Ag-Supported Activated Carbons

Article pubs.acs.org/IECR

Removal Mechanism of Thiophenic Compounds in Model Oil by Inorganic Lewis Acids Jia-jun Gao,†,‡ Hong-qiang Li,†,‡ Hong-xing Zhang,†,‡ Ying-zhou Lu,‡ Hong Meng,‡ and Chun-xi Li*,†,‡ †

State Key Laboratory of Chemical Resource Engineering and ‡College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, P.R. China S Supporting Information *

ABSTRACT: The adsorptive desulfurization ability of four inorganic Lewis acids (AlCl3, FeCl3, ZnCl2, and CuCl) for three thiophenic S-compounds, viz., 3-methylthiophene (3-MT), benzothiophene (BT), and dibenzothiophene (DBT), from their model oils were studied here experimentally at 290 K. The results were explained in terms of the theory of hard and soft acids and bases (HSAB) reasonably. The results show that AlCl3 has excellent removal ability for 3-MT with its adsorbance being 141.4 without toluene and 123.0 mgS/g with 25 wt % toluene. It is noteworthy that 3-MT is removed solely through complexing adsorption; in contrast, BT is adsorbed, accompanied with the formation of oil soluble BT oligomers under catalysis of AlCl3 which can promote the complexation greatly. Further, the addition of benzene and toluene can accelerate the desulfurization rate of 3-MT and BT due to the concentration of aromatics on the adsorbent and the oligomerization between BT and the aromatics. (AC) loaded Cu+ and Pd2+ shows good adsorptivity for BT and its derivatives, and Cu(II)-Y, Ni(II)-Y, Co(II)-Y, and Ce(III)-Y zeolites are effective for removing DBT.9−15 Yu et al.16 explain the interaction between different metal ions exchanged in zeolite and S-compounds by the hard and soft acids and bases (HSAB) theory. Besides, metal−organic frameworks (MOF) material such as MOF-505, as a novel adsorbent, can also adsorb DBT.17,18 In fact, the weak and soft Lewis acids, e.g., ILs, Cu+, and Ag+, undergo the inevitable competitive interaction between thiophenic S-compounds and other aromatics (Lewis soft base) present according to the HSAB theory.19 In order to improve the selectivity of the adsorbent or extractant, therefore, it is necessary to consider the chemical property of both the Lewis acids and the corresponding Scomponents at same time, e.g., their hardness and acidity or alkalinity, since an appropriate acid−base pair is very important. For example, the Ce4+, as a hard Lewis acid loaded on zeolite, shows a strong direct interaction with S-atom of thiophene, while their π-complexation is weak, leading to a high selective desulfurization in presence of soft bases (benzene and 1,5hexadiene).20 Besides, tetranitrofluorenone (TNF), as a strong Lewis soft acid, showed an extremely high adsorptive selectivity factor of 1000 for DBT against the aromatic 1-methylnaphthalene due to the formation of electron donor−acceptor complexes,21 although it is less viable due to its expensiveness and explosive property as a multinitro organics. In this process, DBT and TNF could be deemed as a soft Lewis base and a soft acid, respectively, and apt to form a stable acid−base complex since the basicity of DBT is higher than that of 1methylnaphthalene. In addition to the Lewis acid−base adsorption, a novel adsorbent sulfur-rich activated carbon was

1. INTRODUCTION Fuel combustion has long been one of the main sources of SOx contamination to the air; more and more strict regulations have been set to the limit of the sulfur (S-) content of fuel oils by different organizations. In real fuel oils, there are some thiophenic compounds that can be hardly removed, as shown in Table 1.1 In order to remove the S-component efficiently Table 1. Real Fuel Oils and Corresponding Thiophenic Compounds real fuel oil gasoline jet fuel diesel fuel

thiophenic compounds thiophene (T), 3-methylthiophene (3-MT), benzotiophene (BT) BT, alkylated BTs alkylated BT, dibenzothiophene (DBT), alkylated DBTs

and economically, many desulfurization methods and processes have been explored in the last few decades besides the conventional hydrodesulfurization (HDS), e.g., S-zorb process. The principles underlying the alternative approaches include adsorption, extraction, and reaction of S-compounds to varying reagents, e.g., oxidation, alkylation, and complexation, or combination thereof, so as to make the process viable at mild conditions without high energy consumption and loss of octane value. In recent years, many efforts have been paid to the Lewis acid−base interaction desulfurization. Some ionic liquids (ILs), as weak Lewis soft acids, show good extractive desulfurization ability, e.g., [Bmim]AlCl4, [Bmim]FeCl4, [Emim]BF4, [Bmim]PF6, [Bmim]BF4, [Bmim]DBP, and [Emim]DEP due to the π−π interaction between aromatic S-compounds and ILs.2−7 The sulfur removal mechanism of ILs is also recently studied by molecular dynamics simulation.8 Further, the adsorptive desulfurization (ADS) based on the π-complexation between the metal cation and the thiophenic compounds has been studied extensively. For example, Cu(I)-Y Zeolites and SBA15/Cu(I) can remove thiophene effectively; activated carbon © 2012 American Chemical Society

Received: Revised: Accepted: Published: 4682

December 4, 2011 February 20, 2012 March 5, 2012 March 5, 2012 dx.doi.org/10.1021/ie202831p | Ind. Eng. Chem. Res. 2012, 51, 4682−4691

Industrial & Engineering Chemistry Research

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reported lately.22 The good adsorptive capacity and selectivity of sulfur-rich carbon were attributed to the sulfur−sulfur interaction and the oxidation of DBTs. As stated above, the current study in terms of the adsorptive desulfurization of Lewis acid−base interaction is mainly about adsorbent loaded or modified by the metal ions (Cu+, Ag+, and Ce4+). However, the study that strong Lewis acids react directly with S-compounds has been reported rarely. Considering the strong Lewis acidity of some inorganic metallic chlorides, such as AlCl3 and FeCl3, they might be also applicable as effective adsorbents for the removal of thiophenic S-compounds by Lewis acid−base interactions. Meanwhile, the AlCl3 and FeCl3, as the hard acids, are expected to show good selectivity in the aromatic-rich oil due to their negligible interaction with soft bases (benzene and toluene). Moreover, the AlCl3 and FeCl3 are the common reagents in chemical industry and thus show the low material cost and simple preparation compared with the sorbents reported, e.g., MOF materials and Cu(I)-Y zeolite. The aim of the present paper is to investigate the applicability of some typical Lewis acids, e.g., AlCl3, FeCl3, ZnCl2, and CuCl, for removing different thiophenic S-compounds from model oils, compare the activity of different Lewis acids, and identify the desulfurization mechanisms involved. To approach the Scompounds in the real oils, three thiophenic compounds are selected to mimic the S-compounds in gasoline (3-MT or BT) and diesel (DBT).

component (3-MT or BT) adsorbed as well as its oligomeric derivatives (BT oligomers) formed in situ may be quantitatively analyzed by the high performance liquid chromatography (HPLC) technique. The solid mixture of BT and its oligomers could be obtained for structural identification after the solvent was vaporized. 2.2.3. The Saturated Adsorption of 3-MT. The saturated adsorption curve for toluene free oil was determined by measuring the equilibrium S-contents in model oils with varying initial mass ratios of model oil to a specific Lewis acid by stirring in the flask over 12 h under 290 K, and the experimental method of saturated adsorption in a toluenecontaining system is consistent with that in no toluene system. Moreover, the adsorbance of Lewis acid in this paper is denoted by mgS/g, i.e., the ratio between the adsorbed sulfur mass and adsorbent mass. 2.2.4. The Accumulative Adsorption of 3-MT in TolueneRich Model Oil. The accumulative adsorption curve of AlCl3 was studied by the following experiment. Ten grams of model oil was first mixed with 20-fold AlCl3 by stirring for 10 min at 290 K, and then, equivalent fresh model oil was repeatedly added to the same flask after the adsorbed oil was poured out by decantation. On this basis, the accumulative adsorbance of AlCl3 with respect to each run of uses is obtained in terms of the corresponding S-content in the adsorbed oil samples. 2.2.5. The Desulfurization Investigation in Prepolymerized Model Oil. Thirty grams of model oil, BT-octane, was first mixed with 20-fold AlCl3 (mole ratio for sulfur) by magnetic stirring under 290 K, to which 20 mL of dilute HCl solution was added to destroy the acid−base complex and dissolve AlCl3. The prepolymerized oil containing BT and its oligomers was then obtained by separation operation and drying of anhydrous CaCl2, in which the total S-content was still 1000 μg/g, and then, 7-fold AlCl3 was mixed with 20 g of prepolymerized oil mentioned above by vigorously stirring at 290 K. Five samples (0.2 mL for each) at different times were taken out for S-content analysis. 2.3. Analysis Method. The S-content with respect to 3MT, BT, and DBT in model oils was measured by HPLC using an external standard method (Shimadzu 10A-VP, equipped with UV−vis detector and a C-18 column; wavelength 242 nm for 3-MT, 251 nm for BT, and 310 nm for DBT). For each sample, HPLC analysis was repeated twice to obtain the average S-content, and the minimum detectable S-content was about 0.2 μg/g for 3-MT, 0.2 μg/g for BT, and 0.3 μg/g for DBT. The total S-content for the benzene-containing model oils was determined by a sulfur and nitrogen analyzer (KY3000SN, Jiangyan Keyuan Electronic Instrument Ltd., China) with its minimum detectable S-content being about 0.2 μg/mL. The solid derivatives of the S-component were first dissolved in n-octane and then analyzed by a UV−visible spectrophotometer (Beijing Labtech). Besides, their structure was studied by a 1 H NMR spectrum using a AV400 NMR spectrometer (Bruker) and dimethylsulfoxide (DMSO)-d6 solvent.

2. EXPERIMENTAL SECTION 2.1. Chemical Materials. 3-MT (ACROS, >99%), BT (ACROS, >97%), and DBT (ACROS, >99%) were purchased from J&K Scientific Ltd. Aluminum chloride anhydrous (AlCl3, >99%) and n-octane (AR) were bought from Tianjin Guangfu Fine Chemical Industry Institute. Ferric chloride anhydrous (FeCl3, >99%) was made by Shantou Xilong Chemical Ltd. Zinc chloride (ZnCl2, >98%) and cuprous chloride (CuCl, >97%) were the products of Tianjin Damao Chemicals. Benzene (AR) and toluene (AR) were from Beijing Yili Fine Chemical Ltd. All reagents were used as received. 2.2. The Complexing Desulfurization Process. 2.2.1. The Desulfurization Rate Experiment of the Thiophenic Compounds. The model oil used here is a binary mixture of noctane and an aromatic S-compound, namely, 3-MT, BT, or DBT, with its initial S-content all being 1000 μg/g. 7-fold, 15fold, and 20-fold mole ratios for sulfur of Lewis acids were added directly into the conical flasks under magnetic vigorously stirring (about 200 r/min) at about 290 K, and then five or four samples (0.2 mL for each) within 2 h were taken out from 20 g of model oil for investigating the relationship between time and S-content. Meanwhile, a syringe equipped with a microfiltration membrane was used to filter the solid impurity of the oil. Similarly, 20 g of model oil was also used to take out the two samples beyond 2 h. Further, the toluene or benzene (25 wt %) as a representative of aromatic compounds was used to prepare new model oils (S-content 1000 μg/g) to explore their competitive adsorption of thiophenic compounds. 2.2.2. The Exploration Experiment of Desulfurization Mechanism. To investigate the removal mechanism of Scomponent, 20 mL of dilute HCl solution was added into the reaction system to destroy the acid−base complex, thereby all the S-components released from complex decomposition could be redissolved into the oil, while the Lewis acid was dissolved completely into the water phase, considering that water is a stronger Lewis alkaline than S-compounds. In this way, the S-

3. RESULTS AND DISCUSSION 3.1. Complexing Adsorption of 3-MT by Different Lewis Acids. Four kinds of typical Lewis acids, namely, AlCl3, FeCl3, ZnCl2, and CuCl, were tested for their desulfurization ability for 3-MT from the model oil. The maximum relative error of S-removal mass in oil with respect to the last sample is 1.90% for all the experiments concerning 3-MT, which is described in Supporting Information. From a trial experiment 4683

dx.doi.org/10.1021/ie202831p | Ind. Eng. Chem. Res. 2012, 51, 4682−4691


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Table 2. Desulfurization Yield (%) for 20-Fold Lewis Acids at 2 h AlCl3

FeCl3

ZnCl2

CuCl

3-MT

BT

DBT

3-MT

BT

DBT

3-MT

BT

DBT

3-MT

BT

DBT

99.97

97.09

9.56

98.87

55.27

34.93

1.86

2.31

2.39

2.86

3.94

0.36

Figure 1. S-content versus time and amounts of AlCl3 and FeCl3 added for 3-MT at 290 K.

at 290 K, it is shown that both AlCl3 and FeCl3 have good sulfur adsorption ability, while ZnCl2 and CuCl show negligible desulfurization ability, as shown in Table 2. 3.1.1. 3-MT Removal Activity of AlCl3 and FeCl3. Detailed experiments were performed only for AlCl3 and FeCl3 at 290 K with varying amounts of Lewis acids against 3-MT. The results are shown in Figure 1. As seen from Figure 1a, the S-content decreases drastically with time and the rising amounts of Lewis acid, indicating that AlCl3 is a very efficient adsorbent for 3MT. For example, with 20-fold AlCl3, 80% desulfurization yield can be achieved at 10 min, and a complete desulfurization is obtained within just half an hour. The excellent desulfurization performance is closely related to the formation of a stable acid− base complex between 3-MT and AlCl3, as they come into contacting under vigorously stirring. Meanwhile, the AlCl3 adsorbent changes from loose and light yellow particles into green aggregates tightly stuck to the bottom of the flask due to the formation of liquid complex on the surface of adsorbent, as such clean oil could be obtained simply through decantation and a water scrubbing process. By comparing Figure 1a,b, it is observed that the adsorption behavior of these two Lewis acids follows a similar pattern, but the desulfurization activity of FeCl3 is much lower than that of AlCl3. Similarly, the crystalline granule of FeCl3 changes gradually from its initial black and brown into compact dark green aggregates as the adsorption proceeds. 3.1.2. The Removal Mechanism of AlCl3 and FeCl3. The adsorptive activity of AlCl3 and FeCl3 originates from their strong Lewis acidity and the sulfur alkalinity of 3-MT, which leads to the acid−base complexation. For a specific Scompound, the stronger the acidity of the adsorbent, the stronger is its complexing ability, and accordingly, the better is the desulfurization performance. In effect, the color change of the adsorbents in the adsorption process also implies the formation of a complex between 3-MT and the Lewis acid involved since the neat 3-MT is colorless. In order to verify the complexing adsorption mechanism, the dilute HCl solution was added to the adsorption system to destroy the complex formed, make the S-components returned to the model oil, and prevent the hydrolysis of the Lewis acids. The resulting model oil was analyzed by HPLC; nothing but the peak of 3-MT was detected

(see Figure S2, Supporting Information), and the concentration of 3-MT recovered to its initial value. The results show that the adsorption of AlCl3 and FeCl3 for 3-MT is a purely chemical one or, more specifically, an acid−base complexation process. 3.1.3. Saturated Adsorbance of AlCl3 and FeCl3. The previous results indicate that 3-MT can be removed efficiently by both AlCl3 and FeCl3; therefore, only the adsorption curves of AlCl3 and FeCl3 for 3-MT were determined by measuring the equilibrium S-contents in model oils. As shown in Figure 2,

Figure 2. Saturated adsorbance of 3-MT on AlCl3 and FeCl3 in terms of mgS/g without aromatic compounds at 290 K.

the saturated adsorbance of AlCl3 is as high as 141.4 mgS/g (4.41 mmol/g) for the model oil without aromatic compounds, which is higher than many adsorbents reported, while the adsorption capacity of FeCl3 is only 34.6 mgS/g (1.08 mmol/g) in the same oil. Table 3 compares the saturated adsorbance of AlCl3 and FeCl3 with that of other adsorbents with regard to thiophene or 3-MT from similar model oils.23,24 Moreover, the adsorption curve of AlCl3 and FeCl3 is as steep as a step function, indicating that the saturated adsorbance can be achieved even at extremely low equilibrium concentration of 3MT. Therefore, the complexing desulfurization process of AlCl3 and FeCl3 for 3-MT is of purely chemical nature instead of physical adsorption and thus irrelevant to the surface area and pore size of the adsorbent. 3.1.4. The Accelerating Effect of Toluene on 3-MT Adsorption. To approach the real gasoline, the toluene-rich 4684



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Table 3. Comparison of Saturated Adsorbance between Lewis Acids and Other Adsorbents

Table 4. Adsorptive Rate of AlCl3 and FeCl3 Comparison with That of Ag(I)-Y

saturated adsorbance (mgS/g) adsorbent

model oil

without toluene

AlCl3 10Cu-MCM-4823 FeCl3 Cu(I)/HY-Al2O324

3-MT/n-octane T/i-octane 3-MT/n-octane T/n-octane

141.4 63.0 34.6 10

with toluene 123.0 (25 wt %) 56.5 (10 wt %) − −

adsorbent AlCl3 FeCl3 Ag(I)-Y25 a b

model oil was prepared, and the experiments were carried out at 290 K. Figure 3 compares the influence of toluene on the desulfurization rates of AlCl3 and FeCl3 in the model oil. It is noted that toluene not only does not decrease the desulfurization yield but also accelerates the desulfurization of the adsorbents. Even, 7-fold AlCl3 or FeCl3 can remove completely 3-MT within 30 min, which is much faster than the Ag(I)-Y zeolite reported by Azevedo et al.,25 as shown in Table 4. As a strong Lewis acid, AlCl3 or FeCl3 may interact with toluene, a weak Lewis base, which forms a toluene adsorption layer on adsorbent surface in the presence of abundant toluene since the content of toluene is very high. The as-formed toluene layer may act as an extraction film to facilitate the concentration of 3-MT onto the vicinity of AlCl3 or FeCl3 due to the favorable π−π interaction between toluene and 3-MT, as shown in Scheme 1, which leads to a much higher desulfurization rate. To investigate the adsorption mechanism, similarly, the HCl dilute solution was added into the toluenecontaining model oil, and the resulting oil sample was analyzed by HPLC. The analysis results indicate that 3-MT removal in aromatic-containing model oil is still of acid−base complexation. 3.1.5. The Influence of Toluene on Maximum Adsorbence for AlCl3. The two groups of experiments including saturated adsorption and accumulative adsorption were carried out in toluene-rich oil. Figure 4 shows saturated adsorption curve (solid line) and accumulative adsorption curve (dotted line) of AlCl3. It can be seen that the saturated adsorbance that mounts to 123.0 mgS/g (3.84 mmol/g) is nearly consistent with the maximum accumulative adsorbance being 120.6 mgS/g. As listed in Table 3, the saturated adsorbance of AlCl3 is over 2 times higher than that of 10Cu-MCM-48 with or without toluene.23 Moreover, the saturated adsorbance of 3-MT decreases slightly with adding toluene by the comparison

model oil 3-MT/n-octaneb 3-MT/n-octaneb T/ hexane,cyclohexane

Mads/ Voloila (g/mL)

toluene (vol %)

desulfurization yield at 30 min (%)

0.021 0.024 0.1

20.6 20.6 20.0

97.38 98.32 5

The Mabs means the mass of adsorbent, and Voloil is the volume of oil. The density of model oil (25 wt % toluene) is equal to 0.737 g/cm3.

Scheme 1. Extractive Process of Benzene or Toluene Membrane Formed around AlCl3 Surface

Figure 4. Saturated adsorption and accumulative adsorption curve for 3-MT by AlCl3 with 25 wt % toluene at 290 K.

Figure 3. Influence of toluene on the adsorptive removal of 3-MT by 7-fold AlCl3 and FeCl3. 4685



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Figure 5. The HPLC spectrum for BT and its oligomers.

Figure 6. 1H NMR spectrum for BT monomer and its mixture with BT oligomers.

loose and light yellow particles of AlCl3 gradually became a dark red aggregate stuck to the bottom of the flask, which indicated the formation of a complex between S-compound and AlCl3. Moreover, some new peaks were detected by HPLC in addition to sole BT peak. Thus, the adsorption mechanism of AlCl3 was selected to be investigated deeply. 3.2.1. Oligomerization of BT on AlCl3. When the adsorption was finished, the dilute HCl solution was added into the flask to destroy the complex, and the flask was put aside until two clear phases were obtained. The S-content of the resulting oil phase

between a toluene free and toluene rich system, due to the weak interaction between AlCl3 and toluene or other aromatics as specified. 3.2. Interaction between BT and Lewis Acids. To test the adsorptive desulfurization rate of a Lewis acid to BT, four Lewis acids were added to the model oil to investigate their adsorptive activity. It was found that only AlCl3 had good Sremoval performance, while FeCl3, ZnCl2, and CuCl showed very weak desulfurization activity, especially the last two, as presented in Table 2. Meanwhile, in the adsorption process, the 4686



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was analyzed by HPLC; surprisingly, the BT S-content (321.3 μg/g) was much lower than its initial value (1000 μg/g), and another three peaks were recorded, as shown in Figure 5. With respect to the four peaks, the first one with retention time 4.7 min is for BT, and the remaining ones can only be assigned to the oligomers of BT, i.e., dimer, trimer, and tetramer formed under the catalysis of AlCl3, considering the limited potential reaction approach among the species involved. As stated previously, a solid mixture of BT and its oil soluble oligomers was obtained by evaporating completely the solvent of the oil layer. The solid mixture was analyzed by 1H NMR and UV spectrum and compared with the spectra of neat BT, as shown in Figures 6 and 7, respectively. As shown in Figure 6,

Scheme 2. Oligomerization Mechanism of BT Oligomers and BT−Benzene Dimer

Figure 7. UV spectrum of neat BT and its mixture with BT oligomers.

the 1H NMR spectrum of the mixture is much more complicated than that of the neat BT. A number of peaks are detected in 6.8−7.3 ppm of the spectrum, indicating that the chemical shifts of aromatic protons move to higher fields, which is similar to those on alkyl benzene. Even, some chemical shifts of alkyl protons are found, for example, at 5.16, 3.89, and 3.49 ppm. The 1H NMR results strongly imply that the oligomers produced are nonconjugated. As observed from Figure 7, the UV spectrum of the mixture is also different from that of the neat BT. First, the fine structure of the BT spectrum disappeared due to the presence of different species and the spectra superimposition thereof. Moreover, the absorption band broadens slightly from (190−310) nm to (190−365) nm though the number of units in conjugation decreases, which might be explained by the fact that n−π* transition of nonconjugated oligomers between S-atom and benzene ring occurs more readily than n−π* transition of BT aromatic system. The UV spectrum of the mixture is consistent with its brown madder color since easier n−π* transition leads to the formation of R absorption band. 3.2.2. Mechanism of BT Oligomerization. The formation of the BT oligomers is assumed to be a process containing electrophilic addition and electrophilic substitution under the catalysis of AlCl3 and the presence of trace water. In this process, AlCl3 first combines with H2O forming an ion pair, and then, the H+ is added to the thiophenic ring to form a complex,26,27 making the carbon atom on 3-position of BT positively charged. The 3-position carbonium ion intermediate as an electrophilic agent is apt to attack the aromatic ring of other BT molecules through an electrophilic substitution approach, as presented in Scheme 2a, where the H+ dropped from the benzene ring of BT will combine with OH− to

produce H2O. Further, three different isomers of the BT dimers, as shown in Scheme 2b, can be formed through the above mechanism, which is consistent with the peak appearance of BT dimers in Figure 5. It should be noted that only oil soluble oligomers were detected, which is ascribed mainly to the unfavorable solvent effect of octane for carbonium ion intermediate. In summary, it is the stable carbonium ion of BT, which is similar to the cationic structure of a benzyl carbonium ion, which favors the formation of BT oligomers. In contrast, 3MT cannot form oligomers under present experimental conditions due to the weaker stability of its carbonium ion. Besides, DBT also cannot form oligomers, which might be interpreted by the negligible adsorption and accumulation of DBT on the surface of catalyst AlCl3. 3.3. Removal of BT and Its Oligomers by AlCl3. As the BT adsorption experiments mentioned above, AlCl3 was used to remove BT and its derivatives. For all the continuous sampling experiments of BT, the maximum relative error of remaining sulfur mass in oil with respect to the last sample is 2.09%; for details, see Supporting Information. The results indicate that only AlCl3 has good BT removal ability, coupled with the formation of some oil soluble oligomers. 3.3.1. The Promoting Effect of Oligomerization on Desulfurization Process. As stated previously, oil soluble BT oligomers will be formed under catalysis of AlCl3; however, only dimer was detected in the oil phase, especially at higher mole ratio of AlCl3 to sulfur. This suggests that higher degree 4687



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It can be assumed that the dimer is accumulated by the fast dimerization and then converted and adsorbed steadily. 3.3.3. BT Removal with Time and Amount of AlCl3. Figure 10 shows the variation of S-content with time and amounts of

oligomers like trimer and tetramer show stronger complexing activity with AlCl3 than BT monomer does and, thus, are completely adsorbed on the adsorbent, which is justified by the experimental result. The desulfurization performance of AlCl3 for the prepolymerized model oil is presented in Figure 8 and

Figure 10. Relationship between S-content of BT and time at different amounts of AlCl3 at 290 K.

Figure 8. Relationshiop between desulfurization rate and time for BT oringinal oil and prepolymerized oil by 7-fold AlCl3.

AlCl3 added to the model oil at 290 K. It is evident that the Scontent in oil phase decreases steadily with increasing time and the amount of AlCl3 due to the adsorption of AlCl3 with BT and especially its oligomers. However, in comparison with Figure 1a, it is seen that the sulfur removal activity of AlCl3 with neat BT is much lower than that with 3-MT. For example, with 7-fold AlCl3, the adsorption of BT is negligible within 10 min, while the desulfurization yield of 3-MT exceeds 45%, which sharply contrasts the ability of BT and 3-MT adsorbed by AlCl3 since BT is present dominantly as monomer instead of oligomers within 10 min. The result implies that neat BT has a much lower complexing activity with AlCl3 than 3-MT, while long-term adsorption is largely attributed to the formation of the BT oligomers and their stronger complexing ability with AlCl3. 3.3.4. The Enhancing Effect of Benzene on BT Removal. In order to explore the influence of aromatic compounds on BT removal process, the benzene-rich model oil was used to mix with AlCl3, and the desulfurization performance was compared with the benzene-free system. As shown in Figure 11, benzene can promote the whole desulfurization process. For example, the desulfurization yield for the benzene containing system exceeds 66% at 10 min, while that for the benzene-free system is 0%. This phenomenon may be also related to the benzene

compared with that for the BT original model oil. Obviously, the mixture of BT and oligomers can be removed more easily than the BT monomer, and 97% desulfurization yield can be achieved for the mixture at 60 min in contrast to 46.6% for the neat BT in the same condition. Moreover, AlCl3 was hardly added to the prepolymerized oil when red complex was formed. The strong complexing ability of the oligomers with AlCl3 may be ascribed to their multiple electron donating S-sites, which increases the possibility of forming a stable acid−base complex and even chelate compounds. 3.3.2. S-Content Variation of Different S-Components in Oil. In order to analyze the overall S-content in the oil phase during the adsorption course, the peak area response of different BT oligomers to the S-contents has been determined in terms of the HPLC peak area distribution; for details, see Supporting Information. As an example, the distribution of Scontents with respect to the monomer, dimer, and the total sulfur is presented in Figure 9. The results show that the S-

Figure 9. Variation of S-content with respect to BT monomer, dimer, and the total sulfur in oil with the adsorption of AlCl3.

contents of both BT monomer and the total sulfur in the oil phase decrease steadily with time, while BT dimer S-content increases first and then decreases with time, reaching maximum value at about 30 min, which suggests that the dimerization is a fast process in comparison with the follow-up oligomerization.

Figure 11. Influence of benzene on the desulfurization yield of BT by 7-fold AlCl3. 4688



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Figure 12. The HPLC spectrum for adsorbates released from AlCl3 surface in benzene-containing system.

acid (Al3+ or Ce4+), which may be associated with the direct Smetal bond between Ce4+ and S-atom of thiophene proposed by Song et al.20 Considering the desulfurization process that arose from acid−base complexation in this paper, some experimental results may be elaborated by HSAB theory. Stable complexes are formed favorably between an acid and a base with similar hardness, and the stronger the acid and/or base, the more stable is the complex for a specific pair of acid and base. Among the Lewis acids studied here, their Lewis acidity (electron-withdrawing ability) follows the order AlCl3 > FeCl3 > ZnCl2 or CuCl. Obviously, the negligible desulfurization ability of ZnCl2 and CuCl is mainly ascribed to their weak acidity as presented in Table 2. Meanwhile, the acidic hardness follows the order AlCl3 > FeCl3 > ZnCl2 > CuCl, and only AlCl3 and FeCl3 belong to the hard acid.29 For a strong and hard Lewis acid such as AlCl3, its sulfur removal activity may be interpreted by the properties of organic S-compounds. The Satom on thiophenic compounds has two lone pairs of electrons, of which one is parallel to the plane of the ring, namely, σ lonepair electron, and the other is π lone-pair electron perpendicular to the plane of molecule. On the basis of the quantum chemistry calculation using the WinMOPAC software reported by Shiraishi et al.,30 the sulfur electron density on the σ lone-pair orbital follows the order BT (1.03 ev) > DBT (0.995 ev), while that on the π lone-pair orbital shows an opposite order, i.e., BT (0.835 ev) < DBT (0.955 ev). Considering the result that desulfurization ability of AlCl3 follows the order BT > DBT, being consistent with the order of sulfur electron density on the σ lone-pair orbital, it can be inferred that the complexing desulfurization depends

film present on the surface of AlCl3; see Scheme 1. However, the enhancing mechanism of the benzene film for BT may be different from that for 3-MT. To find out what had happened in the benzene-containing oil, the HCl dilute solution was added to the system to make the adsorbates redissolved to the oil phase. The resulting oil layer was analyzed by HPLC, as shown in Figure 12. Surprisingly, the peaks detected here were much more complicated than that of the no benzene system, which strongly implied the presence of other oligomers. First, the peak at retention time 9 min may be BT−benzene dimer, since its retention time is just between the pure components (benzene and BT) and BT dimers, and some peaks after 27 min might be higher degree polymers. It is noted that the content of BT−benzene dimer in the resulting oil is quite high, which might explain the promoting effect of benzene since the BT− benzene dimer is less conjugated than BT monomer and thus a harder alkaline; see Scheme 2c. The harder alkalinity of BT− benzene dimer can be justified by the fact that its content in the oil phase of the adsorption process is quite low; see Figure 12. 3.4. Complexing Desulfurization Versus HSAB Theory. HSAB principle can explain the stability of many acid−base complexes in many cases.28 For example, Pearson pointed out that soft−soft interaction is frontier-controlled with strong electron transfer, i.e., the electron of ligand converts to empty orbital of metal ion, while π*-antibonding orbital containing some admixed d character is very important, which is consistent with π-complexation between Cu+ and thiophene ring reported by R T. Yang et al.10 Moreover, hard−hard combination turns out to be charge-controlled interaction instead of π-complexation due to relatively unavailable d-orbital electron of the hard 4689



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dominantly on the interaction between σ lone-pair electron and electropositive aluminum atom. In contrast, the extractive performance of the ILs follows the order DBT > BT,5,7 suggesting that the extractive desulfurization is dominated by the interaction between the π lone-pair electron and the conjugated cations of the ILs. Obviously, hard acids should interact directly with σ lone-pair electron, and soft acids combine with π lone-pair electron. Therefore, the thiophenic compounds can be deemed both as hard and soft alkalines, in which σ lone-pair electron represents hard base region, and π lone-pair electron belongs to a soft base part shown in Scheme 3. Moreover, Table 2 listed the desulfurization yield of the

Article

ASSOCIATED CONTENT

* Supporting Information S

S-content calculation of BT oligomers, raw data for the figures, error analysis for the continuous sampling experiments, and HPLC spectrum for 3-MT before and after adsorption. This information is available free of charge via the Internet at http:// pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-10-64410308. E-mail: [email protected]. Notes

Scheme 3. Interaction Mechanism between Thiophenic Compounds and Hard or Soft Acid

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the support from the Fundamental Research Foundation of Sinopec (Grant No. X505015).

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REFERENCES

(1) Song, C. An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel. Catal. Today 2003, 86, 211. (2) Bösmann, A.; Datsevich, L.; Jess, A.; Lauter, A.; Schmitz, C.; Wasserscheid, P. Deep desulfurization of diesel fuel by extraction with ionic liquids. Chem. Commun. 2001, 2494. (3) Zhang, S.; Conrad Zhang, Z. Novel properties of ionic liquids in selective sulfur removal from fuels at room temperature. Green Chem. 2002, 4, 376. (4) Zhang, S.; Zhang, Q.; Zhang, Z. C. Extractive desulfurization and denitrogenation of fuels using ionic liquids. Ind. Eng. Chem. Res. 2004, 43, 614. (5) Nie, Y.; Li, C. X.; Sun, A.; Meng, H.; Wang, Z. H. Extractive desulfurization of gasoline using imidazolium-based phosphoric ionic liquids. Energy Fuels 2006, 20, 2083. (6) Ko, N. H.; Lee, J. S.; Huh, E. S.; Lee, H.; Jung, K. D.; Kim, H. S.; Cheong, M. Extractive desulfurization using Fe-containing ionic liquids. Energy Fuels 2008, 22, 1687. (7) Jiang, X.; Nie, Y.; Li, C.; Wang, Z. Imidazolium-based alkylphosphate ionic liquids-A potential solvent for extractive desulfurization of fuel. Fuel 2008, 87, 79. (8) Liu, X.; Zhou, G.; Zhang, X.; Zhang, S. Molecular dynamics simulation of desulfurization by ionic liquids. AIChE J. 2010, 56, 2983. (9) Hernandez-Maldonado, A. J.; Yang, R. T. Desulfurization of liquid fuels by adsorption via π-complexation with Cu (I)-Y and Ag-Y zeolites. Ind. Eng. Chem. Res. 2003, 42, 123. (10) Hernandez-Maldonado, A. J.; Yang, R. T.; Cannella, W. Desulfurization of commercial jet fuels by adsorption via πcomplexation with vapor phase ion exchanged Cu (I)-Y zeolites. Ind. Eng. Chem. Res. 2004, 43, 6142. (11) Hernandez-Maldonado, A. J.; Yang, R. T. New sorbents for desulfurization of diesel fuels via π-complexation. AIChE J. 2004, 50, 791. (12) Dai, W.; Zhou, Y.; Li, S.; Li, W.; Su, W.; Sun, Y.; Zhou, L. Thiophene capture with complex adsorbent SBA-15/Cu (I). Ind. Eng. Chem. Res. 2006, 45, 7892. (13) Wang, Y.; Yang, R. T. Desulfurization of liquid fuels by adsorption on carbon-based sorbents and ultrasound-assisted sorbent regeneration. Langmuir 2007, 23, 3825. (14) Li, W.; Xing, J.; Xiong, X.; Huang, J.; Liu, H. Feasibility study on the integration of adsorption/bioregeneration of π-complexation adsorbent for desulfurization. Ind. Eng. Chem. Res. 2006, 45, 2845. (15) Bhandari, V. M.; Hyun Ko, C.; Geun Park, J.; Han, S. S.; Cho, S. H.; Kim, J. N. Desulfurization of diesel using ion-exchanged zeolites. Chem. Eng. Sci. 2006, 61, 2599.

strong Lewis hard acids (AlCl3 and FeCl3) for 3-MT, BT, and DBT from model oil. It is seen that complexing desulfurization performance of Lewis hard acids follows the order of 3-MT > BT > DBT, implying that the alkaline hardness follows the order of 3-MT > BT > DBT. In this way, some other phenomena can be interpreted by the point of view mentioned above. The stronger adsorptive activity of BT oligomers and BT−benzene dimer than that with BT monomer may be associated with their increasing σ lonepair electron density of S atom, i.e., the hard alkalinity, due to the breakdown of their aromatic thiophene ring. Meanwhile, AlCl3 shows very good selectivity for 3-MT, which suggests that it interacts weakly with toluene arising from the absence of dorbital electron. Further, FeCl3 shows higher desulfurization yield for DBT than AlCl3, which may be attributed to the fact that the presence of relatively available d-orbital electron, to some degree, results in π-complexation in addition to hard− hard interaction.

4. CONCLUSIONS Both 3-MT and BT in model oil can be removed efficiently by the strong Lewis hard acid AlCl3, virtually through a complexing adsorption mechanism. The saturated adsorbance of AlCl3 for 3-MT is found to be 141.4 mgS/g without toluene and 123.0 mgS/g with 25 wt % toluene, respectively, which is higher than many adsorbents reported heretofore. In contrast to the purely complexing adsorption of 3-MT, BT is mostly converted to its oligomers under the catalysis of AlCl3, which in return enhances the desulfurization effect greatly due to their multiple electron-donating S-sites and accordingly stronger complexing ability. The addition of benzene and toluene can accelerate the desulfurization rate of 3-MT and BT, which is ascribed to the concentration of aromatics on the adsorbent and the oligomerization between BT and the aromatics. 4690



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