936
Energy & Fuels 2000, 14, 936-942
Selective Oxidation of PAH with RuO4 as a Preliminary Step in the Characterization of Polymerized Pitches A. Me´ndez, J. Bermejo, R. Santamarı´a, C. G. Blanco, and R. Mene´ndez* Instituto Nacional del Carbo´ n, CSIC, Apartado 73, 33080 Oviedo, Spain Received March 15, 2000. Revised Manuscript Received April 20, 2000
Components of polymerized pitches are of large molecular size and consequently difficult to characterize. Moreover, pitches with similar degrees of polymerization obtained by different treatments have similar properties but different molecular structures. Selective oxidation with RuO4 has proven to be a suitable method for the elucidation of structural details in other complex carbonaceous substances such as coals, asphaltenes, or kerogens. In this paper the behavior of some model compounds was studied in order to achieve a better understanding of how this specific oxidation affects several possible pitch structures. To improve the efficiency of the reaction, an ultrasonic bath was used. Highly condensed compounds such as coronene and triphenylene were the most reactive, as they were the most oxidized of all the compounds tested. On the other hand, it was found that aryl-aryl bonds are very stable. The ability of RuO4 to distinguish between different molecular structures could be used as a new tool in the chemical characterization of polymerized pitches.
Introduction Ruthenium tetroxide is an electrophilic oxidant extensively used in organic chemistry to obtain a great variety of oxygenated functional groups.1-9 An interesting feature of RuO4 is its ability to oxidize aromatic structures to CO2 and benzene carboxylic acids, selectively, whereas aliphatic and alicyclic structures give rise to aliphatic carboxylic acids. In this way, compounds with condensed aromatic rings are oxidized to benzene polycarboxylic acids, the number of carboxylic groups depending on the condensation degree of the molecule.10 This peculiarity makes RuO4 especially interesting for the study of structural details of the complex molecules present in coals, asphalthenes, or kerogens.11-23 RuO4 * Corresponding author. Fax: 34-98 529 76 62. E-mail: rosmenen@ incar.csic.es. (1) Ilsley, W. H.; Zingaro, R. A.; Zoeller, H. J. Fuel 1986, 65, 12161220. (2) Giddings, S.; Mills, A. J. Org. Chem. 1988, 53, 1103-1106. (3) Carlsen, H. J.; Tsutamu, K.; Martı´n, V. S.; Sharpless, K. B. J. Org. Chem. 1981, 46, 3936-3938. (4) Masaji, K.; Ziffer, H. J. Org. Chem. 1983, 48, 2346-2349. (5) Shuda, P. F.; Cichowicz, M. B.; Heimann, M. R. Tetrahedron Lett. 1983, 24, 3829-3830. (6) Coudret, J.-L.; Waegell, B. Inorga. Chim. Acta 1994, 222, 115122. (7) Hansen, K. C.; Lin, Q.; Aminabhavi, T. M. J. Chem. Soc., Faraday Trans. 1996, 98, 3643-3646. (8) Gao, Y.; Sharpless, K. B. J. Am. Chem. Soc. 1988, 110, 75387539. (9) Nu´n˜ez, M. T.; San Martı´n, V. J. Org. Chem. 1990, 55, 19281929. (10) Lee, D. G.; Van den Engh, M. In Oxidation in Organic Chemistry; Trahanovsky, W. S., Ed.; Academic Press: New York, 1973; Part B, Chapter 4. (11) Stock, L. M.; Tse, K.-H. Fuel 1983, 62, 974-976. (12) Stock, L. M.; Wang, S.-H. Fuel 1985, 64, 1713-1717. (13) Stock, L. M.; Wang, S.-H. Fuel 1986, 65, 1552-1562. (14) Stock, L. M.; Wang, S.-H. Fuel 1987, 66, 921-924. (15) Olson, E. S.; Dienl, J. W.; Froehlich, M. L.; Miller, D. J. Fuel 1987, 66, 968-972. (16) Stock, L. M.; Wang, S.-H. Fuel 1985, 64, 1713-1717.
has an important advantage over other oxidants such H2O2/CF3CO2H.24,25 It is possible to obtain additional information from the amount of CO2 evolved during oxidation.26,27 High carbon yield pitches able to give graphitizable carbons are required for new applications of growing interest, such as carbon fibers,28-29 special graphites,30 and carbon/carbon composites,31,32 etc. Commercial pitches can meet this requirement after thermal polymerization in an inert or oxidative atmosphere.33,34 How(17) Blanc, P.; Valisolalao, J.; Albrecht, P.; Kohut, J. P. Energy Fuels 1991, 5, 875-884. (18) Mallya, N.; Zingaro, R. A. Fuel 1984, 63, 423-425. (19) Mojelsky, T. W.; Ignasiak, T. M.; Frackman, Z.; McIntyre, D. D.; Lown, E. M.; Montgomery, D. S.; Strausz, O. P. Energy Fuels 1992, 6, 83-96. (20) Standen, G.; Boucher, R. J.; Eglinton, G.; Hansen, G.; Eglinton, T. I.; Larten, S. R. Fuel 1992, 71, 31-36. (21) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M. Fuel 1992, 71, 1355-1363. (22) Boucher, R. J.; Standen, G.; Eglinton, G. Fuel 1991, 70, 695702. (23) Boucher, R. J.; Standen, G.; Patience, R. L.; Eglinton, G. Org. Geochem. 1990, 16, 951-958. (24) Hessley, R. J.; Benjamin, B. M.; Larsen, J. W. Fuel 1982, 61, 1085-1087. (25) Liotta, R.; Hoff, W. S. J. Org. Chem. 1980, 45, 2887-2890. (26) Nomura, N.; Artock, L.; Murata, S.; Yamamoto, A.; Hama, H.; Gao, H.; Kidena, K. Energy Fuels 1998, 12, 512-523. (27) Artok, L.; Muraka, S.; Nomura, M.; Satoh, T. Energy Fuels 1998, 12, 391-398. (28) Zeng, S. M.; Maeda, T.; Mondori, J.; Tokumitsu, K.; Mochida, I. Carbon 1993, 31, 407-412. (29) Fritz, J. D.; Pennock, G. H.; Taylor, G. H. Carbon 1991, 29, 139-164. (30) Schmidt, J.; Moergenthaler, K. D.; Brehler, K.-P.; Arndt, J. Carbon 1998, 36, 1079-1084. (31) Savage, G. Carbon-carbon composites; Chapman and Hall: London, U.K., 1992; pp 156-191. (32) Mene´ndez, R.; Granda, M.; Ferna´ndez, J. J.; Figueiras, A.; Bermejo, J.; Bonhomme, J.; Belzunce, J. J. Microsc. 1997, 185, 146156. (33) Blanco, C.; Santamarı´a, R.; Bermejo, J.; Mene´ndez, R. Proceedings of the 24th Conference on Carbon; American Carbon Society: Charleston, NC, 1999; pp 218-219.
10.1021/ef000050p CCC: $19.00 © 2000 American Chemical Society Published on Web 06/17/2000
Characterization of Polymerized Pitches
Energy & Fuels, Vol. 14, No. 4, 2000 937
Figure 1. PAHs oxidized with RuO4.
ever, both kinds of polymerized pitches show different properties, and it is thought that these differences are due to the structure of the constituent macromolecules. Thermal treatment leads to aromatic polymerization with the formation of condensed macromolecules, whereas in air-blowing polymerization, a variable amount of nonplanar oligomers are formed which obstruct or prevent the formation of mesophase. To our knowledge, there is no evidence to clarify the structure of the macromolecules formed by air-blowing of aromatic hydrocarbons. Only the possible presence of large molecules with no structural order and cross-links has been suggested from the results of transit shear rheology, TXRD, and iodine adsorption.35 RuO4 oxidation seems to be a suitable method for acquiring information about the structural differences of both thermally treated and air-blown pitches. Anyway, the main difficulty would probably be in the interpretation of the results since these are dependent on the experimental conditions of oxidation or the effect of ring substituents,1,11,21,27 not to mention the intrinsic complexity of the treated pitches themselves. For these reasons, RuO4 oxidation of model aromatic hydrocarbons was undertaken as a first step in the characterization of the treated pitches. The aim of this work is to study the suitability of RuO4 oxidation in the selective characterization of complex aromatic molecules representative of the structures presumably present in treated pitches. This paper reports on the results of the RuO4 oxidation of aromatic hydrocarbons with different condensation degrees and ring substituents. Comparative oxidation rates and oxidation mechanisms were inferred from the results of gas chromatography (GC) and gas chromatographymass spectrometry (GC-MS) analyses of the organic phase at different reaction times. At the end of the reaction, the organic phase was mixed with the CH2Cl2/diethyl ether extract of the aqueous phase, and the mixture was then methylated with (TMS)Cl (TMS ) tetramethylsilane) in methanol and analyzed by GCMS. Experimental Section Catalytic Oxidation of PAH. All the solvents and products used were of high purity reagent grade. The polycyclic aromatic hydrocarbons (PAHs) are shown in Figure 1. Reactions were carried out in a 100 mL flask covered with a rubber septum. (34) Lewis, I. C. Carbon 1982, 20, 519-529. (35) Mene´ndez, R.; Fleurot, O.; Blanco, C.; Santamaria, R.; Bermejo, J.; Edie, D. Carbon 1998, 36, 973-979.
A 1 mmol amount of each compound was added to a suspension of KIO4 (2 g) and RuCl3‚nH2O (0.025 g) in a biphasic mixture of CCl4 (4 mL), CH3CN (4 mL), and H2O (6 mL), and the mixture was sonicated in an ultrasonic bath at ≈40 °C. Aliquot samples from the organic phase were taken at different times and analyzed by GC in order to control the reaction. Finally the reaction products were filtered, and the solid residue was washed with CCl4 and water. The aqueous and organic phases were separated, and the aqueous phase was extracted two times with 10 mL of CH2Cl2 and another two times with 10 mL of diethyl ether. The organic extracts were joined with the organic phase, dried with Na2SO4, and filtered. The solvents were removed with a rotary evaporator at 40 °C, and 2 mL of MeOH and 1.2 mL of (TMS)Cl were added to the precipitate for esterification. After 48 h, at room temperature, the excess of MeOH/(TMS)Cl was carefully removed using the rotary evaporator. The esterification products were dissolved in CH2Cl2 for analysis by GC-MS. Gas Chromatography. Gas chromatography was carried out in a Hewlett-Packard HP 6890 chromatograph, equipped with a 30m HP-5 capillary column, using hydrogen as carrier gas and a temperature program from 50 to 300 °C at 4 °C min-1. The flame ionization detector (FID) signals were acquired and quantified by peak area measurements. Gas Chromatography-Mass Spectrometry. Analyses were performed in a Finnigan Mat GCQ instrument with a fused-silica capillary column coated with SE-54 stationary phase. The length and internal diameter of the column were 25 m and 0.22 mm, respectively. The temperature program chosen was from 50 to 280 °C at 4 °C min-1, and the carrier gas used was helium (30 cm min-1). The injector temperature was 250 °C, and the detector was of the electron impact type (EI). The temperatures of the transfer time and ion source were 250 and 200 °C, respectively. The instrument was calibrated with perfluorotributhylamine. The components of the esterified samples were identified by comparing the mass spectra with the library data. For undocumented compounds, examination of mass spectral fragmentation and the rule of peak appearance were the strategies employed. High-Resolution Liquid Chromatography. High-resolution liquid chromatography (HPLC) was carried out in a 590 Waters Associates chromatograph with a C18 Kromasil 100 capillary column and a 481 Lambda-Max UV detector (λ ) 210 nm). The mobile phase was a pH ) 3 solution of KH2PO4 in acetonitrile (3:1). CO2 Analysis. During the reaction N2 gas was flowed every 3 h, and the resulting CO2 was purged through Na2SO4/CaCl2 and ascarite containing tubes. Carbon dioxide formation was determined from the increase in weight of ascarite.
Results A series of aromatic hydrocarbons with structures similar to those presumably present in the thermally treated and air-blown pitches were selected for this study (Figure 1). In addition to phenanthrene and
938
Energy & Fuels, Vol. 14, No. 4, 2000
Me´ ndez et al.
Table 1. Compounds Detected by GC (Area %), in the Organic Phase of the Samples at 6, 12, and 18 h of Reaction and in the Esterified Sample, in the Oxidation of Phenanthrene reacn time (h) compd A B C D E F G H a
tra (min) 24.40 34.52 36.70 38.11 39.40 41.76 43.39 44.90
0
6
12
100
90.69 3.59
79.57 12.24
2.31
2.97 0.79 3.80
18
ETb 2.32
2.76
1.66 10.66 44.82 12.60 16.16
81.23 4.01 0.70 5.54 2.10
Retention time. b Esterified sample.
anthracene, the compounds most commonly used as models in studies of pitches and other coal liquids, 9,10-diphenylanthracene was selected as being representative of aryl-aryl bonds which may be present in the olygomeric structures of air-blown pitches. The use of this compound also makes it possible to test the effect of blockage of the reactive 9,10 positions of anthracene on the selective oxidation of RuO4. Triphenylene and coronene were also used as they are representative of condensed pitch aromatic structures which are able to form benzenehexacarboxylic acids by RuO4 oxidation. Oxidation of Phenanthrene. To gain an insight into the reaction mechanism of phenanthrene with RuO4, samples of the organic phase were taken at different times and were analyzed by GC, as mentioned above. At the end of the reaction (18 h) the aqueous phase was extracted with CH2Cl2/diethyl ether, mixed with the organic phase, esterified with (TMS)Cl in methanol (sample ET), and analyzed by GC-MS. Table 1 shows that compounds C, E, and G were formed first and their concentration in the organic phase (estimated from the peak area as area percent of the chromatographed compounds) increased without interruption except for compound C, which shows a maximum at 12 h typical of the intermediate compounds. Compounds F and H were formed after 12 h of reaction, F being the main component of the chromatographable compounds of the 18 h sample. Compounds A and D were only found in the esterified sample (ET), which indicates that they are formed from the carboxylic acids or another kind of compound insoluble in the organic phase which can be transformed into ester. This hypothesis is supported by the marked decrease in the concentration of compound F, and to a lesser extent of compound H in the esterified sample. This suggests that compounds F and H are transformed into compound D during esterification. GC-MS of ET allows several compounds to be identified. The main m/z of the compounds and their structural assignment are shown in Table 2. Compound E was identified by the instrument library data as 9,10phenanthrenequinone with a degree of probability higher than 98%. In the same way, A and D were identified as dimethyl esters of 1,2-benzenedicarboxylic acid (phthalic acid) and 2,2′-diphenyldicarboxylic acid (diphenic acid), respectively. These results are similar to those reported by Stock and Tse,11 but different from those obtained by Masaji and Ziffer4 who reported that the main product from RuO4 oxidation of phenanthrene was 9,10-quinone. Compound C, which was not present
Table 2. Main m/z and Assignment of Compounds compd A B C D E F G H
main m/z
assignt
194, 163, 135
dimethyl phthalate phenanthrene a 270, 239, 211, 196, 180 dimethyl diphenoate 208, 180, 152 9,10-phenanthrenequinone 240, 224, 196, 180, 152 peroxide anhydride of diphenic acid? 216, 188, 160, 144 a 224, 208, 180, 152 diphenic anhydride a
Unidentified compound.
in ET, could be an intermediate compound between B and E, probably a phenanthrenone. A comparison of the spectra of compound E and compound H (Figure 2) strongly suggests that compound H should be assigned to diphenic anhydride. The spectrum of F with the molecular ion 240 and the main m/z of 196 suggests the following fragmentation path:
Compound F, therefore, could be assigned to the peroxy anhydride of the diphenic acid, which, as in the case of compound H, reacts with (TMS)Cl in methanol to form the corresponding dimethyl ester. The main oxidation products were F and H, which are able to form the dimethyl ester D, the latter accounting for 81 area % of ET. The small amount of phthalic acid could have resulted from the slow decomposition of the diphenic acid.27 Furthermore, the mass spectrum of the unidentified compound G (Table 2) suggests the existence of another minor route for the oxidation of phenanthrene. In addition to the GC-MS analysis, the aqueous phase of the 18 h sample was analyzed by HPLC, in an attempt to find benzene polycarboxylic acids insoluble in CH2Cl2/diethyl ether. First, a standard mixture of polycarboxylic acids was chromatographed using the same system. The retention times of the standard mixture are shown in Table 3. The HPLC of the aqueous phase of 18 h showed only three peaks with retention times (tr) of 2.06, 3.17, and 3.53 min, in a proportion of 90, 7, and 3 area %, respectively. The peak of tr ) 3.53 min belongs to the phthalic acid according to Table 3, and that of tr ) 2.06 min must belong to the 1,2,3,4benzenetetracarboxylic acid, the only tetra-acid possible from the oxidation of phenathrene. The other acid is obviously diphenic acid. The small amount of diphenic acid in the aqueous phase adds further proof of the above-mentioned esterification of the anhydride H and the probable presence of peroxy anhydride F present in the organic phase. Oxidation of 2-Ethylanthracene. It is known that in PAHs the carbon atoms adjacent to two quaternary carbons are the most reactive for the reactions of electrophilic substitution36 and the reactions of radical polymerization.37 This type of carbon atom therefore (36) Zander, M. Topics in Current Chemistry; Springer-Verlag: Berlin, Heidelberg, 1990; pp 102-120. (37) Yokono, T.; Miyazawa, K.; Sanada, Y.; Marsh, H. Fuel 1979, 58, 692-694.
Characterization of Polymerized Pitches
Energy & Fuels, Vol. 14, No. 4, 2000 939
Figure 2. Spectra of compounds E and H in the oxidation of phenanthrene. Table 3. Retention Time of Standard Benzenepolycarboxylic Acids carboxylic acid
retention time (min)
1,2,3,4-tetra1,2,4,5-tetra1,2,4-tri1,2,3-tri-
2.14 2.23 2.70 2.86
carboxylic acid 1,3,5-tri1,2-di1,4-di1,3-di-
retention time (min) 2.95 3.53 4.04 4.84
should be more reactive against the highly electrophilic RuO4 than the others. As an example of PAHs with these kinds of carbons, 2-ethylanthracene was used with the aim of finding out also the effect of the presence of an ethyl group. As expected the reaction was very rapid, with most of the substrate transformed after the first 6 h. Compounds identified by GC-MS in the organic phase of the 6 h sample include the isomers 2-ethyl, 9-anthrone and
2-ethyl, 10-anthrone. The predominant component, 80 area % at 6 h and 95 area % at 12 h, was identified by the library data as 2-ethylanthraquinone with a probability higher than 98%. At 18 h only this compound was found by GC of the organic phase. Nevertheless, the reaction was continued up to 39 h in the search for other more oxidized compounds. The organic phase of the 39 h sample after esterification was composed of a mixture of 95% of 2-ethylanthraquinone and 2% of the dimethyl esters of the phthalic acid and the 3-ethylphthalic acid. Ilsley et al.1 and Nomura et al.26 reported the formation of 9,10-anthraquinone as the main product of anthracene oxidation, with only 1 wt % of 1,2,3,4-tetracarboxylic acid. The rapid oxidation of the carbon atoms at the 9,10 positions impedes the formation of tetra-acid. Propionic acid was not found among the reaction products.
940
Energy & Fuels, Vol. 14, No. 4, 2000
Me´ ndez et al.
Figure 3. Spectra of compounds D and F in 9,10-diphenylanthracene oxidation.
Oxidation of 9,10-Diphenylanthracene. The low oxidation rate of 9,10-diphenylanthracene corroborates the high reactivity of the 9,10-carbon atoms of anthracene. Results in Table 4 show a dramatic decrease in reactivity brought about by the introduction of the phenyl groups into these positions. At the same time, the results illustrate the behavior of the aryl-aryl bonds, characteristic of olygomeric structures supposedly present in air-blown pitches. After 60 h of reaction, the concentration of the substrate (K) in the organic phase was 60 area %. GCMS analyses of samples of the organic phase at different reaction times and of ET enable B and D to be identified by means of instrument library data, with a probability higher than 97% as anthraquinone and 1,2-dibenzoylbenzene, respectively. Both compounds, like M, are soluble in the two phases according to the different
concentrations in the sample of the 60 h organic phase and in ET. Compounds A, C, and E are insoluble in the organic phase. The identification of compounds B and D helps in assigning the mass spectra of the other more abundant components of the 60 h sample. The spectrum of A (m/z 240, 225, 209, 181, 163, 152, 105, and 77) with a molecular ion (M) of 240 and a predominant m/z of 163 was assigned to the methyl ester of the benzoylbenzoic acid, on the assumption that the main fragmentation occurs with the loss of the phenyl group. More difficult to assign is the spectrum of compound F (m/z 286, 268, 209, 180, 152, 105, and 78) even though it differs from that of compound D in the m/z 268, which is indicative of a loss of H2O (Figure 3). This spectrum could be assigned to the hydrogenated compound D, benzoyl-2(hydroxybenzyl)benzene. The spectrum of compound C
Characterization of Polymerized Pitches
Energy & Fuels, Vol. 14, No. 4, 2000 941
Table 4. Compound Detected by GC (Area %) in the Organic Phase of the Samples, at 24, 38, 50, and 60 h of Reaction, and in the Esterified Sample, in the Oxidation of 9,10-Diphenhylanthracene reacn time (h) compd A B C D E F G H I J K L M
tra (min) 38.36 39.37 49.94 50.92 51.08 52.98 57.58 59.86 60.69 61.35 61.61 63.89 64.77
a
0
24
38
50
60
ETb
7.41
4.89
13.13
6.20
2.94
8.81
6.33 1.83 30.37 33.32 2.82 0.70
1.58 6.25
1.38
10.20
1.31
72.90
60.32 2.86 1.87
8.81 1.23 5.44
1.16
0.38
100
4.24 2.52 82.96 8.40
73.47 2.20 1.97
1.03
Retention time. b Esterified sample.
(m/z 300, 269, 252, and 223) with m/z 269 as the main peak (consequently with a loss of OCH3) could be the methyl esther of F. The spectra of the intermediate compounds I, J, and H (Table 4) can be assigned to 1-hydroxy-, 2-hydroxy-, and 1,2-dihydroxy-9,10-diphenylanthracene. Oxidation of Coronene and 9,10-Benzophenanthrene (Triphenylene). The aim of the oxidation of these compounds was to test for the formation of benzenehexacarboxylic acid. In the case of triphenylene it was also of interest to investigate the effect of the elimination of the reactive 9,10 positions of phenanthrene. The oxidation of coronene was complete after 18 h. After this time no compounds were found either in the organic or the extract of the aqueous phase after methylation. The HPLC of the aqueous phase gave only one component with tr ) 2.06 min, which confirms the view that this retention time belongs to 1,2,3,4-benzenetetracarboxylic acid, the only tetra-acid possible for coronene. The formation of 1,2,3,4-benzenetetracarboxylic acid as the only product of the oxidation of coronene was corroborated by the amount of CO2 recovered in ascarite, as will be discussed later. For triphenylene, the only compound found in the organic phase between 6 and 30 h of reaction was the substrate. After methylation, the GC-MS of the ET sample showed the presence of dimethylphthalate (4 area %), dimethyldiphenoate (22 area %), and the substrate (73 area %) together with insignificant amounts of two other compounds. As for 9,10-diphenylanthracene, the main effect of the removal of the most reactive 9,10 positions of phenanthrene was the sharp decrease in the oxidation rate. Mojelski et al.19 reported that RuO4 oxidation of triphenylene gave the hexacarboxylic and phthalic acids in a ratio of 1:3, with a conversion of only 28% of the substrate. These results are in agreement with the structure of triphenylene which has one central ring and three peripheral and symmetrical rings. However, the phthalic acid could have originated from the oxidation of the diphenic acid under severe oxidation conditions. CO2 Evolution. Measurement of CO2 evolution might give some indication of the oxidation rate and the extent of the reaction. Figure 4 shows the amount of carbon
Figure 4. Oxidized carbon evolution for coronene and 9,10diphenylanthracene.
that is oxidized to CO2 in relation with the total carbon of the parent compound in two model compounds, the coronene representing a highly condensed aromatic molecule (possibly present in thermally treated pitches) and 9,10-diphenylanthracene representing a molecule that might be present in air-blown pitches, with arylaryl bonds. The first difference that can be observed is the rate of CO2 evolution, which is much higher in the coronene than in the 9,10-diphenylanthracene. Coronene oxidation is completed after 12 h, while the oxidation of 9,10-diphenylanthracene was still not completed after 45 h. Furthermore, 58.2% of the carbon atoms of coronene were converted to CO2 in agreement with the theoretical 58.9% which might be converted if tetra-acid were the only reaction product, thus adding further proof that the tetra-acid is the only reaction product. Discussion The oxidation of phenanthrene and 2-ethylanthracene takes place selectively at the highly reactive 9,10 positions. For 2-ethylanthracene the oxidation occurs quickly and with a high selectivity, giving 9,10-anthraquinone as the main product (85 wt %). As expected, oxidation does not continue because of the high stability of this compound against oxidation due to the deactivating effect of the carbonyl groups on the terminal rings. On the other hand, the 9,10-phenanthrenequinone with two carbonyl groups at adjacent positions is easily oxidized by the insertion of oxygen atoms at the carbonyl carbon atoms, giving the carboxyl anhydride and probably the peroxy anhydride. What is probably the main oxidation mechanism of phenanthrene is shown in Figure 5, with a result similar to that obtained with other common oxidants. The formation of 1,2,3,4-benzenetetracarboxylic acid found in the aqueous phase suggests the possibility of another oxidation route which starts with the oxidation of the terminal rings. The oxidation of 9,10-diphenylanthracene and 9,10benzophenanthrene (triphenylene) is much slower, with most of the substrates still unchanged after long reaction times. This suggests that the effect of the blockage of the reactive positions on the oxidation rate is not balanced by other possible effects of the substituent groups. The blockage of the 9,10 positions of anthracene and phenanthrene gives rise to the appearance of other slower oxidation reactions. Figure 6 displays the probable route of the RuO4 oxidation of 9,10-diphenylan-
942
Energy & Fuels, Vol. 14, No. 4, 2000
Figure 5. Main oxidation route for phenanthrene.
Figure 6. Possible route of 9,10-diphenylanthracene.
thracene. This starts with the oxidation of the carbon atoms at the 1 and 2 positions and ends with the
Me´ ndez et al.
complete oxidation of the ring giving rise to 1,2dibenzoylbenzene. The phenyl-aryl bonds remain unchanged. The resistance to RuO4 oxidation and the presence of the nonreacted phenyl-aryl bonds is of great interest as this reaction could be applied to the structural studies of pitches obtained from different chemical treatments. This is because aryl-aryl bonds are supposedly more abundant in compounds of air-blown pitches than in compounds present in other pitches. On the other hand, the low reactivity of the aryl-aryl bonds contrasts with the high reactivity of condensed aromatic structures. For this reason, the oxidation rate of pitches is probably closely related to the predominant structures of their compounds. These results suggest that RuO4 oxidation can be used to discriminate between the planar structures of the macromolecules formed by the thermal polymerization of the pitch components and those formed by air-blowing polymerization, which can give rise to compounds with aryl-aryl bonds. The oxidation rate can also be measured by the CO2 evolution. Furthermore the total amount of CO2 could be related to pitch structure, which determines the extent of oxidation and the nature of the final compounds. Clear differences are observed between coronene, a highly condensed aromatic compound, and 9,10diphenylanthracene, a compound which contains arylaryl bonds. EF000050P
