Heterogeneous Catalysis of Mixed Oxides: Perovskite and Heteropoly Catalysts PDF
Preview Heterogeneous Catalysis of Mixed Oxides: Perovskite and Heteropoly Catalysts
Elsevier Radarweg29,POBox211,1000AEAmsterdam,TheNetherlands TheBoulevard,LangfordLane,Kidlington,Oxford,OX51GB,UK Firstedition2013 Copyright©2013ElsevierB.V.Allrightsreserved Nopartofthispublicationmaybereproduced,storedinaretrievalsystemortransmittedin anyformorbyanymeanselectronic,mechanical,photocopying,recordingorotherwise withoutthepriorwrittenpermissionofthepublisher. 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At thesametime,themechanismsofthecatalyticfunctionshave attracted much attentionfromthesciencecommunity,andmanyeffortshavebeenmadetoelu- cidatethesemechanisms.Theseeffortshavehelpedthedevelopmentofpracti- cal catalysts. However, the author believes that catalyst materials should be called“catalysts”onlywhentheusefulnessisprovedintheirpracticalapplica- tions.Thismeansthatthescienceofcatalysisismeaningfulonlywhenthesci- ence is useful for the development of practical catalysts. This book is written from this standpoint and intends to provide the fundamental concepts and knowledgeforthedesignofusefulcatalysts. Catalystsareclassifiedintosolidcatalysts,molecularcatalysts,andbiocata- lysts.Mainsolidcatalystsaremetallicandmixedoxidecatalysts.Inaddition, therearesulfides,halides,carbons,organicpolymers,etc.Molecularcatalysts are coordination compounds and organic molecules. Biocatalysts include enzymesandmicroorganisms.Thesubjectofthisbookisthemixedoxidecat- alystswhicharewidelyusedinindustrialprocessesfortheproductionofmate- rialsandtoabatehazardouscompounds. In thisbook,“catalyst design” isdefined as the rational and efficientpro- cedure for the development of useful catalysts. It provides the guidelines for effective trials (and errors) to obtain practically usable catalysts. For many years the author has attempted to establish the guidelines on the basis of the hypothesesthattheyareacquiredfromthestudiesonmixedoxidessatisfying thefollowingconditions:(1)Thestructuresareregular(crystalline)sothatthe surface can be estimated reliably from the bulk structure (that is, well- characterized mixed oxides); (2) important chemical properties of catalysts, namely, acid–base and reduction–oxidation (redox) properties, can be changed widely and systematically while keeping the fundamental structures; and (3) the catalytic performances of the “well-characterized mixed oxides” are not far from the level required for practical applications. The last condi- tion excludes the use of single crystals in our studies. Thisbookiswrittenaccordingtotheideasandhypothesesdescribedabove. Thesubstantialexamplesareadoptedmainlyfromtheworkoftheauthors’group doneintheline,choosingperovskiteandheteropolycatalysts.Therefore,thisis notanattempttocovercomprehensivelythecatalysisofmixed oxides,butthe ix x Preface importantrelevantfindingsthatprecedetheauthor’sstudiesarereferredto.The achievementsofothergroupsinlinewiththeaboveideasarenaturallyincluded. But important mixed oxide catalysts like zeolites, clays, and supported metal complexarenotincludedordescribedtoalimitedextent. This book comprises five chapters. In Chapter 1, a concise introduction is given to the fundamental knowledge of catalysis required to understand the book, including the concept of catalyst design. In Chapter 2, the chemistry of mixed oxides relevant to catalysis, that is, structures, acid–base, and redox properties, is described. Here, the relationships of those properties with the structure and composition of the solid bulk are emphasized and the roles of solid bulk are stressed, although in some cases the surface different from the solid bulk might be more important in catalysis. Then, in Chapters 3 and 4, the chemistry and catalysis of perovskites and heteropoly compounds are described from the viewpoint of catalyst design. Perovskitesandheteropolycompoundsarechosenbecausethey,respectively, representdoubleoxidesandsaltsofoxoacids, thetwomajorgroupsofmixed oxides, and as described in these chapters, both mixed oxides satisfy the above three conditions for the studies of the catalyst design. Chapter 5 is abouttheuseofmixedoxidesforcatalystsupports.Afewinterestingfeatures of mixed oxides when they are used for catalyst support are described. Thus, this book mainly intends to provide the basic concepts of catalyst design taking as examples perovskites and heteropoly compounds, but it is also intended to provide a general view on the catalysis of mixed oxides. It is further hoped that “designed catalysts” will play a significant role in making the society and the environment greener and more sustainable. Theauthordeeplyappreciatesextensivecollaborationsintheresearchonhet- eropolyandperovskitecatalystsduringhiscareerattheUniversityofTokyo,with the late T. Okuhara, N. Mizuno, K. Y. Lee, K. Inumaru, M. Hashimoto, G.Koyano,K.Tabata,H.Tanaka,thelateY.Yoneda,J.Take,S.Nakata,J.M. Dereppe,andE. A.Lombardo,aswellasmanypostdocsand studentswhoare nowprofessorsandengineers.HealsothanksProfessorsY.Kamiya,H.Niiyama, Y.Ono,Y.Izumi,N.Yamazoe,T.Yamase,K.Domen,K.Asakura,R.K.Grass- elli, W. G. Klemperer, M. T. Pope, I. V. Kozhevnikov, A. Corma, the late J. Haber,andthelateW.K.Hallforvaluablediscussionsandsuggestions.Showa DenkoK.K.andToyotaMotorCorp.arehighlyappreciatedforthediscussion onpracticalapplicationsofmixedoxidecatalysts.Thanksarealsoduetothestaff ofElsevierwhohavepatientlyencouragedtheauthortocompletethisbook. Inaddition,theauthorowesalottomanyeminentchemicalscientistsand engineersintheworldfortheirinspirationandguidanceinthestudyofcatal- ysis. Finally, the author expresses his sincere gratitude to his wife Yoshiko and family for warmest supports. Makoto Misono Tokyo, November 2012 Chapter 1 Basis of Heterogeneous Catalysis Makoto Misono Chapter Outline 1.1. CatalystandCatalysis 2 1.2.3. RateEquationof 1.1.1. RateandEquilibriumof CatalyticReaction 11 ChemicalReaction 1.2.4. ReactorTypeandRate andRoleofCatalyst 2 Expression 14 1.1.2. ThreeEssential 1.2.5. ElucidationofReaction FunctionsofCatalyst 3 Mechanism 15 1.1.3. EssenceofCatalytic 1.2.6. Mass andHeat FunctionsBasedon Transfer 17 ReactionMechanism 4 1.2.7. Deactivationof 1.1.4. AShortHistoryof Catalyst 19 IndustrialCatalysts 5 1.2.8. Comparisonof 1.1.5. Classificationof Heterogeneous, Catalysts 8 Homogeneous 1.1.6. PracticalApplications andBiocatalysis 20 ofCatalysts 9 1.3. CatalystDesign 20 1.1.7. Components andShape 1.4. Preparationand ofIndustrialCatalysts 9 Characterizationof 1.2. RateofCatalyticReactionand Catalysts 22 ReactionMechanism 10 References 23 1.2.1. ReactionRate 10 1.2.2. AdsorptiononSolid Surface;Rateand Isotherm 11 Fundamental concepts and principles indispensable to understand the hetero- geneous catalysis of mixed oxides are described in this chapter [1]. StudiesinSurfaceScienceandCatalysis,Vol.176.http://dx.doi.org/10.1016/B978-0-444-53833-8.00001-6 ©2013ElsevierB.V.Allrightsreserved. 1 2 StudiesinSurfaceScienceandCatalysis 1.1 CATALYST AND CATALYSIS Catalystisasubstancethatispresentinasmallamountinthereactionsystem and accelerates the desired chemical reaction(s), but little changes during the reaction. Catalysis is a general term for the function of catalyst. Almost all materials and goods that are daily used are produced through catalytic pro- cesses from various raw materials. Many high-performance catalysts are used, in order to utilize efficiently raw materials, including recycling of used materials, and to utilize efficiently the existing as well as “new” energy sources. Catalysts are used not only for chemicalsyntheticprocessesbutalsoforotherusessuchaselectrodesoffuel cells and batteries. In addition, catalysts are used to protect and improve the environment. The latter catalysts are called environmental catalysts, that is, “KankyouShokubai”inJapanesewhichisanewwordfirstusedbytheauthor in the late 1970s. 1.1.1 Rate and Equilibrium of Chemical Reaction and Role of Catalyst A catalyst changes (accelerates in most cases) the rate of chemical reactions to approach the equilibrium but does not change the equilibrium itself. It is importanttodistinguishbetweenthekinetics(rateofreaction)andthermody- namics (equilibrium constant of reaction). The difference may be evident in Fig. 1.1, which schematically shows the case of a simple reversible reaction 100 %) A ( of n o ati ntr A B e c n Co Higher activity [A] e B A 0 Reaction time FIGURE1.1 Timecourseofareversiblereaction,A$B.Theconcentrationreachesfasterthe equilibriumconcentration[A] overacatalystwithahigheractivity,buttheequilibriumconcen- e trationisthesame. Chapter 1 BasisofHeterogeneousCatalysis 3 of A$B. The rate to approach the equilibrium composition changes from a catalysttoanother,buttheequilibriumfinally attainedisthesame.Ifthefor- wardandreversereactionsarefirstorder,theratesareexpressedbyEq.(1.1)as RateðforwardÞ¼k½A(cid:2), andrate ðreverseÞ¼k0½B(cid:2) (1.1) Attheequilibrium,theratesofforwardandreversereactionsareidentical, then, k½A(cid:2)¼k0½B(cid:2) (1.2) where the ratio of [B]/[A] is being determined by the equilibrium constant, K¼k/k0. With an active catalyst, the rate to approach the equilibrium is fast, andfor alessactivecatalystit isslower.Therefore,both forward andreverse reactions are faster over a more active catalyst at the equilibrium, while both rates are identical for each case. 1.1.2 Three Essential Functions of Catalyst Catalyticactivity,selectivity,anddurability(¼catalystlife)arethethreemost important functions of catalyst. Activity is most fundamental. A reaction which does not occur in the absence of catalyst could proceed in the presence of catalyst. With a more active catalyst, the production rate per volume of reactor becomes larger and the reactor volume can be made smaller. Selectivityisthemostinterestingandattractivefunctionofcatalyst,which selects one (or more) desirable reaction(s) among many reactions that would possibly occur. Choosing a reaction which produces a thermodynamically unfavorablebutvaluableproductisoneofthemostattractivefunctionsofcat- alyst. A fascinating function of catalyst is stereoselectivity that produces one of the two stereoisomers which have thermodynamically the same stability. Theselectivitymaybedividedintotwocategories(Eq.1.3).Oneistoselect oneproductamongseveralpossibleproductsstartingfromonereactingmolecule, e.g.,choosingonlyAtoB(Eq.1.3a).Thismaybecalled(a)product-selectivity. Anothertypeistoselectonereactantinamixtureofseveralreactantschoosing A from a mixture of A and B (Eq. 1.3b). This is called (b) reactant-selectivity (calledsubstratespecificityinenzymaticreactions): (a) A B C, (b) A C ð1:3Þ D B D Durability(orcatalystlife)isindispensableforcommercialcatalysts.Cat- alyst is sometimes defined with a statement that catalyst and its performance donotchangeduringthecatalyticreaction.But,inreality,itchanges(usually itgraduallydeteriorates),andafteracertainperiod,ithastobesubstitutedbya fresh catalyst or reactivated by an appropriate method. Most of commercial 4 StudiesinSurfaceScienceandCatalysis catalysts are synthesized through many elaborate steps and often contain expensiveelements,sothat,unlessthecatalystlifeislongenough,thecatalytic processwouldbeverycostlyandneverbecommercialized.Commercialcata- lysthasusuallyalifeofseveralmonthsto10years,andcontinuousreactivation issometimesnecessaryasinthecaseoffluidizedcatalyticcracking. 1.1.3 Essence of Catalytic Functions Based on Reaction Mechanism Figure 1.2 illustrates schematically the reaction mechanism of oxidation of carbon monoxide catalyzed by palladium (Eq. 1.4). This was demonstrated for a palladium single crystal a long time ago by G. Ertl using techniques, sophisticated at that time, under ultrahigh vacuum. Actual reaction mecha- nismunderordinaryconditionsmaybedifferent,butthisdiagramissufficient to understand the essence of catalytic process: COþð1=2ÞO (cid:3)(cid:3)(cid:3)(cid:3)!CO (1.4) 2 2 Pd Without catalyst 1 CO+––O 2 2 Gas phase With catalyst 250 284 106 CO O ~20 2 O O Gas phase C O C Pd Pd CO and O adsorbed CO adsorbed 2 FIGURE1.2 EnergydiagramofCOþ(1/2)O !CO catalyzedbypalladium.Theinitialandfinal 2 2 statesdonotchangebythepresenceofcatalyst.ThenumbersaretheenergydifferencesinkJ(cid:3)1mol(cid:3)1. Chapter 1 BasisofHeterogeneousCatalysis 5 In this figure, three essential points of catalytic functions are to be noted. First, the oxygen molecule in which two oxygen atoms are strongly bonded (494kJmol(cid:3)1)dissociatesintotwooxygenatomsonthesurfaceofpalladium with a very low energy barrier. This is due to the formation of two oxygen- palladium bonds that occurs simultaneously with the bond breaking of O . 2 Second, the oxygen atoms formed are reactive. If they are too stable, the subsequent reactions would not take place. Third, after one cycle of catalytic oxidation of CO to CO , as in Fig. 1.2, the catalyst surface returns to the ini- 2 tial state. Upon each cycle, one molecule of carbon dioxide is produced. From the second point, the basic principle for the selection of a good cat- alyst is deduced. As stated above, if the oxygen atom adsorbed on the cata- lyst surface is too stable, the subsequent steps become endothermic and would not take place. This is because the initial and final states of the reac- tion (both in the gas phase) do not change by the presence of catalyst. If the first step is too exothermic, the first step may occur very easily, but the fol- lowing steps become inevitably very endothermic and hard to proceed. On thecontrary,ifthefirststepisveryendothermic,thefirststepwouldnottake place. Thus, the key intermediate, oxygen atom adsorbed in this case, must have moderate thermodynamic stability, the energy level being not far either from the initial or from the final state of the reaction. In other words, a good catalyst must have moderate affinity to the key intermediate. In the case of palladium, this condition is satisfied as seen in Fig. 1.2. The appropriate sta- bility of the key intermediate is the first basic principle for selection of a good catalyst. Inalatersection(section2.1.5),theselectionofgoodmetaloxidecatalyst for oxidation will be discussed on the basis of redox (or Mars–van Krevelen) mechanism. The essential idea there is the same as described here. 1.1.4 A Short History of Industrial Catalysts Catalysts have been used for large-scale productions of chemicals since the beginning of the twentieth century. Examples are Pt for SO oxidation, 2 Fe for NH synthesis, and Zn–Cr oxide for methanol synthesis. In the case 3 of Fe catalyst, it took almost 10 years until the industrial production started in 1913, after the Fe catalyst was discovered. During the 10 years, materials andequipmentsnecessaryforthehigh-pressureprocessweredeveloped.This example shows that, in order to industrialize a catalytic process, many exist- ing as well as newly developed technologies have to be integrated. Another goodexampleisso-calledthree-waycatalystfortheemissioncontrolofauto- mobile. In this case, an elaborately synthesized multicomponent catalyst is integrated to an emission control system, combined with a monolith support, an oxygen sensor, and a computer-controlled device for fuel supply. Table 1.1 collects important industrial catalysts which were commercia- lized in the past. 6 StudiesinSurfaceScienceandCatalysis TABLE1.1 MajorIndustrial Catalystsandthe Year ofInvention Year Process(catalyst) Remarks (cid:4)1890 SO oxidation(Pt) SubstitutionofNO catalystand 2 x substitutedbyVoxidecatalyst 1913 NH synthesis(Fe) FixationofN ,high-pressure 3 2 technology 1916 Aceticacidfromacetylene(Hg) Viaacetaldehydeformedbyhydration 1925 Methanolsynthesis(Zn–Cr) Developmentofhigh-pressure technology 1936 Crackingofoil(clay) Gasoline,solidacidcatalyst 1939 High-pressurepolyethylene(O ) Radicalpolymerization 2 (cid:4)1945 Hydroformylationofolefin Oxoprocess (Cocomplex) 1949 Reformingofnaphtha(Pt–alumina) Gasoline,bifunctionalcatalysis 1954 Industrialproductionofzeolite Crystallinesolidacid 1955 Low-pressurepolyethylene Zieglercatalyst (TiCl–AlR ) x 3 1957 Polypropylene(TiCl–AlR ) Stereoselectivepolymerization x 3 1960 Aceticacidfromethylene Wackerprocess (PdCl –CuCl ) 2 2 1963 Ammoxidation(Mo–Bi–O) Allylicoxidationofpropylene, SOHIOprocess (cid:4)1965 Hydrotreatmentofheavyoil HDS,cleanfuel (Mo–Co–S) 1967 Improvedreformingofnaphtha Bimetalliccatalysts (Pt–Re–alumina) (cid:4)1968 Shapeselectivity(zeolite) Isomerization,alkylation 1969 L-Aminoacid(fixedenzyme) Industrialuseofenzyme (cid:4)1970 Improvedammoxidation Multi-componentMo-Bi-O (Mo–Bi–Fe–O) 1973 Aceticacidfrommethanol Monsantoprocess (Rhcomplex) 1974 L-DOPA Asymmetriccatalysisbycomplex (cid:4)1975 SCR(V–Ti–O) Environmentalcatalyst 1976 MTGprocess(zeolite) Diversifiedsourceofhydrocarbons Chapter 1 BasisofHeterogeneousCatalysis 7 TABLE1.1 MajorIndustrialCatalystsandtheYearofInvention—Cont’d Year Process(catalyst) Remarks (cid:4)1976 Emissioncontrolofautomobile Three-waycatalyst(TWC) (Pt,Rh,Pd) 1976 Isopropanolfrompropylene FirstindustrialprocessbyHPA 1985 Acrylamide(bio-organism) Firstindustrialprocessbybiocatalyst 1997 Metallocenecatalystforpolyolefin Single-sitecatalyst (cid:4)2002 Long-lifeautomotivecatalyst Improvedsupportandpreparation 2003 e-Capolactum(gasphase) Greencatalyst (silicalite) (cid:4)2003 High-performanceHDS Sulfur-freefuels HDS,hydrodesulfurization,SCR,selectivecatalyticreductionofNOx,MTG,methanoltogasoline, andHPA,heteropolyacids. Starting molecule: acetylene ethylene methanol Catalyst: Hg salt Pd salt Rh complex Raw material: coal oil variable (natural gas, oil, coal, biomass) SCHEME1.1 Changeofprocessforaceticacidproductionwiththechangesinrawmaterialand catalyst. As seen from the table, many excellent catalysts have been introduced to copewiththesocialneeds,suchaschangesinrawmaterials,desiresforbetter materials, and environmental requirements. A good example may be the production of acetic acid. It was produced from wood in early age, but along with the change in the raw materials, new catalysts were developed (see Scheme 1.1). Another good example is solid acid catalyst. Catalytic cracking of oil to gasoline was first carried out using acidic clay to replace thermal cracking, in order to improve the yield of gasoline. Later synthetic amorphous silica– alumina was introduced in place of clay catalyst, and then it was followed by crystalline zeolites. Now new zeolite catalysts combined with amorphous silica-alumina are commercially used in a fluidized-bed reactor. In the mean time, catalytic performance and understanding of the nature of solid acids made remarkable progress. This was in parallel with the development of oil industry.
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