MolecularsievePrincipleAmolecularsieveisamaterialwithverysmallholesofpreciseanduniformsize.Theseholesaresmallenoughtoblocklargemoleculeswhileallowingsmallmoleculestopass.ApplicationManymolecularsievesareusedasdesiccants.SomeexamplesincludeActivatedcharcoalandsilicagel.ActivatedcharcoalSilicagelMaterials

Molecularsievescanbemicroporous,mesoporous,ormacroporousmaterial.Microporousmaterial(<2nm)Zeolites、Porousglass、Activecarbon、ClaysMesoporousmaterial(2-50nm)SilicondioxideMacroporousmaterial(>50nm)Mesoporoussilica、ZeoliteLTA

Pore??sizePropertiesMolecularsievesareusedasadsorbentforgasesandliquids.Moleculessmallenoughtopassthroughtheporesareadsorbedwhilelargermoleculesarenot.Itisdifferentfromacommonfilterinthatitoperatesonamolecularlevelandtrapstheadsorbedsubstance.Forinstance,awatermoleculemaybesmallenoughtopassthroughtheporeswhilelargermoleculesarenot,sowaterisforcedintotheporeswhichactasatrapforthepenetratingwatermolecules,whichareretainedwithinthepores.Becauseofthis,theyoftenfunctionasadesiccant.Amolecularsievecanadsorbwaterupto22%ofitsownweight.DifferentsfromtheSize-exclusionchromatographyTheprincipleofadsorptiontomolecularsieveparticlesissomewhatsimilartothatofsizeexclusionchromatography,exceptthatwithoutachangingsolutioncomposition,theadsorbedproductremainstrappedbecause,intheabsenceofothermoleculesabletopenetratetheporeandfillthespace,avacuumwouldbecreatedbydesorption.Size-exclusionchromatography(SEC)isachromatographicmethodinwhichmoleculesinsolutionareseparatedbytheirsize,andinsomecasesmolecularweight.ApplicationsMolecularsievesareoftenutilizedinthepetroleumindustry,especiallyforthepurificationofgasstreamsandinthechemistrylaboratoryforseparatingcompoundsanddryingreactionstartingmaterials.Theyarealsousedinthefiltrationofairsuppliesforbreathingapparatus,forexamplethoseusedbyscubadiversandfirefighters.Insuchapplications,airissuppliedbyanaircompressorandispassedthroughacartridgefilterwhich,dependentontheapplication,isfilledwithmolecularsieveand/oractivatedcarbon,finallybeingusedtochargebreathingairtanks.[8]Suchfiltrationcanremoveparticulatesandcompressorexhaustproductsfromthebreathingairsupply.ApplicationsbreathingapparatusInsuchapplications,airissuppliedbyanaircompressorandispassedthroughacartridgefilterwhich,dependentontheapplication,isfilledwithmolecularsieveandactivatedcarbon,finallybeingusedtochargebreathingairtanks.Suchfiltrationcanremoveparticulatesandcompressorexhaustproductsfromthebreathingairsupply.DesiccantRegenerationMethodsforregenerationofmolecularsievesincludepressurechange(asinoxygenconcentrators),heatingandpurgingwithacarriergas(aswhenusedinethanoldehydration),orheatingunderhighvacuum.Regenerationtemperaturesrangefrom175°Cto315°Cdependingonmolecularsievetype.[9]Incontrast,silicagelcanberegeneratedbyheatingitinaregularovento120°C(250°F)for