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ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979
man-week ( 8 ) ,relative size of the fractions for subsequent analyses (for example, Aczel e t al. ( 5 ) chose to define oil as the cyclohexane rather than pentane extract in order to have a larger fraction for mass spectral and other analyses), or consistency with the concept of asphaltene and oil as they relate to the mechanism of coal liquefaction. For example, in our studies of the conversion of asphaltene to pentane extractable oil ( 9 ) , we prefer to employ the separation (definition) described here because it appears that this pentane extraction is more nearly exhaustive than the others. Thus, the separation relates more closely to the properties of the individual molecular species rather than the intractable nature of the coal-derived liquids. This is desirable if we are to understand the mechanism of conversion of asphaltene to oil a t the molecular level. In sum, the choice of the procedure and definition is in the hands of the individual researcher. This work should be taken as an effort to clarify some of the problems associated with that choice.
and Nestor Mazzocco in providing samples of coal-derived liquids from their pilot plant operations.
ACKNOWLEDGMENT We gratefully acknowledge the assistance of Sayeed Akhtar
RECEIVED for review September 25, 1978. Accepted March
LITERATURE CITED (1) M. J. Mima, H. SchuLz, and W. E. McKinstry, "Method for the Determination of Benzene Insolubles, Asphaltenes and Oils in Coal-Derived Liquids", PERC/RI-76/6, Technical Information Center, ERDA (1976). (2) F. K. Schweighardt and B. M. Thomas, Anal. Cbem., 50, 1381 (1976). (3) H. Kutta, E. H. Burk, L. H. Beckberger, and T. S. Chao, "Physical Property Improvement of Coal Liquefaction Products," EPRI Final Report AF-392 (1977). (4) I . Schwager and T. F. Yen, Fuel, 57, 100 (1978). (5) T. Aczel, R. B. Williams, R. J. Pancirov, and J. H. Karchmer, "Chemical Properties of SYNTHOIL Products and Feeds", ERDA Final Report FE 8007, Part 2 (1976). (6) S. Akhtar, N. J. Mazzocco, M. Weintraub, and P. M.Yavorsky, Enerqy -. Commun., 1, 21 (1975). (7) B.C. W r a t h , K. T. Schroeder, and F. W. Steffgen, AM/. Chem.,following paper in this issue. (8) H. Schultz and M. J. Mima, Am. Cbem. Soc., Div. Fuel Cbem., Prepr., 23121. 76 11978). (9) B. C.'Bockrath and R. P. Noceti, Fuel Process. Techno/, 2 , 143 (1979).
12, 1979.
Characterization of Asphaltenes Isolated from a Coal-Derived Liquid Bradley C. B o c k r a t h , " Karl T. Schroeder, and Fred W. Steffgen U.S. Department of Energy, Pittsburgh Energy Technology Center, 4800 Forbes Avenue, Pittsburgh, Pennsylvania 752 13
Coal-derived asphaltenes were isolated from the same liquid product in quantities from 11.8 to 21.7 % depending upon the details of the separation procedure employed. Extensive characterization of these asphaltenes was used to uncover the chemical factors that influence the values of the quantitative determinations. The different asphaltenes were compared according to their molecular weights, molecular size distributions, relative weight of basic subfractions, and relative content of heteroatoms. Also, a subfraction was isolated which is accounted to asphaltenes or oils according to the details of the separation method. Several grams of this asphatlene/oil were further separated by acidlbase fractionation and also by sequential elution by solvent chromatography. The functional group content of this fraction is roughly comparable to that of the total asphaltene fraction on a weight basis. I t s molecular weight is intermediate between those of the oils and asphaltenes.
T h e preceding paper ( I ) reported a n investigation of selected methods of solvent analysis of coal-derived products. It was found that certain variations in these methods of solvent analysis provide different amounts of asphaltenes. Thus, a precise operational definition of asphaltenes must include all of the critical parameters chosen for the analysis. This paper reports our investigation of the character of asphaltenes isolated under different conditions from the same liquid product. Reasons for the quantitative differences in the analytical separations were sought in the qualitative differences of the isolated fractions.
T h e structure of coal-derived asphaltenes has recently become the object of renewed interest (2-5). If the nature as well as the quantity of asphaltenes varies according to the isolation procedure, a rational comparison between asphaltenes or oils isolated by different analytical procedures will be difficult. An element of confusion could thus be added when comparing characterization studies from different laboratories. I t was therefore considered crucial to investigate the nature of the asphaltenes to discover how they differ according to our isolation procedures.
EXPERIMENTAL The source of the asphaltenes used in these experiments was a centrifuged liquid product (CLP), run FB44, batch 56, obtained from the 1 / 2 tpd process development unit at the Pittsburgh Energy Technology Center that had been used for SYNTHOIL research. It was derived from hvAb West Virginia coal by hydrogenation at 26.7 MPa and 450 OC over Harshaw 0402T CoMo catalyst. Molecular weights were obtained both in our laboratories and by Huffman Laboratories by vapor pressure osmometry at 37 "C using tetrahydrofuran (THF) or benzene. Our results were plotted as AR vs. concentration over the range of from 0.5 to 6.0 g/L. The plots were linear, usually with correlation coefficients of 0.99, and the intercepts were near zero. Number average molecular weights were obtained by reference to a plot constructed using benzil as a standard. When THF was used as solvent, it was found necessary to renew the reference drop with each reading to obtain reproducible results. Gel permeation chromatograms were obtained with a Waters Associates model ALC/GPC-244 chromatograph fitted with either three or four 30-cm p-Styragel columns (500 A, 100 A, 100 8, or 1000 A, 500 A, 100 A, 100 A) using THF as the solvent. A refractive index detector was used.
This article not subject to U.S. Copyright. Published 1979 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979 Toluene solubles 0 . 5 m i toluene / g solubles
Oi I
Asphaltene I 5 . 0 m i benzene / g solubles
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Table I. Asphaltene Molecular Weights slurry composition, mL tolueneig solubles yield, % mol wta 0.5 21.7 516 1.o 19.0 527 1.5 14.6 669 2.0 3 .O 5 .O a
13.3
772
12.4
782 958
11.8
Determined in THF.
-
I
I
i
I
Aspholtene / O i l 3 Asphaltene 2 Figure 1. Scheme for separation of toluene solubles into asphaltenes and oils. Asphaltenes 1 and 2 were precipitated by adding pentane until a 20-fold excess over the aromatic solvent was reached. The oil and asphaltene/oil 3 were soluble fractions Dilute solution spectra were obtained in carbon disulfide solutions using either 1-cm quartz or 0.3-cm sodium chloride cells. Asphaltenes were isolated from toluene solubles by the method previously described ( I ) , in which the ratio of solvent to toluene solubles used during the precipitations was varied from 0.5 to 5.0 as described under Results and Discussion. Three different asphaltenes were isolated from the same sample of toluene solubles by the procedure outlined in Figure 1. Asphaltene 1 was precipitated by addition of pentane to a slurry of 0.5 mL toluene per gram of toluene solubles. Asphaltene 2 was obtained by reprecipitation of asphaltene 1 from a slurry of 5.0 mL benzene per gram of asphaltene. A pentane/aromatic ratio of 20/1 was used in both cases. Benzene was used in the second case for convenience in removing solvent from the soluble portion, which we call asphaltene/oil3. After removal of benzene by freeze drying, asphaltene/oil 3 was an orange or amber solid, which became a black tar on standing at room temperature for a few hours or in a freezer overnight. The three asphaltenes were separated into acid/neutral and base components by the addition of dry hydrogen chloride to their benzene solutions according to the method of Sternberg et al. (2). The acid components were separated from the acid-neutral fraction by extraction with Claisen alkali (5.4 N KOH in aqueous methanol) according to the procedure of Husack and Golumbic (5,6). A different separation was provided by fractionating 12 g of
asphaltene/oil 3 on a silica gel column by sequential elution by solvents chromatography (SESC) as described by Farcasiu ( 7 ) . The ratio of sample to silica gel was ca. 1:20 (w/w). Total elution time was about five weeks. Routine ash determinations indicate all fractions in Table IV except 6 and 9 had negligible amounts of ash (
