42
Anal. Chem. 1983, 55, 42-45
Characterization of the Microstructure and Macrostructure of Coal-Derived Asphaltenes by Nuclear Magnetic Resonance Spectrometry and X-ray Diffraction I. Schwager,' P. A. Farmanlam* J. T. K ~ a nV., ~A. Welnberg, and T. F. Yen" School of Engineering, University of Southern California, University Park, Los Angeles, California 90007
Structural characterlratlon studles have been carrled out on coal-derlved asphaltenes from flve major demonstration processes In the Unlted States: PAMCO SRC, Catalytic Inc. SRC, HRI H-Coal, Syntholl, and FMC-COED. Elemental, molecular welght, proton nuclear magnetlc resonance, and functional group analyses have been used to calculate average molecular propertles of the asphaltenes. The macrostructure and crystallite parameters of these asphaltenes were studled by X-ray dHfractlon methods. A comblned NMR-X-ray procedure was used to estimate the size of the average aromatlc structural unit and the number of such aromatlc structural unlts per molecule.
The three direct general processes for converting coals to liquid fuels which have received the most attention in the United States are catalyzed hydrogenation, solvent refining, and staged pyrolysis (1, 2). All of these processes result in coal liquids which contain a fraction known as asphaltene in addition to coal oils. Asphaltenes are defined operationally as the benzene soluble and pentane insoluble part of coal liquid. This fraction consists of highly functionalized, highly aromatic, high molecular weight molecules of the coal-liquefaction products. The purpose of this work is to characterize coal-derived asphaltenes obtained from a wide range of coal liquefaction processes in terms of micro- and macrostructures by use of nuclear magnetic resonance, X-ray diffraction, and other analytical techniques. The study of the composition and structural character of the nitrogen- and oxygen-containing asphaltene compounds present in the liquids is important in understanding problems associated with coal liquid use such as air pollution, health hazards, refining conditions and catalysts, and precursors of useful chemicals.
EXPERIMENTAL SECTION Coal-derived asphaltenes were separated by solvent fractionation from coal liquids produced in five major demonstration liquefaction processes: Synthoil, HRI H-Coal, FMC-COED, Catalytic Inc. SRC, and PAMCO SRC ( 3 ) . Elemental analyses were carried out with standard procedures by the ELEK Microanalytical Laboratories, Torrance, CA, and Huffman Laboratories, Wheatridge, CO. Molecular weights were determined in our laboratory with a Mechrolab Model 301A vapor pressure osmometer. In normal runs, six to eight concentrations, over the range 4-39 (g/L) were employed in the solvents benzene and tetrahydrofuran for extrapolation to infinite dilution ( 4 ) . The amount of phenolic oxygen in the asphaltenes was determined by a silylation procedure. Trimethylsilyl ethers of asphaltenes were synthesized by refluxing the asphaltene with 'Present address: Filtrol Corporation Technical Center, 3250 E. Washington Blvd., Los Angeles, CA 90023. 2Present address: Ralph M. Parsons Co., 100 W. Walnut St., Pasadena, CA 91124. 3Present address: Union Oil Co. of California Research Center, 376 S. Valencia Ave., Brea, CA 92621.
excess 1,1,1,3,3,3-hexamethyldisilazane (HMDS) and catalytic amounts of trimethylchlorosilane and pyridine in benzene ( 5 ) . After removal of liquids by rotary evaporation, the silyl derivatives were freeze-dried from benzene and dried under vacuum to constant weight. The number of trimethylsilyl groups introduced was determined by proton NMR analysis (6) after the silyl derivative was checked by infrared spectrometry in dilute solution to ensure complete removal of hydroxyl groups. The fraction of pyrrole-type nitrogen was determined by a combined gravimetric-infrared method. Separation of nonbasic, pyrrole-type, nitrogen-containing asphaltenes was carried out by solvent elution chromatography on silica gel with benzene followed by further treatment with methyl iodide to remove any residual basic compounds. The methylations were carried out in benzene with a large excess (351) of methyl iodide. The reaction solutions were refluxed for 1week, and any benzene-insoluble product was removed by filtration. The benzene-soluble products were recovered by freeze drying the concentrated benzene solutions. These asphaltenes, which contain essentially no basic (pyridine-like) nitrogen, were then used to establish an infrared correlation between the absorptivity of the N-H stretch and the weight percent pyrrolic nitrogen (7). Proton NMR spectra were run on a Varian T-60 spectrometer. Chloroform-d (99.8%) with or without 1%MeaSi was used as solvent. X-ray diffraction measurements were made on finely ground powders with a General Electric XRD-6 X-ray diffractometer with a Cu K a radiation source. The X-ray techniques used to obtain the reduced intensity were those described previously (8).
RESULTS AND DISCUSSION NMR Analysis. High-resolution proton nuclear magnetic resonance spectrometry was first used by Brown and Ladner (9) for structural characterization of coal pyrolysis products. Other workers have extended this type of analysis to coal extracts (10) and coal hydrogenation products (11-17). A recently published comparison between fa values determined from I 3 C NMR spectra and those estimated from lH NMR data demonstrated that the use of the IH NMR method is reasonably reliable for coal-derived materials (18). The proton NMR spectrum of a typical coal asphaltene is shown in Figure 1. The centers of absorption for different types of protons are marked with arrows: 6 = 7.25, Ha, = aromatic protons; 6 = 2.40, H, = protons a to aromatic rings; 6 = 1.58, H N = naphthenic protons; 6 = 1.25, HR = methylenic protons; 6 = 0.9, HsMe= saturated methyl protons. Brown-Ladner analysis requires that the three areas of absorption centered at 6 = 7.3, 2.4, and 1.2 ppm be assigned to aromatic ring hydrogens (H,), aliphatic hydrogens adjacent to aromatic rings (Ha), and aliphatic hydrogens not adjacent to aromatic rings (H,). The separation point between the Ha and H, protons was chosen a t 6 = 1.73 ppm. Because hydrogen bonded phenolic OH resonances are also believed to be shifted under the aromatic envelope (19, 20), it is necessary to take into consideration the OH concentration in order to correct the H, value. The modified Brown-Ladner equations are used for calculations (21) in this paper. The average molar properties of the asphaltene microstructures, obtained from elemental analyses, molecular weight
0 1982 American Chemical Society 0003-2700/83/0355-0042$01.50/0
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983
43
Table I. Average Molar Properties of Asphaltenesa mol formula
HRI H - C d
FMC-COED
PAMCO SRC
Cat. Inc. SRC
C40.9H35.iNo.7o-
C35.6H28.8No.56
Ca6.iHa4.iNo.33
c36.4H3o.lNo.56
c35.6Ha8.Jo.48
4921.5a 0 ' 1 ° 43 36 18 0.78 0.69 0.35 6.7 1.5 27.8 5.3 0.56 0.55
Oa,,So. 382 32 40 21 0.72 0.79 0.48 7.0 1.5 18.7 3.0 0.78 0.28
o s
01.36SO.07
mol wt H* H* 01
H*
Synthoil
561 30.5 42 25 0.71 0.67 0.45 8.8 1.6 29.2 5.9 0.64 0.53
n
fa
H-ICar 0
RS n CA RA
OHlOtOtal ""atl
a Brown-Ladner method x = y OOH/H.
Is
01.47s0.1
01.39s0,02
0
502 39 38 20 0.76 0.68 0.38 7.2 1.5 27.7 5.4 0.60 0.74
486 44.3 36 16 0.79 0.67 0.34 6.5 1.4 28.3 5.7 0.75 0.58
H*, = H*&obsd) -
Extrapolated infinite dilution values measured in THF.
= 2.
z2
IO
5 008 li
006
tE 004 8
002 00 005
010
015
020
025
,
BO
70
60
* G
50
8
Flgure 1. 'H
30
20
I O
Figure 2.
L 1
035
040
045
050
X-ray diffraction pattern of Catalytic Inc. SRC asphaltene.
00
OPrn
NMR spectrum of PAMCO SRC asphaltene.
determination, and NbvlR are presented in Table I. Hydroxyl oxygen and pyrrolic nitrogen values obtained as described earlier are also included. Nonhydroxylic ether oxygen, and basic pyridine-like nitrogen may be Calculated by difference from total oxygen and nitrogen except for the FMC-COED asphaltene where IR absorption bands vvere observed at 3400 cm-I and 1650 cm-l, which may be assigned to the NH and C=O stretches of amide groups. The average coal-derived asphaltenes generally have number-average molecular weights in the 400-550 range amd are highly arolmatic species having from 71% to 79% of their carbon as aromatic carbon. The average aromatic ring s,ystems, deduced from Haru/Car values, range from about 2 to 4. These molecules are moderately substituted with 34% to 48% of the available aromatic edge carbons being substituted. Saturated substituents are small; the average number of carbons atoms per saturated substituent is between 1.4 and 1.6. The fraction of 0 as OH ranges from 0.56 to 0.78 and ithe fraction of N as pyrrolic NH from 0.28 to 0.74. These 01H/O,d values are more reliable than those reported previously (15,16) due to the more rigorous silylation procedure used and the use of proton NMR analysis (6) instead of the direct silicon analysis method (5). X-ray Diffraction Analysis. X-ray diffraction methods developed by Warren (2?2),Franklin (23,:W) and Diamond (21, 25) have been used by many workers to study the structure of coal (26-28), carbon black structures (29),small aromatic systems in noncrystalline polymers (30),petroleum asphaltenes (81, pitch fractions (31, 32) and oil shale kerogen (33). X-ray analysis was done for the asphaltenes. A typical corrected asphaltene X-ray diffraction curve, plotted on the basis of reduced intensities, is shown in Figure :2. The overlapping peaks in the low angle region, i.e. (sin O ) / h = 0.05-0.20
-
030
iSlNBl/A
(1)
encompass both the y and the (002) bands. The (002) band is generally accepted as representing the spacing between the layers of a condensed aromatic system, whereas the y band is believed to represent the packing distance of saturated structures such as aliphatic chains or condensed saturated rings. The peaks in the high angle region at (sin @ / A = 0.25 and 0.425 can be indexed as the (10) reflection and the (11) reflection, respectively. These bands correspond to the first and second nearest neighbors in ringed compounds, and the shapes of the bands have been studied theoretically (21,34). An X-ray diffraction method, previously used to investigate the structure of petroleum asphaltenes (6) was employed to determine the crystallite parameters of these coal-derived asphaltenes. The repeat distance, representing the spacing between aromatic sheets, d,, was calculated from the maximum of the (002) band by the Bragg relation
The repeat distance representing the spacing between saturated structures, d,, was calculated similarly from the maximum of the y band. The average size of the aromatic clusters perpendicular to the plane of the sheet, L,, was calculated from the width of the (002) band at half maximum by use of the Schemer crystallite size formula (35)
where BlI2is the width of the band at half maximum expressed in terms of (sin B)/h. The number of aromatic sheets associated in a stacked cluster, M , may be calculated from the values of L, and d,
M
= (L,/d,)
+1
(4)
The average layer diameter of the sheets, La,was evaluated by a procedure developed by Yen and co-workers (8) which makes use of Diamond's computed intensities of X-rays dif-
44
0
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983
Table 11. Asphaltene X-ray Cystallite Parameters asphaltene
Laa
LCb
d m C dTd
iMe
Synthoil HRI H-Coal FMC-COED PAMCOSRC Cat. Inc. SRC
10.3 8.2 8.0 10.0 9.4
12.2 13.6 10.6 12.1 10.6
3.7 3.6 3.6 3.6 3.6
4.3 4.8 3.9 4.3 3.9
5.4 4.4 4.9 5.1 5.2
Table 111. Asphaltene Structural Parameters Calculated from X-ray and NMR Data
a La = diameter of aromatic sheet t a! carbons of side L , = diamchains, from Diamond's curve (11)band, A . eter of the aromatic clusters perpendicular to the plane of the sheets, A . d, = interaromatic layer distance, A . d, = interchain or internaphthene layer distance, A e M = average number of aromatic sheets associated in a stacked cluster. x,y
Laa
Synthoil HRI H-Coal FMC-COED PAMCO SRC Cat, Inc. SRC
10.3 8.2 8.0 10.0 9.4
Figure 3. Cross-sectional view of asphaltene model: (w)represents the zigzag configuration of a saturated carbon chaln or naphthenic rlng(s); (-) represents the edge of flat sheets of condensed aromatic ring@).
fracted from randomly oriented perfect aromatic molecules of varying sizes using the Debye radial distribution function (36). Yen and co-workers tested their procedure on both the (10) and (11) bands with a model blend and found that the (11) band afforded better agreement. Therefore, we have calculated La values from the correlation curve between the width of half maximum of the (11) band and Diamond's data. The resulting X-ray crystallite parameters, presented in Table 11, may be used to derive hypothetical cross-sectional model structures for coal-derived asphaltenes (Figure 3). Condensed aromatic sheets are postulated to be stacked on top of each other with the sheets parallel and with aliphatic chains or naphthenic rings protruding from the edges. The results indicate that the average interlayer distance, d,, ranges from 3.6 to 3.7 A, the average interchain distance, d,, is between 4.4 and 5.4 A, and the average stack height of the aromatic clusters perpendicular to the plane of the sheets, L,, ranges between 10.6 and 13.6 A. The average effective number of aromatic sheets associated in a stacked cluster is between 3.9 and 4.8. The average layer diameter of the sheets generally falls between 8.0 and 10.3 A when the (11) band is used in conjunction with Diamond's curve. Combined X-ray-NMR Analysis. The average layer diameter of the sheets, La, is one of the most important structural parameters. Although La is generally considered to be the diameter of the aromatic sheet, this is only strictly correct for wholly aromatic molecules. For such systems, which are kata-condensed, La is related to the number of carbons per aromatic structural unit, CAu, by the following equation (37):
(5)
However, the aromatic systems in coal liquid fractions contain some heteroatoms in place of carbons and contain some rings which may be partially or completely saturated. Therefore, the dimensions of the sheet may be considered to include heteroatoms and a carbons of side chains which are also restricted to the plane of the sheet. In order to correct the above equation for the latter effect, it is necessary to multiply La
La*
CAUC
Nd
7.9 6.6 5.8 7.9 7.6
14.9 12.7 11.5 14.9 14.4
2.0 2.2 1.6 1.9 2.0
a La = X-ray diameter of the aromatic sheet t a! carbons of side chains, A , from Diamond's curve (11)band. La*
= L,[CA/(CA + Rs)]. C A =~aromatic carbons per N = number of structural unit = (La* + 1.23)/0.615. aromatic structural units per molecule = C A / C A ~ .
ACID / NEUTRAL
SHEET_
CA" = (La + 1.23)/0.615
asphaltenes
H
I
8
AMPHOTERIC
00
Flgure 4. Hypothetical average structures of acidheutral and amphoteric Synthoil asphaltene molecules.
+
by the factor CA/(CA Rs) which may be obtained from the preceding NMR analysis. The structural parameter CAu may be divided into the total number of aromatic carbons per molecule, CA,to obtain the number of aromatic structural units per molecule, N.
The results of such calculations are presented in Table 111. For asphaltenes the results agree with those previously deduced from NMR alone. The CAuvalues are generally close to 14 except for FMC-COED which is 11.5. These values are close to the number of aromatic carbons present in a three-ring kata system such as anthracene or phenanthrene. The number of structural units per molecule is close to two for all the asphaltenes except the FMC-COED asphaltene which has an N value of 1.6. The N values of a variety of coal-derived liquid products were determined and found to have N values close to 2. These include not only the asphaltenes but the carbenes and carboids as well (38). Recently Charlesworth (39) proposed a number of structures for coal-derived asphaltenes from Australia. His proposed structures also have N values of approximately 2. These data indicate that these coal derived molecules are ratio condensed systems. Hypothetical Average S t r u c t u r e s . By combining the structural information obtained by NMR and functional group analyses (Table I) with that obtained by the X-ray method (Table 111), it is possible to derive hypothetical average structures for asphaltene molecules. Examples of such molecules, for Synthoil asphaltene, are shown in Figure 4. A comparison of the hypothetical structures shown in Figure 4 is presented in Table IV. The results are seen to agree reasonably well in most cases.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983
Table IV. Comparison of Average Molecular Parameters for Synthoil Asphaltene experimental].~ determiined mol c41H35''0.7 formula 01.4s0.07 mol wt 561 0.31 H* ar 0.42 H*a 0.25 H* n 0.71 0.67 0.45
8.8 1.6 29.1 5.9 1.16 1.93 1.31 14.9 2.0
from hypothetical average structures amphoteric acidheutral C39H35N10280
C41H3701S0
549 0.29 0.40 0.29 0.69 0.71 0.47 9 1.5 28
559 0.30 0.43 0.27 0.68 0.66 0.47 9 1.6 28 7 1.11 2
6 1.11
2 1
1
14 2
14.5 2
From X-raya Determined from I3C NMR analysis,. NMR analysis-considering aromatic ring nitrogen as carbon.
CONCLUSION One of the useful conclusions is that the higher-molecular weight functions of coal liquid products always appear to have their aromatic portions divided into 2 parts ( N = 2). This will automatically cause their aromatic portion to be kata rather than peri in nature. By using these analytical methods and correlations, it is possible to obtain average macro- and microstructural parameters for complex mixtures such as coal liquid asphaltenes. These parameters allow construction of an hypothetical average molecule for the mixture. However, it must be emphasized that values obtained are only average values. It is unlikely that any one molecule in the mixture will actually have the hypothetical structure. But knowledge of the kinds of molecules found in these complex mixtures will aid in the understanding of the physical and chemical behavior of the coal liquefaction product.
ACKNOWLEDGMENT The authors wish to thank the FMC Corporation, Hydrocarbon Research Inc., Catalytic Inc., PAMCO, and the Pittsburgh Energy Technology Center of DOE for generously supplying samples of their coal liquid products.
45
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RECEIVED for review August 4,1982. Accepted September 29, 1982. This research was supported by the United States Department of Energy under Contract No. DE-AC-2276ET10626.
