E.p.r. study of molecular phases in coal

E.p.r. study of molecular Stanistaw

Duber”

and Andrzej

phases in coal

B. Wieckowski

Institute of Molecular Physics of the Polish Academy of Sciences, Smoluchowskiego 17/l 9, 60-l 79 Poznari, Poland “Department of Petroleum and Coal Chemistry of the Polish Academy of Sciences, l-go Maja 62, 44- 100 Gliwice, Poland (Received 15 January 1981)

E.p.r. spectra from coals of various carbon content are reported. The e.p.r. signal from coals in a vacuum consists of two lines with different widths. The results are interpreted using the Larsen-Kovac structural model (Am. Chem. Sot. Div. Fuel Chem. Preprints 1977, 22, 181). The paramagnetic centres, disposed in the macromolecular phase, give a narrow line whereas the spins of the molecular phase are responsible for the broad line. These results provide a good interpretation of the changes in e.p.r. signal-shape under the influence of various organic solvents observed by Yokokawa (Fuel 1968, 47, 273; Fuel 1969, 48,

29). (Keywords: coal; instrumental methods of analysis; spectra)

Numerous e.p.r. studies of natural coals’-’ and also of products obtained by low-temperature pyrolysis of organic compounds ‘- l2 have led to the conclusion that the paramagnetic centres detected are organic freeradicals. Their concentration varies with the rank of coal, increasing to w 102’ spins g-’ for a carbon content of ~94 wt%, beyond which it decreases sharply2*5. The linewidth decreases with the carbon content from it 1 mT at approximately 70 wt% C to 0.1 mT in coals with the maximal spin concentration, and thereafter increases sharply2v5. In many cases7,13*‘4, asymmetric signals and signals with complicated structures have been reported for natural coals. Retcofsky et aLs have studied the signals from two fundamental macerals, vitrinite and fusinite, separated from various coals. The e.p.r. line obtained for vitrinite was broader than that for fusinite. In either case the linewidth decreased with increasing carbon content. In vitrinite, the concentration of unpaired spins increased to a maximum of = 10” spins g-’ and then decreased rapidly. Fusinite failed to exhibit any such correlation between the number of unpaired spins and the carbon content of the maceral. The structural model of bituminous coal recently proposed by Kovac and Larsen’ 5,16 is very well adapted to the interpretation of the e.p.r. signals. Their model involves a molecular (M) and a macromolecular (MM) phase. The MM phase forms a three-dimensional skeleton, consisting of macromolecular fragments, connected by cross-bonds. The M phase is distributed in the pores of the MM phase or on its edges. The model does not state in what way the two phases are interconnected. From studies of extraction of coals with solvents Marzec et al.” have concluded that the forces acting between the M and MM phases are donor-acceptor in nature and lead to the existence of molecular complexes M+MM and M-MM in the coal (the arrow indicates the shift of fractional negative charge). Moreover, they have 00162361/82/050433AM$3.00 @ 1982 Butterworth & Co (Publishers)

Ltd.

proposed a mechanism whereby the solvents act on the coal in a substitution reaction in accordance with the general scheme: M -+ MM + solvent = M + solvent -+ MM

(1)

The present work is concerned with a range of Polish coals of different rank. An attempt is made to analyse the results in terms of the Larsen-Kovac model, with the aim of obtaining better insight into the nature of the paramagnetic centres and their relation to the structural elements of the coal. EXPERIMENTAL The coals studied were ground to a partial size of co.06 mm. Their characteristics are given in Table 1. Figure 1 shows the atomic H/C ratio versusthe carbon content of the coals with corresponding data for three macerals of different coals, studied by van Krevelen and Schuyer’s, to allow a comparison of the Polish coals with some from other countries. A sample of coal, weighing ~20 mg, was placed in a quartz tube and degassed by evacuating to < 1.33 x 10-l Pa which was maintained for 60 inin. The degassed samples were kept in sealed quartz tubes. The measurements were performed with a RadiopanPoznaIi X-band spectrometer with 100 kHz modulation. A ruby crystal was inserted into the resonance cavity as a standard for the position and amplitude of the resonance line. For the resolution of the e.p.r. spectrum into two components, a Fortran digital computer program was established. Following a preliminary analysis, the lines were assumed to be Lorentzian. The program was based on rectilinearization of the functions. The parameters characterizing the Lorentzian function were calculated by the least-squares method.

FUEL, 1982, Vol 61, May

433

E.p.r. study of molecular Table 1 Characteristics

phases in coal: S. Duber and A. 6. Wiqckowski

of the coals studied Proximate Ultimate

32 31 33 34 37 38 35 42

analysis (wt %, daf)

Atomic

C

H

N

s

Ddiff

78.1

5.1

14.8

5.2 5.6 5.8

1.3 1.1 1.5 1.8 1 .3 1.2 0.9 1 .o

0.7

79.7 81.1 83.0 87.1 87.3 89.5 90.8

1 .o 0.9 0.9 0.9 1.6 0.8 1 .o

13.0 10.9 8.5 5.6 5.6 3.8 3.4

5.1

4.3 5.0 3.8

H/C ratio

Moisture (as-received)

0.79 0.78 0.83 0.83 0.70 0.59 0.67 0.50

8.0

7.0 2.8 2.1 1 .o 1.3 1.3 0.8

analysis (wt %I Volatile matter (daf)

34.8 35.9 35.8 36.9 22.0 13.3 19.9 6.6

Ash (as-received)

4.7 5.7 5.8 8.0 6.9 11.9 4.2 9.1

Density in helium, dzo (g cm-3) 1.288 1.272 1.293 1.255 1.289 1.320 1.332 1.372

‘*-

I 75

70

I 80

I I 85 90 Carbon (wt %, daf )

I 95

100

Figure 1 Atomic H/C ratio versus the carbon content (wt %, daf). Macerals: 0, vitrinite; n, exinite; q, micrinite; (van Krevelen and Schuyerle). 0, Polish coals (this work)

The spin-lattice relaxation time, T,, was determined by the method of continuous saturation of the e.p.r. signal. Both Ti and the spin-spin relaxation time, T,, were calculated following the procedure of Poole”. RESULTS

AND DISCUSSION

Figure 2 shows the spectra obtained in measurements without microwave saturation. Coal samples 31 and 32 are characterized by a slightly asymmetric line and lack of any discernible two-component structure to the signal. Sample 33 exhibits an atypical e.p.r. line shape; the line is asymmetric, with smoothly descending wings and a steep slope in the central parts, indicating a signal consisting of at least two lines, one broad and decisive for the wings and the other, narrow, responsible for the steepness of the central part of the line. The e.p.r. signals from samples 34 42 permit similar resolution of broad and narrow line components. Hence, it was assumed that the e.p.r. spectra of all the coal samples consisted of two components. The spectra were processed by computer; the respective g- and (from peak-to-peak) 2AB,,-values are given in Table 2. Both the broad and the narrow lines decrease in width with increasing carbon content of the sample. Their spectroscopic splitting factors, g, differ sufficiently for the sum of the two lines to give a slightly asymmetric line

434

FUEL, 1982, Vol 61, May

Figure 2 saturation. line

E.p.r. spectra from the coals studied without microwave Each spectrum consists of a narrow line and a broad

shape (Figure 2), but, in the case of samples 35,37 and 42 the difference in position was so insignificant that, within the limits of error, the results obtained by computer analysis led to the same values of g. Figure 3 shows the change in spin-spin relaxation time, T2, as a function of the H/C ratio. For bituminous coals with H/C>0.8, the times, T,, characterizing the two systems of spins are short and the difference between them

E.p.r. study of molecular Table 2

Linewidths

2Acls1s, relaxation

times, T,,

T1, and g-factors

phases in coal: S. Duber and A. B. Wigckowski

for the two component

lines of the e.p.r. signals from the coals Narrow

Broad line Sample

2tiIs

32 31 33 34 37 38 35 42

0.638 0.632 0.723 0.614 0.512 0.464 0.529 0.454

‘”

(mT)

line

T, x IO8 (s)

T, x 106 (s)

g

~AI~B,, (mT)

T2 x IO8 (s)

T, x IO’

1 .o 1 .o 0.9 1.1 1.3 1 .4 1 .2 1.5

5.0 2.5 7.9 6.5 3.0 2.6 3.1 2.8

2.0032 2.0032 2.0031 2.0027 2 0036 2.0033 2.0042 2.0037

0.228 0.211 0.189 0.128 0.076 0.070 0.063 0.065

2.9 3.1 3.5 5.1 8.7 9.3 7.9 10.0

16.6 16.4 12.7 14.1 3.2 3.1 3.5 3.2

(s)

g 2.0031 2.0030 2.0028 2.0025 2.0036 2.0034 2.0042 2.0037

I-

---v.,‘\\\ \ \ ‘1’

\A\

1o-g *

-J

O8

0.5

0.8

0

7

0

10-T 0.4

05

Figure 3 Spin-spin relaxation 0, M phase; 0, MM phase

0.6

0.7 H/C

0.8

0.9

time, Tz k), versus atomic

1.0

H/C ratio.

is insignificant. As H/C decreases, 7” increases for both systems. In the case of the spin system related to the broad line these variations are less than for the spin related to the narrow line. As a result of this, the difference between the T, values for the two systems of spins increases with decreasing H/C. The changes in the properties of either system of spins are related to the changes occurring in the chemistry and physics of the phase in which the respective spins are located. Each of these systems of spins, related to the narrow and broad line respectively, characterizes one of the two phases containing the paramagnetic centres. These phases differ as to their chemical and physical structure. The physical properties of either phase are described by the spin-lattice relaxation time, T,, shown in Figure 4 as a function of H/C. The difference in T1 values for the two phases is small for bituminous coals with H/C 20.8. With decreasing H/C the time, T,, becomes shorter: for the phase related to the narrow line the decrease in Tl is greater, leading to a limiting value of Tl = 3 x lo- ’ s.

0.4

0.6

Figure 4 Spin-latticerelaxationtime, ratio. 0, M phase; 0, MM phase

0.7 H/C

0.9

T, Is), versus atomic

1 .o

H/C

If it is assumed that, at room temperature, the spinlattice interaction mechanism is determined essentially by a two-phonon Raman process2’. Such a process is much more effective in a phase with larger regions of crystallattice ordering strongly bound to one another by covalent bonds. Hence, the system of paramagnetic centres with which the narrow line is associated has to be located in the MM phase of the Larsen-Kovac mode1’s,‘6, whereas the spins giving the broad line are located in the M phase. A supplementary argument in favour of the localization of the paramagnetic centres generating the narrow line in the MM phase resides in the narrowness of the line in question ( < 0.01 mT) for coals with H,/C < 0.7 (Table 2). A line as narrow as this must be the result of strong exchange between the spins. The exchange interaction decreases markedly as the distance between the paramagnetic centres increases. Assuming the unpaired electrons to be delocalized over several aromatic rings in the large macromolecules, the condition of strong exchange is

FUEL, 1982, Vol 61, May

435

E.p.r. study of molecular

phases in coal: S. Duber and A. B. Wiqckowski

fulfilled. Also, the fact that oxygen adsorbed on the coal surface strongly affects the increase in width of the narrow line8*12 is evidence in favour of a delocalization of the unpaired electrons associated with the narrow line. A mechanism accounting qualitatively for this effect has been proposed by Ingram*{ on the assumption of delocalization of the unpaired spin. The same conclusion is reached when analysing Figure 3 which shows the changes in spin-spin relaxation time, T,, related to the linewidth. From earlier X-ray studies of coals22 and studies of parameters describing their chemical and physical properties uersus rank,18 the crystallites are small in size along the bedding planes in low-carbon coals (-0.7-1.0 nm on average)22. Coals of this type (samples 31, 32 and 33 of Table I) contained approximately 5 wt% of hydrogen and 13-10 wt% of oxygen. With increasing carbon content the crystallites grow in size to 2.G3.0 nm at the same time, the hydrogen and oxygen contents decrease to -3 wt%, whereas nitrogen remains at a level of 1 wt%. In general, the two phases undergo ordering; during this process, the lines of both phases are reduced in width, more markedly in the case of the phase associated with the narrower line. From the assumption of the existence of two phases as evidenced by the two components of the paramagnetic absorption signal, the narrow line must be assigned to paramagnetic centres in the more highly ordered MM phase. In the opposite case, the general shape of the T2 curves for the two phases (Figure 3), convergent at short times (dashed lines) for H/C values extrapolated close to unity, would converge for longer T2 times. Similarly the spin-lattice relaxation times, T,, of the two systems of spins (Figure 4) converge at H/C = 0.9. This corresponds to TI = 3 x lo-’ s, a value close to that of TI for the initial stage of carbonization of saccharose’. The assignment of the narrow line to paramagnetic centres in the MM phase is corroborated by the results of Retcofsky et al.‘. Fusinite is a maceral with a higher carbon content and a more abundant MM phase than vitrinite from the same coa123, and these authors obtained narrower e.p.r. absorption lines from the former maceral. Yokokawa13*‘4 studied the effects of various organic solvents on the e.p.r. absorption signals from natural coals. His measurements were carried out in air. Some coals, when saturated with ethylenediamine, emitted a modified absorption signal; against the background of the broad line, a well-defined narrow line emerged. On removal of the solvent the intensity of the broad line increased causing the signal to revert partly to its initial form. This complexity of the signal was observed for fusinite as well as for vitrinite. Yokokawa suggested an interpretation of the effect based on the replacement of of ethylenediamine. This by molecules oxygen phenomenon becomes evident in the light of the present results and on the basis of the Larsen-Kovac model’ “16, supplemented by the results of Marzec and co-workers”. Organic solvents acting on coal cause a disruption of the donor-acceptor bonds between molecules of the M phase and of the bonds between the M phase and MM phase, leading to the detachment of small fragments of the MM phase and thus to an enrichment of the M phase24.

436

FUEL, 1982, Vol 61, May

However, the MM phase is maintained essentially by strong covalent bonds. The disruption of donor-acceptor bonds M+MM and M+MM enhances the mobility of the molecules of the M phase and also the probability of recombination of its free radicals. In this way, the narrow line due to the centres of the MM phase becomes apparent against the background of the broad line. The removal of the solvent in vacuum causes a partial reversal to the initial state. ACKNOWLEDGEMENTS The authors wish to thank Professor J. Stankowski, ofthe Institute of Molecular Physics of the Polish Academy of Sciences in Poznali, and Professor Anna Marzec, of the Department of Petroleum and Coal Chemistry of the Polish Academy of Sciences in Gliwice, for their numerous discussions and interest throughout this investigation. Thanks are due to Dr A. Dezor and Dr W. Malinowski, of the Institute of Molecular Physics of the Polish Academy of Sciences in Poznali, for their kind help in performing the numerical computer calculations.

REFERENCES

6 7 8 9 10 11 12 13 14 15 16 17 18

19 20 21 22 23 24

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