Table Of Content

ADSORPTIONANDPOROSITY:AMULTIPLEEQUILIBRIUMANALYSIS By WILLIAMSCOTTKASSEL ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT OFTHEREQUIREMENTSFORTHEDEGREEOF DOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 1998 ThisworkisdedicatedtomyfriendandmentorRussellS.Drago. ACKNOWLEDGMENTS WhenIthinkbackaboutmygraduatecareer,andithasbeenacareer,it hasbeenthepeoplethatIhavemetalongthewaythathavemeantthemost tome. IcametoUFtogainaknowledgeofchemistrytotakemetothelevel 1neededtobeasuccessfuleducatorandscientist,whatIcomeawaywithis somuchmore. 1wouldhketothankthepastandpresentmembersofmygroupformany fondmemories andinsightful discussionsinvolving our research. I would especiallyhketothankMikeMcGilvrayforputtingupwithmymoodsforthe threeyearswesharedalab, andAndrewCottoneforourmanydiscussions aboutpeopleand“culture”. IamalsogratefultoEdWebsterandBenGordon forhsteningtomyrambhngsandgivingtheirhonestopinionsandhelpasI movedforwardwithmyresearch. 1 don’t want to go without thanking Nick Kob who helped me get estabhshed quickly in the group and for the many “Kob” stories that I acquiredduringmytwoyearshvingwithhim. ThankstoDavidKagefor beingagreatfriendandroommateforthelastfouryears. Ithoughtthat1 putupwithalot,butDavidhvedwith bothNickandme! 1wouldhketo thank Bennett Novak for the many evenings spent contemplating and ventingattherotator. ItisremarkabletothinkthattwointeUigentadults m couldspendsomuchtime, andenjoy,sittingatabarthatrotates, drinking beer. Iguessstrangerthingshavehappened. Iwishtoexpressmygratitude toTracieWiUiams,especiallyforheroftenmotivatingwords:“aren’tyoudone withyourdissertationyet?”and“u-huh,Itoldyou!” Iwouldliketothankher forherfriendshipandforalwaysbeinghonestandopenwithme. IamverygratefultoJimHousefromIllinoisStateUniversity. Hefound somethinginmethatIhadyettodiscoverandgavemetheopportunityto exploremanynewandexcitingavenuesinscience andinmyhfe. Iwould also hke to thank Jim for all ofhis help and advice that he has given, especiallyafterRusspassedaway. SpecialthanksgotoDaveRichardsonfor all that he has done for me and the members ofmy group after Russ’s passing. Idonotknowanyonewhocouldhavehandledthatsituationany betterthanDavehas. Ithankyouforyouradvice,andforbeingtherewhenI neededtoaskforhelp. IwouldalsohketothankDanTalham,RussBowers, MikeScott,andJonMartinfortheirhelpfuladviceandguidance. Ofcourse,Ithankmyparents. Theyhavealwaysbeensupportiveofmy decisions,bothgoodandbad. Icannotthankthemenoughforinstillingin methefeehngthatIcoulddoanythingthatIsetmymindto. Ilovethem bothverymuch. AndIthankmybrotherJeffforhisalwayskindwords;“do youhaveajobyet?” Ithankmyfriendandmentor,RussDrago. Iknowthathewillneverget toseethis,andIcannotbegintothankhimenoughforaUthathehasdone IV forme,bothinandoutofthelab. IregretthatIwillneveragaingetthe chanceto“discuss”researchproblemswithhimor“argue”overhiscallsfrom hishaselineaboutshotsonmybasehne duringourweeklytennismatches. Finally, thanks toRuss’s wife Ruthfor treatingme as one ofher own. I thankthemboth. V TABLEOFCONTENTS page ACKNOWLEDGMENTS iii ABSTRACT viii CHAPTERS 1 ABRIEFINTRODUCTIONTOADSORPTIONAND POROSITY 1 Porosity 1 Structure 1 Heterogeneity 2 Adsorption 6 MolecularInteractions 6 ChemicalversusPhysicalAdsorption 7 AdsorbentsandPorousMaterials 9 ApphcationsofPorousSohds 10 Synthesis 12 Characterization 13 ModehngAdsorption 18 Henry’sLaw 18 TheLangmuirIsotherm 19 TheBETIsotherm 20 TheFreundhchIsotherm 21 TheHarkinsandJuraMethod 21 TheDubinin-RadushkevichEquation 23 ConcludingRemarks 24 2 AMULTIPLEEQUILIBRIUMANALYSIS(MEA)OF ADSORPTIONANDPOROSITYINCARBONS 25 TheLangmuirModel 25 TheMultipleEquihbriumAnalysis 28 DefinitionoftheMEA 31 InformationProvidedbytheMEA 36 ExperimentalIsotherms 37 VI AdsorbentsandAdsorptives 37 AdsorptionMeasurements 38 DataAnalysis 39 TheMEAParameters 41 TheMeaningoftheMEAParameters 53 SimulatedAdsorptionIsotherms 54 MonteCarloSimulationandSensitivityAnalysis 56 TheMEASurfaceAreaandPoreVolume 66 TheEnthalpyofAdsorption 70 PredictionofAdsorptionIsotherms 77 ConcludingRemarks 82 3 THEMEAPORESIZEDISTRIBUTION(MEA-PSD) 83 HeterogeneityandPSDs 83 DeterminingPSDs 84 TheHorvath-KawazoeModel 85 Derivation 86 MariwalaandFoley 88 TheMEA-PSD 89 MEAExtensionoftheHorvath-KawazoeModel 90 MEA-PSDofSelectedCarbons 93 Predictingm’sfromtheMEA-PSD 100 ConcludingRemarks 100 4 AMEADESCRIPTIONOFMULTILAYERFORMATION 102 MultilayerFormationonaHomogeneousSurface 102 PoreFiRingandCooperativeAdsorption 104 MEAMultilayers 107 ConcludingRemarks 115 5 SUMMARYANDCONCLUDINGREMARKS 117 APPENDICES A ADSORPTIONDATA 121 B FITTINGROUTINEANDINPUTFORMAT 157 LISTOFREFERENCES 179 BIOGRAPHICALSKETCH 185 vii AbstractofThesisPresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulfillmentofthe RequirementsfortheDegreeofDoctorofPhilosophy ADSORPTIONANDPOROSITY:AMULTIPLEEQUILIBRIUMANALYSIS By WiUiamScottKassel August1998 Chairman:DavidE.Richardson MajorDepartment:Chemistry The goal of the research presented in this dissertation is to gain a fundamentalunderstandingofadsorptive-adsorbentinteractionsinthehope thattheinformationobtainedmaybeapphedtotherationaldesignofnew adsorbentstailoredtospecificadsorptives. Amultipleequihbrium analysis model (MEA) hasbeen developedtointerpret adsorptionisotherms andto showthattheparametersobtainedaremeaningfulwithinthecontextofthe model. Adsorptiveshavebeenselectedthatvaryinsizeandpolarizabfiity,to permitadsorbentcharacterizationintermsofthesephysicalproperties. The equihbriumconstants(Ki’s),capacities(jiis),andenthalpies(-AHi’s)obtained fromtheanalysisofthevariedadsorptivesarethenusedtodirectlycompare viii activatedcarbon adsorbentstoidentifythevariables thatwillbe themost importantinnoveladsorbentsynthesis. Theporesizedistribution(PSD)foraporousmaterialisanessentialpiece of information for selecting a material for a specific application. With crystaUinematerialssuchaszeohtes,thewidthofthedistributionisnarrow and an idea of the average pore size(s) can be determined from crystallography. Thedeterminationofporesizesisahttlemoredifficultfor amorphous materials, for example, carbons, sdicas, and aluminas, which usually have a wide distribution ofpore sizes. The research presented examines a few of the current methods used for determining PSDs and introducesamethodthattakesadvantageoftheinformationprovidedbythe MEA. TheMEAhasbeenapphedtothetransitionfromthefillingofthesmall microporestothefillingofthelargermicropores(>2moleculardimensions) wheremultilayerformationispossible. Thepurposeofthisstudyisnotto necessarilygatherinformationaboutthelargerporesusingMEA, although that may be a result, but to gain insight into adsorption from the hquid phase. To this end, the adsorption ofethane and sulfur hexafluoride at subcriticaltemperaturesisexaminedusingMEA. IX CHAPTER 1 ABRIEFINTRODUCTIONTOADSORPTIONANDPOROSITY One of the more important problems a scientist faces is the characterizationofthematerialstheycreate. Singlemolecules,crystals,and bulk materials all require different methods for determining identity, structure,reactivity,oranyofamyriadofphysicalproperties. Themethods used in characterization must be rehable, accurate, and, probably most importantly, accessible. Thus, when one is developing a characterization methodology,theseconsiderationsmustbetakenintoaccount. Porosity At3q)icalhouseholdspongewithitsinterconnectingseriesofchannelsand holesisanexampleofaporoussohd. Thenumber,type,andsizeofthepores andthewaythattheyarearrangedinthespongerefertoitsoverallporosity. Porosityisabroadtermusedtodescribetheporousstructureofasohdandis usually described by the quantities surface area, pore volume, pore size distribution, andheterogeneity[1,2]. Allofthesetermsareusedtoclassify anddescribeporoussohdsusingmacroscopicconcepts. Structure AcrystaUine material such as a zeohte has aweU-defined andregular structure. The zeohte H-ZSM5 is shown in Figure 1.1a. From 1