704
Energy & Fuels 1993, 7, 704-709
Chemical Constitution of Pocahontas No. 3 Coal? Leon M. Stock' and John V. Muntean Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 Received April 8,1993. Revised Manuscript Received August 31, 1 9 9 9
The available structural information about Pocahontas No. 3 coal, APCSP 5, is assembled and critically evaluated. A chemical structure is presented that portrays this highly aromatic coal with its abundant methyl groups and bridging biaryl and heterocyclic groups.
Introduction The Argonne National Laboratory Premium Sample of Pocahontas No. 3 coal was collected from a 6-ft seam in Buchanan County, Virginia, in 1986and processed for use by the coal chemistry community by a team led by Karl Vorres in 1987. Since that time more than 1600 samples have been distributed and more than 150 research articles concerning its chemical, physical, and spectroscopic properties have appeared in the scientific literature. We have critically evaluated this information and presented a chemical structure for the coal. Composition Pocahontas No. 3 coal is a low-volatile bituminous coal that contains 89% vitrinite, 10% inertinite, and 1% liptinite. The elemental composition was determined in around robin analytical program.172 The composition that is used in this discussion of its structure, CIOOOH5,5010.8N12.5S2.1,differs somewhat from the value given in earlier reports because we have adopted the more accurate value for its water content that was determined by Finseth and his c o - ~ o r k e r s . ~ Aromatic Carbon Atom Distribution The first thorough study of the solid-state magnetic resonance spectrum of Pocahontas No. 3 coal was reported by Solum, Pugmire, and Grants4 They used cross polarization (CP) methods to measure the carbon aromaticity, to derive the distribution of the carbon atoms, and to estimate the average aromatic cluster size. Contemporary research has revealed that special precautions are necessary to achieve quantitative results in solid-state nuclear magnetic resonance spectroscopy of fossil materials."8 There is now a firm body of evidence to show that it is desirable to use an internal standard to ensure the quantitative reliability of the data, to employ the Bloch ~
t Work performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, US. Department of Energy, under contract W-31-109-ENG-38. *Abstract published in Aduance ACS Abstracts, October 15, 1993. (1)Vorres, K. S. "Users' Handbook for the Argonne Premium Coal Sample Program"; Argonne National Laboratory SANL/PCSP-89/1,Oct 1, 1989. (2) Vorres, K. S. Energy Fuels 1990,4, 420. (3) Finseth, D. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1987, 32, 260. (4) Solum, M. S.;Pugmire, R. J.; Grant, D. M. Energy Fuels 1989,3, 187. ( 5 ) Magnetic Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Y., Eds.; American Chemical Society: Washington, DC, 1993. (6) Axelson, D. E. Solid State Nuclear Magnetic Resonance ofFossil Fuels: An Erperimental Approach; Multiscience: Canada, 1985. (7) SpectroscopicAnalysk of CoalLiquids; Kershaw,J. R., Ed.; Elsevier Science: New York, 1989. (8) Muntean, J. V.; Stock, L. M. Energy Fuels 1991, 5,765.
decay (BD) procedure to achieve the most accurate assessment of the distribution of the aromatic and aliphatic carbon atoms, and to reduce the concentration of unpaired electrons in the solid sample to enable the detection of the greatest number of carbon n ~ c l e i Accordingly, .~ our newer work on the solid-state NMR spectrum of the Pocahontas No. 3 coal exploited the finding that tetrakis(trimethy1sily1)silane could be used as a quantitative standard for work in the solid state.10 In addition, we used samarium(11) iodide to decrease the high electron spin density." The recent work of Doetschman and Dwyer on the electron paramagnetic resonance spectrum of this coal certainly provides ample confirmation of the need for concern.12 They measured 3.5 X 1019 spins/g by continuous wave techniques in good agreement with the earlier values of other w o r k e r ~ . ~ JHowever, ~J~ pulse methods suggested that the unpaired electron density was an order of magnitude larger, about 19 X 1019spins/g.12 Finally, we employed Bloch decay techniques throughout the study. The infrared spectrum has also been carefully investigated.15 Solomon and his associates used a curve analysis program to reconstruct the observed spectrum by adding 45 Gaussian absorption peaks with variable positions, widths, and heights. The concentrations of the aliphatic and aromatic hydrogen and hydroxyl hydrogen as well as aliphatic, aromatic, and carbonyl carbon were evaluated for Pocahontas No. 3 coal in this way.15 Generally, the agreement between the findings of this infrared study and the magnetic resonance work is very good. Indeed, the f~ value, 0.849, that was first reportedls is probably too small, subsequent analyses,ls which employ a more appropriate aliphatic carbodaliphatic hydrogen of 2.2, infer that f A may be as large as 0.88. One minor discrepancy concerns the carbonyl carbon concentration; the infrared spectrum suggests that the coal contains carbonyl carbon atoms, whereas virtually all the nuclear magnetic resonance work suggests that there is none. The results that have been obtained in several indefor the fraction of the carbon pendent laboratories4~8~g~l5~'7-24 (9) Muntean, J. V.; Stock,L. M. Energy Fuels 1991,5,767. (10) Muntean, J. V.; Stock, L. M.; Botto, R. E. J. Magn. Reson. 1988, 76. 540. '(11) Muntean, J. V.; Stock, L. M.; Botto, R. E. Energy Fuels 1988,2, 108. (12) Doetachman, D. C.; Dwyer, D. W. Energy Fuels 1992,6,783. (13) Silbernagel, B. G.; Gebhard, L. A.; Flowers, R. A.; Larsen, J. W. Energy Fuels 1991,5, 561. (14) Bowman, M. K. Chapter 32 in ref 5. (15) Rosa, L.; Purski, M.; Lang, D.; Gerstein, B., Solomon, P. Energy Fuels 1992, 6, 460. (16) Solomon, P., private communication. (17) Adachi, Y.; Nakamizo, M. Chapter 13 in ref 5. (18)Hu, J.; Li, L.; Ye, C. Chapter 16 in ref 5. ~~
0887-0624/93/2507-0704$04.00/00 1993 American Chemical Society
Energy &Fuels, Vol. 7, No. 6, 1993 705
Chemical Constitution of Pocahontas No. 3 Coal Table I. Aromaticity of Pocahontas No. 3 Coal specimen original original original original original original original original original original original original original original original THF,acid washed SmI2 original original
method
IR
CP MAS CP MAS
CP MAS CP MAS CP MAS CP MAS CP MAS DNP MAS CP MAS CP MAS DP CP MAS TOSS CP MAS TOSS
BD BD BD BD BD (RIDE)
%cobsd 100
-
57 83
80
-
82 80 90
-
fA
ref
0.86 0.86 0.86
15
0.83 0.86 0.84 0.85 0.85 0.89 0.81 0.87
17
0.83 0.77 0.86 0.89 0.89 0.89 0.89 0.89
15
4 18 19 20 21 22 23 23 23 18 21
8 8 8 20 24
J
J
r
.
.
l
20G
.
.
.
,
t
q
150
,
. .
I
.
.
.
.
100
atoms that are observed, 5% C(obsd), and the aromaticity, Chemical Shift (ppm) in several independent laboratories are summarized in Figure 1. Aromatic region of the solid-state carbon-13 nuclear Table I. magnetic resonance spectrum of Pocahontas No. 3 coal. The The most quantitative work indicates that between 83 spectrum was recorded by using Bloch decay procedures.9 and 90 % of the carbon atoms in this coal can be observed in appropriately designed NMR experiment^.^^^^ The in the results from the experiments at short delay times, NMR observations from many different laborabut the results differed at long relaxation times. Bloch tories4~8J5*17-24 provide f A values between 0.77 and 0.89 with decay techniques revealed there are 115% sp3carbon atoms the Bloch decay experiments favoring the higher value, and 30% sp2 carbon atoms with protons and 59% sp2 0.89, whereas the CP information clusters near 0.86. The carbon atoms without protons. A collaborative effort infrared data suggest an average f~ value of 0.86. Clearly, between the groups led by Gerstein and Solomon provided the experimental values are converging between 0.86 and independent measurements of the hydrogen aromaticity.l6 0.89. Inasmuch as the Bloch decay procedure appears to The combined rotation and multipulse spectroscopy be inherently more accurate than the cross polarization (CRAMPS) procedure indicated that 30 sp2carbon atoms method, we shall adopt the higher f A value, 0.89, in this were bonded to hydrogen atoms in excellent agreement structural analysis. with the results of the dipolar dephasing work on ~ a r b o n . ~ J ~ Although the spectrum is not well resolved, Figure 1, it The infrared data provide exactly the same value.15 Additional magnetic resonance data by Hu, Li, and Ye,l8 is possible to abstract additional information about the Jurkiewicz, Bronnimann, and Maciel,28 and Pan and chemical environment of the carbon atoms through dipolar M a ~ i eare l ~ in ~ reasonable agreement and provide values dephasing measurements in which the chemical character of 27,18 32,18 and 3723for the number of protonated sp2 of the carbon atoms is revealed by the differences in their relaxation properties.2k27 Solum, Pugmire, and Grant carbon atoms. We have adopted the intermediate value of 30 for our structural work. used cross polarization techniques to determine the abundances of protonated and unprotonated aromatic The nonprotonated carbon atoms can be distinguished carbon atoms, the fraction that were at bridgehead by their chemical shifts. Their relative abundances can positions, the number bonded to oxygen, and so forth: be assessed in various ways. We elected to focus attention Subsequently, other workers have employed similar straton the Bloch decay spectrum that was recorded after the egies to unravel the magnetic resonance ~ i g n a l s . l ~ J ~ 1 ~125-ps ~ delay. In brief, this spectrum is devoid of resonances However, in view of the general advantages of the Bloch that could be ascribed to carbon atoms that are bonded decay procedure, it seemed prudent to use this technique to protons. The remaining resonances can, therefore, be in our study of the relaxation properties of the carbon assigned with considerable confidence. Specificially, the nuclei of this ~0al.9Although the resulta of the studies are intensity from 165 to 150 ppm arises from carbon atoms generally in good agreement, several new features appeared that are bonded to heteroatom^.^^ The resonances in the in the Bloch decay spectra. First, the high-field side of 150-129 ppm region originate in two important ways. The the aromatic carbon resonance signal decreased as the group from 138 to 129 ppm arises from methylated delay time increased, while the low-field side of the band aromatic carbon atoms and the group from 150 to 138 showed virtually no change. Second,there were differences ppm contains the other alkylated aromatic carbon atoms in the time dependence. There was excellent agreement and the carbon atoms that link biaryl structures and the bridgehead carbon atoms of the indanes. The assignment (19) MacPhee, J. A.; Kawashima, H.; Yamashita, Y.; Xamada, Y. of the biaryl and heteroaromatic resonances is especially Chapter 17 in ref 5. relevant for this high-rank coal, because, as discussed (20) Franz, J. A.; Linehan, J. C. Chapter 20 in ref 5. (21) Axelaon, D. E.; Botto, R. E. Prepr. Pap.-Am. Chem. Soc., Diu. subsequently, it is deficient in methylene groups. ConFuel Chem. 1988,33,50. sequently, biaryl linkages become important cross-links (22) Jurkiewicz, A,; Wind, R. A.; Maciel, G. E. Fuel 1990, 69, 830. (23) Pan, V. H.; Maciel, G. E. Fuel 1993, 72, 451. in the assembly of the structure. The results are sum(24) Botto, R. E., unpublished results. marized in Chart I. Buchdahl. R. Macromolecules 1975, (25) Schaefer. 3.; Steiskal, E. 0.: fA,
8, 291.
(26) Pines, A.; Gribby, M. G.; Waugh, J. S. J. Chem. Phys. 1973,59, 569. (27) VanderHart, D. L.; Retcofsky, H. L. Fuel 1976,55,202.
(28) Jurkiewicz, A.; Bronnimann, C. E.; Maciel, G. E. Chapter 21 in ref 5. (29) Attalla, M. I.; Vassallo, A. M.; Wilson, M. A. Chapter 7 in ref 7.
706 Energy &Fuels, Vol. 7, No. 6,1993
Stock and Muntean
Chart I Pocahontas Coal
-
89.5 SPZ
n
0.0 Carbonyl
89.5 Aromatic
7.8
2.3
0.3
CH,
CH,
CH
A
30
0.1 C
59.5
I 30
0
8.5 34.5 2.3 5.3 Ethylated Phenolic Methylated Bridgehead and others and others Aromatic Aromatic
8.7 Biaryl
Several independent lines of evidence support these assignments. First, the elemental composition places rather severe restrictions on the possible number of alkyl groups and virtually requires that the resonances in the 150-138 ppm region be assigned to biaryl structures. Second, two studies of the oxidation of Pocahontas coal imply that indanes are pre~ent.~Ofl Third, the results of the ruthenium(VII1) oxidation study and the elemental analysis, especiallythe hydrogen content, also place rather strict boundary conditions on the way in which hydrogen can be distributed and confirm the occurrence of biaryl linkages. Aromatic Cluster Size We adopted the procedure of the Utah group to estimate the average aromatic cluster sizes4 The Bloch decay interrupted decoupling experiments can be used to estimate this quantity from eqs 1and 2, where Xb is the fraction in bridgehead carbon atoms and Cis the number of carbon atoms per molecule.
For clarity, the fraction of bridgehead carbon for linear catanation (benzene, naphthalene, anthracene, etc.) is defined as X’b (eq 1). The data imply an average cluster size of about 30 carbon atoms for this case. Circular catenation (benzene, coronene, etc.) is denoted by X”b (eq 2);this assessment yields 17 carbon atoms per cluster, and the dual model of Solum, Pugmire, and Grant4 implies 20 carbon atoms per cluster. We shall return to this matter subsequently. Information about the aromatic hydrocarbon content of the coal has also been derived from gas chromatographymass spectrometry of extracts of this coal and reaction chemistry by Bartle and Lee and their collaborator^.^^ They carried out several chemical reactions as shown in the equations. coal
-
FeCla H2,10.3 MF’a 290 ‘C, 3 h
product
-
10% KOH
Ha, 600 MPa
product
I
40
80
I 1’
I
I
im
is
I 200
I
90
I
I
zad
3(10
Time and Temperature Figure 2. Gas chromatogram of the polycyclic aromatic hydrocarbon fractionof PocahontasNo. 3 coal after hydrotreatment and base-catalyzed depolymerization.32
The chromatogram, Figure 2, exhibits the customary complexity with a large amount of material underlying an array of sharp peaks from relatively simple compounds such as naphthalene, biphenyl, fluorene, pyrene, and other compounds. Integration of the ion current provides an indication of the relative abundances of the moleculeswith one and two, three, and four or more aromatic rings. The GC observations suggest that these ring sizes occur in the ratio of 1:4:>20. Thus, relatively large aromatic compounds dominate the structure. It is also pertinent that the chromatogram exhibits a sharp decrease after 90 min; this observation provides rather direct evidence that large molecules are present but that they are not volatile under the conditions of the chromatographic experiment. Some additional evidence about the aromatic carbon atom distribution was provided by ruthenium(VI1) oxidation.31 The benzenecarboxylic acids that were formed from aromatic compounds with three or more rings predominated over the benzenedicarboxylic acids.
rings in
4 or more
precursor
rings in precursor
5 or more rings in precursor
Benzenepentacarboxylic acid and biphenyL2,2’,6,6/-teetracarboxylic acid are formed from compounds with five or more aromatic rings and are especially interesting products. The ease with which this coal undergoes C alkylation and the chemical shifts of the C-methylated coal strongly suggests that fluorenylic structures are present.33 This observation is also in accord with the detection of benzofluorenes in the work of Lee and Bartle and their collaborators.32
290OC.lh
Unfortunately, the yields of soluble volatile products were quite low. The fraction of greatest interest, in which the aromatic hydrocarbons were concentrated, constitutes only 7% of the material; nevertheless, the chromatogram of this material provides quite important information about the nature of the coal. (30) Holly,E.D.; Montgomery, R. S. Fuel 1956,3649 and subsequent papers in this series. (31) Stock,L. M.; Wang, S.H. Energy Fuels 1989,3,533. (32) Carlson,R.E.;Critchfield,S.;Vorkink,W.P.;Dong,J. Z.;Pugmire, R. J.; Lee, M. L.; Zhang, Y.; Shabtai, J.; Bartle, K. D. Fuel 1992, 71, 19.
Molecular Weight Distribution Several research groups have examined the mass spectra of extracts of the Pocahontas coal. This work has not been definitive because the coal is quite insoluble, no more than 2-3% of the raw material can be extracted and (33) Chatterjee, K.; Stock,L. M. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1991, 36, 481.
Energy & Fuels, Vol. 7, No. 6, 1993 707
Chemical Comtitution of Pocahontas No. 3 Coal
menta that have been advanced elsewhere suggest that coalification is a net depolymerization and that the molecules which make up higher ranking bituminous coals are much more aromatic but smaller than the molecules that constitute the lower ranking coals.% Heteroatom Content
0
9
11
19
30
67
76
120
191 302 480
Molecular weight x 10-2 Figure 3. Molecular weight distribution of reductively ethylated Pocahontas No. 3 coal.98 Table 11. Molecular Weight Information molecular weight sample original coal base-promoted depolymerization pyridine extract pyridine extract reductive alkylation product
method n u m a v maxvalue ref FIMS FIMS DCI
CD
VPO
475 430
900 800
35
-
500 500
36 36
2800
48000
37
-
35
analyzed. Furthermore, the original coal is not volatile. Solomon and his co-workers reported that only 20 w t % of the coal was vaporized in their study of its pyrolysis and that about 40% of the vaporized material appeared in very low molecular weight gases.34 Nevertheless, the investigations of the molecular weight distribution have provided important information. The results are summarized in Table I1 for convenient comparison. The Stanford Research Institute group studied the FIMS spectra of the raw coal and a sample of an alkylated coal that was 32 7% soluble in pyridine.35 Unfortunately, these samples like the samples that have been studied by Solomonand his co-workersprovided only l e 1 5 % volatile matter. The number average molecular weight of the raw coal was about 500 with ions present at 850. The corresponding values for the alkylated coal were only about 50 smaller. The results of other sophisticated modern mass spectrometric methods such as chemical ionization and desorption chemical ionization exhibit their maximum at lower values and the molecular weight range does not reach 500.36 About 15 years ago,Sternberg and his research group reductively alkylated a different sample of Pocahontas No. 3 coal and reported that 90 % of the coal was soluble in pyridine and obtained the molecular weight distribution that is shown in Figure 3.37 This study strongly suggests that the molecular weight distribution is broad with many large molecules. Unfortunately, the results are not converging, and additional work on the mass spectra of solubilized samples of the premium coal will be necessary to resolve the problem. For the present, it seems most reasonable to adopt the view that the clustered molecules in this coal span a broad distribution of molecular weights ranging from a few hundred to several thousand. Argu(34) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Bassilakis, R. Energy Fuels 1990,4, 319. (35) Malhotra, R., private communication. Muntean, J. V. Ph.D. Thesis, The University of Chicago, 1990. (36) Winans, R. E. J. A n d . Appl. Pyrolysis 1991, 20, 1. (37) Sternberg, H. W.; DelleDonne,L. L.; Pantages, P.; Moroni, E. C.; Morbby, R. E. Fuel 1971,50,432.
The Pocahontas coal contains relatively few heteroatoms with only 12.5 nitrogen, 10.8 oxygen, and 2.1 sulfur atoms per 1000 mol of carbon. Analysis of the nitrogen atom distribution has been carried out by Gorbaty and his coworkers through X-ray photoelectron spectroscopy.a Their results can be best fit to a distribution in which these nitrogen atoms are distributed between four pyridines and eight pyrroles with less than one quaternary nitrogen compound per 1000carbon atoms. More recent soft X-ray work40apparently differentiates between free and hydrogen-bonded pyridines as well as the pyrroles and suggesta two unassociated pyridines, one associated pyridine on an amide, one quaternary nitrogen, and eight pyrroles per 1000 carbon atoms. Analysis of the oxygen atom distribution by O-methylation with methyl-13C iodide and nuclear magnetic resonance analysis suggests that 8 f 2% of the oxygen atoms are in hindered and 10f 2 % in unhindered phenolic compound^.^^ The infrared work implies many more phenolic substances with 45% of the oxygen atoms in phenolic compounds.15 The discrepancy has not been resolved. H
OH
hindered phenol
OH
unhindered phenol
The remainder of the oxygen atoms, between 55 and 80 % of the total oxygen content, appear to be present in heterocyclic ethers. Indeed, Winans and his associates have reported dibenzofurans are rather abundant in the mass spectra of the extracts that were discussed in the previous section.36 It is pertinent that there are no signals in the NMR spectrum that can be attributed to alkyl ethers. Although the evidence about the exact character of the ethers is meager, the current results imply that there may be nine heterocyclic ethers per 1000 mol of carbon. The sulfur heteroatoms are somewhat more completely defined. Although the very low sulfur content of the coal presents a serious experimental challenge, Gorbaty and his associates have examined the X-ray photoelectron and X-ray absorption near-edge structure spectrum of this coal.42 Huffman and his group also investigated the XANES spectrum.43 The XPS work suggests that 100% of the sulfur atoms are present in thiophenic structures. Gorbaty pointed out that the XANES data can best be explained by a distribution of 13% sulfur bonded to aliphatic carbon atoms, 23% bonded to aromatic carbon atoms, and 64% in thiophenic compounds.43 Huffman, on the other hand, concluded that almost 100% of the (38) Stock, L. M. Coal Sci. 1982,1, 161. (39) Keleman, S. R.; Gorbaty, M. L.; Vaughn, S. N.; Kwiatak, P.J. Prepr. Pap.-Am. Chem. Soc., Diu.Fuel Chem. 1993,38, 384. (40) Mitra-Kirtley, S.; Mullins, 0. C.; Branthaver, J.; van Elp, J.; Cramer, S. P. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1993,38, 762. (41) Cheng, C.; Stock, L. M., unpublished results. (42) George, G. N.; Gorbaty, M. L.; Keleman,S. R.; San"e,M.Energy Fuels 1991, 5, 93. (43) Huffman, G. P.; Mitra, S.; Huggins, F. E.; Shah, N.; Vaidya, S., Lu, F. Energy Fuels 1991,5, 574.
708 Energy & Fuels, Vol. 7, No. 6,1993
Stock and Muntean Table 111. Ru(VII1) Oxidation Products acid
monocarboxylic acids ethanoic acid propanoic acid butanoic acid dicarboxylic acids butaneodioic acid 2-methyl-l,4-butanedioic acid pentanedioic acid 2,2-dimethyl-l,I-butanedioic acid P-methy1-1,5-pentanedioic acid hexanedioic acid 2,2-dimethyl-l,5-pentanedioic acid 2,3-dimethyl-l,5-pentanedioic acid heptanedioic acid
14
20
yield, mo1/100mol of C
30
40
Tima (min.)
Figure 4. GC/MC chromatography of the oxidation products of Pocahontas No. 3 coal: (1)1,2-benzenedicarboxylicacid; (2) 3-methylbenzene-l,2-dicarboxylic acid; (3)4-methylbenzene-1,2dicarboxylic acid; (4) dimethylbenzenedicarboxylic acid; (5) dimethylbenzenedicarboxylic acid; (6) dimethylbenzenedicarboxylic acid; (7) 1,2,3-benzenetricarboxylic acid; (8) 1,2,4benzenetricarboxylic acid; (9) methylbenzenetricarboxylicacid; (10) methylbenzenetricarboxylic acid; (11) dimethylbenzenetricarboxylic acid; (12) trimethylbenzenetricarboxylicacid; (13) 1,2,4,5-benzenetetracarboxylicacid; (14) 1,2,3,4-benzenetetracarboxylic acid; (15) 1,2,3,5-benzenetetracarboxylicacid; (16) methylbenzenetetracarboxylicacid + 2,3,2'- biphenyltricarboxylic acid; (17) methylbenzenetetracarboxylicacid; (18) biphenyltetracarboxylic acid; (19)2,6,2/,6'-biphenyltetracarboxylicacid; (20)
benzenepentacarboxylic acid.22 organic sulfur was present in thiophenic gr0ups.~3These three investigations infer that the sulfur atoms are predominantly present in aromatic and heteroaromatic environments and that there certainly are no more than three sulfur atoms per 10 000 mol of C in alkylthioethers or thiols. Aliphatic Structural Elements The f~ values that we consider most appropriate for Pocahontas No. 3 coal suggest that it contains between 11 and 13 aliphatic carbon atoms per 100 mol of C. Unfortunately, the magnetic resonance experiments do not effectivelydefine the degree of substitution of these carbon atoms, but the information about their relaxation properties implies that there are about five methine and methylene groups per 100 mol of C and about six methyl and quaternary groups per 100 mol of C.4 The infrared spectrum of the coal has been carefully studied1SJ6p4 and the results are also consistent with the idea that the coal is unusually rich in methyl groups. Asample of Pocahontas No. 3 coal was oxidized in 1956 by a research group at D o w . ~They ~ employed gas chromatography, but the resolution was poor and only a few products could be confidently identified. However, it is pertinent that indane derivatives were detected. (44)Dyrkacz, C . R., unpublished results, 1991.
1.28 0.25 0.05 0.17 0.10 0.08
0.02 0.09 0.12 0.03 0.03 0.10
The ruthenium(VII1) oxidation reaction of the premium sample provided a relatively simple product distribution with fewer major products and a smaller background of minor products than for lower ranking coals.31*46The results are displayed in Figure 4 and Table 111. The abundances of the volatile carboxylic acids, which were determined by isotope dilution mass spectrometry, indicate that ethanoic acid is produced in especially high yield, 7.3 mol per 100mol of C, a yield that is much greater than that obtained from any other coal. The quantities of propanoic and butanoic acid are, in contrast, low and unexceptional, as are the quantities of the other acids as expected for this highly aromatic coal. However, the yields of the methylated acids are unexpectedly high; for example, the 2-methylbutanedioic acidlbutanedioic acid ratio is 0.62 and the 2-methylpentanedioic acid/pentanedioic acid ratio is 1.1. It was also observed that the methylbenzenecarboxylic acids were formed in significant amounts, about 0.22 mol of methyl groups/100 mol of C. The yields of 3- and
6""'"" WH3 CH3
0.079 md/100 mol of C
0.029 mo1/100 m d of C
0.050 moll100 mol of C
4-methylbenzene- 1,2-dicarboxylicacids were also unusually high compared to benzene-l,2-dicarboxylicacid. The ruthenium(VII1)oxidation system degrades reactive aromatic compounds to benzenecarboxylic acids and aliphatic acids.46 The high yield of ethanoic acid can be related very directly to the high abundance of methylated aromatic compounds. Propanoic and butanoic acid may be formed from pendant ethyl and propyl groups or from bridging groups as illustrated for the propanoic acid.
CH~CH~COZH derived from
or /
ARENE
CH3CHZCH 'ARENE (45) Tse, K. T.; Stock, L. M. Fuel 1983, 62, 974.
Energy &Fuels, Vol. 7,No. 6, 1993 709
Chemical Comtitution of Pocahontas No. 3 Coal The 5 and 6 carbon atom dicarboxylic acids may also arise from more than one structural component in the coal. These substances can be obtained from propano or butano bridging groups or from carbocycles as shown for pentanedioic acid. (ARENE)(CH&(ARENE) H02C(CH2)3C02Hderived from
25
?4
['MI
or
a I
ARENE
The latter assignment is favored because of the large abundance of indane derivatives that were observed in the early study. Quantitatively, the oxidation products account for more than 80 7% of the aliphatic carbon atoms in this coal with 70% of them found in pendant methyl groups. The information from magnetic resonance and infrared spectroscopy as already discussed and pyrolysis" all point to an abundance of methyl groups in this coal. It may also be pertinent that methane continues to evolve from the Pocahontas No. 3 coal in the sealed vials of the premium coal.% Connecting Links Almost every discussion of the chemistry of coal conversion includes commentary on the need to disrupt the methano, ethano, etheral, and thioetheral groups that bridge the aromatic clusters. Few such groups exist in Pocahontas No. 3 coal, yet the average cluster size between 5 and 9 aromatic rings per cluster depending upon the nature of the catenation41gthat is implied by the magnetic resonance data is relatively small, much smaller than implied by the molecular weight data (Table 11). The solution to this puzzle is found in the new evidence from magnetic resonance and ruthenium(VII1) oxidation that reveals the occurrence of biaryl linkages and the heteroatom analysis that suggests the predominance of 5-membered heterocyclic compounds. These results imply that biaryl and heterocyclic structures provide the bridging groups between clusters in this high-rank coal. The straightforward evidence concerning the heteroatoms has already been discussed. The interpretation of the magnetic resonance data deserves further elaboration. Usually, the carbon resonances from 138 to 150 ppm have been ascribed to alkylated sp2 carbon atoms, but this assignment needs to be broadened to alkylated and arylated sp2 carbon atoms. The oxidation experiments define the distribution of alkyl groups and the aliphatic hydrogen atoms in the Pocahontas No. 3 coal. Further, other proton NMR experiments independently define the number of aromatic hydrogen atoms. When all this information is considered, the results virtually require biaryl linkages. Quantitatively, there are 4-5 biaryl bridging units per 100 mol of carbon.
A Structural Representation Even the most elementary findings, for example the array of products in the gas chromatogram, Figure 2, indicate the diversity of the structural elements in this coal. At the present, it is impossible to specify the structures of the aromatic compounds in it. Neither the (46)Vorres, K. S . R e p r . Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1992,37, 1946.
@ m \
/
/
/
'I
\
Figure 5. A plausible structure for Pocahontas No. 3 coal. Plausible aromatic compounds are shown to illustrate a group of structures that accommodate the magnetic resonance data. substitution patterns for the methylated compounds, nor the locations of the connecting links, nor the relative abundances of the aromatic building blocks can be assigned. As we have reported previously, in the face of these difficulties, we adopted the suggestion of K. B. Anderson and presented a block representation of the structure that incorporates the principal concepts. This structural representation is shown in Figure 5 together with plausible aromatic clusters. This representation accommodates the chemical and spectroscopic information that has been presented in the previous sections of this report, but it is not unique. Many other representatives would accomplish the same goal and convey the same information. In a sense, this structure for Pocahontas No. 3 coal may best be regarded as a beginning. It accounts for the key observations and displays them in a chemically convenient manner. As new information becomes available, for example, more definitive information about the aromatic constituents, the structure can be expanded and modified. Coal scientists recognize that the coal's structural diversity is very great, so great that no conventional structural representation will ever satisfy the demand for structural completeness in the traditional sense of the natural product chemist. Yet, these structural representations serve a clear purpose in guiding strategies for the development of greater intellectual understanding of coal and, simultaneously, providing targets for the discovery of new low-severity chemical conversion methods.
