Selective Adsorption for Removing Sulfur from Jet Fuel over Zeolite

and a real jet fuel (JP-8). Among the adsorbents tested, Ce-exchanged Y zeolites exhibited better adsorption capacity of about 10 mg of sulfur/g of ad...
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Ind. Eng. Chem. Res. 2003, 42, 5293-5304

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Selective Adsorption for Removing Sulfur from Jet Fuel over Zeolite-Based Adsorbents 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, Pennsylvania 16802

Adsorbents based on transition metal ion-exchanged Y zeolites (with Cu, Ni, Zn, Pd, and Ce ions) were synthesized and evaluated for the adsorptive desulfurization of a model jet fuel (MJF) and a real jet fuel (JP-8). Among the adsorbents tested, Ce-exchanged Y zeolites exhibited better adsorption capacity of about 10 mg of sulfur/g of adsorbent at 80 °C with a MJF containing 510 ppmw sulfur. The same adsorbent exhibited a sulfur adsorption capacity of about 4.5 mg/g for the real JP-8 jet fuel containing about 750 ppmw sulfur. Desulfurization of MJF under flow conditions at 80 °C showed a breakthrough capacity of about 2.3 mg/g of adsorbent. Ce-exchanged zeolites exhibited higher selectivity for sulfur compounds as compared to the selectivity of aromatics, for which a comparative study indicated that the sulfur compounds are adsorbed over Ce-exchanged Y zeolites via direct sulfur-adsorbent (S-M) interaction rather than via π-complexation. While the selectivity for 2-methyl benzothiophene (2-MBT) was higher in the static adsorption studies, the adsorption selectivity decreased in the order 5-methyl benzothiophene (5-MBT) > benzothiophene (BT) > 2-MBT under dynamic conditions. This trend was correlated to the electron density on sulfur atoms derived from computer-aided molecular orbital calculations. 1. Introduction Gasoline, jet fuel, and diesel fuel are the three major transportation fuels. Currently these fuels contain significant amounts of organic sulfur compounds, up to 300 ppmw in gasoline, 500 ppmw in diesel, and 3000 ppmw in jet fuel.1 Combustion of these fuels in internal combustion engines emits SOx, a major air pollutant. Even at lower concentration, the SOx can poison the catalyst designed for exhaust gas treatment such as NOx reduction catalyst. To reduce emissions of environmental pollutants, the U.S. Environmental Protection Agency (EPA) has announced new regulations that mandate refineries to reduce the sulfur level down to 30 ppmw in gasoline and 15 ppmw in diesel by 2006.2,3 The removal of organic sulfur compounds from transportation fuels is becoming a more and more important issue in recent years not only due to such stringent environmental regulations but also because of the possibility that these fuels can be re-formed on-board or on-site to produce hydrogen-rich gas as a fuel for fuel cells for mobile, portable, and stationary applications.2-6 To reform these liquid fuels for fuel cell applications, the sulfur level should be further reduced close to zero ppm because the presence of even traces of sulfur is a poison to the re-forming catalysts as well as electrode catalysts. Hydrodesulfurization (HDS) using NiMo/Al2O3 and CoMo/Al2O3 catalysts is the conventional process being employed in the refineries worldwide to remove sulfur compounds from the liquid fuels.7 Recently, some new catalyst formulations have been developed to improve the HDS processes.1 As but one example, Turaga and Song8 have reported a novel CoMo/MCM-41 catalyst with improved catalytic performance for the hydrode* To whom correspondence should be addressed. Tel.: (814)863-4466. Fax: (814)865-3248. E-mail: [email protected].

sulfurization of diesel and jet fuel feedstocks. However, the HDS process is highly inconvenient to produce ultraclean (near zero sulfur) transportation fuels, especially for fuel cell applications, as it requires severe reaction conditions such as high temperature and high pressure.1,2 Under these conditions, part of the olefins and aromatics contained in gasoline are saturated, and this decreases the octane number even for conventional engines. Furthermore, the current HDS process is effective to remove only the “easy sulfur” compounds that are present in the liquid fuels to some extent while it is difficult to remove the refractory sulfur compounds present in the diesel and jet fuel feedstock. A new method is therefore needed to remove refractory sulfur compounds from the liquid fuels in order to re-form them to produce hydrogen for fuel cell applications. On the other hand, the selective removal of refractory sulfur compounds is challenging, as they coexist with aromatic hydrocarbons that are present in high concentrations in the range 10-30 wt %. The new sulfur removal technology therefore should be selective for removing only the sulfur compounds without removing aromatic hydrocarbons. Several non-HDS-based desulfurization technologies such as adsorptive desulfurization, charge-transfer complex formation, extraction using ionic liquids, biocatalytic treatment, etc. have been proposed recently for the desulfurization of liquid fuels.1-3,9,10 Conoco Phillips Petroleum Company has recently commercialized a new process called S Zorb technology for the desulfurization of gasoline and diesel, and the process is based on reactive adsorption in the temperature range of 340410 °C under low H2 pressure between 2 and 20 bar.1,2,10 Among the new processes, the adsorptive desulfurization seems more promising for certain applications including on-site and on-board fuel cell systems because

10.1021/ie020995p CCC: $25.00 © 2003 American Chemical Society Published on Web 09/19/2003

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adsorption could be accomplished at ambient temperature and pressure. Several kinds of adsorbents have already been employed for the removal of H2S from flue gas, natural gas, and coal-derived gas.11-13 However, the development of a selective adsorbent for the removal of organic sulfur compounds from transportation fuels is still in its infancy. Most of the reports in the open literature and patents deal with the desulfurization of simple hydrocarbons, vegetable oils, model gasoline, and diesel using mixed oxides or zeolites as adsorbents.14-22 However, the reports on the adsorptive desulfurization of real gasoline, jet fuel, or diesel are scarce in the literature. We have been recently exploring a new process concept for the deep desulfurization of transportation fuels for fuel cell applications by a more efficient and environmentally benign method.1-3 In this process, the sulfur compounds from transportation fuels are removed by a process called “selective adsorption for removing sulfur (PSU-SARS)” at ambient temperature and pressure without using any hydrogen gas. The spent adsorbents are regenerated either by solvent washing or by oxidative regeneration using air. A wide variety of novel materials, including zeolites, mesoporous materials, mixed metal oxides, and supported metal compounds are being evaluated as adsorbents in the SARS process for the production of ultra-clean gasoline, jet fuel, and diesel.1-3,23-25 Because zeolites possess size-selective adsorption property, the zeolite-based adsorbents are interesting and attractive for the desulfurization of liquid fuel. The objective of the present investigation was to explore the potentials of zeolite-based adsorbents for the desulfurization of real jet fuel (jet propellant no. 8 or JP-8), a military logistic fuel, for fuel cell applications. In this paper, we present our preliminary results on the desulfurization of both model and real jet fuels over various metal ion-exchanged zeolites under static condition with continuous stirring. The adsorption capacity determined from the static adsorption experiments was used to screen various metal ion-exchanged zeolites. The adsorption properties of some of the best adsorbents screened were also evaluated under dynamic conditions using a flow apparatus. Attempts were also made to correlate the observed selectivity with electronic factors such as electron density on sulfur atom and bond order between C2 and C3 carbons of the benzothiophene molecules using computer-simulated molecular orbital calculation. To the best of our knowledge, this is the first report dealing with the adsorption desulfurization of model and real jet fuel containing various benzothiophenes as sulfur compounds over zeolite-based adsorbents.24 2. Experimental Method NH4Y zeolite (LZ-Y62, Aldrich Chemicals) having a SiO2/Al2O3 molar ratio of 5 was ion exchanged with various transition metal ions using a 5-fold excess of 0.1 M aqueous solution of respective metal nitrates at 80 °C for 24 h, unless otherwise mentioned. For exchanging with Ce3+, a 2.5-fold excess of 0.1 M Ce(NO3)3 was used. On the other hand, a 1:1 molar ratio of NH4Y zeolite and Pd(NH3)4Cl2 was used for exchanging with Pd2+. After ion exchange, the zeolite suspension was filtered, washed thoroughly using deionized water, dried at 80 °C overnight, and then calcined at 450 °C for 6 h in air atmosphere employing a temperature ramp of 2

°C/min. The KCeY zeolites were prepared by ion exchanging the NH4Y zeolite with KNO3 followed by ion exchange with Ce(NO3)3 solution as described above. To achieve a higher Ce loading, the ion-exchange experiments were repeated at least twice. Chemical compositions of the ion-exchanged zeolites were determined by ICP elemental analysis using a high-resolution magnetic sector ICP-MS spectrometer (Thermo Finnigan; Element 1 model). X-ray powder diffraction (XRD) patterns for some of the representative samples were collected in the 2θ range 5-50° using a Scintag-II XRD instrument equipped with Cu KR radiation. The JP-8 real jet fuel used in the present study (Batch No. 00-POSF-3804) was supplied by the Wright Laboratory of the U.S. Air Force, and it contained about 750 ppmw sulfur. Preliminary tests for desulfurization of model jet fuel (MJF) and real (JP-8) jet fuel were performed in a batch reactor. As reported in a similar study,26 a fuel/ adsorbent ratio of 6 was used in order to quickly screen various ion-exchanged zeolites. About 6 g of the MJF (initial sulfur content ca. 510 ppmw) or JP-8 jet fuel containing about 750 ppmw sulfur and about 1 g of the ion-exchanged zeolite in a 100-mL round-bottom flask were stirred at 80 °C for 4-5 h. The treated MJF or JP-8 was then separated from the adsorbent. The adsorption properties of some of the best adsorbents were further evaluated in a flow system as described earlier.2,23,25 Briefly, the adsorbent was packed in a stainless steel column having an internal diameter of 4.6 mm and a length of 150 mm. The feed (MJF) was flowed through the adsorbent column using a HPLC pump, and the effluent of the column was collected periodically for analysis. Analysis of treated MJF was performed using a Shimadzu GC equipped with a capillary column (XTI5; Restek; 30 m × 0.25 mm × 0.25 µm) and a flame ionization detector (FID). The effluent of the real jet fuel was analyzed by using another Shimadzu GC equipped with a sulfur-selective pulsed flame photometric detector (PFPD). An Antek 9000 series sulfur analyzer with a detection limit of 0.5 ppmw was used for the quantitative analysis and determination of total sulfur in the treated jet fuel. The sulfur content expressed in ppm in this paper is on the weight basis (ppmw). A model fuel containing thiophene and tetrahydrothiophene, each 270 ppmw on a sulfur basis, together with 270 ppmw each of benzene and 1,5-hexadiene in n-decane solvent was also used to investigate the mechanism of sulfur adsorption over Ce-exchanged zeolites. Experiments were performed in the flow system as described above using a liquid hourly space velocity (LHSV) of 12 h-1. Computer-aided molecular orbital calculations of sulfur and naphthalene compounds were performed using a semi-empirical quantum chemistry method (the PM3 method) in computer-aided chemistry (CAChe) molecular orbital package (MOPAC), version 94.27 The PM3 method determines both the optimum geometry and electronic properties of molecules by solving the Schro¨dinger equation using the PM3 semi-empirical Hamiltonians developed by Stewart.28 3. Results and Discussion 3.1. Chemical Composition and Structural Properties of Ion-Exchanged Zeolites. The chemical compositions determined by ICP analyses are sum-

Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 5295 Table 1. Chemical Composition of Metal Ion-Exchanged Zeolitesa

zeolite

metal ion exchanged

HY HCuY HNiY HZnY HPdY HCeYIE-1 HCeYIE-2 HCeYIE-1UCd KCeYIE-1 KCeYIE-2

Cu2+ Ni2+ Zn2+ Pd2+ Ce3+ Ce3+ Ce3+ Ce3+ Ce3+

chemical composition SiO2/ (wt %)b Al2O3 n+ metal M /Al molar ratioc ratiob Si Al ion 30.2 11.8 20.1 8.2 19.3 7.9 nd nd 20.7 8.1 17.2 5.4 16.4 4.8 15.2 5.6 17.1 5.3 13.9 4.4

8.9 1.3 nd 0.7 21.8 37.9 20.9 20.9 26.6

0.46 0.08 nd 0.02 0.78 1.50 0.72 0.76 1.16

4.9 4.7 4.7 nd 4.9 6.1 6.5 5.2 6.2 6.0

a IE-1, first ion exchange. IE-2, second ion exchange. nd, not determined. b Chemical compositions estimated by ICPES analysis. c Number of metal ion exchanged per ion exchangeable site. d Uncalcined sample.

marized in Table 1. The molar ratio of metal ion (Mn+) exchanged to the Al3+ cations (Mn+/Al3+), where Mn+ is the transition metal ion exchanged, indicates the number of metal ions exchanged per ion-exchangeable site. It can be seen that, among the divalent cations exchanged, the Mn+/Al3+ ratio is the highest (0.46) for Cu2+ and lowest (0.02) for Pd2+. If one divalent cation compensates for two aluminum tetrahedra, then the ratios are 2Cu2+/Al3+ ) 0.92, 2Ni2+/Al3+ ) 0.16, and 2Pd2+/Al3+ ) 0.04. The results indicate that, in the present study, the cation-exchange capacities achieved in the exchange with Cu2+, Ni2+, and Pd2+ are about 92, 16, and 4%, respectively. The ion-exchange capacity depends on a number of factors such as cation size and oxidation state, level of hydration of the zeolite, and Si/ Al ratio of the framework.29 These factors determine the location of the cations and also the extent and kinetics of the ion exchange. Thus, the higher ion-exchange capacity of HCuY as compared to HNiY could be due to an effective interaction of Cu2+ ion with zeolite framework as compared to the interaction of Ni2+ ion, which is hydrated. The hydrated Ni2+ ions remain strongly coordinated to water molecules while within the zeolite pore structure.30 Similarly, the Pd2+ ion in the ion exchange solution exists as the [Pd(NH3)4]2+ complex, and this has diffusion limitation to enter into the ionexchange sites.31 The lower ion-exchange capacity of HPdY could also be due to the lower concentration of Pd(NH3)4Cl2 (a 1:1 molar ratio of NH4Y zeolite and Pd(NH3)4Cl2) used for exchanging with Pd2+ as compared to a 1:5 molar ratio used for exchanging with Cu2+ and Ni2+ ions. In the ion exchange with Ce3+, on the other hand, a higher Ce3+/Al3+ ratio of about 0.8 has been achieved. This leads to an over exchange of about 240% assuming that each Ce3+ ion compensates three aluminum tetrahedra. The Ce loading could be further increased, and an exchange capacity of up to about 450% has been achieved by repeating the ion-exchange procedure twice. It should be noted that an over exchange of up to 600% is common in ion-exchanged ZSM-5 samples and has been observed in mordenites and also in Y zeolites, especially when the pH of the exchanging solution is above 6.29 The results indicate that the trivalent cation exchange is more efficient in neutralizing the framework charge in Y zeolites, and this is in agreement with an earlier report.32 It is also interesting to note from the

table that the SiO2/Al2O3 molar ratio remains the same as that of the starting zeolite (ca. 4.9) for ion exchange with divalent cations. On the other hand, there is an increase in the SiO2/Al2O3 ratio to about 6.2 in the case of Ce-exchanged Y zeolites, indicating that in the later case some dealumination occurs, particularly upon calcination (see Table 1). Rietveld refinement of powder X-ray diffraction data for Ce-exchanged Y zeolites also indicated similar dealumination upon calcination.33 The dealumination occurs due to the presence of protons produced in the formation of hydroxyl groups associated with the cerium cations. Such protons would catalyze the reaction of aluminum hydrolysis, which leads to dealumination. The three-dimensional structure of the Y zeolite is generated by connecting sodalite units with a hexagonal prism to give a framework characterized by large, empty cavities known as supercages with a diameter of about 13 Å. It is known that the extra framework cations compensating the framework negative charges occupy a few well-defined sites, namely, sites I, I′, II, II′, and III.29,34,35 Site I is located at the center of the hexagonal prism. Cations located at this site have octahedral coordination, and they are highly populated. Site II is on the six-membered ring face of the sodalite cage on the supercage. Cations located in these sites are 3-fold coordinated to oxygen of the ring. Sites I′ and II′ are located on the opposite side of sites I and II, respectively, inside the sodalite cage and are also 3-fold coordinated. Because sites I, I′, and II′ are not exposed to the supercage and are shielded by framework oxygen, they cannot interact with molecules inside the supercage directly. Site III is located in the supercage near the four-membered rings, and it exists only in the X zeolite. Thus, in Y zeolites, cations present only in site II can interact with molecule inside the supercage. According to the studies on the metal ion exchange in Y zeolite, most of the transition metal ions introduced by ion exchange at room temperature are hydrated inside the supercages. Upon drying and calcination, the ions migrate to various ion-exchange sites. In general, the exchanged transition metal ions preferentially occupy site I in the dehydrated ion-exchanged Y zeolites. The location of Cu2+ or Ni2+ in sites I, I′, II, and II′ of the ion-exchanged Y zeolite is known in the literature.29,30,32,34,35 Migration of a fraction of cerium ions into site I′ has been reported in the dehydrated CeY zeolite.32-34 On the basis of EXAFS investigations, Berry et al.36 have confirmed that thermal treatment of Ceexchanged Y zeolites around 300 °C in a vacuum induces migration of cerium ions into multiple site occupation of the small cages of the zeolite structure. In the case of Pd, on the other hand, the [cis-Pd(NH3)2(O2)2]2+ complex is known to prevail in the temperature up to 250 °C, and they remain in the supercages because they are unable to traverse the windows of 2.2 Å between supercages and sodalite cages.31 Calcination at 500 °C led to the formation of [Pd(O2)4]2+ species and migration of a part of Pd2+ cation into small cages. Powder X-ray diffraction (XRD) patterns of some selected Ce-exchanged zeolites before and after calcination have been collected in order to investigate if the zeolite structure is retained after Ce exchange and also to identify the state of Ce in the ion-exchanged zeolites. Figure 1 compares the XRD patterns of Ce-exchanged zeolites with that of the parent zeolite (HY zeolite derived from NH4Y zeolite). The similarity in the XRD

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Figure 1. Powder X-ray diffraction patterns of Ce-containing zeolites.

patterns, especially in the 2θ range 5-25°, between HY zeolite and cerium-exchanged Y zeolites implies that the original zeolite structure is retained without any significance change upon cerium ion exchange although the chemical analyses results revealed a minor dealumination. The appearance of new diffraction peaks around 29 and 48° 2θ, which are clearly discernible in HCe30YIWI (30 wt % Ce loaded on zeolite by incipient wetness impregnation), suggest the formation of CeO2 phase, and these diffraction peaks correspond to (111) and (220) planes, respectively (JCPDS File No. 43-1002). The ion-exchanged samples (HCeYIE-2UC, HCeYIE1cal, and HCeYIE-2cal) also exhibit broad and less intense diffraction lines in the same 2θ range. Peaks corresponding to CeO2 are very weak in the HCeYIE1cal containing about 22 wt % of Ce as compared to that of HCeYIE-2UC or HCeYIE-2cal containing about 38 wt % of Ce (see Table 1). These diffraction patterns show in general that a part of the Ce in the Ce-exchanged zeolites is bonded to the zeolite framework and that another part is in the form of highly dispersed CeO2, probably located in the mesopores of the zeolite. In the EXAFS investigation of dehydrated Ce-exchanged Y zeolite, a Ce-O distance of 2.51 Å has been observed, which is similar to that between cerium and the oxygen atom of water in the compound Ce(NO3)3‚6H2O.36 The result has been attributed to the presence of cerium ions in site I′ inside the sodalite unit because in this site the cerium ion would be bonded to three oxygen atoms of the six-membered ring and to some water molecules in the sodalite unit. The uncalcined sample (HCeYIE-2UC) also exhibits weak diffraction peaks in the same 2θ ranges as those of calcined samples, indicating that cerium ions are partially bonded to oxygen atoms in the

zeolite framework. This is in agreement with the earlier EXAFS observation of hydrated Ce-exchanged zeolites.36 3.2. Nature of Sulfur Compounds Present in JP-8 and MJF. The real jet fuel, JP-8, is a kerosenelike fuel used in military aircraft, and it is essentially the same as jet A/A1 fuel being used in commercial aircraft. The fuel properties of various advanced aviation fuels, including that of JP-8, can be found in the literature.37,38 Analysis of a paraffinic petroleum-derived JP-8 jet fuel in our laboratory37 indicated that it consists of about 71 vol % paraffin, 19% alkylbenzenes, 6.2% naphthalenes, 3.5% olefins, and about 800 ppmw sulfur compounds. The nature of sulfur compounds present in the JP-8 jet fuel has also been identified in our laboratory using a GC equipped with a sulfur-selective PFPD, and the chromatogram is shown in Figure 2. Individual peaks were identified based on GC-MS and also using some of the authentic sulfur compounds available commercially. It can be seen that JP-8 contains a wide range of alkyl-substituted benzothiophenes (C1 to at least C4) with a total sulfur content of about 750 ppmw. Among them, 2,3-dimethyl benzothiophene (2,3-DMBT), 2,3,7trimethyl benzothiophene (2,3,7-TMBT), and 2,3,5/6trimethyl benzothiophene (2,3,5/6-TMBT) are major constituents. The other sulfur compounds (C1-BT, C2BT, C3-BT, and C4-BT) are isomers of methyl-, ethyl-, or propyl-substituted benzothiophenes. Since most of these sulfur compounds are commercially not available, the authors used benzothiophene (BT), 2-methyl benzothiophene (2-MBT), and 5-methyl benzothiophene (5MBT) as representative organic sulfur compounds to prepare a surrogate mixture to represent the JP-8 jet fuel. The chemical composition of the surrogate mixture or MJF is presented in Table 2. The concentration of each sulfur compound added is around 3.9 mmol/L, and it corresponds to around 170 ppmw sulfur (on the sulfur basis) with a total sulfur content of around 510 ppmw. About 19% of n-butyl benzene is added as representative of aromatics. Since we are interested in comparing the selectivity between benzothiophene sulfur compounds and naphthalenes, an equal concentration (about 3.9 mmol/L) of naphthalene and 1-methylnaphthalene as those of sulfur compounds are added (see Table 2). Another MJF containing about 6 wt % naphthalene compounds as that of the real JP-8 has also been prepared to study the adsorption properties of the ionexchanged zeolites. 3.3. Static Adsorption Studies. 3.3.1. Adsorptive Desulfurization of MJF. The results of static adsorption studies on the desulfurization of MJF over various ion-exchanged zeolites are summarized in Table 3. The H form of zeolite (HY zeolite) obtained by calcining the NH4Y zeolite around 500 °C was also used for comparison purpose. The results show that under the present experimental conditions the adsorption capacity varied between 2 and 3 mg/g of adsorbent depending upon the nature of exchanged cation. Among the adsorbents tested, the Pd-exchanged Y zeolite (HPdY) and Ceexchanged Y zeolite (HCeYIE-1) exhibit better performance adsorbing most of the sulfur compounds. Indeed, the sulfur content in the effluent of the MJF treated with HPdY zeolite is less than the FID detection limit (about 5 ppmw). Interestingly, the HCeYIE-1 shows better selectivity toward sulfur compounds rather than naphthalenes while the HPdY adsorbs naphthalene and 1-methyl naphthalene almost completely along with sulfur compounds. Under the present experimental

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Figure 2. Sulfur selective pulsed flame photometric detector (PFPD) GC chromatogram of real jet fuel (JP-8). The benzothiophene ring and the substitution positions are shown in the inset. Table 2. Composition of Model Jet Fuel (MJF) Used in This Study compound benzothiophene 2-methyl-benzothiophene 5-methyl-benzothiophene naphthalene 1-methyl naphthalene n-butylbenzene n-decane

FW (g/mol)

wt %

134.20 0.071 148.23 0.078 148.23 0.078 128.17 0.068 142.2 0.075 134.22 18.93 142.29 80.70

S content concn (ppmw) (mmol/L) 167.3 170.9 168.8

3.88 3.86 3.86 3.89 3.87 1033.8 4157.2

conditions, even the HY zeolite without any metal also adsorbs some sulfur compounds. The Cu- and Znexchanged zeolites exhibit relatively lower adsorption capacity. This is in contrast to some of the recent reports on the removal of organic sulfur compounds from mineral oil, vegetable oils, and liquid hydrocarbons, wherein both Cu- and Zn-exchanged zeolites exhibited some desulfurization activity.16,26,39 Unlike methyl-substituted benzothiophenes, which are refractory, the nature of sulfur compounds removed in those references are simple organic sulfur compounds such as mercaptans, sulfides, polysulfides, and thiophenes. A recent study showed that Cu1+ in Cu-exchanged Y zeolite is active in the removal of thiophene via π-complexation.20-22 Accordingly, the poor performance of Cu2+, Ni2+, and Zn2+ may be due to the lack of π-complexation with

aromatic sulfur compounds. It is worth noting that in our very recent study40 on the adsorptive desulfurization of a model gasoline over zeolite-based adsorbents, a 3-fold increase in the adsorption capacity has been observed over the Y zeolite co-exchanged with both Cu2+ and Ce3+ (HCuCeY) as compared to the HCeY zeolite without Cu2+ and about a 10 times improvement as compared to the HCuY zeolite without cerium, probably due to the synergistic interaction between Cu and Ce. The data shown in Table 3 indicate that, in all cases, the selectivity for adsorption of 2-MBT is higher than that of BT. The observed selectivity trend and the mechanism involved in the sulfur adsorption over Ceexchanged zeolites will be discussed in the later part of this paper. In an effort to understand the effect of temperature and fuel-to-adsorbent ratio on the sulfur adsorption capacity, experiments were performed over HCeYIE-1, and the results are displayed in Figures 3 and 4, respectively. As can be seen from Figure 3, the amount of sulfur compounds adsorbed increases with increasing temperature. The sulfur adsorption capacity increases from about 2 mg/g of adsorbent at 40 °C to about 6 mg/g at 120 °C. The sulfur content after treatment decreases from the initial 510 ppmw to about 320 ppmw at 40 °C itself, and it further decreases to below 5 ppmw at 120 °C. Under these conditions, naphthalene and 1-methyl

5298 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 Table 3. Adsorptive Desulfurization of Model Jet Fuel over Various Ion-Exchanged Zeolitesa sulfur compds adsorbed (mg/g of adsorbent)

b

naphthalenes adsorbed (mg/g of adsorbent)

adsorbent

BT (4.2)b

2-MBT (4.7)b

5-MBT (4.7)b

naphthalene (4.1)b

1-methyl naphthalene (4.5)b

S adsorption capacity (mg/g of adsorbent)

S content in effluent (ppmw)

HY HCuY HNiY HZnY HPdY HCeYIE-1

3.6 3.1 3.1 2.8 4.2 3.5

4.0 3.8 4.4 3.9 4.7 4.7

4.3 3.9 3.6 3.4 4.7 4.4

3.3 3.0 2.9 2.8 4.1 2.5

4.0 3.7 3.8 3.8 4.5 3.9

2.7 2.4 2.5 2.3 3.0 2.8

65 102 97 131 BT > 2-MBT after the breakthrough point. This trend is in contrast to the results observed in the static adsorption studies wherein the selectivity for the adsorption of 2-MBT is higher than that of other sulfur compounds. These results can be explained with the help of molecular orbital calculations, which will be discussed in the subsequent section. Under similar experimental conditions the HY zeolite, which exhibited good performance in the static experiment, did not show breakthrough point and the outlet sulfur content in the first fraction was about 200 ppmw. This strongly suggests that the metal ions, Ce3+ or Ce4+, exchanged in the Y zeolite contribute significantly to the removal of benzothiophene sulfur compounds from jet fuels. 3.5. Mechanism of Sulfur Adsorption. To develop a selective adsorbent for removing sulfur compounds, it is necessary to understand the nature of interaction between sulfur compounds and adsorbents. Thiophene has two pairs of electrons on the S atom. One pair of electrons is in the six-electron π system, and the other lies in the plane of the ring. Consequently, thiophene can act as either an n-type donor by donating the lone pair of electrons of the sulfur atom to the adsorbent (direct S-adsorbent interaction or direct S-M bond) or as a π-type donor by utilizing the delocalized electrons of the aromatic ring to form a π-complex with the metal or metal ion. At least eight different coordination geometries of thiophene are known in organometallic complexes, and they include direct S-M bond (η1S and Sµ3), π-complex formation using delocalized π electrons of the thiophenic ring (η4 and η5), and geometries involving both direct S-M bonds and π-complexes (η4, Sµ2 and η4, and Sµ3).1,2 This indicates that thiophenic

5302 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 Table 5. Electron Density of Sulfur Atom and Bond Order of the Largest Bonds of Sulfur Compounds Obtained from Computer-Simulated Molecular Orbital Calculations compound

bond ordera

electron density on S atom (no. of electrons per S atom)

thiophene tetrahydrothiophene benzothiophene 2-methyl benzothiophene 5-methyl benzothiophene 1,5-hexadiene benzene naphthalene 1-methyl naphthalene

1.689 0.991 1.769 1.747 1.771 1.973 1.424 1.609 1.607

5.696 6.042 5.739 5.747 5.741

a Bond order of the C-C bond with largest bond order value in the given molecule. In thiophene and tetrahydrothiophene, it is between C2 and C3 carbons as well as C4 and C5 carbons. In benzothiophene, 2-methyl benzothiophene, and 5-methyl benzothiophene, it is the bond order between C2 and C3 carbons. In 1,5-hexadiene, it is the bond order between C1 and C2 as well as C5 and C6 carbons. In benzene, it is the average bond order of C-C bonds. In naphthalene, it is between C1 and C2 carbons; in 1-methyl naphthalene, it is between C5 and C6 carbons.

Figure 10. Breakthrough curves for the adsorption of thiophene, tetrahydrothiophene, benzene, and 1,5-hexadiene over HCeYIE-2 (top panel) and HY zeolite (bottom panel). Initial and outlet concentrations of thiophene and tetrahydrothiophene are on the sulfur basis while for benzene and 1,5-hexadiene they are molecular basis. Adsorption temperature: 80 °C; LHSV: 12 h-1.

sulfur compounds can be removed from the transportation fuels either by the formation of direct S-M bonds or by the π-complexation. In an effort to investigate if the direct S-M interaction or π-complex formation is involved in the adsorption of thiophenic sulfur compounds on the Ce-exchanged zeolites, a model fuel containing thiophene (a sulfur compound containing π system), tetrahydrothiophene (a sulfur compound without π system), each containing about 270 ppmw of sulfur, benzene (non-sulfur aromatic), and 1,5-hexadiene (non-sulfur olefin) has been prepared and used as a feed. Figure 10 exhibits the breakthrough curves for the adsorption of thiophene, tetrahydrothiophene, benzene, and 1,5-hexadiene over HCeYIE-2cal (top panel) and HY zeolite without any metal ion (bottom panel) performed at 80 °C. In this study, a Ag-exchanged Y zeolite, which is known for π-complexation with olefins,22,45 has been used as a representative for removing sulfur via π-complexation. The Ag-exchanged Y zeolite adsorbed all the compounds, namely, thiophene, tetrahydrothiophene, benzene, and 1,5-hexadiene completely (results not shown). The breakthrough points for the adsorption of benzene, thiophene, and 1,5-hexadiene are about 6, 8, and 14 g of the fuel/g of adsorbent, respectively. These results indicated that Ag-exchanged Y zeolite forms π-complexes with benzene, 1,5-hexadiene, and thiophene and that the π-com-

plexation with 1,5-hexadiene being much stronger than that with benzene and thiophene. On the other hand, the Ce-exchanged zeolites show much higher selectivity for the adsorption of thiophene and terahydrothiophene as compared to the adsorption of benzene and 1,5hexadiene (see Figure 10, top panel). The adsorbent does not show significant breakthrough for the adsorption of 1,5-hexadiene and benzene while the breakthrough points for the adsorption of thiophene and tetrahydrothiophene are 5 and 18 g of fuel/g of adsorbent, respectively. These values are much higher as compared to that obtained over HY zeolite without Ce (see Figure 10, bottom panel). These results clearly indicate that sulfur compounds are adsorbed over Ce-exchanged zeolites by a direct S-adsorbent (S-M) interaction (η1S and/or Sµ3) but not by π-complexation as reported recently over Cu(I)Y and AgY zeolites.20-22 Unlike Cu(I) and Ag+ cations, the cerium ions, being an f block element with high positive charge, have a low tendency to form π-complexes with π-bonding ligands.44 The better performance of Ce-exchanged zeolites as compared to other divalent metal ion-exchanged zeolites in the adsorption of sulfur compounds could be due to high polarizability of the Ce4+ cation. 3.6. Computer Simulation. Computer-aided molecular orbital calculations of sulfur compounds and some aromatics and olefins relevant to this work have been performed in an attempt to understand the reason for the observed selectivity trend in the static and dynamic adsorption studies. The methyl groups present in the benzothiophene at 2 or 5 position alter the electron density on the sulfur atom and the bond order between C2 and C3 carbons. In Table 5, the electron density on sulfur atom and the bond order of the largest bonds of the sulfur compounds are compared with those of 1,5-hexadiene, benzene, and naphthalenes. These data have been obtained from computer-simulated molecular orbital calculations. The bond order decreases in the order of 5-MBT (1.771) > BT (1.769) > 2-MBT (1.747) while the electron density on sulfur atom decreases in the order 2-MBT (5.747) > 5-MBT (5.741) > BT (5.739). It has been reported that the electronic factors significantly

Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 5303

influence the reactivity of benzothiophenes in the hydrodesulfurization (HDS) reactions.47 The HDS reaction has been proposed to proceed through two routes:7,47 (1) Direct hydrogenolysis route where the sulfur is eliminated through direct interaction of sulfur atom with active sites of the catalysts without hydrogenation of the aromatic ring. (2) Hydrogenation route in which hydrogenation of olefinic bond or aromatic ring near the S atom takes place followed by the hydrogenolysis of the C-S bond. The reactivity of sulfur compounds in the first route is generally correlated to the electron density on S atom, while in the second route the bond order between C2 and C3 carbons of the sulfur compounds is used. On the basis of the FT-IR investigation, a Lewis-type acid-base interaction between S atom of benzothiophene and surfaces of acid/base catalysts has been proposed.48 In the present study, it has been shown experimentally that the thiophenic sulfur compounds are adsorbed through direct sulfur-adsorbent interaction rather than via π-complexation. Hence, the electron density on S atom is used to correlate the observed selectivity in the desulfurization of MJF in the static and dynamic adsorption studies. The results in the static adsorption study with a residence time of about 5 h indicate that the selectivity decreases in the order 2-MBT g 5-MBT > BT (see Tables 3 and 4 and Figures 3-5). This trend is the same as the electron density on sulfur atom as shown above. However, in the dynamic adsorption study with a shorter residence of about 0.4 h, the selectivity of sulfur compounds after the breakthrough point decreases in the order of 5-MBT > BT > 2-MBT. The lower selectivity of 2-MBT in the dynamic adsorption study may be due to the steric hindrance experienced by the presence of a methyl group in the second position of the BT ring. The steric hindrance in the substituted benzothiophenes and dibenzothiophenes is a well-known factor that strongly affects the kinetics of the hydrodesulfurization reactions.7,46 Work is in progress in order to further improve the adsorption capacity and selectivity for the removal of organic sulfur compounds from jet fuel, gasoline, and diesel fuel. The regeneration of the spent adsorbent by solvent washing and/or oxidative regeneration using air are being investigated currently and will be reported in the future. 4. Conclusions In this paper, a new process called “selective adsorption for removing sulfur (PSU-SARS)” has been reported for the desulfurization of model and real jet fuel (JP-8) over various transition metal ion-exchanged zeolites. NH4Y zeolite exchanged with cerium exhibits better sulfur adsorption capacity and selectivity for the removal of benzothiophene and other substituted benzothiophene aromatic sulfur compounds in jet fuels. The Ce-exchanged zeolite with a cerium loading of 38 wt % exhibited a sulfur adsorption capacity of about 10 mg/g of adsorbent in the desulfurization of MJF containing about 170 ppmw each of BT, 2-MBT, and 5-MBT. Desulfurization of JP-8 real jet fuel containing about 750 ppmw of sulfur on the same adsorbent showed an adsorption capacity of about 4.5 mg/g. The selectivity for the removal aromatic sulfur compounds decreases in the order of 2-MBT g 5-MBT > BT under static conditions with a residence time of about 5 h. This selectivity trend is consistent with the decrease

in the electron density on sulfur atom calculated from computer simulation. On the other hand, in the dynamic adsorption with a residence time of about 0.4 h, the selectivity decreases in the order of 5-MBT > BT > 2-MBT. The lower selectivity of 2-MBT in the later case has been attributed to the steric hindrance due to the presence of a methyl group in the 2 position of the benzothiophene ring. A mechanistic investigation using sulfur compounds with and without π system revealed that the sulfur compounds are adsorbed over Ce-exchanged Y zeolites via direct sulfur-adsorbent (S-M) interaction rather than via a π-complexation. Acknowledgment This work was supported in part by the U.S. Department of Energy under contract with National Energy Technology Laboratory (DOE UCR Program) and in part by the U.S. Department of Defense, Defense Advanced Research Project Agency (DARPA Palm Power Program) under a subcontract with Altex Technologies Corporation. The authors are grateful to the Palm Power Program management for granting permission to publish this work and to Dr. Mehdi Namazian of Altex for many helpful discussions. Literature Cited (1) Song, C.; Ma, X. New Design Approaches to Ultra-Clean Diesel Fuels by Deep Desulfurization and Deep Dearomatization. Appl. Catal. B: Environ. 2003, 41, 207. (2) Ma, X.; Sun, L.; Song, C. A New Approach to Deep Desulfurization of Gasoline, Diesel Fuel and Jet Fuel by Selective Adsorption for Ultra-Clean Fuels and for Fuel Cell Applications. Catal. Today 2002, 77, 107. (3) Song, C. Fuel Processing for Low-Temperature and HighTemperature Fuel Cells; Challenges, and Opportunities for Sustainable Development in the 21st Century. Catal. Today 2002, 77, 17. (4) King, D. L.; Faz, C.; Flynn, T. Desulfurization of Gasoline Feedstocks for Application in Fuel Reforming. Soc. Automot. Eng. 2000, 1, 1. (5) Ruth, L. Vision 21: Energy Plants for the 21st Century. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2002, 47, 659. (6) Krause, T.; Kopasz, J.; Rossignol, C.; Carter, J.; Krumpelt, M. Catalytic Autothermal Reforming of Hydrocarbon Fuels for Fuel Cell Systems. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2002, 47, 542. (7) Whitehurst, D. D.; Isoda, T.; Mochida, I. Present State of the Art and Future Challenges in the Hydrodesulfurization of Polyaromatic Sulfur compounds. Adv. Catal. 1998, 42, 345. (8) Turaga, U.; Song, C. Deep Hydrodesulfurization of Diesel and Jet Fuels Using Mesoporous Molecular Sieve-Supported CoMo/MCM-41 Catalysts. Prepr.-Am. Chem. Soc., Div. Pet. Chem. 2001, 46, 275. (9) Milenkovic, A.; Schulz, E.; Meille, V.; Loffreda, D.; Forissier, M.; Vrinat, M.; Sautet, P.; Lemaine, M. Selective Elimination of Alkyldibenzothiophenes from Gas Oil by Formation of Insoluble Charge-Transfer Complexes. Energy Fuels 1999, 13, 881. (10) Babich, I. V.; Moulijn, J. A. Science and Technology of Novel Process for Deep Desulfurization of Oil Refinery Streams: A Review. Fuel 2003, 82, 607. (11) Kobayashi, M.; Flytzani-Stephanopoulos, M. Reduction and Sulfidation Kinetics of Cerium Oxide and Cu-Modified Cerium Oxide. Ind. Eng. Chem. Res. 2002, 41, 3115. (12) Slimane R. B.; Abbsasian, J. Copper-Based Sorbents for Coal Gas Desulfurization at Moderate Temperatures. Ind. Eng. Chem. Res. 2000, 39, 1338. (13) Zeng, Y.; Kaytakoglu, S.; Harrison, D. P. Reduced Cerium Oxide as an Efficient and Durable High-Temperature Desulfurization Sorbent. Chem. Eng. Sci. 2000, 55, 4893. (14) Khare, G. P. Process for Producing Desulfurization Sorbent. U.S. Patent 6,271,173, 2001.

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Received for review December 9, 2002 Revised manuscript received August 6, 2003 Accepted August 11, 2003 IE020995P