Oxidation and Decarboxylation. A Reaction Sequence for the Study of

Energy & Fuels 1997, 11, 987-997

987

Oxidation and Decarboxylation. A Reaction Sequence for the Study of Aromatic Structural Elements in Pocahontas No. 3 Coal Leon M. Stock* and Marcus Obeng Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, and Department of Chemistry, The University of Chicago, Chicago, Illinois 60637 Received December 26, 1996. Revised Manuscript Received May 29, 1997X

Oxygen in basic solution has been used to oxidize Pocahontas No. 3 coal to a mixture of aromatic carboxylic acids, and copper(I) oxide in N-methylpyrrolidinone-quinoline has been used to decarboxylate the acids to provide a mixture of hydrocarbons. Most pendant alkyl groups have been removed in this sequence and the product distribution is much less complex than the product distributions that are obtained in direct oxidation reactions or in the analysis of coal liquids and extracts. Approximately 25% of the aromatic carbon atoms are retained in the products. The structures of these compounds have been investigated by gas chromatography-mass spectrometry and by high-resolution and laser desorption mass spectrometry. Approximately 150 compounds were detected in the GCMS experiments including a broad array of bi-, ter-, and quateraryls, fluorene and fluoranthene derivatives, polycyclic aromatic hydrocarbons with four, five, and six rings, dibenzo- and dinaphthofurans, and dibenzo- and dinaphthothiophenes. High-resolution mass spectrometry provided additional evidence concerning the principal products and extended the mass range to approximately 400 Da. Laser desorption work revealed that even larger molecules were present in the reaction products with distinct signals at 550 Da and definite intensity extending to 1200 Da. These results are compatible with many other results that have been obtained in the past decade and strongly infer that the relatively high-ranking Pocahontas No. 3 coal has many large, condensed, aromatic clusters that are methylated and connected through biaryl and heterocyclic linkages.

Introduction It has now been more than 10 years since the Argonne premium sample of Pocahontas No. 3 coal was collected and prepared for distribution to the coal chemistry community for study.1 The results of many experimental investigations concerning the chemical constitution of this coal were critically evaluated in 1993.2 The review led to a structural representation that accommodated the available chemical and physical information about its aliphatic, aromatic, and heterocyclic components and emphasized the abundance of aliphatic methyl groups, the bridging nature of biaryl and heterocyclic structural elements, and the existence of large polycyclic aromatic structural units.2 Whereas the information about the aliphatic and heterocyclic constituents could be discussed with some confidence, it was recognized that the experimental observations concerning the aromatic structural elements could not be employed with equal effectiveness to establish the nature of the aromatic components. Some experimental approaches provided average results that were unsuitable for the definition of the number and kind of different aromatic structures, and other approaches provided hopelessly complex arrays of data that could not be successfully partitioned. This feature is well illustrated by the chromatographic results for the Abstract published in Advance ACS Abstracts, July 1, 1997. (1) Vorres, K. S. Energy Fuels 1990, 4, 420. (2) Stock, L. M.; Muntean, J. V. Energy Fuels 1993, 7, 704.

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liquefaction products of the Pocahontas coal.3 The chromatograms typically exhibit numerous sharp signals atop an intense and very broad background of other signals throughout the interval when aromatic compounds with three to eight aromatic rings emerge. Aromatic compounds with four or more rings and their methylated derivatives are clearly present.2,3 But the composition of these mixtures cannot be readily defined because of the very large number of isomeric compounds. Chrysene, for example, provides six monomethyl compounds, but there are more than 30 dimethylchrysenes. The existence of these compounds among many other isomeric benzoanthracenes and benzophenanthrenes and their simple methylated derivatives in Pocahontas coal renders the chromatograms uninterpretable. We elected to approach the problem differently by using chemical oxidation to decompose the coal and simultaneously to oxidize the pendant alkyl groups to obtain an array of carboxylic acids and then to decarboxylate this complex mixture of acids in a second reaction to provide a less complex mixture of hydrocarbons free of pendant groups. This approach was introduced by Entel4 and used by Holly, Montgomery, and Gohlke5 for the study of another sample of Pocahontas coal, but the inadequacy of the gas chromatographic equipment that was available in 1956 and (3) 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. (4) Entel, J. J. Am. Chem. Soc. 1955, 77, 611. (5) Holly, E. D.; Montgomery, R. S ; Gohlke, R. S. Fuel 1956, 35, 56.

© 1997 American Chemical Society

988 Energy & Fuels, Vol. 11, No. 5, 1997

the use of an impure solvent in the decarboxylation reaction prevented the acquisition of the desired information. The technique can, in principle, solve the isomer number problem by converting the initial isomeric carboxylic acids into a simpler set of hydrocarbons that can be analyzed with more confidence, but it was clear from the outset that compromises would have to be made in the selection of the method for the oxidation of the coal. The benzenecarboxylic acids that are obtained in relatively high yields in vigorous oxidation reactions provide very limited information about the large aromatic constituents, and the large, essentially intractable molecules that are obtained under mild conditions cannot be analyzed effectively. Accordingly, we evaluated several different reaction sequences for oxidation and decarboxylation to determine the most effective procedure for the conversion of the coal to polycyclic compounds. Barium hydroxide promoted oxidation by dioxygen and copper(I) oxide catalyzed decarboxylation in N-methylpyrrolidinone-quinoline provided the highest yields of the desired hydrocarbons. The products of this reaction sequence were analyzed by chromatographic and mass spectroscopic methods. Experimental Section Materials. The coal sample was obtained from the Argonne Premium Sample Program.1 The elemental composition of this low-volatile bituminous coal, which contains 89% vitrinite, 10% inertinite, and 1% liptinite, is C1000H575O10.8N12.5S2.1. The solvents were high-purity materials and were analyzed to establish the absence of interfering substances. These substances such as the inorganic hydroxides and copper(I) oxide were obtained from commercial sources. However, quinoline required purification. A mixture of quinoline (100 mL) and 6 M hydrochloric acid (120 mL) was stirred at ambient temperature for 1 h and extracted with methylene chloride (3 × 50 mL). A solution of 10% sodium hydroxide was added slowly until the quinoline was liberated. The quinoline was separated from the aqueous phase, washed several times with water, dried over sodium sulfate, and distilled under reduced pressure. Oxidation of Pocahontas No. 3 Coal in Barium Hydroxide Solution. The oxidation was carried out in 250 mL Parr autoclave equipped with an adjustable-speed motor. A mixture of Pocahontas No. 3 coal (4.51 g) and barium hydroxide (50 g) dissolved in water (120 mL) was thoroughly mixed in the Parr autoclave. Oxygen was introduced and the autoclave heated at 200 °C and a total pressure of 700 psig for 3-5 h. After cooling to room temperature, the gases were vented and the reaction mixture removed from the autoclave. The mixture was acidified to pH ) 1 and the supernatant solution extracted with ethyl acetate (4 × 100 mL). The solid residue also was extracted with ethyl acetate (2 × 100 mL). The combined organic extracts were washed with brine and dried over sodium sulfate and concentrated to give the carboxylic acids (2.31 g). Copper(I) Oxide-Quinoline-N-Methylpyrrolidinone Decarboxylation. A mixture of the organic acids (1.22 g), copper(I) oxide (2.09 g), quinoline (50 mL), and N-methylpyrrolidinone (50 mL) was heated at 250 °C for 12 h. The carbon dioxide yields were determined gravimetrically by adsorption onto ascarite. The mixture was cooled, methylene chloride (200 mL) was added, and the mixture was washed with 10% sodium hydroxide (100 mL). The organic phase was washed successively with water (100 mL), 6 M hydrochloric acid (2 × 100 mL), and water (100 mL). The organic phase was dried over sodium sulfate and concentrated to provide a mixture of hydrocarbons (0.45 g). This product was passed through an

Stock and Obeng Table 1. Oxidation of Pocahontas No. 3 Coal. conditions oxidant 70% nitric acid ceric ammonium nitrate potassium permanganate sodium dichromate

solvent

temp (°C)

time (h)

conversion (wt %)

water water water water

reflux reflux reflux 250

30 48 48 48

68 43 70

aluminum oxide column and analyzed by gas chromatography-mass spectrometry. Oxidation of 4-Methylbenzene-1,2-dicarboxylic Acid. A mixture of 4-methylbenzene-1,2-dicarboxylic acid (200 mg) and barium hydroxide (5.0 g) was dissolved in water (15 mL) and thoroughly mixed in the Parr autoclave. Oxygen was introduced and the autoclave heated at 200 °C and a total pressure of 700 psig for 5 h. After cooling to room temperature, the gases were vented and the reaction mixture removed from the autoclave. The mixture was acidified to pH ) 1.0 and the supernatant solution extracted with ethyl acetate (4 × 50 mL). The combined organic extracts were washed with brine, dried over sodium sulfate, and concentrated to afford a mixture of two acids (180 mg). Gas chromatography-mass spectrometry of a sample (10 mg) that had been treated with diazomethane showed that it contained 67% 4-methylbenzene-1,2-dicarboxylic acid and 33% benzene-1,2,4-tricarboxylic acid. Analyses of Hydrocarbons and Neutral Compounds. Preliminary analyses were performed on a Hewlett-Parkard Model HP 5890 gas chromatograph in the flame ionization mode with a 15 m × 0.25 mm fused capillary column coated with DB-5. The gas chromatography-mass spectrometry analyses of the reaction products were performed on an HP 5890 Series II gas chromatograph with an HP Model 5970 mass spectrometer. The gas chromatograph was equipped with J & W Scientific 60 m × 0.25 mm fused capillary column coated with a film of DB-5. The injection temperature was 300 °C and the oven temperature was programmed to hold at 50 °C for 1 min before the column was heated to 300 °C at the rate of 4 °C/min. Helium was used as the carrier gas and the injection was carried out in the splitless mode. The mass spectrometer was operated at 70 eV in the electron impact mode and was scanned from 10 to 650 amu over 0.5 s. All the mass spectra were scan averaged, and the backgrounds were corrected. The laser desorption mass spectra (LDMS) were acquired on a Kratos MALDI III laser desorption time-of-flight mass spectrometer in both reflection and linear modes. The samples were dissolved in methylene chloride and applied to the sample probe, allowing the solvent to evaporate. An accelerating voltage of 20 kV was used, and about 50 individual spectra were summed to produce the final spectrum. The high-resolution mass spectra (HRMS) were acquired on a Kratos MS 50TC ultrahigh-resolution mass spectrometer operated at 70 eV mode. The samples were analyzed in a direct probe heated from 200 to 400 °C. The 70 eV electron impact spectra were obtained with 15 000 dynamic resolution with scan rate of 10 s/decade.

Results Oxidation. Several different oxidation methods were studied in preliminary experiments. In each case, the coal was oxidized as described in the literature6 and the carboxylic acids were converted to their methyl esters for convenient analysis by gas chromatography. The yields that were obtained with several different reagents are summarized in Table 1. (6) Hayatsu, R.; Scott, R. G.; Winans, R. E. Oxidation of Coal. In Oxidation in Organic Chemistry, Part D; Trahanovsky, W. S., Ed.; Academic Press: New York, 1982; pp 279-352.

Oxidation and Decarboxylation

Energy & Fuels, Vol. 11, No. 5, 1997 989

Table 2. Oxidation of Pocahontas No. 3 Coal with Dioxygen in Basic Aqueous Solution reaction conditions metal hydroxide

temp (°C)

pressure (psig)

time (h)

conversion (wt %)

lithium potassium potassium lead(IV) lead(IV) lead(IV) and potassium barium barium barium

250 250 250 250 200 200

700 900 900 700 700 700

5 5 3 5 3 3

28 58 69 33 37 42

250 250 200

700 700 700

5 3 5

51 51 46

Nitric acid oxidation gave the carboxylic acids in satisfactory yield, but the products were predominantly benzenecarboxylic acids contaminated with nitrobenzenecarboxylic acids rather than the desired polycyclic aromatic carboxylic acids. Oxidation with ceric ammonium nitrate gave unacceptably low yields. Sodium dichromate and alkaline potassium permanganate, both of which are known to oxidize pendant alkyl groups preferentially, also provided benzenecarboxylic acids as the predominant products. Variations in the conditions did not alter the product distribution adequately, and we explored the oxidation of coal with oxygen in alkaline solution to achieve more satisfactory results. This reaction has been used extensively to study the structure and chemistry of coal. We adopted the reaction conditions that were used by Montgomery, Holly, and Gohlke7 in the preliminary phases of our work. The results for Pocahontas No. 3 coal are summarized in Table 2. The yields that were obtained with dioxygen and lithium hydroxide were low. Potassium hydroxide provided quite satisfactory yields, but the products were much too rich in benzenecarboxylic acids to be useful for this investigation. The concept that the heavy metal salts of the first-formed carboxylic acids might be less soluble, and therefore less readily converted into benzenecarboxylic acids, prompted work on barium and lead hydroxides. The results with lead hydroxide were not satisfactory, but the yields with barium hydroxide were very encouraging and the reaction conditions were explored more thoroughly. The reaction regularly provided the carboxylic acids in 45-55% yield with many polycyclic aromatic carboxylic acids. The low product yields are primarily the consequence of the severity of the reaction conditions that lead to the conversion of the coal into carbon dioxide and other small molecule fragments. Decarboxylation. Much of the research on the decarboxylation of the aromatic carboxylic acids has been conducted with their sodium salts at high temperatures under conditions that often lead to secondary reactions that yield complex product distributions. We investigated several different reagents for the decarboxylation of naphthalene-2-carboxylic acid (Table 3). Copper(I) oxide was explored more thoroughly, since it was the only reagent that consistently converted naphthalene-1-carboxylic acid into naphthalene in high yields. The previous work of Cohen, Berninger, and Wood was especially useful in the selection of the (7) Montgomery, R. S.; Holly, E. D.; Gohlke, R. S. Fuel 1956, 35, 60.

Table 3. Yields of Naphthalene during the Decarboxylation of 2-Naphthoic Acid in N-Methylpyrrolidinone at 200 °C for 12 h oxidant

yield

manganese(IV) oxide manganese(III) oxide silver(I) oxide silver(II oxide copper(II) chromite copper(I) oxide

0 3 71 0 54 83

Table 4. Yields of Biphenyl from Copper(I) Oxide Decarboxylation of Diphenic Acid in N-Methylpyrrolidinone (NMP) and Quinoline (Q) in 12 h solvent (vol %) NMP

Q

yield (wt %)

100 83 50 17

0 17 50 83

50 50 79 70

Table 5. Yields of Hydrocarbons from the Copper(I) Oxide Decarboxylation Reaction in 1:1 N-Methylpyrrolidinone (NMP) and Quinoline (Q) at 260 °C carboxylic acid

yield (wt %)

diphenyl-2,2′-dicarboxylic naphthalene-2,3-dicarboxylic anthracene-1,8-dicarboxylic 9,10-dihydroanthracene-1-carboxylic anthraquinone-1-carboxylic

79 73 48 57 55

Figure 1. Capillary gas chromatogram of the hydrocarbons obtained in the oxidation-decarboxylation reaction of Pocahontas No. 3 coal.

reaction conditions for the aromatic acids of interest in this investigation.8 The results, which are shown in Tables 4 and 5, led to the use of copper(I) oxide in a 50:50 solution of N-methylpyrrolidinone and quinoline for the decarboxylation of the acids obtained by the oxidation of coal. The decarboxylation of the carboxylic acids obtained from Pocahontas No. 3 coal proceeded very smoothly. The reactions yielded 15-20% carbon dioxide, indicating that the mixture contained many polycarboxylic acids. Overall, the hydrocarbons and heterocycles that were liberated contained between 22 and 25% of the aromatic carbon atoms in the original coal. Reaction Products. The chromatogram of the reaction products is shown in Figure 1 and the analytical results, including the peak number, retention time, (8) Cohen, T.; Berninger, T. W.; Wood, J. T. J. Org. Chem. 1978, 43, 837.


Stock and Obeng

Table 6. Aromatic Hydrocarbons from the Decarboxylation of the Organic Acids Obtained by the Oxidation of Pocahontas No. 3 Coal peak no.

retention time (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75

21.44 25.65 26.23 28.67 29.17 32.22 32.56 33.44 36.68 36.84 40.30 40.82 41.67 41.95 43.50 44.10 44.75 44.90 45.38 45.56 45.70 46.05 46.18 46.26 46.76 48.28 48.79 49.16 49.42 49.67 49.81 50.05 50.15 50.55 50.81 51.05 51.40 51.58 51.98 52.11 52.43 52.56 52.57 52.80 52.82 52.85 53.06 53.21 53.39 53.51 53.85 54.36 54.47 54.76 55.01 55.32 55.78 55.98 56.15 56.34 56.49 56.63 56.84 57.15 57.35 57.74 57.85 58.19 58.36 58.45 58.65 58.85 59.19 60.01 60.35

mass

structural assignment

relative abundance

128 142 142 154 168 168 168 168 182 182 180 184 178 178 204 230 192 192 192 192 218 218 196 218 204 206 206 202 218 218 218 208 218 202 242

naphthalene 2-methylnaphthalene 1-methylnaphthalene biphenyl 2-methylbiphenyl 4-methylbiphenyl 3-methylbiphenyl dibenzofuran a methylbenzofuran benzophenone fluorenone dibenzothiophene phenanthrene anthracene 1-phenylnaphthalene 1,1′:2′,1′′-terphenyl 2-methylanthracene 2-methylphenanthrene 1-methylanthracene 1-methylphenanthrene a methylphenylnaphthalene a methylphenylnaphthalene a methylbenzophenone a methyphenylnaphthalene 2-phenylnaphthalene 3,6-dimethylphenanthrene a dimethylphenanthrene fluoranthene a methylphenylnaphthalene a methylphenylnaphthalene a methylphenylnaphthalene unidentified a methylphenylnaphthalene pyrene phenylfluorene unidentified benzo[b]naphthofuran unidentified 1,1′:4,1′′′-terphenyl benzo[a]fluorene unidentified benzo[b]fluorene unidentified unidentified a naphthyl phenyl ketone unidentified unidentified unidentified a methylterphenyl unidentified a methylterphenyl unidentified a methylterphenyl a methylterphenyl unidentified a methylterphenyl a methylterphenyl unidentified 1,1′-binaphthyl unidentified a phenylbenzophenone benzonaphthothiophene benzo[c]phenanthrene benzonaphthothiophene 7-H-benz[de]anthracene-7-one benzonaphthothiophene a phenylbenzophenone benzo[a]anthracene chrysene triphenylene 1,2′-binaphthyl 2-phenylphenanthrene a phenylphenanthrene a methylnaphthylnaphthalene a methylnaphthylnaphthalene

very large large small very large small small small small very small very small very small very small very large very small large small small small very small very small very small very small very small very small very large very small very small large small small small very small very small intermediate very small large very small intermediate intermediate very small very small very small very small small very small small small small small small small small very small small small small large small very small very small very small small very small very small small very small very small large very large small small large large very small very small

218 230 216 216 232

244 244 244 244 244 244 254 258 234 228 234 230 234 258 228 228 228 254 254 254 268 268



Table 6. (Continued) peak no.

retention time (min)

76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147

60.69 60.78 60.92 61.14 61.20 61.43 61.57 61.80 62.14 62.34 62.46 63.00 63.11 63.45 63.79 63.94 64.26 64.68 64.78 65.16 65.50 65.79 65.87 65.98 66.12 66.34 66.54 66.82 67.23 67.32 67.59 67.99 68.31 68.54 68.69 68.81 69.02 69.46 70.00 70.35 70.52 70.80 71.07 71.33 71.68 71.85 72.22 72.34 72.69 73.16 73.35 74.10 74.29 74.86 74.94 75.17 75.55 76.31 76.38 76.26 77.31 78.21 80.89 81.23 81.60 82.80 83.49 84.00 87.12 88.63 90.54 91.50

mass 242 242 280 268 242 254 242 254 254 306 306 280 280 268 268 268 252 252 280 268 280 284 252 278 278 266 306 278 306 278 266 266 278 306 320 304 306 304 278 278 280 278 304 276 276 278 304 278 278 276 276 330 304 330 304 304 304 330 318 330 302 302

molecular formula, structural assignment, and an indication of the quantity of the material are summarized in Table 6.

structural assignment 2-methylbenzo[a]anthracene a methylbenzo[a]anthracene a benzannulated terphenyl methylnaphthylnaphthalene 4-methylchrysene phenylanthracene a methylbenzo[a]anthracene 1-phenylphenanthrene 2,2′-binaphthyl 1,1′:2′,1′′:2′′,1′′′-quaterphenyl a quaterphenyl a benzannulated terphenyl a benzannulated terphenyl a dinaphthofuran unidentified a dinaphthofuran a dinaphthofuran benzo[j]fluoranthene benzo[b]fluoranthene a dinaphthofuran unidentified a benzannulated terphenyl unidentified dinaphthothiophene perylene dibenzo[c,g]phenanthrene unidentified a dibenzophenanthrene dibenzofluorene a quaterphenyl a dibenzophenanthrene unidentified a quaterphenyl a dibenzophenanthrene a dibenzofluorene 3-methyl benzo[j]aceanthrylene unidentified benzo[g]chrysene a quaterphenyl unidentified unidentified a benzannulated terphenyl a diphenylphenanthrene a quaterphenyl unidentified unidentified a diphenylphenanthrene dibenzo[b,g]phenanthrene benzo[c]chrysene a benzannulated terphenyl dibenzo[a,j]anthracene a diphenylphenanthrene indeno[1,2,3-cd]chrysene indeno[1,2,3-cd]pyrene pentacene a naphthylanthracene dibenzo[a,c]anthracene picene benzo[ghi]perylene dibenzo[cd,jk]pyrene a diphenylphenanthrene or anthracene a naphthylanthracene a diphenylphenanthrene or anthracene a naphthylanthracene a naphthylanthracene a naphthylanthracene a diphenylphenanthrene or anthracene a diphenylfluorene a diphenylphenanthrene or anthracene a dibenzopyrene a dibenzopyrene

relative abundance very large very small large very small very small very large very small intermediate very large very small very small small small small intermediate small intermediate very large very small large small intermediate intermediate intermediate small very large intermediate intermediate intermediate large small large intermediate large very small large intermediate large large large intermediate intermediate intermediate small intermediate intermediate intermediate small small small intermediate intermediate small very small intermediate intermediate intermediate very small intermediate intermediate intermediate small small very small intermediate small very small very small very small very small very small very small

Aromatic hydrocarbons were obtained in abundance. Naphthalene (1) and its monomethyl derivatives (2 and 3) were obtained in high yield. The formation of the


methyl derivatives in a reaction that was selected to remove the peripheral alkyl groups was perplexing. But a study of the oxidation of 4-methylbenzene-1,2-dicarboxylic acid under the conditions used for the oxidation of coal revealed that two carboxylic groups slow the oxidation of the methyl group with the result that only 33% of the molecules were oxidized to 1,2,4-benzenetricarboxylic acid and 67% of the starting material was recovered.

Under the same conditions, 4-methylbenzenecarboxylic acid was oxidized to benzene-1,4-dicarboxylic acid in 100% yield. Thus, methyl groups are readily oxidized when the aryl ring contains one carboxyl group but are largely preserved when the aromatic ring contains two or more carboxyl groups. Phenanthrene (13) was obtained in much greater yield than anthracene (14). Several monomethyl (18 and 20) and dimethylphenanthrenes (26 and 27) were also formed. Compound 26 is clearly 3,6-dimethylphenanthrene. However, only two monomethylanthracenes (17 and 19) were obtained. The structures of these compounds were established by comparison with authentic materials. Tetracene was absent from the products, but pentacene was formed in a surprising amount. Many benzo and dibenzo derivatives of anthracene and phenanthrene were obtained. All the benzo and three of the dibenzo compounds were identified with confidence by comparison with authentic materials, their mass spectral fragmentation pattern,9 and their behavior during chromatography.10 The annulated anthracenes include benzo[a]anthracene (68) and three monomethyl derivatives (76, 77, and 81), and dibenzo[a,c]anthracene (133). The benzo- and

dibenzophenanthrenes include benzo[c]phenanthrene and chrysene (63 and 69) and picene (134). Pentacene,

(9) Heller, S. R.; Milne, G. W. EPA/NIH Mass Spectral Search System; U.S. Government Printing Office: Washington. Eight Peak Index of Mass Spectra; Chemical Society: London, 1983. (10) Lee, M. L.; Vassilaros, D. L.; White, C. M.; Novotny, M. Anal. Chem. 1979, 51, 768.

Stock and Obeng

dibenzo[a,c]anthracene and picene were identified by comparison with authentic materials. The dibenzophenanthrene derivatives (102, 104, 107, 110, 114, 124, 125, 127) were identified by their retention indices and fragmentation patterns.

Perylene (101) was assigned by comparison with an authentic sample. Two related isomeric components with a molecular ion of 302 were assigned to dibenzo[a,d]pyrene (146) and dibenzo[a,e]pyrene (147).

Pyrene (34) and a dibenzo derivative (136) were also detected.

The structures of (34) and (136) were defined by authentic samples. Biaryls, terphenyls, quaterphenyls, and related compounds are predominant products of the oxidationdecarboxylation sequence. Biphenyl (4) and its three methyl derivatives (5, 6, and 7) were identified by comparison with authentic samples. Two terphenyls, the ortho (16) and para (39) isomers, were also identified by comparison with authentic samples. The meta isomer was shown to be absent. A broad series of six isomeric methylterphenyls (49, 51, 53, 54, 56, 57) and six different quaterphenyls (85, 86, 106, 109, 115, and 120) were detected. 1,1′:2′,1′′:2′′,1′′′-Quaterphenyl (85)



was identified by comparison with several authentic samples of the related isomeric compounds. The structures of the other isomeric quarterphenyls could not be assigned with the same assurance, but their assignments are compatible with their fragmentation patternsand correlate with the elemental compositions from high-resolution mass spectra of the reaction products. Many analogous biaryls were observed, including 1- and 2-phenylnaphthalene (15 and 25) and 1,1′-, 1,2′-, and 2,2′-binaphthyl (59, 71, and 84). These substances were

Figure 2. HRMS spectrum of the hydrocarbons obtained in the oxidation decarboxylation reaction of Pocahontas No. 3 coal. Approximate ring sizes are shown above the spectroscopic results.

established by comparison with authentic substances. 2-Phenylphenanthrene (72) and an isomer (73) were also detected. Other molecules in this series include a monophenyl- (35) and a diphenylfluorene (144). A related set of compounds in this series (78, 87, 98, and possibly 88, 95, and 126) have 280 mass units and are apparently annulated terphenyls such as diphenylnaphthalene. Another family of biaryls with mass 304 were also observed. Some of these eight substances (119, 123, 128, 132, 138, 140, 141, and 142) were produced in relatively large amounts. Inspection of the mass spectra suggests that these compounds include naphthylanthracenes and naphthylphenanthrenes as well as phenylated polycyclic aromatic compounds such as phenylchrysene. The fragmentation patterns of the molecules at mass 330 indicate very clearly that these substances (137, 139, 143, and 145) are diphenylanthracenes and diphenylphenanthrenes. Molecules with five-membered rings were also numerous in the oxidation. Fluorenone (11) was obtained in relatively low yield. Two of the benzofluorenes appear to be benzo[a]fluorene (40) and benzo[b]fluorene (42) on the basis of their mass spectra and retention indices. Benzo[j]fluoranthene (93) and an isomer, benzo[b]fluoranthene (94), were assigned by comparison with authentic compounds.

Two dibenzofluorenes (105 and 111) were also found. The structures were assigned by examination of their mass spectra. Several simple ketones were obtained in the oxidation. Benzophenone (10) and a monomethyl derivatives (23), two phenylbenzophenones (61 and 67), and a phenylnaphthyl ketone (45) were obtained in low yields. The benzophenones were identified by comparison with authentic samples, whereas the structures of the other compounds were assigned on the basis of their fragmentation patterns, which were typical of diaryl ketones with signals at m/z ) 105 corresponding to the benzoyl fragment as well as M - 77 and M - 77 - 29 corresponding to the sequential loss of the phenyl and

formyl fragments, and the appearance of substances with the formula C19H14O in the high-resolution spectra. Relatively few sulfur compounds were observed in the oxidation-decarboxylation products of Pocahontas No. 3 coal in accord with its low abundance of sulfur. The sulfur heterocycles include dibenzothiophene (12), three benzonaphthothiophenes (62, 64, 66), and one dinaphthothiophene (100). These substances were identified on the basis of their fragmentation patterns and the measurements of their masses by high-resolution mass spectrometry. All these compounds showed the M + 2 peak that is typical of thiophene compounds. Many oxygen heterocycles were detected including dibenzofuran (8), a monomethyl derivative (9), benzonaphthofurans (37), and four compounds (89, 91, 92, and 96) with one additional aromatic ring. Unfortunately, many of the reaction products could not be identified. Among this group are the compounds with odd molecular ions of 265, 279, 315, and 341. The substances apparently contain neutral nitrogen atoms, since our attempts to extract them from the reaction products with aqueous acids or to retain them on an acidic adsorbent failed. Additional work will be required before secure structural assignments can be made. The high-resolution mass spectrum (HRMS) of the reaction products is displayed in Figure 2. The spectral information provided supporting evidence for the structural assignments in the GCMS work. Further, the spectrum indicates that highly condensed molecules with molecular weight greater than 300 Da are present and extends the mass range to over 500 Da. The laser desorption mass spectrum (LDMS) of the mixture is shown in Figure 3. This information is in good agreement with the HRMS and GCMS data in the range 200-450 Da. The laser desorption spectrum extends the mass range of well-resolved ions to about 600 Da with spectral intensity clearly continuing to ions with mass-to-charge ratios greater than 1200. Discussion Pocahontas No. 3 coal was converted to carboxylic acids in 69% yield in some reactions with dioxygen in alkaline solution, but benzenecarboxylic acids were the predominant products. More satisfactory yields of the polycyclic aromatic carboxylic acids were obtained when


Stock and Obeng

Figure 3. LDMS spectrum of the hydrocarbons obtained in the oxidation-decarboxylation reaction of Pocahontas No. 3 coal. The inset, which was recorded at different sensitivity, illustrates the extension of the mass range to beyond 1200 mass/charge units.

the reaction was carried out with barium hydroxide even though the overall yield was reduced to 50%. These results, which compare favorably with the observations of other investigators,6 are a consequence of the low selectivity of the reagents, which advantageously convert virtually all the coal molecules into products but disadvantageously convert coal molecules into carbon dioxide and other small fragments of little structural interest. The results from this work and earlier work4-7 indicate that this oxidation reaction is an electrophilic process that converts the peripheral structural elements to deactivating carboxylate groups that preserve the interior aromatic molecular fragments from undesirable oxidation reactions. The use of copper(I) oxide in a mixture of N-methylpyrrolidinone and quinoline enabled the decarboxylation reaction to be carried out at higher temperature than could be realized in quinoline. There were no significant residues, suggesting that the aromatic carboxylic acids from the oxidation of coal were converted completely into hydrocarbons under these conditions. Indeed, the conversions of the oxidized coal molecules appeared to be more quantitative than the conversions of the pure compounds. This observation may be related to the fact that the oxidation products of coal are polycarboxylic acids, whereas monocarboxylic acids were employed in most trial experiments. The yields of the acids, the hydrocarbons, and carbon dioxide suggest that 22-25% of the aromatic carbon content of the original coal is preserved in the final products that were analyzed by gas chromatography and mass spectroscopy. Improvements in the procedure can certainly be made, and the work that is described here constitutes a step toward the elaboration of the aromatic constituents of this coal. The gas chromatograms of the

oxidation decarboxylation products and the high-resolution and laser desorption mass spectra in Figures 1-3 are much less complex than the chromatograms and spectra of other samples of this coal that have been produced by other degradation reactions. This feature is well illustrated by the product distributions for the chemical degradation3 and pyrolysis,11 both of which exhibit an intense continuum of signals throughout a broad mass range. Thus, the desired simplification in the product distribution has been realized even though some methyl groups escape oxidation during the reactions because the newly introduced carboxyl groups reduce their rate of oxidation. The simplification has also been realized by the undesirable loss of aromatic carbon atoms during the oxidation by their conversion into carbon dioxide and other small fragments. In addition, the nature of the reaction requires that many of the hydrocarbons eventually obtained in the reaction were protected from additional oxidation by the introduction of carboxyl groups. Virtually all of the substances observed in the course of this study were produced from larger molecular structures. Studies of the distribution of the aliphatic carbon atoms in Pocahontas coal implied that there were very few bridging methylene groups in the structure.2 The results of the oxidation-decarboxylation sequence are in accord with this suggestion. Only five diaryl ketones including two benzophenones (10 and 23), two phenylbenzophenones (61 and 67), and phenyl naphthyl ketone (45) were identified among the reaction products, and these compounds were obtained in relatively low yield. When present, the etheno structural fragments of many aromatic molecules are selectively oxidized. The selec(11) Malhotra, R. Private communication. Muntean, J. V. Ph.D. Thesis, The University of Chicago, Chicago, IL, 1990.



tivity for the etheno positions originates in the electrophilic character of the oxidants and the inherently high reactivity indices for electrophilic or radical substitution at the etheno positions.12,13 The chemistry is well illustrated by the behavior of phenanthrene and chrysene, which are selectively oxidized at the 9, 10 and 5, 6 positions, respectively.14 We have adopted the postulate that two carbon etheno fragments are selectively oxidized under the conditions of the reaction:

According to this view, compounds 61 and 67 are derived from molecules with at least four aromatic rings. Several lines of evidence suggest that the oxygen and sulfur atoms in this highly aromatic coal are principally contained in furans and thiophenes.2 The new results provide additional information about their constitution. The coal has a very low sulfur content with only about 2 sulfur atoms per 1000 carbon atoms. Consequently, only a few thiophenes were detected and neither the sulfoxides nor the sulfones that might have been produced from a high sulfur coal were formed in detectable amount from this coal. The requirement that the initial oxidation products were polycarboxylic acids suggests that the benzonaphthothiophenes (62, 64, and 66) and the dinaphthothiophene (100) must have resulted from structural elements that had a minimum of five and six aromatic rings, respectively.

This feature is even more apparent in the furans. The more abundant oxygen provided many more molecules (8, 9, 37, 89, 91, 92, and 96) with polycyclic structural elements. The presence of four of the six dinaphthofurans among these products implies that the oxygen atoms are surrounded by diverse structural environments in this coal. Significant signal intensity at mass 318 and 368 in the LDMS spectrum suggests that additional annulated furans may be present in the larger oxidation products. Biaryls, teraryls, and quateraryls were formed in high number and in relatively high abundance. The molecules include biphenyl (4) and its three methyl derivatives, the ortho and para isomers of terphenyl (16 and 39) and six methyl derivatives, and five quaterphenyls (85, 86, 106, 115, and 120). Both phenylnaphthalenes (15 and 25) and several methyl derivatives were detected. In addition, the three binaphthyls (59, 71, and 84) were obtained together with three methyl derivatives. Other binaphthyls appear in the group of teraryl (12) Dewar, M. J. S. The Molecular Orbital Theory of Organic Chemistry; McGraw-Hill: New York, 1969. (13) Issacs, N. S. Physical Organic Chemistry; Longman Science and Technical: London, 1990. (14) Hudlicky, M. Oxidations in Organic Chemistry; American Chemical Society Monograph 196; American Chemical Society: Washington, DC, 1990.

compounds with mass 280 (87, 88, 98, and 126). There were also numerous phenyl, diphenyl, and naphthyl derivatives of anthracene and phenanthrene including 1-phenylanthracene (81) and 1- and 2-phenylphenanthrene (72 and 83). Unfortunately, the structures of other arylanthracenes and arylphenanthrenes could not be determined. However, the products that could be definitely identified provide a basis for the discussion of the chemistry. First, neither 9-phenylanthracene nor 9-phenylphenanthrene was detected among the reaction products. This observation is in accord with the view that the etheno linkages are selectively oxidized under the reaction conditions. The observation is also in accord with the concept that the presence of an aromatic group in the 9 position of these molecules would introduce serious torsional strains in the coal structures. Second, the binaphthyls are produced in the order 2,2′ . 1,2′ > 1,1′. This observation is readily understood on the basis of the suggestion that the oxidation products are formed from molecules with etheno linkages and the fact that the strain energies are highest for the molecules that could produce the 1,1′ isomer and

least for the molecules that produce the 2,2′ isomer.

The extension of these ideas to the phenyl and naphthyl derivatives of anthracene and phenanthrenes rationalizes the formation of the 1- and 2-phenyl isomers rather than the 9-phenyl compound and the preference for 2-phenylphenanthrene relative to the 1-isomer as illustrated in the following structures: The bi-, ter-, and quarteraryls arise in two ways. First, previous analyses of the structural information2 suggested that Pocahontas No. 3 coal contains two to three biaryl bridging groups per 100 carbon atoms, and


it seems certain that some of the biaryls arise from these structure elements inasmuch as the biaryl linkage is particularly stable to oxidation. But we infer that most of the bi-, tri-, and quateraryls are formed in an alternative way through the oxidation of relatively fragile unsaturated two carbon atom etheno connecting groups. Thus, the array of compounds reflects the complexity of this coal structure and strongly suggests that it is assembled from many different polycyclic molecules. Fluorenone (11) and a phenyl derivative (35) are accompanied by six other fluorenes including benzo[a]and benzo[b]fluorene (40 and 42) and a dibenzo compound of unknown structure (111). Fluoranthene (28) and four other related highly condensed aromatic molecules (93, 94 , 129, and 130) with five-membered rings were also obtained in the oxidation reaction in significant abundance.

The Pocahontas coal is known to undergo C-alkylation readily, and it has been postulated that fluorenes, which of course would be oxidized to the corresponding ketones under the conditions of the oxidation experiments, were responsible for the success of these reactions.2 Fluorene derivatives may play a subtle role in the determination of the tertiary structure of this coal because the carbon atoms in the 2, 4a, 5a, and 7 positions in fluorene are not collinear. Consequently, the substitution or annulation at these carbon atoms will disrupt an otherwise linear array. The study was undertaken principally to examine the distribution of the polycyclic aromatic structural elements in Pocahontas coal. The results indicate that naphthalene (1) and the 2-methyl derivative (2) were formed in large amounts. Similarly, phenanthrene and three methyl derivatives (13, 18, 20, and 27) were far more prevalent than the corresponding anthracenes (14, 17, and 19) in accord with many prior results.15 However, this concordance may be misleading because the end products of the oxidation-decarboxylation reaction sequence are necessarily derived from different larger molecular structures. It is pertinent that anthraquinones were absence from the products of these reactions. Many tetracyclic aromatic hydrocarbons were produced in significant amounts including pyrene (34), benzo[a]phenanthrene (63), benzo[a]anthracene (68) and the 2-methyl derivative (76) and an unidentified isomer (15) White, C. M. In Handbook of Polycyclic Aromatic Hydrocarbons; Bjorseth, A., Ed.; Marcel Decker: New York, 1983.

Stock and Obeng

(82), chrysene (69) and the 4-methyl derivative (80), triphenylene (70), and perylene (101). The pentacyclic aromatic molecules were more abundant than the tricyclic compounds. This observation provides additional evidence for the presence of large aromatic structures in this low-volatile, high-ranking bituminous coal. The pentacyclic aromatic compounds include dibenzo[c,g]phenanthrene (102) and dibenzo[b,g]phenanthrene (124), two other dibenzophenanthrenes (107 and 110), benzo[g]chrysene (114) and benzo[c]chrysene (127), pentacene (131), and picene (134). Some molecules with six aromatic rings were also formed in intermediate amounts. This group included benzo[ghi]perylene (135), dibenzo[cd,jk]pyrene (136), and two other dibenzopyrenes (146 and 147). These large aromatic compounds, like the lower molecular weight products, were obtained as their carboxylic acid derivatives and are therefore indicative of the presence of even larger aromatic structures in the coal. The relatively high abundance of the compounds with six to eight rings is confirmed by the results of the HRMS investigation. As shown in Figure 2, the ion currents for these larger molecules exceed the currents observed for the lower mass compounds. The information obtained in the GCMS work provides a basis for a more complete examination of the results obtained from the HRMS and LDMS investigations. The chromatographic information suggests that the oxidation-decarboxylation reaction of the Pocahontas coal provides several relatively prominent homologous series of benzannulated molecules from 128 to 318 Da. These series are identified in Table 7. The HRMS data were examined to provide additional evidence for the presence of these molecules and to provide evidence that the series may extend into a higher mass range. The HRMS method is somewhat less sensitive than the GCMS analysis, and the molecules that were present in small or very small abundance in the chromatographic analysis were only rarely detected in the HRMS spectrum. However, the HRMS spectral data provided excellant confirmatory evidence for many molecules in the series by the identification of M + 1 ions generally associated with the related molecules with a carbon-13 atom or by the detection of M - 2 and M - 4 ions associated with the loss of dihydrogen from the terphenyls and quaterphenyls. Moreover, the high-resolution data extend to higher mass, suggesting that the homologous series continues. The HRMS data are compatible with the presence of about 20 compounds from 128 to about 400 mass units. The knowledge gained about the reaction products from these two methods was used to elaborate the LDMS spectra and to evaluate whether these homologous series extend to much higher mass values. We adopted a conservative approach to avoid an overinterpretation of the data. The procedure used for naphthalene and the benzannulated naphthalenes illustrates the approach. Molecules in the series at 128, 178, 228, and 278 Da were identified in the chromatographic work. The LDMS data suggest that other molecules in the series at 278, 328, 378, and 428 Da are also present in reasonable abundance. The series was truncated when the relative intensity decreased below 25% in the mass range below 350 or below 15% in the mass range above 350 Da. The naphthalene



Table 7. Prominent Molecules in the GCMS, HRMS, and LDMS Spectraa,b GCMS and HRMS

HRMS and LDMS

termination

naphthalene methyl

series

128-178-228-278 142-192-242

278-328-378-428 294-344-394-444

478 (7) 494 (10)

pyrene, fluoranthene methyl

202-252 216

252-302-352-402

452 (11)

biphenyl methyl

154-204-254 168-218-268

254-304-354-404 268-318-368-418

454 (14) 468 (10)

terphenyl methyl

230-280-330 244-294

280-330-380-430 294-344-394-444

480 (7) 496 (7)

quaterphenyl methyl

306 318

306-356-406-456 368-418

a

468 (10) b

The ions observed in the HRMS spectrum and in the GCMS or LDMS spectrum are presented in bold face. The ions that terminate the LDMS series are shown on the right with the relative intensity in parentheses.

series terminates with molecules having nine aromatic rings at 478 Da. As already mentioned, the HRMS results bridge the other two techniques by also establishing that the molecules with the exact mass at 128, 178, 228, 278, and 328 are present. The results for the 10 series of compounds in Table 7 suggest that benzannulated naphthalenes, pyrenes, fluoranthenes, biphenyls, terphenyls, quaterphenyls, and certain methyl derivatives all extend to about 500 Da. The LDMS spectrum extends beyond our artificially imposed limit with well-resolved signal intensity to over 600 Da and signal intensity extending to approximately 1200 Da. Broadly speaking, the results of this study are in accord with the earlier analyses of the magnetic resonance data2,16,17 and the more recent X-ray studies,18 all of which strongly infer that Pocahontas No. 3 coal contains large aromatic structural elements. Although no census of the aromatic components can yet be made (16) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. (17) Muntean, J. V.; Stock, L. M. Energy Fuels 1991, 5, 765. (18) Wertz, D. L. In Proceedings of the 8th International Conference on Coal Science; Pajares, J. A., Tasco´n, J. M. D., Eds.; Elsevier: Amsterdam, 1995; p 51.

and although judgment must be reserved about the interpretation of the structures of the molecules with high mass, the results of the GCMS work as amplified by the information assembled in Table 7 strongly infer that the molecular ions in the LDMS arise from the same types of molecules that have been identified in the GCMS analyses with the series of biaryls, teraryls, and quateraryls accompanied by the extended series of annulated polycyclic aromatic molecules. Inasmuch as these compounds are products derived through an oxidative degradation reaction, the structural elements from which they were obtained are necessarily larger polycyclic molecules indicative of the very high aromaticity of this coal. Acknowledgment. It is a pleasure to acknowledge the contributions of R. E. Winans and J. E. Hunt without whose generous assistance this project could not have been carried out. The work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under Contract No. W-31-109-ENG-38. EF960229T

Oxidation and Decarboxylation. A Reaction Sequence for the Study of

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