Energy 8z Fuels 1991,5, 258-262
258
a spray dryer. The difference is probably caused by additional SOz adsorption in the almost dry solid during the last 8-9 s of residence time. The results in Tables I and I1 and Figure 5 are for the assumed drop size distribution with a maximum size of 70 pm. If some drops are larger than this, a greater driving force is needed for evaporation, and the SO2 removal will be decreased for most cases. For example, if the largest drop size is 90 pm and the residence time is 1.0 s, the approach to saturation is 64 OF, from Figure 3. For 1600 ppm SOz and T i , = 300 OF, the predicted SO2 removal is 45%. For the same inlet conditions and 70 pm maximum
size, the approach to saturation is 27 O F and the predicted SO2removal is 67%. However, if the residence time is 2.0 s, an approach of 20 O F could be used for both cases, and the predicted SOz removal is 72%. The predictions given here are estimates based on limited data. A more thorough laboratory, pilot-plant, and modeling study of the induct process is now being carried out under DOE sponsorship, and a design handbook for the process will be prepared. However, the simple model used here, with corrections to fit current data, may still be useful for quick comparisons or for predicting the effects of changes in plant operation on SO2 removal.
Role of Sulfur Compounds in Fuel Instability: A Model Study of the Formation of Sulfonic Acids from Hexyl Sulfide and Hexyl Disulfide George W. Mushrush,**tJ Robert N. Hazlett,s Robert E. Pellenbarg,t and Dennis R. Hardyt Fuels Section, Code 6180, Naval Technology Center for Safety and Survivability, Naval Research Laboratory, Washington, D.C. 20375-5000, Department of Chemistry, George Mason University, Fairfax, Virginia 22030, and Hughes Associates, University Boulevard West, Wheaton, Maryland 20902 Received October 4 , 1990. Revised Manuscript Received December 7,1990
There are inconsistencies in the literature regarding the role of organosulfur compounds in general, and sulfonic acids in particular, on the oxidative instability of middle distillate fuels. Comparison of published results between investigations is complicated by differences in a large number of variables. Variations in fuel composition, heteroatom distribution, reaction surface, hydroperoxide concentration, dissolved oxygen, and reaction temperature all contribute to the variation in observed results. In an effort to clarify this situation, with respect to sulfonic acid formation, we have examined the tert-butyl hydroperoxide liquid-phase oxidation reactions at 120 "C of hexyl sulfide and hexyl disulfide in deaerated benzene solvent. We report the product slate and its distribution with reaction time and propose a mechanism that is consistent with the observed product slate.
Introduction Oxidative instability is a continuing problem in the utilization of middle distillate fuels. Fuel degradation is observed to occur under long-term, low-temperature storage conditions (storage instability) as well as shortterm, high-temperature stress (thermal oxidative instability).lg Storage instability is usually defined in terms of the formation of insoluble sediments and lacquers with increasing time. There are at present inconsistencies in the literature as to the role of various organoheteroatom (oxygen, nitrogen, and sulfur) compounds on the observed oxidative instability of these fuels. Comparisons of published results are complicated by the large suite of variables observed between fuels. These include, but are not limited to, molecular composition, reaction surface, dissolved oxygen concentration, hydroperoxide content, reaction temperature, heteroatom type and concentration, and the nature of the fuels studied. All of these variables may alter the chemical reaction pathways involved in fuel degradaJ
Naval Research Laboratory. George Mason University. Hughes Associates.
tion. Although degradation products are observed in nonoxidizing atmospheres, degradation is observed to increase dramatically in the presence of oxygen or oxygenreactive species, Le., hydroperoxides." Thus, instability reactions are dependent on the nature of the potential autoxidation pathways. The role of organosulfur compounds in oxidative instability has been the subject of considerable controversy in the literature. Sulfur compounds have been found to act as antioxidants by decomposing peroxides.' Thompson et al. found that elemental sulfur and disulfides were active promoters of instability, while aliphatic mercaptans and sulfides had little effect on the same fuels.a Schwartz (1) Taylor, W. F. Ind. Eng. Chem. Prod. Res. Deu. 1974, 13, 133.
(2) Scott, G. Autoxidation; Elsevier: Amsterdam, 1965; Chapter 3. (3) Goetzinger, J. W.; Thompson, C. J.; Brink", D. W. U.S.Dept. of Energy Report No. DOE/BETC/IC-83/39 1983. (4) Mushrush, G. W.; Hazlett, R. N.; Hardy, D. R. Ind. Eng. Chem. Res., 1987, 26, 662. (5) Taylor, W. F. Ind. Eng. Chem. Prod. Res. Deu. 1974,13,133;19776, 15, 64. (6) Watkins, J. M.; Muahrush,G. W.; Hazlett, R. N.; Beal, E. J. Energy Fuels 1989, 3, 231. (7) Dension, G. Ind. Eng. Chem. 1969,36,477.
This article not subject to U.S.Copyright. Published 1991 by the American Chemical Society
Energy & Fuels, Vol. 5, No.2, 1991 259
Role of Sulfur Compounds in Fuel Instability et al. reported that alkylmercaptans, sulfides, and disulfides accelerated the formation of deposits in cracked gas~lines.~ Several recent investigations have shown that sulfonic acid species are especially deleterious to fuel storage stability.lOJ1 Offenhauer et al. reported that oxidation of thiols to sulfonic acids, and the subsequent acid-catalyzed condensation of basic nitrogen species, was responsible for sediment formation.12 However, there has as yet been no systematic investigation of the various organosulfur functional groups interacting with mild oxidants, such as hydroperoxides, which could lead to the observed solid product. This paper reports on the reaction between a primary autoxidation product, a hydroperoxide, and organosulfur compounds of the type present in middle distillate fuels. Specifically, we examined the tert-butyl hydroperoxide oxidation at 120 "C of hexyl sulfide and hexyl disulfide in deaerated benzene. We report the complete product slate and its variation with increasing reaction time.
Experimental Section Reagents. tert-Butyl hydroperoxide (t-BHP, 90%), hexyl sulfide, and hexyl disulfide were obtained from Aldrich Chemical Co. These reagents were distilled in vacuo to greater than 99.9% purity. CAUTION. We have had no problems with t-BHP distillations in small quantities (15-mL samples) under sufficient vacuum so that the temperature does not rise above 33 "C. The benzene solvent, Aldrich Gold Label, was refluxed and distilled from CaH,. Method. The reagents, 3 X lo4 mol t-BHP and 9 X lo4 mol sulfur compound in 0.6 mL of solvent, were weighed into 6 in. long, 1/4 in 0.d. borosilicate glass tubes that were sealed at one end and fitted at the other with a type 316 stainless steel valve via a Swagelok (flexible graphite ferrules) fitting. The tube was then subsequently flame-sealed below the valve. The ullage volume (0.30 mL) was kept constant for all runs. The deaerated samples were warmed to room temperature and immersed in a Cole-Parmer fluidizer sand bath. The stress temperature (120 "C) was controlled by a Leeds and Northrop Electromax I11 temperature controller. The total pressure during each run was calculated to be 5.1 atm for the benzene solvent. After the reaction period, the sealed tube was quenched to 77 K and opened. The tube was capped and warmed to room temperature, and internal standards were added. The solution was transferred to a screw cap vial (Teflon cap liner) and stored at 0 "C until analysis. Since a typical chromatogram required 90 min, two internal standards were added. One, p-xylene, afforded quantitation for peaks with short retention times and a second, 1-phenyltridecane, for the peaks with longer retention times. The samples were analyzed by two techniques, both based on gas chromatography. Peak identification for both techniques was based on retention time matching with standards and mass spectrometry. In the first, a Varian gas chromatograph Model 3700 with flame ionization detector (FID) and equipped with a 50 m X 0.20 mm i.d. wall-coated open tubular (OV-101)fused silica capillary column gave the necessary resolution to distinctly separate the individual components. A carrier gas flow of 1mL/min was combined with an initial hold at 50 "C for 8 min, and a ramp for 4 deg/min to a final temperature of 260 "C. In the second technique, head space gases formed during the reaction were analyzed by using a Perkin-Elmer Model Sigma 2 gas Chromatograph equipped with a 6-ft 5A molecular sieve column or a 4-ft Poropak/S column. For the gas analysis, the column was operated at 55 "C. The chromatogram was recorded and (8) Thompson, R. 1949,41, 2715.
Results and Discussion t-BHP Products. The thermal decomposition of alkyl peroxides is quite complex. At temperatures of 120 "C or greater, the mechanism of autoinitiated t-BHP decomposition can be depicted as shown in eqs 1-6. The detailed initiation t-BHP propagation (CH,),CO'
(CHJ3CO'
120 'C
(CH3)3CO'
+ 'OH
-
+ t-BHP
@-scission
(CH3)2CO + 'CH3
hydrogen
t-BuOH
+ (CH3)3COO'
B.; Druge, L. W.; Chenicek, J. A. Znd. Eng. Chem.
(9) Schwartz, F. G.; Whisman, M. L.; Ward, C. C. Bull.--US. Bur. Mines 1964,626, 1. (IO) Hazlett, R. N. Fuel Sci. Technol. Znt. 1988, 6, 185. (11) Hiley, R. W.; Pedley, J. F. Fuel 1987, 67, 1124. (12) Offenhauer, R. D.; Brennan, J. A.; Miller, R. C. Znd. Eng. Chem. 1957,49, 1265.
integrated on a Hewlett-Packard Model 3390A reporting integrator. For this procedure, the valve was left on the tube during the reaction at 120 "C, and after the appropriate period, the tube valve was connected directly to a GC gas sampling valve via a Swagelok connection. An external standard waa used for calibration. A pressure gauge measured the pressure in the sample loop at the time of analysis. The sulfur content of the samples was quantitated by using a Tracor Model 565 gas chromatograph equipped with a sulfurspecific Hall electrolyticconductivity detector (ECD). An all-glass GC inlet system was used in conjunction with a 50 m X 0.2 mm i.d. wall-coated open tubular (OV-101)fused silica capillary column which was operated at a temperature program of 70 "C (8 min), and a programmed temperature ramp of 8 "C/min to a final temperature of 260 "C. Air was replaced with oxygen as the reaction gas in the ECD for improved resolution and baseline. Samples were assayed for sulfonic acid content by using a Dionex Model 2000i ion chromatograph fitted with an Omnipac PAX-500 analytical column and the required Anion-MPIC micromembrane suppressor for the analysis. The mobile phase was 40 mmol NaOH in a 20% H20/80% CH30H matrix delivered at 1.5 mL/min. Operating pressure was f1500 psi, within the operating parameters of the 2000i; a higher flow rate would cause back pressure to exceed the capabilities of the 2000i ion chromatograph. The chromatograph was calibrated with standard solutions of benzenesulfonic acid and heptanesulfonic acid in the mobile phase both to quantitate the analyte and to confirm residence times observed from the stressed samples. Chromatograms were collected and peak areas quantitated by a Hewlett-Packard Model 3392 integrator. A material balance was assessed for each compound. The major peaks of the chromatogram (GC and IC) account for approximately 96% of the t-BHP, 97% of the hexyl sulfide, and 95% of the hexyl disulfide. These runs also formed many partially oxidized products. The very small peaks account for another 3-5%. The product distribution was repeatable to 2-3% for each component. Samples were heated for time periods of 15, 30, and 60 min. All reaction tubes were subjected to the same cleaning procedure. They were filled with toluene, cleaned with a nylon brush, rinsed with toluene twice and then with methylene chloride, and dried in air at 150 OC for 8 h. A search of the literature gives a few examples of catalytic behavior with glass ~ystems;'~-'~ however, when a glass tube was filled with crushed borosilicate glass, thus increasing the surface area, the results at 120 "C for the above time periods were not substantially altered.
(13)Benson, S. W. J . Chem. Phys. 1964,40, 1007. (14) Kirk, A. D.; Knox, A. D. Trans. Faraday SOC.1960, 66, 1296. (15) Hiatt, R. R.; Mill, T.; Mayo, F. R. J. Org. Chem. 1968, 33, 1436.
Mushrush et al.
260 Energy & Fuels, Vol. 5, No. 2, 1991 Table I. Mole Percent Conversion for the Reaction of tert-Butyl Hydroperoxide with Hexyl Sulfide and Hexyl Disulfide in Benzene Solvent at 120 O C for a 60-min
Table 111. Mole Percent Converbion for the Reaction of Hexyl Disulfide with tert -Butyl Hydroperoxide in Benzene Solvent at 120 OC for a 60-min Reaction
Reaction
product' acetone tert-butyl alcohol di-tert-butyl peroxide methane isobutylene unreacted t-BHP
conversion, mol %
conversion. mol % hexyl sulfide hexyl disulfide
1.5 65.9 0.7 1.0 3.6 8.7
4.3 30.2 2.2
1.9 1.1
43.1
Based on the starting moles of t-BHP.
Table 11. Mole Percent Conversion for the Reaction of Hexyl Sulfide with tert-Butyl Hydroperoxide in Benzene Solvent conversion, mol %, at reaction time, min hexyl sulfide products' hexyl sulfoxide hexyl sulfone hexyl disulfide hexanal hexane hexene hexyl hexanethiosulfinate unreacted hexyl sulfide trace productsb
15
30
60
74.8
81.6 3.0 0.5 0.3 0.3
0.1
85.9 1.4 0.5 0.2 0.1 0.1 0.1
22.5 6.7
13.9 7.8
1.2
0.3 0.2 0.1 0.1
0.1 0.1 8.2
9.7
'Based on the starting moles of hexyl sulfide. bSummation of small peaks.
mechanism of peroxide decomposition, however, is dependent upon the specific reaction conditions employed, since radical behavior is sensitive to structure, solvent, and stereoelectronic effe~ts.'~J' Small amounts of acetone were observed from the reaction with each sulfur compound run. The greater yield of tert-butyl alcohol (t-BuOH), Table I, compared to acetone definitely shows that hydrogen abstraction was favored over cleavage for t-BHP products under the conditions of this study.'* The quantities in Table I are based on percent conversion from the moles of reactants originally present. Products derived from hexyl sulfide, Table 11,and hexyl disulfide, Table I11 are calculated on the basis of the starting amount of hexyl sulfide or hexyl disulfide. The t-BHP products (for example, t-BuOH) were similarly calculated on the basis of the starting amount of t-BHP. Oxidation products (Le., t-BHP + hexyl disulfide) are calculated on the basis of hexyl disulfide. For increasing reaction time periods, the yield of acetone increased slightly. This indicated that at long reaction ti,mes pscission was a viable competing precess to the predominant hydrogen abstraction process. Of the radicals generated by the processes depicted in steps 1-4,the tert-butylperoxy radical, step 4, was probably the least reacti~e.'~J~ Thus, it would be expected to selectively form termination products. Products that appear to be formed by termination steps involving other radicals such as the alkoxy radical (Le., olefin products) were most likely formed by an SH2 type process. Gaseous Products. The gaseous products formed included isobutylene, methane, and trace amounts (not (16) Walling, C. Free Radicals in Solution; Wiley Interscience: New York, 1957. (17) Hiatt, R. R.; Mill, T.; Mayo, F. R. J. Org. Chem. 1968,33, 1416. (18) Walling, C.; Wagner, P. J. J . Am. Chem. SOC.1964, 86, 3368. (19) Howard, J. A.; Ingold, K. U. Can. J. Chem. 1967,43,785.
hexyl disulfide product' hexyl hexanethiolsulfinate hexyl hexanethiosulfonate hexyl hexanedisulfone x-hexenyl hexyl disulfide(s) hexanesulfonic acid unreacted hexyl disulfide trace productsb hexanethiol hydrogen sulfide hexene butanethiol sulfur dioxide
1.4 10.5 15.6 1.4 0.05 57.1 9.2 methyl sulfide hexane
a Baaed on the starting moles of hexyl disulfide. Summation of small peaks.
quantitated) of sulfur dioxide and hydrogen sulfide. As indicated in Table I, for a 60-min reaction time period, isobutylene was 3.6% and methane was 1.0% for runs employing hexyl sulfide and 1.1%and 1.9%, respectively, for runs with the hexyl disulfide. No free oxygen was observed by gas chromatography in any of the runs. Isobutylene probably resulted from the acid-catalyzed dehydration of tert-butyl alcohol. Acids can be generated by several pathways, as will be shown later. The yield of methane was similar to that of acetone. This was an expected result since both products form via @-scissionof the tert-butoxy radical, step 2. The reactive methyl radical easily abstracts hydrogen from either the hexyl sulfide or hexyl disulfide as shown in later mechanism steps. The lack of measured oxygen does not mean that it was not formed. It could form from a terminating reaction of two tert-butylperoxy radicals and then be consumed immediately by any of several pathways. Hexyl Sulfide Reaction Products. The major product observed from the oxidation of hexyl sulfide by tBHP was hexyl sulfoxide, Table 11. Its yield varied from 74.8% at 15 min to 85.9% at 30 min decreasing to 81.6% at 60 min. As the sulfoxide decreased in yield, the other oxidation product hexyl sulfone increased from 1.2% at 15 min to 3.0% at 60 min. Minor products, yields less than 0.3% , included hexanal, hexyl disulfide, hexene, hexane, and hexyl hexanethiosulfinate. Many trace products were formed. Most of these products were partially oxidized substances that could not be readily identified by mass spectrometry. The major product, hexyl sulfoxide, could result from several mechanisms. The most likely pathway to the sulfoxide would be from the reaction of the t-BHP, step 7, with the starting hexyl sulfide followed by a rapid proton transfer and 0-0bond rupture, step 8.20 Expansion of ( C d l d 8 + C4HgOOH
-
1 % 'I
(cd13)Z~J O--O-C4H9
H
+ C4H@H
(C&l&S-O
I
(7) (8)
the sulfur valence shell is probable in step 8. Other mechanisms involve the reaction of an oxygen-centered radical, such as peroxy, and the sulfide with a concomitant P-scission.21 The stability of the hexyl sulfoxide is illus(20) Curci, R.; Giovine, A,; Modena, G . Tetrahedron 1966,22, 1235, 1227. (21) Rahman, A,; Williams, A. J. Chem. SOC.(B) 1970, 1391.
Energy & Fuels, Vol. 5, No.2, 1991 261
Role of Sulfur Compounds in Fuel Instability trated by the modest decrease in yield at the 60-min reaction time. The usual oxidation product of an alkyl sulfoxide is the corresponding sulfone, step 9. The hexyl (C6H&S-O
+ '00CdHg
-
(C&13)2S-02
+ C4H90'
(9)
sulfone shows only a slight increase in yield from 1.2% at 15 min to 3.0% at 60 min reaction time. The further oxidation of an alkyl sulfoxide to a sulfone, step 10, is 0
believed to occur by a mechanism similar to that in step 8.20The formation of an alkyl sulfone is a facile reaction only in the presence of a strong oxidant or when the reaction is catalyzed by transition-metal ions.22 Sulfoxides with one or more hydrogens on a carbon to the sulfinyl group will undergo thermal decomposition at moderate temperatures to give olefins by a stereospecific cis elimination of a sulfenic acid, RS-OH.= No evidence of sulfenic acids was noted in the present work or in other literature references.% Indirect evidence, however, does implicate a sulfenic acid intermediate. As noted, hexyl sulfide yields hexyl sulfoxide as the major oxidation product. In a model study employing hexyl sulfoxide with t-BHP in benzene solvent at temperatures of 65-120 "C, the following reaction steps, (11)and (12), offered a reasonable explanation for the small amount of sulfonic acid that was observed to form. Products detected by GC/MS included hexyl sulfone from the starting hexyl sulfoxide; from the t-BHP, the same slate of products as depicted in steps 1-6; and by ion chromatography, hexanesulfonic acid. (C6H13)2S-O
+ HOOC4Hg
(C6H13)S-OH
-
+ HOOC4Hg
2(C6H,,q)S-OH
-
+ C4H900' (11)
(C6H13)S03H + C4H90H (12)
Alkyl sulfides are thermally quite stable with C-S bond energies of about 74 kcal/m01.~~ They are also quite unreactive to molecular oxygen.26 Minor products observed from the oxidation of hexyl sulfide included hexyl disulfide, hexylthiosulfinate, hexane, and hexene. The thiyl radical product, hexyl disulfide, and the hexene observed during alkyl sulfide oxidation can be rationalized in terms of 8-scission type reaction (13). The tert-butylperoxy, step 4, or the tert-butoxy, step 1, radicals could abstract the cy or 0 hydrogens to the sulfur atom of an alkyl sulfide. An SH2 mechanism followed by a subsequent 0-scission would yield both an olefin and a hexanethiyl radical, step 13.27p28Results in Table 11, indicated that C&3-S-CH&H.&4H0
+
C4H&'
[C~~I~-S-CH&C~&
-
p-scission is favored over the corresponding a-process. Hexyl disulfide resulted from a dimerization of the hexanethiyl radical, step 14. Thiyl radical products were
-
2C6H13S0 C6H13-S-S-C6H13 (14) observed to be minor products at all reaction times. Consequently, the termination product, hexyl disulfide was observed to be low in yield at all reaction times. Hexyl hexanethiosulfinate probably resulted from an oxidation of the hexyl disulfide by the peroxy radical. On the basis of bond energies, the peroxy radical was the least reactive and most plentiful radical in the system.= Thus, a reaction involving this radical would be favored over the more reactive, but less plentiful tert-butoxy radical, step 15. Only trace amounts of this partially oxidized sul(15) fur-oxy compound were detected in the reaction mixture. The hexanal product could arise from several mechanisms. In a complicated reaction mixture no one reaction step could be invoked to explain the existence of trace products. The most probable among these was the thermal rearrangement of the major product, hexyl sulfoxide.2a Hexyl Disulfide Reaction Products. The hexyl disulfide reaction products observed in Table I11 could all be derived from the partially oxidized disulfide. The major product was hexyl hexanedisulfone at 15.6% followed by the hexyl hexanethiosulfonateat 10.5% and a considerable smaller amount of the singly oxygenated hexyl hexanethiolsulfinate at 1.4%. Hexanesulfonic acid, the final oxidation product of a disulfide, was also found in low yield. Other products included various mixed alkenyl and alkyl hexyl disulfides. For convenience, these are grouped in Table I11 as x-hexenyl hexyl disulfides with a yield of 1.4%. Trace and partially oxidized sulfur products totaled 9.2%. The most likely mechanism to a thiolsulfinate would be from the reaction of the tert-butyl hydroperoxide, step 16,
I ? I Q-O-C4Ho
with the hexyl disulfide followed by a rapid proton transfer and 0-0 bond rupture as depicted for a sulfide.20 A second probable mechanism involves the attack of an oxygen-centered radical, i.e., tert-butylperoxy, on sulfur followed by a /3-scission.21 The tert-butylperoxy radical was probably the most plentiful radical present in the system.% This condition would favor an oxidation reaction, steps 17 and 18, in-
t w
144H000'
SY
+ C 4 H a
Cd+13S*
+
-
P
+ (c&13)82
0
H2C=CHC,Ho
(13)
(22) Henbeat, H. B.; Khan, K. A. Chem. Commun. 1968, 17, 1036. (23) Zuev, Y. S.; Ivanova, S. A.; Mekh. Polim. 1970,3, 539. (24) Shelton,J. R Developments in Polymer Stabilization-$ Applied Science Publiihers: Eesex, England, 1981. (25) Denisov, E. T. Liquid Phase Reaction Rate Constants; IFI/Plenum: New York, 1974. (26) Correa, P. E.; Hardy, G.; Riley, D. P. J . Org. Chem. 1988,53,1695. (27) Migita, T.; Kosugi, M.; T h y m a , K.; Nakagawa,Y. Tetrahedron 1973,29, 51.
I
Cdi3-s-s-c&~
+
C4H00.
(18)
volving this radical over a terminating step involving the very reactive tert-butoxy radical that would be expected, rather, to propagate the chain. The resulting hexyl hexanethiosulfinate once formed is not very stable, reacting (28) Ohno, A.; Ohnishi, Y. Znt. J. Sulfur Chem. 1971,3, 203. (29) Mushrush, G. W.; Hazlett, R. N.; Eaton, H. G. Znd. Eng. Chem. Prod. Res. Deu. 1985.24, 290.
Mushrush et al.
262 Energy & Fuels, Vol. 5, No. 2, 1991
with additional t-BHP or tert-butylperoxy radical to ultimately form the observed stable hexyl hexanedisulfone, step 19-30 + C4H&o'
(C&&-S-S
-
0 0 (c&3)2-s-S
I I
+
I I
c 4 H ~ o ' (1s)
0 0
The stability of this product in step 19 is confirmed by the low yield of sulfonic acid, 0.05 mol % . Sulfonic acid could, however, be formed by other mechanisms. The simplest and probably correct mechanism would involve the scission of the S-S bond from step 19 by the t-BHP resulting in 2 mol of hexanesulfonic acid and two tertbutoxy radicals, step 20. The fate of the tert-butoxy 0 0 (C&&-s-s
I I + I I
C4H@OH
-
2C6H13S03H
+
2C4H@'
(20)
0 0
radical would probably involve an immediate hydrogen abstraction step resulting in more tert-butyl alcohol. Another possible mechanism would involve a disulfide reacting through the sequence represented by step 21. R = (CJ313) RSSR + t-BHP --* [RSOH] RSOzH RS03H (21)
-
-
In the present study, no evidence (GC/MS or ion chromatography) for the presence of another acid species was detected. Thus, step 20 represents a more probable pathway to the sulfonic acid formed. The x-hexenyl hexyl disulfide products could arise from any number of hydrogen abstraction type reactions. Since a hydrogen abstraction step was not possible with the benzene solvent, the starting disulfide represents the most probable molecule for reaction, step 22, with these reactive moieties. tC4H@'
+ (W13)2%
C
hyd-
CeH13-S-S-CH2eH(C4Hd
atrodbn
+
1-BuOH
C~HI~-S-S-CH=CH(C,~)
hydmpn&#racIion
+
1-BuOH
C
(22)
Trace products included many partially oxidized substances that could not be identified. A mechanism that accounted for trace products is dificult to construct. The trace product yield was variable in different runs, indicating that a definitive radical pathway was not operative. However, speculation would indicate that these products could arise from either an S-S bond scission, a probable pathway, or the slightly less probable C-S bond scission. A C-S bond scission in the hexyl disulfide would yield the C6Hl3-S-S' radical. An oxidation process with this in(30) Block, E. Reactiom of Organosulfur Compounds; Academic Press: New York, 1978.
termediate could account for the observed SOz while a hydrogen abstraction reaction would lead to H2S and/or methyl sulfide. This C-S bond scission in the disulfide would also lead to the hexene trace products. An S-S scission would lead to the observed hexanethiol products.
Conclusions The observed deterioration of fuels can manifest itself in many ways, including the formation of insoluble deposits both in storage and in an engine fuel system or nozzle. Organosulfur compounds and hydroperoxides are some of the reactive species in middle distillate fuels which may be involved in the deterioration process. Traces of organosulfur compounds such as sulfonic acids have been implicated in the deposit formation process. This paper examined the tert-butyl hydroperoxide induced oxidation of both hexyl sulfide and hexyl disulfide in deaerated benzene at 120 "C. The reactions gave a common suite of products regardless of reaction time. The yield of individual components, however, varied significantly with reaction time. The major product derived from the t-BHP was tBuOH. In the presence of hexyl sulfide 65.9 mol % conversion of the t-BHP to t-BuOH was observed, while in the presence of hexyl disulfide, only 30.2 mol % was converted. Other observed t-BHP products included methane, acetone, di-tert-butyl peroxide and isobutylene. Theae products were observed to form in the range of 1-4.3 mol 9%. The major product from the hexyl sulfide oxidation was hexyl sulfoxide. Its yield varied from 74.8 mol % at 15 min to 85.9 mol % at 30 min and then decreasing because of secondary reactions to 81.6 mol Ti a t 60 min. Other sulfur-containing products were hexyl sulfone, hexyl disulfide, and hexyl hexanethiosulfinate. The conversion of the hexyl sulfoxide to the hexyl sulfone was the major secondary reaction of the sulfoxide. Hexyl sulfone was observed to gradually increase from 1.2 mol % at 15 min to 3.0 mol 7% at 60 min. The other sulfide oxidation products were present at yields of
