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    Rechargeable Lithium Batteries with Aqueous ElectrolytesAuthor(s): Wu Li, J. R. Dahn, D. S. WainwrightSource: Science, New Series, Vol. 264, No. 5162 (May 20, 1994), pp. 1115-1118Published by: American Association for the Advancement of ScienceStable URL: http://www.jstor.org/stable/2885126

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    mma REPORTSrelativeo thecraterdiametersvery ypical.Meltproductionncreaseswithimpactener-gy, suchthatonlylargecraters reexpectedto produce significantquantitiesof melt(31). Thismelt is deposited sthin veneers,flows,andponds 32), so thatmeltsufficientto affect he spectral eflectivitys expectedto remainnear he surface nlyforrelativelylarge and young craters,postdatingwide-spread esurfacingrom mpactbasins.Sm-rekarand Pieters(33) showed that reflec-tancespectraof the anomalously ed craterrings of Tycho and Copernicus re consis-tent with the presence f iron-bearinglass.Themeltis concentratedear he crater imbecause it is producedfrom the deepestejectedtargetmaterial.Therefore,we ex-pectthatallcratersarger hanabout25 kmin diameter ndpostdating he formation fthe Imbriumasinwill haveanomalouslyednear-rim olors. This generalizations ob-served n the EM-2data for the near-sidecratersCopernicus,Zucchius,Pythagoras,Carpenter,Philolaus, Plato, Archimedes,Theophilus,Langrenus,Plinius, Geminus,Fabricus,and Hainzel, as well as for thefar-side ratersOhm, Vavilov, and Hausen(5). In the caseof Copernican ge craters,the spectral ffectsof fresh oilspartly oun-teract the reddishmelt, but they are stillredder anddarker) han melt-freesoils ofsimilarage and composition.The most re-cent of the largeCopernicanage craters,such as Tycho, Jackson,and Kepler,haverelatively ed anddarkringsascomparedocrater nteriors ndouterejectaandrays.The flybysof the moon by Galileo in1990 and 1992 providedcalibratedmulti-spectral overageof about75%of the lunarsurface n the 400- to 1000-nm spectralregion. The images cover well-character-ized regions of the near side, includingstandard reas orremotesensingand sam-ple sites, thathave enabledthe calibrationofGalileodata orextrapolationo areasonthe northern, eastern, and western limbsand the far side that previouslylackedquantitativemultispectraloverage.

    REFERENCESAND NOTES1. M.J. S. Beltonet al., Science 255, 570 (1992).2. J. W. Head et al., J. Geophys. Res. 98, 17149(1993).3. C. M.Pietersetal., ibid., p. 17127.4. R.Greeley et al., ibid.,p. 17183.5. A. S. McEwen t al., ibid., p. 17207.6. T.B. McCord,M. P.Charette,T. V.Johnson,L.A.Lebofsky,C. M. Pieters, bid. 77, 1349 (1972).7. C. Pieters,Proc. LunarPlanet.Sci. Conf.9, 2825(1978).8. TheSSI ilter et is described by M.J. S. Beltonetal. [Space Sci. Rev. 60, 413 (1992)] and itsrelation to lunar compositionalstudies is de-scribed in (1).9. B. K.Lucchitta,U.S.Geol. Surv.Misc. Invest.Ser.Map 1-1062 1978).10. D. E.Wilhelms,U.S. Geol.Surv.Prof Paper 1348(1987),p. 1.11. W. K.Hartmann nd G. P. Kuiper,LunarPlanet.Lab. Comm.1 (no. 12),plate 12.51 (1962).

    12. T. V. Johnson etal., Proc. LunarPlanet. Sci. Conf.8, 1013 (1977).13. K. A. Howard, D. E. Wilhelms, D. H. Scott, Rev.Geophys. Space Phys. 12, 309 (1974).14. D. E. Wilhelms and J. F. McCauley, U.S. Geol.Surv. Misc. Invest. Ser. Map 1-703 (1971); N. J.Trask and J. F. McCauley, Earth Planet. Sci. Lett.14, 201 (1972); W. R. Muehlberger et al., inApollo 16 Preliminary Science Report, NASASP-315 (1972), p. 6-1; G. E. Ulrich et al., U.S.Geol. Surv. Prof. Paper 1048 (1981), p. 1; J. M.Boyce et al., Proc. Lunar Planet. Sci. Conf. 5, 11(1974); V. R. Oberbeck etal., Moon 12, 19 (1975).15. G. Neukum, Moon 17, 383 (1977); Habilitations-schriff, Ludwig Maximilianis Universitat, Munich(1983); G. Neukum et al., Moon 12, 201 (1975).16. P. H. Schultz and P. D. Spudis, Proc. LunarPlanet. Sci. Conf 10, 2899 (1979).17. J. W. Head and L.Wilson, Geochim. Cosmochim.Acta 56, 2155 (1992).18. B. R. HawkeandJ. F. Bell, Proc. Lunar Planet. Sci.Conf. 12, 665 (1981); J. F. Bell and B. R. Hawke,J. Geophys. Res. 89, 6899 (1984).19. B. R. Hawke et al., Lunar Planet. Inst. Tech.Report 92-09 (1992), p. 14.20. J. W. Head et al., LunarPlanet. Sci. Conf 24, 629(1993).21. D. A. Williams, thesis, Arizona State University(1992); S. D. Kadel, thesis, Arizona State Univer-sity (1993).22. M. P. Charette, T. B. McCord, C. M. Pieters, J. B.Adams, J. Geophys. Res. 79, 1605 (1974); M. P.Charette, L. A. Sodenblom, J. B. Adams, M. J.Gaffey, T. B. McCord, Proc. Lunar Planet. Sci.Conf 7, 2579 (1976).23. M. R. Robinson, B. R. Hawke, P. G. Lucey, G. A.Smith, J. Geophys. Res. 97, 18 (1992).24. D. E. Wilhelms and F. El Baz, U.S. Geol. Surv.

    Misc. Invest. Ser. Map 1-948 (1977).25. J. W. Head et al., in Mare Crisium: A Viewfrom Luna 24, R. B. Merrill and J. J. Papike,Eds. (Pergamon, New York, 1978), pp. 43-74.26. B. L. Barsukov et al., Proc. Lunar Planet. Sci.Conf. 8, 3319 (1977).27. L. R. Gaddis, C. M. Pieters, B. R. Hawke, Icarus61, 461 (1985).28. L. Wilson and J. Head, J. Geophys. Res. 86, 2971

    (1981).29. G. Heiken et al., Geochim. Cosmochim. Acta 38,1703 (1974).30. B. R. Hawke et al., Proc. Lunar Planet. Sci. Conf21, 377 (1991).31.. R. A. F. Grieve et a/., in Impact and ExplosionCratering, D. J. Roddy, R. 0. Pepin, R. B. Merrill,Eds. (Pergamon, New York, 1977), pp. 791-814.32. B. R. Hawke and J. W. Head, ibid., p. 815.33. S. Smrekar and C. M. Pieters, Icarus 63, 442

    (1985).34. We thank the Galileo Project Office, the NationalAeronautics and Space Administration (NASA),and numerous individuals who participated in themission planning and initialdata analysis, includ-ing C. Avis, T. Becker, L. Bolef, N. Bridges, C.Cunningham, E. DeJong, K. Edwards, E. Fischer,R. Garstang, A. Harch, S. Kadel, R. Kirk, J.Moersch, J. Plutchak, M. Robinson, R. Sullivan, W.Sullivan, J. Sunshine, S. Vail, L. Wainio, D. Wil-liams, and J. Yoshimizu. The Galileo data set isavailable on CD-ROM through the NASA Plane-tary Data System. The National Optical AstronomyObservatories are operated by AURA, Inc., undera cooperative agreement with the National Sci-ence Foundation.17 September 1993; accepted 24 March 1994

    Rechargeable LithiumBatteries withAqueous ElectrolytesWu Li, J. R. Dahn,* D. S. Wainwright

    Rechargeable lithium-ionbatteries that use an aqueous electrolyte have been developed.Cells with LiMn2O4and V02(B) as electrodes and 5 M LiNO3in water as the electrolyteprovide a fundamentally safe and cost-effective technology that can compete with nickel-cadmium and lead-acid batteries on the basis of stored energy per unit of weight.

    During the 1970s and 1980s,rechargeablelithiumbatterieswere touted by some re-searchers s providinga possible ong-termsolution o the electricvehicle (EV)batteryproblem. The cells had about twice theenergy density (measuredby watt-hoursstoredper kilogramof battery)of the bestcompeting ambienttemperature atteries.Manycompaniesmoved to commercializethe technology,beginningwith small cellsfor consumerapplications, n view of thelargemarketsanticipated.These cells usedlithium metal as the negativeelectrode,atransitionmetaloxide [suchas MnO2(1)]or chalcogenide suchasNbSe3 (2)] as theW. Li and J. R. Dahn, Department of Physics, SimonFraser University, Burnaby, BritishColumbia, CanadaV5A 1S6.D. S. Wainwright, Moli Energy (1990) Limited, 20000Stewart Crescent, Maple Ridge, British Columbia,Canada V2X 9E7.*To whom correspondence should be addressed.

    positiveelectrode, and a nonaqueous lec-trolytecontainingdissolvedLi ions.The operationof such cells is based onthe abilityof the positiveelectrodematerialto reversibly intercalate i. Intercalationis the insertion of a guest atom (Li, forinstance) into a host solid (such as MnO2),accompaniedby only slight, reversiblestructural hangesin the host. Hosts forintercalationare commonly layeredcom-pounds uch asgraphite 3) or tunnelcom-poundssuch as MnO2or LiMn2O44), inwhich the intercalatedLi can reside be-tweenthe layersor in the tunnels. Interca-lation of Li occurs because the chemicalpotentialof Lican be loweredwhen the Liatom s inserted nto the host, thusformingchemicalbonds.The bindingenergyof Li when interca-lated into a varietyof hosts has been mea-suredwith respect o the bindingenergyofLimetal (Fig.1). In eachof the hosts isted,SCIENCE * VOL. 264 * 20 MAY 1994 1115

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    the Li atomscan diffuse eadilywithin thehost, even whenthe binding nergy snear4eV. (LiF s included n the figure orrefer-ence, but it is not an intercalation om-pound.) BecauseLi can be addedto andremovedrom hesematerials,t is commonto designate hemas Li_(Host),with x des-ignating he Li stoichiometry.The voltage andoperationof cells withnegativeelectrodes f Li metalarenoweasyto understand.During he discharge f thecell, Liat the negativeelectrodedissociatesinto ions and electronsthat move to thepositive electrode throughthe electrolyteand the externalcircuit,respectively.Theions andelectronsmeet at the surface f thepositive electrode,where they intercalatewithin the host material.The open circuitvoltageof the cell is given by the bindingenergyof Li in the host (in electronvolts),dividedby the electroncharge. (Formally,the cell voltage is given by the differencebetween he chemicalpotentialof Li in theintercalationhost and in Li metal, dividedbythe chargeon the electron.)To rechargethe cell, a charger s used to extractelec-trons and the correspondingons fromthehost, andLimetalis replatedon the nega-tive electrode.Rechargeable ells with negative elec-trodesof Li metal can be made that haveexcellent performance haracteristics 1,2). However, it seems difficultto makethem safe (5). At the heart of the safetyproblems the reactivity f metallicLi withthe nonaqueous lectrolytesused in thesecells. Lithiumreactsinstantlywith theseelectrolytesto form a passivating ilm ofinsolublereactionproducts n the Limetalsurface.Once the film s about50 A hick,the reactionstops. Althoughthe film is aLi-ionconductor, t is an electronic nsula-tor that prevents he transport f Li atomsto the electrolyte,oncethefilm thickness sgreaterhan the electrontunneling ength.However, as cells are repeatedlychargedanddischarged,he Li metal becomesveryporous,and the areaof contactbetweenLiand electrolytebecomesvery large. Afterseveralhundredcycles, such cells becomeprone o safetyproblemssuchasproducingintense smoke or even fire) if they reachtemperaturesbove about120?Cbecauseofmechanical relectrical buse.Thisproblemledto a majorproduct ecall n 1989 nvolv-ing cells manufacturedby Moli Energy(Bumaby,BritishColumbia,Canada) (5).MostotherLibattery ntroductions lannedby otherfirmswere then abandoned.To solve the safety problemassociatedwith Limetal,battery cientistseliminatedLimetalbut triedto retainthe highenergydensity attainable with the technology.The so-called Li-ion cells use two differentintercalation hosts as the positive and neg-ative electrodes. For example, Sony Ener-

    6eV --LiF

    Air 5estable e LiNiVO4

    4 eV VLi XNiO2, Li, -XCoO2 VLil - xMn2O4

    t-Li MnO3 eV x 2 LXV(BA~~~~ LiXV02(B)2 eV ULixS2 LixMoS2

    I LixM0?21 eV ,LixWO3

    LixCokeLithiummetal 0 eV L LixGraphite

    Fig. 1. The binding energy of Li,intercalated withina variety of ma-terials, measured relative to thatof Li metal.

    gytec introduced he 3.5-V LiCoO2/carboncell in 1992, andit has been verysuccessful(6). The Sony cell and others like it arecurrentlyhe state-of-the-art owersourcesforconsumer lectronics. n sucha cell, theelectrodematerialsare chosen so that thechemicalpotentialof Li in each differsbyseveralelectron volts, with the result thathigh energydensitycanstill be maintained.Safety s also improved,because he surfacearea of the electrodes remains constantduring ycling, even thoughLi-intercalatedcarbon s almost as reactive as metallicLi(see Fig. 1). It is unclear,however,whethercells largeenoughforEVswill be safe.The Li-iontechnologys alsoexpensive.Nonaqueouslectrolytesave on conductiv-itiesaboutwoorders f magnitudeower hantheir aqueouscounterparts,o cell designsincorporatinghin electrodes0.1 mm) mustbe used. Suitable Li salts, such as LiPF6,are also expensive, as are the microporousfilms used to separate the electrodesandhold the electrolyte. Water mustbe rigor-ously excludedduring ome manufacturingsteps, which leads to additionalcosts.

    Lithiumbatterieswere historically de-signedwith metallicLias one component,which precluded he considerationof wa-ter as an electrolytesolvent. The reactionproductof Li and water,LiOH, is solublein water, so that a passivating ilm doesnot form and a violent reaction occurs.Similarly, in the current generation ofLi-ion cells that useLi-intercalated arbonas the negative electrode,watercannot beusedasthe electrolyte.However, once theLi in an intercalationhost is sufficientlytightly bound (at about3.2 ? 0.2 eV), itwill not react with water to form LiOHand H2 (7).When a Li intercalationcompound sput in water, one must considerwhetherthe reactionLix(Host) xH20'= Host+ xLiOH(aq) x/2H2(g) (1)will occur. We recently showed (8) thatintercalation ompounds hat are unstablein pure water can be made stable in con-centrated LiOH solution. The chemicalpotentialof Li' in solutionincreaseswith

    1.5 [H+]= 1 M [OH-]= 1 M2H20 - 4H+ + 4e + 021.0- Lii-xMn2O4

    0.5 40H-O 2H20 + 02 + 4e> 0.0 2H+ + 2e- -H2 Li1+xMn204wen -0.5 LiXV02(B)

    2H20 + 2e- H2 +20H-(0 -1.02 2Xi -1.5-g -2.0-

    -2.5--3.0- Li -Li++ e-3.5

    Fig.2. Thepotentialsof theindicated reactionsversusthe standard hydrogenelectrode (SHE) in solu-tionswhere [Li+]= 1 0 M.Both acidic and basicelectrolyteswith 1 M H+or1 M OH-, respectively,areconsidered.

    1116 SCIENCE e VOL. 264 * 20 MAY 1994

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    i REPORTSSealinggasket

    CellcapNegativeelectrode

    SeparatorPositiveelectrode

    Cell canFig. 3. A schematic of the coin cell used to testthe aqueous Li-ionchemistry.

    concentrationand tendsto driveEq. 1 tothe left. This suggestshatLi-ioncells withaqueouselectrolytescan be considered orappropriatelyhosenhosts.We mustalso consider he potentials oroxygen and hydrogenevolution that canoccur n aqueous lectrolytes Fig.2). Fig-ure 2 also shows the standardelectrodepotentialfor the reaction

    Li Li+ + e- (2)Figure1 lists the chemicalpotentialsof Liintercalationcompoundsrelative to reac-tion 2, so we can locateelectrodematerialson Fig.2, aswe have donefor the reactionsLiMn2O4= Li,_.Mn204 + xLi+ + xe-and (3)

    V02(B) + xLi+ + xe- ' LixVO2(B)(4)[ B designatesa particularrystal ormofV02 (9).] In an appropriatelyhosenelec-trolyte,reactions3 and4 shouldbe viablewithout excessivesimultaneous roductionof?2 orH2.A cell basedon reactions and4 will have a terminalvoltagenear1.5 V.We synthesizedLiMn204according othe methods described in (10) and V02 (B)accordingo the methodsdescribedn (11)Both of these materials re framework-type

    2.0 . . .. ....

    1.5-0

    0O.5

    0.010 10 20 30 40 50 60 70 80 906100Time(hours)Fig. 4. The voltage plotted versus time as aLiMn2ONO12(B) ell is repeatedly chargedand discharged. The currentsused were ?1mA.The cell test was carriedoutat 30?C.

    intercalation ompounds.Tabletelectrodes8 mmin diameterandwith a total massof0.10 g were preparedrom each material.Carbonblack (10% by weight) and ethyl-ene propylenedienemonomerbinder (3%by weight)were added o the tabletmix toprovide good electrical conductivityandgood mechanical oughness.The mix wasthen pressedn 0.10-gallotmentsn a cylin-dricaldieto the desired hickness 1.0 mm).Coin-type test cells were made with1225 hardwarethe firsttwo numbers ivethe cell diameter n millimetersand thesecondtwo givethe cell heightin tenths ofmillimeters; 1225 cell is 12 mmin diam-eter and 2.5 mm thick). First, the elec-trodes were thoroughlywetted with anaqueous olution thatwas 5.0 M in LiNO3and about 0.001 M in LiOH. A micro-porous polypropyleneseparator (Celgard3500), whichincorporates wetting agent,was used to separate he electrodesas theyweretightlycrimpedn the cell case. Figure3 shows a schematic of the finishedcell.Because he concentrationof OH is 0.001M in this cell, we expectthe potentials orH2 and O2 evolution to shift up by about0.175 V, comparedwith theirpositions n 1M OH-, as shownon the right of Fig. 2.The cellwasthen chargedwith a currentof 1mA. During he charge,Liwasextract-ed fromLiMn2O4, roducingLilxMn2O4,and intercalated nto V02(B), producingLiXVO2(B).Figure 4 shows the voltageprofileduring his chargingandduring henext few charge-dischargeycles. The cellshows excellent reversibility,an averagevoltage near 1.5 V, and a capacityof 10mA hour.We calculated he energydensityof thecell usingthe weightof the electrodes,thecell voltage, and the cell capacity (we didnot include the electrolyte or cell-caseweight). This cell's energy density is 75watt hours/kg.Typically, the active elec-trode weights are about 50% of the totalweightof practical ells, if the SonyLi-ionproduct is used as an example. Thus,practical energy densities near 40watt hours/kg can be expected for thischemistrywhen used in largercells. Fur-thermore,the theoretical energydensityforthis cell is 112wattehours/kg,ssuming0.5 Lipertransitionmetalcan be cycled neach electrodeand that the averagecellvoltageis 1.5 V. These assumptions ouldgive about55 watt-hours/kgn a practicalcell, which s competitivewith bothPb-acid(about30 watt-hours/kg)nd Ni-Cd (about50 wattehours/kg)echnologies.In basic electrolytes, 02 evolution atLil XMn2O4may occur simultaneouslywith the removalof Li fromthe electrode,as indicated in Fig. 2. (This is analogous tothe ?2 evolution that occurs in Ni-Cd cellsduring charging.) The ?2 should then dif-

    fuse to the LiXVO2(B) electrode, where itwill recombine with water and electrons toproduce OH- again. Therefore, it shouldbe possible to select electrolytes of appro-priate pH that allow for efficient charging ofthe positive electrode and that also provideeffective oxygen pressure control.The LiMn2O4/VO2 (B) couple by nomeans represents the optimum electrodepair for the aqueous Li-ion cell. Many typesof Li manganese oxides are known to inter-calate Li in the right chemical potentialrange for this type of cell (12). One ap-proach to an extremely low-cost systemwould be to use LiMn2O4 for both elec-trodes. Because Li can be extracted fromLiMn2O4 (to form Li1_xMn2O4) at 4 Vversus Li and added to LiMn2O4 (to formLi1+XMn204)at about 3 V versus Li, a 1-Vcell would result. Manganese and its oxidesare cheap, plentiful, and less toxic thanNi and Co oxides, so a Li1+xMn2O4/Li1l XMn2O4 ell could have an enormouspotential market.In a 1989 review of about 40 batterytechnologies for EV applications, Ratneret al. (13) concluded that there was nosuitable technology currently available forelectric vehicles. The review consideredfactors such as cost, safety, performance,and environmental friendliness. Today,the search for an acceptable EV batterycontinues. For example, the Ni-metal hy-dride battery developed by Ovonics (War-ren, Michigan) has been shown to attain80 wattehours/kg in sizes acceptable forEVs (14). However, some industry expertscontend that the Ni-metal hydride tech-nology is too expensive and is not envi-ronmentally acceptable for EV applica-tions. It is our opinion that the aqueousLi-ion approach described here showspromise and needs to be included in thehunt for the EV battery.

    REFERENCESAND NOTES1. K. Brandt, paper presented at the Fourth Interna-tional Seminar on LithiumBattery Technology andApplications, Deerfield Beach, FL, 5 to 7 March1989.2. J. Broadhead,n Proceedingsof the Third nnualBatteryConferenceon Applications ndAdvanc-es, California State University, Long Beach, CA,

    12 to 18 January 1988; S. Basu and F. A. Trum-bore, J. Electrochem. oc. 139, 3379 (1992).3. M. S. Dresselhaus and M. Endo, in Graphite Inter-calation Compounds 11,H. Zabel and S. A. Solin,Eds. (Springer-Verlag,Berlin,1992), pp. 347-407; J.R. Dahn et al., Electrochim.Acta 38,1179 (1993).4. M. M. Thackeray et al., J. Electrochem. Soc. 139,363 (1992); M. M. Thackeray and R. J. Gummow,U.S. Patent 5,240,794 (1993).5. For example, see Cellular phone recall maycause setback for Moli, Toronto Globe and Mail(Canada), 15 August 1989.6. T. Nagaura and K. Tozawa, Prog. Batteries SolarCells 9, 209 (1990); G. Stix, Sci. Am. 269, 108(October 1993).7. J. R. Dahn, U. von Sacken, M. W. Juzkow, H.Al-Janaby, J. Electrochem. oc. 138, 2207 (1991).8. W. Li,W. R. McKinnon, J. R. Dahn, ibid., in press.

    SCIENCE * VOL. 264 * 20 MAY 1994 1117

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    9. W. Theobaldet al., J. Solid State Chem.17, 431(1976);D. W. Murphy tal., J . Electrochem.Soc.128, 2053 (1981).10. T.Ohzuku,M.Kitagawa,T.Hirai, . Electrochem.Soc. 137, 769 (1990).11. J. R. Dahn,T. van Buuren,U. von Sacken, U.S.Patent4,965,150 (1990).12. For example, see T. Ohzuku,A. Ueda, T. Hirai,Chem. Express 7, 193 (1992); T. Nohma, T.Saito, N. Furukawa,H. Ikeda,J. PowerSources26, 389 (1989); J. M. Tarascon and D. Guyo-mard,Electrochim.Acta 38, 1221 (1993).

    13. E.Z. Ratner,P.C. Symons,W.Walsh,C. J.Warde,G. L.Hendriksen,Assessmentof batteryechnolo-gies for electric vehicles, contract DE-AC07-761D01 70(U.S.Dept.of Energy,Washington, C,August1989).14. S. R. Ovshinsky,M.A. Fetcenko,J. Ross, Science260, 176 (1993).15. The authorsacknowledge fundingfrom the Na-tional Science and EngineeringResearchCoun-cil of Canada's Operatinggrantprogram.9 February1994; accepted 30 March1994

    Simulatinghe Adsorptionf Alkanes nZeolitesBerendSmit*andJ. Ilija iepmannt

    The configurational-biasMonteCarlotechnique is appliedto simulatethe adsorptionoflong chain alkanes in zeolites. This simulation echniqueis several ordersof magnitudemore efficient hanconventionalmethodsthat can be used to simulatethe adsorptionoflong chain alkanes. The calculated heats of adsorptionare found to be in excellentagreementwithexperimental ata.The resultsshowa surprising hain engthdependenceof the heats of adsorption.This dependence has a simplemolecularexplanation n termsof preferential itingof the long chain alkanes.

    Zeolites are crystalline norganicpolymersthat form a three-dimensional etworkofmicropores.These pores are accessibletovariousguestmolecules.The large nternalsurface, he thermal tability,and the pres-ence of acid sites make zeolites an impor-tant class of catalytic materials or petro-chemicalapplications.Fora rationaluseofzeolites, it is essential to have a detailedknowledgeof the behaviorof the adsorbedmoleculesinside the poresof the zeolites.Unfortunately,such informationis verydifficult to obtain, particularly or longchain hydrocarbonmolecules.Catalytic onversionnside he poresof azeolitecanbe seen schematicallys a three-stepprocess: i) the adsorption nd diffusionof the reactants,(ii) the catalyticconver-sion, and(iii) the diffusion nddesorption fproductsrom he zeolite.The overallactiv-ity and selectivityof a particulareaction sthe resultof a delicatebalanceof thesethreeprocesses.Muchexperimental nd theoreti-cal effort is directed towardobtaining adetailedunderstandingf each of thesestepsat amolecularevel (1). The highselectivityof zeolites mpliesthat the behaviorof theadsorbedmolecules s system-specific.t isthereforeessentialto be able to study thebehaviorof the adsorbedmoleculesof inter-est underreactionconditions.B. Smit,Shell Research B.V.Koninklijke/Shell-Labora-torium,Amsterdam,Post OfficeBox 38000, 1030 BNAmsterdam,Netherlands.J. I.Siepmann,Department f Chemistry,UniversityfPennsylvania,Philadelphia,PA 19104-6323, USA.*Towhomcorrespondenceshouldbe addressed.tPermanentaddress: Department f Chemistry,Uni-versityof Minnesota,207 PleasantStreet SE, Minne-apolis,MN55455, USA.

    Computer imulations,usedwithmolec-ular dynamicsor Monte Carlotechniques,arean attractivealternative o experimentsbecause these methods can, in principle,provide informationfor conditions underwhichexperiments renot feasible.Indeed,over the last few years there has beenconsiderableprogressn simulatingadsorp-tion in zeolites [for a recent review, see(2)]. In practice,however,computer imu-lationshavebeen limitedto atomsor smallmolecular uestmoleculesandcould not beextended to molecules of catalytic rele-vance. These limitationsare discussedbyJune et al. (3); in this work, moleculardynamicswas usedto study he behaviorofbutaneand hexane in the zeolite silicalite.June et al. observedthat the diffusionofthese alkanesis very slow and the rate ofdiffusiondecreaseswith increasingchainlength. Therefore, ong simulationsarere-quired o obtainreliableresults.The Monte Carlo technique s not lim-

    ited by the slowdiffusionof the molecules,becausemoves can be made to arbitrarypositions in the zeolite. For chain mole-cules, however,this is not the casebecausethe probability f findinga positionwithoutoverlapbetween hydrocarbonand zeolitedecreases xponentiallywith chain length.Recently, we have developed a method,configurational-bias onte Carlo, to simu-late chain molecules (4, 5). We demon-strateherethatthisapproachanbeused ostudythe behaviorof long chain hydrocar-bonsin zeolitesand allowsusto addresshemuch debatedquestionof the preferentialadsorption f the n-alkanes n the differentchannelsof silicalite.We usedthe configurational-bias onteCarlotechniqueto studythe adsorption fn-butane to n-dodecane in silicalite. Incontrastto the conventionalMonte Carlotechnique, in the configurational-biasMonte Carlo techniquea molecule is notinsertedat randombut is grown atom byatom such that overlap with the zeoliteatoms is avoided (Fig. 1). This growingprocessntroducesa bias that is removedbyadjustingthe acceptance rules (4-7). Asimulation s performedn cycles,and eachcycle consists of a number of randomlyselected moves: displacementof particles,rotationof particles,partialregrowing f amolecule,andregrowing f a moleculeat arandomly elected position. For the lattertwo moves, the configurational-bias onteCarlotechniqueis used with a total simu-lation consistingof at least 106cycles.Thealkanesaredescribedwith the model of (8).This modelyieldsanaccuratedescription ftheirphasebehavior.FollowingKiselevandco-workers 9), we assume hat the zeolitelattice is rigid and that the zeolite-alkaneinteractions are dominated by dispersiveinteractions.Lattice vibrationscan have apronounced ffecton the diffusionbecausethese vibrationsmay lower the diffusionalbarriers.Because hese barriers ardlycon-tributeto the equilibriumdistribution,weexpect that the effect of lattice vibrationson the equilibrium ropertiess small.Thedetails of the model aregiven in Table 1.

    Fig. 1. Schematic drawingof the growingof an alkane in azeolite ina configurational-biasMonteCarlomove. Theblackcircles represent the atoms of the zeolite, and the whitecircles representthe atomsof the alkane.Seven atomshavebeen grownsuccessfully, and an attempt is made to insertthe eighth.The arrows ndicateseven trialpositionsforwhichthe energy u; s calculated. Outof these seven positionsoneis selected with a probabilitypi = exp(-ui/kBT)/wnew(f)withWnew(f) = -iexp(-uj/kBT), where T sthetemperaturendkBis Boltzmann'sconstant. Similarly, orthe old configurationwe calculate wOId(f) = ljexp(-uj/kBT). This is repeated untilthe entirechainof lengthm has been grown. Itcan be proven(5) that the bias of the growing is removed by the replacement of exp(-AU/kBT) by rIme1Wnew(f)/I2e=1 wold(4) in the acceptancerule.Comparison ithmolecular ynamics howsthatconfigurational-biasMonteCarlo s twoordersof magnitudemore efficient or butane and up to 12orders of maqnitudemoreefficient ordodecane.

    j 0> >

    k0' t

    1118 SCIENCE * VOL. 264 * 20 MAY1994


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