small (ms or less), which is typical for solids containing The rough empirical relationparamagnetic centres”-13. ship indicated in Figure 3 is given by

W;=3.3

x lo-17N,

(4)

1 +27{exp[-2.3

x 10-2(92-%C)2]}

(5)

for %C ~92, o=o,-wn. It can be seen in Figure 4 that in general the results for the 20 coals agree rather well with the previous results, with the exception of the premium coal 501, for which a much larger enhancement factor (43) was obtained than found in other coals of similar rank. Again, this is due to the fact that this coal had never been exposed to air. It can be shown’ that (PH- 1) is proportional to (N,)( W,“)-‘, and that the unusually small value of WF before air exposure is largely responsible for the large enhancement factor. Indeed, if P, is calculated by using the values of N, and WF obtained after air exposure of this coal prior to degassing, a value of 32 is predicted, close to the measured value of 29. This value is in close agreement with those of other coals of similar rank (see Figure 4). The carbon aromaticity,

(f& The carbon aromaticity has been determinedi’ by means of ‘H-13C cross polarization (CP). Here the of aromaticity, Vg)cp, is defined as the total number carbons with sp and sp2 hybridization to the total number of carbons, including those with sp3 hybridization The ‘H polarization prior to CP was increased by ‘H

Ok,, 65

,

,

,

,

,

70

75

80

85

90

.,I 95

%C (dmmfl

‘H DNP enhancement,factor,

P,

Dynamic nuclear polarization phenomena other solids containing unpaired electrons

832

FUEL,

1990,

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in coal and have been

Figure 4 Plot of the ‘H DNP enhancement factor, P,, as a function of the carbon content, %C. Symbols as in Figure I. The solid curve represents the empirical relation given by Equation (5)

Magnetic

0

20

40 9% VM

resonance

60

(dmmf)

Figure 5 Plot of the i3C aromaticity obtained via DNP-CP, Vg),,, as a function of the per cent volatile matter, %VM. Symbols as in Figure I. The solid curve represents the empirical relation given by Equation (6)

DNP. Magic-angle spinning (MAS) was not applied, to avoid possible intensity distortions associated with CP under MAS conditions7.‘3. In a 13C DNP-CP experiment the resonances of the aromatic and aliphatic carbons of coal overlap substantially. In this work the aromaticity was estimated by digital subtraction of the spectrum of a highly aromatic sample from the measured spectrum of the coal of interest, yielding the spectrum of aliphatic carbons only7. Wind et ~1.~found an empirical relationship between (Qcp determined in this manner and the percentage of volatile matter, %VM. This relation is shown in Figure 5, where the solid curve is given by the following equations (Qcp=

1-4.7

x 10-3VM

for VM > 40%

et al.

The authors are grateful to the Fulbright Foundation for a grant to A. Jurkiewicz, and to the US Department of Energy for partial support of this work under Contract No. DE-FG22-85PC80-506. REFERENCES

5

(6b)

6

It can be seen in Figure 5 that in general the aromaticities obtained for the Polish and premium coals are in agreement with this relationship, with the exceptions of the Polish coals WK-22 and WK-26; those two coals have distinctly smaller aromaticities than other coals of similar rank. This might mean that these coals have been subjected to different geochemical conditions during their formation or originate from different starting materials. (Unusually low aromaticities have also been observed in some Chinese coals16,“, a fact that might be due to the same reasons).

I 8 9 10 11 12 13 14

15

CONCLUSIONS 16

For the Polish coals, the same general trends are found between magnetic resonance parameters and parameters obtained via proximate and ultimate analyses as were

coals: A. Jurkiewicz

ACKNOWLEDGEMENTS

(6a)

x 10-3(VM-40),

to characterize

found in the previous investigation of 60 coals of other origins. The main exceptions are the carbon aromaticities of two Polish coals, which are considerably lower than aromaticities of other coals of similar rank, perhaps reflecting different geochemical conditions or origins for the formation of these coals. For the premium US coals that had not been exposed to air prior to degassing, deviations are found between their magnetic resonance parameters, N,, WF and P,, and the corresponding parameters of other coals of similar rank. A much better agreement is obtained if the premium coals are exposed to air for a few months prior to evacuation. This means that irreversible processes occur in the premium coals during air exposure and implies that similar processes have taken place in the other coals, which have all been subjected to the effects of air for a long time. It should be interesting to investigate whether air exposure also changes other coal properties, such as conversion properties. To investigate general trends in coals, a large number and wide range of samples should be used. Otherwise, apparently ‘clear’ relationships found for a limited number and range of coals, may not hold up under examination of a wider range of samples, e.g. samples previously unexposed to air.

+ 1.5 x 10p2sin(2nVM/26), for VM < 40%

(&p=0.808-9.2

parameters

17

Van Krevelen, D. W. in ‘Coal’, Elsevier, Amsterdam, The Netherlands, 1969 ‘Analytical Methods for Coal’ (Ed. C. Karr, Jr.), Academic Press, New York, USA, 1978 ‘Coal Science’ (Eds. M. L. Gorbaty, J. W. Larsen and I. Wender), Academic Press, New York, USA, 1982 ‘Magnetic Resonance. Introduction, Advanced Topics and Applications to Fossil Energy’ (Eds. L. Petrakis and J. P. Fraissard), D. Reidel, 1984 Axelson, D. E. in ‘Solid State NMR of Fossil Fuels. An Experimental Approach’, Multiscience, 1985 Davidson, R. M. ‘Solid State NMR Studies of Coal’, Report No ICTlS/TR 32, IEA Coal Research, London, UK. 1986 Wind, R. A., Duijvestijn, M. J., van der Lugt, C. et al. Fuel 1987, 66, 876 Wind, R. A., Duiivestiin, M. J., van der Lugt, C. et ul. Proqr. NMR Spctr. 1985, 17, 33 Wind. R. A.. Anthonio. F. E.. Duiivestiin. M. J. et al. J. Mum. _ Res. 1983, 52, 424 Wind, R. A., Jurkiewicz, A. and Maciel, G. E. Fuel 1989,68,1189 Lowe, I. J. and Tse, D. Phys. Rev. 1968, 166. 279 Tse, D. and Lowe, I. J. Phys. Rev. 1968, 166, 292 Snape. C. E., Axelson, D. E., Botto, R. E. Edal. Furl 1989,68,547 Wind, R. A., Duijvestijn, M. J., van der Lugt, C. cl ul. in ‘Magnetic Resonance. Introduction, Advanced Topics and Applications to Fossil Energy’ (Eds. L. Petrakis and J. P. Fraissard). D. Reidel, 1984, D. 461 Pines, A.,‘Gibby, M. G. andwaugh, J. S. J. Chem. Phys. 1973. 59, 569 Retcofsky, H. L. in ‘Coal Science’ (Eds. M. L. Gorbaty, J. W. Larsen and I. Wender), Academic Press, New York, USA, 1982, p. 43 Ye, C., Wind, R. A. and Maciel, G. E. Scient~fica Sini1.u 1988. A31, 968 I

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