888
Anal. Chem. 1982, 5 4 , 688-691
Determination of Sulfur in Asphalts by Selective Oxidation and Photoelectron Spectroscopy for Chemical Analysis Jean-Mlchel Rulz, Brlan M. Carden, LOUISJ. Lena,” and Emlle-Jean Vincent Instltut de P6trol6ochlmle et de Synth6se Organlque Industrielle, Facult6 des Sciences et Techniques de Saint J6r6me. Rue Henri Poincar6, 13397 Marseille, France
Jean-Claude Escaller Centre de Recherche Elf-France, Solalze B.P. No. 22, 69360 Saint Symphorien d‘Ozon, France
The characterlratlon of sulfur In asphalts Is lnvestlgated by coupllng selectlve reactlons and photoelectron spectroscopy for chemical analysis. The speclflclty of the two followlng reactlons has been studled on test compounds: oxldatlon of sulfldes wlth lert-butyl hydroperoxlde In chloroform (a); reduction by llthlum aluminum hydrlde of sulfones in tetrahydrofuran (b). These reactlons have been used wlth an asphalt, dlrecily for (a) or after a preoxldatlon wlth mchloroperbenzolc acld for (b) to determlne, uslng photoelectron spectroscopy, which klnd of sulfur was Involved. An analyllcal scheme sums up the dlfferent results: cyclic sulfides = 67 % , thlophenlc sulfldes = 54 %, cyclanlc sulfldes = 13 % , alkyl and aryl alkyl sulfldes = 33 %
.
The determination of sulfur in both petroleum and other samples has been carried out through various techniques, such as oxidative titrimetry of thioethers ( I ) , radiometry (2), spectrophotometric titration (3), and Auger electron spectroscopy (4). However none of these methods have been able to both identify and quantify the different types of sulfur in a given sample and, more precisely, to distinguish between aliphatic and thiophenic sulfur compounds in a complex mixture at low concentrations. Recently, chemists have shown an increasing interest for X-ray photoelectron spectroscopy (ESCA). We have applied this technique to the analysis of sulfur compounds in an asphalt (“Arabian Light asphalt”). The ESCA spectrum displays signals which correspond to ejection of electrons possessing particular “binding energies” from some nuclei of the sample. The binding energies of the electrons are given in terms of chemical shifts expressed in electron volt (eV). They will vary for the same electron belonging to the same atom when the latter is present in different oxidation and hybridization states. For sulfur the most exhaustive study was completed by Lindberg et al. (5). These authors determined the S (2p) binding energies of 136 compounds, a representative number of which are listed on a correlation chart in which it appears that all compounds fall within a range of 10 eV. The initial idea was to obtain the ESCA spectrum of asphalt and to compare it with the spectra of model compounds of the asphalts sample. ESCA signals are quantitative and the accuracy of their integration is 5%. The ESCA spectrum was recorded and is shown in Figure 1. It is obvious that a simple analysis of the spectrum and an indentification of the sulfur compounds of the sample is impossible. The S (2p) spectrum obtained appears as a single peak centered at 164.2 eV (related to carbon peak) with a half-height width of approximately 3 eV. We have therefore proposed to modify selectively the state of oxidation of sulfur in the asphalt to change the binding O003-27OO/82/O354-0688$0 1.2510
Chart I
energies of the S (2p) electrons and thus to obtain different peaks on the ESCA spectrum. The most evident choice, but not the only possible one, was the selective oxidation of specific sulfur atoms into either sulfoxides or sulfones because the correlation chart indicates that the binding energies of sulfoxides and particularly of sulfones are significantly larger than the ones of sulfides. Choice of Model Compounds. To test experimental conditions it was first necessary to choose suitable test compounds which would be typical of the sulfur compounds existing in asphalt. Attention must be drawn to the exhaustive study carried out by the American Petroleum Institute about the sulfur compounds in crude oils (6). These compounds were classified in four principal subdivisions: thiols, aromatic sulfides, long chain sulfides, and cycloalkyl sulfides. For aromatic sulfides the use of high-resolution mass spectrometry has allowed the identification of different sulfur compounds in various high boiling petroleum fractions. Bodin (7) has identified in a hydrocarbon distillate (between 300 and 450 “C) compounds of the types shown in Chart I. On the other hand, the amount of thiols and disulfides in our asphalt is quite low (8). Bearing in mind all these considerations,the sulfur compounds representative of our asphalt were the following ones: dibenzothiophene, diphenyl sulfide, dibenzyl sulfide, di-n-hexyl sulfide, n-hexyl phenyl sulfide, diphenyl disulfide, dibenzyl disulfide. Two remarks can be made here. The choice of dibenzothiophene for the thiophenic series, including thiophene itself and benzothiophene, is justified by the fact that it is the most reactive of the series toward mild oxidizing agents. Dipole moment data indicate that the lone pair electron density around the sulfur atom decreases in the following order (9):
The choice of two aliphatic sulfides, namely, the dibenzyl sulfide and di-n-hexyl sulfide, enables one to establish whether the reactivities of the sulfur atoms toward oxidizing agent are similar or not. It could be argued that the benzyl group lowers lone pair electron density, such a phenomenon would reduce 0 1982 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 54, NO, 4, APRIL 1982
snllides
I 157,s
I 164,2
I
170,5
cV
Figure 1. Study of the nature of sulfur of an asphalt. S (2p) ESCA spectrum of the asphalt.
the reactivity of the dibenzyl sulfide atom to oxidation. Similar arguments could be put forward for the two chosen disufides. Choice of Oxidation Agents. For the oxidation of sulfur compounds to the corresponding sulfoxides and sulfones (IO), various methods are available. Oxidative methods have previously been employed to determine sulfur compounds, for example, bromine water (II), pot.a?,siumbromate (U), and hydrogen peroxide (13). However, it would be difficult to apply all these methods to samples of asphalt since these reactions are carried out in water or in other solvents where asphalts are not soluble. Then, only fiiding a selective oxidizing agent soluble in organic solvents where asphalts themselves are soluble will allow the oxidation of asphalts. We have thun chosen: (1) m-chloroperbenzoic acid (MCPBA) which is now widely used in preparation of sulfoxides and sulfones and (2) tert-butyl hydroperoxide (TBHP) which is a much milder oxidizing reagent. Choice of Experimental Proceedings. First we tried a selective oxidation of sulfides into sulfones, because, as it has been previously mentioned, it should have been better to find a selective oxidation of saturated sulfur into sulfones than into sulfoxides to obtain a more accurate result by ESCA, but we did not succeed. C h the contrary, we have found experimental conditions in which all sulfides are quantitatively oxidized into sulfones (oxidation with MCPBA). We have then iinvestigated two other reactions: very mild oxidation with tert-butyl hydroperoxide to oxidize only sulfur in saturated structures into sulfoxides; reduction with lithium aluminum hydride (LiAIH,) of sulfones into sulfides, because according to Bordwell and McKellin (14, sulfones in pentacyclic cycles can he reduced by LiAlH4 more easily than the others. A quantitative preoxidation of sulfides into sulfones is done with m-chloroperbenzoic acid. EXPERIMENTAL SECTION Sulfides. Sulfides are commercially available (Fluka and Aldrich). n-Hexyl phenyl sulfide has been synthesized by phase transfer catalysis from thiophenol and n-hexyl bromide: the phase transfer catalyst is hexadecyltributylphosphonium bromide. Sulfoxide and Sulfone Syntheses. Sulfoxides. A solution (20 mL) of m-chloroperbenzoic acid (acid/sulfide = 1) in chloroform is slowly added to a solution of sulfide (2 g) in chloroform (50 mL). This mixtwe is heated during 3 h at reflux temperature and then cooled, the solution is washed with a sodium bicarbonate solution (3 X 50 mL), dried on MgSO,, and then filtered and evaporated to give the crude sulfoxide. Sulfones. The method in identical with a ratio of acid/sulfide of 2.5. Oxidation with tert-Butyl Hydroperoxide (TBHP). Oxidation of Model Compounds. Reactions of model compounds with TBHP (TBHP/sulfide = 10) have been carried out this way. A sample of sulfides (0.0018 mol) is dissolved in 15 mL of chlo-
689
roform and stirred at room temperature; TBHP (0.018 mol) i s then added. Analyses are done by gas chromatography. Gas Chromatography. Chromatograms are realized on an 1.G.C. 120 FB under isothermal conditions, using a 1.5-m Chromosorb P/AW 80/100 (10% of SE 30) column. Oxidation of Asphalt with TBHP. Asphalt is precipitated from an Arabian crude oil using n-pentane. TBHP (TBHP/S = 10) is added to a solution of asphalt (10 g) in chloroform (150 mL). The mixture is stirred at room temperature during 5 days, washed with a sodium bicarbonate solution, and then evaporated and dried under vacuum with P205. m -Chloroperbenzoic Acid (MCPBA) Oxidation and Lithium Aluminum Hydride (LiAlH,) Reduction. Reduction of Model Sulfones. In EL250-cm3reactor, 40 mL of tetrahydrofuran (THF) and LiAlH, were introduced and heated to THF reflux, the ratio LiA1H4/sulfonebeing 10 for 0.5 g of sulfone. A solution of sulfone with an internal standard (terphenyl or triphenylmethane) (0.8 g) in THF (20 mL) is prepared (100 fiL being used for gas chromatography) and added to the reaction mixture through a dropping funnel. The dropping funnel i s washed with 10 mL of THF. Samples of the reaction mixture (1mL) are taken after half an hour; 1drop of diluted hydrochloric acid and 1mL of methylene chloride are then added. Chromatograms are realized on an Intersmat I.G.C. 12!0 equipped with a S.E. 30 column. Oxidation of Ashpizlt with m-Chloroperbenzoic Acid. mChloroperbenzoic acid (MCPBA) (10 g) is added to a solution of asphalt (10 g) in chloroform (150 mL). The solution is heated at chloroform reflux temperature during 3 h and then allowed to cool at room temperature, washed with a sodium bicarbonate solution, evaporated, and dried under vacuum, the yield is 10.5 g of oxidized asphalt. The stoichiometric ratio (MCPBA/S) is 3.2. Reduction of the Oxidized Asphalt by Lithium Aluminum Hydride. In a 300-cm3reactor, a sample of the oxidized asphalt (4 g) is dissolved in THF and heated to THF reflux temperature. Lithium aluminum hydride (2.85 g) (LiAlH,/S = 10) is added and refluxed during 3 h. After allowing the mixture to cool at room temperature, an acidic hydrolysis is done. Asphalt is extracted with methylene chloride. The solution is filtered,washed, and dried. ESCA Spectra. ESCA spectra are recorded on a Vacuum Generator ESCA using a magnesium X-ray source (Ka). The spectrometer is linked to a P.D.P. 8 digital computer. The reference is the C (IS) peak (285 eV). The asphalt samples are powders, so, they are deposited and flattened on a sample loader constituted by a 70 mesh stainless grid. Samples are introduced first in the preparation chamber where a secondary vacuum is3 realized and then in the ionization chamber. Several spectra of the same sample are studied to notice an eventual modification under the X-ray effect; no change had been observed after 4 h. RESULTS AND DISCUSSION Oxidation w i t h tert -butyl hydroperoxide. tert-Butyl. hydroperoxide (TBHE’) has been used to remove sulfur from coal (15). A model compound for coal, dibenzothiophene, gave a yield of 9% when oxidized with TBHP. Because of the poor yield, we performed the reaction with TBHP at a lower temperature in chloroform. We have done many reactions on the three main models, dibenzothiophene, diphenyl sulfide, and dibenzyl sulfide to determine the best reaction conditions by changing the stoichiometry and the oxidation time. The yields were obtained by gas chromatography and are given in Table I. The Table I shows that the thiophenic sulfur is not at all oxidized, the diphenyl sulfide is slightly oxidized whereas the dibenzyl sulfide is oxidized with a yield of 95% with 10 mol equiv of TBHP after a 5-day reaction. To be able to quantify the yields of oxidation by gas chromatography, it is necessary to have the coefficients of response of both starting materials and products relative to a standard compound; so all the sulfoxides and the sulfones of the three model compounds were prepared.
690
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL
1982
Table I. Yield of Oxidation ( W ) of Different Models with tert-Butyl Hydroperoxide in Chloroform test compds dibenzothiophene diphenyl sulfide
stoichiometry TBHP/S
1 day
5 days
0 0 0
0 0 0
3.7 1.0 10.0 3.7 7.0
test co mp ds
30min
dibenzothiophene sulfone m-hexylphenyl sulfone di-n-hexyl sulfone
100
3.1 7.0
14
10.0
30
10 40
1h
3h
14
20
30
10
16
29
slllfoilas
5 6
10.0
dibenzyl sulfide
Table 111. Yield of Reduction of Sulfones into Sulfides (%)
k
I I
15
95
Table 11. Molecular Yield of the Different Species Appearing after the Treatment % at the end of
the reaction sulfoxide sulfone
test compds dibenzothiophene diphenyl sulfide dibenzyl sulfide di-n-hexyl sulfide diphenyl disulfide dibenzyl disulfide
158p
0
16 78 64 0 0
le4,35
0 0 13 29 0 0
17q35
Figure 2. Study of the nature of sulfur of an asphalt. S (2p) ESCA spectrum of the asphalt oxidized by tert-butyl hydroperoxide. When using experimental conditions of 10 mol equiv of TBHP and a 5-day reaction, we obtained the yields given in Table 11. I t can be observed that the aromatic sulfides and the disulfides are not a t all oxidized whereas the dibenzyl sulfide and the di-n-hexyl sulfide are oxidized with a high yield (>90%) to give, mainly, sulfoxides. The diphenyl sulfide gives intermediate results (between thiophenic and dialkyl sulfides). The ESCA spectrum of the asphalt sample oxidized under the same conditions was recorded and is shown in Figure 2. Two peaks can be observed, one centered a t 164.35 eV corresponding to the sulfides and the other one centered at 165.35 eV corresponding to the sulfoxides contained in the oxidized asphalt sample. A base line correction followed by a manual deconvolution into two peaks was drawn. The results indicate that 54% of the sulfur has remained unoxidized while 46% is oxidized to sulfoxide; the amount of sulfones in the sample is very low. Thus, in the Arabian Light asphalt, about 54% of total sulfur is involved in thiophenic forms of aromatic hydrocarbons and about 46% in saturated structures. Preoxidation with m -Chloroperbenzoic Acid and Reduction with Lithium Aluminum Hydride. By coupling a selective oxidation of non thiophenic sulfur with tert-butyl
163,7
168,2
SV
Flgure 3. Study of the nature of sulfur of an asphalt. S (2p) ESCA spectra of the asphalt ( I ) oxidized with m-chloroperbenzoic acid and of the preoxklized asphatt (11) reduced with lithium aluminum hydride. hydroperoxide and ESCA, we have shown that sulfur in the Arabian Light asphalt is quantitatively more involved in aromatic structures than in saturated ones. Of course, the low difference of chemical shifts in ESCA between sulfides and sulfoxides is annoying for an accurate quantitative determination of the two structural types. A bibliography on quantitative analysis of sulfoxides has shown that no analytical technique could allow a better accuracy than ESCA. As we have not found specific reactions to oxidize saturated sulfides into sulfones and as, on the contrary, we have determined conditions for which all sulfides are quantitatively oxidized into sulfones, the specificity of the reduction of sulfones into sulfides with lithium aluminium hydride has been studied. Oxidation with MCPBA. Reactions were performed on dibenzothiophene, dibenzyl sulfide, and diphenyl sulfide using MCPBA (3.2 mol equiv MCPBA/S); 95-100% of oxidation to the sulfones occurred for the three model compounds. Those yields were calculated by using gas chromatography, small quantities of the sulfides were observed when using thin-layer chromatography. These experiments show that MCPBA is a very strong oxidizing agent for organic sulfur compounds. Reduction with LiAlH,. If sulfoxides (16) are easily reduced into sulfides by numerous reducing agents, on the contrary sulfones (17) show a certain stability. According to Bordwell and McKellin (14) sulfones involved into pentacyclic cycles are easily reduced by lithium aluminum hydride in ether at 35 OC whereas sulfones involved in hexacyclic cycles and in parafinic chains are reduced only at a higher temperature in high boiling point ethers and with very long reducing times. Use of tetrahydrofuran, in which asphalt is perfectly soluble, could be an excellent compromise between ethylic ether and high boiling point ethers. Pentacyclic sulfones should be easily reduced while the other sulfones should remain as sulfones. Reactions were performed on three model compounds with a stoichiometric ratio LiAlHJS = 10. The yields of these reductions were determined by gas chromatography and given in Table 111. These results perfectly agree with those of Bordwell (14); the sulfone of dibenzothiophene being reduced in less than 1 h. The n-hexyl phenyl sulfone and di-n-hexyl sulfone are reduced only with a yield of 30% and 29% after 3 h. We have thus oxidized a sample of asphalt with MCPBA in chloroform (MCPBA/S = 3.2) and reduced a part of this oxidized sample
1
Anal. Chem. 1982, 54, 691-697
oxidized with
69 1
ACKNOWLEDGMENT Thanks are due to $J.C. Maire and J. Baldy of University of Laws, Economic and Sciences of Aix-Marseille, for establishment of ESCA results, and to F. L. Rigaud (I.P.S.O.I.) for helping us to write this publication in English. LITERATURE CITED
IQ
sulfones easily reduoel by
eyolio inlfides
Figure 4. Study of the nature of sulfur sulfur in the Arabian1 Light asphalt.
of an asphalt. Repartition of
with LiA1H4 (LiA1H4/S = 10). The ESCA spectra of these two samples are shown in Figure 3. MCPBA oxidized the sulfur of asphalt with high yields; 84% of the sulfur being under the state of sulfone and 13% under the state of sulfoxide. After a reduction with LiA1H4for 3 h, one can observe that about 77% rrulfur is again under the state of sulfide. Twenty-three percent of the sulfur is still under the state of sulfones. As the reduction with lithium aluminum hydride is not perfectly specific, to estimate alkyl and aryl alkyl sulfides, it is necessary to use results from Table 111: after 3 h, 70% of the alkyl and alkyl aryl model sulfones are still under the state of sulfones. Therefore the amount of alkyl and aryl alkyl sulfides present in the asphalt is 23/0.7%, Le., 33%. The complement (67%) is involved in cyclic sulfides which are either cyclanic or thiophenic. All the various results are summed up in Figure 4.
(1) Casallnl, Claudlo; Cesarano, Giulia; Mascellanl, Giuseppe Anal. Chem. 1977, 4 9 , 1002-1004. (2) Clements, David M. Anal. Chem. 1977, 4 9 , 1148-1152. (3) Burgasser, A. J.; Sinqley, K. F.; Colaruotolo, J. F. Anal. Chem. 1977, 4 9 , 1987-1989. (4) Schoeffel, James A.; Hubbard, Arthur T. Anal. Chem. 1977, 49, 2330-2336. (5) Lindberg, Bernt J.; Hamrin, Kjell; Johansson, G.; Gellius. Ulrlck; Fahlman, Anders; Nordllng, Carl; Slegbahn, Kal, Phys. Scr. 1970, 1 , 288-298. (6) Rall, H. T.; Thomson, C. J.; Coleman, H. J.; Hopklns, R. L. A.P.I. RP60, Rept. 22, Bu. 659, 1972. (7) Bodin Evellne, ThCe Docteur IngQnieur, Marselile, 1977. (8) Elf, Centre de Recherche Elf Solarize, unpublished report, 1978. (9) Hartough, H. D.; Meisel, S. L. “Compounds wlth Condenses Thlophene Rlngs” The Chemistry of Heterocyclic Compounds”; Intersclence: New York 1954; Chapter 1. (10) “Organic Compound of Sulfur, Selenlum, and Tellurium”; Chemical Society: London; Vol. l , 2, 3. (11) Sampey, John R.; Slagle, Kenneth H.; Reid, Emmet E. J . Am. Chem. SOC.1932, 3401-3404. (12) Belcher, Ronald; Gawargious Youssef A.; Mc Donald, Alison M. G., Mlcrochlm. Acta 1966, 1114-1121. (13) Puickaisky, Charles 5. Anal. Chem. 1969, 4 1 , 843-844. (14) . . Bordwell, F. G.; McKellln, W. H. J . Am. Chem. SOC. 1951, 73, 2251-2253. (15) La Count, Robert B.; Frledman, Sidney J . Org. Chem. 1977, 42, 2751-2754. _ _ _ .
(18) Om, S. “Organic Chemistry of sulfur”; Oae S., Ed.; Plenum: New York, 1977; Chapter 8. (17) Truce, W. E.: Klinger, T. C.; Brandt, W. W. “Organic Chemistry of Sulfur”; Oae S., Ed.; Plenum: New York, 1977; Chapter 10.
RECEIVED for review July 1,1980. Resubmitted and Accepted November 12, 1981. We are indebted to the Elf Research Center of Solaize and C.N.R.S. for financial support.
Structure-Sensitive Search-Match Procedure for Powder Diffractiom Ludo K. Frevel Department of Cheniktty, The Johns Hopkins University, Baltimore, Maryland 2 12 18
Current procedures for ldentifylng a crystalilne phase involve searching for its standard powder pattern In the comprehendve data base available from the Joint Committee on Powder Diffraction Standards. If the unknown phase is not represented In the powder dlffractlon file or If the unknown is an extenslve solid solution, then the empirical flngerprlnt matchlng procedures yield no posltlve Identlkatlon. To remedy thls Iimltation,, the author has resorted to a structuresensitive quasl-Invariant which serves to order the powder dlffraction standards Into various Isomorphous groups. To test the efficacy ,of ldentifylng an isomorphous prototype, a core file of 907 powder patterns has been compiled, the SEARCH FILE being restricted to the common crystaiilne phases encountered in Industry, the ubiquitous crystalline mlnerais, the slmpler lntermetaillc phases, and prototypes of the more common crystal structures. A detailed search-match procedure is descrlbed and Illustrated.
The various search-match procedures for powder diffraction 0003-2700/82/0354-0691$01.25/0
(1-11) are predicated on empirical “fingerprint” matching. When a selected data base of 200 or less standard powder patterns is used for a particular identification, such as identifying copper minerals (2), a simple ordering of the standards in an ascending (or descending) order of the most intense d spacing, d(Il), suffices for spotting the standards required to match the powder pattern of a mineral sample submitted for X-ray diffraction analysis. However, if one uses the comprehensive powder diffraction file (PDF) of the Joint Committee on Powder Diffraction Standards (12),one finds that their current search procedures often are time-consuming and, in the absence of supplemental data such as elemental data, lead to a sizable subfile (10-100 standards) for matching the pattern of a multiphase genuine unknown. Part of the difficulty can be ascribed to the marginal accuracy of old data obtained with 57.3-mm Debye-Scherrer cameras. A more troublesome factor is the ubiquitous encounter with dilute solid solutions of the real world as in the case of minerals, corrosion products, scales, precipitates, alloys, catalysts, etc. To cope with this eventuality, the available search techniques have to widen the Ad error window, thus enlarging the subfile 0 1982 American Chemical Society
