Ind. Eng. Chem. Res. 2004, 43, 1081-1089
1081
Desulfurization of Diesel Fuels via π-Complexation with Nickel(II)-Exchanged X- and Y-Zeolites Arturo J. Herna´ ndez-Maldonado and Ralph T. Yang* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136
Desulfurization of a commercial diesel fuel (297.2 ppmw S) by different nickel(II)-exchanged faujasite zeolites was studied in a fixed-bed adsorber operated at ambient temperature and pressure. The zeolites were prepared by both liquid-phase (LP) and solid-state (SS) ion-exchange (IE) methods. In general, the adsorbents tested for total sulfur adsorption capacity at breakthrough followed the order: Selexsorb CDX (alumina)/Ni(II)-Y (SSIE-500) > Selexsorb CDX (alumina)/Ni(II)-X (LPIE-RT) > Ni(II)-Y (SSIE-500) > Ni(II)-X (LPIE-RT) > Ni(II)-Y (LPIE135). The best adsorbent, Selexsorb CDX (alumina)/Ni(II)-Y (SSIE-500) [layered bed of 25 wt % activated alumina followed by Ni(II)-Y] is capable of producing 19 cm3 of diesel fuel per gram of adsorbent with a weighted average content of 0.22 ppmw S. These low-sulfur fuels are suitable for fuel cell applications. The sorbents were fully regenerated in one step using air at 350 °C, which simplifies possible implementation for many applications. GC-FPD results showed that the π-complexation sorbents selectively removed highly substituted thiophenes, benzothiophenes, and dibenzothiophenes from diesel, which is not possible using conventional hydrodesulfurization (HDS) reactors or direct sulfur-metal interaction-based zeolites such as Ce(IV)-Y. The high sulfur selectivity and high sulfur capacity of nickel(II)-zeolites were due to π-complexation. Introduction The quality of commercial fuels and emissions from refineries is one of the main targets of current environmental regulations. New regulations by the U.S. federal government mandate a reduction in gasoline and diesel sulfur levels to 30 and 15 ppm, respectively, from the current levels of 300-500 ppmw, to be implemented by 2006.1,2 Removal of sulfur-containing compounds is currently achieved by hydrodesulfurization (HDS), a catalytic process operated at elevated temperatures (300-340°C) and pressures (20-100 atm H2) using CoMo/Al2O3 or Ni-Mo/Al2O3 catalyst.3 HDS is highly efficient in removing thiols, sulfides, and disulfides, but it is less effective for aromatic thiophenes and thiophene derivatives, especially those containing functional groups that hinder the sulfur atoms. Because of this problem, the sulfur compounds that remain in the transportation fuels are mainly thiophene, benzothiophene, dibenzothiophene, and their alkylated derivatives. To reduce the sulfur content to meet the new regulations, the reactor size needs to be increased by factors of 5-15.4 Faced with the severely high costs of compliance, a surprising number of petroleum refiners are seriously considering reducing or eliminating production of transportation fuels.5 Future fuel cells will also require deep-desulfurized fuels, if at all possible, with zero sulfur content.2 For example, methanol-based fuels for on-board fuel cell applications require the use of a fuel with a sulfur content of activated carbon/Cu(I)-Y > Selexsorb CDX/ Cu(I)-Y. The layered beds can be fully regenerated using hot air streams followed by reduction of the copper species.12 In the π-complexation mechanism, the cations can form the usual σ bonds with their s orbitals, and in addition, their d orbitals can back-donate electron density to the antibonding π orbitals of the sulfur rings. It was determined that π-complexation is stronger with the thiophenic ring than that with the benzene ring, which makes the process suitable for the desulfurization of commercial fuels. More information on the latter7-12,20-22 and additional π-complexation work by our group can be found elsewhere.23-28 In the present work, we focus on the use of nickel(II)-based sorbents for the desulfurization of commercial
10.1021/ie034206v CCC: $27.50 © 2004 American Chemical Society Published on Web 01/21/2004
1082 Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004
diesel fuels via π-complexation. Ni2+, which has the electronic configuration 1s22s22p63s23p63d84s0 is known for its π-complexation ability.29 Ni2+ was introduced into X- and Y-type zeolites via ion-exchange techniques. For the ion exchange, we used both liquid-phase and solidstate ion-exchange techniques to obtain different loadings of nickel(II) ions. In addition, the uses of alumina as a guard bed and sorbent regeneration methods are discussed. The diesel fuel tested in this work was fully characterized for sulfur content using gas chromatography-flame photometric detection (GC-FPD) techniques. Experimental Section Adsorbent Preparation. The starting adsorbent materials used in this study were Na-Y zeolite (Si/Al ) 2.43, Strem Chemicals), NH4-Y zeolite (Si/Al ) 2.40, Strem Chemicals), 13-X zeolite (Si/Al ) 1.25, Linde), and Selexsorb CDX (Alcoa Industrial Chemicals), all in powder form. H-Y zeolites were obtained after calcination of NH4-Y with air at 450 °C. All zeolites were modified by either liquid-phase (LP) or solid-state (SS) ion-exchange (IE) techniques. For clarity, the as-prepared zeolites will be individually identified with corresponding “LPIE” and “SSIE” appendages. Also, the preparation temperature will be specified where appropriate. Ni(II)-Y(LPIE) and Ni(II)X(LPIE) were prepared by ion exchanging Na-Y and 13X, respectively, with NiCl2‚6H2O aqueous solutions for 48 h at either room temperature (RT) or 135 °C. For the latter, the zeolite and aqueous solutions were placed in an autoclave and slowly heated to the desired temperature. During the ion-exchange process at room temperature, the pH was kept at a value of approximately 6 to avoid hydrolysis of the nickel(II) species in solution.30 After ion exchange, the powder crystals were recovered by filtration and washed with 4 L or more of deionized water to remove excess NiCl2. Afterward, the sorbents were dried at 90°C for 24 h and stored in tightly sealed vials. The exchanged zeolites, which were white powders initially, had a greenish color at the end of the nickel ion-exchange process. Cerium-exchanged zeolites were obtained following procedures found elsewhere.18 NH4-Y zeolite was ion exchanged twice using Ce(NO3)3 aqueous solutions at 80°C. The final product was also recovered by filtration and washed with copious amounts of deionized water. The solid-state ion-exchange (SSIE) method was used to obtain zeolites with high nickel contents. This method has the advantage that it allows metal cations to be introduced into extraframework positions without the presence of hydrolyzed species that might result from aqueous exchanges. As first step, H-Y zeolites (obtained after calcination of NH4-Y zeolites) and NiCl2‚6H2O were manually mixed. The amount of nickel(II) used corresponded to the maximum theoretical cationexchange capacity (CEC) for the zeolite. Afterward, the powder/salt mixture was placed inside a reactor, heated from room temperature to 150 °C at 1 °C/min, and held at that temperature for 4 h, all in a dry oxygen atmosphere. The temperature was then increased to 500 °C also at 1 °C/min and held at that set point for 6-12 h. The oxygen gas was pretreated for moisture removal using 3-A type zeolite beds. The gas flow rate was kept constant at about 140 cm3/min. After being heated, the zeolite was allowed to slowly cool to room temperature, again in dry oxygen.
For layered-bed experiments, a thin layer of Selexsorb CDX alumina was placed on top of the nickel-exchanged zeolites. Selexsorb CDX is an activated alumina specially formulated by Alcoa Industrial Chemicals for the adsorption of polar organic compounds including sulfurbased molecules (mercaptans, sulfides, disulfides, thiophenes), nitrogen-based molecules (nitriles, amines, pyridines), and oxygenated hydrocarbon molecules (alcohols, glycols, aldehydes, ketones, ethers, peroxides). Reagents and Standards. Commercial diesel samples were obtained from a gas station located in Ann Arbor, MI. The actual sulfur content was measured by gas chromatography techniques (please refer to the Gas Chromatography Analysis section). Thiophene, benzothiophenes (BTs), dibenzothiophene (DBT) standards were purchased from Sigma-Aldrich. Elemental Analysis. All nickel(II) sorbents were characterized using neutron activation analysis (NAA) in the research nuclear reactor of the Radiation Center at Oregon State University (Corvallis, OR) following standard procedures. Fixed-Bed Adsorption/Breakthrough Experiments. All dynamic adsorption or breakthrough experiments were performed in custom-made vertical quartz adsorbers equipped with a supporting glass frit as described elsewhere.8,9 The setup consisted of a low-flow liquid pump, Kynar compression fittings, feed tanks, and a heating element. Initially, the sorbents were loaded inside the adsorber, and heated in situ using dry gases to avoid exposure to atmospheric moisture. Faujasite-type zeolites, as well as many other aluminosilicates, are highly hydrophilic in nature, and it is imperative to avoid any contact with moisture before adsorption tests to achieve the highest sulfur loadings possible. The nickel-based sorbents were heated to 350 °C at 1 °C/min in inert gas atmosphere and held at this temperature for at least 18 h. For the Ce(III)-Y zeolite, the sorbent was heated to 450 °C in dry air at 2 °C/min and held at this temperature for 6 h.18 The gases used for activation were pretreated inline before contacting the sorbent using a 3A-type zeolite. After the activation treatment, the nickel- and cerium-zeolites under study were allowed to cool to room temperature and 80 °C, respectively, under gas flow. The beds were also tapped to ensure proper packing. The feed was then switched to a commercial-grade diesel, and effluent samples were collected at regular intervals until saturation was achieved (i.e., until the effluent total sulfur concentration equaled the influent total sulfur concentration), which depended on the adsorption dynamics and the amount of adsorbent. Dry air was used for calcination regeneration tests in which the temperature was controlled by a PID temperature controller to within (1 °C. The nickel- and cerium-zeolites were regenerated at 350 and 450 °C, respectively. Gas Chromatography Analysis. All of the samples collected during the breakthrough experiments were analyzed using a Shimadzu GC-17A v3 unit equipped with an EC-5 capillary column and a flame photometric detector (FPD). More details about the GC analysis can be found elsewhere.9,11 Peak identification information for sulfur compounds present in diesel was gathered after using standards and by comparisons of elution times with data available in the literature.31-35 For standards, stock solutions consisting of thiophene, BTs, or DBT in sulfur-free
Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004 1083
Figure 1. GC-FPD chromatogram of a commercial diesel fuel. Table 1. Composition Data for Sorbents Obtained from Neutron Activation Analysis (NAA)a molar ratios adsorbent
2Ni/Al
Si/Al
unit cell compositiona
Ni(II)-Y (LPIE-RT) Ni(II)-Y (LPIE-135) Ni(II)-Y (SSIE-500) Ni(II)-X (LPIE-RT)
0.50 0.62 1.00 0.38
2.43 2.43 2.40 1.25
Ni14Na28(Al56Si136O384) Ni18Na20(Al56Si136O384) Ni29(Al57Si135O384) Ni17Na52(Al86Si106O384)
a
Unit cell composition given for dehydrated zeolites.
n-octane were further diluted to a known concentration and then injected for elution time determination. Using the three standards (thiophene, BTs, and DBT), it was first established that the peak area per S atom was statistically the same for these three molecules at similar sulfur concentrations. After obtaining calibration data at different concentration levels, the diesel fuel total sulfur content was estimated by carefully adding all of the individual peaks areas. The total sulfur content of the fuel was 297.2 ppmw S. Figure 1 shows a detailed FPD chromatogram for the diesel fuel tested. Detectable thiophenic sulfur peaks from standards were recorded at concentrations down to approximately 20 ppbw S (or 50 ppbw thiophene). Nitrogen Equilibrium Adsorption Isotherms. The sorbents were tested for surface area measurements to verify crystallinity. As expected, the values obtained were similar to those of a crystalline faujasite zeolite (700-800 m2/g). This information was obtained from liquid nitrogen (at -196°C) equilibrium isotherms and following standard procedures found elsewhere. The equilibrium data was gathered using a Micromeritics ASAP 2010 static volumetric analysis unit. Information on this type of analysis and calculations is beyond the scope of this study. More information can be found elsewhere.36-38 Results and Discussion Adsorbent Characterization. Elemental analyses of some of the adsorbents were achieved by NAA. All of the zeolites were in hydrated conditions before testing. As seen in Table 1, ion-exchanging Na-Y with aqueous solutions containing Ni2+ species resulted in incomplete
ion exchange. If it is assumed that one Ni2+ cation compensates for two aluminum tetrahedral charges, then, for our case, the ion exchange resulted in only 50 and 62% substitution of the original sodium ions for Ni(II)-Y (LPIE-RT) and Ni(II)-Y (LPIE-135), respectively. The remaining sodium ions were compensating for the other aluminum tetrahedra charges; in other words, the (2Ni + Na)/Al ratio should be unity. For the sample using solid-state exchange techniques, the exchange was complete (i.e., 2Ni/Al ) 1.0) as expected. Fixed-Bed Adsorption Experiments. After in situ activation of the adsorbent bed, a commercial diesel feed (297 ppmw S) was allowed to contact the beds, and the effluent total sulfur content was monitored periodically. In situ activation was necessary for evaluation of the sorbents’ desulfurization capabilities without any exposure of the hydrophilic zeolites to atmospheric air. It should be mentioned that, after activation, the nickel(II)-zeolites were pink in color, as opposed to the greenish color observed after ion exchange, which indicated the sorbents were dehydrated.39 Breakthrough adsorption curves were generated by plotting the transient total sulfur concentration normalized by the feed total sulfur concentration versus the cumulative fuel volume normalized by the total bed weight. The adsorption amounts (normalized per adsorbent weight) were obtained after solving the following equation40
qbreakthrough or saturation )
(
)(
FfuelXi v˘ madsorbent MWsulfur
)∫ [ t
0
1-
]
C(t) dt (1) Ci
where q is the total amount of sulfur adsorbed (mmol/ g), v˘ is the feed volumetric flow rate (cm3/min), Ffuel is the fuel density (g/cm3) at room temperature, Xi is the total sulfur fraction (by weight) in the feed, Ci is the total sulfur concentration in the feed (ppmw S), madsorbent is the weight of the sorbent bed (g), MWsulfur is the molecular weight of sulfur, and C(t) is the effluent total sulfur concentration (ppmw S) at any time t (min). The integral part on the right-hand side of eq 1 is the area above the breakthrough curve at any time t. Table 2 summarizes the results obtained for total sulfur break-
1084 Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004 Table 2. Breakthrough Loadings for Total Sulfur from Diesel on Fresh Sorbents
adsorbent Ni(II)-Y (LPIE-RT) Ni(II)-Y (LPIE-135) Ni(II)-Y (SSIE-500) Selexsorb CDX/ Ni(II)-Y (SSIE-500)b Ni(II)-X (LPIE-RT) Selexsorb CDX/ Ni(II)-X (LPIE-RT)b Ce(IV)-Y (LPIE-80) a
breakthrough loading (mmol/g)a
saturation loading (mmol/g)a
0.085 0.120 0.158 0.191
0.204 0.213 0.289 0.331
0.143 0.161
0.251 0.281
0.043
0.122 b
Loading amounts normalized by total bed weight. Adsorbent bed contained two layers: the first layer was alumina Selexsorb CDX (CDX), and the second layer was Ni(II)-Y or Ni(II)-X zeolite. CDX accounted for 25% of the bed weight.
Figure 3. Faujasite zeolite framework with cation sites.
Figure 2. Breakthrough of total sulfur in fixed-bed adsorbers with Ni(II)-Y (LPIE-RT) (b) and Ni(II)-X (LPIE-RT) (O) adsorbents, for diesel feed at room temperature. Ci is the total sulfur concentration of the feed.
through and saturation adsorption amounts in fresh activated sorbents. Figure 2 shows the resulting breakthrough curves for the Ni(II)-Y (LPIE-RT) and Ni(II)-X (LPIE-RT) beds. The zeolites are capable of deep desulfurizing (i.e., achieving total sulfur contents of less than 1 ppmw) about 11.44 and 19.24 cm3 of diesel per gram of sorbent, respectively. This corresponded to adsorptions of 1.10 and 1.94 thiophenic molecules per zeolite unit cell, respectively. For both sorbent cases, sulfur saturation loading is reached after the processing of approximately 70 cm3 of diesel per gram of sorbent. It should be mentioned that recent reports on jet fuel desulfurization with Ni(II)-Y containing only 4 nickel ions per unit cell indicate that the zeolite is capable of removing 0.08 mmol of thiophenic sulfur per gram of zeolite at saturation.18 Although diesel instead of jet fuel was tested in the present work, the aforementioned value is lower than the amount obtained with our Ni(II)-Y (LPIE-RT) sorbent, which is about 0.20 mmol per gram of zeolite (see Table 2). The organosulfur species present in jet fuel are mostly substituted and nonsubstituted benzothiophenes. Diesel contains not only these species but also refractory dibenzothiophenes, which are difficult to remove. Also, our sorbent has 14 nickel ions per unit cell, 71% more ions than the zeolite used for the jet fuel
desulfurization work. This accounted for the large difference in adsorption capacities. The desulfurization performance differences observed in Figure 2 should result from the ion-exchange characteristics of the zeolites. Because X-type (Si/Al ) 1.25) zeolites have 54% more cation sites than Y-type (Si/Al ) 2.43) zeolites, the former were expected to offer more exposed nickel(II) ions and, as a result, higher adsorption capacities. For thiophenic molecules to be adsorbed, the adsorption has to occur in the zeolite supercages, and the cation sites exposed to these regions are (following the Smith nomenclature41) site II (SII), site III (SIII), site III′ (SIII′), and site U (SU). The cation sites for faujasite zeolites are portrayed in Figure 3. Only a handful of zeolites have been identified to contain sites of type SU,42 not including nickel(II)-exchanged zeolites. X-ray diffraction studies for aqueous-phase nickel(II)-exchanged Y-zeolites showed the nickel cations prefer sites SI, SI′, SII, and SII′.43-45 No site SIII occupancy was ever observed. Also, it was determined that the nickel ions in activated nickel(II)-zeolites occupy exposed sites when the cation content in the zeolite is greater than 12 cations per unit cell. Nevertheless, the migration of Ni2+ ions to exposed sites upon the adsorption of molecules has been studied before.39 Pyridine adsorption on Ni(II)-Y zeolites dehydrated at 300 °C causes nickel ions located in hexagonal prism windows (SI′) to migrate because of strong interactions with aromatic rings. This is also expected to occur when thiophenic rings are adsorbed onto nickel(II)-zeolites particularly because of the strong π-complexation mechanism. For the case of aqueous-phase exchanged Ni(II)-X zeolites, Bae and Seff found that nickel ions preferentially occupy sites SII and SIII′.46 Thus, the cation positions in X-zeolites seem to favor the adsorption of thiophenic molecules as shown in Figure 2. It should be mentioned that, for all nickel(II)-zeolites, the color of the powders changes from pink to brown during the desulfurization process. The color change is gradual and occurs along the length of the bed. Herna´ndezMaldonado and Yang observed similar color changes during the desulfurization of a diesel fuel with Cu(I)-Y zeolites.8 It was determined that the color change was
Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004 1085
Figure 5. Breakthrough of total sulfur in fixed-bed adsorbers with Ce(IV)-Y (LPIE-80) (0), Ni(II)-Y (LPIE-135) (2), and Ni(II)-Y (SSIE-500) (4) adsorbents, for diesel feed at room temperature. Ci is the total sulfur concentration of the feed.
Figure 4. Breakthrough of benzothiophene (O), dibenzothiophene (b), 4-methyldibenzothiophene (4), 4,6-dimethyldibenzothiophene (2), and 2,4,6-trimethyldibenzothiophene (]) in a (A) Ni(II)-Y (LPIE-RT) or (B) Ni(II)-X (LPIE-RT) adsorbent, with diesel feed at room temperature. Cj(t) and Cji are the sulfur concentrations for the individual molecules in the effluent and feed, respectively.
due to the complex formed between the cuprous ions and the organosulfur molecules. Figure 4 shows the breakthrough adsorption of specific organosulfur molecules during the desulfurization of a commercial diesel also with Ni(II)-Y (LPIE-RT) and Ni(II)-X (LPIE-RT) beds. It is clear that both zeolites are capable of removing refractory thiophenic compounds [e.g., 4-methyldibenzothiophene (4-MDBT), 4,6dimethyldibenzothiophene (4,6-DMDBT), and 2,4,6trimethyldibenzothiophene (2,4,6-TMDBT)], which remain largely intact during traditional HDS processes. However, the exposed cations in Ni(II)-X (LPIE-RT) zeolites are capable of removing the organosulfur molecules without much discrimination between them. This is possible because of the π-complexation interactions between the exposed cation and the aromatic rings. Thus, substituted thiophenic rings with methyl groups surrounding the sulfur atom can also be easily removed, as opposed to HDS, which relies on attacking the sulfur atom directly. It should be mentioned that the sodium ions remaining in the zeolite structure after ionexchanging Na-X and -Y zeolites for nickel are not responsible for the deep-desulfurization process. Herna´ndez-Maldonado and Yang showed that Na-Y zeolites are not capable of deep desulfurizing liquid fuels because of the lack of a π-complexation capability.8 To increase the adsorption capacity and selectivity of Ni(II)-Y, it was desired to test other methods of increasing the concentration of nickel ions per unit cell. One
way to accomplish this is by increasing the ion-exchange temperature. Olson ion exchanged faujasite zeolites with nickel(II) ions to almost 100% capacity by using a temperature of 90 °C and for 60 days.43 Certainly, a higher exchange temperature should accomplish the process in a smaller time window. Figure 5 shows the resulting diesel desulfurization breakthrough curves for a zeolite prepared after ion exchanging Na-Y with nickel(II) at 135 °C. Ni(II)-Y (LPIE-135) shows a considerable increase in total sulfur adsorption loading before breakthrough occurs. The capacity increased by 41% when compared to the value obtained with Ni(II)-Y (LPIE-RT) (please refer to Table 2). This corresponds to approximately 1.55 organosulfur molecules per zeolite unit cell. These results clearly indicate not only that more nickel(II) ions were added to extraframework positions but also that these ions were located in exposed sites as discussed earlier. Ion exchange at higher temperatures to obtain high nickel loadings in X-zeolites resulted in the collapse of the framework as determined from nitrogen adsorption at -196 °C. The surface area was approximately 200 m2/g, which indicates a loss of crystallinity; thus, the sorbent was not tested for desulfurization. Collapse of Co(II)-X zeolites caused by exchange at high temperatures was observed by Bae and Seff.47 The same collapse possibly occurs during exchange for Ni2+ into X-zeolite at high temperatures. The solid-state ion-exchange (SSIE) technique is an excellent way of exchanging cations in a more direct fashion. The advantages of SSIE48 over conventional ion exchange from aqueous media include (i) avoiding the use of large volumes of salt solution, (ii) avoiding the problem of discarding waste salt solution, and (iii) allowing the metal cations (which are small) to be introduced through narrow windows or channels that would impede or prevent the ion exchange of solvated cations (which are larger) from aqueous solution. The SSIE process to obtain Ni(II)-Y involves the following reaction scheme heat
2H+Y(s) + NiCl2(s) 98 Ni2 + Y(s) + 2HCl(v)
(2)
The H-Y zeolite and nickel(II) chloride were mixed thoroughly and heated in dry air, oxygen, or vacuum to
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Figure 6. Breakthrough of benzothiophene (O), dibenzothiophene (b), 4-methyldibenzothiophene (4), 4,6-dimethyldibenzothiophene (2), and 2,4,6-trimethyldibenzothiophene (]) in a (A) Ni(II)-Y (SSIE-500) or (B) Ce(IV)-Y (LPIE-80) adsorbent, with diesel feed at room temperature. Cj(t) and Cji are the sulfur concentrations for the individual molecules in the effluent and feed, respectively.
induce the ion exchange. This method was successfully used to obtain 100% exchanged and overexchanged Ni(II)-Y zeolites.49,50 Thorough mixing of the solid species is crucial for successful ion exchange, but it could be detrimental to the zeolite framework if not performed carefully. For instance, Auroux et al.51 and Xie and Kaliaguine52 studied the effect of ball milling zeolites for different amounts of time. They found that, even after 10 min of mixing, the process generated sufficient localized heating to reduce the surface area by 19%. For our preparation, the zeolite and the nickel source were mixed by hand without applying any pressure to the powders. After the SSIE of H-Y with NiCl2 to obtain Ni(II)-Y (SSIE-500), the final sorbent was used to desulfurize a fresh diesel feed, and the results are shown in Figure 5. Ni(II)-Y (SSIE-500) is capable of deep desulfurizing approximately 22 cm3 of diesel per gram of sorbent. This corresponds to increases of 85, 32, and 10% increase in breakthrough loading when compared to Ni(II)-Y (LPIERT), Ni(II)-Y (LPIE-135), and Ni(II)-X (LPIE-RT), respectively (see Table 2). As shown in Figure 6A, the SSIE zeolite removed all compounds without distinction among the different organosulfur molecules present in the diesel, including the refractory compounds. SSIE to obtain Ni(II)-X was not tried because of the low thermal stabilities of zeolites containing high concentrations of aluminum atoms per unit cell.42
Fuel desulfurization via chemical adsorption using cerium-exchanged zeolites was recently claimed.18 The process, which involves direct interactions with the sulfur atoms of the thiophenic molecules, was tested for the desulfurization of a model jet fuel at 80 °C; the results for a real diesel sample were not reported. Using recipes identical to that reported in the aforementioned literature,18 we prepared cerium-exchanged Y-zeolites (Ce-Y) and tested them for the desulfurization of a commercial diesel fuel. The sorbent was activated in dry air at 450 °C, and the fuel desulfurization process was performed at 80 °C. The elevated temperature was necessary because the process relies on a chemical reaction. It should be pointed out that, after the activation and desulfurization steps, the sorbent color was yellowish and amber, respectively. The yellow color corresponds to cerium(IV), and we will refer to this zeolite as Ce(IV)-Y (LPIE-80). The amber color obtained after desulfurization is indicative of a complexation mechanism. The total sulfur breakthrough curve for diesel treatment with Ce(IV)-Y (LPIE-80) is shown in Figure 5. Integration of the area above the curve indicates that the sorbent is capable of removing 0.013 and 0.122 mmol of thiophenic sulfur per gram of zeolite at breakthrough and saturation, respectively. For a model jet fuel, this zeolite was capable of removing 0.072 mmol of thiophenic sulfur per gram of zeolite18 under the same conditions as used for the diesel treatment mentioned above. The lower desulfurization capacity observed for diesel treatment is probably due to the lack of an interaction between the cerium ions and the sulfur atom in the refractory sulfur compounds, which are abundant in the diesel fuel (refer to Figure 1). In these compounds, the sulfur is difficult to access because of steric hindrance imposed by the methyl groups adjacent to the sulfur atom. Further evidence of this difficulty can be found in Figure 6B, which shows the breakthrough of individual organosulfur molecules during the diesel desulfurization. The figure shows clearly that the Ce(IV)-Y (LPIE-80) sorbent has more selectivity toward nonsubstituted thiophenes (e.g., BTs and DBT) than toward substituted ones. Abundant refractory compounds such as 4,6-DMDBT leave the bed almost instantly when compared to BTs and DBT. Layered Beds. Recently, Yang et al., Herna´ndezMaldonado and Yang, and Herna´ndez-Maldonado et al. used guard beds to further increase the total adsorption capacity when using π-complexation adsorbents.7,9-12 They found that adding a thin layer of either activated carbon or alumina or both on top of Cu(I)-Y zeolites not only increases the deep-desulfurization capacity, but also sharpens the concentration wave fronts. A more detailed discussion on the usage of layered beds for different applications can be found elsewhere.53 The best two sorbents tested here, Ni(II)-Y (SSIE-500) and Ni(II)-X (LPIE-RT), were also tested with alumina guard beds. Figure 7 shows diesel desulfurization breakthrough curves after treatment with Selexsorb CDX/Ni(II)-Y (SSIE-500) and Selexsorb CDX/Ni(II)-X (LPIE-RT). In all cases, the activated alumina accounted for 25 wt % of the total bed weight. A summary of the breakthrough and saturation adsorption loading capacities for these sorbents is also reported in Table 2. For both sorbents, the adsorption capacities increased considerably when compared to the case without a guard bed. The greatest adsorption capacity, however, was obtained with the
Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004 1087
Figure 7. Breakthrough of total sulfur in fixed-bed adsorbers with Selexsorb CDX/Ni(II)-X(LPIE-RT) ([) and Selexsorb CDX/Ni(II)Y(SSIE-500) (]) adsorbents, for diesel feed at room temperature. Ci is the total sulfur concentration of the feed.
Figure 8. Breakthrough of benzothiophene (O), dibenzothiophene (b), 4-methyldibenzothiophene (4), 4,6-dimethyldibenzothiophene (2), and 2,4,6-trimethyldibenzothiophene (]) in a Selexsorb CDX/ Ni(II)-Y (SSIE-500) adsorbent, with diesel feed at room temperature. Cj(t) and Cji are the sulfur concentrations for the individual molecules in the effluent and feed, respectively.
Selexsorb CDX/Ni(II)-Y (SSIE-500) matrix. Selexsorb CDX activated alumina is known to adsorb refractory and nonrefractory compounds from diesel, but not to deep desulfurization levels.11 Also, activated carbons and aluminas are known to remove mainly aromatic molecules such as benzene. Takahashi and Yang concluded that, at low concentrations, polarizability interactions play an important role in the adsorption of benzene molecules in both activated carbon and alumina.20 At high concentrations, more thiophene molecules can be adsorbed because of a pore-filling mechanism. Figure 8 shows the resulting adsorption breakthrough behaviors of individual organosulfur compounds also in a Selexsorb CDX/Ni(II)-Y (SSIE-500) layered bed. It is clear that the bed adsorbed more of these compounds than did the Ni(II)-Y (SSIE-500) bed alone and that it did so in a selective fashion. The actual sulfur levels attained during desulfurization with all of the nickel(II)-zeolites and the layered beds are shown in Figure 9. One gram of Ni(II)-Y (SSIE-
Figure 9. Total sulfur contents of desulfurized diesel during breakthrough in different nickel(II)-zeolites based on a feed with 297 ppmw S total sulfur content.
Figure 10. Total sulfur contents of desulfurized diesel during breakthrough in different layered beds based on a feed with 297 ppmw S total sulfur content.
500) is capable of producing about 14 cm3 of diesel fuel with a sulfur content of 0.256 ppmw S, which is suitable for use in fuel cell reforming units. When used in a layered-bed configuration, the Selexsorb CDX alumina and the Ni(II)-Y (SSIE-500) zeolite are capable of producing about 19 cm3 of diesel fuel with a sulfur content of 0.220 ppmw S (Figure 10). Sorbent Regeneration. Ni(II)-Y (SSIE-500) was tested for regeneration after saturation with diesel organosulfur molecules. The regeneration was achieved in a single step: calcination in dry air at 350 °C for 6 h. After calcination, the nickel-exchanged sorbent recovered its original color (after activation), which was pink. Afterward, a fresh untreated diesel feed was allowed to contact the sorbent at room temperature, and the second desulfurization cycle was monitored for sulfur elution. Figure 11 shows the total sulfur adsorption breakthrough curve and compares it to the results from the first cycle. The regeneration scheme was capable of recovering the full original sulfur sorption capacity in one step. This is an advantage over the regeneration scheme found by Herna´ndez-Maldonado et al. for Cu(I)-Y zeolites, which were also used for the desulfu-
1088 Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004 Table 3. Breakthrough and Saturation Loadings for Total Sulfur from Diesel on Regenerated Sorbentsa
adsorbent
regeneration method
Ni(II)-Y (SSIE-500) Ce(IV)-Y (LPIE-80)
airc aire
stage
breakthrough loading (mmol/g)
saturation loading or total sulfur removed (mmol/g)
sorbent breakthrough capacity recovery (%)b
second cycled second cycled
0.157 0.032
0.275 0.091
99.4 74.4
a Refer to Table 2 for fresh adsorbent breakthrough loading capacities. b Recovery calculated as follows: recovery (%) ) 100 x (second cycle breakthrough loading/first cycle breakthrough loading). c Regeneration in air at 450 °C for 6 h. d Second cycle desulfurization with fresh, untreated diesel fuel feed. e Regeneration in air at 350 °C for 6 h.
X-ray diffraction studies indicate that Ce-Y zeolites lose crystallinity after calcination processes.18 It was likely that the collapse of the structure did not stop after the original activation step and continued during the regeneration step. Conclusion
Figure 11. Breakthrough of total sulfur in fixed-bed adsorbers with fresh (4) and regenerated (b) Ni(II)-Y (SSIE-500) adsorbents, with diesel feed at room temperature. Adsorbent regenerated in air at 350 °C.
Our results have shown that nickel(II)-exchanged zeolites are superior adsorbents for the removal of all sulfur compounds from commercial diesel fuels, according to dynamic fixed-bed adsorption experiments. When used with a guard bed, solid-state ion-exchanged Ni(II)-Y provides, by far, the best adsorption capacities both at the breakthrough point and at saturation. For alumina acting as a guard bed, the sorbent is capable of processing 19 cm3/g of diesel with an average sulfur content of 0.220 ppmw. In general, the adsorbents tested displayed the following order for total sulfur adsorption capacity at breakthrough: Selexsorb CDX (alumina)/ Ni(II)-Y (SSIE-500) > Selexsorb CDX (alumina)/Ni(II)-X (LPIE-RT) > Ni(II)-Y (SSIE-500) > Ni(II)-X (LPIE-RT) > Ni(II)-Y (LPIE-135). Desulfurization tests with Ce(IV)-Y zeolites18 indicated that the sorbents have only selectivity toward nonsubstituted thiophenes, which are not abundant in diesel fuels. The adsorption limitations are due to the lack of direct sulfur-metal interactions with refractory compounds. Nickel-exchanged zeolites interact via π-complexation with thiophenic aromatic rings rather than by attacking sulfur atoms directly. Ni(II)-Y zeolites used for desulfurization are also fully regenerated using dry air at 350 °C. Because of the stability of nickel(II) ions, there is no need for further activation steps after calcination. Acknowledgment
Figure 12. Breakthrough of total sulfur in fixed-bed adsorbers with fresh (0) and regenerated (9) Ce(IV)-Y (LPIE-80) adsorbents, with diesel feed at room temperature. Adsorbent regenerated in air at 450 °C.
rization of a diesel fuel via π-complexation.12 The copperzeolites required both a calcination step and a reduction step for complete regeneration. Figure 12 shows diesel desulfurization breakthrough curves for the regenerated Ce(IV)-Y (LPIE-80) sorbent. The sorbent was regenerated under the same conditions as used for its activation (i.e., using dry air at 450 °C for 6 h).18 A second desulfurization cycle shows that the sorbent adsorption capacity diminished by 74% (see Table 3). This could be due to strong interactions of the cerium ions with the adsorbed organosulfur or, more likely, low zeolitic framework stability with Ce cations.
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Received for review October 24, 2003 Revised manuscript received December 11, 2003 Accepted December 17, 2003 IE034206V
