TS-1

896

Energy & Fuels 2006, 20, 896-902

Oxidative Desulfurization of Organic Sulfur in Gasoline over Ag/TS-1 Lingyan Kong,† Gang Li,*,† Xiangsheng Wang,†,‡ and Bo Wu§ Department of Catalytical Chemistry and Engineering, Dalian UniVersity of Technology, Dalian 116012, P.R.C., State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, Dalian 116012, P.R.C., and Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R.C. ReceiVed August 8, 2005. ReVised Manuscript ReceiVed January 5, 2006

An effective Ag/TS-1 (0.06 wt %) catalyst for oxidative desulfurization of organic sulfur in gasoline was prepared by equal-volume impregnation. The Ag species is highly dispersed, and obvious particles of the Ag species cannot be observed directly in the TEM image. The EDX mapping of a chosen line 1 in the STEM image of the catalyst by line scanning confirmed the existence of the Ag species. It was not homogeneously or randomly dispersed in any part of the titanium silicalite. In fact, they prefer to deposit around Ti species of TS-1 catalyst. A large amount of Ag loading will adversely influence the performance of TS-1 because it will sterically hinder the oxidation of organic sulfur. The oxidative desulfurization of FCC gasoline was carried out over the Ag/TS-1 catalyst with H2O2 as the oxidant and water as the solvent. The results showed that the sulfur content in FCC gasoline was lowered from 136.5 to 18.8 µg/g after 4 h.

Introduction Oxidative desulfurization (ODS) provides a promising deep desulfurization process alternative to traditional HDS, which suffers cost efficiency to meet the more stringent sulfur level in liquid fuels all over the world. The ODS process is very efficient for removing benzothiophene, dibenzothiophene, and their corresponding alkyl derivatives in gas oil.1-6 For the gasoline-range product, however, the ODS process encounters many problems. One obvious problem is the oxidation of thiophene and alkyl thiophenes, which are the typical organic sulfur compounds in gasoline. Thiophene and its alkyl derivatives were reported as impossible to be oxidized by H2O2 under mild conditions because of the lower electron density of the sulfur atom in a thiophene ring.2 Fortunately, our previous study shows that the selective oxidation of thiophene and its alkyl derivatives in n-octane could be achieved using TS-1 as the catalyst.7,8 It is interesting to note that thiophene could be oxidized to sulfuric * Corresponding author. Tel: +86-411-8368-9065. Fax: 86-411-83689065. E-mail: [email protected]. † Department of Catalytical Chemistry and Engineering, Dalian University of Technology. ‡ State Key Laboratory of Fine Chemicals, Dalian University of Technology. § Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences. (1) Zannikos, F.; Lois, E.; Stournas, S. Fuel Process. Technol. 1995, 42, 35. (2) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W.; Ishihara, A.; Imai, T.; Kabe, T. Energy Fuels 2000, 14, 1232. (3) Mure, T.; Fairbridge, C.; Ring, Z. Appl. Catal. A 2001, 219, 267. (4) Collins, F. M.; Andrew, R. L.; Christopher, S. J. Mol. Catal. A 1997, 117, 397. (5) Yatzu, K.; Yamamoto, Y.; Furuya, T.; Miki, K.; Ukegawa, K. Energy Fuels 2001, 15, 1535. (6) Vasily, H.; Fajula, F.; Bousquet, J. J. Catal. 2001, 198, 179. (7) Kong, L. Y.; Li, G.; Wang, X. S. Catal. Lett. 2004, 92, 163. (8) Kong, L. Y.; Li, G.; Wang, X. S.; Wang, Y. Chin. J. Catal. 2004, 25, 89.

acid over TS-1 by H2O2 under mild conditions only when water or tert-butyl alcohol was used as the solvent (eq 1): TS-1

C4H4S + H2O2 98 H2SO4 These results revealed the possibility of removing organic sulfur from gasoline by the ODS process. It should be noted that the oxidation product was nearly 100% sulfuric acid, and only trace amounts of sulfones formed under the reaction conditions, which can be analyzed in the water phase. The carbons of the thiophene molecule were transferred mainly to styrene, which can be detected in the oil phase.8 However, there are still problems in removing organic sulfur from gasoline by ODS. Our previous investigation indicates that titanium silicalite (TS-1) alone does not show efficient selective oxidation of thiophene in FCC gasoline. Therefore, further study is required to improve the selectivity of TS-1. Recently, Yang9,10 reported that the Ag+-modified zeolite Y, which is prepared by the ion-exchange method with Na-Y as the starting material, could adsorb sulfur compounds from commercial fuels selectively. Although TS-1 has no ionexchange site, there are alternative ways applicable for Ag loading onto the TS-1 catalyst and thus for achieving the goal of desulfurization of gasoline by ODS under mild conditions. On the basis of this consideration, in this paper we investigated the Ag/TS-1 catalyst prepared by impregnation and its catalytic performance in oxidative desulfurization of gasoline. The physical characteristics of Ag/TS-1 were also studied by the STEM-EDX technique. (9) Yang, R. T.; Herna’ndez-Maldonado, A. J.; Yang, F. H. Science 2003, 301, 79. (10) Herna’ndez-Maldonado, A. J.; Yang, R. T. Ind. Eng. Chem. Res. 2003, 42, 123.

10.1021/ef050252r CCC: $33.50 © 2006 American Chemical Society Published on Web 03/04/2006

OxidatiVe Desulfurization of Organic S in Gasoline

Energy & Fuels, Vol. 20, No. 3, 2006 897

Experimental Section 1. Catalyst Preparation. A nanosized titanium silicalite, used as the starting support, was prepared according to ref 11 The typical preparation procedures of nanosized TS-1(Si/Ti ) 30) can be described as follows. An aqueous solution of tetrapropylammonium hydroxide (TPAOH 1 mol/L, Selfmade) was added to tetraethylorthosilicalite (TEOS, AR, Beijing Chemicals, China) at room temperature with stirring. In another container, TPAOH, 2-propanol (IPA, AR, Zhengzhou chemicals, China), and water were added to tetrabutylorthotitanate (TBOT, AR, Beijing Jinlong Chemicals, China). After hydrolysis, the two mixtures were mixed directly and stirred for 1-6 h at 333-368 K; obtained was a mixed gel with a composition of SiO2/xTiO2/yTPAOH/zIPA/mH2O, where x ) 0.020.03, y ) 0.2-0.35, z ) 1.0-1.1, and m ) 25-50. This gel was then transferred to a thick Teflon bottle and kept undisturbed for 12-24 h at 443 K under autogenous pressure. The resulting solid was recovered by centrifugation, washed, dried, and finally calcined at 813 K for 5-8 h. The Ag/TS-1 catalyst was prepared by equal-volume impregnation onto the above titanium silicalite. A quantity of 2 mL of an aqueous solution (0.003 M) of AgNO3 (AR, China Chemicals, Shanghai branch) was added dropwise to 1 g of TS-1 at room temperature. The obtained wet solid sample was dried at 383 K in the absence of light for 6 h, and then it was calcined at 813 K for 4 h. The obtained Ag/TS-1 needs no further treatment before use. 2. Catalyst Characterization. The catalyst morphology study was performed with a Tecnai G2 F30 STEM/TEM with a Schottky field emission gun (FEG), a Gatan Ultrascan CCD digital camera, and a high-angle annular dark-field (HAADF) detector, operating at 200 kV in the STEM mode. The probe resolution is 0.20 nm for a point and 0.10 nm for a line. STEM-energy-dispersive X-ray spectrometry (EDX) was used for elemental mapping through scanning a chosen line and for the elemental concentration determination of a single point (5 cubic nanometers) in Ag/TS-1. Spectra were processed with ES Vision 4.0 software (ES Vision, Emispec Systems Inc., Tempe, AZ). The background signal for EDX line scanning was automatically reset. The data of the sample were recorded and analyzed by the computer. A UV-vis spectrum was obtained on a SHIMADZU UV-240 spectrometer, using silicalite-1 (S-1) as the reference. 3. Reaction Conditions and Analytical Methods. The oxidation of sulfur compounds was performed in a water-bathed jacket flask equipped with a condenser. In the typical run, the water bath was first heated and stabilized to the desired reaction temperature (333 K). A quantity of 10 mL of model gasoline with a sulfur content of 1000 µg/g for model gasoline (model gasoline was obtained by dissolving thiophene in n-octane, n-octane + n-octene (volume ratio ) 7:3), n-octane + 1,5-hexadiene (volume ratio ) 9.7:0.3), n-octane + benzene (volume ratio ) 7:3), or n-octane + methylbenzene (volume ratio ) 7:3), respectively, was prepared. The obtained model gasoline or FCC gasoline (Fushun Petroleum Company, China) was added to the water-bathed jacket flask. Then, 0.05 g of the catalyst and 10 mL of water containing H2O2 at a H2O2/sulfur molar ratio of 4:1 were added to the reactor. The resulting mixture was stirred for 4-24 h at the reaction temperature and analyzed periodically. Catalysts were centrifuged off, and the organic phases were analyzed by GC-8810/FID with a capillary column (SE-54, 15 m × 0.25 mm, 0.33-µm film thickness) and HP-6890N/FPD with a capillary column (HP-5, 30 m × 0.32 mm, 0.25-µm film thickness).

Figure 1. TEM image of the Ag-modified TS-1 catalyst.

Figure 2. UV-vis spectrum of theAg-modified TS-1 catalyst.

1. TEM and the UV-Vis Characterization of the Ag/TS-1 Catalyst. Figure 1 shows the general morphology of the Agmodified TS-1 catalyst as determined by STEM/TEM characterization. The scale for the image was generated automatically,

and the size of 0.8 mm in the image stands for 1 nm of the sample. From this highly resolved image we can see that the parent TS-1 catalyst we made has an average crystal size of 80 nm. The Ag species was highly dispersed on the TS-1, and there are no observable Ag particles on it. The UV-vis spectrum of the Ag/TS-1 does not show any difference from that of the original TS-1, and no Ag species could be observed at 400800 nm (Figure 2).12 2. STEM/EDX Study of the Ag/TS-1 Catalyst. Although neither are there any obvious Ag particles in the TEM image nor can the Ag species on TS-1 be observed by XRD and UVvis characterization for the lower Ag loading (0.06 wt %), the existence of the Ag species was confirmed by STEM/EDX analysis. Figure 3 was the STEM/HAADF image of a typical particle containing the Ag species. Its corresponding EDX mapping was obtained by scanning line 1 from the starting point up in the dark area down to the end of the line in the bright area. The line in EDX mapping at position 0.058 µm was taken corresponding to the point A on line 1 in the STEM/HAADF image. Figure 4 shows the energy disperse spectrum of point A, and the elemental composition of point A is listed in Table 1. Figure 5 shows the STEM/HAADF image of the Ag/TS-1 catalyst and the corresponding EDX mapping of line 1 at point B. On the basis of the obtained EDX mapping of line 1, we can see that the dark area consists only of the elements Si and O, and the results indicate that in this area there are no Ag and Ti species. When scanning to the gray particle, at position 0.048 µm in EDX mapping, the signal for element Ag is emerging and reaching its first peak at 0.058 µm, which was taken at point A shown in Figures 3 and 4. The elemental analysis at

(11) Wang, L. Q.; Wang, X. S.; Guo, X. W.; Li, G.; Xiu, J. H. Chin. J. Catal. 2001, 22, 513.

(12) Wang, R., Ph.D. Dissertation, Dalian University of Technology, 2004.

Results and Discussion

898 Energy & Fuels, Vol. 20, No. 3, 2006

Kong et al.

Figure 3. STEM/HAADF image of the Ag/TS-1 catalyst and the corresponding EDX mapping of line 1 at point A.

Figure 4. Eergy disperse spectrum of Ag/TS-1 at point A.

the point (5 nm) shows in Table 1, point A. The point is composed of three elements with an atomic ratio of O/Si/Ag ) 4.3:1:2.5. There is no titanium element in this gray particle. However, the signal for the titanium element begins to go up immediately after this point. And the signal peaked at position 0.066 µm when electrons scanned to point B showing in Figure 5. Figure 6 shows the energy disperse spectrum at point B. The analysis at this point (Table 1, point B) shows four elements with an atomic ratio of O/Si/Ag/Ti ) 2.09:1:0.11:0.04. The signal for the elements Ag and Ti disappear at position 0.09 µm. The EDX mapping analysis of other sites containing the element Ag also shows that the Ag species was not

Table 1. Elemental Composition of the Ag/TS-1 Analyzed by EDX Mapping sample

element

atomic %

point A

O (K) Si (K) Ti (K) Ag (K) O (K) Si (K) Ti (K) Ag (K)

55.474 12.823 0.000 31.702 64.605 30.905 1.244 3.243

point B

homogeneously dispersed on any part of the surface of TS-1. Instead, it deposited preferentially around the Ti sites of TS-1 when the Ag loading was as little as 0.06 wt %. The Ag species



Figure 5. STEM/HAADF image of the Ag/TS-1 catalyst and the corresponding EDX mapping of line 1 at point B.

Figure 6. Energy disperse spectrum of Ag/TS-1 at point B.

formed a close interaction with the TS-1, and no separated Ag particles could be observed. 3. Oxidative Desulfurization of Model Gasoline. Ag/TS-1 was used as the catalyst for the oxidation of model gasoline. Figure 7 shows the blank test of thiophene oxidation without the addition of H2O2, in which the thiophene removal from the n-octane was attributed to the adsorption of thiophene onto the solid catalyst. It shows that the adsorption capacity does not differ obviously between Ag/TS-1 and TS-1, since the Ag loading was much lower compared with Ag-Y.9-10 No thiophene oxidation could be observed over the Ag/HZSM-5 prepared exactly like Ag/TS-1. But there are obvious differences in selective oxidation of thiophene in various model gasoline between Ag/TS-1 and the

TS-1. From Figure 8, it can be seen that TS-1 shows selectivity in n-octane, n-octane + benzene, and n-octane + methylbenzene. This means that thiophene can be oxidized in the presence of alkane and aromatics over TS-1 by H2O2 in water. But in the presence of alkenes such as n-octene, TS-1 shows no activity toward thiophene oxidation. That H2O2 was not consumed was analyzed by determining the initial and the final amount of H2O2 in the reaction system. These results indicate that neither thiophene nor alkene can be oxidized under the condition. The activity for thiophene oxidation is also very poor in n-octane + 1,5-hexadiene. When using Ag/TS-1, the activity for thiophene oxidation in n-octane + n-octene increased significantly (Figure 9). The result revealed the selective oxidation of thiophene in the presence of alkene because of the Ag species loaded on


Kong et al.

Figure 7. Adsorption of thiophene onto the solid catalyst. Model gasoline: thiophene/n-octane.

Figure 10. Effect of Ag loading on selective oxidation of thiophene in n-octane.

Figure 8. Oxidation of thiophene over TS-1 in various model gasolines.

Figure 11. Effect of Ag loading on selective oxidation of thiophene in n-octane + n-octene.

Figure 9. Oxidation of thiophene over Ag/TS-1 in various model gasolines.

TS-1. Ag/TS-1 exhibits better performance in model gasoline composed of thiophene/n-octane + 1,5-hexadiene. Ag/TS-1 also shows higher activity than TS-1 in oxidation of thiophene in n-octane. It can be seen from Figure 8 that the thiophene conversion was only 42.2% over TS-1 in 30 min, while that for Ag/TS-1 was 78.4% in 30 min (Figure 9). The catalytic

performance does not differ much for the model gasoline consisting of the alkane + aromatics between the two catalysts. 4. Effect of the Amount of Ag Loading on TS-1. The catalytic performance of TS-1 for selective oxidation of thiophene has been improved by impregnating Ag onto TS-1. In model gasoline composed of thiophene in n-octane (Figure 10), the catalyst with Ag loading of 0.06 wt % shows the highest activity with a thiophene conversion of 78.4% in 30 min. For the catalyst with Ag loading of 0.6 and 0.01 wt %, respectively, the catalytic activity does not change much compared with that of the TS-1 catalyst. In model gasoline composed of thiophene in n-octane + n-octene (Figure 11), the catalytic activity decreased with the amount of Ag loaded onto the catalyst, although TS-1 without Ag shows no activity. The results indicate that the Ti species of TS-1 probably is overlaid by a large amount of the Ag species. The proper Ag amount is necessary for preparing Ag/TS-1 catalyst with higher activity and selectivity. 5. Oxidative Desulfurization of FCC Gasoline. The above results show that the Ag-modified TS-1 catalyst was able to catalyze the selective oxidation of thiophene using H2O2 in various model gasolines. It was then expected that this catalyst would also be active in the ODS of real gasoline. The organic sulfur compound in the real gasoline used was shown in Figure 12. The main types of its organic sulfur are thiophene and its



Figure 12. Organic sulfur compounds present in Fushun FCC gasoline. Table 2. Composition of Fushun FCC Gasoline entry

typical compound

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 group composition

C4 iso-alkane + alkene C4 n-butane C4 alkene C5 iso-pentane C5 alkene C5 n-pentane C5 alkene + cycloparaffin, C6 alkene + iso-alkane C6 iso-alkane C6 alkene (1-hexene) C6 n-alkane (hexane) C6 cycloparaffin, C6 alkene + iso-alkane C6 aromatics “benzene” C6 cycloparaffin, C7 alkene + iso-alkane + cycloparaffin C7 iso-alkane C7 cyclo-olefin, C8 iso-alkane C7 n-alkane (n-heptane) C7 cyclo-olefin, C8 iso-alkane C7 aromatics (toluene) C8 iso-alkane, C7, C8 cycloparaffin C8 n-alkane (n-octane) C8 alkene, C8, C9 cycloparaffin C8 aromatics (ethylbenzene) C9 cycloparaffin + iso-alkane C8 aromatics (m,o-xylene) C9 iso-alkane + cycloparaffin C8 aromatics (p-xylene) C9 cycloparaffin C9 n-alkane (n-nonane) C10 iso-alkane n-alkane, C9 aromatics (n-propylbenzene) C9 aromatics (include 1,3,5-trimethylbenzene) C9 aromatics (1,2,4-trimethylbenzene) C10 cycloparaffin C10 n-alkane (n-decane) C9 aromatics, C10 aromatics, C11 iso-alkane + n-alkane + aromatics, C12 iso-alkane + n-alkane alkenes aromatics alkanes

calculated RON NO

alkyl derivatives. Benzothiophene and its alkyl derivatives, which should appear at 13.6 min under the settled analytic condition or after that, were not present in this Fushun FCC gasoline. The total feed sulfur for the original Fushun FCC gasoline was 136.5 µg/g. The gasoline with a sulfur content of 371.9 µg/g was obtained by adding thiophene and 2-methylthiophene to the original Fushun FCC gasoline. The composition of gasoline before and after the oxidation is listed in Table 2. The catalyst used in oxidative desulfurization of FCC gasoline was Ag/TS-1 with 0.06 wt % Ag loading. The result of sulfur removal from FCC gasoline is shown in Table 3. As shown in Table 3, Ag/TS-1 exhibited high activity for desulfurization of FCC gasoline, while TS-1 does not show any selective sulfur removal. The results indicate that the loading

original gasoline

sample after oxidation

1.9172 0.6779 2.2561 7.9659 1.1003 1.769 12.6102 8.6361 1.5177 1.4088 10.4085 1.5383 1.376 6.1009 3.5542 1.007 7.945 2.3653 8.1977 1.4921 2.6293 0.8449 0.1607 3.7619 2.1721 1.0909 1.2573 0.3772 0.2863 1.272 1.2738 1.0308 0.1777 0.1856 2.7105 41.14 11.29 47.57 92.02

0.6714 0 0.9038 5.9628 0.8079 1.34 11.0413 8.5738 1.4755 1.4573 10.4781 1.9252 0.8537 6.9971 4.2315 1.1286 8.7037 3.0155 8.774 1.9061 3.6353 0.9234 0.313 3.5795 2.5547 1.2763 1.4276 0.4004 1.0015 1.3362 1.3859 1.0704 0.3309 0.1964 0.7083 41.24 13.28 45.48 89.82

Table 3. Oxidative Desulfurization of FCC Gasoline over Ag/TS-1 sulfur content in FCC gasoline (µg/g) catalyst

original

after oxidation

degree of desulfurization (%)

Ag/TS-1

371.9 136.5 136.5

181.2 18.8

51.2 86.2

TS-1

of Ag does improve the selectivity of TS-1 catalyst for effective oxidation of thiophenes. The presented results show a new Ag-modified TS-1 catalyst for selective oxidation of thiophenes in gasoline. We proposed that the existence of Ag active species was responsible for the selective adsorption of the organic sulfur, and then the sulfur


was oxidized at the Ti sites. The resultant sulfuric acid desorbs from the active site and enters into the solvent water. This allows the active site to be recovered for further reaction. In conclusion, we obtained useful results by investigating the physical chemistry property of Ag-modified TS-1 catalyst. The existence of Ag species was identified by STEM/EDX characterization. It was not randomly scattered on the surface of TS-1 but preferentially deposited around Ti sites of TS-1. This location makes it possible to combine the selectivity of Ag species toward sulfur in the presence of alkenes and the activity of the Ti species in TS-1, and to achieve the goal of ODS. It can be concluded that the introduction of a small amount of Ag does improve TS-1 selectivity toward sulfur compounds and enables the effective oxidative desulfurization of gasoline. The Ag dispersion is important, as is also proper Ag loading. Too

Kong et al.

much loading of Ag would influence the performance of TS-1 adversely, since it would be dispersed around the active site (Ti site) and might overlay the active species or sterically hinder the oxidation of organic sulfur. Also it should be noted that although we have obtained useful results, there is still much need to investigate further to obtain a better understanding of its mechanism. Acknowledgment. The financial support of the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (No. 200346) and the Program for New Century Excellent Talents in University and National Natural Science Foundation of China (No. 20406005) is gratefully acknowledged. EF050252R

TS-1

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