Desulfurization of Commercial Jet Fuels by Adsorption viaπ-Complexation with Vapor Phase Ion Exchanged Cu(I)-Y Zeolites

Arturo J. Hernandez-Maldonado and Ralph T. Yang*

Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136

William Cannella

ChevronTexaco Energy Research and Technology Company, Richmond, California 94802

Desulfurization of a commercial jet fuel by vapor phase ion exchange (VPIE) Cu(I)-Y adsorbentswere studied in a fixed-bed adsorber operated at ambient temperature and pressure. When usedwith an activated alumina guard bed, Cu(I)-Y(VPIE) provides by far the best adsorptioncapacities. In general, the adsorbents tested for total sulfur adsorption capacity at breakthroughfollow the order Selexsorb CDX < Cu(I)-Y(VPIE) < Selexsorb CDX/Cu(I)-Y(VPIE). The bestadsorbent, Selexsorb CDX/Cu(I)-Y(VPIE) (layered bed of 25 wt % activated alumina followedby Cu(I)-Y), is capable of producing 38 cm3 of jet fuel per gram of adsorbent with a weightedaverage content of 0.071 ppmw-S. The sorbent is also capable of producing about 50 cm3 of jetfuel with a sulfur content of less than 1 ppmw-S. These low-sulfur fuels are suitable for fuel cellapplications. Regeneration tests have shown the sorbent capacity can be fully recovered usingcalcination/reduction methods. Gas chromatography-flame photometric detection results showthat the π-complexation sorbents selectively adsorb highly substituted benzothiophenes fromjet fuel, which is not possible by using conventional hydrodesulfurization reactors. The highsulfur selectivity and high sulfur capacity of Cu(I)-Y(VPIE) were due to π-complexation.

Introduction

Due to worldwide environmental mandates, refinersare facing the challenge of producing increasinglycleaner fuels.1 In 1998, the European Union firstmandated new sulfur specifications for drastically re-duced levels that started to be phased in from the year2000.2 Similar regulations were legislated in the U.S.and elsewhere soon after. The EPA Tier II regulationsrequire reductions of sulfur in highway diesel from thecurrent average of 500 ppmw to 15 ppmw by June 2006,and that in gasoline from 350 ppmw to 30 ppmw byJanuary 2005.1 However, refinery pipelines would prob-ably require even lower sulfur content in their fuels,below 10 ppmw, to compensate for possible contamina-tion from the higher sulfur concentration of someproducts and the inherent measurement variability.3For aviation fuels, the current maximum sulfur limitis 3000 ppmw,4 but regulation on this type of fuels islikely to become more stringent as governments con-tinue to study sulfur emission sources around the globe.

According to the U.S. Northeast States for Coordi-nated Air Use Management [NESCAUM], airports aremajor local sources of air pollution due to heavy emis-sions from aircraft.5,6 In 1999, NESCAUM reported SO2emissions as high as 230 tons/year. These numbers areexpected to keep climbing over the next few years and,thus, it will be imperative to produce ultralow sulfurjet fuels to help reduce emissions.7

Removal of sulfur-containing compounds is currentlyachieved by hydrodesulfurization (HDS), a catalyticprocess operated at elevated temperatures (300-340 °C)and pressures (20-100 atm of H2) by using Co-Mo/

Al2O3 or Ni-Mo/Al2O3 catalyst.8 HDS is highly efficientin removing thiols, sulfides, and disulfides but is lesseffective for aromatic thiophenes and thiophene deriva-tives, especially those containing functional groups thathinder the sulfur atoms. Because of this problem, thesulfur compounds that remain in the transportationfuels are mainly thiophene, benzothiophene, diben-zothiophene, and their alkylated derivatives. To reducethe sulfur content to meet the new regulations, thereactor size needs to be increased by factors of 5-15.9Furthermore, a severe hydrotreatment side effect willbe a further reduction in the “natural” lubricity of theresulting fuel blend.10,11 As a result, refineries need toadd certain additives to restore the original lubricity,this to avoid mechanical problems in engines.

Future fuel cells will also require deeply desulfurizedfuels, if at all possible with “zero” sulfur content.12 Forexample, methanol-based fuels for on-board fuel cellapplications required the use of a fuel with sulfurcontent <1 ppmw in order to avoid poisoning anddeactivation of the reformer catalyst.13 To use aviationfuels, which for military fuel cell applications are idealbecause of their ready availability,14 the sulfur concen-tration should preferably be below 0.1-0.2 ppmw. Thismakes HDS an inappropriate solution and, thus, the useof adsorption to selectively remove the sulfur compoundsat ambient conditions an excellent option.

There is an ongoing effort to develop new adsorbentsto remove the thiophenic compounds from aviation fuels.Velu et al. performed dynamic adsorption experimentswith Ce(IV)-Y zeolites to desulfurize a 510 ppmw-Smodel jet fuel (i.e., fuel made up of some of thecompounds present in real jet fuels) at 80 °C andatmospheric conditions.15 The sorbent was capable ofremoving 2.3 mg of sulfur per gram of zeolite beforebreakthrough occurred. Ma et al. studied fixed-bed

* To whom correspondence should be addressed. Tel.: (734)936-0771. Fax: (734) 764-7453. E-mail: yang@umich.edu.

6142 Ind. Eng. Chem. Res. 2004, 43, 6142-6149

10.1021/ie049533x CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 08/26/2004

adsorption of thiophene compounds from commercialfuels using an undisclosed transition metal compound(5 wt % loading) supported on silica gel.16,17 For jet fuel(490 ppmw-S), they obtained a saturation adsorptioncapacity of 0.015 g of sulfur per cm3 of adsorbent andalso showed that breakthrough occurred at about 20 cm3

effluent volume for about 3.2 cm3 of the metal-loadedsilica gel. In a recent work, Pillon studied the stabilityof jet fuels after treating them using clays and activatedaluminas.18 Although the work was not intended tostudy the sorbents’ ability to remove sulfur, the authorreported that the clays and activated aluminas reducedthe sulfur content of the fuel from 2300 ppmw to about2100 and 1500 ppmw, respectively, after processing 30cm3 of fuel per gram of sorbent.

Most recently, our group disclosed the use of cuprouszeolites to deeply desulfurize commercial liquid fuels viaπ-complexation at room temperature and atmosphericpressure.19-25 Recently, Hernandez-Maldonado and Yangused vapor phase ion exchanged (VPIE) cuprous zeo-lites to deep-desulfurize commercial diesel fuels (297.2ppmw-S).24 The sorbent is capable of processing about29 cm3 of diesel fuel per gram of zeolite with a sulfurcontent of less than 1 ppmw. After treating the first18 cm3 of fuel, the sulfur concentration was 0.032ppmw-S, which is already lower than the requirementsimposed for fuel cell applications. The sorbent beds canbe fully regenerated using hot air streams followed byreduction of the copper species. In the π-complexationmechanism the cations can form the usual σ bonds withtheir s orbitals and, in addition, their d orbitals canback-donate electron density to the antibonding πorbitals of the sulfur rings (see Figure 1). It was de-termined that π-complexation is stronger with thethiophenic ring than with the benzene ring, whichmakes the process suitable for commercial fuel desulfu-rization. More information on the latter and additionalπ-complexation work by our group can be foundelsewhere.9,19-27

In the present work, we focused on the use of copper-(I) zeolites for deep desulfurization of a jet fuel. The sor-bents were prepared using vapor phase ion exchangetechniques to obtain high copper(I) loadings. This tech-

nique permits direct exchange of cuprous cations andachievement of this at a 100% level, which is impossibleto obtain using traditional aqueous phase ion ex-changes.28 In addition, the use of activated alumina asa guard bed and sorbent regeneration methods are dis-cussed. The jet fuel tested in this work was fully char-acterized for sulfur content using gas chromatography-flame photometric detection (GC-FPD) techniques.

Experimental Section

Adsorbent Preparation. The starting adsorbentmaterials used in this study were NH4-Y zeolite (Si/Al) 2.40, Strem Chemicals) and Selexsorb CDX (AlcoaIndustrial Chemicals), all in powder form. H-Y zeoliteswere obtained after calcination of NH4-Y with dry airat 450 °C at 1 °C/min.

The zeolites were prepared following proceduresreported previously.24,29-31 Layers of the proton-formzeolite and CuCl (99.99%, Sigma-Aldrich) were loadedinto a reactor, each separated by thin quartz wool walls.Direct contact of the zeolite and salt is not desiredbecause of hydrolysis of the copper species in thepresence of adsorbed water.32 The reactor was thenheated in an inert, dry atmosphere, from room temper-ature to 200 °C at 1 °C/min, and the temperature washeld at that point for 4-6 h. Afterward, the temperaturewas slowly increased above the normal melting pointof CuCl (430 °C) and kept at around 650-700 °C foranother 10 h.24 The CuCl excess was completely evapo-rated from within the voids of the zeolite, and this wascorroborated by the absence of the salt from its originalreactor location at the end of the VPIE process and bya white crystalline ring formed at the reactor outlet.Spoto et al.31 also made these observations during aVPIE process to produce Cu(I)-ZSM-5. The zeolite wasthen treated in oxygen at 200 °C for 4 h before coolingto room temperature. After recovery, all the as-preparedzeolites were analyzed for elemental composition usingneutron activation analysis (NAA).

For layered bed experiments, a thin layer of SelexsorbCDX alumina was placed on top of the copper(I)exchanged zeolites. Selexsorb CDX is an activatedalumina specially formulated by Alcoa Industrial Chemi-cals for the adsorption of polar organic compoundsincluding sulfur-based molecules (mercaptans, sulfides,disulfides, thiophenes), nitrogen-based molecules (ni-triles, amines, pyridines), and oxygenated hydrocarbonmolecules (alcohols, glycols, aldehydes, ketones, ethers,peroxides).

Reagents and Standards. A commercial jet fuelsample (364.1 ppmw-S) was used during the desulfur-ization tests. The fuel was completely characterized bythe manufacturer, including the total sulfur content,which was obtained using X-ray fluorescence and gaschromatography techniques. Table 1 summarizes all theproperties of the jet fuel sample used in this work. Itshould be mentioned that the jet fuel sample used in

Figure 1. Copper ions occupying faujasite six-ring window sites(A); σ-donation of π-electrons of thiophene to the 4s orbital ofcopper(I) (B); d-π* back-donation of electrons from 3d orbitals ofcopper(I) to π* orbitals of thiophene (C). Here 3d represents dxy,dyz, or dxz, or three of the five 3d orbitals.

Table 1. Composition Data for Commercial Jet Fuel andJP-8

content

compound commercial jet fuel JP-8

aromatics 20.5 vol % 19%paraffins + naphthenes 78 vol % 80%olefins 1.5 vol % 2%nitrogen 5.88 ppmw variablesulfur 364.10 ppmw variable

Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6143

this study has characteristics very similar to those ofJP-8 fuels. The latter are military fuels, which arecurrently considered for many fuel cell applications.14

Elemental Analysis. Cu(I)-Y adsorbents were char-acterized using NAA in the research nuclear reactor ofthe Phoenix Memorial Laboratory at the University ofMichigan. The data were obtained from a 1-min core-face irradiation delivered via pneumatic tube to alocation with an average thermal flux of 2.13 × 1012

neutrons/cm2/s. Following irradiation, two separatecounts were made, one after a 13-min decay (for Al andCu) for 500 s.

The concentration of aluminum was determined basedon comparison with three replicates of the standardreference material NIST1633A (coal fly ash); the deter-mination of Cu content was evaluated relative to high-purity copper wire. Data reductions for NIST1633Awere based on NIST-certified values.

Fixed-Bed Adsorption/Breakthrough Experi-ments. All dynamic adsorption or breakthrough experi-ments were performed in vertical custom-made quartzadsorbers equipped with a supporting glass frit asdescribed elsewhere.21 The setup consisted of a low-flowliquid pump, Kynar and Teflon compression fittings,feed tanks, and a heating element. Initially, the sorbentswere loaded inside the adsorber, and heated in situusing dry gases to avoid exposure to atmosphericmoisture. Faujasite type zeolites as well as many otheraluminosilicates are highly hydrophilic in nature, andit is imperative to avoid any contact with moisturebefore adsorption tests to achieve the highest sulfurloadings possible. For this reason, all gases used werepretreated for moisture removal by flowing them through3A-zeolite beds. Activation of the zeolite to obtainCu(I)-Y(VPIE) was performed in situ in a verticalreactor as follows: heating to 350 °C in oxygen or airto remove bulk and adsorb water followed by cooling toroom temperature followed then by heating to about 231°C at 10 °C/min in an inert gas stream containing 5 vol% hydrogen. The last temperature leg was held for anadditional 15 min before cooling to room temperature.The sorbent bed was then washed with a sulfur-freehydrocarbon (e.g., n-octane) to remove any entrappedgas. After allowing the liquid hydrocarbon head todisappear, the jet fuel was allowed to contact the bed.The effluent was sampled periodically until saturationwas achieved (i.e., when the effluent total sulfur con-centration equaled the influent one), which dependedon the adsorption dynamics and the amount of adsor-bent.

Dry air was used for calcination regeneration testsand the temperature (350 °C) was controlled by a PIDtemperature controller to within (1 °C. After this, theoxidized copper ions were reduced again using heliumgas containing 5% hydrogen at the same conditionsdescribed above.

Gas Chromatography Analysis. All the samplescollected during the breakthrough experiments wereanalyzed using a Shimadzu GC-17A v3 unit equippedwith an EC-5 capillary column and a flame photometricdetector (FPD). More details about the GC analysis canbe found elsewhere.21,33

Peak identification information for sulfur compoundspresent in the jet fuel was gathered after using stan-dards and by elution time comparison with data avail-able in the literature.4,15-17 For standards, stock solu-tions consisting of thiophene, benzothiophene (BT), or

dibenzothiophene (DBT) in sulfur-free n-octane werefurther diluted to a known concentration and theninjected for elution time determination. After obtainingnonlinear calibration data at different concentrationlevels, the jet fuel total sulfur content was estimatedby carefully adding all the corresponding concentrationvalues of the individual peaks and comparing theresulting value to the one reported by the supplier.Figure 2 shows a detailed FPD chromatogram for thejet fuel tested. Detectable thiophenic sulfur peaks fromstandards were recorded at concentrations down toapproximately 20 ppbw-S (or 50 ppbw thiophene). NoDBT or substituted-DBTs were detected in our jet fuelsamples. It should be mentioned that no quenchingeffect resulting from hydrocarbons was ever observedduring the GC-FPD analysis.

Results and Discussion

Vapor Phase Ion Exchanged Adsorbent. Thecopper zeolites used in previous desulfurization studieswere prepared by liquid phase ion exchange (LPIE) ofNa-Y with copper(II) nitrate (followed by reduction toCu+) and had a unit cell composition of Cu21Na14-[Al56Si136O384].19-25 If all the copper is reduced to Cu+,then 38% of the charge compensation comes fromcuprous ions and the other 62% from free hydroxylgroups or extra lattice oxygen (ELO).34,35 Complete ionexchange in aqueous phase is usually difficult becauseof the hydration state of the cupric ions and because ofcomplex hydrolysis mechanisms that arise during theprocess. Thus, other means of introducing copper(I) ionsdirectly are essential. Guidry and Price29 and Spoto etal.30,31 used vapor phase ion exchange (VPIE) techniquesto fully exchange ZSM-5 (MFI) zeolites with copper(I)ions. Recently, Hernandez-Maldonado and Yang appliedthese techniques to produce a Cu(I)-Y zeolite fordesulfurization of a diesel fuel.24 The process is sum-marized by the following stoichiometric reaction:

These techniques were also used here to produce afaujasite zeolite with high cuprous content. Elementalanalyses of the cuprous adsorbent (from NAA) indi-cated the zeolite had the following unit cell content:Cu54H3[Al57Si135O384]. This unit cell contains more thantwice the copper cation amount prepared by aqueousphase ion exchange.

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

H+Y(s) + CuCl(v) f Cu+Y(s) + HCl(v) (1)

6144 Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004

To avoid disproportionation of Cu+ to Cu2+ andCu0 in the presence of atmospheric air, the zeoliteneeded to be oxidized to a Cu2+ state. During theoxidation stage the cupric ions react to form oxoniumions [Cu-O-Cu]2+, which could further interact withmoist air to form stable [Cu(OH)]+ ions.29 Guidry andPrice and Hernandez-Maldonado and Yang showed thatCu+ species in zeolites could be easily recovered by usinga reducing atmosphere.24,29 This is essential since cupricions are not capable of undergoing π-complexation.Temperature programmed reduction experiments showedthat all the zeolite cupric ions were reduced to Cu+ afterthe sorbent was heated to 231 °C at 10 °C/min in a 5vol % hydrogen in inert gas atmosphere.24

Fixed-Bed Adsorption Experiments. After in situactivation of the adsorbent bed, a commercial jet fuelfeed (364 ppmw-S) was allowed to contact the sorbentbed and the effluent total sulfur content was monitoredperiodically. In situ activation was necessary for evalu-ation of the sorbents’ desulfurization capabilities with-out any exposure of the hydrophilic zeolites to atmo-spheric air. It should be mentioned that, after activation,the copper(I) zeolites were “white” in color as opposedto the greenish color observed after ion exchange, whichindicated the sorbents were dehydrated and all thecopper(II) ions reduced.36,37

Figure 3 shows a chromatogram depicting the “evo-lution” of the thiophenic peaks as a function of bothGC-FPD retention time and fixed-bed adsorption sam-pling time. The remarkable difference in sulfur peakpopulation between the samples is worth noting. Forthe first 40 cm3 of fuel treated per gram of adsorbentthere are almost no detectable sulfur peaks comparedto the original untreated sample (see Figure 2). Thiswas due to π-complexation interaction between orga-nosulfur molecules and Cu+ ions. A detailed discussionon this will be presented shortly.

Breakthrough adsorption curves were generated byplotting the transient total sulfur concentration normal-ized by the feed total sulfur concentration versuscumulative fuel volume normalized by total bed weight.The adsorption amounts (normalized per adsorbentweight) were obtained after solving the following equa-tion:38

where q is the total sulfur adsorbed amount (mmol/g),v_ is the feed volumetric flow rate (cm3/min), Ffuel is the

fuel density (g/cm3) at room temperature, Xi is the totalsulfur fraction (by weight) in the feed, Ci is the totalsulfur concentration in the feed (ppmw-S), madsorbent isthe weight of the sorbent bed (g), MWsulfur is themolecular weight of sulfur, and C(t) is the effluent totalsulfur concentration (ppmw-S) at any time t (min). Theintegral part on the right-hand side of eq 2 is the areaabove the breakthrough curves at any time t. Table 2summarizes the results obtained for total sulfur break-through and saturation adsorption amounts in fresh andregenerated sorbents.

Figure 4 shows a quantitative total sulfur break-through curve for desulfurization of the jet fuel withCu(I)-Y(VPIE) sorbents. As shown in Table 2, thesorbent is capable of removing 0.40 mmol of sulfur pergram of sorbent before breakthrough is observed. Afterthe concentration of sulfur in the effluent matched thatof the influent, the sorbent removed 0.72 mmol of sulfurper gram of zeolite. These numbers translate to six (6)and eleven (11) adsorbed thiophenic molecules perzeolite unit cell for breakthrough and saturation, re-spectively. According to Maxwell et al.,39 X-ray datashow dehydrated Cu(II)-Y zeolites have about 25% ofthe copper ions occupying, following Smith’s nomencla-ture,40,41 site II and about 14% in site III. Given thecomposition of our zeolite [Cu54H3(Al57Si135O384)], only11 Cu+ cations are undergoing π-complexation withthiophenic molecules, which accounts for 20.3% of thetotal copper ions available. The latter is in excellentagreement with the findings of Maxwell et al. for siteII occupancy. However, there are approximately 19% ofexposed cuprous ions that are probably occupied byother molecules (e.g., nitrogen hetereocycles) capable of

Figure 3. GC-FPD chromatograms as a function of fixed-bedadsorption/breakthrough experiment sampling time. The jet fuelentering the fixed bed had a concentration of 364.1 ppmw sulfur.

Table 2. Adsorption Loading Amounts for Total SulfurRemoved from Jet Fuel

adsorbent

breakthroughloadinga

(mmol/g)

saturationloadinga

(mmol/g)

Cu(I)-Y(VPIE) 0.395 0.723Selexsorb CDX 0.089 0.484Selexsorb CDX/Cu(I)-Y(VPIE)b 0.521 0.796Selexsorb CDX/Cu(I)-Y(VPIE)

(regenerated)b,c0.514 0.761

a Loading amounts normalized by total bed weight. b Adsorbentbed contained two layers: the first layer was alumina SelexsorbCDX (CDX) followed by Cu(I)-Y zeolite. CDX accounts for 25 wt% of the bed weight. c Regeneration in air at 350 °C for 6-12 hfollowed reduction in an inert gas stream containing 5 vol %hydrogen at 231 °C.

Figure 4. Breakthrough of total sulfur in a fixed-bed adsorberwith Cu(I)-Y(VPIE), Selexsorb CDX γ-Al2O3, or Selexsorb CDX/Cu(I)-Y(VPIE) adsorbent, for jet fuel feed at room temperature.Ci is the total sulfur concentration of the feed.

q(breakthrough or saturation) )

( v_madsorbent

)( FfuelXi

MWsulfur) ∫0

t [1 -C(t)Ci

] dt (2)

Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6145

undergoing π-complexation. Takahashi et al.26 andHernandez-Maldonado and Yang42 have shown alreadythat copper π-complexation sorbents could adsorb aro-matics and nitrogen heterocycles, which are competingfor adsorption sites during the desulfurization process.The former, however, did not adsorb as strongly asorganosulfur molecules as evidenced from empirical andtheoretical results.43 Thus, it is believed the othercopper(I) sites are probably occupied by organonitrogenmolecules.

Most recently Velu et al. used aqueous phase pre-pared Ce(IV)-Y zeolites to desulfurize a jet fuel.15 Theprocess, which involved direct interaction with thesulfur atoms of the thiophenic molecules, was tested fordesulfurization of both real and model jet fuels at 80°C. The zeolite was capable of removing 0.14 and 0.31mmol of total sulfur per gram of zeolite for real andmodel fuels, respectively. The lower desulfurizationcapacity observed for the real jet fuel treatment isprobably due to lack of interaction between the ceriumions and the sulfur atom in the refractory sulfurcompounds, which are abundant in the jet fuel (refer toFigure 2). In these compounds, the sulfur is difficult toaccess because of steric hindrance imposed by themethyl groups adjacent to the sulfur atom. The increasein capacity for the test with the model jet fuel wasprobably due to the use of thiophenic molecules exhibit-ing “less” steric hindrance toward the sulfur atoms. Formodel fuels, Velu et al. used 2-methylbenzothiopheneand 5-methylbenzothiophene molecules as representa-tive molecules for thiophenes.15

Ma et al. studied fixed-bed adsorption of thiophenecompounds from jet fuels using an undisclosed transi-tion metal compound (5 wt % loading) supported in silicagel.16,17 They obtained a saturation adsorption capacityof 0.015 g of sulfur per cm3 of adsorbent which can becompared directly to our results. Assuming that thedensity of the Cu(I)-Y is close to that of Na-Y (∼1.3g/cm3), which is lower than the actual value becausesodium is lighter than copper, then the observed satura-tion capacity in our case is approximately 0.029 g ofsulfur per cm3 of adsorbent. Ma et al. also showed thatbreakthrough occurs at about 20 cm3 effluent volumefor about 3.2 cm3 of the metal loaded silica gel comparedto 44 cm3 effluent volume for only 0.75 cm3 volume ofCu(I)-Y. It should be mentioned that Ma et al. probablydevelop the sorbent based on catalyst compositionsoriginally used for HDS. A similar approach was alsotaken by Khare (Phillips Petroleum Co.), who developeda process called S Zorb in which the sulfur atomspresent in the thiophenic compounds are removed attemperatures in the 340-420 °C range under a hydro-gen atmosphere.44,45 They use a proprietary adsorbent,which is believed to be comprised of a bimetallicpromotor (cobalt, molybdenum, etc.) dispersed on asupport formed by zinc oxide and alumina or silica gel.The main limitation of HDS legacy sorbents is thatthese rely on attacking the sulfur atom directly. Thus,any steric hindrance offered by refractory compoundswill limit the sorbent’s capability.

Figure 5 shows adsorption breakthrough of selectedorganosulfur molecules, including jet fuel refractorythiophenes, in Cu(I)-Y(VPIE) zeolites. The sorbentoffers more selectivity toward refractory compoundssuch as 2,3,7-trimethylbenzothiophene (2,3,7-TMBT)and 2,3,6-trimethylbenzothiophene (2,3,7-TMBT). Cop-per(I) exchanged zeolites interact via π-complexation

with thiophenic aromatic rings instead of attackingsulfur atoms directly, which explains why the sorbentis able to remove organosulfur compounds that are leftunreacted after hydrodesulfurization.

Layered Beds. Yang et al., Hernandez-Maldonadoand Yang, and Hernandez-Maldonado et al. used guardbeds to further increase total adsorption capacity whenusing π-complexation adsorbents.19,22,23 It was foundthat adding a thin layer of either activated carbon oralumina or both on top of Cu(I)-Y zeolites increases thedeep-desulfurization capacity. Although the highestsulfur adsorption capacities for two-layered beds werereported for matrixes having activated carbon as aguard bed, it was found that aluminas were moresuitable because of their stability toward calcinationtreatment during sorbent regeneration.23 A more de-tailed discussion on the usage of layered beds fordifferent applications can be found elsewhere.46,47

Before testing layered beds, it was desired to studythe desulfurization capability of the activated aluminaalone. The alumina used during this study was aSelexsorb CDX γ-Al2O3 (Alcoa), which was described inthe Experimental Section. After activating the aluminain situ at 350 °C using dry air, the sorbent was exposedto a fresh untreated jet fuel feed (364.1 ppmw-S) at roomtemperature. Figure 4 shows the resulting total sulfurbreakthrough curve after the fuel treatment with thesorbent. The organosulfur molecules break throughalmost instantly when compared with the results ob-tained with Cu(I)-Y(VPIE) sorbents, but the aluminaalone does adsorb sulfur (see Table 2). After reachingsaturation (at about 100 cm3/g), the sorbent is capableof removing 0.48 mmol of sulfur per gram of alumina.This not surprising since Selexsorb CDX activatedalumina is known to adsorb refractory and nonrefrac-tory compounds from diesel. However, the sulfur re-moval is not even close to deep desulfurization levels(<1 ppmw-S).22 In a recent work Pillon used activatedaluminas to treat jet fuels and study the thermalstability of the fuels afterward.18 For a jet fuel contain-ing 2300 ppmw-S and 15 ppmw-N, the author foundthat the aluminas were able to reduce the amount ofsulfur and nitrogen to about 1500 ppmw-S and at least2 ppmw-N, respectively, after treating about 33 cm3 offuel per gram of sorbent. Although the aluminas werenot able to remove sulfur to deep-desulfurization levels,Pillon’s study evidences the sorbent is capable of ad-sorbing organosulfur and organonitrogen, with selectiv-ity toward large molecules as also recently demon-

Figure 5. Breakthrough of methylbenzothiophene, 2,3-dimeth-ylbenzothiophene, 2,3,7-trimethylbenzothiophene, or 2,3,6-tri-methylbenzothiophene in Cu(I)-Y(VPIE). C(t) and Cj are theindividual molecules of sulfur concentration in the effluent andfeed, respectively.

6146 Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004

strated by Hernandez-Maldonado and Yang.22,42 Thus,the use of aluminas as guard beds should definitely helpincrease the overall desulfurization capacity when usedin layered bed matrixes. It should be mentioned thatboth activated carbons and aluminas are also known toremove mainly aromatic molecules such as benzene.Takahashi and Yang concluded that at low concentra-tions polarizability interactions played an importantrole for adsorption of benzene molecules in both acti-vated carbon and alumina.26 At high concentrations,more thiophene molecules were adsorbed because of apore-filling mechanism.

Figure 6 shows breakthrough adsorption curves forselected organosulfur molecules during desulfurizationof a commercial jet fuel with Selexsorb CDX activatedalumina. Although the molecules are not removed todeep-desulfurization concentration levels as in theCu(I)-Y(VPIE case (see Figure 5), the alumina doesadsorb them and in a selective fashion. The break-through order followed that of molecular weights (orpolarizabilities); the smaller molecules broke throughearlier.

For the layered sorbent tests, the matrix consistedof a bed of 25 wt % activated alumina followed byCu(I)-Y(VPIE). The adsorbent combination was acti-vated also in situ using the same conditions as thosefor the Cu(I)-Y(VPIE) desulfurization tests mentionedbefore. After allowing a fresh jet fuel feed to contact thematrix, the total sulfur concentration in the effluent wasmonitored, and the results are shown in Figure 4. Usingeq 2, it was determined that the dual-layer combinationis capable of removing 0.52 and 0.80 mmol per totalgrams of bed at breakthrough and saturation, respec-tively. This corresponds to an increase of 32% and 10%in breakthrough and saturation loadings, respectively,when compared to the results obtained for a singleCu(I)-Y(VPIE) bed. Thus, the usage of alumina as aguard bed to Cu(I)-Y(VPIE) increases considerably theamount of sulfur adsorbed before breakthrough hap-pens, which indicates that the guard bed possiblyremoved some large aromatic molecules that werepossibly competing for π-complexation sites when theCu(I)-Y(VPIE) adsorbent was used alone. It should bementioned that of the breakthrough curves shown inFigure 4 the Selexsorb CDX/Cu(I)-Y(VPIE) one is thesharpest. This phenomenon was observed by Hernan-dez-Maldonado and Yang and Hernandez-Maldonado etal. during desulfurization of a diesel fuel using aqueousphase ion exchanged copper zeolites coupled with acti-

vated carbon and/or aluminas as guard beds.22,23 It wasdetermined that the increase in sharpening of the wavefronts was due to elimination of partial pore and/or siteblockage caused by some fuel large molecules in thezeolite.

Individual organosulfur molecule breakthrough curvesare shown in Figure 7. The layered bed configurationremoves all organosulfur molecules, with a slight se-lectivity toward the large thiophenic refractory com-pounds. It should be mentioned that all the wave frontsare sharper as mentioned above for the total sulfurbreakthrough. This evidences that the use of a layeredbed possibly permits the use of more π-complexationsites in the zeolites since many competing molecules arenow retained by the guard bed.

Desulfurized Jet Fuel Sulfur Content. Actualsulfur levels attained during desulfurization with allthe sorbents are shown in Figure 8. One gram ofCu(I)-Y(VPIE) is capable of producing about 38 cm3 ofjet fuel with a sulfur content of 0.299 ppmw-S, which issuitable for usage in fuel cell reforming units. Whenused in layered bed configuration, the Selexsorb CDXalumina and the cuprous zeolite are capable of produc-ing about 38 cm3 of jet fuel with a sulfur content of 0.071ppmw-S.

Sorbent Regeneration. Based on breakthroughloading capacity, the best sorbent, Selexsorb CDX/

Figure 6. Breakthrough of methylbenzothiophene, 2,3-dimeth-ylbenzothiophene, 2,3,7-trimethylbenzothiophene, or 2,3,6-tri-methylbenzothiophene in Selexsorb CDX γ-Al2O3. C(t) and Cj arethe individual molecules of sulfur concentration in the effluentand feed, respectively.

Figure 7. Breakthrough of methylbenzothiophene, 2,3-dimeth-ylbenzothiophene, 2,3,7-trimethylbenzothiophene, or 2,3,6-tri-methylbenzothiophene in Selexsorb CDX γ-Al2O3/Cu(I)-Y(VPIE).C(t) and Cj are the individual molecules of sulfur concentrationin the effluent and feed, respectively.

Figure 8. Total sulfur content of desulfurized jet fuel duringbreakthrough in different sorbents for a feed with 364.1 ppmw-Stotal sulfur content.

Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6147

Cu(I)-Y(VPIE), was tested for regeneration after satu-ration with fresh untreated jet fuel organosulfur mol-ecules. The regeneration was achieved in a two-stepfashion: (a) calcination in dry air at 350 °C for 6 h; (b)copper(II) species reduction to copper(I) using heliumwith 5% hydrogen at 231 °C (as explained in the Ex-perimental Section). After regeneration was completed,the sorbent recovered its original color (after activation),which was white. Afterward, a fresh untreated jet fuelfeed was allowed to contact the sorbent at room tem-perature and the second desulfurization cycle wasperformed. Figure 9 shows the total sulfur adsorptionbreakthrough curve and compares it to the first cycleresults. The regeneration scheme was capable of recov-ering all of the original sulfur sorption capacity in onestep. As shown in Table 2, after regeneration, thesorbent removed 0.514 and 0.761 mmol of sulfur pergram of bed at breakthrough and saturation conditions,respectively. The slight difference in saturation loadingsbetween the first and second cycles is due to possibleexperimental errors. However, even under these cir-cumstances, the regeneration scheme and sorbent com-binations are capable of recovering 96% of the originalsaturation loading, which is remarkable and ideal forcyclic processes.

Conclusion

Results have demonstrated that vapor phase ionexchanged copper(I) type-Y zeolites are superior adsor-bents for removal of all sulfur compounds from com-mercial jet fuels, based on dynamic fixed-bed adsorptionexperiments. When used with an activated aluminaguard bed, Cu(I)-Y provides by far the best adsorp-tion capacities both at breakthrough point and atsaturation. The matrix is capable of processing 38 cm3/gof jet fuel with an average sulfur content of 0.071ppmw or 71 ppbw. In general, the adsorbents testedfollowed the following order for total sulfur adsorp-tion capacity at breakthrough: Selexsorb CDX <Cu(I)-Y(VPIE) < Selexsorb CDX/Cu(I)-Y(VPIE). Alladsorbents show some capacity for the refractory sulfurcompounds such as 2,3,7-trimethylbenzothiophene. Nev-ertheless, the cuprous zeolites were the only sorbentscapable of removing such compounds to deep-desul-furization levels.

The activated alumina, when used as guard bed toCu(I)-Y(VPIE), promotes sharpening of the concentra-tion wave fronts, and this is excellent for complete bed

usage. Other layered bed combinations need to bestudied in the future to fully understand this phenom-enon.

For commercial applications, sorbent regeneration isa key element. Our tests indicate that the best regen-eration strategy for copper(I)/π-complexation basedlayered bed consists of regenerating the spent sorbentswith air at 350 °C followed by reduction of the copperspecies. It was found that Selexsorb CDX/Cu(I)-Y canbe fully regenerated.

Acknowledgment

We are grateful to many helpful discussions with Dr.Alice He of ChevronTexaco Energy Research and Tech-nology Company. Neutron activation analysis (NAA)was conducted in the Ford Nuclear Reactor of thePhoenix Memorial Laboratory at the University ofMichigan. Dr. Leah Minc of the Michigan PhoenixMemorial Project conducted the analysis. The authorsthank Stephen D. Stamatis and Adam D. Urban fortheir help. Partial support by NSF and DOE is grate-fully acknowledged.

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Received for review May 29, 2004Revised manuscript received July 10, 2004

Accepted July 15, 2004

IE049533X

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