Energy & Fuels 1987,1, 111-120
111
One- and Two-Dimensional NMR Methods for Elucidating Structural Characteristics of Aromatic Fractions from Petroleum and Synthetic Fuels David J. Cookson and Brian E. Smith* Melbourne Research Laboratories, T h e Broken Hill Proprietary Co., Ltd., Mulgrave, Victoria 3170, Australia Received September 3, 1986. Revised Manuscript Received November 14, 1986
A range of one-dimensional and two-dimensional NMR methods have been used to elucidate the structural characteristics of a set of monoaromatic fractions separated from petroleum, coal, and shale-derived distillate fuels. Gated spin-echo (GASPE) spectral intensities yield a variety of chemically comprehensible average structure parameters. These include the number of carbon atoms per molecule, the number of side chains per molecule, the number of carbon atoms per side chain, the number of side chain branches per molecule, and the number of side chain rings per molecule. Additional structural characterization has been sought through more detailed spectral interpretation. The assignment of NMR resonances has been facilitated by the use of selective heteronuclear correlation (SHECOR) spectra, coupled and decoupled heteronuclear correlation 2D NMR spectra, the heteronuclear relayed coherence transfer (RELAY) 2D NMR method, and homonuclear correlation (COSY) 2D NMR spectra. Wherever possible, spectral assignments have been supported by a range of NMR information including CH, type ( n = 0-3), lH chemical shift, 13C chemical shift, lH-13C resonance connectivity, lH-lH resonance connectivity, and lH multiplicity. As a result, the average numbers of a-,6-, and y C H 3 groups per molecule have been calculated. Also, a range of specific structures has been identified, including methyl, ethyl, propyl, butyl, tetralin ring, indan ring, and 1-methylindan ring side chains. Overall, the information collected is sufficient to identify structural similarities and differences for the samples investigated.
Introduction It is well-known that liquid products from fossil fuel sources such as petroleum, oil shale, and coal are difficult to characterize in terms of chemical structure, primarily because they consist of large numbers of structurally diverse compounds. In the present work, attention has been focused on a set of samples that have been chromatographically separated from distillate fuels such that they consist of hydrocarbons with one aromatic ring per molecule, boiling in the range of approximately 190-340 "C. Despite these simplifying features, such samples still present a formidable analytical problem. Inspection of gas chromatograms indicates that hundreds of individual compounds are present. Component molecules may contain approximately 9-18 carbon atoms. Compounds with the same number of carbon atoms may differ in the number of hydrogen atoms or, if not, they may differ in isomeric form. Identification of individual compounds is not attempted here. Rather, the intention is to identify submolecular structures and to derive average structural information. NMR spectroscopy is widely applied to the characterization of fossil fuel products,lV2in part because it provides a potential route to quantitative chemically comprehensible information pertinent to the whole sample under consideration. Usual approaches involve subdividing a conventional NMR spectrum into regions, each of which is assumed to be associated with a single type of substructure. Although a variety of spectral subdivisions have been proposed (see, for example, ref 3-12) and although specific proposals may be questioned: it is apparent that simple application of conventional NMR spectroscopy can provide useful information about complex samples.
* Author to whom correspondence should be addressed.
Nevertheless there are obvious limitations inherent in such approaches. Since only conventional NMR spectra are recorded these procedures necessarily rely solely on chemical shift information. It is often difficult to know how to subdivide spectra in such a way that structural inferences are both detailed and valid. In the present work the objective has been to obtain structural information with minimal use of assumptions. To do this it has been necessary to employ a range of one-dimensional (1D) and two-dimensional (2D) NMR techniques in addition to conventional (1D) 'H and 13C NMR spectroscopy. In recent years, selected multiplet 13C NMR procedures have become more widely applied to fossil fuel p r o b l e m ~ . ' ~ J ~The - ~ ~GASPE selected multiplet (1) Retcofsky, H. L.; Link, T. A. Analytical Methods for Coal and Coal Products; Karr, C., Jr., Ed.; Academic: New York, 1978, Vol. 2, Ch. 24. (2) Magnetic Resonance, Introduction, Advanced Topics and A p plications t o Fossil Energy; Petrakis, L., Fraissard, J. P., Eds.; D. Riedel Dordrecht, The Netherlands, 1984. (3) Brown, J. K.; Ladner, W. R. Fuel 1960,39, 87-96. (4) Cookson, D. J.; Smith, B. E. Coal Science and Chemistry; Volborth, A., Ed.; Elsevier: Amsterdam, in press. (5) Solash, J.; Hazlett, R. N.; Hall, J. M.; Nowack, C. J. Fuel 1978,57, 621-528. . - - .- -.
(6) Bartle, K. D.; Ladner, W. R.; Martin, T. G.; Snape, C. E.; Williams, D. F. Fuel 1979,58,413-422. (7) ODonnell, D. J.; Sigle, S. 0.;Berlin, K. D.; Sturm, G. P.; Vogh, J. W. Fuel 1980,59, 166-174. (8) Takes", Y.: Watanabe, Y.: Suzuki, T.:Mitsudo. T.:Itoh, M. Fuel 1980, 59, 2g3-259. (9) Maekawa, Y.; Yoshida, T.;Yoshida, Y. Fuel 1979, 58, 864-872. (10) Snape, C. E.; Ladner, W. R.; Bartle, K. D. Anal. Chem. 1979,51, 2189-2198. (11) Awadalla, A. A.; Cookson, D. J.; Smith, B. E. Fuel 1985, 64, 1097-1107. (12) Snape, C. E.; Smith, C. A.; Bartle, K. D.; Matthews, R. S. Anal. Chem. 1982,54, 20-25. (13) Cookson, D. J.; Smith B. E. Fuel 1983,62, 34-38. (14) Cookson, D. J.; Smith B. E. Fuel 1983, 62, 39-43. (15) Cookson, D. J.; Smith B. E. Fuel 1983,62,986-988.
0887-0624/87/2501-0111$01.50/0 0 1987 American Chemical Society
Cookson and Smith
112 Energy & F u e l s , Vol. 1, No. 1, 1987
m e t h ~ d yields ' ~ ~ ~quantitative ~ abundances of CH, (n = 0-3) groups. It will be shown that for monoaromatics this information can be used to define a range of structurally explicit characteristics. In order to obtain further specific structural information, spectral interpretation is necessary. For complex samples t h i s is an inherently difficult task, and it is especially desirable to bring together a combination of different types of NMR evidence i n order to substantiate proposed interpretations. In principle, 2D NMR techniqueszkz7 and
the 1D SHECOR NMR methodzs can provide needed information. Despite the apparent potential of such methods in the fossil fuels/synfuels area,29they have received relatively little a t t e n t i o n to date.4J1Js,z*31 Of course, notwithstanding their potential, limitations can also be foreseen. For example, 2 D NMR information is not usually q u a n t i t a t i v e in nature. It is desirable to transfer the qualitative implications of 2D data to quantitative 1D spectra. In the fossil fuels/synfuels area we are only aware of one other studyzz in which CH, abundances have been used to deduce secondary structural information, we are unaware of a n y other application of coupled heteronuclear 2D NMR or 2D RELAY NMR techniques, and the COSY,31 SHECOR,4J'pz9and decoupled heteronuclear correlation NMR30,31techniques have been applied only sparingly. Since the application of these techniques in the current work is novel, it is especially important to cross-check the self-consistency of conclusions. A suite of samples has therefore been studied (seven diesel fuels and two kerosenes) in order to allow spectral and structural comparisons. Interest in the specific samples under investigation derives from a recent study showing that variations in a number of distillate fuel properties can be explained quantitatively in terms of fuel c o m p o s i t i ~ n . For ~ ~ example, for distillates of a given boiling range, the abundances of n-alkanes, branched plus cyclic saturates, and aromatics control kerosene smoke point ( A S T M D1322) and diesel aniline point (ASTM D611). It is of interest to understand the structural characteristics of these compound classes. Monoaromatic fractions are investigated here. Saturates have been studied previously.z2
Experimental Section Sample Origins. Monoaromatic fractions have been chromatographically separated from diesel and kerosene fuels in the manner described p r e v i ~ u s l y . They ~ ~ are given numerical des-
(16) Snape, C. E. Fuel 1982,61, 775-777. (17) Snape, C. E. Fuel 1982,61, 1164-1167. (18) Dalling, D. K.; Haider, G.; Pugmire, R. J.; Shabtai, J.; Hull, W. E. Fuel 1984,63, 525-529. (19) Barron, P. F.; Bendall, M. R.; Armstrong, L. G.; Atkm, A. R. Fuel 1984,63, 1276-1280. (20) Dereppe, J. M.; Moreaux, C. Fuel 1985,64, 1174-1176. (21) Gerhards, R. Magnetic Resonance. Introduction, Advanced Topics and Applications t o Fossil Energy; Petrakii, L., Fraissard, J. P., Eds.; D. Reidel: Dordrecht, The Netherlands, 1984; pp 377-407. (22) Cookson, D. J.; Smith, B. E. Anal. Chem. 1985, 57, 864-871. (23) Cookson, D. J.; Smith, B. E. Org. Magn. Reson. 1981,16,111-116. (24) Freeman, R.; Morris, G. A. Bull. Magn. Reson. 1979, 1, 5-26. (25) Freeman, R. Proc. R. SOC.London, A 1980,373, 149-178. (26) Nagayama, K. Adu. Biophys. 1981,14, 139-204. (27) Bax, A. Two-Dimensional Nuclear Magnetic Resonance in Liquids; D. Riedel: Dordrecht, The Netherlands, 1982. (28) Cookson, D. J.; Smith, B. E. J. Magn. Reson. 1983,54,354-362. (29) Cookson, D. J.; Smith, B. E. Fuel 1982,61, 1007-1013. (30) Young, D. C.; Galya, L. G. Liq. Fuels Technol. 1984,2,307-326. (31) Snape, C. E.; Ray, G. J.; Price, C. T. Fuel 1986, 65, 877-880. (32) Cookson, D. J.; Latten, J. L.; Shaw, 1. M.; Smith, B. E. Fuel 1985, 64, 509-519.
Table I. List of Monoaromatic Fractions" in source diesel/ kerosene fuel wt %
sample no.
origin
distillation range, " C
7
Diesels petroleum 232-316 petroleum 232-316 petroleum 232-316 petroleum 232-316 brown coal 232-316 oil shale 230-340 black coal 250-300
12 12 12 13 45 36 60
8 9
Kerosenes petroleum* 190-232 black coal' 190-250
12 68
1 2 3 4 5 6
Fractions from the same source fuels have been studied in ref 33 (same numbering system) and ref 22 (different numbering system). Aromatic samples 1-9 in the present work correspond to saturates fractions 1, 3, 5, 6,7, 8, 9, 10, and 11, respectively, in ref 22. *Same source as diesel sample 1. 'Same source as diesel sample 7.
ignations in Table I (1-7 for diesels, 8 and 9 for kerosenes). Samples 1-4 and sample 8 are derived from geographically disparate Australian petroleum sources as follows: samples 1 and 8, Gippsland Basin;sample 2, Cooper Basin;sample 3, Surat Bash, sample 4, North West Shelf. Samples 5-7 and sample 9 are synfuels. Their sources are as follows: sample 5, a brown coal tar (AC Char Pty Ltd.); sample 6, a shale oil (Julia Creek, Australia); samples 7 and 9, anthracene oil (Koppers Australia Pty. Ltd.). In all cases the synfuels were obtained by hydrotreating the above feedstocks, followed by distillation. NMR Measurements. All spectra were recorded at room temperature on a modified Bruker WP200 NMR spectrometer operating at 50.33 MHz for 13C and 200.13 MHz for 'H. Chemical shifts are reported relative to internal tetramethylsilane. Quantitative conventional (1D) 'H N M R experiments were carried out as described elsewhere.34 Typically, a 90" pulse flip angle (9.5 pa) and a 204 relaxation delay period were employed. Solution concentrations of
