Ind. Eng. Chem. Res. 2001, 40, 1213-1224
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A Novel Desulfurization Process for Fuel Oils Based on the Formation and Subsequent Precipitation of S-Alkylsulfonium Salts. 1. Light Oil Feedstocks Yasuhiro Shiraishi, 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 novel desulfurization process for light oils has been investigated, based on the precipitation of S-alkylsulfonium salts produced by the reaction of sulfur-containing compounds with alkylating agents (CH3I and AgBF4). Dibenzothiophenes (DBTs) and benzothiophenes (BTs), present in the light oils, were methylated by the addition of the alkylating agents, in the presence of dichrolomethane, to be removed as precipitates under moderate conditions. A semiempirical molecular orbital (MO) calculation shows that the desulfurization reactivity for both DBTs and BTs increases with increasing electron density on the sulfur atom, lying parallel to the plane of the molecules, and that the desulfurization activity for BTs decreases with electron density (bond order) on the unsaturated bond in the thiophene ring. The desulfurization of light oil, containing a high amount of aromatic hydrocarbons, was relatively difficult, because the methylation of aromatics occurs competitively with the S-methylation reaction. Desulfurization was, however, accelerated by an increase in the quantity of the alkylating agents added, such that the sulfur content of the light oils was decreased successfully to less than 0.005 wt %. The resulting light oil products confirm that the proposed process is satisfactory for application to the deep desulfurization of light oils. The desulfurization reactivity of the individual sulfur-containing compounds in light oils was found to depend on the aromatic content of the light oils. This is because the S-methylsulfonium salts are converted to parent sulfur-containing compounds by reaction with aromatic hydrocarbons. Introduction There has been much recent interest in the deep desulfurization of light oil feedstocks, as the sulfur oxy acids (SOx), contained in diesel exhaust gas, are a cause of both air pollution and acid rain. To protect the environment against contamination, the sulfur level in diesel fuels is therefore presently strictly limited to 0.05 wt % in Japan and Europe, and this limit will certainly be tightened to 0.005 wt % soon. The U.S. government plans to tighten the limit to 0.0015 wt % in 2006. The current technology for hydrodesulfurization (HDS) can desulfurize aliphatic and acyclic sulfur-containing compounds quite adequately, when adopted on the industrial scale. The above process, however, is limited, for the treatment of benzothiophenes (BTs) and dibenzothiophenes (DBTs), especially DBTs having alkyl substituents on their 4 and/or 6 positions.1,2 Thus, the production of light oil of very low level sulfur inevitably requires severe high-energy conditions and the use of especially active catalysts. An alternative desulfurization process, able to be operated under moderate conditions and without the requirements of hydrogen and catalysts, is therefore strongly required. Acheson and Harrison3,4 have reported that BTs and DBTs are successfully methylated by iodomethane (CH3I), in the presence of silver tetrafluoroborate (AgBF4), to give rise at room temperature to crystalline powders of S-methylated benzothiophenium and diben* To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +81-6-6850-6273. Tel.: +816-6850-6272.
zothiophenium tetrafluoroborates, respectively. These are highly polarized (water-soluble) and are insoluble in nonpolar hydrocarbon solvents. Thus, such a synthetic method, if applied to the desulfurization of nonpolar light oil, might be able to remove the BTs and DBTs specifically from the light oils as precipitates under moderate conditions. This has now been demonstrated, thus enabling the new desulfurization method for BTs and DBTs from light oil to be presented here. In this work, three light oils of differing sulfur and aromatic contents were employed to examine the feasibility of the process. Attempts were made to correlate the desulfurization reactivity of the BTs and DBTs with their electronic parameters, as estimated by MO calculations. The validity of this calculation was confirmed through a comparison with the results for actual light oils. The effects of the addition of aromatic hydrocarbons on the desulfurization of BTs and DBTs and their selectivity were also studied. Finally, the properties of the product light oils were measured, and the applicability of the present process to the refining of light oils was examined in detail. Experimental Section 1. Materials. Benzothiophene (BT), 2-methyl-BT, dibenzothiophene (DBT), iodomethane (CH3I), silver tetrafluoroborate (AgBF4), naphthalene, tetralin, and n-tetradecane were purchased from Wako Pure Chemical Industry, Ltd., and were used as received. 2,8Dimethyl-DBT was synthesized via the methylation of 2,8-dibromo-DBT5 using CH3I, according to the proce-
10.1021/ie000547m CCC: $20.00 © 2001 American Chemical Society Published on Web 01/18/2001
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Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001
Table 1. Properties and Composition of Feed and Treated Light Oils straight-run light gas oil (LGO) density at 288 K hydrogen carbon sulfur nitrogen
(g/mL) (wt %) (wt %) (wt %) (ppm)
saturated fraction aromaticsb one-ring two-ring
(vol %) (vol %)
distillationc IBP 10 vol % 20 vol % 30 vol % 50 vol % 70 vol % 80 vol % 90 vol % FBP
(K)
cetane index
(-)
commercial light oil (CLO)
light cycle oil (LCO)
feed
producta
feed
producta
feed
producta
0.8548 85.6 13.0 1.380 160.0
0.8489 85.7 13.8 0.015 25.3
0.8313 85.5 13.5 0.179 80.4
0.8374 85.2 13.4 0.039 26.1
0.8830 87.9 11.5 0.132 243.1
0.8952 86.0 11.3 0.050 100.6
75.4
75.7
77.9
78.2
33.7
32.9
14.9 9.7
14.7 9.6
17.5 4.6
17.1 4.7
36.4 29.9
37.1 30.0
482 528 545 557 583 608 624 643 679 58.3
468 526 545 558 604 611 627 647 681 60.7
449 485 510 528 555 583 598 618 642 60.6
457 493 515 532 559 584 599 618 642 59.1
443 473 481 490 508 537 553 570 594 28.2
417 474 482 492 511 538 554 571 596 25.1
a
Desulfurization conditions were as follows: feed light oil, 15 mL; temperature, 303 K; reaction time, 11 h (with 15 mL of dichloromethane); [CH3I]initial ) 20-fold concentration for sulfur content of the feed light oils; [AgBF4]initial ) 2-fold concentration for sulfur content of the feed light oils. b By the JPI-5S-49-97 normal-phase HPLC method. Three- and greater-than-three-ring aromatic compounds are present only in trace quantities ( CLO > LCO. As shown in Table
Figure 9. (a) 1H NMR and (b) 13C NMR spectra for the precipitate obtained by the desulfurization of CLO.
Figure 10. Variation in the remaining percentage of sulfur in light oils in the presence of differing quantities of AgBF4. The resulting sulfur contents of the light oils, following desulfurization, are also shown in this figure. Reaction time, 11 h; temperature, 303 K; [CH3I]initial ) 20-fold molar excess for the sulfur content of feed light oils.
1, the light oils contain large amounts of aromatic hydrocarbons. The quantities of the one- and two-ring aromatics, as compared to the sulfur content of the feed light oils, are determined to be 10.8 and 7 (vol % of aromatics/wt % of sulfur) for LGO, 97.8 and 25.7 for CLO, and 275.8 and 226.5 for LCO, respectively. The desulfurization efficiency of the light oils is thus seen to decrease with increasing aromatic content of the light oils. These results suggest that aromatic hydrocarbons significantly affect the desulfurization of light oils.
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 1221
Figure 11. Effect of the addition of tetralin and naphthalene on the desulfurization yield from tetradecane and on the selectivity, R, for (a) DBT and (b) BT. Reaction time, 11 h; temperature, 303 K; [BT or DBT]initial ) 54 mM; [CH3I]initial ) 1080 mM; [AgBF4]initial ) 108 mM.
2.2. Effect of Aromatic Hydrocarbons on Desulfurization. The effect of aromatic hydrocarbons on the desulfurization of sulfur-containing compounds from tetradecane solutions was then studied. Tetralin and naphthalene were used as model components for the one- and two-ring aromatics present in light oils9,11 and were added to the tetradecane, together with DBT or BT for the desulfurization experiments. The results are summarized in Figure 11, where a selectivity of the desulfurization yield of DBT and BT from tetradecane,9 R, is defined as
R ) [desulfurization yield of sulfur compound]/ ([desulfurization yield of sulfur compound] + [reaction yield of aromatic compound]) (2) The desulfurization yields of both DBT and BT are seen to decrease with increasing amount of both tetralin and naphthalene added. Naphthalene prevents the desulfurization more significantly than tetralin for both DBT and BT. The selectivity values, R, also decreased slightly in these experiments, thus indicating that the these aromatics are converted to other compounds during the reaction. When the resulting tetradecane solution, obtained from the experiments for both DBT and BT in the presence of naphthalene, was analyzed by GC-MS, four new peaks, having molecular ions at m/z 142, 156, 170, and 184, appeared in the gas chromatogram. These peak
compounds were thus identified as mono-, di-, tri-, and tetramethylated naphthalenes, respectively. In the absence of DBT and BT, these four products were still obtained. As shown in Figure 6e, naphthalene has a relatively large electron density distribution (bond order) on the C1-C2 and C3-C4 bonds in its aromatic ring. The hydrogen atom for naphthalene is therefore likely to be substituted by the methyl groups of CH3I during reaction with the alkylating agents to form these methylated products, as also for BT. In the experiments with tetralin, four products consisting of mono-, di-, tri-, and tetramethylated tetralins were produced. As shown in Figure 6f, tetralin also has a large electron density distribution (bond order) on the C4-C4a and C8a-C1 bonds in its aromatic ring, and tetralin is thus also likely to be substituted by methyl groups to form these methylated products. From these results, it appears that, in the present desulfurization process, an electrophilic substitution by the methyl groups of CH3I on the aromatic hydrocarbons occurs competitively with the S-methylation reaction of the sulfur-containing compounds, such that the desulfurization of aromatic-rich light oil is significantly prevented. These findings are consistent with the results for the desulfurization efficiency for light oils, as shown in Figure 10. The present process is thus shown to be more effective for the desulfurization of low-aromatic-content light oils than for aromatic-rich ones. 2.3. Desulfurization Reactivity of Sulfur Compounds and the Property of the Products. The desulfurization reactivities of the sulfur-containing compounds from actual light oils were compared with those obtained by the MO calculations. The variations in the compositions of the individual sulfur-containing compounds in light oils following desulfurization in the presence of differing quantities of AgBF4 are shown in Table 4. The data are plotted in Figure 12 as a function of the carbon number of the alkyl substituents, n for DBT and m for BT. As shown in Figure 12a.i and ii, the remaining portion for both DBTs and BTs in LGO has a tendency to increase with an increase in the carbon number of the alkyl substituents. These tendencies agree reasonably well with those obtained by the MO calculation, as shown in Figure 5c for DBTs and in Figure 8c for BTs. For CLO, the remaining percentage for the DBTs is seen to increase up to the carbon number C3 but to decrease in the range of carbon number C3-C6, as shown in Figure 12b.i. For LCO, the remaining percentage for both DBTs and BTs appears to decrease with increasing carbon number of the alkyl substituents, as shown in Figure 12c.i and ii. These tendencies differ from the results obtained by the MO calculations. Thus, the aromatic hydrocarbons probably affect the desulfurization reactivity for both alkyl-substituted DBTs and BTs. To clarify the cause of this, the variation in the desulfurization yield of alkyl-substituted DBTs and BTs in the presence of aromatic hydrocarbons was studied using a model light oil. The results are summarized in Figure 13, in which a reactivity ratio, η, the ratio of the desulfurization yield of sulfur-containing compounds in the presence of aromatic compounds to that obtained in the absence of aromatic compounds, is defined as
η ) [desulfurization yield of DBT or BT in the presence of aromatics]/ [desulfurization yield of DBT or BT in the absence of aromatics] (3)
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Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001
Table 4. Quantities of Benzothiophenes (BTs) and Dibenzothiophenes (DBTs), Having Different Carbon Numbers of Substituents, in (i) Feed and (ii) Treated Light Oilsa straight-run light gas oil (LGO)
a
commercial light oil (CLO)
light cycle oil (LCO)
species
(i) (wt %)
(ii) (wt %)
(i) (wt %)
(ii) (wt %)
(i) (wt %)
(ii) (wt %)
