Compositional Studies of an Asphalt and Its Molecular Distillation Fractions by Nuclear Magnetic Resonance and Infrared Spectrometry R. V. Helm and J. C. Petersen Laramie Petroleum Research Center, Bureau of Mines, U . S. Department of the Interior, Laramie, W y . Molecular distillation of a Wilmington (California) asphalt that was prepared in the laboratory gave fractions whose sulfur, oxygen, nitrogen, and aromatic contents increased with molecular weight. Paraffinic content was nearly constant with molecular weight. The increase in aromatic content with molecular weight was offset by a decrease in the naphthenic content. Although the total oxygen content increased with molecular weight, the acid oxygen decreased. A correlation between the naphthenic content and the acid oxygen was shown. Methylene chloride, as a solvent, permitted the resolution of the 1700 cm-1 carbonyl band of the asphalt and its fractions into three discrete bands at 1740,1700, and 1655 cm-l. Carboxylic acids appear responsible for the 1740 cm-1 band in the asphalt and its distillation fractions. These acids were found to be associated in the asphalt. A model naphthenic acid, cyclohexanecarboxylic acid, associated with functional groups in the asphalt.
THE COMPLEX NATURE of asphalt creates problems in the separation and characterization of asphalt components beyond those encountered in the characterization of distillate fractions from petroleum and other bitumens. Its high molecular weight, heterogeneous nature, and high viscosity require techniques for examination different in nature from those normally employed in the characterization of the lower molecular weight materials. This paper reports the molecular distillation of a Wilmington (California) asphalt and the spectroscopic and chemical evaluation of the asphalt and its distillation fractions. To reduce the chances of compositional changes, the asphalt used in this study was carefully prepared in our laboratory. The crude oil from which it was prepared was carefully protected from the time of collection through the preparation of the asphalt. The asphalt was distilled into five fractions and a residue, under conditions that reduced compositional changes to a minimum. Carbon type distribution was determined by nuclear magnetic resonance (NMR) spectra in conjunction with elemental analyses of the fractions. Methylene chloride (CH2C12)was used in infrared (IR) studies as a solvent to break up molecular association and resolve the 1700 cm-l (carbonyl) region of the IR spectra into three discrete bands. In addition, acid numbers were determined on the fractions and a relationship between the acid number, the IR absorbance at 1740 cm-l, and the naphthenic carbon ~~
Table I.
Sulfur
Nitrogen Oxygen Determined by activation analysis.
1100
ANALYTICAL CHEMISTRY
EXPERIMENTAL Preparation of Asphalt. The Wilmington (California) crude oil from which the asphalt was prepared has been described ( I ) . The original crude oil was protected from high temperatures, oxygen, and light from the time of collection through preparation of the asphalt. The asphalt was prepared by isothermal distillation ( 2 ) under vacuum to a temperature of 200 "C (500 "C at 760 Torr) in our laboratory and was 51.6 wt of the crude oil. This asphalt had a number-average molecular weight of 600 and a viscosity of 99.5 x l o 3poises at 77 OF,0.05 sec-l shear rate. Elemental analysis of this asphalt is given in Table I. Structural parameters derived from NMR data (reference 3, asphalt 4), and analyses by inverse gas-liquid chromatography and the Kleinschmidt chromatographic separation (reference 4 , asphalt 4, and reference 5 , asphalt L-4) have previously been reported in other studies of this asphalt. Molecular Distillation. Approximately 5.5 kg of the Wilmington asphalt was distilled in a Jena Glaswerke allglass molecular still similar to that described by Frank and Kutsche (6). Five fractions were collected isothermally at 25" intervals between 175 and 275 "C at Torr. Residence time in the still was approximately 15 seconds at each temperature. Nitrogen was used as an inert blanket gas, and the samples were sealed in glass ampoules. NMR Spectra. The NMR spectra were obtained using a Varian A-60 spectrometer modified t o be equivalent to the Model A-60A. All NMR spectra were obtained on 10% carbon tetrachloride (CCIJ solutions of the samples using tetramethylsilane as the internal standard. N o spectrum was obtained on the residue because it was partially isoluble in CCI,. Infrared Spectra. IR spectra, except for the differential IR spectra discussed below, were obtained with a PerkinElmer Model 21 double-beam spectrophotometer. Spectra were obtained on solutions of approximately 5 by weight of the asphalt and fractions in CCI, or CH2C12. One-tenth cm sodium chloride cells were employed using solvent compensation. Spectra were also obtained on CH2C12solutions of benzoic, cyclohexanecarboxylic, and undecanoic acids as model compounds, and a n additional spectrum was obtained on a CC1, solution of cyclohexanecarboxylic acid.
~ _ _ _ _ _
Elemental Analysis of Wilmington Asphalt
Element Carbon Hydrogen
content is reported. Carboxylic acid groups were found to associate with asphalt molecules. Some relationships are shown for other polar constituents with respect t o carbon type and nitrogen and sulfur content.
Wt
z
85.38 10.63 2.08 1.08 0.81a
(1) R. V. Helm, D. R. Latham, C. R. Ferrin, and J. S. Ball, Chern. Eng. Data Ser., 2, 95 (1957). (2) W. E. Haines, Proc. Arner. Petrol. hzst., Sect VIIZ, 42, (8), 51 ( 1962). (3) J. W. Ramsey, F. R. McDonald, and J. C. Petersen, Znd. Eng. Chern.,Prod. Res. Decelop., 6,231 (1967). (4) T. C. Davis, J. C . Petersen, and W. E. Haines, ANAL.CHEM., 38, 241 (1966). ( 5 ) T. C . Davis and J. C. Petersen, ibid., 39, 1852 (1967). (6) W. A. Frank and H. D. Kutsche, ibid., 36, 2167 (1964).
Table 11. Analyses of Wilmington Asphalt and Molecular Distillation Fractions Distillation tempperature Molecular Wt Z Elemental analyses wt % Material weightb of asphalt C H S N 100 85.38 10.63 600 Whole asphalt 2.08 1.08 31 . O 85.98 11.50 175 370 1.57 0.50 Fraction 1 200 475 10.5 85.82 11.46 Fraction 2 2.04 0.74 Fraction 3 225 535 8.4 85.14 10.99 2.20 0.88 6.2 84.95 10.84 Fraction 4 250 650 2.30 0.99 6.4 85.15 10.74 275 710 Fraction 5 2.30 1.11 1650 37.5 84.67 9.66 Residue 2.54 1.73 a At a pressure of Torr. b Vapor pressure osmometric in CCh. c Determined by activation analysis.
Cyclohexanecarboxylic acid was added to fraction 2, and spectra were obtained on a thin film and on solutions in CH2C12 and CC1,. Differential IR spectra were obtained on fraction 2 and cyclohexanecarboxylic acid. These differential spectra were obtained with a Perkin-Elmer Model 521 double-beam spectrophotometer using 0.05-cm potassium bromide matched cells. The following spectra were obtained: 1 . 6 x by weight of cyclohexanecarboxylic acid in CC14compensated with cc14 in the reference beam; 1.6% by weight of cyclohexanecarboxylic acid plus 10% by weight of fraction 2 inCC14 compensated with 10% by weight of fraction 2 in cc14 in the reference beam; 10% by weight of fraction 2 in cc14 compensated with CC14 in the reference beam; 10% by weight of fraction 2 in CC14 plus 1.6% by weight of cyclohexanecarboxylic acid in CCl, compensated with 1.6 % by weight of the acid in CCll in the reference beam. Acid Number Titrations. Methylene chloride solutions of the asphalt and fractions, cyclohexanecarboxylic acid added to fraction 2, and cyclohexanecarboxylic acid, were titrated for acid number with KOH in isopropanol following the ASTM D 664-58 procedure. Following titration the solvents were evaporated, the residues dissolved in CH2C12 and the IR spectra obtained in a manner described previously for the nontitrated samples. RESL'LTS AND DISCUSSION
In our work, the whole Wilmington asphalt was distilled for these studies so that the fractions would be more representative of the asphalt, although deasphaltening is a usual procedure before distillation. O'Donnell (7) reported the distillation and analysis of fractions from a California coastal asphalt both before and after separation of asphaltenes with isopentane. Considerably more waxy material was taken overhead when the whole asphalt was distilled. O'Donnell found that the asphaltenes separated with isopentane contained material with molecular weights lower than the molecular weights of some of the heavier distillation fractions. Description of Molecular Distillation Fractions. Molecular distillation of the Wilmington asphalt gave five distillate fractions, comprising approximately two thirds of the asphalt, and a residue. The first fraction was a fluid, amber-green material, with succeeding fractions becoming darker and more viscous. The residue was a black, hard, brittle material that became semifluid at 275 "C. Data on the whole asphalt and its fractions are given in Table 11. Composition of Samples. Elemental analyses of the molecular distillation fractions (Table 11) show the carbon-hydrogen (7) G. 0. ODonnell, ANAL.CHEM., 23, 894 (1951).
0 0
0.81 0.51 0.73 0.76 0.79 0.86 1.13
C/H 8.0 7.5 7.5 7.7 7.8 7.9 8.8
Table 111. Carbon type Distribution in Wilmington Asphalt and Molecular Distillation Fractions Per cent carbon type determined by NMR Paraffinic Naphthenic Aromatic Material 49 22 29 Whole asphalt Fraction 1 48 29 23 Fraction 2 47 28 25 Fraction 3 51 19 30 Fraction 4 54 18 28 Fraction 5 53 19 28 49 Residuea 15 36 Values by difference-sample partially insoluble in NMR solvent.
ratio to increase steadily with molecular weight, the ratio being highest for the residue, indicating an increase in aromaticity or ring condensation with increasing molecular weight. The sulfur, nitrogen, and oxygen content also increase with the molecular weight. Sulfur showed the smallest relative increase. Carbontype distribution in the original asphalt and in the distillate fractions was calculated from the integrated NMR spectra of the samples, using the method of Ramsey and co-workers (3) employing a combination of the methods of Brown and Ladner (8) and of Williams (9). Because of partial insolubility, values for the residue were calculated by difference. The compositional data on the fractions and residue (Table 111) show the per cent paraffinic carbon to be nearly constant at about 50% in all samples. A decrease in per cent naphthenic carbon with increasing molecular weight from 29% in fraction 1 to 15% in the residue is offset by an increase in the aromatic carbon content of the samples. The increase in aromatic content parallels the increase of the carbon-hydrogen ratios shown by elemental analyses (Table 11). The relationship of acid number to the oxygen content of the asphalt and its fractions is shown in Table IV. The acid number decreases with increasing fraction number (molecular weight). Assuming one carboxyl group per molecule, the asphalt and its fractions each contain 2 to 3 mol carboxylic acids as indicated in the table. Although the total oxygen content increases with fraction number, the weight per cent oxygen as titratable acids decreases. The per cent of the total (8) J. K. Brown and W. R. Ladner, Fuel, 39, 87 (1960). (9) R. B. Williams, American Society for Testing Materials, Special Technical Publication No. 224, 168 (1957). VOL. 40, NO. 7, JUNE 1968
1101
a . 1
1
I
I
I
1
I
l
l
iI
oxygen which is attributed to carboxylic acids decreases from 34% in fraction 1, to 7% in the residue. Studies of the 1700 cm-l Absorption Region. IR studies were conducted on the asphalt and its fractions in the 1700 cm-1 region in which the carbonyl stretching frequency of acids occurs. IR spectra of asphalts generally show a broad carbonyl absorption in the 1700 cm-1 region when examined as films, in KBr pellets, in Nujol mulls, or as solutions in CC14. Although carboxylic acids were shown by titration to be present in the asphalt and its fractions, their presence could not be distinguished from other carbonyl absorption in this region. The broad 1700 cm-I. band and the absence of a free acid band at about 1740 cm-1 suggest that the acids present are associated, for instance hydrogen bonded. Methylene chloride has been shown to reduce the intramolecular hydrogen bonding in amides of nicotinic acid (IO), and therefore it was chosen as the spectral solvent in our work in an attempt to reduce molecular association in the asphalt samples and further resolve the broad carbonyl absorption band. Figure 1 compares the IR spectra of Wilmington asphalt dissolved in CC14 and CH2C12. Curve C, using CC14 as the solvent, shows only the broad carbonyl band at about 1700 cm-l. Curve A in the figure illustrates the resolution (10) W. Barbari and L. Bernardi, Tetrahedron, 21,(9), 2453 (1965).
obtained when the asphalt is dissolved in CH2Cl2. Bands at about 1740, 1695, and a shoulder at about 1655 cm-1 are observed. This demonstrates the ability of CH2C12to break up molecular association-possible hydrogen bondingwithin the asphalt. Spectra in CH2C12of the various fractions from the distillation also showed these three regions clearly defined. With a technique available for resolving the three bands in the 1700 cm-l region, studies were made to determine which of these bands were related to the carboxylic acids present. The samples from the determination of acjd numbers were examined by IR in CHzClzsolutions. A typical IR spectrum of one of these is shown in Figure 1, curve B. Titration removed the 1740 cm-l band, increased the intensity of the 1695 cm-l band, and left the 1655 cm-' band essentially unchanged. The increase in intensity and the broadening of the low frequency side of the 1600 cm-1 region in the titrated sample is typical of alkali metal carboxylates which are formed on titrating the acids. Fraction 1 from the distillation showed two bands after titration, one at about 1600 and the other at about 1570 cm-l. The latter is again attributed to the formation of alkali metal carboxylates on titration. Thus, the 1740 cm-l band in Wilmington asphalt appears to be caused entirely by carboxylic acids. The absence of the 1740 cm-l band in the solvent free asphalt shows that the carboxyl groups are associated. Model Acid Studies. Because the data in Tables I11 and IV suggested a relationship between the naphthenic carbon content and the acid number, a study using aromatic, naphthenic, and aliphatic acids was undertaken to help elucidate the nature of the acids in the Wilmington asphalt. Infrared spectra of solutions of benzoic, cyclohexanecarboxylic, and undecanoic acids in CHzClzgave absorption bands for the free acids at 1730, 1740, and 1750 cm-1, respectively. Dimer bands also occurred at frequencies about 35 cm-l lower than the free monomer bands. The correspondence between the absorption of cyclohexanecarboxylic acid and the acid band at 1740 cm-I in the asphalt and its fractions suggests that the acids in the asphalt are naphthenic in character. Acid Interactions. To provide insight into the asphalt carboxyl group interactions in the asphalt, the interactions of the model acid, cyclohexanecarboxylic acid, with itself and with fraction 2 were studied by IR. Fraction 2 was chosen because of its high acid number. Curves A and B of Figure 2 illustrate the greater ability of CHzClzover CCL to break up the relatively strong hydrogen bonding of the cyclohexanecarboxylic acid dimer. This is evidenced by the larger free acid band at 1740 cm-l and the smaller dimer band at about 1700 cm-l in the CHzClzsolution. When the acid was added to a CH2C12solution of fraction 2, both the free and dimer bands of the model acid are still evident as shown by rein-
Table IV. Relationship of Acid Number to Oxygen Content of Wilmington Asphalt and Molecular Distillation Fractions Whole Fraction Material Asphalt 1 2 3 4 5 Residue Acid number" 2.47 3.08 3.41 3.07 2.60 1.84 1.34 Mole per cent acidsb 2.6 2.0 2.9 2.9 3.0 2.3 4.0 Weight per cent total oxygen
0.81
Per cent of total oxygen as acids 0
b
17
0.51
34
ASTM Method D 664-58 in CHtCh solution. Calculated on the basis of one acid group/molecde.
1102
ANALYTICAL CHEMISTRY
0.73
27
0.76
23
0.79
19
0.86
14
1.13
7
Gl
4 m
'
0.1
-
0.2
-
0.3
-
0.4
-
D
""I 0.5
-.-
1,800
1,700
1,600
1,800
1,700
1,600
F R E Q U E N C Y , cm-'
Figure 3. Differential IR spectra in CCl,
z
t
05I
l
l
1,800 1,700 1,600 FREQUENCY, cm-I
I
l
1,800 1,700
l
1,600
Figure 2. 1700 cm-1 Region of cyclohexanecarboxylic acid in different systems Curve A , 0.5 g/l cyclohexanecarboxylic acid in CHZCI~;Curve B, 0.5 g/1 cyclohexanecarboxylic acid in CCId; Curve C, fraction 2 plus 0.8 wt cyclohexanecarboxylic acid, thin film; Curve D, 63.8 g/l fraction 2 in CHZCI~;Curve E , fraction 2 plus 0.8 wt cyclohexanecarboxylic acid, 62.4 g/1 in CHzClt; Curve F, fraction 2 plus 0.8 wt cyclohexanecarboxylicacid, 62.4 g/l in CCla
forcement of the bands in fraction 2 at about 1740 cm-l and 1700 cm-I, respectively (compare curves D and E ) . However, when the acid was added to a cc14 solution of fraction 2 (curve F ) only a weak shoulder is evident at 1740 cm-l on the steep slope of the 1700 cm-I band. The resolution of this shoulder appears almost the same in the mixture of the acid and fraction as it does in a CC14 solution of the fraction only (compare with curve C, Figure 3), thus suggesting a negligible contribution from the free acid band of the cyclohexanecarboxylic acid. The question arises as to whether the inability to see a contribution from the free acid band of cyclohexanecarboxylic acid in curve F, Figure 2, is because it is superimposed on the steep slope of the 1700 cm-1 band or because the acid associates with the asphalt fraction or further dimerizes with itself. The spectra shown in Figure 3 indicate that there is considerable interaction between the carboxylic acid and the asphalt. This is demonstrated by curves A and B. Curve A is a differential spectrum of the acid in CCld with CCld compensation. Curve B is a differential spectrum of the acid in the presence of 10 wt Z of fraction 2 with both solvent and asphalt compensation. Thus, the changes in curve B, com-
Curve A , 1.6 wt cyclohexanecarboxylic acid-solvent compensated; Curve B, 1.6 wt cyclohexanecarboxylic acid plus 10 wt fraction 2-solvent and fraction 2 compensated; Curve C,10 wt fraction 2-solvent compensated: Curve D,fraction 2 plus 1.6 wt cyclohexanecarboxylic acid-solvent and acid compensated
z
z z
pared with curve A , result from interactions of the cyclohexanecarboxylic acid with the asphalt. Note the loss of both free acid and acid dimer bands in curve B, and a broadening of the band at about 1700 cm-l because of the acid-asphalt interaction. Curves C and D in Figure 3 further support the spectra shown by curves A and B. Curve C is a differential spectrum of fraction 2 in ccl4 with solvent compensation; curve D shows the addition of cyclohexanecarboxylic acid to a solution of fraction 2 with both solvent and acid compensation. The loss of both free acid and dimer bands, observed in curve B, is seen in curve D as absorption minimums at 1740 cm-I and 1700 cm-l, respectively. These IR studies show that carboxylic acid groups associate with asphalt molecules in CC14 solution. In the absence of solvent, the cyclohexanecarboxylic acid would be expected to associate with the asphalt. Therefore, this association should contribute significantly to the absence of a free acid band in the film spectrum of the acid-asphalt fraction mixture shown in curve C, Figure 2. RECEIVED for review January 29, 1968. Accepted March 19, 1968. Presented at the Division of Petroleum Chemistry, 155th Meeting, ACS, San Francisco, Calif., March-April 1968. Reference to specific brand names is made for identification only and does not imply endorsement by the Bureau of Mines. Work done under a cooperative agreement between the Bureau of Mines, U S . Department of the Interior, and the University of Wyoming.
VOL. 40, NO. 7, JUNE 1968
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