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Energy & Fuels 1996, 10, 1187-1188
1187
Formation of Dimethyl Polysulfides via Reaction of Methanol and Elemental Sulfur Curt M. White,* Paul C. Rohar, Jr., Leonard J. Shaw, and Leo W. Collins Pittsburgh Energy Technology Center, P.O. Box 10940, Pittsburgh, Pennsylvania 15236 Received April 29, 1996. Revised Manuscript Received July 3, 1996X
Dimethyl polysulfides have been reported in the supercritical methanol extracts of coal and in the pyrolysis products of a high-sulfur Spanish coal that was first extracted with a mixture of dichloromethane and methanol. Dimethyl polysulfides and other products are shown to be formed by the reaction of methanol with elemental sulfur at elevated temperature and pressure. This reaction is a possible source of some of the dimethyl polysulfides observed by others in hightemperature methanol extracts.
There have been several recent reports of the identification of individual polysulfides, including dimethyl disulfide, dimethyl trisulfide, and dimethyl tetrasulfide, following thermal treatment of coal.1-4 Specifically, using GC-MS Lee and Fullerton1 identified dimethyl disulfide and dimethyl trisulfide in the supercritical methanol extracts of the coals they studied. Muchmore et al.2,3 studied the supercritical methanol/KOH desulfurization of nitric acid treated coal by combined GCMS. They positively identified carbonyl sulfide, hydrogen sulfide, methyl thiol, methyl sulfide, and dimethyl disulfide. Dimethyl trisulfide and other polysulfides were tentatively identified on the basis of mass spectral data alone but could not be confirmed due to a lack of pure analytical standards. The authors claimed that the “sulfur is removed from the organic structure by direct reaction with the alcohol or its degradation product”.3 We have determined that dimethyl polysulfides are formed via reaction of methanol with elemental sulfur, both of which were present during the supercritical methanol extraction of coal conducted by Lee and Fullerton.1 The coals Muchmore et al.2,3 studied had been exposed to air and thus probably contained elemental sulfur as a result of reaction of pyrite with air.5 In fact, Muchmore et al. report the presence of a category of sulfur compounds in the coals they studied as group I compounds, which include elemental sulfur.2 To determine if the dimethyl polysulfides observed by Lee and Fullerton and Muchmore et al.2,3 could be artifacts, elemental sulfur (0.20 g) and methanol (0.5 mL) were placed in a stainless steel tube (2.5 mL) and sealed. The tube and its contents were placed in an oven at 240 °C for 5 h and then cooled to room temperature. Analysis of the reaction mixture using * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, September 1, 1996. (1) Lee, S.; Fullerton, K. L. Fuel Sci. Technol. Int. 1992, 10, 11371159. (2) Muchmore, C. B.; Chen, J. W.; Kent, A. C.; Liszka, M. Proceedings, International Conference on Coal Science; New Energy and Industrial Development Organization: Tokyo, 1989; pp 193-196. (3) Tao, W. L.; Hippo, E. J.; Muchmore, C. B. Proceeding, International Conference on Coal Science; Butterworth-Heinemann Ltd., Oxford, U.K., 1991; pp 1013-1016. (4) Sinninghe Damste, J. S.; de las Heras, F. X. C.; de Leeuw, J. W. J. Chromatogr. 1992, 607, 361-375. (5) White, C. M.; Lee, M. L. Geochim. Cosmochim. Acta 1980, 44, 1825-1832.
open-tubular gas chromatography equipped with a sulfur selective flame photometric detector (FPD) showed that dimethyl disulfide, dimethyl trisulfide, and dimethyl tetrasulfide were formed (Figure 1 and eq 1). GC-
(1)
MS analysis at comparable conditions was used to confirm the identity of the labeled peaks in Figure 1. We suspect that some of the dimethyl polysulfides observed by both Lee and Fullerton1 and Muchmore et al.2,3 were present as a result of this reaction. The observation of dimethyl polysulfides in the pyrolysis products of a Spanish coal by Sinninghe Damste et al.4 using pyrolysis GC-MS is also questionable. They observed dimethyl disulfide, dimethyl trisulfide, and dimethyl tetrasulfide in the pyrolysis products of Rubielos coal. They mention the possibility of dimethyl polysulfide formation by secondary reactions but indicate that this is unlikely. We suspect that formation of dimethyl polysulfides by secondary reactions may be more likely than originally thought. Sinninghe Damste et al.4 studied five coal samples. Only one sample, the Rubielos coal, was found to contain dimethyl polysulfides. The Rubielos coal studied was Soxhlet extracted with a mixture of dichloromethane-methanol (2:1) for 36 h before the insoluble portion was subjected to pyrolysis GC-MS. Coals that contain significant amounts of pyrite and that have been exposed to air often contain elemental sulfur as a result of the oxidation of pyrite.5 Further, the insoluble coal residue that remains after solvent extraction typically contains significant amounts of the extraction solvent, even after exhaustive drying. Therefore, we suspect that the Rubielos coal used in the pyrolysis GC-MS experiments contained both methanol and elemental sulfur. The dimethyl polysulfides observed when this methanolextracted air-oxidized coal residue was pyrolyzed may have been formed by the reaction of methanol and elemental sulfur and not originally present in the coal. The polysulfides shown in Figure 1 were difficult to elute, because they were thermally sensitive. The column was operated at high helium average linear velocity to assist in eluting the polysulfides at lower
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1188 Energy & Fuels, Vol. 10, No. 6, 1996
Figure 1. Chromatogram of the methanol and elemental sulfur reaction mixture showing dimethyl polysulfide products. A Tracor 570 GC equipped with a sulfur selective FPD and a 25 m × 0.32 mm fused silica column coated with a 1.24 µm film of CP-Sil-8CB (Chrompack) column eluted with helium having an average linear velocity of 66.5 cm/s was used. The sample, 2 µL, was introduced using splitless injection over a 1.3 min splitless period. The initial temperature was 55 °C for 4 min, then programmed to 280 °C at 2 °C/min, and held for 8.5 min. The splitless injector was 250 °C, and the FPD was 230 °C.
temperatures than would be required at lower average linear velocities. At lower helium average linear velocities the polysulfide peaks either could not be eluted or could not be eluted as sharp peaks. We suspect that higher polysulfides may have been present in the reaction mixture but were not eluted because they were thermally unstable at the temperature required to elute them.
White et al.
To eliminate the possibility that the dimethyl polysulfides observed in the gas chromatogram shown in Figure 1 were formed in the injection port of our GC, a blank reaction was performed. Elemental sulfur and methanol were added to the stainless steel tube reactor in the same proportions as described above and allowed to stand at room temperature for 5 h. The tube was unsealed and an aliquot injected into the gas chromatograph using conditions identical to those used to generate the chromatogram in Figure 1. No peaks corresponding to dimethyl polysulfides were observed in this blank. A monograph documenting the reactions of elemental sulfur with organic compounds including aliphatic alcohols has been published.6 Specifically, methanol is reported to react with elemental sulfur in the presence of ultraviolet light or in the presence of catalysts. Methanol is reported to react with excess sulfur ar 550 °C in the gas phase to yield carbonyl sulfide and carbon disulfide.6 Elbanowski has reported that the photochemical reaction of methanol with elemental sulfur yields dimethyl sulfide and a trace of methane.7 To our knowledge, the chemistry between methanol and elemental sulfur described here to produce dimethyl polysulfides has not been previously reported. EF9600671 (6) Voronkov, M. G.; Vyazankin, N. S.; Deryagina, E. N.; Nakhmanovich, A. S.; Usov, V. A. Reactions Of Sulfur With Organic Compounds; Pizey, J. S., Ed.; Consultants Bureau a Division of Plenum Publishing: New York, 1987; p 189. (7) Elbanowski, M. Phosphorus Sulfur 1978, 5, 111-116.
