Handbook of Green Chemistry, Volume 2: Heterogeneous Catalysis PDF
Preview Handbook of Green Chemistry, Volume 2: Heterogeneous Catalysis
Contents 1 Zeolites in Catalysis 1 (Stephen H. Brown) 2 Sol–Gel Sulfonic Acid Silicas as Catalysts 37 (Adam F. Lee and Karen Wilson) 3 Applications of Environmentally Friendly TiO Photocatalysts in Green 2 Chemistry: Environmental Purification and Clean Energy Production Under Solar Light Irradiation 59 (Masaya Matsuoka and Masakazu Anpo) 4 Nanoparticles in Green Catalysis 81 (Mazaahir Kidwai) 5 ‘Heterogreeneous Chemistry‘ 93 (Heiko Jacobsen) 6 Single-site Heterogeneous Catalysts via Surface-bound Organometallic and Inorganic Complexes 117 (Christophe Copéret) 7 Sustainable Heterogeneous Acid Catalysis by Heteropoly Acids 153 (Ivan Kozhevnikov) 8 The Kinetics of TiO -based Solar Cells Sensitized by Metal Complexes 175 2 (Anthony G. Fitch, Don Walker, and Nathan S. Lewis) 9 Automotive Emission Control: Past, Present and Future 197 (Robert J. Farrauto and Jeffrey Hoke) 10 Heterogeneous Catalysis for Hydrogen Production 223 (Morgan S. Scott and Hicham Idriss) 11 High-Throughput Screening of Catalyst Libraries for Emissions Control 247 (Stephen Cypes, Joel Cizeron, Alfred Hagemeyer, and Anthony Volpe) 12 Catalytic Conversion of High-Moisture Biomass to Synthetic Natural Gas in Supercritical Water 281 (Frédéric Vogel) j 1 1 Zeolites in Catalysis StephenH.Brown 1.1 Introduction Acidcatalysisasamodernscienceislessthan150yearsold.Fromitsinception,acid catalysishasbeenexploredasameansofproducingfuels,lubesandpetrochemicals. Ordinaryhomogeneousacids,bothinorganicandorganic,neverprovedindustrially usefulattemperaturesmuchabove150(cid:1)C.Thefirstreportsofaluminosilicatesolid acidcatalystsinvolvedtheuseofclaysaftertheturnofthecentury.Theinspirationfor the first commercial synthetic aluminosilicate catalysts came from work done co precipitatingsiliconandaluminumsaltsduringWWIbyaSunOilchemist[1].The Brønsted acid site in these materials is most often represented as in Scheme 1.1. Usefulfeaturesofthisnoveltypeofacidversushomogeneousliquidacidsweretheir high temperature stability, moderate acidity (roughly equivalent to a 50% sulfuric acidsolution),solidandnon-corrosivecharacterandregenerabilitybyairoxidation. Thesefeaturesenabledacidcatalyzedreactionsofchemicalstobecontemplatedata greatlyextendedrangeoftemperatures(upto600(cid:1)C)andmetallurgies. Scheme1.1 TheBrønstedacidsiteofanaluminosilicate. Thefirst embodiments ofmany modernrefining processes includingheavy oil cracking,naphthareformingandlightgasoligomerizationdidnotusecatalysts[2]. Assoonasthesethermalprocessescommercialized,explorationoftheuseofsolid acidcatalystsensuednaturally. Becauseofthekeyroleplayedinthedevelopmentoftheautomotiveindustry,heavy oilcrackingtogasolineprovidedafocalpointfortheearlydevelopmentofheteroge- neousacidcatalysis.Temperaturesabove400(cid:1)Candpressuresbelow3atmospheres are thermodynamically favorable for the conversion of heavy oils to light hydro- carbonsrichinolefins.Acceptableheavyoilcrackingratesareachievedwithouta HandbookofGreenChemistry,Volume2:HeterogeneousCatalysis.EditedbyRobertH.Crabtree Copyright(cid:1)2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim ISBN:978-3-527-32497-2 j 2 1 ZeolitesinCatalysis catalyst at temperatures above 600(cid:1)C. This was the basis of the thermal cracking process. Thermal cracking produces high yields of methane and aromatic hydro- carbons. The goal of researchers was to find a catalyst that could crack heavy hydrocarbons selectively to gasoline with only minimal formation of gases with molecularweightsoflessthan30.Duetothermodynamicconstraints,thecatalyst had to be effective at a temperature above 400(cid:1)C. In order to avoid unselective thermalcracking,thecatalysthadtobeeffectivebelow550(cid:1)C. Thediscoveryintheearly1920sbyHoudrythatacidactivatedclayswereactiveand selectiveinthistemperaturewindowwasabreakthrough[2].Inthe1930sand1940s methods were developed and commercialized to produce high surface area man- madealuminosilicatesthatweresignificantlyimprovedcatalysts.Examinationofthe aluminosilicatecatalystsledtotheunderstandingthattheactivesitewasaBrønsted acid[3]. Atthetimeofthediscoveryofsyntheticzeolitesintheearly1950s,onlytwoclasses ofsolidBrønstedacids(solidphosphoricacidandaluminosilicates)werebeingused commerciallytoproducecommodityfuelsorpetrochemicals[4].Thecommerciali- zationofsilica-richsyntheticzeolitesintheirhydrogenformrepresentedabreak- throughforscientistsandorganizationsinterestedintheproductionoffuels,lubes andpetrochemicalsattemperaturesabove200(cid:1)C.Likeamorphousaluminosilicates, zeolite Brønsted acid sites are active and stable up to 600(cid:1)C. Shortly after Union Carbidesdiscoveryofsyntheticzeolitesinthelate1940s,MobilOilresearchersin catalytic cracking of heavy oil investigated zeolites as potential catalysts [5]. The zeoliteknownasfaujasite(FAU)wasfoundtobethreetofiveordersofmagnitude moreactivethanamorphousaluminosilicates.Unmodified,FAUwastooactiveto beuseful.WhentheactivityofFAUwastunedbyionexchangewithrareearthcations and/orbyreducingaluminumcontent,itwasfoundtohaveadramaticallydifferent selectivitytocrackedproducts.OptimizedsamplesofFAUzeolitesproducedalmost 5%lessC -gasesandcokeandincreasedgasolineyieldsbymorethan10wt%.Over 2 thecourseofthepast50years,evolvingheavyoilcrackingcatalystsandhardware havebeencontinuouslydecreasingcokeandC -gasyieldswhileincreasingtheyield 2 ofgasoline. The commercialization of zeolite catalysts for heavy oil cracking unleashed the creativeabilitiesofeveryorganizationinterestedinproducingfuelsandpetrochem- icalsusingacidcatalystsbetween250and600(cid:1)C.Closeto23processeshavebeen commercialized(Table1.1).Abouttwo-thirdsoftheprocesseshadnorealprecedence usinghomogeneousacids.Theotherthirdinvolveddisplacementofhomogeneous andamorphousacidcatalysts.Introductionofzeolitecatalystsfortheproductionof commoditieshasproceededatasteadypace.Eachcommercializationhasprovided anopportunityforzeolitescientiststofindimprovedcatalysts. 1.1.1 TheEnvironmentalBenefitsofZeolite-enabledProcesses The petroleum industry has been subject to environmental drivers for many decades[6].Innovationsintechnology,somedrivenbymorerestrictiveregulations, j 1.1 Introduction 3 Table1.1 Listofzeoliteprocesses. Process Reactortype Temperaturerange,(cid:1)C TolueneþC9þaromatics Fixed 350–450 MSTDP Fixed 350–450 Cumeneviatransalkylation Fixed 150–200 Ethylbenzeneviatransalkylation Fixed 230–260 Ethylbenzene Fixed 180–250 Cumene Fixed 100–150 Fluidcatalyticcracking(FCC) Fluid 500–550 ZSM-5inFCC Fluid 500–550 Gasoilhydrocracking Fixed 350–475 Distillatehydrocracking Fixed 280–350 Distillatedewaxing Fixed 350–450 Waxhydrocracking Fixed 290–370 Waxhydroisomerization Fixed 300–350 Gasolineoctaneenhancement Fixed 350–450 Reformateupgrading Fixed 450–550 Lightparaffinisomerization Fixed 240–300 Buteneisomerization Fixed 350–450 Xyleneisomerization Fixed 400–470 Lightparaffinaromatization Moving 450–550 Methanoltogasoline Fixed 300–400 Methanoltoolefins Fluid 400–500 Aromaticsfeedtreating Fixed 150–250 Caprolactam Fluid 350–450 have continuously increased the efficiency of refining processes. The trend is to producefuelshavinglowerconcentrationsofheteroatomsandpolynucleararomatics (oftenreferredtoascleanfuels)thatcanbeburnedtocarbondioxideandwaterwith increasinglyloweremissionsofNO,SO andparticulatebyproducts. x x For decades, nearly the entire hydrocarbon content of a barrel of oil feeding a refineryorpetrochemicalcomplexhasbeenconvertedtosalableproductsorusedfor fuelatthemanufacturingsite.Distillationofcrudeoillargelysplitsitintostreams with the boiling ranges of the fuels sold to consumers and businesses (gasoline, diesel, fuel oil, etc.). The quantities of the streams produced by distillation rarely match market demand. Processes using zeolite catalysts have reduced the effort requiredtoconvertstreamsthatareoversuppliedbysimplecrudeoildistillationinto undersupplied products. Optimized zeolite catalyzed processes are often high technologyoperations.Performancecanbesensitivetotheperformanceofneigh- boring units. Operating multiple zeolite-catalyzed processes can provide refiners with an incentive to continuously work to bringthe refinery closer to steady state operation. Adoption of these high technology processes and work practices has helpedrefinerstosteadilyincreasetheamountofcleanfuelproductsproducedfrom eachbarrelofoil,therebyreducingemissionsofCO ,NO,SO andparticulatesand 2 x x increasingenergyefficiency. j 4 1 ZeolitesinCatalysis Zeolite catalysts are remarkably efficient. Each weight unit of zeolite produces between3000and500000weightunitsoffuelorpetrochemicalproductbeforeits lifetime ends and it is removed from catalyst service. As a result, relatively small volumesofspentzeolitecatalystsareproduced.Thereareoftenotherusesforspent catalysts, such as an ingredient for cement. In the many cases where reuse is an option,therearelittle/nocatalystwastedisposalcosts. Catalyticcracking(alsoknownasfluidcatalyticcrackingorFCC)isbyfarthemost economicallyimportantprocessintherefiningandpetrochemicalsindustryandwill bedescribedinsomedetailtoallowthegreenaspectstobehighlighted.Worldwide, FCCunitsprocessalmost20millionbbl/dayoffeedstock(almost30%ofthecrudeoil produced) and FCC catalysts generate $1 billion in sales [7]. The remarkable performanceoftheFCCprocessisachievedbybothoptimizingthezeolitecatalyst andthereactordesign.AschematicofanFCCunitisprovidedinFigure1.1. TheFCCcatalystspendsmostofitstimeinalarge,cylindricalregenerationvessel typically15metersindiameterand40meterstallholding300tonsofacoarsepowder catalyst comprised of a bell-shaped distribution of spheres between 15 and 120 micronsindiameter.Thevesselistypicallyheldat15–25psigand620to700(cid:1)C.Airis continuouslyblownupfromthebottomofthevesselandiscarefullydistributedto provideuniformcontactingwiththesolids.Whenproperlyengineered,upflowing gasesmixwiththecoarsecatalystpowdertoformamixturewhichbehaveslikea fluid.Thereactioncarriedoutintheregenerationvesselisthecombustionofthe solidcarbonaceousreactionbyproductsthataccumulateonthecatalystduringthe crackingreaction.TheFCCcatalystenterstheisothermal,back-mixedregeneratorat thereactiontemperature(about550(cid:1)C)andisheatedtotheregeneratortemperature bytheheatofcombustionofthecoke. Because of its fluid-like properties in the presence of a flowing gas stream, the catalystwillflowsmoothlyoutofthebottomoftheregenerator,upa2meterdiameter Figure1.1 FCCreactorprocessflowdiagram. j 1.2 GeneralProcessConsiderations 5 pipe(calledariser)whereitcontactstheheavyoilfeedstockandthenbackintothetop oftheregenerationvessel.Atypicalcatalystcirculationrateforaunitfilledwith300 tonsofcatalystwouldbe3000tons/hour.Anaveragecatalystparticletravelsthrough theriseronceevery5or6minutes.Heavyoilfeedstockisheatedtoabout300(cid:1)Cand sprayedintothecirculatingcatalyst(620to700(cid:1)C)atthebottomoftheriser.Feed vaporization is accomplished by direct contact with the hot zeolite catalyst. The gaseousproduct isremovedutilizingcyclonesat thetopof theriser.Feedstockis typicallyfedintotheriserattwicethetotalcatalystinventoryandonefifththecatalyst circulation rate (e.g. catalyst circulation of 3000tons/h and a feed throughput of 600tons/h).Thetotaltimeoffeedstockandcatalystcontactisseveralseconds.About 5wt% of thefeedstock(nomore, noless) mustbe convertedto thecarbonaceous solids (coke) that are required to provide the energy input needed to drive the feedstock vaporization and the endothermic reaction. A typical catalyst particle containsabout1wt%cokeoncatalystuponenteringtheregenerator. Thirtytofiftypercentofabarrelofcrudeoilboilsabovetheendpointofgasoline and automotive diesel fuels. The FCC unit converts much of this material into gasolineanddieselfuelswithroughly80wt%selectivity.Another5to10%oftheC - 4 productsareeasilyconvertedintohighqualitygasolineinasecondstep,resultingin anoverallselectivitytogasolineanddieselfuelsof85to90%.Fivewt%ofthefeedis convertedtocokewhichisusedtosupplymostofthefuelfortheunit(regeneration and separations). The remaining ca. 5–10% of the byproducts are mostly low molecularweightgases(<35)andpropane. Catalyst,oilfeedstockandairaretheonlysignificantinputstotheprocess.The removalrateofspentcatalystisroughly2wt%perday(6tons/dayfroma300ton inventory). The catalyst is often used as an ingredient in cement manufacture. If necessary,thegasesproducedintheFCCregeneratoraretreatedtomeetemissions specificationsforNO,SO andparticulates. x x 1.2 GeneralProcessConsiderations AsillustratedbytheFCCexample,zeolitesareimportantgreentechnologiesthatare used in processes conducted on a large scale. Zeolite processes with products producedinquantitiesof<50000000kg/yearmakeanegligiblecontributiontothe overallenvironmentalcreditsachievedbyzeolites.Zeoliteprocessescarriedoutona largescalearelistedinTable1.1.Mostoftheseprocessesproduceplastics,lubricants or fuels. The significant production volumes required place many practical con- straintsonproductionmethods.Commoditymaterialsalmostwithoutexceptionare producedfromcommodityrawmaterials(usuallyfossilfuels)inonetofourcatalytic steps.Eachsteptakesplaceinreactorsofasizewhichcanbeconvenientlyfabricated, transportedanderectedandthereactormustbeabletoruncontinuouslyorsemi- continuouslyfor>1yearwithoutshut-down.Threereactortypesareemployed:fixed bed,fluidbedandmovingbed.Commodityprocessestypicallyproducebetween0.5 and5productvolumesperreactorvolumeperhour. j 6 1 ZeolitesinCatalysis 1.3 ZeoliteFundamentals Basicinformationaboutthestructuresandcompositionsofknownzeolitesisreadily obtainedbyconsultingtheInternationalZeoliteAssociation(IZA)structureatlas[8] (availableontheInternet)ortheHandbookofMolecularSieves[9]. AlmostallofthezeolitecatalystsusedintheprocesseslistedinTable1.1sharea number of basic features. They are silica rich (Si:Al>5 and <50). They are manufactured (capableofbeingsynthesized inthelab)andtheycontain10or12 memberedringchannelsystems.Up-to-dateinformationaboutzeolitestructuresis availablefromtheIZAonlinestructureatlas[10].Atthetimeofwriting,theatlas containedatotalof180knownstructuretypeseachassignedauniquethreeletter code.Sinceaninfinitenumberofzeolitestructuresarepossible,currentlyavailable samplesareanegligiblefractionoftotalpossiblestructures.15ofthese180structure typesarereadilysynthesizedinthelaboratorywithSi:Al>5and<50andwitha10or 12memberedringchannelsystem(Table1.2).FAU,EMT,FER,LTLandMORwere synthesized firstatUnionCarbide.MEL,MFI,MFS,TON,MTT,MTW,BEAand MWWwerefirstsynthesizedatMobil. Manymoreman-madezeoliteframeworksareintheIZAdatabasecontaining10 and/or12memberedring systems.Thesematerials are notincluded inTable 1.2 becausethetypematerialsarepuresilica.ExamplesincludeCFI,CON,DON,IFR, ISV,SFE,SFF,STF,STTandVET. Most molecules with less than 7 carbons have critical diameters less than the 5.5 angstrom diameter typical of a 10-ring zeolite pore and can freely diffuse. Manylargermoleculesalsohavecriticaldiameterslessthan5.5angstroms.Even moleculesthesizeof1,3,5tri-isopropylbenzenecandiffuseinto12-ringzeolites Table1.2 IZA10and/or12ringzeolitestructureswithSi:Albetween5and50. Structurecode Ringsize Diffusionalpotential EUO(ZSM-50) 10 1D MTT(ZSM-23) 10 1D TON(ZSM-22) 10 1D NES 10by10 2D FER(ZSM-35) 10by8 2D MFS(ZSM-57) 10by8 2D MWW(MCM-22) 10by10 2D MEL(ZSM-11) 10by10 3D MFI(ZSM-5) 10by10 3D MTW(ZSM-12) 12 1D LTL 12 1D MOR 12by8 2D BEA 12by12by12 3D EMT 12by12by12 3D FAU 12by12by12 3D j 1.3 ZeoliteFundamentals 7 withporediametersexceeding7angstroms.Thismeansthatthevastmajorityof molecules present in distilled petroleum fractions can diffuse in and out of zeolites containing 12-membered rings. Larger ring structures, such as 18-ring VPI-5, have a pore diameter of 12 angstroms. These pores are so big that many small molecules fit in side-by-side and ordinary molecules can not be discrimi- nated by molecular size. Once pore sizes have reached >12–20 angstroms, acid sitesinsidetheporescanbeconceptualizedinthesamefashionasacidsiteson amorphousaluminosilicatesoronzeolitesurfaces.Poressolargeplacefewsteric constraints on the polymerization of large molecules into larger deactivating oligomeric structures. StructuresofthezeoliteframeworkslistedinTable1.2areprovidedattheIZA website.Althoughallthestructurescontain10or12ringpores,eachstructurehas manyuniqueaspects.Eachringsystemhasitsownuniquesizeandshape.Some zeolites, e.g. FAU, have large internal cavities, while others contain only one dimensionalcylindricalpores(e.g.LTL).Becausethereareonlyahandfulofunique structures, it should not be surprising that there is often a large difference in performancewhenthesestructuresareapplied. 1.3.1 OtherProperties Attemperatures>200(cid:1)CzeoliteBrønstedacidprotonsdelocalize[11].Attempera- tures>550(cid:1)CdehydroxylationisinitiatedandBrønstedacidactivityislost[12].The presence of steam can retard dehydroxylation. Activity loss by dehydroxylation is commonlyreversibleasthedehydroxylatedzeolitecanrehydrateatlowtemperature andresumeitsoriginalstructure. Zeolites exchanged with polyvalent metal ions (typically nitrate salts) become acidicuponthermaldehydrationandnitratedecomposition[13–16].WeakBrønsted acidsitescanformbyhydroxylationofthemetalcation.Themechanismisbelievedto proceedbyassociationofthecationwithaspecificframeworkaluminumaccompa- nied by dissociation of water to form a hydroxyl group attached to the cation (Scheme1.2).Forthisreason,theadditionofpolyvalentcationstozeolitesdirectly impactsthenumberofBrønstedacidsitesandtotalzeoliteporevolumebuthasonlya minorimpactonthestrengthoftheremainingBrønstedacidsites.Furthermore, zeolitescontainingpolyvalentcationsareconsiderablymorecomplexbecauseboth themetalcationsandtheprotonsaremobileandbecausemanymetalionsaremore active for redox reactions than silicon and aluminum. At reaction temperatures between250and500(cid:1)Cthesefeaturesgenerallyleadtoincreasedratesofhydrogen transferreactionsandmorerapiddeactivationexplainingthelimiteduseofzeolites exchangedwithpolyvalentcations. Scheme1.2 ExampleofaweakBrønstedacidsiteinametal- exchangedzeolite. j 8 1 ZeolitesinCatalysis 1.3.2 NumberofAcidSites Loewensteins rule forbids the formation of Al(cid:2)O(cid:2)Al bonds in zeolite struc- tures [17]. Therefore, the potential number of acid sites equals the number of aluminum atoms in any reference unit of a zeolite crystal. High silica zeolites (Si:Al>20)cangenerallybesynthesizedandconvertedtothehydrogenformwith minimaldeviationfromtheidealizedstructure.Forthesematerialsthenumberof acidsitesdeterminedbyanalyticaltechniquesagreeswellwiththenumberofacid sitesderivedfromasimpleanalysisofbulkaluminumcontent. 1.3.3 AcidStrength Allofthecatalystsusedinthereviewedprocessesarealuminosilicates.Theoverallacid strengthofahydrogenformaluminosilicatezeolitedependsuponaluminumdistri- bution. Acidityassociated with analuminumtetrahedra is strongerwith a smaller numberofnearaluminumatoms[18–20].Forthisreason,zeoliteswithSi:Alratios between1and10canhaveavarietyofacidsitestrengths.However,carefulstudieswith ZSM-5demonstratedthatacidsiteswith0and1nextnearestneighboraluminumsare verycloseinacidsitestrength[21].MostoftheacidsitesinzeoliteswithSi:Alratios>10 haveonlyasmallnumberoftheirsiteswithmorethanonealuminumnextnearest neighborsleadingtouniformacidsitestrength.Thestrengthofthissitehasbeenwell characterizedbyNMRandIRprobesofsimplesorbatesallowingtheconclusiontobe reachedthattheacidsitestrengthissimilartothatof70%sulfuricacid[22–24].Careful studiesofmodelcompoundreactionsuncomplicatedbymasstransferlimitationsor fastsecondaryreactionprovidefurthersupportforuniformacidsitestrength[25–27]. Atthepresenttime,aluminosilicatezeolitesremaintheonlyclassofcrystalline solidBrønstedacidstofindbroaduseintheproductionofcommoditychemicals. Although a wide range of materials with alternative framework compositions are known,fewcommercialuseshavebeenfoundforthesematerials. Zeoliteframeworksandnovelframeworksbasedonaluminophosphatebuilding blocks were discovered at Union Carbide in the early 1980s [28–30]. When phos- phorussitesaresubstitutedwithsilicon,aBrønstedacidisformed.Theacidsitein thesematerialsisweakerthananaluminosilicateacidsite. 1.4 ReactionMechanisms 1.4.1 HydrocarbonCracking Academicworkinthe1930sand40selucidatedhowAlCl –astrongLewisacid–is 3 convertedwhendissolvedinhydrocarbonfractionstoaworkingcatalyticspecieswith
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