220
Energy & Fuels 1988,2, 220-223
Formation of Polycyclic Thiophenes from Reaction of Selected Polycyclic Aromatic Hydrocarbons with Elemental Sulfur and/or Pyrite under Mild Conditions+ Curt M. White,* Louise J. Douglas, and Charles E. Schmidt Division of Coal Science, Pittsburgh Energy Technology Center, P.O. Box 10940, Pittsburgh, Pennsylvania 15236
Michael Hackett Physical Science Department, California University of Pennsylvania, California, Pennsylvania 15419 Received September 14, 1987. Revised Manuscript Received November 4, 1987 The origin of polycyclic thiophenes in coal is unknown. Such compounds may have slowly formed during the coalifcation process via reaction of polycyclic aromatic hydrocarbons (PAH) with elemental sulfur, pyrite, and/or hydrogen sulfide. To test the feasibility of this hypothesis, sealed-tube reactions of various PAH with elemental sulfur and/or pyrite were carried out at 115-118 "C. In some instances, small amounts of either vanadium pentaoxide, ferric sulfate, or bentonite clay were included as an additive. After approximately 6 months, the contents of the tubes were analyzed by low-voltage high-resolution mass spectrometry and by combined gas chromatography-mass spectrometry. Many of the reaction tubes contained a small amount of polycyclic thiophene. These producta are formed by insertion of a sulfur bridge into the starting PAH,and a similar reaction could have occurred during coalification, leading to polycyclic thiophenes in coal.
Introduction Condensed thiophene compounds are estimated to constitute 40%-70% of the organic sulfur in bituminous coals.' Elucidating the origin of thiophenic sulfur will further our understanding of the chemistry of the coalification process. A possible explanation or partial explanation of the origin of condensed thiophenes may lie in reactions of elemental sulfur, pyrite, and/or hydrogen sulfide with hydrocarbons during the coalification proc e s ~ . ~ Geochemists '~ have generally subscribed to the belief that organic sulfur compounds found in crude oil may have originated from reactions involving simple orand, ganic compounds and elemental ~ u l f u r . Chatterjee5 ~ more recently, Duran et al.e have shown that elemental sulfur is absent from pristine coals but is prevalent in weathered coal. This does not mean that the organosulfur constituents in coals could not have been formed by reaction of hydrocarbons with elemental sulfur. The formation of thiophenes from the reaction of hydrocarbons and elemental sulfur could have occurred in early stages of the coal's development when elemental sulfur was present. Alternatively, the organosulfur compounds in coal could result from reaction of hydrocarbons with another sulfur source, such as hydrogen sulfide. There are many examples of reactions of elemental sulfur with various classes of compounds, including terpenes, steroids, alkylated aromatic hydrocarbons, amino acids, and humic acids. Weitkamp' reported that the reaction of limonene and sulfur (1h at reflux temperature) gives cyclic sulfides and trace amounts of 3,6-dimethylbenzo[b]thiophene. Douglas and Maire found that cholesterol and sulfur react (30 days a t 150 OC) to give hydrocarbons possessing the benzene, naphthalene, and
* Author to whom correspondence should be addressed.
Reference in this work to any specific commercial product, pro-
cess, or service is to facilitate understanding and does not necessarily
imply its endorsement or favoring by the United States Department of Energy.
phenanthrene ring systems. Unidentified sulfur products were also obtained. DeRoo and Hodgson9 reported that the reaction between equimolar quantities of sulfur and ethylbenzene (several days refluxing a t 130 "C) gives several identifiable sulfur compounds, including 2,4-diphenylthiophene and phenylbenzodithiophenes. An additional reaction between sulfur and ethylbenzene was carried out (20 days a t 30 "C)in which 10 sulfur compounds were detected by gas chromatography. Two of these were identified as 2,4-diphenylthiophene and 1phenylbenzo[ 1,2-b:4,3-bldithiophene, respectively. Przewocki, Malinski, and SzafraneklO studied the reaction of sulfur with toluene a t 200 "C in an argon atmosphere. Included in the products after reaction periods of more than 48 h were 2-phenylbenzo[ b]thiophene, tetraphenylthiophene, and 8-(diphenylmethyl) benzo[b]thiophene. The present study tests the hypothesis that elemental sulfur or pyrite can react with polycyclic aromatic hydrocarbons (PAH), both with and without an additive, under mild conditions over extended periods of time to give polycyclic thiophenes via direct insertion of sulfur. The formation of such compounds from biaryls and angularly condensed arenes by direct insertion of heterosulfur bridges has been demonostrated by Klemm et al." using (1) Attar, A. Fuel 1978,57,201-212. (2) White, C. M.; Lee, M. L. Geochim. Cosmochim. Acta 1980, 44, 1825-1832. ~. ~. (3) White, C. M.; Douglas, L. J.; Perry, M. B.; Schmidt, C. E. Energy Fuels 1987, 1, 222-226. (4) Hunt, J. M. Petroleum Geochemistry and Geology; W. H.Freeman: San Francisco, CA, 1979. (5) Chattejee, N. N. Q. J. Geol.,Min. Metall. Soc. India 1942,14,1-8; Chem. Abstr. 1942,41, 1410g. (6) Duran J. E.; Mahesay, S. R.; Stock L. M. Fuel 1986,65,1167-1168. (7) Weitkamp, A. W. J. Org. Chem. 1959,81,3430-3434. (8) Douglas, A. G.; Mair, B. J. Science (Washington, D.C.)1966,147, 499-501. (9) DeRoo, J.; Hogdson, G. W. Chem. Geol. 1978,22, 71-78. (10)Przewocki, K.; Malinski, E.; Szafranek, J. Chem. Geol. 1984,47, 347-360. ~
~
(11) Klemm, L. H.; Karchesy, J. J.; McCoy, D. R. Phosphorus Sulfur 1979, 7, 9-22.
0881-0624/88/2502-0220$01.50/0 0 1988 American Chemical Society
Formation of Polycyclic Thiophenes
Energy & Fuels, Vol. 2, No. 2, 1988 221
hydrogen sulfide and a heterogeneous catalyst (usually a metallic oxide). The temperatures involved (450-630 OC), however, are considerably higher than the 200 O C or less
Scheme I
to which coals are believed to have been exposed.
Experimental Section Six PAH (biphenyl, phenanthrene, 1-methylphenanthrene, chrysene, triphenylene, and 2-phenylnaphthalene) were allowed to react individually with sublimed elemental sulfur and with pyrite (MCB Manufacturing Chemists, Inc., granular, -292 micron), in each instance both with and without an added 'catalyst". Vanadium pentaoxide, ferric sulfate, or a bentonite clay (National Premium 75-pm bentonite, NL Baroid/NL Industries, Inc.) were used as additives. The quantities of reagents used were as follows: elemental sulfur, from several milligrams to -0.2 g; PAH, from several milligrams to -0.1 g; pyrite (FeS2), from -0.04 to 0.2 g; and Fez(SO&, V2OS,and bentonite, from -0.01 to 0.02 g. The purity of the starting PAH was determined by high-resolution gas chromatography using a sulfur-specific flame photometric detector (FPD), combined gas chromatography-mass spectrometry (GC-MS),and low-voltage high-resolution mass spectrometry (LVHRMS). Methylene chloride was employed as the solvent for samples analyzed by GC-MS and high-resolution gas chromatography. The reactants were placed in small glass tubes (9-10 cm long by 5-10 mm 0.d.) and carefully mixed, and the tubes were sealed by torch. No attempt was made to exclude air. Care was taken to ensure that no reactants contacted the glass near the seal. Several duplicates of most tubes were prepared and placed in an oven set at 115-118 OC. Because of a limited supply of 1methylphenanthrene, no duplicate tubes containing it were prepared. After approximately 6 months, the tubes were opened and the contents analyzed by low-voltage high-resolution mass spectrometry (LVHRMS) and in some cases by combined gas chromatography-mass spectrometry (GC-MS). The low-voltage high-resolution mass spectra were obtained on a Kratos MS-50 high-resolution mass spectrometer interfaced to a Kratos DS-55 data system. The samples were introduced into the ion source of the mass spectrometer via an all-glass heated-inlet system (AGHIS) equipped with a 1-L expansion volume and a molecular leak in the 8ource transfer line. The inlet system and transfer line were maintained at 300 O C , and the ion source was maintained at 250 OC. Typically, a few milligrams of the reaction mixture were taken from a tube and placed in the solids inlet vessel of the AGHIS along with an aliquot of the low-voltage calibration standard. The calibrating standard was in an open-ended glass capillary added to the solids inlet vessel. The calibrant used in this study was similar to that described e4~lier.a'~The mass calibration standard in the open-ended glass capillary was added to each sample to be analyzed only moments before analysis by LVHRMS. A heater maintained at 300 "C was then placed over the solids inlet vessel and vaporized both sample and standard into the AGHIS. The mass spectrometer was operated at 1part in 25000 resolution dynamic resolving power. The ionizing voltage was maintained at 11.75 eV to enhance the detection of molecular ions and to minimize fragmentation. The mass calibration standard used during LVHRMS analysis was a mixture of halogenated aromatics that readily ionizes under low-voltage ionizing conditions and that possesses mass defects large enough that they do not interfere with hydrocarbon ions from the sample. High-resolution gas chromatography employing a dual flame ionization detector and a sulfur-specific flame photometric detector (FID/FPD) was performed by using a Tracor 570 gas chromatograph equipped with a Spectra-Physics Model 4270 integrator and a Westronics dual-pen recorder. A 29 m X 0.21 mm i.d. fused-silicacapillary column wall-coated with a 0.25-rm film of SE52 was used with He carrier gas at a 54.9 cm/s average linear velocity. The column oven was programmed from 40 to 310 OC at 4 OC/min after a 2-min hold at 40 OC. (12)White, C.M. In Handbook of Polycyclic Aromatic Hydrocarbons; Bjorseth, A., Ed.:Marcel Dekker: New York, 1983;pp 525-616. (13)Aczel, T.;Allan, D. E.; Harding, J. H.; Knipp, E. A. Anal. Chem. 1970,42,314-347.
1
(andlor
\
u
s
'
J
Combined GC-MS analysis were performed by using a Hewlett-Packard 5985 GC-MS system employing a 70-eV ionizing voltage and a quadrupole mass filter. The electron multiplier voltage was maintained at 2400 V as the instrument scanned from 40 to 350 amu every 3.5 s. The same chromatographic conditions employed during high-resolution gas chromatographic analysis were used during GC-MS analysis.
Results and Discussion Of the six PAH used as starting materials, two are biaryls and four are angularly condensed arenes. The primary interest was to determine whether, under the conditions of reaction, sulfur could be inserted directly into the PAH via a dehydro-l,4-cycloadditionto give polycyclic thiophenes, even in trace amounts. Of the six PAH studied, only phenanthrene did not yield a detectable sulfur-bridged product. The experimental results are presented in Table I. Where reactions occurred as indicated in Table I, the products are thought to have formed as shown in Scheme I. The last sulfur-bearing structure shown would not, of course, be a polycyclic thiophene. The product structures indicated here are speculative to the extent that they are based solely on molecular formulas provided by mass spectrometry. Conclusions reached concerning the actual presence of a product corresponding to a given molecular formula were based upon precise mass measurements made by LVHRMS (Table 11). In all cases where LVHRMS indicated that a polycyclic thiophene was formed, that reaction mixture was also analyzed by combined GC-MS. In 13 out of 16 instances (Table I), GC-MS led to observation of a chromatographic peak having both the expected retention time and the expected mass spectral fragmen-
White et al.
222 Energy & Fuels, Vol. 2, No. 2, 1988
Table I. Summary Description of the Reaction Results (Reaction Tubes Heated at 115-118 "C for 6 Months) PAH 2-phenylnaphthalene biphenyl chrysene phenanthrene triphenylene 1-methylphenanthrene 2-phenylnaphthalene biphenyl chrysene phenanthrene 2-phenylnaphthalene biphenyl chrysene phenanthrene triphenylene 2-phenylnaphthalene biphenyl chrysene phenanthrene 1-methylphenanthrene biphenyl phenanthrene triphenylene 2-phenylnaphthalene chrysene phenanthrene 1-methylphenanthrene
S source
additive
no. of reacn tubes analyzed 1 2 2 2 2 1
bentonite bentonite bentonite bentonite
a a
no no yes yes
1
a a
1 1 1
yes no
1 1 2 1 1
a a
2 2 2 2
a a
Yes no Yes
Yes no Yes no no no
1
Fez(S0A3 Fe2(S04)3 Fez(S04)~ bentonite bentonite bentonite bentonite
2-phenylnaphthalene biphenyl chrysene phenanthrene triphenylene 1-methylphenanthrene
S-bridged product obsd by LVHRMS GC-MS
1 1 1 1 2 1 1
Yes no no no no no no no no no
1 1 1 1 1 1
a The sulfur-bridged product was observed by LVHRMS analysis of these reaction mixtures. These results have been omitted from this table because the sulfur-bridged product is formed in the mass spectrometer by reaction of Sa and biaryls.
tation pattern for the compounds listed in Table 11. The absolute ion intensities from the LVHRMS of the sulfur-bridged products were low. Under the reaction conditions employed, polycyclic thiophenes were formed in small amounts. In three cases, shown in Table I, the observation of a polycyclic thiophene product by LVHRMS was not confirmed by GC-MS. The failure to confirm these observations by GC-MS may simply be a consequence of the low concentrations of the products. For the present purposes, observation of products by either LVHRMS or GC-MS was interpreted as a positive result. The purity of the starting materials was checked by LVHRMS, GC-MS, and high-resolution gas chromatography employing a dual FID/FPD. The PAH are commonly contaminated with trace amounts of polycyclic thiophenes.14Js Trace amounts of organosulfur compounds were contaminants in some of the PAH starting materials. For example, dibenzothiophene was found in phenanthrene. However, in no case were the products ultimately formed as a result of a reaction studied here found as a contaminant in the starting material. To determine if the polycyclic thiophenes were formed in the mass spectrometer, freshly prepared mixtures of the individual PAH and elemental sulfur were analyzed by LVHRMS and capillary gas chromatography. As in all (14) Karcher, W.; Depaus, R.; van Eijk, J.; Jacob, J. In Polynuclear Aromatic Hydrocarbons; Jones, P. W., Leber, P., E&.; Ann Arbor Science: Ann Arbor, MI, 1979; pp 341-356. (15)Karcher, W.; Nelen, A.; Depaus, R.; van Eijk, J.; Glaude, P.; Jacob, J. In Polynuclear Aromatic Hydrocarbons; Cooke, M., Dennis, A. J., Ma.; Battelle: Columbus, OH, 1981; pp 317-327.
Table 11. Typical Analytical Results Obtained during Analysis of the Reaction Products probable reacn product
mol formula
measd mass
calcd mass
millimass unit dev
184.0347
184.0347
0.0
258.0500
258.0503
0.3
258.0507
258.0503
0.4
234.0500
234.0504
0.4
222.0505
222.0504
0.1
288.0773 288.0774 193.9832 193.9827 162.0311 162.0305
0.1 0.5 0.6
other instances, mass standards used in LVHRMS were not included in these mixtures until moments before LVHRMS analysis when a small amount of calibrant was added in an open-ended glass capillary to an aliquot of the mixture. These freshly prepared mixtures of starting
Formation of Polycyclic Thiophenes material were purposely not heated before analysis. When an aliquot of the freshly prepared mixtures of elemental sulfur and biphenyl and one of elemental sulfur and 2phenylnaphthalene were analyzed by LVHRMS, the corresponding polycyclic thiophenes shown in Table I1 were observed. Immediate analysis of another aliquot of the same freshly prepared mixtures of the biphenyl and sulfur mixture and of the 2-phenylnaphthalene and sulfur mixture by high-resolution gas chromatography showed no trace of the corresponding polycylic thiophene. These polycyclic thiophenes were not present in the PAH blanks; therefore, they must have formed by a reaction with sulfur in the mass spectrometer. This only occurred in the two cases where the PAH was a biaryl. When freshly prepared mixtures of sulfur and the remaining PAH were analyzed by LVHRMS, no products were observed. Reaction of sulfur with biphenyl and 2-phenylnaphthalene in the spectrometer masks the ability of LVHRMS to properly detect the corresponding polycyclic thiophenes formed during long-term, low-temperature reactions. Since no artifacts were observed by capillary gas chromatographic analysis of the reaction mixtures, the formation of polycyclic aromatic thiophenes from biphenyl and 2-phenylnaphthalene under long-term, low-temperature conditions was monitored by using capillary gas chromatography and/or combined gas chromatography-mass spectrometry. However, as can be seen from Table I, verification by GC-MS was good but not perfect in this series of experiments. The conclusion that the biaryls react with elemental sulfur under conditions of long-term mild thermolysis is supported by GC-MS data alone, since the expected polycyclic thiophene products are formed in the mass spectrometer during LVHRMS analysis. Our results show that many of the reaction tubes containing elemental sulfur done or in combination with an additive gave a sulfur-bridged product, as indicated in Table I. Reaction of PAH with pyrite alone gave no detectable sulfur compounds under the conditions employed. A number of products appeared to be formed in trace amounts in addition to the polycyclic thiophenes. Three sulfur-containing ~ O ~ S - C ~ ~C10H6S2, H ~ ~ S and , CIOH8Swere observed by LVHRMS during analysis of reaction products that contained elemental sulfur, alone or in combination with FeS2, bentonite, or V205. (In 13 cases, all three ions appeared, and in eight cases, two out of the three ions appeared.) This was true regardless of whether a polycyclic thiophene had formed. However, these same three ions were not observed when the reaction products were analyzed by GC-MS. The LVHRMS shows the absence of these compounds in the starting PAH (with the exception of C10H8Spresent in the biphenyl). These ions were not observed when the PAH starting materials were analyzed individually by LVHRMS but were observed by LVHRMS when the elemental sulfur used in the reactions was analyzed. Ultrasonic extraction of the elemental sulfur with methylene chloride for a few minutes and analysis of the soluble portion by GC-MS failed to reveal the presence of these compounds in the extract. When elemental sulfur enriched in % was analyzed by itself using LVHRMS, the C20H1434S, C ~ O H ~and ~ ~CloHa4S S ~ , ions were observed, as illustrated in Table 11. These ions are apparently formed in the mass spectrometer via reaction
Energy & Fuels, Vol. 2, No. 2, 1988 223 of elemental sulfur and the mass calibration standards used in the LVHRMS experiment. Reaction of bromonaphthalene, chloronaphthalene, and iodonaphthalene, present in the low-voltage mass ~ t a n d a r d , ' ~ Jwith ~ elemental sulfur is a probable source of the C d l 4 S , C1&S2, and C1JI8S ions observed in the LVHRMS spectra of the reaction mixtures and of the elemental sulfur.
Conclusions Polycyclic thiophenes are formed from the reaction of PAH and elemental sulfur, both with and without an additive, or from PAH and pyrite with an additive over extended time periods at temperatures below the generally accepted upper limit for that of the coalification process. Previous research has shown that sulfur bridging occurs in various types of arenes, including PAH, in significant yield when hydrogen sulfide, high temperatures (450630°C), and metallic oxide catalysts are used and when elemental sulfur a t high tempertures is used both with and without an additive." The concept that polycyclic thiophenes found in coal extracts may be derived from reaction of PAH with elemental sulfur, pyrite, and/or hydrogen sulfide within the coal macromolecule over geological periods of time has been previously proposed.2 The present work demonstrates the chemical feasibility of this concept under mild thermal conditions. Furthermore, during the course of this investigation it was shown that elemental sulfur reacts with the commonly employed LVHRMS mass standard to give C20H14S,CloH6S2,and ClOH8SThe reaction of aromatic hydrocarbons with a sulfur source is not the only explanation that accounts for the fact that a large portion of the organosulfur constituents in cod are thiophenes. Thiophenes are formed via biogenic pathways and frequently occur in plants. Thus, a-terthienyl has been found in marigolds and related plants that often contain other related thiophenes.16J7 Thiophene acetylenes have been isolated from many plants of the Compositae species.18 Thus, the polycyclic thiophenes in coal could results from the diagenesis of these naturally occurring thiophenes. Acknowledgment. We acknowledge the assistance of J. Malli, Jr., in obtaining the LVHRMS results. The work of M.H. was performed under appointment to the Faculty Research Participation program administered by Oak Ridge Associated Universities for the U.S. Department of Energy. Registry No. Vz06,1314-62-1; Fe2(SO4),, 10028-22-5; S, 7704-34-9; 2-phenylnaphthalene2612-94-2; biphenyl, 92-52-4; chrysene, 218-01-9;phenanthrene,85-01-8;triphenylene, 217-59-4; 1-methylphenanthrene, 832-69-9; pyrite, 1309-36-0; dibenzothiophene, 132-65-0; chryseno[4,5-bcd]thiophene,72076-98-3; triphenyleno[l,l2-bcd]thiophene,68558-73-6;benzo[blnaphtho[2,1-d]thiophene, 239-35-0; l-methylphenanthro[4,5-bcd]thiophene, 88114-01-6. (16) Zeichmeister, L.; Seaae, J. W. J. Am. Chem. SOC. 1947, 69, 273-275. (17) Beny, J. P.; Dhawan, S.N.; Kagan, 3.;Sundlass, S.J. Org. Chem. 1982,47, 2201-2204. (18) Bohlmann, F.; Burkhardt, T.; Zdero, C. Naturally Occurring Acetylenes; Academic: London, 1973; pp 61-74.
