Ind. Eng. Chem. Res. 2001, 40, 1225-1233
1225
A Novel Desulfurization Process for Fuel Oils Based on the Formation and Subsequent Precipitation of S-Alkylsulfonium Salts. 2. Catalytic-Cracked Gasoline Yasuhiro Shiraishi, Kenya Tachibana, Yasuto Taki, Takayuki Hirai,* and Isao Komasawa Department of Chemical Science and Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan
A desulfurization process, based on the formation and subsequent precipitation of S-alkylsulfonium salts using alkylating agents (CH3I and AgBF4), has been applied to the desulfurization of catalytic-cracked gasoline (CCG). The desulfurization reactivity of each sulfur compound (thiol, disulfide, benzothiophene, tetrahydrothiophene, and thiophenes) in n-decane solution, as a model gasoline, was compared with that obtained from actual CCG. The sulfur compounds in CCG are methylated by the addition of the alkylating agents under moderate conditions and are removed as the precipitates of the corresponding S-alkylsulfonium salts. By employing this new process, the sulfur content of the CCG was decreased from 100 ppm to less than 30 ppm. The benzothiophene in CCG was found to be the most difficult compound to desulfurize, whereas the thiophenes were the most difficult compounds for the model gasoline. Although the olefin concentration was decreased significantly following desulfurization, the resulting CCG demonstrated as high an octane number as the feed CCG. The results thus suggest that the proposed process is satisfactory for application to the desulfurization of CCG. Introduction Catalytic-cracked gasoline (CCG) is produced by fluid catalytic cracking units from vacuum gas oil or atmospheric residue and is one of the major components of motor gasoline, because of its high concentration of olefins. CCG, however, contains a high level of sulfur, and therefore, the desulfurization of CCG is becoming increasingly important from an environmental point of view. The sulfur content of commercial gasoline is limited presently to 100 ppm in Japan and 200 ppm in Europe, and these limits will certainly be tightened to 30 ppm soon.1 The current hydrodesulfurization (HDS) technology can easily desulfurize the sulfur-containing compounds in CCG, but it causes a simultaneous hydrogenation of the olefins.2,3 In the development of a more selective desulfurization process, a new approach, not limited to HDS technology, is required. As a result, the new concept of a desulfurization process for CCG based on the combination of the photochemical oxidation of the sulfur-containing compounds and liquid-liquid extraction of the resulting reaction compounds has been proposed.4 In our previous paper,5 a novel desulfurization process for light oils was investigated, based on the precipitation of S-alkylsulfonium salts produced by the reaction of sulfur-containing compounds with alkylating agents (CH3I and AgBF4). The dibenzothiophenes and benzothiophenes in light oils are methylated by the addition of the alkylating agents, in the presence of dichrolomethane under moderate conditions, and are removed as precipitates. Thus, in this way, the sulfur content of the light oils can be decreased successfully to less than * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +81-6-6850-6273. Tel.: +81-6-6850-6272.
0.005 wt %. This new process is applied here to the desulfurization of CCG. To clarify the desulfurization reactivity of the sulfur-containing compounds, n-decane solutions containing each sulfur compound are used to represent a model gasoline. The relative desulfurization reactivities of the various sulfur compounds have been correlated with electronic configuration parameters obtained by semiempirical molecular orbital (MO) calculation, and the key factors governing the reactivity of the sulfur compounds have been clarified. The desulfurization of the actual CCG was then carried out, and the feasibility of the process was confirmed. Finally, the properties of the resulting CCG were compared with those of the feed CCG and also with the properties of the material obtained according to the previous photochemical desulfurization process and the applicability of the present process to the refining process for CCG was examined. Experimental Section The model sulfur-containing compounds and the actual CCG material employed were identical to those used in a previous photochemical desulfurization study.4 The properties of CCG are summarized in Table 1. The desulfurization of CCG was carried out according to a previously described procedure,5 with slight modifications. To prevent the evaporation of CCG, the reaction was carried out at a temperature of 273 K, using an ice bath. AgBF4 was added to the CCG (100 mL), and then CH3I was added carefully dropwise via a syringe over a period of 10 min and under a nitrogen atmosphere. For the desulfurization of light oils,5 a dichloromethane solvent was added to the light oil in order to accelerate the desulfurization. CCG has a low boiling point range of 314-469 K, which overlaps that of dichloromethane. Thus, dichloromethane could not be used in the present
10.1021/ie000548e CCC: $20.00 © 2001 American Chemical Society Published on Web 01/18/2001
1226
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001
Table 1. Properties and Composition of (i) Feed Catalytic-Cracked Gasoline (CCG) and Oils Following Desulfurization (ii) by the Present Process and (iii) by the Previously Proposed Photochemical Process4
total sulfur
(ppm)
compositionc saturates naphthenes aromatics olefins
(vol %)
distillationc IBP 10 vol % 30 vol % 50 vol % 70 vol % 90 vol % 95 vol % EP
(K)
research octane number
(-)
(i) feed
(ii) producta
(iii) productb
99.4
28.8
28.3
32.7 7.9 16.4 43.0
33.3 10.2 20.0 36.5
34.6 9.7 15.6 40.1
314.5 331.0 352.0 368.5 393.0 438.5 454.5 469.5
319.5 341.0 366.0 387.0 408.5 445.5 468.5 482.0
328.0 338.5 357.0 377.0 404.5 445.5 458.0 469.5
90.6
88.4
88.1
a
Desulfurization conditions are as follows: temperature, 273 K; reaction time, 5 h; [AgBF4]initial and [CH3I]initial ) 10- and 20fold molar excess for the initial sulfur concentration of the feed CCG, respectively. b Data are cited from the previous paper.4 c By the JIS K 2536 gas-chromatographic method.
study. Following stirring for 5 h, the byproducts AgI and S-alkylsulfonium salts, which precipitated simultaneously at the bottom of the flask, were recovered completely by decantation from the resulting CCG. The S-alkylsulfonium salts were separated from the AgI by the addition of dichrolomethane and were then analyzed following evaporation and recrystallization. n-Decane solutions (model gasoline, 50 mL), each containing an individual sulfur compound (54 mM), were also employed in order to clarify the relative reactivity of sulfur compounds for the desulfurization studies. The analyses of the resulting gasoline and precipitate were carried out according to the previously described method.5 The concentrations of the individual sulfur-containing compounds in CCG were analyzed by gas chromatography with atomic emission detection (GC-AED), according to the previously described procedure.4 The electron density on the sulfur atom, for thiophenes, was calculated by means of the WinMOPAC ver. 2.0 software (Fujitsu Ltd.).5 Results and Discussion 1. Desulfurization of Sulfur-Containing Compounds from Decane. 1.1. Products and Reaction
Pathway for Sulfur Compounds. The individual sulfurcontaining compounds in CCG were identified in a previous paper.4 As shown in Table 2, CCG contains a large quantity of alkyl-substituted thiophenes (72%) and smaller amounts of disulfides, thiols, tetrahydrothiophenes, and benzothiophene. To examine the feasibility of the present desulfurization process, eight kinds of sulfur-containing compounds, as listed in Table 3, were each dissolved in n-decane and used to represent model gasoline for the desulfurization experiments. The addition of alkylating agents (CH3I and AgBF4) to the decane solution was found to give an insoluble precipitate for all of the sulfur compounds. Most of the precipitates, especially for thiophenes, were very oily and hardly crystallized at room-temperature conditions, such that, in order to recover the precipitate completely, the resulting decane solution was required to be cooled to 273 K. The analytical data obtained for each precipitate are summarized in Table 3. As described in a previous paper,5 benzothiophene (no. 5) is removed from decane solution as the corresponding S-methylsulfonium salt. The IR spectra for all of the precipitates show a strong absorption band at the range of 1000-1100 cm-1, which is attributable to a BF4- counterion. The 1H and 13C NMR spectra for the precipitates obtained from thiophenes (nos. 1-4) and tetrahydrothiophene (no. 6) agree reasonably with those of corresponding S-methylsulfonium salts synthesized by standard procedures.6,7 n-Butanthiol (no. 7), however, gives rise to a n-butyldimethylsulfonium tetrafluoroborate8 as the only product. The hydrogen atom of n-butanthiol is highly polarized and is easily substituted by the methyl group of CH3I to produce intermediate n-butylmethyl sulfide, as shown schematically in Scheme 1. This is then methylated via SN2 displacement by the CH3I-AgBF4 complex,9,10 to form the final n-butyldimethylsulfonium salt. The precipitate obtained from diethyl disulfide (no. 8), as shown in Figure 1, exhibits a complicated 1H NMR spectrum. The spectrum consists of two sharp singlets at 2.70 and 2.75 ppm and a large number of peaks at 1.3-1.5 and 3.0-3.5 ppm, thus indicating that this precipitate is a mixture of sulfonium salts with differing structures. The diethyl disulfide is first methylated by alkylating agents to form an (ethylmethylthio)ethylsulfonium salt intermediate compound, as shown in Scheme 2.1. This compound then reacts rapidly with other disulfides, with an accompanying S-S bond cleavage, to give rise to ethylmethyl sulfide (Et-S-Et) and (diethylthio)ethylsulfonium salt [Et-S+-(S-Et)2], as
Table 2. Relative Quantities of Individual Sulfur-Containing Compounds (i) in Feed Catalytic-Cracked Gasoline (CCG) and in the Oils Obtained Following Desulfurization in the Presence of (ii) 10-Fold and (iii) 20-Fold Molar Excesses of AgBF4 for the Initial Sulfur Concentration of the Feed CCGa (i) feed CCG
(ii) following desulfurization
(iii) following desulfurization
sulfur compounds
sulfur (ppm)
sulfur (ppm)
remaining (%)
sulfur (ppm)
remaining (%)
thiophene disulfides, thiols C1-thiophenes tetrahydrothiophene C1-tetrahydrothiophenes C2-thiophenes C3-thiophenes C4-thiophenes benzothiophene total sulfur
5.3 5.7 15.1 2.8 4.1 21.1 19.6 16.1 9.6 99.4
0.9 0.1 1.8 0.3 1.1 1.8 4.7 9.1 9.0 28.8
17.0 1.8 12.0 10.7 26.8 8.5 24.0 56.5 93.8
0.7 0.1 1.0 0.2 1.0 1.1 2.9 5.3 5.2 17.5
13.2 1.8 6.6 7.1 24.4 5.2 14.8 32.9 54.2
a
[CH3I]initial ) 20-fold molar excess for the initial sulfur concentration of the feed CCG.
2,5-dimethyl thiophene
2,3,5-trimethyl thiophene
benzothiophene
tetrahydrothiophene
n-butanthiol
diethyl disulfide
3
4
5
6
7
8
C2H5-S-S-C2H5
n-C4H9-SH
structure
87.9
100
96.8
48.8
41.7
24.4
39.3
33.4
desulfurization yield (%)
mixture (8 compounds)
product
oil
oil
256-259
426-428
oil
oil
oil
oil
mp (K)
148, 142, 139, 132, 27.4 (S+-CH3), 15.0, 13.8, 11.5 31.9 (S+-CH3), 124.6, 127.7, 128.5, 131.6, 133.7, 134.5, 142.1, 143.0
6.90 (s, 1H), 3.08 (s, 3H, S+-CH3), 2.37 (s, 3H), 2.26 (s, 3H), 2.11 (s, 3H)
3.22 (s, 3H, S+-CH3), 7.34 (d, J ) 5.6 Hz, 1H), 7.76 (t, J ) 8.4 Hz, 1H), 7.85 (t, J ) 6.8 Hz, 1H), 7.92 (d, J ) 5.6 Hz, 1H), 7.99 (d, J ) 8.0 Hz, 1H), 8.20 (d, J ) 8.4 Hz, 1H) 3.57-3.43 (m, 2H), 3.30-3.20 (m, 2H), 2.69 (s, 3H, S+-CH3), 2.38-2.19 (m, 4H) 3.18 (t, 2H, J ) 7.69 Hz), 2.78 (s, 6H, S-CH3), 1.78-1.68 (m, 2H), 1.52-1.41 (m, 2H), 0.962 (t, 3H, J ) 7.32 Hz)
142, 138, 27.2 (S+-CH3), 14.1
6.89 (s, 2H), 3.12 (s, 3H, S+-CH3), 2.40 (s, 6H)
43.7, 26.4 (S+-CH3), 25.4, 22.1, 13.5
45.9, 29.1 (S+-CH3), 26.0
145, 144, 137, 128, 28.1 (S+-CH3), 13.9
128, 126, 20.2 (S+-CH3)
NMR data (product)
13C
7.40 (m, 1H), 7.34 (m, 1H), 7.12 (s, 1H), 3.15 (s, 3H, S+-CH3), 2.45 (s, 3H)
7.55 (s, 4H), 3.17 (s, 3H, S+-CH3)
NMR data (product)
1H
Desulfurization conditions: reaction time, 5 h; temperature, 273 K; [sulfur compound]initial ) 54 mM; [AgBF4]initial ) 108 mM; [CH3I]initial ) 1080 mM.
2-methyl thiophene
2
a
thiophene
compound
1
no.
Table 3. Desulfurization Yields from Decane Solution and Products Obtained for the Various Sulfur-Containing Compoundsa
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 1227
1228
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001
Figure 1. 1H NMR spectra for the precipitate obtained by the reaction of diethyl disulfide with the alkylating agents.
Scheme 1. Reaction Pathway for n-Butanthiol with Alkylating Agents Figure 2. Correlation between the desulfurization yield of the differing thiophene compounds from decane solution and the electron density on the sulfur atom for the thiophenes. The electron density is calculated for (a) the sulfur σ lone-pair orbital, (b) the sulfur π lone-pair orbital, and (c) all of the occupied orbitals on the sulfur atom. 1, thiophene; 2, 2-methylthiophene; 3, 2,5dimethylthiophene; 4, 2,3,5-trimethylthiophene.
shown in Scheme 2.2. Helmkamp et al.11 and Smallcombe and Caserio12 have reported that (dimethylthio)methylsulfonium salt [(Me)2-S+-S-Me] reacts with dimethyl disulfide (Me-S-S-Me) to produce dimethyl sulfide (Me-S-Me) and (dimethylthio)methylsulfonium salt [Me-S+-(S-Me)2], according to the same reaction pathway, as shown in Scheme 2.2. Alkylsulfides and -disulfides are reported to be methylated by both thiosulfonium and sulfonium salts.11 These reactions might therefore be expected to occur also in the present experiments and are shown in Schemes 2.4 and 2.7. The precipitate obtained from diethyl disulfide can therefore be expected to be a mixture of eight compounds (Scheme 2.a-h), and each resonance in the 1H NMR spectrum can therefore be explained, as shown in Figure 1. From these results, all of the sulfur compounds, when dissolved in decane solution, are shown to be removed successfully as precipitates. 1.2. Desulfurization Reactivity. The desulfurization yields of the sulfur-containing compounds from decane solution are shown in Table 3. Desulfurization of nbutanthiol, diethyl disulfide, and tetrahydrothiophene proceeds very effectively, with almost 100% desulfurization yield being attained. The desulfurization reactivity of the sulfur-containing compounds lies in the ranking order n-butanthiol > tetrahydrothiophene > diethyl disulfide > benzothiophene > thiophenes, with nonsubstituted thiophene being the most difficult compound to desulfurize, according to the present process. As shown in Table 2, the thiophenes constitute the main sulfur-containing compounds in CCG. It is therefore necessary to study the desulfurization reactivity of the thiophenes in further detail.
Figure 3. Calculated energy-level diagram and schematic representation of electron density distribution for thiophene.
As previously described,5 the S-methylation reactivity depends on the electron density on the sulfur atom of the sulfur compounds. To clarify the reactivity of the thiophenes, the electron density on the sulfur atom, as calculated by the MO method, was correlated with the obtained desulfurization yield. The results are summarized in Figure 2. As shown in Figure 3.i, the frontier orbitals for thiophenes are sulfur π lone-pair orbitals of the HOMO, with the electron density lying perpendicular to the plane of the thiophene molecule, as occurs also in the case of dibenzothiophenes and benzothiophenes.5 Thus, any relationship between the desulfurization yield and the electron density, as calculated for this π orbital, is hardly apparent, as shown in Figure 2a. As for dibenzothiophenes and benzothiophenes,5 thiophenes also have a sulfur σ lone-pair orbital with the electron density lying parallel to the
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 1229 Scheme 2. Reaction Pathway for Diethyl Disulfide with Alkylating Agents
plane of the molecules and on a lower-energy-level orbital than the HOMO, as shown in Figure 3.ii.13,14 However, no relationship between the electron density, as calculated for this σ orbital, and the desulfurization yield is again apparent, as shown in Figure 2b. The previous study revealed that the S-methylation reaction for dibenzothiophenes and benzothiophenes with CH3I occurs in a direction parallel to the molecules, because of steric hindrance caused by the aromatic and thiophenic rings.5 Thiophenes have a rather smaller molecular structure than the above compounds. It is considered that the approach between the thiophenes and CH3I therefore occurs more easily than it does the above compounds, such that the occupied orbitals other than the sulfur σ orbital also contribute to the Smethylation reaction of the thiophenes. Thus, when the net electron densities on the sulfur atom, as calculated for all of the occupied orbitals on the sulfur atom, are correlated with the desulfurization yield, a linear relationship is obtained for nonsubstituted, 2-methylated, and 2,3,5-trimethylated thiophenes, as shown in Figure 2c. However, the reactivity for 2,5-dimethylthiophene is not included in this correlation. Figure 4 shows the electron density distribution of the alkyl-substituted thiophenes. The phase of the sulfur σ lone-pair orbital for 2,5-dimethylthiophene, as shown in Figure 4.ii.b, differs from that for the other thiophenes. This suggests that the σ orbital for 2,5-dimethylthiophene consists of antibonding orbitals, whereas for the other thiophenes,
Figure 4. Schematic representation of the electron density distribution for (i) the sulfur π lone-pair orbital and (ii) the sulfur σ lone-pair orbital for (a) 2-methylthiophene, (b) 2,5-dimethylthiophene, and (c) 2,3,5-trimethylthiophene.
it consists of bonding orbitals. As a result, a lower desulfurization yield is obtained for 2,5-dimethylthiophene than expected by MO calculation. 1.3. Effect of Hydrocarbons on Desulfurization. As shown in Table 1, olefinic, naphthenic, and aromatic hydrocarbons form the main constituents of CCG. The effect of these hydrocarbon compounds on the desulfurization of sulfur-containing compounds was therefore investigated using a model gasoline solution. 1-Octene and 1,7-octadiene, tetralin, and Decalin were employed
1230
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001
Table 4. Variations in (i) Desulfurization Yield from Model Gasoline and (ii) Selectivity, r, for Sulfur-Containing Compounds in the Presence of Hydrocarbon Compoundsa no addition
1-octene
1,7-octadiene
tetralin
decalin
(i)
(i)
(ii)
(i)
(ii)
(i)
(ii)
(i)
(ii)
sulfur compounds
(%)
(%)
(-)
(%)
(-)
(%)
(-)
(%)
(-)
tetrahydrothiophene thiophene 2-methylthiophene 2,5-dimethylthiophene 2,3,5-trimethylthiophene benzothiophene
96.8 33.4 39.3 24.4 41.7 48.8
99.1 55.5 64.8 64.5 84.3 81.6
0.966 0.901 0.938 0.882 0.900 0.905
98.1 63.6 65.2 34.8 79.4 49.9
0.978 0.655 0.681 0.613 0.568 0.500
100 41.7 41.0 22.0 34.5 9.8
0.995 0.993 0.998 0.972 1.000 0.503
100 32.8 35.6 18.6 28.3 9.7
0.997 0.995 0.999 0.988 1.000 0.520
a
Desulfurization conditions are identical to those in Table 3. [Hydrocarbon compound]initial ) 540 mM.
as the model compounds representing the mono- and di-olefins and aromatic and naphthenic hydrocarbons, respectively.2-4 In these experiments, 540 mM of each compound was added to decane solution, together with 54 mM of the particular sulfur compound, and then the mixture used for desulfurization. The results are summarized in Table 4, where the desulfurization selectivity, R,4,5 for each sulfur-containing compound is defined as
R ) [desulfurization yield of sulfur compound]/ ([desulfurization yield of sulfur compound] + [reaction yield of hydrocarbon compound]) (1) The addition of Decalin and tetralin was found to decrease the desulfurization yields for almost all of the sulfur compounds. GC-MS analysis reveals that the Decalin is converted by reaction with the alkylating agents to form mono-, di-, tri-, and tetramethylated Decalins, as was found also for the case of tetralin.5 MO calculations reveal that the Decalin has a large electron density distribution on its naphthenic ring, as occurs also for the case of tetralin.5 The decrease in the desulfurization yields, therefore, results as a consequence of the methylation of these hydrocarbons occurring competitively with the S-methylation reaction.5 Both the desulfurization yield and the selectivity for benzothiophene, as shown in Table 4, are seen to decrease more significantly than those for other compounds. This is because hydrocarbon compounds, having a large electron density, are methylated by the Smethylbenzothiophenium salt to produce parent benzothiophene (demethylation reaction), as has been described previously.5 The demethylation reactivity depends on the nucleophilicity of the S-methylsulfonium salts, with the higher selectivity for thiophenes being obtained because of their lower nucleophilicity.9 When olefins are present, the desulfurization yield increases but the selectivity decreases, as shown in Table 4. The decrease in selectivity with 1,7-octadiene is greater than that for 1-octene. The precipitates obtained have a high viscosity, and most are insoluble in both dichloromethane and acetonitrile polar solvents. The precipitates were also obtained in the absence of sulfur compounds, thus suggesting that they are produced by reaction of the olefins with the alkylating agents. Figure 5a and b shows the 1H NMR spectra for the feed and for the precipitate of 1,7-octadiene. All of the signals observed in the spectrum for the feed 1,7octadiene (Figure 5a) are diminished by the reaction, and broad signals attributable to methyl (Hγ) and methylene (Hβ) protons appear at 0.5-1.0 and 1.0-2.0 ppm, as shown in Figure 5b. The other signals appearing at 4.0-6.0 ppm (Figure 5b.i) are probably attribut-
able to the olefinic protons. In the IR spectrum for the precipitate, essentially no absorption band, attributable to counterion BF4-, could be observed. These findings strongly suggest that the precipitate obtained involves different types of unsaturated bonds and methyl and methylene protons on its macromolecular (polymeric) structure. The electron densities on the unsaturated bonds of olefins are very large, such that electrophilic substitution by the methyl groups of CH3I occurs easily to form a carbonium ion.9 This initiation reaction probably causes a chain olefin polymerization to produce polymerized material. Figure 5c shows the 1H NMR spectrum for the precipitate obtained by the reaction of 1,7-octadiene in the presence of thiophene. As shown in Table 3, the 1H NMR spectrum for pure S-methylthiophenium salt shows distinctive resonances for the methyl protons of S+-CH3 at 3.17 ppm and for the thiophenic protons at 7.55 ppm. When 1,7-octadiene was present, a large number of methyl protons of S+-CH3 and thiophenic protons appeared in the spectrum, and the intensity of the thiophenic protons became very weak. These results indicate that several S-methylthiophenium salts of differing structures are contained within the precipitate. The broad proton signals (Figure 5b.i) for the precipitate of 1,7-octadiene are seen to shift to a lower magnetic field when in the presence of thiophene, as shown in Figure 5c.ii. This indicates that the condition of the olefinic protons within the polymer matrixes is changed by the presence of thiophene. During the polymerization of 1,7-octadiene with alkylating agents, it is therefore suggested that thiophene and S-methylthiophenium salt are involved in the polymer matrixes, accompanied by saturation of their thiophenic rings. Consequently, the desulfurization yield of sulfur compounds is probably accelerated by the presence of olefins, as shown in Table 4. 2. Desulfurization of Sulfur-Containing Compounds from CCG. 2.1. Desulfurization of CCG. The applicability of the present process to the desulfurization of actual CCG was then examined. The addition of alkylating agents to the CCG, as in the case for light oils,5 leads to the formation of a dark brown viscous liquid at the bottom of the flask. The variation in the total sulfur content of CCG, for various amounts of AgBF4 added, is shown in Figure 6. The desulfurization yield of CCG is increased with the quantity of the AgBF4 added, as was also the case for light oils,5 and the sulfur content of CCG is decreased successfully from 100 ppm to less than 20 ppm, in the presence of 20-fold molar excess of AgBF4 based on the initial sulfur concentration of the feed CCG. This sulfur content accords with prospective regulations for a limit of 30 ppm,1 thus
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 1231
Figure 6. Variation in the sulfur content of CCG as a function of increasing quantity of AgBF4. Reaction time, 5 h; temperature, 273 K; [CH3I]initial ) 20-fold molar excess for the sulfur content of feed CCG.
Figure 5. 1H NMR spectra for (a) feed 1,7-octadiene and the precipitate obtained by the reaction of 1,7-octadiene with alkylating agents (b) in the absence and (c) in the presence of thiophene, and (d) precipitate obtained by the desulfurization of CCG.
indicating that the present process is effective for the desulfurization of CCG and light oil feedstocks.5 Figure 5d shows the 1H NMR spectrum for the precipitate obtained from the desulfurization of CCG. A strong resonance, due to methyl protons adjacent to the S+ atom, appears at 3.00-3.60 ppm, which agrees reasonably well with those in the spectra of the Salkylsulfonium salts for the model sulfur compounds, as shown in Table 3. The IR spectrum for the precipitate
obtained from the CCG showed a strong absorption band at 1000-1100 cm-1, which is attributable to a BF4- counterion. These results suggest that the resulting sulfur compounds occur as S-alkylsulfonium salts. The spectrum also shows a strong resonance (Figure 5d.iii) at 6.7-7.1 ppm, which closely resembles that for the spectrum of 1,7-octadiene in the presence of thiophene, as shown in Figure 5c.ii. Thus, the results suggest that the precipitate obtained from CCG contains polymerized materials produced by the reaction of the olefins with the alkylating agents, and that several sulfur compounds and their sulfonium salts are involved in the polymer matrixes. The CCG recovery, following desulfurization in the presence of 10- and 2-fold molar excess of CH3I and AgBF4, respectively, based on the initial sulfur concentration of the feed CCG, was 89%. This low recovery of CCG, as compared to that for light oils (92-95%),5 is thus due to the polymerization of the olefins. 2.2. Desulfurization Reactivity of Sulfur-Containing Compounds in CCG. The variations in the compositions of the individual sulfur-containing compounds in CCG, following desulfurization, are shown in Table 2, and they confirm that the desulfurization of thiophene, disulfides, thiols, and tetrahydrothiophene proceeds effectively. As shown in Table 3, this result also agrees reasonably well with that obtained with decane solution. The desulfurization efficiency lies in the ranking order of disulfides, thiols > tetrahydrothiophenes > thiophenes > benzothiophene, with benzothiophene being the most difficult compound to desulfurize. The low desulfurization yield for benzothiophene results because (i) the S-methylation reaction is prevented by the presence of naphthenic and aromatic hydrocarbons and (ii) the thiophenium salts produced are converted to parent benzothiophene through the demethylation reaction caused by these hydrocarbon compounds, as shown in Table 4. The remaining proportion of the thiophenes in CCG, as shown in Table 2, has a tendency to decrease with increasing carbon number of the alkyl substituents for C0-C2, but then to increase for C2-C4. As shown in Figure 2 and Table 3, this tendency differs from that obtained for decane solution. Thus, the hydrocarbon compounds in CCG probably affect the desulfurization reactivity of the thiophenes. To clarify this, the data in Table 4 for the variation in the desulfurization yield of
1232
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001
the previously studied photochemical desulfurization processes, which decrease the octane number by 7-10 and 2.5 units, respectively.1,4 The results therefore suggest that the present process is applicable as a new desulfurization process for CCG. Conclusion
Figure 7. Variation in the reactivity ratio, η, for thiophenes from decane, in the presence of tetralin and Decalin. The value of η represents the ratio of the desulfurization yields of thiophenes obtained in the presence and in the absence of hydrocarbon compounds, based on the data shown in Table 4.
thiophenes from decane solution in the presence of hydrocarbon compounds are summarized in Figure 7 in terms of a reactivity ratio, η,5 for thiophenes defined as
η ) [desulfurization yield in the presence of hydrocarbon compound]/[desulfurization yield in the absence of hydrocarbon compound] (2) The relative decrease in the reactivity ratio, η, for thiophenes is increased by addition of tetralin and Decalin and by increasing carbon number of the alkyl substituents on the thiophene molecule. The effect of the naphthenic and aromatic hydrocarbons thus results in a different desulfurization reactivity for the thiophenes in the actual CCG (Table 2) as compared to that obtained in model gasoline (Table 3). However, for the desulfurization of benzothiophenes and dibenzothiophenes,5 the reactivity ratio, η, for these compounds increases with increasing carbon number of the alkyl substituent, and these results differ from those obtained above for the thiophenes. The reason the reactivity ratio, η, for thiophenes is decreased in the presence of the hydrocarbon compounds with an increase in the carbon number of alkyl substituents, however, is not made clear by the present experiments. 2.3. The Property of Product CCG. The properties and composition of the CCG obtained by the present desulfurization process were examined by comparing them with those for the feed CCG and product CCG obtained using the previous photochemical desulfurization process.4 The data for each CCG sample are summarized in Table 1. The results show that the olefin content of CCG is decreased significantly by the present process, as the olefins are converted by the reaction with alkylating agents to polymerized materials. The product CCG for the new process also exhibited a slightly higher distillation range compared to those of the feed and the photochemically treated CCG. This is because, in the new process, the naphthenic and aromatic hydrocarbons are converted to methylated products having higher boiling points, as was also found in the case for olefins. However, the research octane number for the CCG produced was decreased by only 2.2 units. This compares with the current hydrodesulfurization (HDS) and
A novel deep desulfurization process for light oils, based on the formation and the subsequent precipitation of S-alkylsulfonium salts using alkylating agents (CH3I and AgBF4), has been applied to the desulfurization of catalytic-cracked gasoline (CCG) and the following results have been obtained. (1) Sulfur-containing compounds (thiophenes, benzothiophene, tetrahydrothiophene, thiol, and disulfide), when dissolved in n-decane as a model solution, are methylated by the alkylating agents to form the corresponding S-alkylsulfonium salts, which are removed successfully as precipitates. Thiophenes are the most difficult compounds to desulfurize from the decane solution, with their order of reactivity increasing with the net electron density on the sulfur atom. (2) Sulfur-containing compounds in CCG are removed as precipitates, such that the sulfur content of CCG is decreased from 100 ppm to less than 30 ppm, by the presence of a 20-fold concentration ratio of AgBF4 and CH3I to that of the sulfur content of the feed CCG. Benzothiophene is the most difficult compounds to desulfurize, as the S-methylation reaction is prevented by the presence of naphthenic and aromatic hydrocarbons and the sulfonium salts produced are converted to parent benzothiophene by the demethylation reaction. (3) The large quantity of olefins in the CCG is converted by reaction with the alkylating agents to form polymerized materials, such that the olefin concentration of CCG is significantly decreased following the desulfurization process. The decrease in the research octane number for the product CCG, however, is smaller than that obtained according to the current hydrodesulfurization process and to the previously proposed photochemical desulfurization process, thus suggesting that the present method is satisfactory for application to the desulfurization of CCG. Acknowledgment The authors acknowledge the members of the Idemitsu Kosan Co. Ltd. and Kinuura Research Center for the JGC Corporation for their help in the analyses of boiling points, hydrocarbon compositions, and octane numbers for CCG. The authors are grateful for the financial support of Grants-in-Aid for Scientific Research (09555237 and 12555215) from the Ministry of Education, Science, Sports and Culture, Japan, and Showa Shell Sekiyu Foundation for Promotion of Environmental Research. Y.S. is grateful for the Research Fellowship of the Japan Society for the Promotion of Science (JSPS) for Young Scientists and for the British Council Grants for JSPS Fellows visiting the United Kingdom. Nomenclature R ) selectivity for desulfurization yield of sulfur-containing compounds defined in eq 1 η ) reactivity ratio of sulfur-containing compounds defined in eq 2
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 1233
Literature Cited (1) Upson, L. L.; Schnaith, M. W. Low-Sulfur Specifications Cause Refiners to Look at Hydrotreating Options. Oil Gas J. 1997, Dec, 47. (2) Hatanaka, S.; Yamada, M.; Sadakane, O. Hydrodesulfurization of Catalytic Cracked Gasoline. 1. Inhibiting Effects of Olefins on HDS of Alkyl(benzo)thiophenes Contained in Catalytic Cracked Gasoline. Ind. Eng. Chem. Res. 1997, 36, 1519. (3) Hatanaka, S.; Yamada, M.; Sadakane, O. Hydrodesulfurization of Catalytic Cracked Gasoline. 2. The Difference between HDS Active Site and Olefin Hydrogenation Active Site. Ind. Eng. Chem. Res. 1997, 36, 5110. (4) Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. Visible LightInduced Desulfurization Process for Catalytic-Cracked Gasoline Using an Organic Two-Phase Extraction System. Ind. Eng. Chem. Res. 1999, 38, 4538. (5) Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. A Deep Desulfurization Process for Fuel Oils Based on Formation and Subsequent Precipitation Method of S-Alkylsulfonium Salts. 1. Light Oil Feedstocks. Ind. Eng. Chem. Res. 2001, 40, 1213. (6) Acheson, R. M.; Harrison, D. R. The Synthesis, Spectra, and Reactions of Some S-Alkylthiophenium Salts. J. Chem. Soc. C 1970, 1764. (7) Umehara, M.; Kanai, K.; Kitano, H.; Fukui, K. The Synthesis of S-Alkylsulfonium Salts of Tetrahydrothiophenes. Nippon Kagaku Zasshi 1962, 83, 1060 (in Japanese); Chem. Abstr. 1963, 59, 11, 398.
(8) Wanzel, T. J.; Zaia, J. Organic-Soluble Lanthanide Nuclear Magnetic Resonance Shift Reagents for Sulfonium and Isothiouronium Salts. Anal. Chem. 1987, 59, 562. (9) Lowe, P. A. In The Chemistry of the Sulphonium Group Part 1; Stirling, C. J. M., Ed.; John Wiley & Sons: New York, 1981; Chapter 11. (10) Trost, B. M.; Melvin, L. S., Jr. Sulfur Ylides; Academic Press: New York, 1975; Chapter 2. (11) Helmkamp, G. K.; Cassey, H. N.; Olsen, B. A.; Pettitt, D. J. Alkylation of Organic Disulfide. J. Org. Chem. 1965, 30, 933. (12) Smallcombe, S. H.; Caserio, M. C. Kinetics of SulfurSulfur Bond Cleavage in Methylated Methyl Disulfide by Nuclear Magnetic Resonance. J. Am. Chem. Soc. 1971, 93, 5826. (13) Isoda, T.; Takase, Y.; Isumi, N.; Kusakabe, K.; Morooka, S. Deep Desulfurization of Refractory Sulfur Species in Gas Oil Using Skeletal Isomerization Reaction over Zeolite Catalyst (Part 1) Quantum Chemical Analysis of Desulfurization Reactivity of Alkyldibenzothiophenes. J. Jpn. Pet. Inst. 1998, 41, 318 (in Japanese). (14) Palmer, M. S.; Rowe, S.; Harris, S. Ab Initio Molecular Orbital Study of Organometallic Complexes Containing Benzo[b]thiophene. Organometallics 1998, 17, 3798.
Received for review June 7, 2000 Revised manuscript received October 11, 2000 Accepted November 28, 2000 IE000548E
