Dihydroaromatic structure of Illinois No. 6 Monterey coal

Dihydroaromatic No. 6 Monterey

structure coal

of Illinois

N. C. Deno, Kenneth W. Curry, Sarbara A. Greigger, A. Daniel Jones, Walter G. Rakitsky, Karen Ann Smith, Karen Wagner, and Robert D. Minard Department of Chemistry, Pennsylvania State University, University Park, PA 16802, USA (Received 6 November 1979)

Coal from the Monterey mine (Illinois No. 6 seam) has been oxidatively fragmented by trifluoroperoxyacetic acid. The major products were acetic, malonic, and succinic acids; benzene di, tri, tetra; and penta-carboxylic acids; and two compounds provisionalli identified as the epoxides of ethylene tri- and tetra-carboxylic acids. These products are in best accord with a structure for Monterey coal in which dihydrobenzene units are frequently interspersed in a polymer composed of condensed aromatic chains and clusters. The methyl and hydroxyl substituents complete the dominant pattern.

proposals of Given’, there has been a concept that the vitrinite maceral in coals could be represented by the structural formulae familiar to organic chemistry. The contrasting view is that coals consist of such a complex mixture of component structures that no single or simple representation is possible. We have investigated this question using trifluoroperoxyacetic acid (per TFA) to fragment the coals oxidatively’. Oxidations with per TFA contrast sharply with oxidations by O,, HNO,, MnVII, and Cr VI. The latter reagents attack the benzylic hydrogen leading to much destruction of aliphatic structure. The per TFA attacks the aromatic ring so that aliphatic structure is often preserved despite destruction of the aromatic rings’. In the present study the products from Monterey coal were examined with special emphasis on identification of the products and determination of the structural units in the coal responsible for their formation. Starting with the pioneering

EXPERIMENTAL Coal sample

The sample, a typical bituminous coal, came from Monterey mine (Illinois No. 6 seam) and was identical with that used in earlier studies’. Oxidation procedure

The reagent is prepared from 8 cm’ CF,COOH, 10 cm3 of 3O’i/, aqueous H,O,, and 5 cm3 97% H,SO,. Stirring with a magnetic bar is started and 0.5 g of <20 US mesh coal is added. A condenser is attached and the temperature slowly raised until a mild exothermic reaction begins as evidenced by gentle reflux. A few coals and other substrates may give an immediate mild exothermic reaction. The temperature is held at 6&7O”C for 1 h. A potassium iodide test for peroxides may be negative after 1 h in which case 4 cm3 of 30’% H,O, are added and heating at 60-70°C continued for 15 h. 0016~2361/80/100694-05S2.00 0 1980 I PC BusinessPress

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The excess peroxide is destroyed by adding small increments of 10 wt y0 Pt catalyst until the test for peroxides is negative. The catalyst is then removed by filtration. Although the proton magnetic resonance (p.m.r.) spectrum of the filtrate shows line broadening due to iron salts, the resolution is sufficient with Monterey coal to allow a satisfactory estimate from peak areas of the amounts of acetic, malonic, and succinic acids present. Convenient internal standards were pivalic acid or 2,2dimethyl-2-silapentane-5-sulfonate (DSS). The filtrate is distilled at 2 kPa and 90°C and the distillate condensed at 15°C. The p.m.r. spectrum of the distillate is highly resolved, and the amount of acetic acid is determined again. Propionic and butyric acids would have distilled under these conditions and they were not detected by p.m.r. Some broadened p.m.r. bands were initially attributed to propionic acid* but this now appears to be incorrect. The nonvolatile fraction consists largely of polycarboxylic acids. These are converted to methyl esters by stirring the residue for 2 h at 60°C with 6 cm3 of 14% BF, in methanol. The methyl esters are dissolved in CH,Cl, and the solution washed with water, 10% NaCO,, and water. After drying over MgSO,, the solution is decanted and the CH,Cl, removed on the rotary evaporator. Subsequent studies with authentic samples of succinic and some of the benzenepolycarboxylic acids showed that the methyl esters form nearly quantitatively but only 3& 80% are recovered. The methyl ester of benzene-1,2,4,5tetracarboxylic acid is recovered with 2&50x efficiency and the recovery is even less with the benzene penta-acid. Studies are in progress to improve recoveries, using salting out and multiple extractions. Checks are also in progress to ensure that esterification is complete with highly hindered polyacids. For the present, yields of acetic, malonic, and succinic acids are reliable when derived from the p.m.r. spectra. Yields from gas chromatogram (g.c.) peak areas are semiquantitative.

Dihydroaromatic Tab/e

7 Capillary

Relative retention timea

g.c. of methyl esters

Relative peak heightb

MWc (methyl ester)

6.6 8.0 11.3 12.2 15.0 20.5 38.3 50.1

45 4 6 100 7 9 3 16

204 218a

52.0 58.0 64.4 65.9 80.8

11 60 16 28 21

218b 194a 194b 194c 276

102.5 105.2 129.3 135.9 138.7 142.1 146.8 165.6

60 120 17 54 58 13 13 13

252a 252b 297 310a 310b 31oc 308 368

Identification

(parent acidId

malonic (methylmalonic) (maleicl succinic (methylsuccinic) (benzoic) ethanetriCOOH epoxide of ethylenetriCOOH propane-1,2,3-triCOOH benzene-1.2diCOOH benzene-1,4diCOOH benzene-1,3diCOOH epoxide of ethylenetetraCOOH benzene-1,2,4-triCOOH benzene-1,2,3-triCOOH benzene-l ,2,4,5tetraCOOH benzene-l ,2,3,4-tetraCOOH benzene-1,2,3,5-tetraCOOH

mol) of toluene with 0.10

mol

Mols of CF&OOH

Mols of H+O,

Yield of acetic acid (wt o/o)

0.0875 0.0875 0.0875 0.0438 0.0219

0.0875 0.1150 0.0438 0.0875 0.0875 0.0875

79 77 71 76 76 75

0.0120

coal: N. C. Deno et al.

nature of the oxidation, the variable temperature, and the presence of free radicals from cleavage of peroxides heightened concern over reproducibility. The data in Table 2 on toluene and Table 3 on indan show that yields are not affected by modest variations in the ratios of reactants, although it must be remembered that the absence of H,SO, causes large changes2. Of particular significance were runs on indan in which addition of 2 wt % Fe III carbonate or 2 wt ‘x oxide did not affect yields from the oxidation of indan. To determine the effect of temperature toluene was added to an oxidation mixture over a 10 min period at 5”C, and the mixture was heated slowly, at a rate of 60°C h- I. At 35°C a yellow colour began to develop in the polar phase and at 40°C the mixture became one phase and the colour deepened to brown; at 70°C the solution became colourless. The yield of acetic acid (p.m.r. analysis) was 69:/,; this is comparable to the 71-797” yields reported in Tuble 2 and shows that there is no advantage in careful control of temperature during addition nor to conducting the reaction at temperatures below 70°C. Finally, three identical runs were conducted on the coal. No significant variation in yields was detected.

benzenepentaCOOH

a These are measured from the appearance of the solvent peak. No further peaks appeared out to retention times of 200 b In addition to this list, the following minor peaks were observed (retention time is followed by the peak height in parenthesis): 8.2 (3). 9.7 (41, 18.8 (31, 54.5 (2). 72.4 (7), 93.2 (3). 97.5 (5). 98.1 (6). 101.0 (2). 113.2 (5). 115.2 (31, 117.3 (3), 118.2 (3). 119.2 (4), 127.2 (5), 140.9 (41, 148.2 (31, and 157.2 (2) c These are MWvalues determined from chemical ionization mass spectra d Identifications in parenthesis are those resting only on g.c. retention time coincidences with authentic samples

Table 2 Oxidation of 0.5 g (0.054 of hydrogen peroxide

structure of Illinois No. 6 Monterey

Gas chromatography The data in Table 1 were obtained on a 30 m glass capillary column supplied by Supelco Inc. (Bellefonte, PA). It was used with a Varian 1200 instrument equipped with flame ionization detector. The stationary phase was a methyl silicone, SP-2100, and the carrier gas was nitrogen. The flame ionization detector gives peak areas that are approximately proportional to weight. The estimates of amounts can be improved by calculating effective carbon numbers3, but truly quantitative results must await improved recoveries of esters and empirical calibration with authentic samples. Reproducibiliry studies There was concern that products and yields might be sensitive to minor changes in reaction conditions and changes in amounts of reagents. The heterogeneous

Mass spectra Data on the chemical ionization mass spectra are not explicitly reported. In general methyl esters give a set of P -31, P+l, P+29, and P+41 peaks. The P-31 arises from loss of methanol from P + 1. The P + 1 arises from protonation of P by CH:. The P+29 and P +41 peaks are smaller and arise by addition of C,H: fragments to P. From these peaks, the MWof P can be assigned and the products recognized as methyl esters. The appearance of only one such set shows that the g.c. peak was homogeneous, at least in respect of non-isomeric impurities. Where products had the same MW they are distinguished by adding a, b, or c to the M Wto indicate increasing g.c. retention times. The M Wvalues appear in Tub/e 1. Some of the esters were subjected to chemical ionization by isobutane. This gave only the P + 1 peak. The electron impact mass spectra showed strong peaks at m/e 59 (CH,OC=O+) and P-31 (RCrO’) and confirmed the identification as methyl esters. YIELDS The p.m.r. spectrum of the reaction mixture was used to calculate the fraction of the hydrogen and carbon of the coal which was observed in products. For the hydrogen, 2.8 wt 7; appeared in the methyl group of acetic acid, 2.1 wt ‘g appeared in the methylene group of malonic acid, and 9.8 wt ‘A was observed in the c-3.46 region. For carbon, 1.4 wt y0 was observed in the two carbons ofacetic Tab/e 3 Oxidation of 0.5 g (0.042 of hydrogen peroxide

mol) of indan with 0.071

mol

Yield (wt %I MOI of CF$OOH

Mol of H,S04

Succinic acid

Glutaric

0.055 0.055 0.055 0.0275 0.0138 0.0069

0.055 0.0715 0.0275 0.055 0.055 0.055

28 27 27 27 26 25

39 38 36 39 39 39

FUEL, 1980, Vol 59, October

acid

695

Dihydroaromatic

structure of Illinois No. 6 Monterey

acid, 2.4 wt “/;, was in the three carbons of malonic acid, and 7.4% in the four carbons of succinic acid. Assuming that the remaining absorption from O-3.46 is due to CH, units, a total of 14.4 wt “/, of the carbon of the coal is accounted in aliphatic CH, and CH,. The p.m.r. data show that only a small proportion of aliphatic carbon appears as products other than acetic, malonic, and succinic acids. The g.c. data in Table 1 show the same result. Methylmalonic and methylsuccinic acids are the only additional products containing aliphatic hydrogen and their combined areas (4+ 7) are small compared with the 45 and 100 for malonic and succinic acids. This shows that no major aliphatic product is undetected in the g.c. Despite the problems in obtaining quantitative results from the g.c. data, an estimate can be made of the amount of carbon observed in products. Using data in Table 1, calculating yields relative to succinic acid, and using an average effective carbon number3 of 8 for products of M W 218 and higher, about 16.6 wt % of the carbon of Monterey coal appears in products of M W218 or higher. Combining this with the 14.4 wt % carbon from the smaller products, a total of 31 wt y0 of the carbon of the coal is observed in products. This is a reasonable number in view ofthe model studies which showed that about twothirds of aliphatic carbons and less than half of the aromatic carbons can be expected to appear in products recognizable by p.m.r. and g.c. IDENTIFICATION

OF PRODUCTS

The p.m.r. spectra of acetic, malonic, and succinic acids and of dimethyl malonate and succinate were identical with spectra of authentic samples. The g.c. retention times of the dimethyl esters of malonic, maleic, succinic, methylmalonic, and methylsuccinic acids were identical with those of authentic samples. In addition, there was corroborative evidence from mass spectra. The series of M W 194,252,3 10, and 368 are the methyl esters of benzene di-, tri-, tetra-, and penta-carboxylic acids. The M W of each peak was determined from the mass spectra. The relative retention times have been reported4. Authentic samples of methyl esters were available for direct comparison for: benzene I,2 and 1,4dicarboxylic acids; benzene-1,2,4$tetracarboxylic acid, and benzenepentacarboxylic acid. The electron-impact mass spectra of the methyl esters of benzene-1,2,4tricarboxylic acid and benzene-1,2,4,5-tetracarboxylic acid agreed with literature values’. The two species of M W252 were shown not to be the trimethyl ester of benzene-1,3,5-tricarboxylic acid, which has a different mass spectrum’ and a different g.c. retention time. The product of M W276 is identified as the tetramethyl ester of the epoxide of ethylenetetracarboxylic acid. The high-resolution mass spectrum indicated that the formula was C,H,,O,. The M W of the deuterated ester was 288 showing that it was a tetramethyl ester. A synthetic sample was prepared from the tetramethyl ester of ethylene-tetracarboxylic acid using the procedure described for epoxidizing tetracyanoethylene6. The 276 species from the coal and the synthetic 276 had the same g.c. retention times. The m/e peaks in the mass spectrum of 276 with relative heights at least 7% of the base peak are given here in increasing order; the relative heights are in parenthesis, first for synthetic 276 and second for 276 from

696

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coal: N. C. Deno et al. coal; m/e 53 (16, l), 59 (100, loo), 67 (7,2), 69 (33,63), 75 (7, 5),83(13,59),105(14,90),111(7,2),133(36,20),173(9,60), and 229 (7, 4). The agreement is only fair. The mass spectrum of a deuterated sample indicated that m/e 59 was 69 was OCCHCO+, and 135 was CH30CO+, (CH,O),C+. This epoxide ofethylenetetracarboxylic acid and its tetramethyl ester have not been previously reported so that the identification rests on the MW the mass spectrum, and the method of preparation. The synthetic sample was stable to per TFA. The 2 18a peak is tentatively identified as the trimethyl ester of the epoxide ofethylenetricarboxylic acid. The M W of 218 indicated one less carbomethoxy group than 276 and the mass spectra showed similarities: m/e 59 (53), 69 (87) 75 (43) 115 (100) 127 (30), 131 (60), 159 (83), 171 (43), and 187 (23). The m/e peaks at 59,69, and 75 are common to the mass spectra of both compounds 218a and 276. The mass difference of 58 between 218 and 276 suggests that them/epeaksof115and171inthemassspectrumof218a correspond to 173 and 229 in the spectrum of 276. The 218a species was specifically shown not to be an ester of a propanetricarboxylic acid: the mass spectrum differed from the spectra of four of the five possible isomers (preparations referenced later) and the fifth isomer (l,l,l) was dismissed as too unstable. The 218b peak is identified as the trimethyl ester of propane-1,2,3-tricarboxylic acid. An authentic sample was prepared by esterification of the free acid (Aldrich Chemical Co.). The nine strongest peaks in the mass spectrum were m/e 127 (loo), 126 (66), 187 (55), 59 (49), 186 (29), 99 (22), 95 (22), 154 (20), and 113 (18). All of these are found in the mass spectrum of 218b from coal and in comparable intensity. The g.c. retention times were identical. As a precaution, the 1,1,2, 1,1,3, and 1,2,2 isomers of propanetricarboxylic acid were prepared by literature methods 7m9. The mass spectra of their methyl esters showed significant differences from that of 218b. The 204 peak was the trimethyl ester of ethanetricarboxylic acid. An authentic sample was prepared” which matched 204 in g.c. retention time and in electron impact and chemical ionization mass spectra. Methyl benzoate was identified by M W and g.c. retention time. Two products had odd-numbered MW 239 and 297, which indicated that they contain one nitrogen atom. The difference of 58 in their MWsuggest that they differ by a simple carbomethoxy group. By esterifying with CD,OH, it was shown that the 239 species has two and 297 three, carboxyls that esterify. It has not been possible to assign unambiguous structures from mass spectra although fragmentation patterns suggest that two carboxyls are in a cis maleic acid type arrangement, and the M Wis in accord with a five-membered lactone ring fused to a pyrrole polycarboxylic acid. The identification of the products from model compounds depended largely on g.c. retention times. In the case of unexpected products such as the succinic acid from phenanthrene and the adipic acid from dihydrophenanthrene, identifications were reinforced with comparisons of mass spectra. STRUCTURE

OF MONTEREY

COAL

The most significant result of many model studies is that the products from per TFA oxidation of 5,12-dihydronaphthacene (I) duplicate the majority of products from

Dihydroaromatic Tab/e 4 A comparison 5,12dihydronaphthacene

of products from per TFA (I) and Monterey coal Relative

oxidation

structure

area of g.c. peak of methyl ester

lb

Ic

Id

coal

Malonic acid Benzoic acid EthanetriCOOH epoxide of ethvlenetriCOOH propane-1,2,3-triCOOH Benzene-l ,2diCOOHa Benzene-l ,4diCOOH Epoxide of ethylenetetraCOOH Benzene-l ,2,4-triCOOH Benzene-l ,2,4,5-tetraCOOH

23 48 3 11 6 60 la

7 22 2 13 6 60 a

a 12 0 15 la 60 11

45 9 3 16 11 60 16

13 41 25

9 34 22

3 35 61

21 60 54

a Arbitrarily set at 60 b Peak heights from a capillary column c Areas from a 0.25 inch (6.35 mm) packed column d In this experiment, the initial CHzClz extraction was aided by saturating the aqueous layer with NaCI. The relative increase in the tetramethyl ester of benzene-l ,2,4,5+etracarboxylic acid is evident. Areas were obtained on a 0.25 inch (6.35 mm) packed column

esters of products

Product

Naphthalene

Benzene-1,Sdi COOH (75), malonic succinic (4). maleic (2). others (15)

Anthracene

Benzene-1,2di COOH (431, maleic (101, others (3). anthraquinone (43)

Phenanthrene

Benzene-1,2di COOH (491, succinic (18). benzoic (121, maleic (41, malonic (1). others (16)

2-methoxyphenanthrene

Benzene-1,2di COOH (41). succinic (15), malonic (6), others (38)

Chrysene

Benzene-1,2di COOH (181, WV218 1181, succinic (121, malonic Ill), benzene-l ,3d.i COOH (8). benzene-l ,4di COOH (B), epoxide of ethylenetetra COOH (8). benzene-1,2,4-tri COOH (4). others (13)

Product

Perylene

Benzene-l

Benzene-1,2di COOH (16), maleic (16). 2carbomethoxybutyrolactone ? (161, a carbomethoxyphthalide ? (16). phthalide (71, benzene-1,2,4-tri COOH (5). epoxide of ethylenetetra COOH (3), others (21)

2-anthroic

Benzene-1,2,4-tri COOH (3B), benzene1,Zdi COOH (25). others (37)

1,2,3,4-tetrahydro-lnaphthoic acid

malonic,

1,2,3,4-tetrahydro-2naphthoic acid

Butane-1.2.4~tri COOH (27), succinic (231, propane-1.2.3~tri COOH (la), others (32)

Fluoranthene

Maleic (15), benzene-1,4di COOH (151, benzene-1,2di COOH (14), benzoic (9), epoxide of ethylenetetra COOH (5). benzene-l ,2,3-tri COOH (4). a carbomethoxyphthalide ? (41, a dicarbomethoxyphthalide ? (41, others 130)

Pyrene

Succinic (14), benzene-l ,4di COOH 110). benzoic (8). benzene-1,2,4-tri COOH (B), three dicarbomethoxyphthalides ? (7, 3,6), two methyl tricarbomethoxyphthalides ? (5.6). two MW21B 16,4), benzene-l ,2di COOH (5). epoxide of ethylenetetra COOH (4). others (14)

,4di

COOH

(241, benzoic

(141,

benzene-1,2di COOH (111, epoxide of ethylenetetra COOH (8). two of WV236 (5, 8). benzene-1,2,4-tri COOH (81, a dicarbomethoxyphthalide ? (8). a carbomethoxyphthalide (6), others (8) lndan

Succinic plus a little maleic (43), glutaric (39). malonic (4), cyclopentene-1.2di COOH (4). others (10)

4 and 5-hydroxyindan

Similar to the above

Tetralin

Cyclohexene-1.2di COOH (37). adipic (24). glutaric (16), succinic (41, maleic (2), malonic (1). others (16)

5 and 6hydroxytetralin

Similar to the above

I-methvltetralin

Methylsuccinic (19). succinic (17). malonic (15), methylmalonic (141, 2methylglutaric

(5), others (30)

9,lOdihydroanthracene

Benzene-l ,2di COOH succinic (14), benzoic

9,lOdihydrophenan threne

Succinic (31). 3-(2’~carboxyphenyl)propionic (27). benzene-1.2di COOH (91, MWl98 (91, adipic (a), maleic (5). benzoic (5). malonic (4). others (2)

1 naphthoic

acid

Benzene-l ,2,3-tri COOH (31). benzene1,2di COOH (11). others (58)

2-naphthoic

acid

Benzene-l ,2,4-tri COOH (591, benzene, 1,2di COOH (18). others (23)

(41,

Benz [al anthracene

glutaric,

Model

from

Model

acid

coal: N. C. Deno et al.

Table 5 -cont.

of

Product

Tab/e 5 Relative g.c. peak areas of methyl per TFA oxidation of model compounds

of Illinois No. 6 Monterey

Naphthalene-2,3dicarboxylic acid

(70), maleic and (5). others (11)

Benzene-l ,2,4,5-tetra COOH (35), benzene-l ,2di COOH (25). benzene1,2,4-tri COOH (20). phthalide (20) Maleic acid (501, benzene-l ,2di COOH (341, phthalidea-COOH 15). homologue of phthalide+COOH ? (6). a methyl ester of Mw218 (5)

Monterey coal, and the relative amounts are comparable (Table 4). Of particular significance is the formation of benzene- 1,2,4-tricarboxylic acid (252a) and benzene1,2,4,5-tetracarboxylic acid (310a). These are uncommon products from per TFA oxidations and are not produced from polyalkylbenzenes’*” or polyaromatic structures (Table 5). Even the epoxides, 218a and 276, are duplicated as are the unusual ethane and nronanetricarboxylic acids, 204 and 218b.

and succinic acids

cont.

I

II

The formation of 252a and 310a from I was foretold by the results of oxidation of naphthalene carboxylic acids and 9,10-dihydroanthracene. Early model studies’ had established that carboxyl substituents render benzene rings inert to oxidation by per TFA. The formation of benzene-1,2,3- and 1,2,4-tricarboxylic acids from l- and 2napthoic acids, and the formation of benzene- 1,2,4-triacid and 1,2,4,5_tetraacid from napthalene-2,3-dicarboxylic acid (Table 5), were all expected. However, the 252 and 310 series of products cannot arise from naphthalene carboxylic acid units in coal because such structures cannot be interior units in a polymer and because of the paucity of carboxyl groups in bituminous coals.

FUEL, 1980, Vol 59, October

697

Dihydroaromatic

structure

of Illinois

No. 6 Monterey

The puzzle was to discover an interior unit that would degrade to an aromatic dicarboxylic acid. Early considerations were anthracene and phenanthrene units in which an end ring is activated, but the failure of 2methoxyphenanthrene and 1,4-chrysoquinone to form benzene-triand tetra-acids (Table 5) dismissed this possibility. The key to the puzzle was the behaviour of 9,10This benzene- 1,2dihydroanthracene. formed dicarboxylic acid and benzoic acid as the dominant products (T&e 5). Thus, a dihydroanthracene unit in the interior of a polymer would cleave leaving an aromatic diacid. Further oxidation would destroy all but the carboxyl substituted ring as with the naphthalene carboxylic acids and the anthracene carboxylic acid (Table 5). The results with I confirmed this analysis. Note that 9,10dihydroanthracene does not produce benzene-triand tetra-acids by itself, but only as a unit in the interior of a polyaromatic polymer. Monterey coal gave a few products in addition to those formed from I. Acetic acid arises from arylmethyl groups which are scattered throughout the polyaromatic structure. When such methyl groups are attached to an aliphatic benzyl carbon, methylmalonic and methylsuccinic acids are produced and these are minor products from the coal (Table 1). Succinic acid is believed to arise largely from 9,10-dihydrophenanthrene units in coal much as it does from 9,10-dihydrophenanthrene itself (T&/e 5). Benzene-1,2,3-tricarboxylic acid (252b) and benzene-1,2,3,4-tetracarboxylic acid (3 lob) are believed to arise from dihydroanthracene units in which the fourth ring is an (u)-fusion as in compound II rather than the(b)fusion of compound I. The final conclusion is that Monterey coal is a polymer composed of fused benzene rings in which dihydrobenzene units are interspersed at frequent intervals, perhaps every third to fifth ring. Dihydrobenzene units were also a prominent feature of the vitrinite structure proposed by Given’, and there are strong reasons from bonding theory for favouring such structures. If hydroaromatic rings are to be interspersed in a polyaromatic polymer, dihydrobenzene units are more stable than tetrahydro or octahydro units since the dihydro unit maximizes the number of bonds in benzene rings and concomitantly maximizes resonance energy and thermodynamic stability (the opfor terminal rings). would be true posite Hexahydronaphthalene units would also be stable, but the aliphatic polyacids to be expected from such units were not detected. The formation of benzenepentacarboxylic acid (Table l), and the many reports of the formation of benzenepenta- and hexa-carboxylic acids from oxidation of coals with O,, HNO,, Mn VII, and Cr VI indicate that some of the benzene rings are fused at three or more carbons as in pyrene, perylene, and graphite. Dihydro derivatives are also the stable form of hydroaromatics in such clusters, and such structures can be viewed as 9,10-dihydro-

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coal: N. C. Deno et al. phenanthrene variants which would produce succinic acid on per TFA oxidation. This aromatic-dihydroaromatic structure readily accommodates organic nitrogen and sulphur by interspersing pyridine, pyrrole, and thiophene rings. In fact the 239 and 297 products appear to be pyrrole derivatives. This structure is in accord with X-ray diffraction studies which show the absence of graphite spacings and indicate that aromatic clusters contain no more than 2-4l 2 or 3-5’ 3 contiguous benzene rings. The aromatic-dihydroaromatic structure also explains products from thermal cracking of coal. The weakest bonds are at the dihydro units and these cleave to form relatively stable benzyl radicals. These and the aryl radicals formed simultaneously abstract hydrogen from dihydro units to form arylmethyl and additional benzene rings. The frequent occurrence of the dihydro units leads to aromatic products of largely l-3 rings, with or without methyl substituents. The loss of hydrogen from the dihydro units creates larger aromatic domains which form char or coke. The amount of arylmethyl groups in coal can be estimated from the yield of acetic acid. Since the typical yield from arylmethyl model compounds is 70 wt “/, and since only one of the two carbons of acetic acid comes from arylmethyl, the best estimate for the wt % C present as arylmethyl is the wt :/, C observed as acetic acid (1.4 wt ‘I/,)divided by 2 and 0.7. This gives 1.O wt % for Monterey coal.

ACKNOWLEDGEMENT This work was supported by grants from the Electric Power Research Institute and the Department of Energy. We are grateful for this support. We are also grateful to Professor Peter H. Given (College of Earth and Mineral Sciences, Pennsylvania State University) for a careful review of this paper.

REFERENCES 1 2 3 4

5 6 7 8 9 10 11 12 13

Given, P. H. Fuel 1960, 39, 147; 1961, 40, 427 Deno, N. C., Greigger, B. A. and Stroud, S. G. Fuel 1978,57,455 Grob, R. L. ‘Modern Practice of Gas Chromatography’, Wiley and Sons, N.Y., 1977, p. 254 Hayatsu. R., Winans, R. E., Scott, R. G., Moore, L. P. and Studier, M. H. Naturr 1978, 275, 116: and preprints and unpublished work kindly supplied by M. H. Studier Heiler, S. R. and Milne, G. W. A. EPA/NIH Mass Spectra Data Base, US Govt. Printing Office, Washington, D.C., 1978 Linn, W. J. Ory. Syn. Co/l. Vo[. V 1973, 1007 Bischoff, C. A. and Emmert, A. Chrm. Ber. 1882, 15, 1107 Marvel, C. S. and Stoddard, M. P. J. Org. Chem. 1938, 3, 201 Bischoff, C. A. and von Kuhlberg, A. Chrm. Ber. 1890, 23, 635 Bischoff, C. A. Chem. Ber. 1896,29, 967 Hart, H. Accounts Chem. Rrs. 1971, 4, 337 Hirsch, P. B. Phil. May. Roy. Sot. 1960, 252A, 68 Scaroni, A. W. and Essenhigh, R. H. Am. Chem. Sot., Din Fuel Chem., Preprints 1978, 23, 124