分子筛(英文)

Molecularsieve李瑞琦应化01Principle•Amolecularsieveisamaterialwithverysmallholesofpreciseanduniformsize.Theseholesaresmallenoughtoblocklargemoleculeswhileallowingsmallmoleculestopass.Application•Manymolecularsievesareusedasdesiccants.SomeexamplesincludeActivatedcharcoalandsilicagel.•Activatedcharcoal•SilicagelMaterialsMolecularsievescanbemicroporous,mesoporous,ormacroporousmaterial.•Microporousmaterial(2nm)Zeolites、Porousglass、Activecarbon、Clays•Mesoporousmaterial(2-50nm)Silicondioxide•Macroporousmaterial(50nm)Mesoporoussilica、ZeoliteLTAPore​​sizeProperties•Molecularsievesareusedasadsorbentforgasesandliquids.Moleculessmallenoughtopassthroughtheporesareadsorbedwhilelargermoleculesarenot.Itisdifferentfromacommonfilterinthatitoperatesonamolecularlevelandtrapstheadsorbedsubstance.•Forinstance,awatermoleculemaybesmallenoughtopassthroughtheporeswhilelargermoleculesarenot,sowaterisforcedintotheporeswhichactasatrapforthepenetratingwatermolecules,whichareretainedwithinthepores.•Becauseofthis,theyoftenfunctionasadesiccant.Amolecularsievecanadsorbwaterupto22%ofitsownweight.DifferentsfromtheSize-exclusionchromatography•Theprincipleofadsorptiontomolecularsieveparticlesissomewhatsimilartothatofsizeexclusionchromatography,exceptthatwithoutachangingsolutioncomposition,theadsorbedproductremainstrappedbecause,intheabsenceofothermoleculesabletopenetratetheporeandfillthespace,avacuumwouldbecreatedbydesorption.•Size-exclusionchromatography(SEC)isachromatographicmethodinwhichmoleculesinsolutionareseparatedbytheirsize,andinsomecasesmolecularweight.Applications•Molecularsievesareoftenutilizedinthepetroleumindustry,especiallyforthepurificationofgasstreamsandinthechemistrylaboratoryforseparatingcompoundsanddryingreactionstartingmaterials.•Theyarealsousedinthefiltrationofairsuppliesforbreathingapparatus,forexamplethoseusedbyscubadiversandfirefighters.Insuchapplications,airissuppliedbyanaircompressorandispassedthroughacartridgefilterwhich,dependentontheapplication,isfilledwithmolecularsieveand/oractivatedcarbon,finallybeingusedtochargebreathingairtanks.[8]Suchfiltrationcanremoveparticulatesandcompressorexhaustproductsfromthebreathingairsupply.Applications•breathingapparatus•Insuchapplications,airissuppliedbyanaircompressorandispassedthroughacartridgefilterwhich,dependentontheapplication,isfilledwithmolecularsieveandactivatedcarbon,finallybeingusedtochargebreathingairtanks.Suchfiltrationcanremoveparticulatesandcompressorexhaustproductsfromthebreathingairsupply.•DesiccantRegeneration•Methodsforregenerationofmolecularsievesincludepressurechange(asinoxygenconcentrators),heatingandpurgingwithacarriergas(aswhenusedinethanoldehydration),orheatingunderhighvacuum.•Regenerationtemperaturesrangefrom175°Cto315°Cdependingonmolecularsievetype.[9]Incontrast,silicagelcanberegeneratedbyheatingitinaregularovento120°C(250°F)fortwohours.•However,sometypesofsilicagelwillpopwhenexposedtoenoughwater.Thisiscausedbybreakageofthesilicasphereswhencontactingthewater.THEEND