Quantitative 13C NMR study of structural variations within the vitrinite and inertinite maceral groups for a semifusinite-rich bituminous coal

PII: SOO16-2361(97)00260-3

Fuel Vol. 77, No. 8, pp. 805-813, 1998 0 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0016.2361/98 $19.01I+O.o0

Quantitative 13C NMR study of structural variations within the vitrinite and inertinite maceral groups for a semifusinite-rich bituminous coal M. Mercedes Maroto-Valerlya, Darrell N. Taulbeeb, John M. Andr6sena, James C. Howerb and Colin E. Snapeaf* aUniversity of Strathclyde, Department of Pure and Applied Chemistry, Fuel Chemistry Research Group, Glasgow Gl IXL, UK bCenter for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, KY 40511-84 10, USA (Received 7 October 1997)

To determine the structural variation within vitrinite and inertinite maceral groups, fractions with purities over

90% in vitrinite and semifusinite were obtained by density gradient centrifugation from a medium volatile Australian bituminous coal and the bulk structural compositions of the maceral concentrates were determined by the quantitatively reliable single pulse excitation (SPE) solid state 13CNh4R technique. As previously reported for coals and chars, the aromaticities determined by cross polarisation are often lower than those by SPE, due to the unfavourable spin dynamics. As expected, the aromaticities of the vitrinite fractions are significantly lower than those of the semifusinite ones, but the aromaticity, the fraction of non-protonated aromatic carbon and the number of rings per cluster all increase with density within both the maceral groups. The vitrinite and semifusinite fractions contain 3-6 and 9 to over 15 aromatic rings, respectively. Methyl groups account for greater proportions of the aliphatic carbon with increasing density. These structural trends are consistent with the variations evident in random reflectance. 0 1998 Elsevier Science Ltd. All rights reserved (Keywords:

vitrinite; semifusinite;

density gradient centrifugation

(DGC); solid state 13CNMR; single pulse excitation

(SW)

INTRODUCTION The variations in chemical composition between the maceral groups are reflected in density differences, which is the basis for the two separation techniques used, namely sink-float and density gradient centrifugation (DGC). The first of these, which was introduced by van Krevelen et al. ‘, requires a number of steps to separate reasonably pure macerals and is thus time consuming. The DGC technique developed by Drycasz and Horwitz2 allows the separation of several density fractions, so reducing the time required. The procedure requires two preliminary steps to guarantee success, namely (i) crushing the sample to micron size (6 pm) to assure that the particles are monomaceral in nature, and (ii) demineralisation by conventional HCl/HF treatment3, since the presence of mineral matter increases the average density of the coal matrix. However, this

* To whom correspondence should be addressed. ’ Present address: Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, KY 4051 l-8410, USA.

procedure has limitations in that (i) the small particle size used increases the possibility of error in the microscopic analysis, and (ii) only ca. 2 of sample is processed in each 4F separation. Taulbee et al. ’ overcame these by increasing both the particle size up to 100 mesh, and the amount of sample processed per batch to 8 g. The increase in particle size clearly gives rise to less pure maceral concentrates and there is the possibility of a greater retention of mineral matter after demineralisation. However, on the other hand, the larger particle size enables the maceral compositions of the DGC fractions to be determined with much greater certainty. Maceral concentrates separated by the density methods described above have been characterised in a number of investigations4,6-17. Elemental data obtained for pure macerals have been reported for coals6 and for oil shales4T7. The atomic H/C ratios decrease continuously with increasing density, suggesting a concomitant increase in aromaticity. Nitrogen and oxygen contents are lower for the lighter liptinite fractions than for vitrinite and huminite fractions. Atomic S/C ratios are independent of the density

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‘3C NMR study of bituminous

Table 1

coal: M. M. Maroto-Valer

Density cut points and petrographic

Coal/fraction no.

et al.

data for the initial coal, demineralised

Cut point (g cm-‘)

vitrinite (vol.%)

coal and the fractions from the preliminary Semifusinite (vol.%)

separation

Fusnite (vol.%)

Initial coal *

63.4

22.8

13.3

Demineralised (DEM) a

68.5

22.0

9.0 2.5

1”

1.25

92.4

4.0

2

1.27

90.8

7.7

1.5

1.28

92.0

6.0

2.0

1.29

88.0

10.0

2.0

1.30

81.0

15.0

4.0

1.31

76.0

21.5

2.5

1.32

75.5

21.5

3.0 0.5

8

1.33

66.0

33.5

9

1.34

45.5

53.0

1.5

10

1.35

29.8

66.0

4.2

11

1.36

19.0

75.0

6.0

12

1.37

4.0

89.0

7.0

13

1.38

2.0

92.0

6.0

14

1.39

2.0

87.0

11.0

15

1.40

2.0

89.0

9.0

16

1.42

3.5

83.0

13.5

17

1.44

1.5

67.5

31.0

18

>1.44

1.5

56.5

42.0

a Both the coal and the deminemlised sample contain 0.5% of liptinite, while fraction 1 contains 1.1 vol.% of liptinite.

for kerogen (oil shale) maceral sarr~ples~~~, but these decrease with increasing density in an analogous manner to the I-I/C ratios for coal macerals6. The aromaticities of coal macerals have been characterised extensively, using the early densimetric methods’ and, more recently, by 13C cross olarisation/magic angle spinning (CPMAS) P NMR*- ‘. These investigations have all indicated that the aromaticity increases in the order liptinite < vitrinite < inertinite, which agrees with the results of Fourier transform infrared (FTIR) studies4,‘2*‘3. Furthermore, combined chemical alkylation with 13C enriched methyl iodide and 13C NMR have shown that the degree of substitution of aromatic rings decreases from liptinite < vitrinite < inertinite14. Electron spin resonance (ESR) studies have indicated that the concentrations of free radicals follow the trend: liptinite < vitrinite < intertinite15’16. However, while the structural variations between different maceral groups are well established, the structural diversity that might exist within a given maceral group has not been addressed by solid state r3C NMR in a quantitative manner thus far, although FTIR has also suggested that profound variations can occurl’. Although solid state 13C NMR is generally considered to be the best approach to obtain quantitative carbon skeletal parameters on coals and their insoluble derivatives, such as chars, the CP technique often discriminates against aromatic carbon17-23. The strategy of using a combination of a relatively low magnetic field strength to minimise problems with spinning sidebands and the simple, albeit insensitive, Bloch decay or single pulse excitation (SPE) technique is now generally recognised as the best approach for obtaining quantitative 13C NMR results on coal~‘~-~~ and their derivatives, such as chars and coal-tar pitches23. However, only CP has thus far been applied to maceral concentratess- il. In this investigation, high purity vitrinite and semi-fusinite maceral concentrates (over 90% purity) have been obtained by DGC from a medium volatile Australian coal with a relatively high inertinite content, and

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the quantitative SPE 13C NMR methodology has been used to elucidate structural variations within the vitrinite and semifusinite groups. A preliminary account of this investigation can be found in the proceedings of the 1997 International Conference on Coal Science24. In addition, the maceral fractions have a particle size range (<75 pm) large enough for meaningful carbonisation experiments, and the use of high temperature ‘H NMR to determine the contributions from vitrinite and semifusinite to the overall plasticity development for the whole coal is described elsewhere24.25.

EXPERIMENTAL Coal and demineralisation The medium volatile Australian bituminous coal (RQ,~ = 1.14, V.M. of 23%) was chosen for the maceral study because of its high relatively concentration of semifusinite (Table 1). The coal was ground to <75 pm, and demineralised using conventional HCUI-IF treatments3. Approximately 50 g was weighed and placed in a glass beaker with 750 cm3 of 6 N HCl. The mixture was stirred for 24 h under nitrogen. The solution was then filtered aided by vacuum suction, washed with distilled water, and transferred to a Teflon beaker. A mixture of 500 cm3 I-IF (4849%) and 250 cm3 of HCl(6 N) was then added to the coal. The solution was stirred overnight for around 24 h under nitrogen. The solution was filtered and rinsed with distilled water until the filtrate was neutral. The coal was dried overnight under 60 kPa vacuum at 50°C. All the acid treatments were carried out under a nitrogen atmosphere to minimise the risk of oxidation. After demineralisation, the recovered sample was again sieved to <75 km to circumvent any possible agglomeration that could have taken place during demineralisation. The ash content of the recovered sample was determined (ASTM-D3174-93) to ascertain the extent of demineralisation achieved.

‘3C NMR study of bituminous

& a z

10

Spectroscopic

1.26

1.29

1.32

1.35

1.38

1.41

1.44

Density cut I g cm3 Figure 1

Recoveries obtained in the preliminary separation

Density gradient centrifugation The procedure used for the DGC separation has been described previously by Taulbee and coworkers4*5*7. In summary, a Beckman model J2-21 fitted with a 1.9 dm3 capacity titanium JCF-Z zonal core rotor was used. CsCl gradients were loaded stepwise to the outer wall of a spinning rotor (2000 rpm) at a rate of 50 cm3 min-’ using a rotary peristaltic pump Masterflex 7016. The density measurements were made using a portable Mettler DMA 35 oscillating meter. Typically, between five and six density gradients were employed, the densities and volumes being tailored according to the densities targeted. A preliminary separation (PS) was carried out using 6 g of demineralised coal dispersed in CsCl solution of density 1.10 g cme3. The mixture was vigorously stirred with a magnetic bar and was ultrasonically dispersed using a Fisher model 300 sonic dismembrator during transfer to the centre of the rotor at a rate of 25 cm3 min-‘. For the large scale 10 g of sample was dispersed in runs, a proximately Y 175 cm solution. In the PS, the CsCl solution employed contained a surfactant, Brij-35 (polyoxyethylene (23) lauryl ether [C 12H25(OCH2)230H]), to avoid possible agglomeration. The runs LS-1, LS-2 and LS-3 were carried out without surfactant, and in LS-4, ca. 2 g of Brij-35 was added to the coal dispersion. After loading, the rotor speed was increased to 16 000 ‘pm, and the slurry spun for one hour, after which the speed was slowly reduced to 2000 rpm. A high density fluorocarbon (Fluorinert FC-43, 3M Corp.) was then introduced to the outer wall, so the dispersion forming the gradient is forced to exit through the portable density meter to determine the collection cut points. The resultant fractions were then filtered, rinsed three times with deionised water, diluted in 100 cm3 of water, ultrasonicated, refiltered, rinsed again three times, and finally left to dry overnight at 50°C under 70-80 kPa vacuum. Petrographic

et al.

grit sandpaper successively, followed by fine polishing with a 0.3 pm alumina suspension on a Texmet polishing cloth, and finally by a 0.005 pm alumina suspension on a silk polishing cloth. A minimum of around 200 counts was taken in each pellet using a X 32 objective under oil immersion, and blue light fluorescence was employed to verify the identification of liptinite. In addition, the reflectance was measured for the vitrinite rich fractions, as described elsewherez6.

Semifwinite zone

15

coal: M. M. Maroto-Valer

characterisation

Petrographic analysis was performed on each density fraction using a Leitz microscope, according to standard procedures (ASTM-D2797-85). Micropellets were prepared using a low fluorescence epoxy resin (90 cm3) and hardener (3 cm3) to set the mixture. To minimise the amount of sample required for analysis, a hole of 1 cm i.d. and 1.5 cm deep was drilled in the pellets to introduce a mixture of sample, epoxy and hardener. A polisher Ecemet III grinder was used for polishing the pellets, employing 240 and 600

analyses

Eight samples were chosen for detailed characterisation by NMR and ESR from the large scale runs without surfactant. These included fractions rich in vitrinite of varying density, designated DGC-1V to DGC-4V (1.26- 1.29 g cme3), an intermediate fraction, DGC-6 with similar proportions of vitrinite and semifusinite, two concentrates of semifusinite, DGC-9S and DGC- 10s (1.36- 1.39 g cmm3), and the most dense fraction, DGC- 11 (>1.39 g cmm3). The letters V and S in the nomenclature used refer to vitrinite and semifusinite concentrates with purities over 90%. A Bruker MSL-100 spectrometer with MAS at 5.0 kHz was used for the 13C NMR measurements as described previously21-24. SPE and CP measurements were carried out to determine the proportion of aromatic carbon. The 13C thermal relaxation times (T1 values) were determined using the Torchia pulse sequence27. Dipolar dephasing experiments were also conducted to establish the concentrations of non-protonated aromatic carbon and these were used to calculate the bridgehead aromatic carbon concentrawith a field tions2’*23. A Jeol JES-FE ESR instrument strength of 0.34 T was used for the determination of free radical concentrations (N,)23.

RESULTS

AND DISCUSSION

Preliminary separation Prior to the DGC separation, the Australian coal was demineralised, the ash content being reduced from 9.4% (db) to 0.22% (db), with a recovery close to 96%. Table 1 lists the petrographic compositions of the initial and demineralised (DEM) coal. A preliminary separation was conducted in which 6 g of demineralised sample was divided into 18 density fractions. Table 1 lists the density cut points and the petrographic composition of the resultant fractions. There was little material with densities below - 1.23 g cm-3. No attempt was made to collect the pellet, and the total weight recovery was 94%. Figure 1 shows the variation of yield with density. Three different zones can be identified. In the first, the highest recovery was obtained at low density, with over 59% being recovered in the range 1.25- 1.29 g cm-3 (corresponds to fraction numbers 2,3 and 4, with recoveries of 11, 24 and 22%, respectively). The second zone starts after the fourth fraction, where there is a sharp decrease in recovery possibly signalling the end of the vitrinite range. Then, the yield continues to fall continuously until a density of 1.38 g cmp3 is reached (fraction no. 13). Finally, for densities between 1.38 and 1.44 g crnW3 (fraction nos. 14- 1S), the yields show a much more gradual decrease. TabZe 1 list the petrographic compositions of the 18 fractions from the PS. Extremely good enrichments of over 90% were obtained in the two macerals targeted, namely vitrinite and semifusinite. As expected, the liptinite was

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60-

-

et al.

VitriniteI %

!z 2.

1.26

1.27

1.28

1.29

1.33

1.34

1.35

1.36

1.37

1.39

1.42

Density cut ! g cm3

Figure 2

Table 2

Variation of maceral concentrations

with density for the LS fractions

Recoveries from the large scale runs (LS-1, LS-2 and LS-3)

Fraction no. ’

LS-1

Density (g cm-?

LS-2 (%)

(g)

(g)

LS-3 (%)

(g)

(%)

1 (1)

1.26

0.244

3.1

0.270

2.8

0.279

3.0

2 (2)

1.27

0.932

11.7

0.936

9.7

0.860

9.4

3 (3)

1.28

2.437

30.6

2.748

28.4

2.279

24.9

4 (4)

1.29

1.710

21.4

1.834

19.0

2.040

22.3

1.30-1.33

1.366

17.1

2.344

24.2

2.283

25.0

6 (9)

1.34

0.369

4.6

0.373

3.9

0.322

3.5

7 (10)

1.35

0.217

2.7

0.291

3.0

0.283

3.1

8 (11)

1.36

0.220

2.8

0.213

2.2

0.215

2.3

5 (5-S)

1.37

0.158

2.0

0.172

1.8

0.197

2.2

10 (13-14)

1.38-1.39

0.227

2.8

0.241

2.5

0.204

2.2

11 (15-18)

b1.39

0.052

0.7

0.212

2.2

0.176

1.9

0.041

0.4

0.013

0.1

9 (12)

12

Pellet

0.039

0.5

7.973

Total

9.674

9.153

a The numbers in parentheses indicate the fractions of the PR that were combined in the large-scale runs.

only observed in the lightest fraction, and due to its low abundance (< 1% in the coal), it was not possible to achieve a significant enrichment. Vitrinite was enriched to around 90% in fractions 1-4, with densities of 1.23-1.29 g cmm3. The primary impurity was semifusinite in fractions 5- 11 (densities 1.29-1.36 g cmm3), which progressively replaces vitrinite as the predominant maceral (as suggested from Figure I). The density range of 1.37- 1.42 g cme3 (fractions 12-16) is the semifusinite domain (purities over 83%), where the main impurities are vitrinite and fusinite for the least dense fractions, and fusinite for the most dense ones (fractions 14-16). As indicated above, this domain is indicated in Figure 1 by a tail in the weight recovery profile through this density range. Finally, the last two fractions showed some enrichment in the most dense maceral, fusinite, but the most abundant component was still semifusinite due to its relative high concentration in the coal compared to fusinite (22% cf. 9%). Large scale separations On the basis of the PS, large scale runs were carried out, where the density cut points were tailored according to the petrography data obtained for the 18 fractions analysed from the PS. It was necessary to carry out three runs to acquire sufficient sample (0.4 g of semifusinite-rich fraction) for

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NMR analysis. Table 2 lists the recoveries for these runs (LS- 1, LS-2 and LS-3), the total recoveries being 80,97 and 97%, respectively. The lower recovery for LS-1 was due to the losses that occurred during the filtration of fraction no. 5. Table 2 indicates that a similar weight recovery distribution was obtained in all three runs. The density cut points were slightly modified in relation to those used for the PS (Table 1) by reducing the number of mid-range fractions collected, where no significant enrichment was achieved. Thus, fraction 5 in the large scale runs corresponds to fractions 5-8 in the PS, and where a transition exists between vitrinite and semifusinite. Fraction 10 is a combination of fractions 13 and 14 in the PS, due to the petrographic similarity of these two fractions. Finally, it was decided to combine all the fractions with densities > 1.39 g cmp3 (fractions 15-18 in the PS) into fraction 11 for the large scale runs to obtain sufficient sample for analysis. Although no surfactant was used in these runs, the overall trend is similar to that observed in the PS (Figure 1). This can be attributed to the fact that the coal was re-sieved (<75 pm) after demineralisation and the sample was thoroughly dispersed during loading to hinder particle agglomeration (which otherwise is achieved by using surfactant). Due to the similar recovery trend obtained in the three runs, it was decided to bulk the corresponding

13CNMR study of bituminous

60

et al.

coal: M. M. Maroto-Valer

m DGC-1V

?? DGC_zV 8 DGC-3V

?? DGC-4V ~~~___

0.800-0.899

0.900-0.999

1.000-l ,099

1.100-1.199

1.200-1.299

1.300-l ,399

Random reflectance Figure 3

Histograms

Table 3

Petrographic

of random reflectance for the vitrinite fractions (DGC- 1V to DGC-4V)

compositions

Fraction number

of the fractions from the large-scale Cut point (cme3)

Vitrinite (vol.%)

separation runs Semifusinite (vol.%)

Fusinite (vol.%)

la

1.26

92.8

4.2

2

1.27

94.8

4.7

0.5

3

1.28

95.3

4.0

0.7

4

1.29

91.8

7.5

0.7

5

1.30-1.33

71.0

26.0

3.0

6

1.34

40.5

56.0

3.5

7

1.35

33.5

62.0

4.5

8

1.36

23.5

71.5

5.0

9

1.37

5.1

90.7

4.2

10

1.38-1.39

3.8

90.0

6.2

11

>1.39

3.5

78.5

18.0

0.5

a Fraction 1 also contains 2.5 vol.% of liptinite.

Table 4

Elemental compositions

and atomic ratios for the samples investigated

Density

DGC-IV

Elemental composition (daf)

(g cmm3)

%C

%H

%N

86.0

4.5

1.OO-1.26

86.3

4.9

Coal DEM b

by 13C NMR Atomic ratios %O”

H/C

1.5

8.1

0.63

0.014

0.075

1.8

7.1

0.68

0.017

0.066

N/C

o/c

DGC-2V

1.26- 1.27

86.4

5.1

2.2

6.3

0.71

0.022

0.058

DGC-3V

1.27- 1.28

86.1

4.7

1.7

7.5

0.66

0.017

0.070

DGC4V

1.28-1.29

86.5

4.6

1.5

7.4

0.64

0.015

0.069

DGC-6

1.33-1.34

86.1

3.9

1.4

6.5

0.54

0.014

0.059

DGC-9S

1.36-1.37

89.0

3.9

1.2

5.9

0.53

0.011

0.053

DGC-1OS

1.37-1.39

88.2

3.8

1.2

6.8

0.51

0.012

0.062

>1.39

90.2

4.0

0.9

5.0

0.53

0.008

0.041

DGC-I 1

a Determined by difference. b DEM, demineralised coal.

density fractions from each run. These bulked samples are designated as DGC-1 to DGC-11. Table 3 and Figure 2 present their petrographic data. There are four vitrinite-rich fractions (DGC-1V to DGC4V) and two semifusinite-rich fractions (DGC-9S and DGC-10s) with purities over 90%. As expected, the maximum concentration of fusinite was achieved in the most dense fraction, DGC-11, but it was

only 18% due to its low abundance (-10%).

in the parent

coal

Random reflectance measurements Figure 3 shows the histogram of the random reflectance for the vitrinite fractions, DGC-1V to DGC4V. As expected for these high purity fractions, the histograms

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‘3C NMR study of bituminous coal: M. M. Maroto-Valer et al.

25-8

_ ‘00

Density I g cme3 Figure 4

Variation of the carbon thermal relaxation constants and spin concentrations with density

Table 5 ’3C thermal relaxation times and ESR-determined spin concentrations for the samples investigated by 13CNhIR

DEM a DGC-1V DGC-2V DGC-3V DGC-4V DGC-6 DGC-9S DGC- 10s DGC-11

ti (s) 24.0 24.0 26.0 27.0 24.0

N,

(1019 spins g-‘)

12.0 7.3 7.8 6.0

3.5 3.0 2.5 2.5 3.0 5.5 7.5 8.0 8.5

aDEM, demineralised coal. ,. expand only over a small range, but they shift to higher reflectances with increasing density. The mean random reflectances are 1.07, 1.15, 1.12 and 1.22 for DGC-IV to DGC4V, respectively. Although the mean random reflectances for samples DGC-2V and DGC-3V are similar, the differences between DGC-1V and DGC-2V and between DGC-3V and DGC4V are significant, since the reflectance differences are more than the calculated standard deviation of 0.06. These differences in reflectance indicate that structural variation exists within the vitrinite maceral group. Elemental analysis Table 4 list the elemental compositions and the atomic H/C, N/C and O/C ratios for the samples investigated. The analysis for the raw coal and the demineralised sample (DEM) are very similar, with somewhat lower C content for the DEM sample, possibly due to some carbonates having been removed. As expected, for the vitrinite fractions (DGC-1V to DGC4V), the C contents are lower than for the inertinite fractions (DGC-9S and DGC-10s) with the atomic H/C ratios being higher, as previously reported by Dyrkacz et aL6 and Taulbee et a1.28. Furthermore, the I-I/C ratios decrease with density for the vitrinite fractions, with values ranging from 0.71 to 0.64 for the most and least dense fractions, respectively. However, the H/C ratios for density fractions above 1.34 g cme3 show very small variations (0.51-0.54), despite their significantly different petrographic

810

Fuel 1998 Volume

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. . . 1,. 200

.

1..

.

.

1..

100 PPM

.

1..

.

1..

0

.

Figure 5 13C SPE spectra for the two vitrinite concentrates, DGC-1V (bottom) and DGC-4V (middle), and for the semifusinite fraction DGC-9S (top)

characteristics. The N/C and O/C ratios also decrease with density. Although these are more subject to error, due to the small concentrations of N present and the 0 contents being estimated by difference, the data indicate that heteroatomic concentrations decrease with increasing density. In the case of oxygen, phenols are more likely to be most abundant in the least dense fractions, but the proportion of furanic 0 is likely to increase with density, as reported for coals of increasing rank by using a combination of analytical (MS and XI’S) methods29. ESR and NMR relaxation studies Table 5 list the 13C thermal relaxation times (fi) and the concentrations of free radical (N,) for the fractions investigated. The Torchia experiments were conducted primarily to ascertain the delay in the SPE experiments, and therefore the c values listed correspond to the longer component obtained for the aromatic carbon when two were evident. Figure 4 shows the variation of the thermal relaxation constant fi and N, with density. The demineralised sample (DEM) has a higher free radical concentration

‘3C NMR study of bituminous

Table 6

coal: M. M. Maroto-Valer

et al.

Carbon distributions for the samples investigated by SPE and CP (values in parentheses, contact time of 1 ms) ‘k NMR f methmetmerhylene a

fa( 2 0.01)

f methyl b

fmhyllfai c

DEM d

0.85

(0.73)

0.10

(0.20)

0.05

(0.07)

0.33

DGC-1V

0.79

(0.74)

0.14

(0.19)

0.07

(0.07)

0.33

DGC-2V

0.80

(0.75)

0.14

(0.17)

0.06

(0.08)

0.3

DGC-3V

0.83

(0.78)

0.11

(0.15)

0.06

(0.08)

0.35

(0.34)

DGC-4V

0.85

(0.80)

0.10

(0.13)

0.05

(0.07)

0.36

(0.35)

DGC-6

0.90

(0.85)

0.06

(0.09)

0.04

(0.06)

0.40

(0.40)

DGC-9S

0.90

(0.86)

0.05

(0.07)

0.04

(0.06)

0.44

(0.47)

DGC-IOS

0.90

(0.86)

0.06

(0.08)

0.04

(0.06)

0.40

(0.42)

DGC- 11

0.91

(0.86)

0.04

(0.08)

0.05

(0.06)

0.55

(0.42)

(0.26) (0.27)

1

(0.32)

a Aliphatic carbon in the region 24-45 ppm. Fraction of the total carbon. b Aliphatic carbon in the region O-24 ppm. Fraction of the total carbon. ’ Fraction of aliphatic carbon that is methyl. d DEM, demineralised coal.

Table 7 Proportions of non-protonated aromatic carbon and bridgehead aromatic carbon and the number of rings per aromatic cluster determined from the SPE and CP (values in parentheses, using contact time of 3 ms) 13CNMR measurements f”on.Drot ( * 0.02)

C&q( t 0.03)

N “IX?a

DEM b DGC-1V

0.62 0.60

(n.d.) ’ (0.69)

0.38 0.28

5 3

DGC-2V

0.66

(0.73)

0.35

4

DGC-3V

0.67

(0.69)

0.41

5

DGC-4V

0.70

(0.67)

0.47

6

DGC-6

0.67

(0.65)

0.51

7

DGC-9S

0.68

(0.70)

0.53

9

DGC-1OS

0.74

(n.d.)

0.59

12

DGC-11

0.77

(0.69)

0.64

17

a Number of aromatic rings per cluster, considering b DEM, demineralised coal ’ n.d., not determined.

circular catenation34,

than the initial coal (3.5 X 1019 spins g-’ cf. 2.4 X 1019 spins g-l). This might arise from some additional radicals being generated by removal of exchangeable cations3’. The four vitrinite rich fractions (DGC-1V to DGC-4V) have similar N, values of ca. 2.5 X 1019 spins g-‘. However, when vitrinite is not the predominant maceral, N, increases with density (T&e 5). Even for the two semifusinite concentrates (DGC-9S and DGC-lOS), there is an increase in N, from 7.5 to 8.0 X 1019 spins g-‘, with the most dense fraction having the highest N,, i.e. 8.5 X 10” spins g-’ (Table 5). The rise in N, with density could be connected to the increase in the size of the aromatic domains (see below). The thermal relaxation time constants for carbon seem to clearly be dependent upon the concentration of free radicals. the vitrinite-rich fractions with the same N, have similar relaxation time constants close to 24 s (Table 5 and Figure 4). With increasing concentration of free radicals the fi values decrease, reaching a minimum of 6 s for DGC11. This indicates that relaxation via unpaired electrons can be regarded as the predominant mechanism for the DGC fractions studied, as also found previously for partially carbonised coals23*3’. Carbon distributions Figure 5 shows the 13C SPE spectra for two of the vitrinite concentrates, DGC-1V and DGC4V, and for the semifusinite fraction, DGC-9s. Previous NMR8-‘* and FTIR4*‘*‘* studies have reported that the aromaticity of the vitrinite maceral group is smaller than that of the inertinite group. Figure 5 shows that this is also the case when

’

and derived from SPE experiments.

comparing the vitrinite (f, = 0.80) and semifusinite concentrates (fa = 0.90). Furthermore, the phenolic band observed around 150 ppm in the spectra of the vitrinite fractions is much loss prominent for the semfusinite concentrates. There are also differences in the distribution of aliphatic carbon in that the proportion of methyl (O25 ppm) is higher for the semifusinite fraction (Figure 5). Table 6 lists the carbon distributions for the fractions investigated, where the values in brackets were determined by CP experiments. Figure 6 shows the variation in the aromaticity values (both CP and SPE) with density. As previously reported for coals20-22, semicokes31, chars and pitches23, the aromaticities estimated by CP are much lower than those obtained by SPE for the maceral concentrates. The difference is ca. 10 mole % carbon for the vitrinite concentrates compared to ca. 5% for the more highly aromatic semi-fusinite concentrates. A previous CP study on semifusinite has reported an aromatic&y value as low as 0.40, even though the H/C ratio was only 0.6832, which is a similar value to those of the vitrinite concentrates and thus implying an aromaticity of close to 0.80. The fmethyl/fal values from CP are in most cases fairly similar to those derived from SPE experiments. Rotationally mobile methyl groups require relatively long contact times to cross polarise fully and discrimination against methyl groups has been reported for pitches23, where the SPE technique assigned -65% of the aliphatic carbons as being methyl, while CP experiments with short contact times (0.5-1.0 ms) gave only -35%. The aromaticities of the vitrinite-rich fractions increase to

Fuel 1998 Volume

77 Number 8

811

13CNMR study of bituminous

,x

:;

g $ c 2

coal: M. M. Maroto-Valer et al.

0.85-

0.80-

a 0.75 -

0.70], 1.25

, , , 1.2%

, . 1.31

, . * (. 1.34

. , *. 1.40

1.37

,

I

1.43

1.25

1.28

1.31

1.34

1.37

1.40

1.43

Density I g cm3

Density I g cm3

Figure 6 Variation in the carbon aromaticity (CP and SPE) with density for the samples investigated

that observed for bituminous coals, in going from low to high rank with the empirical equation being virtually identica122. Furthermore, when the DGC fractions were included in a much larger set of bituminous coals, the relationship still held33, indicating that it is independent of both maceral distribution and geological provincialism.

0.92 0.90 %

0.88

.[

0.86

g

0.84

Bridgehead carbon and ring cluster size

0.82 0.80 0.78 0.50

0.55

0.60

0.65

0.70

0.75

H/C ratio

Figure 7 Variation of carbon aromaticity with H/C ratio for the DGC samples investigated

a significant extent with density (Figure 6 and Table 6). the aromaticity of the least dense fraction, DGC-1V (l.OO1.26 g cmw3) is 0.79 compared to 0.85 for the most dense fraction, DGC4V (2.28-1.29 g cmp3). However, the aromaticity of the fractions with densities > 1.34 g cme3 does not change significantly, although there is a considerable difference in their petrographic characteristics. This is consistent with the similar H/C ratios for these samples (0.51-0.53, Table 4). In addition, the increase in the proportion of aliphatic carbon that is methyl with density is reflected in Table 6. For the vitrinite concentrates, DGC-1V to DGC4V, ca. one third is methyl, but for the semifusiniterich fractions (DGC-9S and -lOS), this increases to ca. 40% and then to over 50% for the most dense fraction (DGC-11). Figure 7 shows the variation in carbon aromaticity, fa, (SPE values) with atomic WC ratio. For all the samples investigated, there is an extremely good correlation (regression, 0.95) between these two parameters. Previous studies have reported a similar dependence of the aromaticity (determined by variable contact time CP experiments), but the scatter is significantly higher than for the values observed here14. Indeed, the relationship between I-I/C and aromaticity for the DGC fractions parallels exactly

812

Figure 8 Variation of the bridgehead aromatic carbon and ring size cluster with density

Fuel 1998 Volume

77 Number

8

and bridgeTable 7 lists the fractions of non-protonated head aromatic carbon (Caa), and the numbers of sixmembered rings involved in the aromatic clusters (Nri,,ss) assuming circular catenation34 that were determined by SPE and CP (values in brackets, 3 ms contact time) for the DGC fractions. Within experimental error, the demineralised sample has a similarf, (0.85 vs. 0.87),f,,,,, (0.62 vs. 0.62) and CBR (0.38 vs. 0.42) as the initial coal. The vitrinite concentrate, DGC-3V, has the closest aromatic structural parameters to those of the initial coal, probably due to the fact that this mid-range density fraction was the most abundant from the DGC separation, accounting for over 25% w/w. As for the overall aromaticity, the aromatic structural parameters values derived from CP are often lower than those obtained by SPE. For example, Pugmire et al. lo have reported values for fnon_protof 0.44 and 0.57 for vitrinite and inertinite concentrates with H/C ratios similar to the DGC fractions investigated. These values are considerably lower than those reported here; they were determined using CPDD experiments, with only one dephasing time of 40 PCS being used. Since both aromaticity and the fraction of nonprotonated aromatic carbon increase with density (Tables 6 and 7), there are fewer substituents on the aromatic clusters for the more dense fractions and these are predominantly methy135. Figure 8 shows the variations in bridgehead aromatic carbon (C,,) and aromatic ring cluster size with density. CBR increases continuously with density, but the increase is more pronounced for the vitrinite samples. The number of rings per aromatic cluster also increases with density, with the four vitrinite concentrates containing 3-6 aromatic rings per cluster and the semifusinite fractions containing 9-12 rings. The increases in aromaticity and the degree of condensation of the aromatic structure within the vitrinite maceral group cannot be assigned to different macerals, since the vitrinite distribution in the initial coal comprises

13CNMR study of bituminous

85% telocollinite and only 15% desmocollinite. However, the increases do relate to the increasing reflectance for telocollinite, as indicated qualitatively by the lighter particle colour for DGC-4V compared to DGC-1V and quantitatively by the increase in the random reflectances values obtained. Comparable differences have been observed by Dyrkacz et al. in the fluorescence colour of exinites6 that change from yellow to brown as density increases. The two semifusinite concentrates characterised have similar aromaticities (f, = 0.90 for both, Table 6), but the number of rings per cluster differ markedly (9 and 12 for DGC-9S and DGC- 1OS, and 17 for DGC- 11 which only contains 18% v/v fusinite, Table 7), indicating that significant structural variations also exist within the semifusinite group. Therefore, although maceral groups are clearly more homogeneous in character than coals, they are still quite structurally diverse. In view of the homogeneity that exists in lignin and other major precursors of coal, it is considered that varying rates of coalification make a significant contribution to the structural variations within the maceral groups.

CONCLUSIONS Vitrinite and semifusinite concentrates with purities over 90% have been obtained by DGC from a medium volatile bituminous coal using a particle size up to 75 pm. The ESRdetermined free radical concentrations, N,, followed the trend vitrinite < inertinite, and all the vitrinite fractions have similar values of N,, ca. 2.5 X 1019 spins g-l, independent of their densities. The 13C thermal relaxation times are dependent heavily upon the free radical concentrations in that they decrease with increasing N,. As previously reported for coals, chars and semicokes, the aromaticities determined by CP are often lower than by SPE for the maceral concentrates. As expected, the aromaticity is significantly lower for the virinite fractions than for the semifusinite ones. Indeed, a good linear correlation was obtained between the atomic H/C ratios and SPE-derived aromaticities, independent of their maceral composition. The fractions of non-protonated and bridgehead aromatic carbon increase with density, together with the fraction of aliphatic carbon that is methyl. The vitrinite fractions contain 3-6 aromatic rings compared to over 9 for the two semifusinite fractions. The structural variations within the vitrinite maceral group are related to the increase in the reflectance of telocollinite, the dominant maceral present, with density.

6

10

11

13 14 1.5

16 17

18

19

20 21

1 2 3 4 5

Dormans, H., Huntjens, F. and van Krevelen, D. W., Fuel, 1957, 36, 321. Drycasz, G. R. and Horwitz, E. P., Fuel, 1982, 61, 3. Bishop, M. and Ward, D. L., Fuel, 1958, 37, 191. Taulbee, D. L., MS. thesis, University of Kentucky, 1986. Poe, S. H., Taulbee, D. L. and Keogh, R. A., Org. Geochem., 1989, 14, 307.

Drykacz, G.R, Bloomquist, C. A. A., Rustic, L. and Horwitz, E. P., in Am. Chem. Sot. Symp. Ser. No. 252, eds. Winans, R. E. and Crelling, J. C., Chapter 5, p. 62, 1985. Taulbee, D. L., Seibert, E. D., Barron, L. S. and Keogh, R. A., Energy and Fuels, 1990, 4, 254. Retcofsky, H. L. and Vanderhart, D. L., Fuel, 1978,57,421. Axelson, D. E., Solid State Nuclear Magnetic Resonance of Fossil Fuels: an experimental approach. Multiscience Pub., Canada, 1985. Pugmire, R. J., Woolfenden, W. R., Mayne, C. L., Karas, J. and Grant, D. M., in Am. Chem. Sot. Symp. Ser. No. 252, eds. Winans, R. E. and Crelling, J. C., Chapter 6, p. 79, 1985. Axelson, D. E. and Parkash, S., Fuel Sci. and Technol., Dyrkacz, G. R., Bloomquist, C. A. A. and Solomon, P. R., Fuel, 1982, 63, 536. Riesser, B., Starsinic, M., Squires, E., Davis, A. and Painter, P. C., Fuel, 1984, 63, 1253. Choi, C., Muntean, J. J., Thompson, A. R. and Botto, R. E., Energy and Fuels, 1989, 3, 528. Silbemagel, B. G., Gebhard, L. A. and Bloomquist, C. A. A., in Am. Chem. Sot. Symp. Ser. No. 252, eds. Winans, R. E. and Crelling, J. C., Chapter 8, p. 121, 1985. Silbemagel, B. G., Gebhard, L. A., Dyrkacz, G. A. and Bloomquist, C. A. A., Fuel, 1986, 65, 558. Snape, C. E., Axelson, D. E., Botto, R. E., Delpuech, J. J., Tekely, P., Gerstein, B. C., Pruski, M., Maciel, G. E. and Wilson, M. A., Fuel, 1989, 68, 547. and references therein. Silbemagel, B. G. and Botto, R. E., in Botto, R. E. and Sanada, Y. (Eds.) Magnetic Resonance of Caronaceous Solids, Adv. in Chem. Ser. No. 229. Am. Chem. Sot., Washington, DC, 1993 and references therein. Wind, R. A., Maciel, G. E. and Botto, R. E. in Botto, R. E. and Sanada, Y. (Eds.) Magnetic Resonance of Caronaceous Solids, Adv. in Chem. Ser. No. 229. Am. Chem. Sot., Washington, DC, 1993, and references therein. Franz, J. A., Garcia, R., Linehan, J. C., Love, G. D. and Snape, C. E., Energy and Fuels, 1993,7, 639. Love, G. D., Law, R. V. and Snape, C. E., Energy and Fuels, 1993, 7, 639.

22 23

Maroto-Valer, M. M., Love, G. D. and Snape, C. E., Fuel, 1994, 73, 1926. And&en, J. M., Maroto-Valer, M. M. and Snape, C. E., Fuel,

24

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ACKNOWLEDGEMENTS

REFERENCES

et al.

1986, 4, 45.

12

27 28

The authors thank the European Coal and Steel Community (Contract Nos. 7220-EC/870 and 7220-EB/845) and the Basque Government (studentship for MMMV) for financial support.

coal: M. M. Maroto-Valer

29

1996, 75, 1721.

Maroto-Valer, M. M., Taulbee, D. N., And&en, J. M., Hower, J. C. and Snape, C. E., Proc. 1997 Int. ConJ on Coal Sci., Essen, Germany, Vol. II, p. 969. Maroto-Valer, M. M., Taulbee, D. N., And&en, J. M., Hower, J. C. and Snape, C. E., Energy and Fuels, submitted. Keogh, R. A., Taulbee, D. N., Hower, J. C., Chawka, B. and Davis, B. H., Energy and Fuels, 1992, 6, 614. Torchia, D. A., J. Magn. Reson., 1978, 30, 613. Taulbee, D. L., Poe, H. P., Robl, T. and Keogh, R. A., Energy and Fuels, 1989, 3, 662. Winans, R. E., Kim, Y., Hunt, J. E. and McBeth, R. L., Coal Sci. and Technol.,

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Silbemagel, B. G., Gebhard, L. A., Plowers, R. A. and Larsen, J. W., Energy and Fuels, 1991, 5, 561. Maroto-Valer, M. M., Snape, C. E., Willmers, R. R., Atkinson, C. J. and Loudon, K. W. G., Proc. 1995 Int. Con& on Coal Science, Vol. 1, 1995, Oviedo, Spain; Energy and Fuels, submitted. Wilson, M. A., Hanna, J. V., Cole-Clarke, P. A., Greenwood, P. F. and Willett, G. D., Fuel, 1992, 71, 1097. Maroto-Valer, M. M., And&en, J. M. and Snape, C. E., Fuel, in press. Solum, M. S., Pugmire, R. J. and Grant, D. M., Energy and Fuels, 1989, 3, 187. Vasallo, A. M., Lockhart, N. C., Hanna, J. V., Chamberlain, R., Painter, P. C. and Sobkowiak, M., Energy and Fuels, 1991, 5, 477.

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