AN AMERICAN CHEMICAL SOCIETY JOURNAL VOLUME 1, NUMBER 5
SEPTEMBER/OCTOBER 1987
0 Copyright 1987 by the American Chemical Society
Articles Aliphatic Elements of Structure in Petroleum Resids Malvina Farcasiu* and Burton R. Rubin? Central Research Laboratory, Mobil Research and Development Corporation, Princeton, New Jersey 08540 Received April 9, 1987. Revised Manuscript Received June 15, 1987
The paper repork a recently developed semiquantitative method for determination of the condensed saturated rings and their partially aromatic derivatives in petroleum resids. This method involves the aromatization of the saturated rings followed by UV spectroscopic determination of the newly formed polyaromatic structures. It is found that condensed polycycloalkane structures are important in heavy petroleum fractions. The existing methods for the identification of aliphatic elements of structure are critically reviewed. The discussion addresses the inherent ambiguities of defining chemical structures for complicated mixtures of high molecular weight compounds such as petroleum resids.
Introduction A better characterization of complicated mixtures, typified by the heavy end in petroleum, should certainly help the quest for better methods and processes for upgrading and using them. There are many ways to characterize very complicated mixtures, such as petroleum fractions’ and petroleum products. One approach is to measure some specific property (viscosity, density, octane number, level of toxicity, etc.) by an established analytical method, and compare the results with an accepted “good”value for the ,particular product. I t is important that the results of the analysis be as precise and reproducible as required. On the basis of these results the product can or cannot be sold. Moreover, the same set of properties can usually be obtained from different chemical compositions. Another way to characterize the properties of the same mixtures is based on the understanding of their process behavior, the understanding of their structure-properties relationship, or both. In this case the characterization involves understanding chemical structures. There are, however, real difficulties associated with this approach. The most basic is that the “chemical structure” of a complicated mixture cannot be rigorously defined. In the case of oil fractions, it is generally more informative Present address: Albert Einstein Medical School, New York,
NY. 0887-0624/87/2501-0381$01.50/0
to identify elements of chemical structure and their relative abundance than to try to know the structure of any particular molecule in the structurally dispersed mixture. A large variety of physical and chemical techniques can be used to obtain such information, but the results will invariably contain some degree of ambiguity about their meaning. Understanding this inherent ambiguity in any particular experimental result and accepting the limitations of each individual analytical method are essential to understanding and eventually controlling the chemistry of complicated mixtures. Physical methods of investigation have been introduced and are now routinely used. They give a great deal of information about structural elements present in the components of the mixture. Nonetheless, some structural elements in these mixtures, even when present in large amounts, are simply “invisible” to the currently used physical methods of investigation. Ignoring their presence and chemical reactivity could have serious consequences for the design and control of many industrial processes. This is particularly true for petroleum high molecular weight fractions. It may be time to return to the more tedious chemical methods of characterization in order to provide additional compositional information. We have developed two (1) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1980.
0 1987 American Chemical Society
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382 Energy &Fuels, Vol. 1, No. 5, 1987 ~~
substrate
Table I. Dehvdronenation of Model ComDounds catalyst; reaction time, h PdIC; 3.5 PdIC: 24 sulfur; 3.5
03 4%
4%
a9
sulfur; 24
15%
13%
03 18% 79% 26%
12%
95 %
28%
100%
chemical methods that could give structural information about the aliphatic part of petroleum resids, information which cannot be obtained by the use of physical mehods alone. We were concerned specifically with the following elements of structure in petroleum heavy hydrocarbons: (a) the lengths of aliphatic chains substituted to aromatic moieties (determined by transalkylation reactions), on which we reported earlier;24 (b) the degree of condensation of saturated rings (determined by dehydrogenation reactions), which is discussed here. Both of the above-mentioned chemical methods give only semiquantitative results due to the complexity of mixtures and also to the fact that the yield (product of conversion and selectivity) is seldom 10070,as in most organic reactions. This yield limitation translates into numbers that are probably only an inferior limit to the actual ranges of the content of structural elements identified by us. It is then quite significant that the values we find here for the condensed polycycloalkane structures present in the mixtures are large enough to indicate an important contribution of condensed saturated rings to the structure and reactivity of heavy oil fractions. The presence of condensed saturated structures in fractions in the lower molecular weight range can be evidenced by mass spectroscopy by the examination of certain fragmentation peaks typical to steranes and hopanes (see, for example, the excellent papers of Boduszynski5). In the case of high molecular weight (>goo) mixtures, how(2) Farcasiu, M.; Forbus, T. R.; LaF’ierre, R. B. Prep.-Am. Chem. Soc., Diu.Pet. Chem. 1983,28(2), 279. ( 3 ) Farcasiu, M. Fuel Process. Technol. 1986, 14, 161. (4) Farcasiu, M.; Forbua, T. R.; Rubin, B. R. Energy Fuels 1987, I , 28. (5) (a) Boduszynski, M. M. ACS Prepr.-Am. Chem. Soc., Diu.Pet. Chem. 1985, 30, 626. (b) Boduszynski, M. M. Energy Fuels 1987,1, 2.
ever, in which molecules contain many different elements of structure, the fragmentation pattern in the mass spectra cannot be used with any degree of confidence for structure identification. Our results were secured not only by the chemical method we developed, but by a combination of it with physical methods, such as 13C NMR and UV spectroscopy, field-ionization mass spectroscopy (FIMS), and vaporphase osmometry. This way we obtained a more reliable picture of the structural features of the materials investigated. Experimental Section Gas chromatographic analyses were recorded on a HewlettPackard Model 7620A instrument equipped with a 10 ft, 10% SP-2100 chromatographic column. GC/MS measurements were performed with a Hewlett-Packard Model 7825 GC/MS instrument with a 10% OVlOl capillary column. 13CNMR spectra were recorded on a JEOL JNM-FX6OG spectrometer. Ultraviolet spectra were recorded on a Hewlett-Packard Model 8540A UV/vis spectrophotometer. The solvent used in the UV measurements was spectrophotometric grade cyclohexane (Aldrich). T h e elemental analyses were performed by Galbraith, Inc. Decalin and tetralin were purchased from Aldrich, 6-n-hexyltetralin was synthesized and 2-n-dodecyl-9,lO-dihydrophenanthrene (PS0142) was obtained from API Project 42. Saturated hydrocarbon and aromatic hydrocarbon (plus thiophene-containing compounds) were separated from furfural-extracted Arabian Light vacuum resid and Minas vacuum resid by liquid chromatography over Woelm alumina (basic alumina Super 1). The ratio alumina: sample was 5 0 1 by weight. The eluents were n-heptane and n-heptane-toluene 955 v Jv. Dehydrogenation reactions were performed by placing a mixture (2:l w/w) of hydrocarbon and either sulfur (Baker) or P d / C (MCD; minimum 9% Pd) in a thick-walled glass tube, sealing the tube, and heating the tube at 320 OC in an oven for a specified reaction time. The tube was then broken and its content dissolved in spectrophotometric grade cyclohexane. Yields for reactions
Energy &Fuels, Vol. 1, No. 5, 1987 383
Structure in Petroleum Resids 1.o
I
r
O8
Table 111. Dehydrogenation of Heavy Hydrocarbons (Sulfur, 320 “C, 24 h) % di% tri5% tetrastarting material aromatics aromatics aromatics Arabian Light Bright Stock 1. saturated hydrocarbons init 0.0 0.0 0.0 after dehydrogenation 5.0 10.0 4.8 2. aromatic hydrocarbons init 12.1 0.6 0.0 after dehydrogenation 8.2 13.8 3.0
CONCENTRATION 5.38 x I W M
0.2
Minas 1085 OF+ Resid
I
200
240
280
320
380
400
Wavelength (nm)
Figure 1. UV spectra of cyclohexane solutions of the saturated hydrocarbon fraction of an Arabian Light vacuum resid (A) initial spectrum;
(B)spectrum after dehydrogenation.
Table 11. “Typical” Extinction Coefficients extinction coeff (e), M-’cm-I ,A, = , ,x = A,, = structure (concn) 221 nm 254 nm 343 nm 0 117 000 0 diaromatics (cz) 63000 0 triaromatics ( C J 20000 57 000 49 000 tetraaromatics (c4) 79 000 with model compounds were determined by gas chromatography and the structure of reaction products by GC/MS. T h e experimental results obtained in the dehydrogenation of model compounds are given in Table I. The products from liquid chromatographic fractions from oils were analyzed by W spectroscopy (Figure 1).
Results and Discussion A. Dehydrogenation Reactions of Model Compounds. We conducted dehydrogenation reactions6 of several model compounds in the presence of sulfur and Pd/C (Table I). It can be seen that dehydrogenation with sulfur gives higher yields than dehydrogenation with Pd/C for all substrates tested. Furthermore, no disproportionation of the substrate, ring closure of the side chains, or incorporation of sulfur into the substrate was observed. As expected, the dehydrogenation by sulfur was found to occur fast for ring systems that were partially aromatized. After 24 h of sulfur-induced dehydrogenation, the decalin was 15% transformed to naphthalene, with no tetralin present. However, after only 3.5 h, tetralin was 79% converted to naphthalene and 95% of n-hexyltetralin converted to n-hexylnaphthalene. Under the same conditions, 2-n-dodecyl-9,lO-dihydrophenanthrene is fully aromatized. We conclude that the extent of aromatization of saturated hydrocarbon fractions will be lower than for the mono- and diaromatic fractions which contain hydroaromatic structures. B. Semiquantitative Method for the Calculation of the Relative Concentration of Di-, Tri-, and Tetraaromatic Structures in DehydrogenationProducts. “Typical extinction coefficients” of polyaromatics at 221, 254, and 343 nm (Table 11) were estimated from a large number of UV spectra of various di-, tri- and tetraa r ~ m a t i c s .These ~ “typical extinction coefficients” were then used to express UV absorption of dehydrogenated hydrocarbon mixtures (See, for example, Figure 1)as linear combinations of UV spectra of typical di-, tri- and tetra(6) Fu, P. P.; Harvey, P. G. Chem. Rev. 1978, 78, 317. (7) Perkampus, H. H., Sandeman, I, Eds. UVAtlas of Organic Compounds; Plenum: New York, 1967.
1. saturated hydrocarbons
init after dehydrogenation 2. aromatic hydrocarbons init after dehydrogenation (8 h)
0.0 2.8
na 5.3
0.0 2.6
0.0
7.8 5.6
0.0
2.6 3.5
aromatics at these three wavelengths. Monoaromatics were omitted from the calculation because of the overlap of absorbance in the 200-210-nm region between monoaromatics and alkyl chains present in large concentration. The average molecular weight of the mixture was considered unchanged after the dehydrogenation reaction, in agreement with the results for each model compound investigated. It could be noted that the type and degree of alkyl substitution do in fact affect the position of the maximum, particularly for tetracyclic condensed aromatics.8 Thus pyrene has a maximum at 335 nmga,1’2‘,3’4’-tetrahydro3,4-benzopyrene at 346 nmgb;and 1,2,2a,3,4,4a,5,6-octahydrocoronene at 350 nm.899c None of these shifts, however, is such as to produce an overlap with the absorption in the range of 254 nm, common to triaromatics and tetraaromatics; therefore, the calculation is not affected by them. Conversely, no substitution pattern in triaromatics shifts the adsorption maximum (“254nm”) enough to come close to the adsorption maximum of tetraaromatics (343 nm). On the basis of Ai = measured absorbance of solution of heavy hydrocarbon mixture at i nm, and cj = calculated concentration of j-aromatics in a solution of the heavy hydrocarbon mixture, the following values were calculated concentration of tetraaromatics, c4 = A343/49000; concentration of triaromatics, c3 = [A254- (c4 X 57000)]/ 63000; concentration of diaromatics, cz = [Azzl- (c3 X 20000) - ( ~ X4 79000)]/117000. The choice of ”typical extinction coefficients” (Table 11) is necessarily arbitrary, and the absolute concentrations of polyaromatics in an oil that are calculated by the above method naturally depend upon this arbitrary choice. However, the concentrations generated by the above calculations are generally valid for the purpose of comparison by the virtue of the following facts: 1. We chose “typical”extinction coefficients,rather than just averaging values for a large number of aromatics tabulated in the literature.’ While the differences between the two were not appreciable for any of the classes considered, the typical values were weighted toward (poly)cyclic systems known as being more probably produced from the natural precursors of petroleum. 2. As discussed above, small structural differences between polyaromatic systems (such as degree of alkyl sub(8)Observation made by a reviewer. (9) Friedel, R. A.; Orchin, M. Ultrauiolet Spectra of Organic Compounds; Wiley: New York, 1951: (a) No. 472;(b) No. 478; (c) No. 479. (10)Kvenvolden, K. A., Ed. Geochemistry of Organic Molecules; Benchmark Papers in Geology; Dowden, Hutchinson & Ross: Stroudsburg, PA, 1980; Vol. 52.
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1
PARAFFINS
SATURATEDHYDROCARBONS ARABIAN LIGHT VACUUM RESID
12.0 40
I 50
I 60
I 70 CAR0ON NUMBER
1 80
1 90
100
Figure 2. Hydrogen content vs. chemical structure for high molecular weight hydrocarbons: (1)CnZn+2paraffins;(2) C,HZ, mononaphthenes;(3) C,H2,-4 trinaphthenes; (4) CnH2,4 alkylalkylnaphthalenes; monoaromatics,tetranaphthenes;( 5 ) CnH2n-12 (6) CnH2,-14 alkyldiphenyls.
stitution or size of substituents) do not greatly affect, A or t values. 3. Calculations performed with extreme individual values of extinction coefficients found in tables' gave numerical results for the content of various classes of polyaromatics that did not alter the conclusions of our analysis. Nevertheless, interpretation of the UV spectra should be performed with caution, and our method is only semiquantitative (Table 111). C. Carbon Skeleton in Heavy Fractions of Oils. The main tools for investigating the chemical composition of complicated mixtures with a large number of components and no large amount of any individual constituent are elemental analysis, determination of average molecular weight and molecular weight distribution, chromatographic and spectroscopic methods, and chemical methods. We will discuss the use of some of these methods for obtaining information about the carbon skeleton of heavy hydrocarbons in oil. The elemental analysis gives the ratio between the elements in a mixture and as such is a very powerful tool that limits speculation about the possible structures actually present. The average molecular weight, together with elemental analysis, gives an average formula for the mixture or a separated fraction of it. We present in Figure 2 the hydrogen percent content as a function of the carbon number for different classes of substances. These are calculated numbers based on chemical structure and as such establish boundaries on what can be said about the chemical components present in a given mixture. Let us consider an apparently simple case: the saturated hydrocarbon fraction of a furfural-extracted Arabian Light vacuum resid. For an average molecular weight of 1000 (-70 carbon atoms/molecule) and a determined hydrogen content of 13.3%, the average molecular formula is C7JIlm, i.e. CnH2n-l,,. Even if we consider the possibility that the actual H content is, for example, 13.6% (the upper limit of the error of the elemental analysis), the average formula would be C70H132, i.e. CnH2n-s. This result is rather dramatic because it is consistent with an average molecule of saturated hydrocarbon containing five to six rings/molecule. Of course, there are molecules with less than this number, even paraffins with no rings at all, but to maintain the average, there must also be molecules containing more than six saturated rings per molecule. Field ionization mass spetroscopy (FIMS) data also show the presence of naphthenes in the same satu-
I ' " ' ~ " " I ' " ' ~ " " 1 " " ~ " " I " ' ' ~
60
40
20
0
PPM
Figure 3. Quantitative 13C NMR spectrum of the saturated hydrocarbon fraction of an Arabian Light vacuum resid (thesharp, identifiable peaks representing -30% of all carbons).
rated hydrocarbon fraction. There are some limitations in using FIMS data. One is related to the quantitative interpretation of the data. The lack of appropriate model compounds did not allow the determination of response factors for different classes of saturated hydrocarbons. Another limitation is related to the fact that FIMS identifies only molecular ions. Because of that, for example, a paraffin with the molecular formula of C70H142 and a polynaphthalene with formula C71H130cannot be distinguished by the use of FIMS. The NMR quantitative analysis of the same saturated LC fraction evidenced that about 30% of the carbon is present as long aliphatic chains. The rest of the carbons give a broad unidentifiable envelope of peaks (Figure 3). No signal or absorption over the base line was observed beyond 60 ppm downfield from Me&. Therefore, the fraction did not contain any aromatic carbons within the sensitivity of 13C NMR method. (The low-field portion of the spectrum showing only the base line, was not reproduced in Figure 3). The results described indicate already the presence of a significant number of saturated rings in the LC fraction examined. They do not indicate, however, whether those rings are isolated or condensed, or whether they are sixmembered or five-membered. Our dehydrogenation experiments, combined with the UV analysis of the products, gave answers to these questions. When the saturated hydrocarbons from a furfural-extracted Arabian Light vacuum resid were dehydrogenated (Table 111, Figure l),the UV spectra indicated the formation of about 20% of polyaromatic hydrocarbons. The 13CNMR spectrum of the same dehydrogenated fraction reveals that 18% of the carbons (an average of about 13 carbons per 1000-Damolecule) are aromatic. Thus, there are on average 13 ring carbons per molecule of saturated hydrocarbon fraction. The actual number is likely to be greater since a saturated fraction is not expected to have been fully aromatized (see model compounds results). Also it should be mentioned that the five-membered rings as such or condensed with six-membered rings are not aromaticized under the experimental conditions used! Their
Energy & Fuels, Vol. 1, No. 5, 1987 385
Structure in Petroleum Resids presence in naturally occurring steanes and hopanes is well-known.lo If we consider the aromatic fraction of the same furfural-extracted resid, the situation is even more complicated. Aromatic hydrocarbons cannot be separated from thiophenic sulfur compounds. A liquid chromatographic fraction has in this case a general average formula of C,,,Hl,&30,,e Approximately 10% of the carbon is aromatic (13CNMR data) and the majority of the molecules contain sulfur. Some possible structures based on these data are shown below. For an average molecule containing benzothiophene, the formula will be
03
C82H122
side chain CnH2n-2
Le., approximately two saturated rings should be present in the aliphatic substituents. For an average alkyl benzene
the aliphatic part should contain two to four saturated rings/molecule. Again these are the average data. Of course, the sample contains molecules with no saturated ring, with one saturated ring, and with several saturated rings, but the hydrogen content indicates an appreciable number of saturated rings overall. All these data have an important consequence for the way we can or cannot use different methods of characterization. We refer in particular to the mass spectroscopic methods, especially FIMS.l1 FIMS provides information about the molecular ions present in a sample. In general it is surmised that no fragmentation of the molecular ions occurs even for high molecular weight components of oil fractions. FIMS gives information about the so-called molecular weight profile. There is a possibility of different response factors for molecules with different structures, so the actual molecular weight distribution could be somewhat different from what we see. For a complicated mixture (such as the fraction we are discussing now) containing molecules with alkyls, saturated and aromatic rings, and/or heteroaromatic rings, FIMS cannot give useful information about the chemical structure. The number of possible components with the same molecular weight is endless, and the method cannot give any data about their possible structure. If we look at the data, we know that the fraction contains about 10% aromatic carbons (13C NMR) present as mono and diaromatic structures (UV, Table 111) and cyclic saturated structures (information provided by a combination of elemental analysis and 13C NMR) and that the molecular weight is around lo00 (VPO, FIMS). Table 111 indicates that 12% of the molecules contain diaromatic rings, which account for less than 2% of all carbons, instead of the 10% aromatic carbons evidenced by 13C NMR. The rest of the aromatic carbons existing in the other 88% of the molecules must be present as substituted benzene rings, which are not measured by our UV spectra (see Experimental Section). We checked next whether the saturated rings are isolated or condensed among themselves and with the aromatic rings. For this purpose we dehydrogenated this fraction (Table 111) and found that it aromatizes very (11) Beckey, H. D. Angew. Chem., Znt. Ed. Engl. 1969,8,623.
easily, consistent with the presence of hydroaromatic structures. After dehydrogenation, the content of polyaromatic hydrocarbons increased from 13% to -25%. The triaromatic hydrocarbons, nonexistent in the initial fraction, now represent almost 14% of the product. As we have mentioned before, it is known6that five-membered rings do not aromatize under the reaction condition. It is plausible that compounds containing analogues of steranes (I) and hopanes (11)are present as partially aro-
-
n
I
I1
matized structures in these fractions and form tri- and tetraaroamtics after aromatization. When the same methodology is applied to LC fractions of an aliphatic resid (Minas),the results are consistent with the presence of less saturated rings (Table 111). D. Limitations of t h e Dehydrogenation Method. There are several characteristics of the dehydrogation method that make it, at best, semiquantitative: 1. The conversions are different for different structures and far from quantitative for condensed saturated rings. The situation is better for hydroaromatic structures. An improvement of the dehydrogenation yield, with no reduction in selectivity, is certainly possible and should be tried. 2. Five-membered saturated rings are not aromatized under the reaction conditions.6 Data for lower boiling point fractions prove both the presence and the lower frequency of five-member-ring structures compared with six-member-ring structures.’ It is likely that the errors introduced by these still “invisible” structures are not too large. 3. Bridged condensed saturated structures such as adamantane do not aromatize under the reaction conditions used. Adamantane has been first separated from petroleuml.12 It is the most stable tricycloalkane and can be formed by isomerization from any tri~ycloalkane’~ as well as from bicycloalkanes as decalins.14 It is very reasonable to expect derivatives of adamantane and other bridged cycloalkanes in heavy ends of oils. For the moment, however, these structures remain “invisible” to our methods of investigation and we are not even able to gauge the size of our error. It is important to observe that all of the sources of errors discussed are in the same direction; Le., the content of cyclic saturated rings will be underestimated. E. Why It Is Important To Know the Structure of the Saturated Rings in Petroleum Heavy Ends. There are at least two practical reasons why knowledge about the presence and abundance of saturated cyclic structures is important: (1) for processing of heavy ends (or fractions of them, for example furfural-extracted fractions used in lube manufacture), the chemistry, shape and size of hydrocarbons in heavy ends being very much affected by the presence of saturated cyclic structures that are quite reactive under the conditions of many industrial proce~ses.’~ (12) Landa, S.; Machacek, V. Collect. Czech. Chem. Commun. 1933, 5 , 1. (13) Fort, R. C.; Schleyer, P.v. R. Chem. Reu. 1964,64,277. Whitlock, H. H., Jr.; Siefken, M. W. J. Am. Chem. SOC.1968, 90, 4929. Farcasiu, M.; Hagaman, E. W.; Wenkert, E.; Schleyer, P.v. R. Tetrahedron Lett.
1981,22, 1501. (14) Farcasiu, M., manuscript submitted for publication.
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Energy & Fuels 1987,1, 386-391
(2) for petroleum exploration to the extent that it is affected by conclusions based on biomarkers. Biomarkers are constituents of petroleum that are used as an indication of oil origin and maturation.1°J5J6 ManY of them are steranes and hopanes and their partially aromatized derivatives. The presence of the partially aromatized derivatives is considered a measure of oil maturation (supposed to be a thermal aromatization reaction taking place over “geological” time). Our results indicate the existence in the heavy fractions of petroleum of sizable amounts of species containing four or five condensed rings per molecule, of which three to four are aromatized. Such polycyclic structural elements indicate that highly substituted, heavy derivatives of steranes and hopanes can be present in residua in quantities much larger than in the fractions from which steranes and hopanes are isolated for use as biomarkers. It has not been demonstrated that light and heavy steranes and hopanes have the same degree of aromatization, and therefore the use of biomarkers separated from a narrow fraction of petroleum should be viewed with great caution. It should also be noted that the current use of biomarkers discounts, perhaps too lightly, the possibility of geocatalyzed reactions.” The presence of adamantoid (15)Ourisson, G.; Albrecht, P.; Rohmer, M. Sci. Am. 1984,251, 44. (16) Schmid,J.-C.; Connan, J.; Albrecht, P. Abstracts of Papers, 192nd National Meeting of the American Chemical Society, Anaheim, CA; American Chemical Society: Washington, DC, 1986; GEOC 46.
structures in petroleum12speaks, however, for such a cata1y~is.l~ Conclusions We began this paper by arguing that no rigorous definition is available (or could be available) for the chemical composition of complicated mixtures, and some uncertainty is always associated with the subject. It appears to us, however, that the chemistry of such mixtures can be reasonably predicted on the basis of an understanding of the predominant or important structural elements present. No single method of investigation can be successful in the elucidation of these structural elements. The combination of the chemical methods of dehydrogenation (reported here) and transalkylation2s4 with physical, particularly spectroscopical, methods has proved useful in the determination of condensed, nonbridged, six-membered saturated rings and of alkyl groups bonded to aromatic rings in fossil fuels. Acknowledgment. We are grateful to Prof. J. W. Larsen for inviting us to write this paper and to Mobil Research and Development Corp. for financial support. Registry No. Decalin, 91-17-8; tetralin, 119-64-2; 2-(n-dodecyl)-9,10-dihydrophenanthrene,55401-77-9;6-(n-hexyl)tetralin, 56598-72-2. (17) Goldstein, T. P., personal communication.
Fluorescence of Extracts of Daw Mill Coal H. B. Aigbehinmua, J. R. Darwent, and A. F. Gaines* Birkbeck College, University of London, London W C l E 7HX, England Received December 1 , 1986. Revised Manuscript Received March 23, 1987
Unlike their ultraviolet absorption spectra, fluorescence spectra of suspensions of Daw Mill coal (high-volatile bituminous A, H/C = 0.78), its pyridine extract, its supercritical toluene extract, and the solubility fractions of the supercritical toluene extract show resolved peaks in the 270-450-nm regions. The major emissions of the suspensions of Daw Mill coal were in the 370- and 440-nm regions, and further work is needed to determine the origin of this fluorescence. In contrast the fluorescence of mono-, di-, and polyaromatic materials corresponded to the molecular structures present, and accordingly the peaks present in the fluorescence of supercritical toluene extracts, asphaltenes, and benzene insolubles can be interpreted in terms of their constituent compounds.
Introduction It is well established that 7040% of the carbon atoms in coal are aromatic;’ however, no simple analytical technique has been published for defining the individual aromatic ring systems present in coal and coal liquids. U1traviolet absorption spectrometry, the obvious technique for distinguishing aromatic structures, gives uninformative results when applied to coal systems, probably because much of the light is scattered and not absorbed.2 Re(1) van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1961. (2) Gethner, J. J. Chem. SOC., Faraday Trans. 1 1986,81, 99.
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cently, however, work3 has shown that both pyridine extracts of coal and suspensions of coal in cetyltrimethylammonium bromide (CTAB) gave fluorescence spectra in which the excitation spectra showed peaks that were attributed to the aromatic systems present. Previous studies of the fluorescence of coals are summarized in ref 3. The aim of the present work was to extend our studies to investigate the possibility of using fluorescence as an analytical technique for determining aromatic ring systems (3) Clark, E. R.; Darwent, J. R.; Demirci, B.; Flunder, K.; Gaines, A. F.; Jones, A. C. Energy Fuels, following paper in this issue.
0 1987 American Chemical Society
