Handbook of Green Chemistry, Volume 2: Heterogeneous Catalysis

326 Pages·2009·0.7 MB·English

by  Robert H. Crabtree

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)
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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
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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
Carbide’sdiscoveryofsyntheticzeolitesinthelate1940s,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,
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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.
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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.
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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.
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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
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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.
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8 1 ZeolitesinCatalysis
1.3.2
NumberofAcidSites
Loewenstein’s 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
Academicworkinthe1930’sand40’selucidatedhowAlCl –astrongLewisacid–is
3
convertedwhendissolvedinhydrocarbonfractionstoaworkingcatalyticspecieswith