Energy & Fuels 1988,2, 37-48
37
Ruthenium Tetraoxide Catalyzed Oxidation of Maceral Groups Chol-yoo Choi, Shih-Hsien Wang, and Leon M. Stock* Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, and Department of Chemistry, The University of Chicago, Chicago, Illinois 60637 Received August 17, 1987. Revised Manuscript Received September 29, 1987
Pure liptinite, vitrinite, and inertinite group maceral fractions from a representative high-volatile A bituminous coal have been oxidized with ruthenium(VII1) to produce a broad series of aliphatic carboxylic and benzenecarboxylic acids. Chromatographic and mass spectroscopic procedures were used to identify these compounds, and isotope dilution techniques were used to establish the yields of certain key products. The product distributions from the individual macerals differ considerably. Generally speaking, the observations for the benzenecarboxylic acids indicate the dominance of 1,2-fused benzene rings and suggest that the aromatic structures in the three different maceral groups are similar. The broad array of aliphatic acids obtained from the three macerals imply that the aliphatic structural components of the vitrinite and inertinite maceral groups are similar with hydroaromatic compounds predominating whereas the liptinite group contains polymethylene chains that bridge aromatic structures. Thus, the differences between the inertinite and vitrinite arise from variations in the concentrations of similar types of aromatic and hydroaromatic structures whereas the liptinites contain other quite different aliphatic substances.
Introduction Oxidative procedures have been widely used to degrade complex coal macromolecules to simpler molecules that are more suitable for chromatographic and spectrometric studies. Knowledge of these products and their relative abundances often provide information concerning the original aromatic and aliphatic structures in the coals. A broad array of contributions in this area have been discussed in a recent review.l Generally, the peripheral aliphatic structures of aromatic fragments are oxidized to yield aromatic carboxylic acids. Far fewer methods selectively yield aliphatic carboxylic acids. Den0 and his associates developed one of these methods?-' They report that the sulfuric acid catalyzed reaction of peroxytrifluoroacetic acid with coals in trifluoroacetic acid at 70 "C selectively oxidizes the aromatic ring, leaving the aliphatic structures However, this procedure has limited use as a method for the elucidation of structures because the harsh reaction conditions cause other undesirable side
reaction^.^*^^^ Ruthenium tetraoxide offers an alternative choice for the selective oxidation of the aromatic structural elements. This reagent oxidizes alkylaromatic compounds to the corresponding alkanecarboxylic acids.1° For example, phenylcyclohexane is converted to cyclohexanecarboxylic acid.ll Until rather recently, low conversions limited the usefulness of ruthenium(VII1) ion as a catalyst for the oxidation of aromatic compounds. However, Sharpless and his co-workers found that the addition of acetonitrile to the reaction medium enables the achievement of high yields of carboxylic acids in catalytic reactions.12 His discovery led to the wider use of ruthenium tetraoxide for coal oxidation.'"ls The products include a wide array of aliphatic and benzene carboxylic acids and carbon dioxide. More than 100 substances are obtained from the oxidations of coals of various rank.lsJe The major products are
* To whom correspondence should be addressed at Argonne National Laboratory or The University of Chicago. 0887-0624/88/2502-0037$01.50/0
short-chain aliphatic monocarboxylic acids, straight-chain aliphatic dicarboxylic acids and benzenecarboxylic acids with 1,2-, 1,2,3-, 1,2,4-, 1,2,3,4-, and 1,2,4,5-substitution patterns. The nature of the alkyl fragments in the products mirror the alkyl groups in the coals. These include arylmethanes, hydroaromatic compounds, and substances with short and long alkyl chains.lg The degree of coalification is very important. The product distribution clearly depends on rank; lower rank coals produce larger quantities of aliphatic acids than higher rank coals. The situation for the aromatic acids (1)Hayatsu, R.; Scott, R. G.; Winans, R. E. In Oxidation in Organic Chemistry; Trahanovsky, W. S., Ed.; Academic: New York, 1982;Part
D. (2)Deno, N.C.; Greigger, B. A.; Messer, L. A.; Meyer, M. D.; Stroud, S. G. Tetrahedron Lett. 1977,1703-1704. (3)Deno, N.C.; Greigger,B. A.; Stroud, S. G. Fuel 1978,57,455-459. (4)Deno, N.C.; Curry, K. W.; Greigger,B. A,; Jones, A. D.; Rakitsky,
W. G.; Smith, K. A.; Wagner, K.; Minard, R. D. Fuel 1980,59,695-698. (5)Deno, N. C.; Greigger,B. A.; Jones, A. D.; Rakitsky, W. G.; Smith, K. A.; Minard, R. D. Fuel 1980,59,699-700. (6)Deno, N. C.; Jones, A. D.; Koch, C. C.; Minard, R. D.; Potter, T.; Sherrard, R. S.;Stroh, J. G.; Yevak, R. J. Fuel 1982,61,490-492. (7)Deno, N. C.; Jones, A. D.; Owen, D. 0.; Weinschenk, J. I., 111. Fuel 1985,64,1286-1290. (8) Liotta, R.; Hoff, W. S.J. Org. Chem. 1980,45,2887-2890. (9) Hessley, R. K.; Benjamin, B. M.; Larsen, J. W. Fuel 1982, 61, 1085-1087. (10)Lee, D. G.; van den Engh, M. In Oxidation in Organic Chemistry; Trahanovsky, W. S.,Ed.; Academic: New York, 1973;Part B. (11)Caputo, J. A.; Fuchs, R. Tetrahedron Lett. 1967,4729-4731. (12)Carlsen, P. H.J.; Katauki, T.; Martin, V. S.;Sharpless, K. B. J. J. Org. Chem. 1981,46,3936-3938. (13)Stock, L. M.; Tse, K. T. Fuel 1983,62,974-976. (14)Tse, K. T. Ph.D. Dissertation, The University of Chicago, Chicago, IL, 1985. (15)Stock, L. M.; Wang, S. H. Fuel 1985,64,1713-1717. (16)Stock, L.M.: Wan& S. H. Fuel 1986,65,1552-1562. (17)Mallya, N.; Zingaro, R. A. Fuel 1984,63,423-425. (18)Olson, E. S.;Diehl, J. W. P r e p . Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1984,29(6), 217-220. (19)Tschamler, H.; de Ruiter, E. In Coal Science; Given, P. H., Ed.; Advances in Chemistry 55; American Chemical Society: Washington, DC, 1966.
0 1988 American Chemical Society
Choi et al.
38 Energy & Fuels, Vol. 2, No. 1, 1988
differs; lignite and subbituminous coal provide less of these Table I. P u r i t y and Density of the Maceral Groups acids than higher rank bituminous coals. It may be anmaceral erouD Duritv. % densitv ranee. e cm-3 ticipated that the product distribution also depends on the liptinite 96 1.178-1.207 maceral composition of the coal. The liptinites, which vitrinite 99 1.282-1.310 contain larger quantities of aliphatic material than viinertinite 99 1.359-1.390 trinites or inertinites,20are especially interesting substances for selective oxidation. In brief, the idea that the macerals obtain additional qualitative and quantitative information of the liptinite group contain aliphatic chain structures was about the aliphatic and aromatic structures in the maceral suggested by Tschamler and de Ruiter on the basis of groups. elemental analyses in 1966.19 More recent work using solid-state 13C nuclear magnetic resonance spectroscopy Experimental Section confirmed the idea and led to the assignment of the aliEquipment. The 13Ccross-polarization-magic angle spinning phatic resonance near 30 ppm to methylene groups in long spectra were recorded on a Bruker CXP-100 instrument (2.3T, ~hains.~l-~' 13C frequency 25.18 MHz). The infrared spectra were recorded Although many investigators have oxidized coal, only on a Nicolet 20 SXB Fourier transform spectrometer using poa few workers have studied macerals. The chromic acid tassium bromide pellets. The gas chromatography-mass spectrometry results were obtained by using a Hewlett-Packard 5790A oxidation of tasmanite kerogen, which is believed to be gas chromatograph equipped with an OV-1701capillary column fossil green algae, yields large quantities of aliphatic mono(0.25pm X 50 m) or a Tenax packed column (0.32 mm X 1.8 m) and dicarboxylic acids and small quantities of aromatic and a VG 70-250 mass spectrometer. acids.28 The authors suggest that the aliphatic acids are Coal and Maceral Samples. The macerals were separated produced by the oxidation of a randomly cross-linked from the West Virginia Upper Kittanning Seam coal, designated saturated hydrocarbon polymer. The alkaline peras PSOC-732, obtained from The Pennsylvania State University manganate oxidation of alginite macerals provides sigCoal Sample Bank. The coal was ground, demineralized, and nificant quantities of aliphatic mono- and dicarboxylic separated into maceral groups by density gradient centrifugation acids as well as low yields of aromatic acids.29 The oxitechniques as described p r e v i ~ u s l y . ~The ~ elemental and pedation of sporinites from British bituminous coals in altrographic analyses of the coal and the representative maceral groups were described in a previous rep0rt.3~ Several adjacent kaline permanganate solution yields long-chain unof the maceral groups separated by density gradient branched aliphatic mono- and dicarboxylic a c i d ~ . ~ O ~ ~ fractions l centrifugation were combined to provide pure maceral group Solvent extraction and saponification of the original sposamples for the oxidation experiments. The density ranges and rinites give only low quantities of aliphatic acids. Thus, purities are shown in Table I. neither free carboxylic acids nor esters, for example, triReagents. T h e solvents and reagents used in the oxidation glycerides, appear to be present in this sporinite. It is experiments were obtained commercially and purified as neceasary pertinent that large quantities of aromatic carboxylic acids as described in previous reports.lbls Isotopically labeled carboxylic are produced in the oxidation of the sporinites. Reports acids were used to identify the key products and to provide concerning the nitric acid oxidations of the alginites, quantitative data. Labeled Carboxylic Acids. Ethanoic-d3 acid-d (Aldrich), sporinites, virtinites, and inertinites from several bitubutanedioic-2,2,3,3-d4 acid (Cambridge Isotope), and 1,2minous coals are also available.32 Sporinite and alginite benzene-3,4,5,6-d4-dicarboxy1ic acid (Cambridge Isotope) were react with nitric acid to yield aliphatic dicarboxylic acids obtained commercially. Propanoic-2,2-d2 acid and butanoic-2,2-d2 and minor quantities of aromatic polycarboxylic acids. T h e acid were prepared from methylpropanedioic acid and ethylvitrinites produce both short-chain aliphatic and aromatic propanedioic acid, respectively, according to the method described carboxylic acids and the inertinite gives mostly aromatic by Murray and Williams.3s Dr. S. Ray Mahasay generously acids. Clearly, the oxidation products of these macerals provided the sample of 1,2,4,5-benzene-3,6-d2-tetracarboxylic acid. differ uniquely. In view of the fact that oxidations with RutheniumTetraoxide Oxidation of Maceral Groups. The chromic acid, alkaline permanganate, and nitric acid are oxidation of macerals was accomplished by using the procedure known to degrade aliphatic chains, we elected to study the described p r e v i o u ~ l y . ' ~ -In ~ ~a typical experiment, a maceral concentrate (200mg), ruthenium(II1) trichloride trihydrate (12 more selective ruthenium(VII1) ion catalyzed oxidation to (20)Van Krevelen, D. W. Coal; Elsevier: New York, 1961. (21)Wilson, M. A.; Pugmire, R. J.; Karas, J.; Alemany, L. B.; Woolfenden, W. R.; Grant, D. M.; Given, P. H. Anal. Chem. 1984,56,933-943. (22)Karas, J.; Pugmire, R. J.; Woolfenden, W. R.; Grant, D. M.; Blair, S.Int. J. Coal Geol. 1986,5,316-338. (23)Pugmire, R. J.; Zilm, K. W.; Woolfenden, W. R.; Grant, D. M.; Dyrkacz, G. R.; Bloomquist, C. A. A.; Horwitz, E. P. Org. Geochem. 1982, 4 , 79-82. (24)Retcofsky, H.;VanderHart, D. L. Fuel 1978,57,421-423. (25)Zilm, K.W.; Pugmire, R. J.; Larter, S.R.; Allan, J.; Grant, D. M. Fuel 1981,60,717-720. (26)Maciel, G. E.;Sullivan, M. J.; Petrakis, L.; Grandy, D. W. Fuel 1982,61,411-414. (27)Axelson, D. E. Solid State Nuclear Magnetic Resonance of Fossil Fuels: An Experimental Approach; Multacience: Montreal, Canada, 1985. (28)Simoneit, B. R.; Burlingame, A. L. Geochim. Cosmochim. Acta 1973,37,595-610. (29)Allan, J.; Bjorey, M.; Douglas, A. G. Phys. Chem. Earth 1980,12, 599-618. (30)Allan, J. Ph.D. Dissertation, The University of Newcastle upon Tyne, Newcastle, Great Britain, 1975. (31)Allan, J.; Larter, S.R. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1981,26(1),26-30. (32)Winans, R. E.; Dyrkacz, G. R.; McBeth, R. L.; Scott, R. G.; Hayatsu, R. Roc.-Int. Kohlenwiss. Tag., 1981, 1981.
mg, 0.05 mmol), and sodium periodate (4g, 19 "01) were added to a mixture of carbon tetrachloride (8 mL), acetonitrile (8 mL), and water (12 mL) and stirred for 16 h at room temperature. Quantitative isotope dilution analyses were performed in the following way. Ethanoic-d3 acid-d (10.3 mg, 163 pmol), propanoic-2,2-d2 acid (3.3 mg, 43 pmol), and but&noic-2,2-d2acid (1.1 mg, 12 pmol) were added to the reaction mixture to determine the yields of the volatile carboxylic acids. Butanedioic-2,2,3,3-d, acid (3.3 mg, 27 pmol), 1,2-benzene-3,4,5,6-d4-dicarboxylic acid (2.1 mg, 12 pmol) and 1,2,4,5-benzene-3,6-d2-tetracarboxylic acid (1.0mg, 3.9 pmol) were added to the reaction mixture to assess the yields of selected aliphatic and benzenecarboxylic acids. Previous experiments established that labeled acids were equilibrated in the multiphase mixture after being stirred for 6 h.16,16 The color of the organic phase a t the end of the reaction was yellow-brown for the liptinites and black for the other macerals. The mixture was filtered, and the acids were extracted exhaustively into diethyl ether and dried with sodium or magnesium ~~
~
~
~~
(33)Dyrkacz, G. R.; Horwitz, E. P. Fuel 1982,61,3-12. (34)Choi, C. Y.; Dyrkacz, G. R.; Stock, L. M. Energy Fuels 1987,1, 280-286. (35)Murray, A., III; Williams, D. L. Organic Synthesis with Isotopes; Interscience: New York, 1958; Part 11, pp 1265-1266.
Energy & Fuels, Vol. 2, No. 1, 1988 39
Ru04-Catalyzed Oxidation
0.9
0.2
1
1
i
1
4000
3600
3200
2800
2400
2000
1600
1200
800
400
WAVENUMBER (CM-l)
Figure 2. FT-IR spectra of PSOC-732 (a) liptinite, (b) vitrinite, (c) inertinite, and (d) whole coal. 1 . . . . I . . . . I . . . . ( 1 . . . I . I I .
200
150
100
50
I . I . . I
0
PPM
Figure 1. lsC CP/MAS spectra of PSOC-732 (a) liptinite, (b) vitrinite, (c) inertinite, and (d) whole coal. sulfate. The volatile organic reagents were carefully removed by a rotary evaporator at 40 OC. The products were identified by gas chromatography-mass spectrometry (GC-MS) techniques. The volatile monocarboxylicacids were separated on a Tenax packed column (0.32 mm X 1.8 m) by using the following temperature program: initial temperature 80 O C , temperature hold for 1min, temperature increase to 270 O C at 6 OC/min. One scan per second was recorded for the low-resolution electron-impact mass spectral analyses. The relative quantities of labeled and unlabeled ethanoic, propanoic, and butanoic acids were determined by measuring the integrated intensities of the peaks at m/e pairs 63 and 60,76 and 74, and 62 and 60,respectively. Approximately25 scans of each GC peak were recorded because the labeled and unlabeled acids exhibited different retention timea. The results were integrated to determine the relative quantities of the labeled and unlabeled acids. For the analyses of the nonvolatile acids, diazomethane (0.35 M in diethyl ether) was added to a concentrated solution of the reaction products. The ether was removed carefully by using a rotary evaporator,and the methylation was repeated. After three cycles, the producta were separated on an OV-1701 capillary column (0.25 pm X 50 m). The temperature program for the separation was as follows: initial temperature 50 OC, temperature hold for 1min, temperature increase to 270 OC at 7 OC/min. One scan per second was recorded for low-resolution electron-impact mass spectral analyses. The quantities of dimethyl butane-l,rl-dioate, dimethyl benzene-1,2-dicarboxylate,and tetramethyl benzene-1,2,4,5tetracarboxylate were determined by measuring the integrated intensities of the m/e peak pairs 115 and 119,163 and 167, and 279 and 281, respectively. A totalof 10-15 scansof each GC peak were recorded. The relative amountsof the labeled and unlabeled acids were defined by the integrated intensities of these data. Known mixtures of pure compounds were studied to establish
the suitability of these procedures. The recorded spectra were compared with the data in the National Bureau of StandardsLibrary and other literature sources. Authentic samples of many carboxylic acids and esters were obtained from Dr. R. Hayatsu and Dr. R. E. Winans. NMFt Analyses. The 13CNMR spectra were recorded at 2.3 T (25.18 MHz for 13C) on a Bruker Instruments spectrometer, Model CXP-100, in the pulse Fourier transform mode with quadrature phase detection. The ceramic sample spinners had an internal volume of 300 p L and were spun at approximately 4 kHz. Operating parameters in cross-polarizationexperiments included a spectral width of 10 kHz, a 90' proton pulse width of 4.2 ps (60-kHz proton-decoupling field), an acquisition time of 20 ms, a pulse repetition time of 1s and a total accumulation of lo00 transients. In a typical experiment,200 words of memory was allocated for data acquisition and was then increased to 4K (2K real data) by zero filling. Before Fourier transformation of the data, the interferogram was multiplied by a decreasing trapezoidal window function after the f i i t 20 data points. Optimum resolution enhancement was achieved by Gaussian, trapezoidal fiiteringof the free induction decay using the trapezoidal window function described above and a Gaussian broadening function of 0.5.
Results and Discussion The maceral samples used in this investigation were separated from a high-volatile A bituminous coal, PSOC732. Adjacent density fractions of very pure maceral groups that had been obtained by density gradient centrifugation were combined t~ provide the starting materials for the oxidation experiments. The density ranges and petrographic purities of these materials are presented in Table I. Although the maceral samples used in this work span a slightly broader density range than the samples used in earlier studies,%there is no evidence to suggest that their chemical or physical properties differ in any significant way from the materials used in the previous work. The nuclear magnetic resonance and infrared spectra of the macerals are shown in Figures 1 and 2.
Choi et al.
40 Energy & Fuels, Vol. 2, No. 1, 1988 Table 11. 'F NMR Aromaticity (fa)Values of the Macerals maceral group whole coal liptinite vitrinite inertinite
Wt%C
fa
82.4 84.2 82.1 85.0
0.78 0.59 0.79 0.87
The solid-state 13C CP/MAS spectra of the maceral groups are scaled to the intensity of the largest signal. While the absolute intensities of the signals cannot be compared, the f a values for the four samples can be measured. The results, which are summarized in Table 11, reveal that there are major differences in the aromatic carbon atom contents of these materials. Recent carboncounting experiments in our laboratories suggest that the fa values obtained by CP/MAS techniques underestimate the aromatic carbon content of coals. However, the fa values determined in CP/MAS experiments for different materials are usually proportional to values determined in more precise experiments. The maximum signal intensity in the aliphatic region for the liptinite group is near 31 ppm with a much less intense but distinct signal at 18 ppm. The aliphatic regions of both vitrinite and inertinite maceral groups show two major signals of similar intensities at 30 and 20 ppm. The observation that there are two well-resolved aliphatic resonances for the vitrinite and inertinite from this coal suggests that there are two distinct aliphatic structures. The resonances centered near 30 ppm and at 20 ppm are compatible with the presence of methylene groups in long aliphatic chains and pendant methyl groups on aromatic structures, respectively. Although the aromaticity of the whole coal is similar to that of vitrinite, the aliphatic region of the whole coal is not resolved into two distinct resonances. The failure to resolve this resonance in the whole coal results from the contribution of the rather intense 30 ppm signal of the liptinite, which broadens the signal. The infrared spectra shown in Figure 2 have been scaled to illustrate the absorbance for 1mg of material/cm2 and have been corrected for background scattering and electronic absorptions by using conventional routines. The spectrum of the whole coal provides averaged information. As a consequence, this spectrum is more similar to the spectrum for the dominant component, vitrinite, than to the spectra of the other macerals. The most prominent differences among the macerals occur in the aliphatic C-H region at 2950-2850 cm-', the aromatic C = C region at 1600 cm-l, the CH, and CH2 regions between 1450 and 1350 cm-', and the aromatic C-H deformation region at 900-700 cm-l. These spectra are compatible with the idea that the functional group distribution is similar among the macerals with a distinctly dissimilar distribution of aliphatic and aromatic carbon atoms. Accordingly, we examined the relative abundance of aromatic and aliphatic hydrogen atoms. Quantitative analyses of the infrared spectra of coal are difficult due to the presence of many different types of structures, each of which has a different absorption coefficient. Nevertheless, many investigators have estimated the relative proportions of aromatic and aliphatic hydrogen atoms in coal by the adoption of average values for the aromatic and aliphatic C-H absorption coeffi~ients.3(+~~ ~~
~~
(36)Solomon, P.R In Coal Structure; Gorbaty, M. L.; Ouchi, K., Eds.; Advances in Chemistry 192;American Chemical Society: Washington, DC, 1981. (37)Brown, J. K. J . Chem. SOC. 1955,744-752. (38)Durie, R.A.; Shewchyk, Y.; Sternhell, S.Fuel 1966,45,99-113.
Table 111. Distribution of Aliphatic and Aromatic Hydrogen and Carbon Atoms in the Macerals maceral group HAI/Htot HAr/Htnt HOH/Htot HAI/CAI HAr/CAr whole coal 0.55 0.39 0.06 1.67 0.33 liptinite 0.76 0.21 0.03 1.71 0.33 vitrinite 0.61 0.33 0.06 1.98 0.28 inertinite 0.38 0.58 0.04 1.61 0.37
The total hydrogen atom concentration , H in a coal can be expressed as Hbt = HA1+ Hh + HOH, where HM, Hh, and HOH are the aliphatic, aromatic, and hydroxyl hydrogen atom concentrations in the sample. The integrated area, A, under the absorption band in the infrared spectrum is related to the corresponding hydrogen concentration by an absorption coefficient, e, such that HA, = tMAM,*Hh = tdh, and HOH = eOHAOH. Quantitative measurements of the hydroxylic content of coals have been reported by the direct measurement of the intensity of the 0-H a b s ~ r p t i o n . ~ However, *~l residual absorption due to the presence of water in potassium bromide pellets even after prolonged heating may lead to To circumvent these large errors in this difficulties, we used alkylation reactions to measure the hydroxylic hydrogen atom content. The results, which are summarized in a subsequent table, were presented in a previous report.34 Quantitative determinations of the aliphatic and aromatic hydrogen atom concentrations require suitable values of and ek In brief, Brown estimated the absorption coefficients of the aromatic and the aliphatic C-H stretching modes at 3030 and 2925 cm-l, respectively, using various model compounds and adopted an average value of 0.5 for t k / e M for the analysis of coal spectra even though the ratios ranged from 0.3 to 1.0 when pure compounds were used.37 Other workers have used proton NMR measurements of coal extracts to establish average infrared absorption coefficient^.^^,^^.^^ Solomon calibrated the absorption coefficients for the aliphatic and aromatic C-H regions using regression analyses based on a series of model compounds, coals, tars, and chars under the assumption that the absorption coefficients were constant for most coal products as well as model compounds.36In a later study, Solomon and his associates examined a wider group of coals and coal products in the same way and derived different absorption coefficients for different types of The observed absorption in a particular region frequently arises from the superposition of many absorptions, each with a distinct absorption coefficient, and the use of a single extinction coefficient to analyze the integrated intensity in this region is que~tionable.,~ Nevertheless, Solomon's approachNgMwas adopted to gain perspective on the aliphatic and aromatic hydrogen atom content of the macerals. The integrated areas of the aliphatic C-H stretching modes between 3000 and 2800 cm-' and the aromatic C-H out-of-plane bending modes between 920 and 680 cm-' in the spectra were determined. (39) Painter, P. C.; Snyder, R. W.; Starsinic, M.; Coleman, M. M.; Kuehn, D. W.; Davis, A. In Coal and Coal Products: Analytical Characterization Techniques; Fuller, E. L., Jr., Ed.; ACS Symposium Series 205;Am5rican Chemical Society: Washington, DC, 1982. (40)Oaawa, Y.;Shih, J. W. Fuel 1971,50,53-57. (41)Solomon, P. R.;Carangelo, R. M. Fuel 1982,61, 663-9. (42)Snyder, R. W.; Painter, P. C.; Havens, J. K.; Koenig, J. L. Appl. Spectrosc. 1983,37,497-502. (43)Brown, J. K.; Ladner, W. R. Fuel 1960,39,87-96. (44)Retcofsky, H.L. Appl. Spectrosc. 1977,31,116-121. (45)Solomon, P. R.;Hamblen, D. G.; Carangelo, R. M. In Coal and Coal Products: Analytical Characterization Techniques; Fuller, E. L., Jr., Ed.; ACS Symposium Series 205; American Chemical Society: Washington, DC, 1982.
Ru04-Catalyzed Oxidation The absorption coefficients of c d = 746 and e h = 686 determined by Solomon for bituminous coals were used for the whole coals, vitrinites, and inertinites. Aliphatic hydrocarbons exhibit higher absorbances and the absorption coefficients of eAl = 710 and eh = 541 recommended by Solomon for highly aliphatic coals including lignites and subbituminous coals were used for the liptinite. The average number of hydrogen atoms on the aliphatic and aromatic carbon~atomsmay be derived from the results of the elemental analyses, the infrared spectra, and the NMR spectra. The results are shown in Table 111. Small, but significant differences are observed in the values for Hm/Cd and HAr/Ck The variations in the HA1/Cdvalues strongly suggest that there are significantly different combinations of methine, methylene, and methyl groups in the three macerals. This feature of the results will be examined in more detail in the subsequent discussion. The Hh/C* values are quite similar for all the macerals groups; approximately one-third of the aromatic carbon atoms in these materials are protonated. However, the observation that the vitrinite has the lowest Hh/Ch value suggests that this material has a somewhat higher degree of substitution than the other macerals. Recent results using 13C CP/MAS NMR dipolar dephasing techniques, which discriminate between protonated and nonprotonated carbon atoms on the basis of their differences in relaxation characteristics, indicate similar quantities of protonated and nonprotonated aromatic carbons for a series of coals and m a c e r a l ~ . ~All ~ ,the ~ ~current ~ ~ ~ results support the broad view that the aromatic structures are condensed and highly substituted. Ruthenium(VII1) Oxidation. The whole coal and the maceral group fractions were oxidized with ruthenium(II1) trichloride as a catalyst in the presence of sodium periodate in a mixture of acetonitrile, water, and carbon tetrachloride at ambient temperature.13 The reactions of the macerals had to be carried out on a small scale. Preliminary work revealed that reproducible results could be achieved with 100-200 mg of PSOC-732 coal, and ae a consequence, all the reactions were performed on this scale. The oxidations of the small ( 5 3 pm) particles were rapid, and no significant differences were observed in the reaction rates of the whole coal or the separated macerals. Even the liptinite group fraction that had a relatively low fa value, 0.59, was readily oxidized. The behavior of this sporinite-rich maceral group contrasts strongly with the very slow oxidation rate of pure resinite. The reaction products were treated in several different ways. In one series of experiments, the yields of the low molecular weight, monocarboxylic acids were measured. In another series of experiments, the initial reaction products were converted into their methyl esters for analyses. Gas chromatographic-mass spectroscopic techniques were very suitable for qualitative work. However, the difficulties encountered in the quantitative determination of the volatile acids and esters led to the adoption of an isotope dilution technique. Known quantities of labeled acids or esters were added to the mixture near the end of the reaction and stirred for 6 h to guarantee that these compounds were distributed in the multiphase mixture in the same way as the oxidation products from the maceral groups. After the reaction mixture was worked (46) Pugmire, R. J.; Woolfenden, W. R.; Mayne, C. L.; Karas,J.; Grant, D. M. In Chemistry and Characterization of Coal Macerals; Winans, R. E., Crelling, J. C., Eds.; ACS Symposium Series 252; American Chemical Society: Washington, DC, 1984. (47) Pugmire, R. J.; Soderquist, A.; Burton, D. J.; Beeler, A. L.; Grant, D. M. €'roc.-Int. Conf. Coal Sci., 1985 1986,114-111.
Energy & Fuels, Vol. 2, No. 1, 1988 41 Table IV. Yields of Ethanoic, Propanoic, and Butanoic Acid from PSOC-732 Coal Maceral Groups Determined by IsotoDe Dilution Analyses yield, pmol of acid/g of maceral acid coal 1iDtinite vitrinite inertinite ethanoic 676 578 1003 627 propanoic 161 173 195 99 butanoic 18 20 27 3
up, the relative amounts of labeled and unlabeled acids were determined by gas chromatography-mass spectroscopy as described in the Experimental Section. Monocarboxylic Acids. In the first series of experiments, we observed that the oxidation produced all the straight-chain acids from ethanoic to dodecanoic acid but that the three lowest molecular weight acids constituted more than 90 mol % of the monocarboxylic acids. Dodecanoic acid appears in unexpected large amounts in the oxidation produds of vitrinite and inertinite. Its presence must be attributed to the oxidation of residual amounts of poly[oxyethylene(23)] lauryl ether, the surface active agent used in the maceral separation p r ~ c e d u r e . Con~~ siderably larger quantities of the straight-chain acids with more than four carbon atoms are produced from the liptinite sample than from the other maceral groups. The low molecular weight, branched monocarboxylic acids, for example 2-methylbutanoic acid, are formed in low yields. These acids are considerably more abundant in the oxidation products of the liptinite and vitrinite macerals than in the inertinite sample. Quantitative data for the three most abundant monocarboxylic acids are presented in Table IV. While we are not yet able to rationalize all these data, there are certain prominent trends. First, the quantities of the low molecular weight acids obtained from whole PSOC-732 coal are comparable with the quantities of these acids obtained from Illinois No. 6 coal.15 Second, distinctive variations are apparent in the yields of the acids from the different maceral groups. Specifically, whereas all three low molecular weight acids were obtained in greatest abundance from the vitrinite group, the quantity of ethanoic acid produced from this maceral was almost 2-fold greater than that produced from the other maceral groups. The yields of propanoic and butanoic acid from the inertinite material are especially small when compared with those from the other maceral groups. The aliphatic monocarboxylic acids may arise from the oxidation of alkylaromatic structures (eq l ) , 1,l-diarylalkanes (eq 2), and ethers (eq 3).49 Work with Illinois No.
-
-
R-Ar R-CO2H Ar-CHR-Ar RCH(CO,H),
(1)
RCOzH
(2)
RCH20Ar RCOzH (3) 6 coal suggests that arylmethanes and arylpropanes are primarily responsible for the formation of ethanoic and butanoic acid, but that diarylpropanes contribute in a very significant way to the formation of propanoic acid.I5 If the same tendencies exist for PSOC-732, the observations presented in Table IV suggest that arylmethanes are considerably more abundant in the vitrinite than in the liptinite or inertinite groups and that the diarylpropanes (48) Paly[oxyethylene(23)]lauryl ether is oxidized to the corresponding ester, which is subsequently converted to dodecanoic acid.
-
CH3(CHz)i1(OCHzCHz)230H CHs(CWloC02H (49) Ilsley, W. H.; Zingaro, R. A.; Zoeller, J. H., Jr. Fuel 1986, 65, 1216-1220.
Choi et al.
42 Energy & Fuels, Vol. 2, No. 1, 1988
0
200
400
I. 600
800
1000
1200
1400
1600
1800
2000
2200
Scan Number in MS Experiment
Figure 3. GC-MS chromatogram of the methyl esters of the oxidation products of PSOC-732 whole coal.
are less abundant in the inertinite group than in the liptinite or vitrinite macerals. The diarylpropanes may be depleted in the inertinite because such structural elements have tertiary benzylic hydrogen atoms that would have been oxidized during the conversion of the plant materials to the substances that were the precursors of the inertinites. The relatively high yield of ethanoic acid observed for the vitrinites is in accord with the results obtained for other vitrinite-rich coal ~amp1es.l~ Thus,our results suggest that the arylmethane concentration in the vitrinite significantly exceeds that in the other principal maceral groups. It is important to note that only about 50% of the methyl groups in Illinois No. 6 coal that can be detected by infrared spectroscopy or nuclear magnetic resonance are converted into ethanoic acid in the ruthenium(VII1) oxidation r e a ~ t i 0 n . lThe ~ methyl groups in hydroaromatic components such as 1-methylindan or 2-methyltetralin are oxidized to other aliphatic carboxylic acids during this reaction. The appearance of methyl substituted dicarboxylic acids such as 2-methylpentane-1,5-dioic acid in the reaction products and the finding that coals that have been dehydrogenated under mild conditions produce substantially larger amounts of ethanoic acid are in accord with this view (eq 4 and 5).15 Thus, a high yield of eth-
CH3
I
CH3
I
anoic acid implies a high concentration of methylaromatic and methylheterocyclic compounds in the maceral. Fur-
ther support for this view is provided by the broad array of methylbenzenecarboxylic acids produced in the oxidation.50 These substances result from the nonselective oxidation of alkylaromatic compounds. For example, 2butylnaphthalene is oxidized to pentanoic acid (46%) benzene-1,2-dicarboxylic acid (58%), and 4-butylbenzene-1,2-dicarboxylic acid (43%).14 Indeed, the findings that the yield of ethanoic acid in the vitrinite exceeds the yield of propanoic acid by a factor of 10 and that there are readily detectable methylbenzene carboxylic acids but few, if any, ethylbenzenecarboxylic acids indicate the dominance of methyl groups in the vitrinite maceral group. Theories of coalification must, therefore, account for the conversion of the lignin polymer into methylaromatic and heterocyclic compounds rather than into ethyl or propyl derivatives. The long-chain aliphatic monocarboxylic acids observed in the liptinite and vitrinite samples probably arise from sources such as those shown in eq 1-3. This feature of the work will be discussed subsequently. Methyl Esters. The complex mixtures of aliphatic and aromatic acids were converted to the corresponding methyl esters for convenient identification and measurement of the product distribution. The gas chromatograms of the products of the whole coal and the maceral groups are shown in Figure 3-6, and the principal products are listed in Table V. Although the separation of the methyl esters achieved by capillary gas chromatography were generally very good, the large array of products occasionally resulted in overlapping peaks. For example, dimethyl benzene-1,3-dicarboxylate and dimethyl nonanedioate have virtually identifical retention times and coemerge near scan 1136. Examination of the mass spectra established that the ox(50)The methylbenzenecarboxylic acids have also been detected by Hayatsu and his associates.’
Energy & Fuels, Vol. 2, No. 1, 1988 43
Ru04-Catalyzed Oxidation 100
EO
60
40
20
0
0
500
1000
2000
1500
2500
Scan Number in MS E x p e r i m e n t
Figure 4. Gas chromatogram of the oxidation products of PSOC-732 liptinites. The numbers correspond to the acids in Table V.
51,52
I
EO U
c
E 3
V
e
2
60
-
40
59
1
20
62
';
0 0
500
1000
1500
2000
2500
Scan Number i n M S E x p e r i m e n t
Figure 5. Gas chromatogrdm of the oxidation products of PSOC-732 vitrinites. The numbers correspond to the acids in Table V.
idation product of the whole coal that emerged in this region W M a mixture of these materials. A second analysis using different chromatographic conditions resolved the mixture into 55% of the aliphatic ester and 45% of the benzenedicarboxylate. Further study of the oxidation
products of the maceral groups revealed that the material that emerged in scan 1136 from the liptinite was virtually pure dimethyl nonanedioate, that from the inertinite was equally pure dimethyl benzene-1,3-dicarboxylate, and that from the vitrinite was a mixture consisting of about 75%
44 Energy &Fuels, Vol. 2, No. 1, 1988 approx compd scan n0.O 1 60 2 135 3 135 4 164 242 5 6 289 7 343 8 355 9 378 10 415 11 423 12 471 13 520 14 551 15 557 16 571 17 580 18 619 19 661 20 687 21 696 22 744 23 764 24 796 25 808 26 825 27 840 28 884 29 916 30 924 31 956 32 979 1027 33 1034 34 35 36 37
1078 1106 1135
Choi et al.
Table V. Methyl Esters Detected by GC-MS Techniques approx identificationb compd scan n0.O identificationb butanoic acid 38 1137 nonane-1,g-dioic acid 2-methylbutanoic acid 39 1161 butanetricarboxylic acid (t) 3-methylbutanoic acid 40 1181 3-methylbenzene-l,2-dicarboxylicacid pentanoic acid 41 1223 4-methylbenzene-1,2-dicarboxylicacid dichloroethanoic acid 1235 decane-1,lO-dioicacid 42 hexanoic acid 43 N-nitroso-p-toluenesulfonamide 1250 44 diiodomethane pentanetricarboxylic acid (t) 1265 45 1323 carboxy-a-oxobenzeneaceticacid (t) 3-iodopropanoic acid 46 5-methylhexanoic acid (t) 1328 undecane-1,ll-dioic acid 2,2-dimethylpropane-1,3-dioic acid 47 hexanetricarboxylic acid (t) 1360 heptanoic acid 1418 dodecane-1,12-dioicacid 48 cyclohexanecarboxylicacid heptanetricarboxylic acid (t) 49 1450 butane-1,4-dioic acid benzenetricarboxylic acid 50 1467 2-methylbutane-1,4-dioicacid benzenetricarboxylic acid 51 1489 octanoic acid benzenetricarboxylic acid 52 1495 2,2-dimethylbutane-l,4-dioic acid (t) 53 1503 tridecane-1,13-dioicacid benzoic acid 54 1542 octanetricarboxylic acid (t) 2,3-dimethylbutane-l,4-dioic acid methylbenzenetricarboxylic acid 55 1547 pentane-l&dioic acid 56 1562 methylbenzenetricarboxylic acid nonanoic acid 57 1574 methylbenzenetricarboxylic acid 2-methylpentane-1,5-dioic acid 58 1587 tetradecane-1,14-dioic acid 2,2-dimethylpentane-1,5-dioic acid (t) 59 1600 methylbenzenetricarboxylicacid (t) 2,3-dimethylpentane-1,5-dioic acid 60 1609 dimethylbenzenetricarboxylic acid (t) hexane-1,6-dioic acid 61 nonanetricarboxylic acid (t) 1626 decanoic acid 62 methyldicarboxy-a-oxobenzeneaceticacid (t) 1656 2-methylhexane-l,6-dioicacid 1677 63 pentadecane-l,15-dioic acid 3-methylhexane-1,6-dioic acid 1724 64 methyldicarboxy-a-oxobenzeneaceticacid (t) 2,2-dimethylhexane-l,6-dioic acid (t) 1777 65 hexadecane-1,16-dioicacid heptane-1,7-dioic acid benzene-1,2,3,4-tetracarboxylic acid (t) 1810 66 undecanoic acid benzene-1,2,4,5-tetracarboxylic acid 1844 67 4-methylheptane-1,7-dioic acid (t) 1882 68 benzene-1,2,3,5-tetracarboxylic acid (t) 3-methylheptane-1,7-dioic acid (t) 1947 69 methylbenzenetetracarboxylic acid (t) octane-l,d-dioic acid 1982 70 methylbenzenetetracarboxylicacid (t) dodecanoic acid and propane-1,2,3-tricarboxylic 71 methyltricarboxy-a-oxobenzeneaceticacid (t) 2099 acid 2123 72 methyltricarboxy-a-oxobenzeneaceticacid (t) benzene-1,4-dicarboxylicacid 2182 73 methyltricarboxy-a-oxobenzeneaceticacid (t) benzene-1,2-dicarboxylicacid 2402 74 benzenepentacarboxylic acid benzene-1,3-dicarboxylicacid
"The scan numbers are shown on the abscissa in Figures 4-6. bTentativeidentifications are designed by (t).
of the benzene derivative and 25% of the aliphatic ester. There is a broad background between scans 1000 and 2000 in three of the four chromatograms of the methyl esters. The origin of this background was investigated. An ether solution of the methyl esters was extracted with dilute aqueous sodium hydroxide to determine whether the background resulted from residual carboxylic acids, for example, a trimethyl ester of a tetracarboxylic acid. Only traces of acidic materials were extracted, and the chromatogram of the extracted material was virtually identical with that of the original sample. Hence, unmethylated materials are not responsible for the broadness of these spectra. Further, alterations of the chromatographic conditions had little influence on the appearance of the chromatograms. Therefore, we attribute the broad background to the superposition of many different, unresolved reaction products. The chromatograms of the liptinite and vitrinite group products exhibit substantial amounts of unresolved materials, while the inertinite does not. This result suggests that the aliphatic structural elements that are present in greater abundance in the liptinite and vitrinite materials are primarily responsible for the complexity of the chromatograms. The results imply that the more aromatic, inertinite maceral group obtained from PSOC-732 has a more uniform and simpler macromolecular structure than the related liptinite and vitrinite maceral groups. Inspection of the chromatograms in Figures 3-6 reveals that the whole coal provides the most diverse array of products, whereas the individual maceral groups provide
considerably less complex mixtures of materials. Qualitatively, the liptinite group provides the greatest yield of aliphatic materials with readily detectabIe amounts of all the linear aliphatic monocarboxylic acids with 1-12 carbon atoms as well as certain branched monocarboxylic acids. The linear dicarboxylic acids with as many as 18 carbon atoms are also found among the reaction products. Lesser amounts of the benzenecarboxylic acids are produced from the liptinite group. In contrast, the inertinite group produces much less aliphatic materials but rather large amounts of benzenedi-, benzenetri-, and benzenetetracarboxylic acids. The differences in the product distributions are most striking and provide strong support for the viewpoint often expressed by Dyrkacz in his presentation& that maceral separation is a necessary first step in structural investigations of petrographically heterogeneous coals. Quantitative studies were performed by using gas chromatographic methods to determine the yields of certain of the more abundant products. Isotope dilution analyses were employed to determine the yields of three major products, butane-1,4-dioic acid, benzene-1,2-diacid. carboxylic acid, and benzene-1,2,4,5-tetracarboxylic The results are summarized in Table VI. The aliphatic acids are obtained in strikingly different amounts from the three maceral groups. The quantitative order liptinite > vitrinite > inertinite is readily under(51) Dyrkacz, G. R.; Choi, C. Y.; Stock,L. M. Prepr. Pup.-Am. Chem.
Soc., Diu. Fuel Chem. 1987, 32(1),448-456.
Energy &Fuels, Vol. 2, No. 1, 1988 45
Ru04-Catalyzed Oxidation 36
100
80 U
C bl
u
a
U
8
60
13
H
67
a 5
5
4
40 62
.2 ?I
U rl
2 20
0 0
500
1000 SCM
1500
2000
2500
N u m b e r i n MS E x p e r i m e n t
Figure 6. Gas chromatogram of the oxidation products of PSOC-732 inertinites. T h e numbers correspond to the acids in Table V.
standable on the basis of the difference in aliphatic content of these materials. The higher yields of the benzene acids from the inertinite group is also expected, but the finding that essentially equivalent amounts of the benzene acids are produced by the liptinite and vitrinite maceral groups is somewhat unexpected. The information presented in Figures 3-6 and Tables V and VI indicates that straight chain a,w-dicarboxylic acids are prominant among the aliphatic acids. All the acids of this type with 4-16 carbon atoms are readily detected in the oxidation products of the liptinite maceral group. Vitrinite provides the straight-chain dioic acid series to nonanedioic acid and inertinite the series to octanedioic acid. Significant amounts of the branched-chain dioic acids are also found among the oxidation products of all the macerals. The branched derivatives are all methyl-substituted straight-chain dioic acids such as 2methylpentane-1,bdioic acid and 2,3-dimethylpentane1,Bdioic acid. Aliphatic acids with ethyl or other larger alkyl substituents are not formed in detectable amounts. Although the propane-l,&dioic acids with enolizable hydrogen atoms are unstable under the reaction conditions,14 2,2-dimethylpropane1,3-dioicacid was detected among the reaction products of all three macerals. 2,2-dimethylbutane-1,4-dioic acid, which also has a quaternary carbon atom, is present too. It seems premature to speculate upon the origins of these kinds of compounds, but it is appropriate to note that quaternary carbon atoms are clearly present in PSOC-732 coal. These findings contrast with the results that have been obtained on whole coals with the use of less sensitive nuclear magnetic resonance techniques. Cyclohexanecarboxylic acid is also present in small quantities in the oxidation products. The presence of alicyclic acids among the oxidation products of kerogen and coal have been reported by other investigator^,^^^^ but none of these acids appear to have been individually
Table VI. Quantitative Yields of Selected Products yield: pmol of acid/g of maceral acid liptinite vitrinite inertinite Aliphatic Acids butane-1,4-dioicb 114 42 80 2-methylbutane-l,4-dioic 50 19 38 pentane-1,5-dioic 103 31 53 2-methylpentane-l,&dioic 17 42 54 hexane-1,6-dioic 93 15 35 14 1 2-methylhexane-l,&dioic 6 12 9 3 3-methylhexane-1,6-dioic 24 100 9 heptane-1,7-dioic 17 77 3 octane-1,3-dioic 8 bdl 68 aonane-l,g-dioic bdl bdl 61 decane-1,lO-dioic bdl bdl 45 undecane-1,ll-dioic bdl 47 dodecane-lJ2-dioic bdl bdl bdl 37 tridecane-1,13-dioic bdl bdl 31 tetradecane-l,14-dioic 14 bdl bdl pentadecane-1,15-dioic hexadecane-1,16-dioic 8 bdl bdl Benzene Acids 1 benzoic benzene-1,2-dicarboxylicb 43 benzene-l,3-dicarboxylic bdl benzene-1,4-dicarboxylic bdl 3 3-methylbenzene-1,2-dicarboxylic 4 4-methylbenzene-1,2-dicarboxylic 76 benzene-l,2,3- and benzene-l,2,4-tricarboxylic benzene-1,2,4,5-tetracarbo~ylic~ 43 benzene-1,2,3,4-tetracarboxylic 12
3 46 4 3 8 14 92
7 79 6 4 2 7 117
48 14
73 17
"The designation "bdl" indicates that the yields was below the detection limit. The yields of these products were determined by isotope dilution analyses.
identified previously. In this connection, it is pertinent to note that alicyclic diacids and triacids may contribute to the background between scans 1250 and 2000 in the
46 Energy & Fuels, Vol. 2, No. 1, 1988
chromatograms of the liptinite and vitrinites (Figures 4 and 5). Many oxidation procedures degrade long-chain aliphatic acids and diacids to smaller mo1ecules;l however, most such acids appear to be more stable under the conditions employed in the ruthenium(VII1) oxidation rea~ti0n.l~ Consequently, the striking observation that there is a regular decrease of the peak areas corresponding to the straightchain dicarboxylic acids in the oxidation products of liptinite, as shown in Figure 4, implies that the relative quantities of the acids are related to the actual distributions of the aliphatic dicarboxylic acids and, therefore, to the true polymethylene chain lengths in the macerals. Several types of structures may produce the aliphatic dicarboxylic acids. I t should be noted that it is unlikely that these acids were present in the pure maceral fractions because these materials were extracted with organic solvents and treated with acidic and basic solutions to remove mobile materials prior to oxidation. In addition, other investigators who have tried to extract free carboxylic acids have had only limited To illustrate, Allan and Larter found less than 10 ppm of mono- and dicarboxylic acids upon saponification of sporinites and v i t r i n i t e ~ .Therefore, ~~ it is unlikely that the large quantities of aliphatic dicarboxylic acids could be formed via the hydrolysis of esters or triglycerides during the oxidation reaction with ruthenium(VII1). Several lines of evidence suggest that the aliphatic dicarboxylic acids may most 1 isibly form from diarylalkanes, hydroaromatic comp(CH2ds, or ethers as illustrated in eq 6-9. Although little i nown about the types of
-
ArOCH2(CH2),Ar
H02CCH2CH2CH2C02H
-
ArCH2CH2CH2Ar H02CCH2CH2CH2C02H
ethers in these macerals, a substantial proportion of these ethers in coals and some macerals are heterocyclic or diaryl ether^.^^^^^ These structures do not oxidize to form aliphatic dicarboxylic acids. The 13Cnuclear magnetic resonance spectra of the PSOC-732 macerals (Figure 1)show little or no signal intensity in the 60-75 ppm region, which would indicate the presence of sp3 carbon atoms bonded to oxygen atoms.% In view of this evidence, we conclude that the aliphatic diacids are produced by the oxidation of hydroaromatic compounds (eq 8) and diarylalkanes (eq 9). Both kinds of compounds could produce the aliphatic acids that are observed in the reaction products, but it is unlikely that the long-chain aliphatic acids originate from thermodynamically unstable large cyclic hydroaromatic structures. The low abundance of acids larger than nonane-1,9-dioic acid in vitrinite and inertinite suggests that the longer straight-chain dicarboxylic acids observed in the liptinite are probably derived from some component unique to the original spores, probably from the abundant paraffinic substances in sp~ropollenin.~' (52) Given, P. H.; Peover, M. E.; Wyss, W. F. Fuel 1960,39,323-340. (53) Given, P. H.; Peover, M. E.; Wyss, W. F. Fuel 1965,44,425-435. (54) Wachowska, H. M.; Nandi, B. N.; Montgomery, D. S.Fuel 1979, 58, 257-263. (55) Stock, L. M.; Willis, R. S. J. Org. Chem. 1985, 50, 3566-3573. (56) Alemany, L. B.; Stock, L. M. Fuel 1982, 61, 1088-1094.
Choi et al.
The methyl-substituted aliphatic dicarboxylic acids form an interesting series. Large amounts of 2-methylbutane1,4-dioic, 2- and 3-methylpentane-l,5-dioic, and 2- and 3-methylhexane-1,6-dioic acids are found. However, few methyl-substituted long-chain dicarboxylic acids are detected. Even in the case of the liptinite group where the straight-chain dicarboxylic acid series extended to hexadecane-l,l6-dioic acid, only very small quantities of methyl-substituted heptanedioic acid or the larger acids were observed. This fact suggests that most of these lower molecular weight branched acids were derived from the oxidation of methylated hydroaromatic structures such as 9-methyldihydrophenanthrene,1-and 2-methylindan,and 1- and 2-methyltetralin. The absence of methylated long-chain diacids supports the previous conclusion that the related unmethylated materials arise from diarylalkanes or triaryl alkanes (eq 2 and 8) present in the cutinite and sporinite in the liptinite maceral group from PSOC-732. Aliphatic tricarboxylic acids and other polycarboxylic acids are also produced in the oxidation of macerals. While the precise structures of these acids have not yet been established, enough information is available to define their principal structural elements. These substances may arise from the oxidation of branched structures (eq lo), related kinds of cyclic branched structures (eq 111, and partially aromatic cyclic aliphatic compounds (eq 12). The liptinite coal-ArCH2CH(Ar-coal)CH2Ar-coal H02CCHZCH(C02H)CH2CO2H (10)
-
-
eW
A
r -ccal
H02CCH2CH(C02H)CH2CO2H
(11)
\
(HO2CCH&~)(HO$)CHCH(C~)CH2CH~CO2H) (12)
maceral group yields the largest array of tricarboxylic acids, which range from propane-1,2,3-tricarboxylicacid to nonanetricarboxylic acid. Vitrinite yields fewer tricarboxylic acids than liptinite, and only butanetricarboxylic acid to hexanetricarboxylic acid can be detected among the products. No tricarboxylic acids were detected among the oxidation products of inertinite. This product distribution is compatible with the idea that specific components of the sporinite and cutinite are uniquely responsible for the rather high abundance of these materials. The yield data for selected benzenecarboxylic acids presented in Table VI indicate that these substances are the most abundant products in the oxidation of vitrinite and inertinite. These acids clearly arise from the oxidation of fused polycyclic aromatic systems. Studies of representative substituted and unsubstituted compounds with two to four aromatic rings establish that the aromatic rings that are activated for electrophilic substitution are selectively oxidized (eq 13).13J4 1-methoxynaphthalene benzene-1,2-dicarboxylicacid (13)
-
The predominance of the 1,Zfusion pattern, which was previously reported for benzenecarboxylicacids produced from the oxidation of whole coal,16was also observed for all three maceral groups. The major products are benzene-1,2-dicarboxylic, benzene-1,2,3- and benzene-1,2,4(57) Stock, L. M.; Wang, S. H., unpublished results.
Ru04-Catalyzed Oxidation Table VII. Yields per 100 Aliphatic Carbon Atoms of 10 Abundant AliDhatic Acids yield, mmol of acid/100 mol of aliphatic C acid liptinite vitrinite inertinite 2012 6785 ethanoic 6938 propanoic 1348 602 1069 552 397 454 butane-1,4-dioic 2-methylbutane-1,4-dioic 262 174 206 pentane-1,5-dioic 367 356 339 2-methylpentane-l,5-dioic 291 187 186 hexane-1,6-dioic 243 324 162 2- and 3-methylhexane-1,6-dioic 80 43 125 100 349 167 heptane-1,7-dioic 33 267 118 octane-l&dioic ~~
Table VIII. Yields per 100 Aromatic Carbon Atoms of Four Abundant Benzene Acids yield, mmol of acid/100 mol of aromatic C acid liptinite vitrinite inertinite benzene-1,2-dicarboxylic 103 85 127 3- and 4-methylbenzene17 40 14 1,2-dicarboxylic benzene-1,2,3- and 183 169 189 benzene-1,2,4-tricarboxylic benzene-1,2,4,5-tetracarboxylic 103 89 118
tricarboxylic, and benzene-1,2,4,5-tetracarboxylicacids. Naphthalenes, acenaphthenes, phenanthrenes, anthracenes and other polycyclic aromatic substances oxidize to produce these substances. Only small quantities of benzoic, benzene-1,3- and benzene-1,Cdicarboxylic acids are found. This fact indicates that biphenyls or terphenyls are much less abundant than fused aromatic structures. Methyl-substituted benzenecarboxylic acids such as 3and 4-methylbenzene-l,2-dicarboxylicacids are more predominant in vitrinite groups than in either liptinite or inertinite groups. As mentioned earlier, these acids arise from the oxidation of methylaromatic structures where the ring containing the methyl group is preserved. Inasmuch as the yield of ethanoic acid is greater in the oxidation of the vitrinite than of liptinite or inertinite, these two observations establish that aryl methyl groups are much more abundant in the vitrinite of PSOC-732. Quantitative comparisons of the abundances of the aliphatic and aromatic acids provide further information about the constitution of the macerals. The relative quantities of the aromatic and the aliphatic carbon atoms in the macerals were estimated from the solid-state 13C CP/MAS NMR data. These results and the formation presented in the previous tables were used to calculate the yields of the 10 most abundant aliphatic acids, which account for 20%, 18%) and 15% of the aliphatic carbon atoms in the liptinite, vitrinite, and inertinite, respectively. The results for the three maceral groups are summarized in Table VII. The corresponding data for the benzene acids are given in Table VIII. This comparison reveals that the yields of these aliphatic acids from the vitrinite and the inertinite are remarkably similar and that ethanoic acid is, by far, the most abundant product. As already discussed, this observation reflects the high concentration of aryl methyl groups in these two macerals. In contrast, ethanoic acid represents a much smaller portion of the aliphatic acid formed from the liptinite group. Long aliphatic fragments are much more abundant in this maceral group. This feature is best illustrated through the estimated aliphatic CH3, CHz, and CH group contents of the three macerals shown in Table IX. These data reveal that the methylene/methyl ratio
Energy & Fuels, Vol. 2, No. 1, 1988 47 Table IX. Aliphatic Group Distribution" % aliuhatic g r o w maceral group CH3 CH2 CH liptinite 14 84 2 vitrinite 51 45 4 inertinite 60 37 3
"The group distributions are based upon the yields of the aliphatic acids identified in Table VI.
is about 5-fold greater for the liptinite group. The yields of the benzene acids produced from every 100 mol of aromatic carbon atoms of each maceral are shown in Table VIII. The yields of unsubstituted aromatic acids are similar for liptinite and inertinite while vitrinite shows a slightly lower amount of these acids. However, when the total amount of unsubstituted and alkyl-substituted acids are taken into consideration, the similarity in the results becomes apparent. For example, the sum of the quantities of the benzene-1,2-dicarboxylicand the methylbenzene1,2-dicarboxylic acids are similar for all three macerals. The results imply that similar aromatic structures are present in each of these macerals, with the difference being that methyl substituents on such structures are more common in vitrinite than in the other macerals, but it is clear that many different aromatic and heteroaromatic compounds may produce the same benzene acids. Further study will be required to resolve this structural feature. Conclusion Analyses of the oxidation products of pure maceral groups provide new insightful information about their aromatic and aliphatic structures. The macerals all produce similar types of benzene acids. The aromatic structures in the macerals occur predominantly with 1,2-fused benzene rings. The relative quantities of aromatic acids produced from the aromatic carbons in each maceral are also similar. This fact implies that the types of aromatic structures in the different macerals are quite similar, but this tentative conclusion requires more definitive study. A wide array of aliphatic acids are found in the oxidation products of the macerals. The yields of aliphatic acids correlate with the aliphatic carbon content of these macerals. More important, significant differences in the aliphatic structural units are implied. The liptinites contain large quantities of long polymethylene chains, which probably bridge aromatic structures. The results are also consistent with the idea that a considerable portion of the aliphatic material in the vitrinite occurs in hydroaromatic structures and pendant methyl groups. Although the inertinite maceral group oxidizes to yield less aliphatic acids than the other macerals, the quantities and types of aliphatic acids produced from the aliphatic carbon atoms in this maceral are strikingly similar to the results of the vitrinite. Thus, the differences between inertinite and vitrinite may be due to the differences in the relative concentrations of similar types of aromatic and aliphatic structures. In contrast, the liptinites contain significantly different aliphatic structures although the aromatic structures may be similar to those in vitrinite and inertinite. Acknowledgment. We wish to acknowledge that the solid-state NMR spectra were recorded by J. V. Muntean. The research was supported by the Office of Basic Energy Sciences, Division of Chemical Sciences, U S . Department of Energy, under Grant 86-ER-13573. Registry No. 1, 107-92-6; 2, 116-53-0; 3,503-74-2; 4, 109-52-4; 5, 79-43-6; 6, 142-62-1; 7, 75-11-6; 8, 141-76-4; 9, 628-46-6; 10,
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Energy & Fuels 1988,2, 48-58
595-46-0; 11, 111-14-8; 12, 98-89-5; 13, 110-15-6; 14, 498-21-5; 15, 124-07-2; 16, 597-43-3; 17,65-85-0; 18, 13545-04-5; 19, 110-94-1; 20,112-05-0; 21,617-62-9; 22,681-57-2; 23,17179-91-8; 24,124-04-9; 25,334-48-5;26,626-70-0; 27,3058-01-3;28,763-06-4; 29, 111-16-0; 30, 112-37-8; 31, 10200-27-8; 32, 10200-31-4; 33, 505-48-6; 34, 111324-47-1; 35,100-21-0; 36,88-99-3; 37, 121-91-5; 38,123-99-9; 39, 65891-27-2; 40, 37102-74-2; 41, 4316-23-8; 42, 111-20-6; 43,
66021-66-7; 44, 111324-41-5; 45, 111324-42-6; 46, 1852-04-6; 47, 111324-43-7; 48, 693-23-2; 49, 110063-44-0; 50, 27252-21-7; 53, 505-52-2; 54, 111348-90-4; 55, 67595-78-2; 58, 821-38-5; 60, 70174-69-5; 61, 111324-44-8; 62, 111324-45-9; 63, 1460-18-0; 65, 505-54-4; 66, 476-73-3; 67,89-05-4; 68,479-47-0; 69,67595-79-3; 71,111324-46-0; 74, 1585-40-6;H&C02H, 64-19-7;H&CH&02H, 79-09-4; R u O ~20427-56-9. ,
Dipolar-Dephasing 13C NMR Studies of Decomposed Wood and Coalified Xylem Tissue: Evidence for Chemical Structura1 Changes Associated with Def unctionalization of Lignin Structural Units during Coalification Patrick G. Hatcher US. Geological Survey, 923 National Center, Reston, Virginia 22092 Received July 30, 1987. Revised Manuscript Received October 5, 1987
A series of decomposed and codified gymnosperm woods was examined by conventional solid-state 13Cnuclear magnetic resonance (NMR) and by dipolar-dephasing NMR techniques. The results of these NMR studies for a histologically related series of samples provide clues as to the nature of coalification reactions that lead to the defunctionalization of lignin-derived aromatic structures. These reactions sequentially involve the following: (1)loss of methoxyl carbons from guaiacyl structural units with replacement by hydroxyls and increased condensation; (2) loss of hydroxyls or aryl ethers with replacement by hydrogen as rank increases from lignin to high-volatile bituminous coal; (3) loss of alkyl groups with continued replacement by hydrogen. The dipolar-dephasing data show that the early stages of coalification in samples examined (lignin to lignite) involve a decreasing degree of protonation on aromatic rings and suggest that condensation is significant during coalification at this early stage. An increasing degree of protonation on aromatic rings is observed as the rank of the sample increases from lignite to anthracite.
Introduction The study of coal’s chemical structure has traditionally relied on detailed characterization of whole coal by several chemical and spectroscopic techniques. Despite such extensive studies having been conducted over several decades, we still have only a limited knowledge of coal’s chemical composition, because coal is such a complex substance composed of a multitude of different plant remains, each of which introduces heterogeneity and complexity to coal’s chemical structure. To simplify the task of defining coal’s chemical composition and the processes that transform plant remains to coal, several earlier studies1s2focused only on woody xylem tissue. This tissue is commonly coalified to a maceral known as vitrinite, a major component of most coals. Examining the chemical structural evolution of xylem tissue as it is converted to vitrinite provides an insight into the processes that can collectively be described as coalification. In most cases, the early diagenetic phase of coalification, the peat stage, involves degradation of cellulosic components of wood and selective preservation of lignin-like components.l” However, specific examples can (1) Hatcher, P. G.; Breger, I. A.; Earl, W. L. Org. Geochem. 1981,3,
be found where cellulosic substances survive the early stages of decomposition in coalified wood and persist over geologic time.2v6p7 No cases have been reported where cellulosic substances survive in coals having ranks greater than subbituminous coal. Thus, while cellulosic materials can rarely be found in lignitic coals, or brown coals, these substances are geologically unstable and are usually selectively degraded at the peat stage. In most cases lignin is altered to vitrinite during coalification; consequently, we must seek to examine the chemical structural alteration of lignin. Nuclear magnetic resonance studies of a sample series of coalified logs and/or stems that increase in degree of coalification from peat to lignite and to higher ranks have shown that lignin-like components of wood become defunctionalized, first losing methoxyl groups and then losing aryl ether and phenolic groups.2 With coalification increasing to high-volatile bituminous coal, the xylem tissue is converted to aromatic and aliphatic structures containing little, if any, substitution by oxygen-containing functional groups. This study focuses on the nature of changes that lead to the defunctionalization of lignin. The solid-state 13C NMR data provide direct evidence that defunctionalization is the major alteration, but details of the reactions are
49.
(2) Hatcher, P. G.; Breger, I. A.; Szeverenyi, N. M.; Maciel, G. E. Org. Geochem. 1982,4, 9. (3) Fischer, F.; Schrader, H. Brennst.-Chem. 1921, 2, 37. (4) Hedges, J. J.; Cowie, G. L.; Ertel, J. R.; Barbour, R. J.; Hatcher, P. G. Geochim. Cosmochim. Acta 1985, 49, 701.
(5) Spiker, E. C.;Hatcher, P. G. Geochim. Cosmochim.Acta. 1987,51, 1385. (6) Mitchell, R. L.; Ritter, G. J. J. Am. Chem. SOC.1934, 56, 1603. (7) Russel, N. J.; Barron, P. F. Int. J . Coal Coal. 1984, 4 , 119.
This article not subject to U S . Copyright. Published 1988 by the American Chemical Society
