ZEOLITE-BASED ADSORBENTS FOR DESULFURIZATION OF JET FUEL BY SELECTIVE ADSORPTION S. Velu, Xiaoliang Ma and Chunshan Song* Clean Fuels and Catalysis Program, The Energy Institute and Department of Energy and Geo-Environmental Engineering The Pennsylvania State University 209 Academic Projects Building, University Park, PA 16802 E-mail:
[email protected] Introduction Gasoline, jet fuel and diesel are the three major types of transportation fuels. Currently, these fuels contain significant amounts of sulfur, up to 300 ppmw in gasoline, 500 ppmw in diesel and 3000 ppmw in Jet fuel1. Combustion of these fuels in IC engines emits SOx, a major air pollutant. Owing to the stringent environmental regulations imposed in recent years, the removal of sulfur compounds from transportation fuel is becoming more and more important issue. In addition, the possibility that these fuels can be reformed catalytically onboard or onsite to produce H2 for fuel cells to drive automobiles or residential uses in the near future, demands reduction of sulfur content to near-zero level.2 Hydrodesulfurization (HDS) is the conventional method being employed to remove the sulfur compounds. However, this method is incapable of removing some of refractory sulfur compounds present in jet fuels and diesel.1 Furthermore, the HDS process is operated at high temperature and pressure. Under these conditions, a part of olefines and aromatics contained in the fuels are saturated and as a consequence the octane number decreases substantially. On the other hand, adsorptive desulfurization is a new challenge to remove sulfur compounds from the transportation fuels, because adsorption would be accomplished relatively at lower temperature and pressure.3 The success in this method, however, depends on the development of highly selective adsorbents, because the commercial adsorbents are not desirable for this application. Several kinds of adsorbents have been reported for the removal of H2S from the flue gas, natural gas and coal-derived gas.4 However, the development of a selective adsorbent for the removal of organic sulfur compounds from the transportation fuels is still in its infancy. There are only a few reports on the use of transition metals such as Ni, Cu, Co or in some cases noble metals (Pt and Pd) supported on Al2O3, SiO2, ZnO or mixture thereof.1 These metal and metal oxide components bind organic sulfur compounds, which results in metal sulfide formation. One disadvantage is that they must be used with expensive components (metal or support) in significant quantities. Further, they are relatively more difficult to regenerate for subsequent runs. On the other hand, adsorption by zeolites could offer a powerful means for the development of methods for super purification, as they possess "size-selective" sorption property. We are exploring SARS (selective adsorption for removing sulfur) process concept in our laboratory. The objective of the present investigation is to explore some selective zeolite-based adsorbents for the removal of organic sulfur compounds from transportation fuels. We present here some of our preliminary results on the desulfurization of JP-8 jet fuel over transition metal ionexchanged Y-zeolites. Experimental Commercially available NH4 Y-zeolite (Aldrich; SiO2/Al2O3 molar ratio = 5) was ion-exchanged with various transition metal ions using 3 to 5 fold excess amounts of 0.1 M metal nitrates at 80oC for 24 h. After ion-exchange, the zeolite suspension
was filtered, washed with copious amount of deionized water and dried at 80oC overnight and then calcined at 450oC for 6h in air atmosphere employing a temperature ramp of 2oC/min. Adsorption experiments were performed in a batch reactor. A JP-8 jet fuel sample was supplied by the Wright Laboratory of the US Air Force. About 6 g of the JP-8 jet fuel and 1g of the ion-exchanged Zeolite in a 100 ml round-bottom flask were stirred at 80oC for 4-5 h.5 The treated jet fuel was then separated from the adsorbent and analyzed by a GC equipped with a pulsed flame photometric detector (PFPD) using benzothiophene as an internal standard. Results and Discussion The PFPD gas chromatogram of the JP-8 jet fuel untreated and treated using CeY-zeolite and PdY-zeolite are shown in Figure 1, while the sulfur adsorption data are gathered in Table 1. The major sulfur compounds present in the JP-8 jet fuel are 2,3-DMBT, 2,3,7TMBT, 2,3,5/6-TMBT with a net sulfur content of 736 ppm. The intensity of these peaks decreases significantly after treatment with ion-exchanged zeolite. Among the adsorbents tested, the Ceexchanged and Pd-exchanged zeolites exhibit a better sulfur adsorption capacity (50 to 60 %). The HY-zeolite, without a metal also exhibits some sulfur adsorption capacity of about 30 %. On the other hand, the adsorption capacity of Cu-exchanged and Znexchanged zeolite is negligibly low under the present experimental conditions. Table 1 Desulfurization of JP-8 jet fuel over Zeolite-based adsorbents Amount of Sulfur Adsorption capacity x 104 Adsorbent removed (g of sulfur / g of adsorbent) (Weight %) 20 45 NiY-Zeolite 3 7 CuY-Zeolite 0 0 ZnY-Zeolite 27 60 CeY-Zeolite 26 58 PdY-Zeolite 13 29 HY-Zeolite It is also interesting to note from Figure 1 that the relative concentration (peak intensity) of 2,3-DMBT and 2,3,7-TMBT (2,3DMBT/ 2,3,7-TMBT ratio) is around 2.4 in the untreated jet fuel as well as in the fuel treated with CeY-zeolite. Similar results were also observed in the chromatograms of fuel treated with NiY-zeolite and HY-zeolite. On the other hand, this ratio is close to unity in the fuel treated with PdY-zeolite. This indicates that the PdY-zeolite is more selective towards 2,3-DMBT. The difference in the selectivity of sulfur adsorption between Ce-exchanged and Pd-exchanged zeolites indicates that the mode of adsorption of sulfur compounds over PdYzeolite is probably different from that of others. The methyl group in the 7-position of the 2,3,7-TMBT would cause a steric hindrance thereby inhibiting the interaction between the sulfur atom and the adsorbent. A systematic study is currently underway in order to understand the mechanism of sulfur adsorption over zeolite-based adsorbents. The study will also be extended to gasoline and diesel fuels and detailed results will be presented. Conclusions Zeolite Y exchanged with Ce and Pd exhibits a higher sulfur adsorption capacity compared to other transition metal ion exchanged zeolites in the desulfurization of Jet fuel containing around 736 ppm of sulfur. The PdY-zeolite exhibits a higher selectivity for the adsorption of 2,3-DMBT. As Ce is less expensive
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than Pd, the Ce-exchanged zeolite may be considered a more promising candidate for deep desulfurization of transportation fuels. Acknowledgments.. This work was supported in part by US Department of Energy and in part by the US Department of Defense. We gratefully acknowledge the financial support.
2,3-DMBT JP-8
References 1 Song, C, and Ma, X, Appl.Catal.B Environ. (Submitted); Song, C. Am. Chem. Soc. Div. Fuel Chem. Prepr., 2001, 46 (1), 8. 2 Brown, L.F., Intl.J.Hydrogen Energy, 2001, 26, 381. 3 Yang, R.T., Takahashi, A., and Yang, F.H., Ind.Eng.Chem.Res. 2001, 40, 6236. 4 Slimane, R.B., and Abbsasian, J. Ind.Eng.Chem.Res. 2000, 39, 1338. 5 Kulprathipanja, S., Nemeth, L.T. and Holmgren, J.S. 1998, U.S.Patent: 5,807,475.
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Retention time (min) Figure 1 PFPD Chromatograms of untreated JP-8 jet fuel and that treated with CeY-, and PdY-zeolites
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