New Sorbents for Desulfurization of Diesel Fuels via π Complexation

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New Sorbents for Desulfurization of Diesel Fuels via π Complexation: Layered Beds and Regeneration Arturo J. Herna´ ndez-Maldonado, Stephen D. Stamatis, and Ralph T. Yang* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136

Alice Z. He and William Cannella Chevron-Texaco Energy Research & Technology, Richmond, California 94802

Desulfurization of a commercial diesel fuel by different layered adsorbents and regeneration of the latter were studied in a fixed-bed unit operated at ambient temperature and pressure. In general, a layered bed consisting of 12 wt % activated carbon, 22 wt % of activated alumina (Selexsorb CDX), followed by Cu(I)-Y, activated carbon/Selexsorb CDX/Cu(I)-Y, is capable of producing 41 cm3 of desulfurized diesel fuel/g of adsorbent. The matrix is capable of processing 27 cm3 of deep-desulfurized diesel with a weighted average content of 76 ppbw S. These lowsulfur fuels are suitable for fuel cell applications. For layered-bed regeneration, it was determined that calcination of the adsorbed sulfur moieties with air at 350 °C followed by autoreduction of the copper species recovered all of the original desulfurization capacity when activated aluminas are used as a guard layer. Solvent elution experiments indicate that carbon tetrachloride and N,N-dimethylformamide are suitable solvents to recover all of the adsorbed organosulfur species. Introduction The present paper is a continuation of our research on desulfurization of commercial liquid fuels using π-complexation sorbents.1-5 Sulfur in transportation fuels remains a major source of air pollution. Because of government mandates worldwide, refiners must produce increasingly cleaner fuels.6-8 Another need for deep desulfurization is for applications in fuel cells. Gasoline or diesel is the ideal fuel for fuel cells because of its high energy density, ready availability, safety, and ease for storage. However, to avoid poisoning of the catalysts for the fuel processor and that in the electrode of the fuel cell, the sulfur concentration should be preferably below 0.1-0.2 ppmw. Recently, Herna´ndez-Maldonado and Yang showed that both Cu(I)-Y and Ag-Y zeolites are excellent adsorbents for thiophene sulfur removal from benzene and/or n-octane mixtures.2 In another work, they demonstrated that Cu(I)-Y zeolites are superior sorbents for sulfur removal from commercial gasoline and diesel fuels.1,3-5 It was found that a layered bed containing activated carbon (upstream, 15 wt %) and copper(I) faujasite is capable of desulfurizing about 24 and 34 cm3 of gasoline and diesel fuel, respectively, per gram of bed. The sulfur content in the desulfurized effluent was found to be less than 1 ppmw (sulfur basis). Deep desulfurization of fuels with Cu(I)-Y sorbents is possible because of a complexation mechanism between the cation and the thiophenic rings (see Figure 1). In the π-complexation mechanism, the cations can form the usual σ bonds with their s orbitals and, in addition, their d orbitals can backdonate electron density to the antibonding π orbitals of the sulfur rings. It was determined that π complexation is stronger with the thiophenic ring than with the benzene ring, which makes the process * To whom correspondence should be addressed. Tel.: (734) 936-0771. Fax: (734) 764-7453. E-mail: [email protected].

Figure 1. View of a copper (Cu+) faujasite zeolite R cage with adsorbed thiophene rings. Copper ions are located in sites of type II.

suitable for commercial fuel desulfurization. More information on the latter1-5,9-11 and additional π-complexation work by our group could be found elsewhere.12-20 Layered or multiple beds have been used for many years to enhance adsorption capacities, especially for pressure-swing adsorption (PSA) applications. Bakr and Salem21 and Salem and Hamid22 studied desulfurization of naphtha using activated carbon and 13X zeolite and proposed the use of a two-bed strategy based on equilibrium isotherm data: an activated carbon bed and a 13X zeolite bed operating at 80 °C and room temperature, respectively. However, there is no data on performance of such combination or level of desulfurization attained. Chlendi and Tondeur modeled layered beds consisting of activated carbon and 5A zeolite to purify

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hydrogen streams.23 Carbon was used to preferentially adsorb carbon dioxide and methane while the zeolite adsorbed nitrogen. The authors observed that the layered bed of sorbents had a complex dynamic behavior associated with the passage of the concentration wave fronts from one layer to another, analogous to refraction, diffraction, dispersion, and interference of light rays penetrating in two media. They mathematically correlated the phenomena to equilibrium isotherms. To the best of our knowledge, the layered-bed dynamics, however, are not understood at the experimental level. Herna´ndez-Maldonado and Yang studied individual thiophenic compound breakthrough/adsorption behavior during desulfurization of diesel fuels with an activated carbon/copper faujasite zeolite layered bed.3,5 The activated carbon not only enhanced the breakthrough capacity of the zeolite but also sharpened the concentration wave fronts of all thiophenic molecules. Efficiency of desulfurization using sorbents is determined mainly by its adsorption capacity, selectivity for organosulfur compounds (i.e., thiophenes), durability, and regenerability.24 For the last two characteristics, there are at least two common approaches: (1) desorption or elimination of the sulfur species by using gases or vacuum at moderately high temperatures (e.g., burning in air) and (2) solvent elution techniques. The former was used successfully for regeneration of spent Cu(I)-Y after sulfur removal from n-octane feeds.2 Calcination with air at 350 °C followed by autoreduction of the copper cations recovered all of the original desulfurization capacity. Phillips Petroleum Co. developed a process called S Zorb in which the sulfur atoms present in the thiophenic compounds are removed at temperatures in the 340-420 °C range under a hydrogen atmosphere. They use a proprietary adsorbent, which is believed to be comprised of a bimetallic promotor (cobalt, molybdenum, etc.) dispersed on a support formed by zinc oxide and alumina or silica gel.25,26 This proprietary material is also regenerated using air at moderate temperatures and is then reactivated using hydrogen prior to the next desulfurization cycle. For solvent elution technique options, Savage et al. used toluene at 100 °C to regenerate an activated carbon used to remove adsorbed sulfur compounds.27 They tested a mid-distillate stream containing 1200 ppmw S and were able to reduce the sulfur content to about 25 ppmw S (based on minimum sulfur detectability). In this paper, we discuss π-complexation sorbent regeneration after desulfurizing a commercial diesel fuel stream using solvent elution techniques or air burning/ calcination. For the former, we tested carbon tetrachloride (CCl4), N,N-dimethylformamide [HCON(CH3)2], methanol (CH3OH), and toluene [(C6H5)CH3] solvents. We present alternate layered-bed configurations and discuss the advantage of using alumina as a guard bed for regeneration purposes. The fuels tested in this work were fully characterized for sulfur content using gas chromatography-flame photometric detection (GCFPD) techniques. Experimental Section Adsorbent Preparation. The starting adsorbent materials used in this study were Na-Y zeolite (Si/Al ) 2.43, Strem Chemicals), activated alumna Selexsorb CDX (Alcoa Industrial Chemicals), and type PCB activated carbon (Calgon Corp.). Selexsorb CDX is an

activated alumina specially formulated by Alcoa Industrial Chemicals for the adsorption of polar organic compounds including sulfur-based molecules (mercaptans, sulfides, disulfides, and thiophenes), nitrogenbased molecules (nitriles, amines, and pyridines), and oxygenated hydrocarbon molecules (alcohols, glycols, aldehydes, ketones, ethers, and peroxides). This material was supplied in pellet form and was crushed into powder for use during tests. Cu(I)-Y [or reduced Cu(II)-Y] was prepared by first ion exchanging Na-Y with a Cu(NO3)2 aqueous solution for 48 h followed by reduction of Cu2+ to Cu+. More details about sorbent preparation, including autoreduction of the copper ions, can be found in our previous reports1-5,9,17 and elsewhere.28-33 Reagents and Standards. Diesel samples were obtained from a gas station located in Ann Arbor, MI. The average total sulfur concentration for the particular diesel brand was reported to be 430 ppmw. The actual sulfur content, however, was measured by GC techniques (please refer to the Gas Chromatography Analysis section). Thiophene, benzothiophene (BT), dibenzothiophene (DBT), carbon tetrachloride (CCl4), N,Ndimethylformamide (DMF), methanol, and toluene standards were purchased from Sigma-Aldrich. Elemental Analysis. Cu(II)-Y sorbents were characterized using neutron activation analysis in the research nuclear reactor of the Phoenix Memorial Laboratory at the University of Michigan. The data were obtained from a 1-min core-face irradiation delivered via a pneumatic tube to a location with an average thermal flux of 2.13 × 1012 neutrons/cm2/s. Following irradiation, two separate counts, lasting for 500 s, were made: one after a 13-min decay (for Al and Cu) and a second count after a 1-h and 56-min decay (for Na). The concentrations of Al and Na were determined based on comparisons with two replicates of the standard reference material NIST1633A (coal fly ash); the determination of the Cu content was evaluated relative to a high-purity copper wire. Data reductions for NIST1633A were based on NIST certified values. Fixed-Bed Adsorption/Breakthrough Experiments. All adsorption/breakthrough experiments were performed in a vertical custom-made quartz adsorber equipped with a supporting glass frit as described elsewhere.2,3,5 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 (at 450 °C) in a flowing helium gas. The gases used for activation were pretreated inline prior to contacting the sorbent using a 3A-type zeolite. After activation treatment, the adsorbent under study was allowed to cool to room temperature under inert gas and then tapped to ensure proper packing. Next, a sulfur-free hydrocarbon was allowed to flow through the sorbent to eliminate any entrapped gas. After the adsorbent was wetted for several minutes, the feed was switched to a commercialgrade diesel. Effluent samples were collected at regular intervals until saturation was achieved, which depended on the adsorption dynamics and the amount of adsorbent. For regeneration experiments with solvents, these were poured on top of the spent layered beds and the head of liquid was maintained constant. Effluent samples were periodically analyzed for sulfur elution. Air was used for calcination regeneration tests and the temper-

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Figure 2. GC-FPD chromatogram of a commercial diesel fuel.

ature controlled by a proportional-integral-derivative temperature controller to within (1°C. After calcination, the zeolite part of the layered bed was oxidized and needed to be autoreduced again with an inert gas at 450 °C. GC 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. More details about the GC analysis could be found elsewhere.3,5 Peak identification information for sulfur compounds present in diesel was gathered after using standards and by retention time comparison with data available in the literature.34-38 For standards, thiophene, BT, and DBT solutions were diluted in sulfur-free n-octane to a known concentration and then injected for retention time determination. Using the three standards (thiophene, BT, and DBT), it was first established that the peak area per S was statistically the same for these three molecules at similar sulfur concentrations. After calibration data were obtained at different concentration levels, the diesel fuel total sulfur content was estimated by carefully adding up all individual peak areas. The total sulfur content of the fuel was 297.2 ppmw S and not 430 ppmw S as reported above. Figure 2 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. All sorbents were tested for surface area and cumulative pore volume measurements. This information was obtained from liquid-nitrogen (at -196 °C) equilibrium isotherms and by following standard procedures found elsewhere. The equilibrium data were 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.39-41 Results and Discussion Layered Beds. After in situ activation of the sorbents (in situ), a commercial diesel fuel was allowed to contact the fixed bed at room temperature and conditions. Figure 3 shows total sulfur breakthrough curves for a Cu(I)-Y zeolite and several layered bed configurations including activated carbon and Selexsorb CDX

Figure 3. Breakthrough of total sulfur in a fixed-bed adsorber with Cu(I)-Y (9), activated carbon/Cu(I)-Y (0), Selexsorb CDX/ Cu(I)-Y (4), or activated carbon/Selexsorb CDX/Cu(I)-Y (O) adsorbents, for diesel feed at room temperature. Ci is the total sulfur concentration of the feed. Table 1. Properties of Sorbents Used during Desulfurization and Regeneration Tests adsorbent

BET surface area (m2/g)

total pore volume (cm3/g)

Cu(I)-Y (Si/Al ) 2.43)a Selexsorb CDX (γ-Al2O3)b activated carbon (PCB)

778 494 1115

0.374 0.421 0.538

a Unit cell ) Cu Na (Al Si b 21 14 56 136O384). Material contains a proprietary modifier (source: Aluminum Co. of America, Alcoa).

together with Cu(I)-Y. Our group has previously tested diesel desulfurization with Cu(I)-Y, activated carbon, Selexsorb CDX, and activated carbon/Cu(I)-Y beds.3,5 Only the breakthrough curves for Cu(I)-Y and activated carbon/Cu(I)-Y are shown because they are the only ones relevant to the present study and needed for comparison purposes. All breakthrough and saturation amounts presented throughout the paper were calculated after integration of the areas above the individual total breakthrough curves. Herna´ndez-Maldonado and Yang showed that activated carbon, when used as a guard bed to Cu(I)-Y, increases the desulfurization capacity by 28% and sharpens the concentration wave fronts of all of the sulfur (thiophenic) molecules, including refractory ones such as 4,6-dimethyldibenzothiophene (4,6-DMDBT) and 4-methyldibenzothiophene (4-MDBT).3,5 When using a single activated carbon bed, it was shown the diesel fuel sulfur broke through almost instantly. Despite this, the carbon displayed some adsorption capacity for thiophenic compounds.5 Figure 3 compares these results to the ones obtained for alternate layered-bed configurations, in particular, Selexsorb CDX/Cu(I)-Y and activated carbon/Selexsorb CDX/Cu(I)-Y, also during desulfurization of a diesel fuel. The amount of alumina used in the Selexsorb CDX/Cu(I)-Y layered bed accounts for 25 wt % of the total weight, which is higher than the weight content used for the activated carbon layered bed (15 wt %). The rationale behind this was to provide a similar surface area and pore volume capacity in the top layer. Table 1 shows BrunauerEmmett-Teller (BET) surface areas and total pore volumes as determined from nitrogen equilibrium adsorption at -196 °C. Activated carbons and aluminas

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Table 2. Breakthrough Loadings for Total Sulfur from Diesel on Fresh Sorbents adsorbent

breakthrough loading (mmol/g)d

Cu(I)-Y AC/Cu(I)-Ya CDX/Cu(I)-Yb AC/CDX/Cu(I)-Yc

0.167 0.235 0.221 0.290

a The adsorbent bed contained two layers: the first layer was PCB activated carbon (AC) followed by Cu(I)-Y zeolite. AC accounts for 15 wt % of the bed weight. b The adsorbent bed contained two layers: the first layer was alumina Selexsorb CDX (CDX) followed by Cu(I)-Y zeolite. CDX accounts for 25 wt % of the bed weight. c The adsorbent bed contained three layers: the first layer was PCB activated carbon (AC) followed by alumina Selexsorb CDX (CDX) followed by Cu(I)-Y zeolite. AC and CDX account for 12 and 22 wt % of the bed weight, respectively. d Loading amounts were normalized by total bed weight.

are known to remove not only thiophenic molecules but mainly aromatic molecules such as benzene. Takahashi and Yang concluded that at low concentrations polarizability interactions played an important role for adsorption of benzene molecules in both activated carbon and alumina.9 At high concentrations, more thiophene molecules were adsorbed because of a porefilling mechanism. Figure 3 shows that activated alumina, like activated carbon, also increases the breakthrough adsorption capacity of the Cu(I)-Y zeolite. Breakthrough adsorption capacities for all layered beds are shown in Table 2. Because the deep-desulfurization process is accomplished by the Cu(I)-Y zeolite, the activated alumina is actually diminishing the adsorption of aromatics downstream in the zeolite as well as adsorbing certain thiophenes. Herna´ndez-Maldonado and Yang found that the Cu(I)-Y zeolite removed about 0.3 sulfur molecules (thiophenic) per copper(I) cations based on only 50% autoreduction of the original cupric ions.3,9 These cations have to be located at sites II and III (based on the Smith nomenclature42) in order to be accessible to thiophenic molecules. According to Maxwell and DeBoer, X-ray data show that dehydrated Cu(II)-Y zeolites have about 25% of the copper ions occupying site II and about 14% in site III.43 Given the composition of our zeolite [Cu21Na14(Al56Si136O384)], it was determined that approximately three Cu+ cations are undergoing π complexation with thiophenic molecules, which accounts for 28% and 14% of the cuprous and total copper ions available, respectively. The latter is in excellent agreement with the findings of Maxwell and DeBoer for site III occupancy. However, there are 72% of cuprous ions that are probably occupied by other molecules (e.g., nitrogen hetereocycles) that could also do π complexation. These results indicate that the copper zeolite was responsible for most of the sulfur removal and that the guard beds were probably just enhancing the process. This was proven recently by Herna´ndez-Maldonado and Yang, who tested several commercial sorbents, including aluminas and other zeolites, for fuel desulfurization.5 Cu(I)-Y was the best sorbent (without guard beds) by far. A three-layered-bed strategy was also tested here. The bed consisted of (starting upstream) activated carbon (12 wt %), Selexsorb CDX (22 wt %), and Cu(I)-Y. This order follows that of the average pore size in each sorbent/layer (i.e., activated carbon > Selexsorb CDX > Cu(I)-Y) and should remove molecules with similar size distributions. As seen in Figure 3, this combination

Figure 4. Total sulfur content of desulfurized diesel during breakthrough in different layered beds based on a feed with 297 ppmw S of total sulfur content.

Figure 5. Breakthrough of BT (O), DBT (b), 4-MDBT (4), or 4,6DMDBT (2) in an activated carbon/Selexsorb CDX/Cu(I)-Y layered bed, with diesel feed at room temperature.

is by far the best for sulfur removal from diesel fuels, increasing both breakthrough and saturation capacities and sharpening the sulfur concentration wave front. The bed is capable of producing approximately 41 cm3 of desulfurized diesel/g of bed. Figure 4 shows the actual sulfur content (aggregated) of the desulfurized commercial diesel after treatment with different layered beds. The activated carbon/Cu(I)-Y and Selexsorb CDX/ Cu(I)-Y beds offer about the same sulfur content, but the activated carbon/Selexsorb CDX/Cu(I)-Y matrix can produce as much as 27 cm3 of diesel/g of bed with a sulfur content of as low as 76 ppbw S. Figure 5 shows the individual breakthrough curves of thiophenic compounds, including the refractory 4,6DMDBT and 4-MDBT, from the activated carbon/ Selexsorb CDX/Cu(I)-Y bed. All sulfur molecules are removed selectively regardless of their structural characteristics (i.e., position of methyl groups, aromatic rings), which is impossible to achieve using current commercial hydrodesulfurization (HDS) reactors. This is possible because thiophene molecules are believed to undergo π complexation with copper(I) in a flat configuration.5,9 During HDS the molybdenum (Mo) atom

Ind. Eng. Chem. Res., Vol. 43, No. 3, 2004 773 Table 3. Breakthrough and Saturation Loadings for Total Sulfur from Diesel on Regenerated Sorbentsa adsorbents AC/Cu(I)-Y

regeneration method airb CCl4c DMFc methanolc toluenec

CDX/Cu(I)-Y

airb

stage

breakthrough loading (mmol/g)

saturation loading or total sulfur removed (mmol/g)

sorbent breakthrough capacity recovery (%)f

second cycled sulfur elimination second cycled sulfur elimination second cycled sulfur elimination second cycled sulfur elimination second cycled second cycled

0.200 N/A 0.176 N/A 0.095 N/A 0.059 N/A 0.066 0.214

0.339 0.346e 0.297 0.346e 0.187 0.292e 0.166 0.152e 0.180 0.366

85.1 75.0

a

40.6 25.0 28.1 97.1

b

Refer to Table 2 for fresh adsorbent breakthrough loading capacities. Regeneration in air at 350 °C for 6 h followed by autoreduction in helium at 450 °C. c Regeneration at room temperature. d Second cycle desulfurization with fresh untreated diesel fuel feed. e Total amount of sulfur removed during regeneration. During the first desulfurization cycle, the AC/Cu(I)-Y bed adsorbed an average of 0.381 mmol of S/g of bed. f Recovery was calculated as follows: recovery % ) 100(second cycle breakthrough loading)/(first cycle breakthrough loading).

Figure 6. Sulfur elution during regeneration of a spent activated carbon/Cu(I)-Y fixed-bed adsorber with CCl4 (b), DMF (O), methanol (4), or toluene (0). Ci is the maximum sulfur concentration at the effluent.

needs to bond the sulfur atom directly, which is sterically hindered by the methyl groups.44 It should be mentioned that the concentration wave fronts observed in activated carbon/CDX/Cu(I)-Y or CDX/Cu(I)-Y for all molecules are also sharper when compared to the ones obtained with Cu(I)-Y or activated carbon/Cu(I)Y.3,5 This could be observed in Figure 3 as well. Sorbent Regeneration Using Solvents. For this part, we focused initially on the regeneration of the activated carbon/Cu(I)-Y layered bed because the combination was disclosed previously but no major regeneration study was done. For regeneration tests, two techniques were chosen: (1) solvent elution and (2) air calcination. For the former, four different solvents were used: CCl4, DMF, methanol, and toluene. After saturation of the beds with sulfur molecules (from a diesel fuel), the solvents were allowed to flow through the fixed beds in a constant head pressure fashion. During the solvent elution, the outlet sulfur concentration was also monitored as a function of time, and the results are plotted in Figure 6. The total sulfur removal capacity was obtained after integration of the areas below the sulfur elimination curves, and the results are summarized in Table 3. CCl4 and DMF completely removed all sulfur adsorbed during the first desulfurization cycle of a diesel fuel. Methanol and toluene, however, were capable of removing about 85 and 45%, respectively. Nevertheless, the methanol solvent removed almost 74% of the original total sulfur faster than any of the other solvents did. Because of its high polarity, CH3OH is known to adsorb strongly in hydrophilic zeolites45 and

even in some dealuminated (hydrophobic) ones.46 Because methanol probably does not adsorb as strong as thiophenes in activated carbon, some adsorbed thiophenic molecules in the top layer probably remain after solvent elution, and this accounts for the incomplete regeneration. Lee et al. used activated carbons to remove sulfur from diesel-contaminated methanol for fuel cell applications, and the carbons were capable of removing 0.85 mg of sulfur/g of sorbent.47 It should be mentioned here that the spent copper(I) zeolite layeredbed color, when exposed to methanol, changed to bluish dark from the black observed during desulfurization.2,3 This was additional evidence for some sulfur displacement during methanol elution throughout the zeolite layer. For DMF, the color changed to dark purple, and this indicates some type of interaction because the solvent is also capable of undergoing π complexation (because of the double bond present between the oxygen and carbon atoms). For CCl4 and (C6H5)CH3, the zeolite color remained dark. CCl4 as supplied contained a small content of a single-light sulfur contaminant as determined by our GC-FPD analysis (less than 0.5 ppm S) that was apparently adsorbed. CCl4 is known to contain carbon disulfide impurities (CS2) because the former is usually produced from chlorination of the latter,48 but this type of sulfur species should be easily displaced by heavier refractory thiophenic molecules during a second desulfurization cycle. For regeneration with toluene, Figure 6 shows that not all of the thiophenic sulfur is desorbed, which is again evidence that the π complexation with aromatics is not as strong. More information about the solvent interaction with the bed during regeneration should arise after second cycle diesel desulfurization tests, and this will be discussed next. After it was verified that sulfur was no longer eluting from the beds (see Figure 6), the solvent was allowed to exit completely and a flow of a diesel fuel was then allowed to enter the beds for a second cycle. The resulting sulfur breakthrough curves are shown in Figure 7. The best capacity recovery was obtained after regeneration with CCl4, but the results indicate that not all thiophenic molecules could displace the solvent molecules for adsorption sites. Figure 8a shows that the layered bed had some preference toward the refractory compounds [4-MDBT, 4,6-DMDBT, and 2,4,6-trimethyldibenzothiophene (2,4,6-TMDBT)] over the nonsubstituted ones. The bed regenerated by DMF was capable of desulfurizing only 10 cm3 of diesel/g of bed. The low capacity was due to π complexation between DMF and

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Figure 7. Breakthrough of total sulfur in an activated carbon/ Cu(I)-Y fixed-bed adsorber regenerated with CCl4 (b), DMF (]), methanol (4), or toluene (0), for diesel feed at room temperature. Ci is the total sulfur concentration of the feed.

the copper ions. Figure 8b shows that only heavy compounds such as 2,4,6-TMDBT persisted to adsorb after DMF regeneration. Also shown in the figure is some sort of dispersion (wave fronts are not sharp),

which was not observed by Herna´ndez-Maldonado and Yang during diesel desulfurization with fresh activated carbon/Cu(I)-Y beds.3,5 This could be a result of competitive π complexation between DMF and the thiophenic compounds. Sulfur breakthrough after methanol regeneration shows little adsorption capacity recovery, but there was a dip at around 60 cm3/g (see Figure 7) that may indicate that some thiophenic molecules were trying to displace the polar methanol molecules. Figure 8c shows the selectivity among the individual sulfur compounds, and breakthrough occurs relatively fast. After regeneration of activated carbon/Cu(I)-Y with a toluene feed, a second adsorption cycle recovers only 0.07 and 0.180 mmol of sulfur capacity/g of bed at breakthrough and saturation capacity. Keeping in mind that not all of the sulfur adsorbed during the first cycle was removed by toluene elution (see Figure 6), the quantities shown in Table 3 indicate that more sulfur was adsorbed during the second cycle than was removed by toluene. As commented on before, not all of the copper sites are being occupied by thiophene molecules; thus, some other species (i.e., aromatics) are occupying the extra sites.

Figure 8. Breakthrough of BT (O), DBT (b), 4-MDBT (4), 4,6-DMDBT (2), or 2,4,6-TMDBT (]) in an activated carbon/Cu(I)-Y layered bed after regeneration with CCl4 (A), DMF (B), methanol (C), or toluene (D), with diesel feed at room temperature.

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Figure 9. Breakthrough of total sulfur in a fixed-bed adsorber with regenerated activated carbon/Cu(I)-Y (O) or Selexsorb CDX/ Cu(I)-Y (9), with diesel feed at room temperature. Adsorbent regenerated in air at 350 °C followed by reactivation in helium at 450 °C.

Now the refractory organosulfur compounds have the opportunity to displace them and adsorb, and this should account for the increase in sulfur removal observed during the second cycle after regeneration with toluene. Figure 8d also shows that the bed prefers the refractory compounds after regeneration with toluene. Sorbent Regeneration with Air at Moderately High Temperature. Fresh activated carbon/Cu(I)-Y was saturated with sulfur during desulfurization with a diesel fuel (see Figure 3) and then regenerated with flowing air at 350 °C for at least 6 h followed by autoreduction. The latter was required because most of the copper ions were oxidized during the regeneration process with air and copper(I) species were needed for π complexation. After reactivation, the layered beds were again exposed to diesel feed and the sulfur breakthrough in the exit was monitored. Figure 9 shows the resulting breakthrough curve. Herna´ndez-Maldonado and Yang2 and Khare25,26 used similar approaches to regenerate desulfurization sorbents. For the former, a Cu(I)-Y zeolite (no guard bed) was saturated first with thiophene molecules (from a model fuel) and then fully regenerated with air at 350 °C followed by autoreduction. The S Zorb process removes sulfur species from fuels at high temperatures under a hydrogen atmosphere at high pressures. Afterward, the spent adsorbent is regenerated using nitrogen at temperatures of 37-538 °C followed by contact with air at 427-650 °C.25,26 It should be mentioned that their adsorbent also needs to be reduced before reuse. Table 3 shows the calculated breakthrough and saturation adsorption amounts after bed regeneration with air. For activated carbon/Cu(I)-Y, only 85% of the original capacity is recovered, and this is due to simultaneous burning/calcination of the activated carbon layer. During tests, some reddish and gray spots developed in the outer surface of the carbon, which is most certainly an indication of burning. A possible solution to this problem is testing of other layered-bed configurations such as Selexsorb CDX/Cu(I)-Y, which was discussed in a previous section. After regeneration of spent Selexsorb CDX/Cu(I)-Y with air at 350 °C followed by autoreduction, a diesel feed was allowed again to flow though the bed and the sulfur breakthrough monitored as well (see Figure 9). This time 97.1% of the original capacity was recovered. Given the possibility of small experimental errors, the bed sorbents were actually fully regenerated (100%) because the saturation adsorption

capacities coincide. Success with the activated alumina was expected because this type of material has a high thermal stability and, in addition, is often activated using hot air streams. The Cu-Y zeolite redox process during regeneration with air followed by autoreduction needs to be discussed more to explain the charge-balancing phenomena and how the zeolite does not undergo dealumination. Valyon and Hall32 and Petunchi et al.49 studied redox effects on Cu(II)-Y zeolites prepared also by liquid-phase ion exchange. IR and NMR data showed the appearance of extra-lattice oxygen (ELO) introduced during the ionexchange process. This ELO compensates for aluminum tetrahedra charges during reduction of cupric ions to cuprous ones, and the process is fully reversible. Dealumination was observed only at temperatures high enough to cause dehydroxylation. Thus, the proposed oxidative regeneration/autoreduction scheme to regenerate π-complexation sorbents and recover adsorption capacity should not jeopardize the durability of the sorbents under multiple-cycle desulfurization processes. Conclusion Our results have shown that layered beds based on π-complexation sorbents offer great breakthrough and saturation adsorption capacities. The matrixes also promote sharpening of the concentration wave fronts, and this is excellent for complete bed usage. It is believed that the mechanisms involved in the π-complexation layered beds are related to the theoretical findings of Chlendi and Tondeur for other layered-bed configurations. The best layered-bed combination for deep desulfurization was activated carbon/Selexsorb CDX/Cu(I)-Y. This matrix is capable of processing 41 cm3 of deep-desulfurized diesel/g of sorbent. For commercial applications, sorbent regeneration is a key element. Our tests indicate that the best regeneration strategy for π complexation based on a layered bed consists of regenerating the spent sorbents with air at 450 °C followed by autoreduction of the copper species. It was found that Selexsorb CDX/Cu(I)-Y can be fully regenerated while the activated carbon/Cu(I)-Y bed can only recover 85% of the original adsorption capacity. Solvent elution techniques were also studied, and only CCl4 was capable of recovering a considerable part of the original adsorption capacity. However, all of the regeneration liquid solvents undergo some type of interaction with the layered beds, which affect readsorption of organosulfur molecules during the second desulfurization cycle of a diesel fuel. Acknowledgment Partial support by NSF and DOE is gratefully acknowledged. Literature Cited (1) Yang, R. T.; Herna´ndez-Maldonado, A. J.; Yang, F. H. Desulfurization of Transportation Fuels with Zeolites Under Ambient Conditions. Science 2003, 301, 79. (2) Herna´ndez-Maldonado, A. J.; Yang, R. T. Desulfurization of Liquid Fuels by Adsorption via π-Complexation with Cu(I)-Y and Ag-Y Zeolites. Ind. Eng. Chem. Res. 2003, 42, 123. (3) Herna´ndez-Maldonado, A. J.; Yang, R. T. Desulfurization of Commercial Liquid Fuels by Selective Adsorption via π-Complexation with Cu(I)-Y Zeolite. Ind. Eng. Chem. Res. 2003, 42, 3103.

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Received for review September 3, 2003 Revised manuscript received October 15, 2003 Accepted November 21, 2003 IE034108+