Compositional Determination of Acidic Species in Illinois No. 6 Coal

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Energy & Fuels 2004, 18, 1424-1428

Compositional Determination of Acidic Species in Illinois No. 6 Coal Extracts by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Zhigang Wu, Ryan P. Rodgers, and Alan G. Marshall* Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310-4005, and Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306 Received March 15, 2004. Revised Manuscript Received June 9, 2004

Coal acids constitute a source of corrosion in coal refining, and carboxylic acids are implicated in retrograde reactions in coal liquefaction. Here, we use electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to provide detailed molecular compositions of acidic species in a pyridine extract as well as in hexane-soluble (coal acids) and hexane-insoluble (acidic asphaltenes) fractions designed to concentrate the acidic components. All three extracts contain similar major heteroatomic classes. For a given heteroatomic class, asphaltenes > pyridine extract > coal acids in aromaticity; the combined acid fractions exhibit wider distributions in carbon number and double bond equivalents than the pyridine extract. Although the pyridine extract contains the widest variety of organics, additional acidic species are found in the other two extracts. The chemical compositions of coal acids are now accessible in unprecedented detail and promise to provide a starting point for more rational analysis of coal origin and processing.

Introduction The presence of acidic constituents considerably increases corrosion in high-temperature distillation units in petroleum refining. The carboxylic acids of various heteroatomic classes (i.e., different numbers of N, O, and S atoms) in heavy immature petroleum crude oil have been identified, including naphthenic (alicyclic) acids, naphthenoaromatic acids, and aliphatic acids.1-4 Those acids are believed to be a primary cause of corrosion in oil refining equipment.5-7 Severe corrosion has been observed in coal liquefaction equipment when acid fraction, basic fraction, and water-soluble chlorides are present simultaneously.8 Moreover, carboxylic acids are major factors in retrograde reactions in the coal lique* Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Jones, D. M.; Watson, J. S.; Meredith, W.; Chen, M.; Bennett, B. Determination of naphthenic acids in crude oils using non-aqueous ion exchanges solid-phase extraction. Anal. Chem. 2001, 73, 703-707. (2) Seifert, W.; Teeter, R. Identification of polycyclic naphthenic, monoaromatic, and diaromatic crude oil carboxylic acids. Anal. Chem. 1970, 42, 180. (3) Seifert, W.; Teeter, R. Identification of polycylic aromatic and heterocyclic crude oil carboxylic acids. Anal. Chem. 1970, 42, 750. (4) Dzidic, I.; Somerville, A. C.; Raia, J. C.; Hart, H. V. Determination of naphthenic acids in California crudes and refinery wastewaters by fluoride-ion chemical ionization mass spectrometry. Anal. Chem. 1988, 60, 1318-1323. (5) Ramljak, Z.; Solc, A.; Arpino, P.; Schmitter, J.; Guiochon, G. Separation of acids from asphalts. Anal. Chem. 1977, 49, 1222-1225. (6) Miller, S.; Gardiner, M.; Ward, C. In-line inspection detects early cracking on Canadian crude-oil line. Oil Gas J. 1998, 96, 90. (7) Filby, R.; Olsen, S. A comparison of instrumental neutronactivation analysis and inductively-coupled plasma-mass spectrometry for trace-element determination in petroleum geochemistry. J. Radioanal. Nucl. Chem. 1994, 180, 285-294.

faction process.9-13 Acids are surface active (i.e., they lower the surface tension of the medium and accumulate at interfaces) and water-soluble; thus, they can readily propagate into the environment.14 Therefore, a detailed chemical composition of acids in coal should enable a better understanding of the behavior of coal during liquefaction and processing, leading to rationally based improvements in those treatments. Crude oils are considered acidic if their total acid number (TAN) exceeds 0.5 mg KOH/g by nonaqueous titration. However, TAN does not reliably predict the corrosivity of crude oil and does not identify the causative agents. Carboxylic acids can be isolated by simple base extraction15 or exposure to KOH-impregnated silica (8) Bagga, P. S.; Harris, C. F.; Baumert, K. L. Corrosion of fractionation towers in coal liquefaction plants. Energy Res. Abstr. 1984, 9, 38. (9) Manion, J.; McMillen, D.; Malhotra, R. Decarboxylation and coupling reactions of aromatic acids under coal-liquefaction conditions. Energy Fuels 1996, 10, 776-788. (10) Suuberg, E.; Lee, D.; Larsen, J. Temperature-dependence of crosslinking processes in pyrolyzing coals. Fuel 1985, 64, 1668-1671. (11) Serio, M.; Hamblen, D.; Markham, J.; Solomon, P. Kinetics of volatile product evolution in coal pyrolysissexperiment and theory. Energy Fuels 1987, 1, 138-152. (12) Solomon, P.; Serio, M.; Despande, G.; Kroo, E. Cross-linking reactions during coal conversion. Energy Fuels 1990, 4, 42-54. (13) Gozmen, B.; Artok, L.; Erbatur, G.; Erbatur, O. Direct liquefaction of high-sulfur coals: effects of the catalyst, the solvent, and the mineral matter. Energy Fuels 2002, 16, 1040-1047. (14) Lai, J. W. S.; Pinto, L. J.; Kiehlmann, E.; Bendell Young, L. I.; Moore, M. M. Factors that affect the degradation of naphthenic acids in oil sands wastewater by indigenous microbial communities. Environ. Toxicol. Chem. 1996, 15, 1482-1491. (15) Barth, T.; Moen, L.; Dyrkorn, C. Comparison of acid numbers and carboxylic acid molecular compositions in biodegraded and normal crude oils. Abstr. Pap. Am. Chem. Soc. 1998, 3, 134-136.

10.1021/ef049933x CCC: $27.50 © 2004 American Chemical Society Published on Web 07/16/2004

Acidic Species in Illinois No. 6 Coal

gel.5,16,17 Carboxylic acids and phenols have also been extracted quantitatively with anionic surfactants18,19 Naphthenic acids in the polar fraction have been obtained by passage through a cyano-modified silica column after elution of nonpolar hydrocarbons.20 Nonaqueous ion exchange-based methods have long been used to isolate acids.1,21-23 Carboxylic acids isolated from crude oil and coal have been analyzed by infrared spectroscopy,5,22 gas chromatography,15,17 and mass spectrometry.2-4,22-25 However, the compositional complexity of the carboxylic acids fraction has prevented detailed analysis. For example, some aliphatic acids can be resolved by GC/MS, but a large “unresolved complex mixture” in the chromatogram is not completely resolved, even when examined by high-resolution mass spectrometry.23 Preparative thin-layer chromatography coupled with mass spectrometry has the same problem.26 We previously reported that negative-ion ESI FT-ICR MS resolves more than 10 000 compositionally distinct compounds (enabling assignment of chemical formulas (CcHhNnOoSs) to more than 7500 species) in a pyridine extract of Illinois No. 6 coal.27 In that work, we used pyridine because it has the highest extraction efficiency for organics in coal. Here, by chromatographic isolation of two acid fractions, we are able to characterize the acids more completely than by pyridine extraction alone. The acids are concentrated in different fractions according to their degree of saturation, so that species undetectable in the pyridine extract are rendered observable. Experimental Methods Isolation of Acid Fractions. The methods for isolation of acid and acidic asphaltene fractions have been described previously by Qian et al.28 Here we applied the method to coal (16) Douglas, A. G.; Powell, T. G. Rapid separation of fatty acids from fossil lipids by impregnated adsorbent thin-layer chromatography. J. Chromatogr. 1969, 43, 241-246. (17) Jaffe, R.; Albrecht, P.; Oudin, J. Carboxylic-acids as indicators of oil migration. 2. Case of the Mahakam Delta, Indonesia. Geochim. Cosmochim. Acta 1988, 52, 2599-2607. (18) Seifert, W.; Teeter, R.; Howells, W.; Cantow, M. Analysis of crude oil carboxylic acids after conversion to their corresponding hydrocarbons. Anal. Chem. 1969, 41, 1639-1646. (19) Seifert, W.; Teeter, R. Carboxylic acids in a California petroleums identification of structure type. Chem. Ind. 1969, 41, 1464. (20) Yepez, O.; Lorenzo, R.; Callarotti, R.; Vera, J. Reaction kinetics of Venezuelan naphthenic acids with high surface iron. Abstr. Pap. Am. Chem. Soc. 1998, 3, 114-122. (21) Jewell, D.; Weber, J.; Bunger, J.; Plancher, H.; Latham, D. Ionexchange, coordination, and adsorption chromatographic separation of heavy-end petroleum distillates. Anal. Chem. 1972, 44, 1391-1395. (22) Green, J.; Hoff, R.; Woodward, P.; Stevens, L. Separation of liquid fossil-fuels into acid, base and neutral concentrates. 1. An improved nonaqueous ion-exchange method. Fuel 1984, 63, 1290-1301. (23) Tomczyk, N.; Winans, R.; Shinn, J.; Robinson, R. On the nature and origin of acidic species in petroleum. 1. Detailed acid type distribution in a California crude oil. Energy Fuels 2001, 15, 14981504. (24) Schulten, H.; Marzec, A.; Czajkowska, S. Mass-spectrometric and chemometric hemometric studies of thermoplastic properties of coals. 2. Field-ionization mass spectrometry of coals. Energy Fuels 1992, 6, 103-108. (25) Rudzinski, W.; Oehlers, L.; Zhang, Y. Tandem mass spectrometric characterization of commercial naphthenic acids and a Maya crude oil. Energy Fuels 2002, 16, 1178-1185. (26) Seifert, W.; Teeter, R. Preparative thin-layer chromatography and high-resolution mass spectrometry of crude oil carboxylic acids. Anal. Chem. 1969, 41, 786. (27) Wu, Z.; Jernstro¨m, S.; Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Resolution of 10, 000 compositionally distinct components in polar coal extracts by negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2003, 17, 946-953.

Energy & Fuels, Vol. 18, No. 5, 2004 1425 samples with minor adjustments. Five grams of Illinois No. 6 coal (Argonne National Laboraotry) was dissolved in 40 mL of toluene:methanol (70:30 v/v) and loaded onto aminopropyl silica (Sigma-Aldrich, Bellefonte, PA). After filtration and solvent washing, the silica with coal sample (∼0.7 g) was dissolved in 40 mL of toluene spiked with 30% acetic acid and then extracted in a sonicator water bath for 45 min. After filtration, the extract was water washed to remove residual acetic acids and then rotary evaporated to remove solvent. The dark yellowish residue was first dissolved in 30 mL of hexane and passed through a silica column (Sigma-Aldrich, Bellefonte, PA). More hexane was added to wash the residue until the solvent was colorless. The hexane-soluble fraction is designated as the “acids fraction” (∼0.04 g), and the hexane-insoluble fraction is designated as “acidic asphaltenes” (∼0.01 g) subsequently dissolved in toluene. Sample Preparation. Prior to analysis by ESI FT-ICR MS, a solution of 10 mg of coal acid or acidic asphaltene sample was dissolved in 3 mL of pyridine and then diluted with 17 mL of methanol. One milliliter of the solution mixture was removed and spiked with 10 µL of pure (99.9%) ammonium hydroxide. Pyridine extraction of the same coal sample is described elsewhere.27 All solvents were HPLC grade (Fisher Scientific, Pittsburgh, PA). All preparatory work was done in a hood. Mass Analysis. Negative-ion ESI FT-ICR mass spectra were acquired with a home-built mass spectrometer equipped with a 22 cm diameter bore horizontal 9.4 T magnet (Oxford Corp., Oxney Mead, England)29-32 as described previously.27 Time-domain data sets were processed and Hanning-apodized, followed by a single zero-fill before fast Fourier transformation and magnitude calculation. Frequency was converted to massto-charge ratio (m/z) by the quadrupolar electric trapping potential approximation33,34 to generate the m/z spectra. Mass spectrometry was conducted in a well-ventilated lab equipped with a flexible exhaust pipe.

Results and Discussion Figure 1 shows broadband mass spectra of the acid fraction, acidic asphaltenes fraction, and pyridine extract of Illinois No. 6 coal. Because all detected ions are singly charged (based on the observed unit m/z spacing between chemically identical species containing 12Cc vs 13C12C 35 we shall henceforth denote each ion by its c-1), (28) Qian, K.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Resolution and Identification of 3000 crude acids in heavy petroleum by negative-ion microelectrospray high field Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2001, 15, 1505-1511. (29) Senko, M. W.; Hendrickson, C. L.; Pasa-tolic, L.; Marto, J. A.; White, F. M.; Guan, S.; Marshall, A. G. Electrospray ionization FTICR mass spectrometry at 9.4 T. Rapid Commun. Mass Spectrom. 1996, 10, 1824-1828. (30) Senko, M. W.; Canterbury, J. D.; Guan, S.; Marshall, A. G. A high-performance modular data system for FT-ICR mass spectrometry. Rapid Commun. Mass Spectrom. 1996, 10, 1839-1844. (31) Blakney, G. T.; van der Rest, G.; Johnson, J. R.; Freitas, M. A.; Drader, J. J.; Shi, S. D.-H.; Hendrickson, C. L.; Kelleher, N. L.; Marshall, A. G. Further improvements to the MIDAS data station for FT-ICR mass spectrometry. In Proceedings of the 49th American Society of Mass Spectrometry Conference on Mass Spectrometry & Allied Topics; American Society of Mass Spectrometry: Chicago, 2001. (32) Senko, M. W.; Hendrickson, C. L.; Emmett, M. R.; Shi, S. D.H.; Marshall, A. G. External accumulation of ions for enhanced electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. J. Am. Soc. Mass Spectrom. 1997, 8, 970-976. (33) Ledford, E. B., Jr.; Rempel, D. L.; Gross, M. L. Space charge effects in Fourier transform mass spectrometry. II. Mass calibration. Anal. Chem. 1984, 56, 2744-2748. (34) Shi, S. D.-H.; Drader, J. J.; Freitas, M. A.; Hendrickson, C. L.; Marshall, A. G. Comparison and interconversion of the two most common frequency-to-mass calibration functions for Fourier transform ion cyclotron resonance mass spectrometry. Int. J. Mass Spectrom. 2000, 195/196, 591-598.

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Figure 3. Mass scale expansion of the negative-ion ESI FTICR mass spectrum for species of nominal mass, 619 Da. The fully saturated naphthenic acid is the most abundant species. Figure 1. Broadband negative-ion ESI FT-ICR mass spectra of Illinois No. 6 coal samples. (Top) Coal acids fraction. (Middle) Acidic asphaltene fraction. (Bottom) Coal pyridine extract.

Figure 2. High-mass segment of the negative-ion ESI FTICR mass spectrum of saturated naphthenic acids from the coal acids fraction. Homologous series of compounds differing in degree of alkylation are evident from the characteristic mass spacings of 14.0157 Da in mass (CH2).

mass in Daltons rather than its mass-to-charge ratio (m/z). Negative-ion ESI FT-ICR MS resolves ∼10 000 compositionally distinct compounds in the acidic asphaltene fraction, ∼5000 in the acid fraction, and ∼10 000 in the pyridine extract. Virtually no species above 600 Da are found in the pyridine extract whereas numerous compounds of 600-750 Da are seen in both acidic fractions. Homologous Series and Compound Class. The coal acids mass spectrum (Figure 2) exhibits homologous series of saturated carboxylic acids differing by multiples of 14.01565 Da (i.e., CH2). It is worth noting that those species are not seen in the mass spectrum of the pyridine extract under identical experimental conditions. Mass scale expansion over a 1 Da interval at 619 Da (Figure 3) resolves 29 compositionally distinct compounds, of which 28 could be identified by matching to within 1.0 ppm of a putative elemental composition. The highest mass peak corresponds to the fully saturated carboxylic acid, C31H23O2-. Other classes including O3 and O4 are also represented. Interestingly, the species with highest mass defect (i.e., highest saturation) are most abundant: in general, the coal acids fraction is the least aromatic of the three. (35) Senko, M. W.; Beu, S. C.; McLafferty, F. W. Automated assignment of charge states from resolved isotopic peaks for multiply charged ions. J. Am. Soc. Mass Spectrom. 1995, 6, 52-56.

Figure 4. Mass scale expansion of the negative-ion ESI FTICR mass spectrum for species of nominal mass, 437 Da. Note the higher compositional complexity for the pyridine extract relative to the two acid fractions.

Comparison of Acid Fractions and Pyridine Extract. At low mass, the pyridine extract yields more detectable compounds, as shown in Figure 4, mass segment at 437 Da: 24 resolved components in the pyridine extract compared to 12 and 9 for acidic asphaltenes and the coal acid fraction. However, (see the species at 407 Da in Figure 5), some acids of low abundance and/or low pyridine extraction efficiency are detected only in the coal acid fraction (C27H51O2- and C29H43O-) or only in the coal acids and acidic asphaltenes fraction (C28H39O2- and C27H35O3-). C26H31O4- is not present in the coal acid fraction but is found in the acidic asphaltenes fraction and in the pyridine extract. Generally, the fractionation process isolates and concentrates acidic species from coal. Therefore, by concentrating acids in different coal fractions, we are able to recover and identify a more complete acid composition. Compound Type and Carbon Distribution. Figure 6 shows compound type distributions for the pyridine extract and coal acid fraction. The pyridine extract distribution was scaled relative to the summed abundances of all O3 species as 100%, and the coal acids distribution was scaled relative to the summed abundances of all O2 species as 100%. (The acidic asphaltenes distribution (not shown) has a much higher relative abundance of O3 species (see below).) O3 and O2S classes are considered to be acids with oxygen or sulfur het-

Acidic Species in Illinois No. 6 Coal

Figure 5. Mass scale expansion of the negative-ion ESI FTICR mass spectra for species of nominal mass, 407 Da from coal acids, acidic asphaltenes, and pyridine extract of Illinois No. 6 coal. Some acid species are detectable only in the two acid fractions, and the combined acid fractions contain more acid compounds than the pyridine extract alone.

Figure 6. Type distributions DBE (rings plus double bonds or double bond equivalents) for the coal acids fraction and pyridine extract of Illinois No. 6 coal. The O3 class is dominant in the pyridine extract whereas the O2 class dominates in the coal acid fraction. Compounds in the pyridine extract are more unsaturated than in the acid extract. (The molecular structures are representative and cannot be directly determined by mass alone.)

erocycles. The O3 and O2S distributions in both pyridine extract and coal acid fraction are roughly Gaussian (i.e., not bimodal), suggesting a single dominant aromatic core structure (rather than two or more nonfused cores) for those two classes. On the other hand, the O2 distribution is bi- or trimodal with up to six aromatic rings, suggesting the presence of at least two different core structures.28 Note that naphthenic acids (i.e., acids containing fused cyclohexane rings) and aromatic acids can have the same number of rings and double bonds and overlap in exact mass and are thus unresolvable by mass measurement alone. Interestingly, the coal acid fraction also contains a high percentage of saturated aliphatic acids. The type (rings plus double bonds) distribution differs markedly from that for acids in a Chinese heavy crude oil.36 Naphthenic acids are likely the most prevalent acids in Chinese crude; in coal, saturated straightchain acids are dominant, and mono-aromatic acids are much more abundant than naphthenic acids. Thus, the

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Figure 7. Type distributions for sulfur-containing classes in coal acids and acidic asphaltenes from Illinois No. 6 coal. The acidic asphaltenes fraction is more aromatic and more compositionally complex.

Figure 8. Carbon number distributions for O2 species with 19 rings and double bonds for acidic asphaltenes and the pyridine extract of Illinois No. 6 coal. The carbon number distribution for acidic asphaltenes is shifted to higher mass relative to that for the pyridine extract.

composition of organic acids in coal differs qualitatively from that of crude oil. Sulfur-containing acids have low (but different) relative abundances in both coal acids and coal asphaltenes fractions. The major sulfur-containing acids classes (see Figure 7) found in the coal acids fraction are OS, O2S, and O3S, whereas acidic asphaltene has mainly O2S, O3S, and O4S. The compounds concentrated in coal acids fraction evidently have fewer heteroatoms (and simpler aromatic cores) than those concentrated in acidic asphaltenes fraction. Moreover, the DBE (rings plus double bonds or double bond equivalents) distribution shows that acidic asphaltenes extend to larger aromatic cores than do coal acids. The carbon number distribution for O2 species with 19 rings and double bonds (Figure 8) in the acidic asphaltenes fraction is shifted to higher carbon number relative to that for the pyridine extract. Similarly, the carbon number distribution for the less aromatic com(36) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Qian, K.; Robbins, W. Identification of acidic NSO compounds in crude oils of different geochemical origins by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry. Org. Geochem. 2002, 33, 743-759.

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ponents of the coal acid fraction (not shown) is wider than for compounds of the same double bond equivalents in the pyridine extract. Overall, all three extracts share similar major heteroatomic classes, but (for a given heteroatomic class and aromaticity) the combined acid fractions display wider distributions in carbon number and double bond equivalents than the pyridine extract. The less aromatic acids are enriched in the coal acid fraction, whereas the more aromatic acids are concentrated in the acidic asphaltenes fraction. For a given heteroatomic class, asphaltenes > pyridine extract > coal acids in aromaticity. Although the pyridine extract contains the widest variety of organics, additional acidic species are found in the other two extracts. In this paper, we extend our characterization of Illinois No. 6 coal by comparing a standard pyridine extract with two alternative fractions designed to concentrate acidic components: coal acids and acidic

Wu et al.

asphaltenes. The resulting detailed compositional analysis of coal acids provides detailed distributions of heteroatomic classes, aromaticity, and alkylation of coal. That kind of information establishes a fundamental basis for assessing the role of those acids in coal processing. Acknowledgment. We thank Daniel McIntosh for machining all of the custom parts required for the 9.4 T instrument constructions, John P. Quinn for the maintenance of the instrument, and Christopher L. Hendrickson for help in optimizing instrument-operating parameters. This work was supported by the NSF National High Field FT-ICR Facility (CHE-99-09502), Florida State University, and the National High Magnetic Field Laboratory in Tallahassee, FL. EF049933X