Identification of Alkylbenzo[b]thiophenes in a Wasson, Texas, Crude Oil Distillate Boiling 200 to 250 “C R. L. Hopkins, H . J. Coleman, C. J. Thompson, and H. T. Rall’ Bartlesville Petroleum Research Center, Bureau of Mines, U.S. Department of the Interior, Bartlesville, Okla. 74003 By a combination of chemical, chromatographic, and spectroscopic methods, 22 benzo[b]thiophenes were identified in Wasson, Texas, crude oil. Five compounds comprise 82% by weight of the total benzothiophenes identified. A marked order of preference for the position of the alkyl substituents on the benzothiophene ring was noted: 2 > 3 > 7 2 4 > 6. Although no alkyl substitution in the 5-position has been found such substitution could be present in small amounts. Knowledge of the benzothiophene content of crude oils i s of practical value to the petroleum scientist, not only in the areas of production, rocessing, and storage, but also to aid in fundamentarstudies related to the origin and composition of petroleum.
ALTHOUGHBENZO[b]THIOPHENES ConstitUte a major Class of sulfur compounds in petroleum, only in recent years have individual members of this family been identified. Richter et al. (1) isolated and identified the parent compound, benzo[b]thiophene, in Santa Maria Valley, California, crude oil. More recently, Coleman et al. (2) identified benzo[b]thiophene and its 2- and 3-methyl homologs in Wasson, Texas, crude oil. Other members of the series could not be identified as readily because of the inability of gas-liquid chromatography (GLC) to separate all benzothiophene isomers from each other and from associated naphthalenes. Additional techniques were needed for positive identifications. Two techniques were particularly useful : desulfurization by Raney nickel and reduction by calcium in liquid ammonia to benzenethiols. Benzo[b]thiophenes are desulfurized readily and almost quantitatively with Raney nickel in boiling or near-boiling methanol to alkylbenzenes having the same carbon skeletal structure. The alkylbenzenes are then separated by GLC and identified by infrared (IR) or ultraviolet and mass spectroscopy. Some alkylbenzenes may be produced from two benzo[b]thiophene precursors in the C gand CIOrange or from as many as four in the C,, series. While a complete list of alkylbenzenes with all possible benzo[b]thiophene precursors is too long for inclusion in this report, some examples are cited. n-Butylbenzene has only one possible benzo[b]thiophene precursor, 2-ethylbenzo[b]thiophene. sec-Butylbenzene may originate from 3-ethylbenzo[b]thiopheneor from 2,3-dimethylbenzo[b]thiophene. Fortunately, these two benzothiophenes are separable by GLC, and by that means the presence of both isomers was established with the 2,3dimethyl isomer predominating.
Reduction with calcium metal in liquid ammonia was especially useful in distinguishing between certain isomeric benzo[b]thiophenes which produce the same alkylbenzene by desulfurization. By this method it was possible to distinguish between 5-nlethyl- and 7-methylbenzo[b]thiophenes both of which yield 1-ethyl-3-methylbenzene by desulfurization but yield different benzenethiols by reduction :
C
c C
‘Q
C C a i n ”3)
‘0-c-c -SH
Cain N H 3
I
I
C
C
In like manner 3,7-dimethylbenzo[b]thiophenewas found in substantial amount; whereas the 3,5-isomer could not be detected. Reaction with mercuric acetate has been used by other investigators (3) to derivatize and identify benzothiophenes. Our experiments with pure compounds show that only those benzothiophenes unsubstituted on the thiophene ring react readily with mercuric acetate. Those compounds having only a 2-substituent will react, but much less readily. Because of the limited amount of sample available only one application of this method was used in this investigation, but the results added confirmatory evidence for several identifications. Theoretically, some of the alkylbenzenes produced by Raney nickel desulfurization could arise from benzo[c]thiophenes (S@
I; however, the presence of benzo[c]-
Retired, Bartlesville Petroleum Research Center, Bureau of Mines, U. S. Dept. of the Interior, Bartlesville, Okla.
thiophenes is unlikely because of their chemical instability. The absence of hydrogen sulfide in the calcium-ammonia reduction products is further evidence of the absence of benzo[clthiophenes in the fractions under study. In a continuation of the studies of the sulfur compounds in
(1) F. P. Richter, A. L. Williams, and S. L. Meisel, J. Amer. Chem. SOC.,78,2166 (1956). (2) H. J. Coleman, C. 3. Thompson, R. L. Hopkins, N. G. Foster, M. L. Whisman, and D. M. Richardson, J. Chem. Eng. Data, 6, 464 (1961).
(3) R. W. King, F. A. Fabrizio, and A. R. Donnell, “Gas Chro-
matography, Third International Symposium,” Academic Press, Inc., New York, N. Y., 1961, pp 149-62.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
0
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Table I. Benzo[b]thiophenes Identified in Wasson Crude Oil
fl
1 l 66 0I -l
No. Benzo[b]thiophenes 1. Benzo[b]thiophene 2.
3. 4.
A d s a r b e n l - H - 4 1 a l u m i n a i i l u i d grade) Column diameter - 2 inches
5.
6. 7. 8. 9. 10.
11. 12. 13.
14. 15. 16.
desorplion
Begin
benzene-pentare elution
17. I
10
20
30
I
40 50 60 WEIGHT, pel cent
I
I
!
7C
83
90
18. 100
19. Figure 1. Adsorptogram of aromatic fraction of Wasson 200-250 "C distillate
20. 21. 22.
Wasson, Texas, crude oil, the identification of 22 benzo[b]thiophenes, including the three previously identified, are reported here.
3-Methyl2-Methyl4-Methyl7-Methyl2,3-Dimethyl3-Ethyl3,7-Dimethyl-;possibly 3,53,6-Dimethyl2,7-Dimethyl-;possibly 2,52-Ethyl2,6-Dimethyl2,4-Dimethyl4,7-Dimethyl2-Methyl-3-ethyl3-Ethyl-5-methyl- and/or 3-ethyl-7-methyl- and/or 2,3,5-trimethyl-and/or 2,3,7-trimethyl2,3,6-trimethyl- and/or 3-ethyl-6-methyl2-Methyl-5-ethyl- and/or 2-methyl-7-ethyl- and/or 5-propyl- and/or 7-propyl2-Methyl-4-ethyl- and/or &propyl2,4,7-Trimethyl2-Ethyl-5-methyl- and/or 2-ethyl-7-methyl2-Ethyl-4-methylz; 11-22 Total
z
Wt in crude
Wt of benzothiophenes
...
...
z
0.00095 0.00248
...
1 :::::;:
9.2 24.0 ..,
1.0
0.00009 0.00101 0.00037 0.00013 0.00094 0.00004 0.00006
29.0 1.9 0.9 9.8 3.6 1.2 9.1 0.4 0.6
0.00038
3.7
0.00051 0.01032
100.0
0.00020
5.0
EXPERIMENTAL
Reference Samples. 3-Methyl-, 3-ethyl-, 2,3-dimethyl-, 3,5-dimethyl-, and 3,7-dimethylbenzo[b]thiopheneswere synthesized by dehydrocyclization of appropriate arylketosulfides with polyphosphoric acid at 120 "C. The arylketosulfides were prepared by coupling 2-chloroketones with sodium aryl mercaptides. 7-Methyl-, 5-methyl-, 5-ethyl-, and a mixture of 4- and 6-methylbenzo[b]thiophenes were prepared in like manner from the aryl mercaptodiethylacetals. All 10 dimethylnaphthalenes were obtained from commercial sources. Six Cl0 alkylbenzenes and 19 C11 alkylbenzenes were synthesized. Most of these compounds were made by hydrodeoxygenation of the appropriate phenone or arylalkyl carbinol over copper chromite catalyst at 200250 "C. Other alkylbenzenes (approximately 25) were commercial or API standard samples. Equipment. Two Perkin-Elmer vapor fractometers, Model 854, were used with a variety of columns and substrates. Dow Corning 550 silicone oil and Reoplex 400 (polypropylene glycol adipate) were the most generally useful substrates, but Ucon LB-550X was especially useful for analyzing the alkylbenzenes from Raney nickel desulfurization. A Perkin-Elmer Model 21, double-beam, infrared spectrophotometer was used for all IR spectral measurements. A modified Consolidated Model 21-102C mass spectrometer was used for mass spectral determinations. Preparation of Sample. The benzothiophenes were concentrated from a 200-250 "C distillate by adsorption on silica gel followed by graded elution from alumina to remove the aromatic-polar portion. An adsorptogram of this second step is shown in Figure 1. Alumina adsorption fractions 19-26 were subjected to a series of precipitations at -78 OC with anhydrous HI to remove sulfides ( 4 ) . The residue from these treatments was reacted with 1,3,5-trinitrobenzene to 2042
e
adduct naphthalenes and benzothiophenes which were recovered from the adduct by adsorption on alumina. The product, consisting of naphthalenes, benzothiophenes, and less than 1% of biphenyls, as determined by mass spectrometry, was distilled into 10 fractions and a residue using a 50plate spinning band column. The processing steps are diagramed in Figure 2. DISCUSSION AND RESULTS
Nineteen benzo[b]thiophenes, in addition to the three previously reported (2), were identified in the 10 distillation fractions and residue of the 200-250 "C concentrate. These compounds are listed in Table I. Five of these identifications are ambiguous in that the identified alkylbenzene could be derived from two or more isomeric benzothiophenes. Because of this ambiguity, the identifications are listed as and/or and the member believed to be present in greatest amount on the basis of observed structural trends is indicated by boldface type. The carbon-number distribution of the benzothiophenes in the distillate fractions is approximately as follows : fractions 1-7, C9HBS;fractions 8-10, CloWloS; residue, CIOHIOS CiiHizS. Fractions 1-7. Figure 3 shows gas-liquid chromatograms of distillation fractions 1 and 6 obtained with a ' 1 4 in.-0.d. by 25-ft stainless steel column of Reoplex 400 on 30-42 mesh
+
(4) R. L. Hopkins, H. J. Coleman, C. J. Thompson, and H. T. Rall, Bureau of Mines Rept. of Inv. 6458, Bartlesville, Okla., 1964.
ANALYTICAL CHEMISTRY, VOL. 41,NO. 14, DECEMBER 1969
H I EXTRACTION
TREAT WITH TRINITROBENZENE
Filtrate
H I EXTRACTION
Nofe
I
I
Dashed-line construction i n chart indicates individual processing of fractions 21, 22 t 23, 24, and 25,
I
TRINITROBENZENE TREAT WITH
c "A"
I
REGENERATE A N D DISTILL Fraction
1
Fraction
"A" (Further
"8"
treatnent of sample at these points not pertinent t o present r e p a t .
Residue
2
I
CRYSTALLIZE
WHIRLING BAND DISTILLATION (3 mm) Fraction
Residue
1
t
2 3 4 5 6 7 8 9 1 0
Benzolblt hiophene
Figure 2. Treatment of Wasson 200-250 "C adsorption fractions
Supersupport GC-22 (20:lOO loading) at 180 "C and 70 ml/ min helium flow. 2-Methyl- and 3-methylbenzo[b]thiophenes (reference points C and F ) , identified previously (2), appear prominently in these chromatograms. However, an examination of retention time data for 4-, 5-, 6-, and 7-methylbenzo[blthiophenes indicates that if present they would not be resolved sufficiently for direct observation. Fraction 1 was therefore examined by both Raney nickel desulfurization and reduction by calcium in liquid ammonia. Approximately 0.1 gram of sample was added to about 1 gram of activated Raney nickel in 2-3 ml of methanol in a test tube. The contents of the tube were mixed thoroughly, stoppered, and placed in a water bath at 60-63 "C for about 30 minutes. The liquid was decanted, rinsed with methanol, and centrifuged three times; then 0.5 ml of pentane was added. Water was then added, and the tube was again centrifuged. The pentane layer was removed with a dropper, most of the pentane evaporated and the resulting product injected into the chromatograph. The bottom panel of Figure 4 shows the chromatogram of the desulfurization products of fraction 7. The GLC peaks for ethylbenzene, isopropylbenzene, and n-propylbenzene are the predominant features of this chart, but inflections at 46
minutes and 57 minutes indicate the presence of small amounts of 1-ethyl-3-methyl- and/or 1-ethyl-4-methylbenzene and of 1-ethyl-2-methylbenzene and/or sec-butylbenzene, respectively. Chromatographing on a column of Ucon LB-550-X established that the peak at 57 minutes results primarily from 1-ethyl-2-methylbenzene derived from 4methylbenzo[b]thiophene. Similarly, the peak at 46 minutes results from 1-ethyl-3-methylbenzene whose source could be either §-methyl- or 7-methylbenzo[b]thiophene. Calcium in liquid ammonia reduces benzo[b]thiophenes to substituted benzenethiols without cleaving or rearranging the carbon skeletal structure (5-7). This reaction was employed to differentiate between 5- and 7-methylbenzo[b]thiophenes because the resulting thiols are separable by GLC. About 0.5 gram of sample was added to an excess of calcium dissolved in about 5 ml of liquid ammonia, and about § ml of ethyl ether was added. In a few minutes, reduction was complete then (5) H. Boer and Ph. M. Duinker, Rec. Truu. Chim., 77, 346 (1958). (6) . , J. Van Schooten, J. Knotnerus. H. Boer, and Ph. M. Duinker, ibid., 77,935 (1958). (7) , , J. Knotnerus, Ph. M. Duinker, and J. Van Schooten, J . Inst. Petrol., 47, 317 (1961).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
c
2043
6R
50 TIME, minutes
60
40
I
170
160
150
140
I30
methanol was added dropwise to discharge the blue color of excess calcium. The ammonia was alloyed to boil off (exposure to air must be avoided to prevent oxidation of the thiols to disulfides), and the product was acidified with hydrochloric acid. The acidified product was extracted with pentane and the pentane layer was extracted with 20 % NaOH to separate the benzenethiols from hydrocarbons. The extract was acidified with HCl and again extracted with pentane. The mixtures of benzenethiols produced in this manner froin fractions 1, 4, and 7 were chromatographed on a in. by 6-ft. polypropylene glycol column at 155 "C and 50 ml/min of helium sweep gas to produce the chromatograms shown in Figure 5 . Reference retention times of the benzenethiols were obtained from the product of the treatment of pure benzo[b]thiophenes of authentic structure. Evidently Peak 3 in fraction 1 was derived from 7-methylbenzo[b]thiophene. Th's conclusion is confirmed by comparison of the infrared spectra, Figure 6, of material trapped from this peak with that of 2-ethyl-6-methylbenzenethiol produced by reduction of 7-methylbenzol[b].hiophene. The preceding data do not necessarily exclude the presence of traces of 5-methylbenzo[b]thiophene because the occurrence of a small amount of Z-ethyl-5-methylbenzenethio1, derived from it, would be obscured by the 2-n-propylbenzene-
34
I
I
120
Paq! ing V l s c o L-699" on 30-40 "Chromosorb" Temperature Column 1/4 inch by 15 f e e t stainless s i e e l
9 Q 0G C a r r i e r posHelium, 60ml/min
Fraction 7
I
2044
e
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14,DECEMBER 1969
I
I
Fraction 7
I
1
i)
1
Table 11. Products of Desulfurization and Benzo[b]thiophene Precursors in Fractions 8-10 Alkylbenzene Identified by Precursor 1-Ethyl-2-methylGLC 4-Methylsee-ButylGLC, UV 2,3-Dimethyl-and 3-ethyP I-Methyl-3-isopropyl- GLC, UV, IR 3,s- and/or 3,7-dimethyl-* 1-Methyl-4-isopropyl- GLC, IR 3,6-Dimethyl1-Methyl-3-propylGLC, IR 23- and/or 2,7-dimethyLb 1- methyl-2-propylGLC, IR 2,4-Dimethyl1,4-Dimethyl-2-ethyl- GLC 4,7-Dimethyla Both 2,3-dimethyl- and 3-ethylbenzo[b]thiopheneswere identified by selective GLC trapping followed by desulfurization by Raney nickel. 2,3-Dimethylbenzo[b]thiophene predominates. b Reduction by Ca in NHs shows that 3,7- and 2,7-dimethylbenzo[b]thiophenes predominate over the 3,5- and 2,5-isomers.
Fraction I
i 80
75
70
65
55 50 TIME, minutes
60
45
40
35
30
Figure 5. Gas-liquid chromatograms of benzenethiols resulting from calcium reduction of distillation fractions 1,4, and 7
thiol peak (2). If present, however, the amount is much less than that of 7-methylbenzo[blthiophene. Fractions 8-10. The Cla benzo[b]thiophenes and CE naphthalenes appear in substantial quantities in distillation fraction 8 (Figure 7) and constitute most of fractions 9 and 10. The fractions are too complex to permit direct isolation and identification of any compounds by GLC alone; consequently, the identifications are based on Raney nickel desulfurization, mercuric acetate reaction, and calcium-ammonia reduction. Figure 4 shows chromatograms of the alkylbenzenes from the desulfurization of fractions 7, 8, 9, 10, and residue. The indicated retention times were obtained from reference hydrocarbons and do not necessarily imply identification of these hydrocarbons in the desulfurized products. The identifications of individual hydrocarbons were made by GLC trapping of appropriate portions of each sample followed by mass, infrared, and ultraviolet spectroscopic analyses of the trapped material. The identified hydrocarbons, the benzo[blthiophene precursor, and the method of identification are listed in Table 11.
The calcium-ammonia reduction to benzenethiols was useful in resolving two ambiguous situations which involve either 5 - or 7-substitution. 3,7-Dimethylbenzo[blthiophene was the principal precursor of l-methyl-3-isopropylbenzene, but the possible presence of the 3,Sisomer was undetermined because the thiol derivative was insufficiently resolved from 2-sec-butylbenzenethiol. These results are illustrated in Figure 8, which shows chromatograms of the thiols produced from fractions 8, 10, and residue. The retention times indicated were established from the reduction products of reference benzothiophenes. There is also evidence that the 2,7-dimethylbenzo[blthiophene is more abundant than the 2,5-isomer. The region of the charts at about 71 minutes where the derivative of 2,5-dimethylbenzo[b]thiophene should be visible is in a valley; therefore little, if any, of this compound was present. The 2,7-isomer at 73 minutes appeared too near the 2-butylbenzenethiol peak (75 minutes) for good resolution but was apparently fairly well resolved in the residue fraction, which contains little 2-butylbenzenethiol, Reaction with mercuric acetate was tried as a means of isolating certain benzothiophenes selectively. A 0.2-ml
I I
P L L - 1 - -
130
WAVE L E N G T HI microns
Figure 6. Comparison of infrared spectrum of reference thiol and spectrum of thiol produced from a benzothiophene isolated from crude oil
120
ll0
I
100 90 T I M E , minu'es
Sesidue
L I t
80
J
~
I
, j ,
1
70
60
50
Figure 7. Gas-liquid chromatograms of fractions 8, 9, 10, and residue on Reoplex 400 column, 180 "C, 70 ml per min helium flow
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
*
2045
Table 111. Products of Desulfurization and Possible Benzo[b]thiophene Precursors in Residue for C1l Benzo[b]thiophenes Alkylbenzene Precursor 1-Methyl-3-sec-butyl2,3,5-Trimethyl- and/or 2,3,%trimethyl-and/or 3-ethyl-5-methyl- and/or 3-ethyl-7-methyl1-Methyl-4-sec-butyl2,3,6-Trimethyl- and/or 3-ethyl-6-methyl1-Ethyl-3-propyl2-Methyl-5-ethyl- and/or 2-methyl-7-ethyl- and/or 5-propyl- and/or 7-propyl1-Ethyl-2-propyl2-Methyl-4-ethyl- and/or Cpropyl2-Ethyl-5-methyl- and/or 1-Methyl-3-butyl2-ethyl-7-methyl1-Methyl-2-butyl2-Ethyl-4-methyl3-Phenylpentane2-Methyl-3-ethyl1,4-Dimethyl-2-propyl2,4,7-Trimethyl-
1
90
80
70 60 TIME, m i l u t e s
50
40
30
Figure 8. Chromatograms of the products of reduction [by Ca(NHs)el of fractions 8, 10, and residue from a Wasson 200-250 "C distillate
sample of fraction 9 was treated and a minute amount of the recovered crystalline material was regenerated with H2S and desulfurized. The product was chromatographed by GLC using an argon ionization detector, and one principal peak resulted. However, because the peak occurred in a region where several alkylbenzenes overlap it could not be identified by GLC alone, and the sample was too small for spectroscopic identification. The mercuric acetate did concentrate 4,7dimethylbenzo[b]thiophene as shown by an enhanced 1,4dimethyl-2-ethylbenzene peak in the GLC chromatogram thereby moving its identification from probable to positive. Residue. The principal features of the residue fraction were the increase in the concentration of 2,3-dimethylbenzo[blthiophene and the appearance of significant amounts of C1l benzothiophenes. As with previous fractions, desulfurization by Raney nickel was the principal method used in investigating the residue. GLC alone was inadequate for identifying the Cll alkylbenzenes, so trapping of individual peaks or areas for I R analysis was used. Effluents were collected in a special needle (8) and chilled with dry ice to -78 "Cand the samples were transferred by centrifugation to a n infrared microcell for analysis. When necessary, repetitive runs were made to assure enough material in each trap for analysis. The alkylbenzenes derived from the residue and their benzo .hiophene precursors are listed in Table 111. CONCLUSIONS
Two compounds, 2-methyl- and 2,3-dimethylbenzo[b]thiophenes comprise almost 54 % of the benzothiophenes in the 200-250 "C boiling range distillate. Three other compounds, 3-methyl-, 2,7-dimethyl-, and 2,4-dimethylbenzo[b]thiophenes comprise another 28 %. An interesting parallel exists between the structures of the benzothiophenes described in this report and those of the 1(8) R. F. Kendall, Appl. Spectrosc., 21, 31 (1967).
2046
thiaindans (2,3-dihydrobenzo[b]thiophenes) identified in the same distillate fraction (9). Most of the ring structures present in one exist also in the other and in much the same order of magnitude. The major difference is the existence of geminal disubstitution in the thiaindans which, of course, cannot exist in benzothiophenes. ACKNOWLEDGMENT
The authors are indebted to R. F. Kendall and D. E;. Hirsch for the spectral assistance rendered in the course of this investigation. The following benzo[b]thiophenes were provided by D. S. Rao, Petroleum Research Fund Fellow, Bureau of Mines, Laramie, Wyo., Petroleum Research Center: 2,7-dimethyl-, 2-ethyl-, 7-ethyl-, 2,5,7-trimethyl-, 2-ethyl-5-methyl-, 2-ethyl7-methyl-, 2-n-propyl-, 2,7-diethyl-, 2-ethyl-5,7-dimethyl-, and 2-n-propyl-7-ethylbenzo[b]thiophene.A sample of 4,7dimethylbenzo[b]thiophene was provided by F. G. Bordwell, Northwestern University, Evanston, Ill. A. W. Weitkamp, American Oil Co., Whiting, Ind., provided 3,6-dimethylbenzo[b]thiophene. RECEIVED for review August 11, 1969. Accepted October 2,1969. Investigation performed as part of American Petroleum Institute Research Project 48 on "Production, Isolation, and Purification of Sulfur Compounds and Measurement of their Properties" which the Bureau of Mines conducted at Bartlesville, Okla., and Laramie, Wyo. Reference to specific makes and models of equipment is made for identification only and does not imply endorsement by the Bureau of Mines. (9) C. J. Thompson, H. J. Coleman, R. L. Hopkins, and H. T. Rall, ANAL.CHEM., 38,1562 (1966).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
