Catalysis by Zeolites

Transcription

1 EUCHEME /08/0 Catalysis by Zeolites CRERG - IBB Department of Chemical Engineering Instituto Superior Técnico IST, Lisbon Carlos Henriques

2 EUCHEME /08/0. Introduction to Zeolites. Composition, Porous Structure and Active Sites. Shape-Selectivity with Zeolites. Zeolites in Industrial Catalysis a. Catalytic cracking b. Hydrocracking c. Metanol to gasoline (MTG) 5. Zeolites and Green Chemistry

3 Zeolites EUCHEME /08/0 Swedish mineralogist A.F. Crönstedt upon rapidly heating, the mineral Stilbite produces large amounts of steam from adsorbed water hydrated calcium aluminium silicate (natural zeolite) ZEOLITE (zeo boiling; lithos stone) A.F. Crönstedt, Akad. Handl. Stockholm,8 (756) 0

4 Zeolites EUCHEME /08/0 Crystalline alumino-silicates, with regular open tridimensional nanosized porous framework ZSM-5 (MFI) MORDENITE (MOR)

5 What is so special about zeolites? EUCHEME /08/0 Zeolites have pores with nanosized dimensions ( nm) Shape Selectivity As crystalline materials, zeolites present a narrow range of pore sizes gives better selectivity than non-crystalline materials 5

6 What is so special about zeolites? EUCHEME /08/0 Ion-exchange properties Acidity Transition metals Catalytic Active sites 6

7 Pore Size EUCHEME /08/0 Kinetic Diameters of molecules, when compared with Zeolite Pore Sizes 0.8 Zeolite-X, Y 0.6 ZSM-5 0. Zeolite-A 0. helium argon xenon methane propane butane ethene propene -butene iso-butane iso-octane cyclopropane cyclohexane benzene p-xylene o-xylene 0 ammonia water hydrogen carbon monoxide carbon dioxide oxygen nitrogen 7 Kinetic diameter (nm)

8 Zeolite Framework () EUCHEME /08/0 Zeolite framework is composed of SiO and AlO tetrahedral units (Al, Si T- atoms), sharing oxygen between every two consecutive units 8

9 Zeolite Framework () EUCHEME /08/0 Arrangement of Primary Building Units solid tetrahedral oxygen tetrahedral negative charge TO O - Al or Si -oxygen 9

10 Zeolite Framework () EUCHEME /08/0 How zeolites are built 0

11 Zeolite Framework () EUCHEME /08/0 Cations (Na +, NH, H +, transition metals) located inside the channels or cavities of zeolites, to balance negative charges in the framework: Exchange Positions Zeolite sodium form

12 Zeolite Framework (5) EUCHEME /08/0 FAU structure (Y zeolite) Exchange sites. nm Al positions Framework and EFAL Supercage ~.7 nm

13 Zeolite Framework (6) EUCHEME /08/0 Y zeolite (FAU structure). nm 0.7 nm 0.77 nm ZSM-5 (MFI structure)

14 LTA NAT MOR ANA LTL TON SEM photos W.J. Mortier, Leuven Zeolites Morphology EUCHEME /08/0

15 Why Zeolites as Catalysis? EUCHEME /08/0 acidity (both Brönsted protons and Lewis electrons acceptors acid sites) ion-exchange capacity shape selectivity (separations, catalysis) confinnement effects (small cages act like nano- reactors) allows the stabilization of particular species of metal active sites acceptable stability of both framework and active sites 5

16 Key Structural Features in Zeolites EUCHEME /08/0 Zeolites can be synthesized with a wide range of pores sizes and shapes Composition (Si/Al ratio) can be modified during synthesis or by post-synthesis synthesis treatments (dealumination; desilication) Pure silica zeolites (e.g., silicalite, with MFI structure) tend to be hydrophobic High alumina content zeolites have significant amount of charge balancing extra- framework cations (in exchange positions) and have a very high affinity to polar molecules: hydrophilic 6

17 Composition of Zeolites EUCHEME /08/0 Zeolites are crystalline microporous alumino-silicates, constituted by a tridimensional arrangement of TO tetrahedra, linked by oxygen atoms, forming different construction units and large frameworks, where identical blocks constitute unit cells : M n+ x/n (AlO ) x (SiO ) y n M cation charge x+y number of tetrahedra per unit cell y/x atomic Si/Al ratio 7

18 Porous Structure () EUCHEME /08/0 Porous Structure: most zeolites have pores in a -0 Å (0.- nm) range They can act as sieves, allowing to separate molecules by their size Molecular Sieves 8

19 Porous Structure () EUCHEME /08/0 Most zeolites are classified accordingly to the number of oxygen atoms in the opening ( ring ) of larger pores: small-pore zeolites, with 8-membered oxygen rings and a free diameter of -.5 Å medium-pore zeolites, with 0-member oxygen rings and a free diameter of.5-6 Å large-pore zeolites, with -member oxygen rings and a free diameter of 6-8 Å 9

20 Porous Structure () EUCHEME /08/0 Porous Structures: letters code, accordingly to IZA definition: FAU, MFI, MOR, BEA, supercages hexagonal prysms sodalite cages Porous Structure of some zeolites: a) FAU b) MOR c) MFI 0

21 Active Sites in Zeolites () EUCHEME /08/0 Active sites in zeolites Each zeolite can be obtained with a large range of compositions, both during synthesis or by post-synthesis modifications Furthermore, different compounds can be introduced (or even synthesized) inside zeolite pore system Consequently, zeolite can behave like acid catalysts, basic catalysts, redox catalysts, metal catalysts, bi (multi)-functional catalysts

22 Active Sites in Zeolites () EUCHEME /08/0. Acid Catalysts: cracking, isomerization Several hydrocarbon reactions are catalysed by Brönsted acid sites (proton donators) Lewis acid sites (electron acceptors ex: extraframework Al species) often seem not participate directly in the reactions mechanism, but can increase Brönsted acidity strength

23 Active Sites in Zeolites () EUCHEME /08/0 in acidic catalysis, zeolites activity depend on the number of protonic sites and on their intrinsic activity; active sites located in very small micropores, where reactants cannot access, are inactive; even for accessible sites, different issues related to reaction intermediate species can determine catalysts activity; important parameters are also acid strength and acid density (namely for bimolecular reactions)

24 Active Sites in Zeolites () EUCHEME /08/0 Acid Catalysts sodium form ion-exchange ammonium form thermal treatement Brönsted Acid Sites dehydroxylation (- H O) Brönsted acidity Lewis Acid Sites Basic sites

25 Active Sites in Zeolites (5) EUCHEME /08/0 Acid Catalysts Protonic acid sites comes essentially from bridged hydroxyl groups in the framework.. Al-(OH)-Si (no Al-(OH)-Al exist: Lowenstein rule) The maximum concentration of protonic acid sites is equal to the concentration of Al atoms in the framework of the zeolite BUT the real concentration is usually lower: noncrystalline fraction, dehydroxylation or even dealumination, during thermal treatments at high temperature (T> ~50ºC) decrease Al species in framework. 5

26 Active Sites in Zeolites (6) EUCHEME /08/0 Acid Catalysts Acid sites strength in zeolites is much higher than in amorphous silica alumina: A (zeolite) B (amorph. Si/Al) 6

27 Active Sites in Zeolites (7) EUCHEME /08/0 Acid Catalysts in zeolite structures (A), Al-O and Si-O bonds are almost equivalent and the strong interaction Al-O result in a weaker bond O-H, increasing the strength of the proton on the contrary in amorphous silica-alumina (B), the acid site is represented by a silanol with a weak acid-base interaction between OH group and the Al atom 7

28 Active Sites in Zeolites (8) EUCHEME /08/0 Acid Catalysts there is a relationship between the protonic acid strength and the angle of TOT bonds (T = Si or Al). The higher the angle, the strongest the acid sites: H-MOR (º-80º) > H-MFI (º-77º) > H-FAU 8º-7º 8

29 Active Sites in Zeolites (9) EUCHEME /08/0 Acid Catalysts protonic acid sites strength depend on their proximity: is maxima when sites can be considered isolated (Barthomeuf et al.), i.e., when sequences Al- O-Si-O-Al does not exist (but mainly...si-o-al-o-si-o- Si ) relationship between acidic activity and Al concentration has been shown for different reactions and it is only observed for pure protonic zeolites, without any extra-framework aluminium species, which are obtained by dealumination 9

30 Active Sites in Zeolites (0) EUCHEME /08/0. Metal Catalysts redox and hydrogenation sites Metal catalysis in zeolites is carried out by metallic species introduced in zeolite pores or framework during synthesis or post-synthesis modifications 0

31 Active Sites in Zeolites () EUCHEME /08/0 Metal Catalysts Metal Species can be located: in the zeolite framework, like Silicalite- (TS-), a pure titanium MFI structure molecular sieve; in extra-framework locations; in exchange positions (transition metals able to change their valence); impregnated in the pore system (as when metal oxides are introduced).

32 Active Sites in Zeolites () EUCHEME /08/0 Bifunctional Catalysts different catalytic processes run in the simultaneous presence of different type of active sites (catalytic functions) multifunctional catalysis bifunctional catalysts e.g.,acidic and hydrogenating functions, are mainly used in refining processes: light alkanes isomerization, hydrocracking, catalytic dewaxing, aromatization of light alkanes, isomerization of C 8 aromatic fraction

33 Active Sites in Zeolites () EUCHEME /08/0 Bifunctional Catalysts Hydrogenating/dehydrogenating catalytic functions are usually fulfilled by: dispersed transition (Ni) or noble metals (Pt, Pd, ) in exchange positions in the zeolite or in another support (Pt/Al O ) metal (Ni, Co, Mo) sulphides, when sulphur is present in the feed metal oxides (Ga O /MFI) as in the aromatization of C 6 -C 7 cuts

34 EUCHEME /08/0 How did nanoporosity influences catalysis? Two ways, mainly:. Shape selectivity P.B. Weisz et al., J. Phys. Chem. 6 (960) 8. Confinement effects E. G. Derouane, J. M. André, A. A. Lucas, J. Catal 0 (988) 58 Zeolite cages, channels and channels intersections really act like nanoreactors

35 Shape Selectivity () EUCHEME /08/0 Shape Selectivity is a particular type of selectivity that is originate in the fact that active sites, in zeolites, are included in a microporous framework with dimensions similar to those of molecules of reactants and products This micropore framework is constituted by cages, channels and channels intersections, that can really be considered as nanoreactors (or molecular reactors) 5

36 Shape Selectivity () EUCHEME /08/0 It means that their shape and size, the shape and size of the inter-connecting rings will determine the selectivity of catalyzed reactions This micropore system will also influence the activity and stability of zeolite catalysts Furthermore, this particular type of catalyst structure (framework) allows the stabilization of particular, well defined and relatively homogeneous types of active sites (i.e., metal active sites) 6

37 Shape Selectivity () EUCHEME /08/0 Consequently, it becomes a real possibility, with zeolite catalysts i. to tune active sites properties ii. to choose the appropriate zeolite structure in order to obtain, for a given reaction (or set of reactions) an ideal catalyst on base of a scientific approach 7

38 Shape Selectivity () EUCHEME /08/0 Molecular Sieving Reactant Selectivity Branched molecules do not access channels nc 6 /ic 6 separation + nc 6 transformation Product Selectivity Bulker products are formed insid pores but their exit is hampered by slower diffusion toluene dismutation w/ increasing p-xy 8

39 Shape Selectivity (5) EUCHEME /08/0 I - Molecular sieving of reactants Reactant type shape selectivity: competitive cracking of n- octane and,,- trymethylpentane, the last being too bulky to enter the pores of the zeolite and is hindered from reaching the active sites inside pores. - n-octane, on the contrary, is readily converted 9

40 EUCHEME /08/0.5 nm First Shape Selectivity to be found is in the base of Selectoforming Process (Mobil) Erionite Zeolite Selective dehydration of -butanol over a 5A zeolite (0.5nm pores), in a mixture with -butanol or iso-butanol Selective hydrogenation of n-butene in a mixture with isobutene over a Pt-5A catalyst 0

41 . Shape Selectivity (6) EUCHEME /08/0 i) Molecular Sieving of Reactants (cont) This Molecular Sieving phenomena is based in the difference of the rate of diffusion of the considered molecules (that can be expressed by the ration between their Diffusion Coefficients D A /D B ): Shape Selectivity usually occurs when this ratio tends to infinite one of the molecules doesn t enter in the pores, so it cannot diffuse!!! Nevertheless, lower ratios can also configure a Shape Selectivity behaviour, depending on the relative rates of diffusion and reactivity

42 . Shape Selectivity (7) EUCHEME /08/0 ii) Molecular Sieving of Products Applied by Mobil to p-xylene synthesis over ZSM-5 zeolite processes: Product Selectivity: Toluene dismutation Bulkier products can be formed inside pores but their exit is hampered by slower diffusion benzene o-xylene m-xylene p- xylene

43 . Shape Selectivity (8) EUCHEME /08/0 ii) Molecular Sieving of Products the formation of bulkier products become limited by their desorption diffusion coefficient of p-xylene is several orders greater than o- or m-xylenes in ZSM-5 catalyst furthermore, o- and m- isomers, trapped inside zeolite structure, are converted in p-xylene for other reactions, when inter-conversion is not rapid, entrapped species concentration can increase and deactivate the catalysts

44 . Shape Selectivity (9) EUCHEME /08/0 ii) Molecular Sieving of Products Toluene Ethylene Product shape-selectivity: acid catalyzed alkylation of Toluene with Ethylene (aromatic C 8 formation) Both reactants are small enough to enter the zeolite pores, but from the potential products (o-, m- and p-ethyltoluene), only the slim p-ethyltoluene is small enough to leave the pore system

45 . Shape Selectivity (0) EUCHEME /08/0 Molecular Sieving Selectivity depends on the relative reaction and diffusion rates the diffusion coefficients ratio (size of molecules and pores sizes) length of diffusion (zeolite crystallites size) There are the possibility of deactivate external (non-shape selective) active sites, by coke deposition, selective poisoning (bulkier bases), Increasing global Shape Selectivity 5

46 . Shape Selectivity () iii) Transition-State Selectivity EUCHEME /08/0 Reaction intermediates and/or transition states are sterically limited by available space near active sites Dimethy-benzene dismutation over HMOR catalysts diphenyl - methane intermediates Reactants and products can easilly diffuse in catalyst structure, BUT intermediairy species formation, in the vicinity of active sites (cages, channels, channels intersections) are sterically limited: no,,5 trimethylbenzene is formed!!! 6

47 . Shape Selectivity () iii) Transition-State Selectivity In an appropriate size zeolite no room exists for the formation of bimolecular reaction intermediates: no toluene or trimethyl benzenes are observed!!! 7

48 . Shape Selectivity () EUCHEME /08/0 iii) Transition-State Selectivity Transition State Shape Selectivity: m-xylene can undergo acid-catalyzed isomerization into p-xylene (and o-xylene, omitted from the figure) and transalkylation into toluene and one of the trimethylbenzene isomers. Transalkylation is a bimolecular reaction and bulkier intermediate species are formed, when compared with monomolecular isomerization Shape Selective catalysts hampered their formation 8

49 . Shape Selectivity () EUCHEME /08/0 iii) Transition-State Selectivity Contrary to Molecular Sieve effect, Transition-State Selectivity does not depends on crystallites sizes, on relative rates of reaction and diffusion, on Diffusion Coefficients ratio, Only depends of the porous structure of the zeolite and the size of transition-state species. 9

50 . Shape Selectivity (5) EUCHEME /08/0 iii) Transition-State Selectivity This transition-state effect can co-exist with molecular sieve effects (namely products); Transition-State Selectivity mainly concerns all transformations that occur by inter-molecular (bimolecular) reactions that originate intermediary species bulkier than those arising from monomolecular reactions, for similar reactants. 50

51 Shape Selectivity (6) EUCHEME /08/0 iii) Transition-State Selectivity This can explain the key role of the porous structure of zeolite catalysts on reactions mechanism, in the case that both type of interactions (intra- or inter-molecular) are possible. 5

52 Shape Selectivity (7) EUCHEME /08/0 iii) Transition-State Selectivity Transition-State Selectivity also plays a key role in what heavy adsorbed products (coke) formation is concerned: Coke formation occurs via bimolecular steps as condensation reactions, that are very sensitive to steric constraints Coke rate formation is strongly dependent of the size of zeolite structure: smaller cavities do not favours coke formation inside pores 5

53 Shape Selectivity (8) EUCHEME /08/0 Confinement Effect () Confinement Effect Is due to the strong interaction between zeolite frameworks and molecules: zeolites act as solid solvents One of the most important consequences of this solid solvent effect, is that the concentration of reactants is much higher inside the structure than it is outside This concentration of reactants inside the pores presents a positive effect on reactions rates This effect evidences the role of interaction forces between molecules and zeolite framework. 5

54 Shape Selectivity (7) EUCHEME /08/0 Confinement Effect () The very important increase of reactants concentrations inside zeolite pores is one of the explanations for the high activity of zeolite catalysts, when compared with other structures Other important issue related to Confinement Effect is the fact that bimolecular reactions play a major role in zeolite catalysts when compared with other structures: zeolites allowed to put in evidence, for the first time, the bimolecular character of different reactions. 5

55 Shape Selectivity (8) EUCHEME /08/0 Confinement Effect () Kinetic models must take into account, besides classic chemisorption steps on reaction mechanisms, physisorption steps on micropores (Langmuir model) Assuming a simple first order reaction A B, the correct expression for the reaction rate is not r = k.[a] but is better described by Langmuir- Hinshellwood kinetics: r = k. θ A (θ A fraction of sites occupied by A) r = k.k A.[A]/( + KA.[A] + K B.[B]) 55

56 Shape Selectivity (Conclusions) EUCHEME /08/0 Shape Selectivity phenomena clearly highlight the important role of the size and shape of pore, cages and channels that constitute the zeolite framework Zeolite nanopore structure (succession of cages act as nanoreactors) makes zeolite unique tools for the development of selectivity in heterogeneous catalysis 56

57 Shape Selectivity (Conclusions) EUCHEME /08/0 In all processes that use zeolites as catalysts, their activity, selectivity and stability depends not only on the type of active sites, but also on their location inside the zeolite structure. Shape Selectivity properties of zeolites constitute one of the main reasons for the applications of these solids in catalysis 57

58 Zeolites in Industrial Catalysis () EUCHEME /08/0 Zeolites are widely used both in refining and petrochemical industries We will look for three different processes where zeolites are used Catalytic Cracking (acid catalysis) Methanol to Gasoline (MTG/Mobil) (acid catalysis with shape selectivity) Hydrocracking of paraffins (bifunctional catalysis) 58

59 Zeolites in Industrial Catalysis () EUCHEME /08/0 Simplified flowsheet diagram of refinery operation 59

60 Zeolites in Industrial Catalysis () EUCHEME /08/0 Catalytic Cracking () Catalytic Cracking - Fluidized Catalytic Cracking/FCC intends to transform heavy products in a refinery process (mainly from vacuum distillation) into lighter products: gasoline, C -C 7 olefins FCC catalyst is mainly a FAU zeolite in acidic (protonic) form: USHY (Ultra-Stable HY) Ultra-Stability arises from controlled dealumination (during preparation) leading to a more resistant framework (high T and H O presence) 60

61 Zeolites in Industrial Catalysis () EUCHEME /08/0 a. Catalytic Cracking FCC processes are quite flexible, as they can transform different fractions (paraffinic, naphthenic or aromatics). One of the key points on these process is the formation of significant amounts of heavy unsaturated polyaromatic surface products (coke, -.5% of the feed) Catalysts Deactivation Regeneration 6

62 Zeolites in Industrial Catalysis (5) EUCHEME /08/0 Catalytic Cracking Coke rapidly deactivates the catalyst that needs to be continuously regenerated by coke burning Heat released during regeneration is recovered for the endothermic cracking process 6

63 Zeolites in Industrial Catalysis (6) EUCHEME /08/0 atalytic Cracking FCC Process ºC coke -.5% Catalyst stripper ºC ºC FEED Pre heating ºC; coke < 0.05% ºC 6

64 Zeolites in Industrial Catalysis (7) EUCHEME /08/0 Catalytic Cracking - Main Reactions Compounds Reaction Products Paraffins cracking paraffins + olefins cracking olefins cyclization naphthenes Olefins* isomerization iso-olefins (hydrogen transfer) iso-paraffins hydrogen transfer paraffins cyclization, condensation dehydrogenation coke cracking olefins Naphthenes dehydrogenation cyclo-olefins (hydrogen transfer) aromatics isomerization naphthenes w/different rings cracking of side chain unsubstituted aromatics + olefins transalkylation alkylaromatics Aromatics dehydrogenation, polyaromatics (dehydrogenation condensation) coke condensation * - mainly from cracking but also as charge impurities 6

65 Zeolites in Industrial Catalysis (8) EUCHEME /08/0 Catalytic Cracking- The Mechanism () The mechanism of catalytic cracking proceeds via the formation of charged organic species carbocations as intermediate species 65

66 Zeolites in Industrial Catalysis (9) EUCHEME /08/0 Catalytic Cracking- The Mechanism () st step corresponds to the formation of carbocations (R + ) - Initiation: i) hydride abstraction on a Brönsted site ii) hydride abstraction on a Lewis site iii) paraffin protonation on a Brönsted site (formation of penta-coordinated species) iv) olefins (as impurities of the feed or arising from thermal cracking) protonation 66

67 Zeolites in Industrial Catalysis (0) EUCHEME /08/0 Catalytic Cracking- The Mechanism (). Initiation (Hydride abstraction) (carbocation) R -CH -CH -R + H + Z - R -CH C + H - R + Z - + H (paraffin) (Brönsted site) (Hydride abstraction) (carbocation) R -CH -CH -R + L + R -CH -C + H - R + HL (paraffin) (Lewis site) (Protonation) (carbocation) R - CH = CH - R + H + Z - R -CH C + H - R + Z - (olefin/feed) (Brönsted site) 67

68 Zeolites in Industrial Catalysis () EUCHEME /08/0 Catalytic Cracking- The Mechanism () nd step corresponds to the cracking of C-C bonds via β-scission of formed carbocation (cracking takes place in the C-C bond in β position, considering the positive charge) 68

69 Zeolites in Industrial Catalysis () EUCHEME /08/0 Catalytic Cracking- The Mechanism (5) Chain Propagation (hydrogen transfer reactions) R -CH -C + H-R + R -CH -CH -R R -CH -CH -R + R -CH -C + H-R (carbocation) (paraffin) (paraffin) (carbocation) Cracking (β scission) R -CH C + H - R R + + CH = CH - R (carbocation) (carbocation) (olefin) Chain Termination FCC - important source of olefins Occurs when the surface carbocations are desorbed (as olefins) and the Brönsted sites of the catalyst are regenerated: R + R = + H + 69

70 Zeolites in Industrial Catalysis () EUCHEME /08/0 FCC catalysts - The HY zeolite () Sodalite cage TO tetrahedra T = Si, Al Final Structure: Sodalite cages linked by hexagonal prisms, in a tetrahedral arrangement, defining supercages (Å) 70

71 Zeolites in Industrial Catalysis () EUCHEME /08/0 FCC catalysts - The HY zeolite () The industrial cracking catalyst: 5-0% HY zeolite in a ceramic matrix (alumina, silica, amorphous silica-alumina) This matrix intents to protect zeolite crystallites: (i) from abrasion, (ii) retaining metal species (V, Ni as asphaltenes) and (iii) N - containing molecules (acidity poisons) from HC feed 7

72 Zeolites in Industrial Catalysis (5) EUCHEME /08/0 FCC catalysts - The HY zeolite () The fresh zeolite continuously fed to the raiser is rapidly deactivated; It spends ~80% of time inside regenerator (coke burning (680 ºC < T < 750 ºC), in the presence of steam; Under this conditions, zeolite suffers dealumination (extraction of aluminium atoms from the framework). Such a phenomena, if in a large extent, can destroy the zeolite structure collapse 7

73 Zeolites in Industrial Catalysis (6) EUCHEME /08/0 FCC catalysts - The HY zeolite () To avoid this: a previous controlled and limited dealumination treatment (high temperature steaming followed by acid leaching of extra-framework aluminium atoms, EFAL) is performed during preparation Ultra Stable Y zeolite elimination of EFAL atoms is important, as their presence increases protonic acidity strength and coke and gases formation, become too important. this controlled dealumination also results in the formation of some mesoporosity, what facilitates the diffusion of bulkier molecules 7

74 Zeolites in Industrial Catalysis (7) EUCHEME /08/0 FCC catalysts - The HY zeolite (5) Dealumination also results on a decrease of acid sites, although an increase of acid strength of remain sites is normally observed As acid sites become more isolated, (less sites density), so bimolecular reactions like hydrogen transfer are reduced: FCC products become more olefinic and less aromatic. Also condensation reactions (coke formation) are reduced 7

75 Zeolites in Industrial Catalysis (8) EUCHEME /08/0 FCC - CONCLUSIONS Very important process to obtain transportation liquid fuels from crude; Y zeolite in acidic form are normally used in a dealuminated-stabilized form Continuous regeneration of catalyst is required to maintain its activity (coke formation) Heath balance is reached in industrial processes: heath released during regeneration is enough to balance endothermal reactions 75

76 Zeolites in Industrial Catalysis (9) EUCHEME /08/0 FCC - CONCLUSIONS Hydrogen transfer reactions (bimolecular reactions) increase paraffins and aromatics yield This effect is more important in zeolites, due to: i) the concentration of reactants increases (confinement effect) ii) the density of acid sites is higher in zeolites Aromatic rings are not transformed in catalytic cracking Release of pollutants (NOx, SOx, COx) is a major concern 76

77 Zeolites in Industrial Catalysis (0) EUCHEME /08/0 Simplified flowsheet diagram of refinery operation 77

78 Zeolites in Industrial Catalysis () EUCHEME /08/0 Hydrocracking - Bifunctional Catalysts () Transformation of heavy feedstocks in the presence of a high pressure of hydrogen (~00 bar) Hydrocracking presents a higher flexibility in what concerns both the charges to transform: from heavy gasoline to cuts from vacuum distillation, in a refinery the obtained products: gasoline but mainly middle distillates (jet, diesel, fuel, lube oil) accordingly to the market 78

79 Zeolites in Industrial Catalysis () EUCHEME /08/0 Hydrocracking - Bifunctional Catalysts () An important issue concerns catalyst selectivity and stability: high H pressure increases the hydrogenation of unsaturated compounds no unsaturated compounds are obtained as products (mono)- aromatic rings are transformed under such conditions, via hydrogenation coke precursors are also hydrogenated stability catalysts, fixed bed reactors are used (~ years lifetime) 79

80 Zeolites in Industrial Catalysis () EUCHEME /08/0 Hydrocracking - Bifunctional Catalysts () Hydrocracking catalysts are Bifunctional: they possess two different catalytic functions (i) an acid function cracking and isomerization (ii) a metallic function hydrogenation / dehydrogenation Acid function zeolite in acidic form (HY) Metallic function noble metals (Pt, Pd), Ni, or metal sulfides (Co, Ni, Mo) 80

81 IMPORTANT REMARK EUCHEME /08/0 CATALYTIC FUNCTION A Catalytic Function is a set of a given type of active sites that catalyze a given reaction (catalytic cycle) 8

82 Zeolites in Industrial Catalysis () EUCHEME /08/0 Hydrocracking Mechanism () (ex: n-c 7 ) Products Feed + H -H + H -H + H -H M M M H + Z - H + Z - H + Z - H + Z - H + Z - H + Z - H + Z - H + Z - + H M Bifunctional transformation of n-heptane (n-c 7 ): (m-c 6 ) methylhexanes; (dm-c 5 ) dimethylpentanes; (C + ) carbocation; (O) olefins X 8

83 Zeolites in Industrial Catalysis (5) EUCHEME /08/0 Hydrocracking Mechanism () n-paraffins are dehydrogenated into olefins on metal sites olefins are transformed into carbocations on acid sites formed carbocations can undergo: isomerisation (skeletal) cracking by β-scission of isomerised compounds ( cracking is consecutive to isomerization ) iso- and n-olefins are hydrogenated into isoand n-paraffins in metal sites 8

84 Zeolites in Industrial Catalysis (6) EUCHEME /08/0 Hydrocracking Mechanism () Balance between the two catalytic functions: Strong hydrogenating function will result in diminishing coke formation and, consequently, an increase of catalysts stability Ratio [A A /A H ] can be adjusted to optimize activity and selectivity: an optimum in the balance between both functions results in the maximum activity the selectivity can be tuned to favour lighter or middle distillate products (more or less cracking) 8

85 Zeolites in Industrial Catalysis (7) EUCHEME /08/0 Hydrocracking - Effect of hydrogenation/acidity ratio () the performance of a bifunctional catalyst depends on the balance of both catalytic functions when A H (hydrogenation) is weak, the rate limiting step can be (i) the initial dehydrogenation on n-paraffins or (ii) the hydrogenation of isoolefins. Consequently, catalyst activity increases with A H when A A (acidity) is weak, the rate limiting step is the isomerization of n-olefins, so catalyst activity increases with A A 85

86 Zeolites in Industrial Catalysis (8) EUCHEME /08/0 Hydrocracking - Effect of hydrogenation/acidity ratio () Catalyst activity initially increases with npt/na. n-decane transformation on Pt/HFAU After a given value ( 0.0) it remains constant: this means that there is enough metal sites in order to fed all acid sites into intermediary olefins, maintaining the maximum activity. A From this point on, catalyst activity depends on acidity npt/na 86

87 Zeolites in Industrial Catalysis (9) EUCHEME /08/0 Hydrocracking Reactions in the presence of a hydrocracking catalyst, further hydrogenation of aromatics take place, followed by naphthene ring opening (hydrodecyclization) hydrogenation reactions lead to total (or partial) saturation of olefinic and aromatic hydrocarbons 87

88 Zeolites in Industrial Catalysis (0) EUCHEME /08/0 Hydrocracking Process High stability catalysts fix-bed reactors Simplified flow diagram of a singlestage, dual catalysts hydrocracking process 88

89 Zeolites in Industrial Catalysis () EUCHEME /08/0 Conversion Processes: FCC vs. Hydrocracking 89

90 Zeolites in Industrial Catalysis () EUCHEME /08/0 Methanol to Gasoline -MTG Integrated processes NG Syngas MeOH Olefins (UOP) Gasoline (Mobil) Reaction pathway: -H O MeOH dimethyl-ether light olefins HC (alkanes, aromatics) ZSM-5 : light olefins C, C MTG on H/MFI catalysts 70ºC, bar Products distribution as a function of contact time 90

91 Zeolites in Industrial Catalysis () EUCHEME /08/0 Methanol to Gasoline Mechanism () Methanol DME st step formation of DME on acidic sites of zeolite 9

92 Zeolites in Industrial Catalysis () EUCHEME /08/0 Methanol to Gasoline Mechanism () nd step formation of a C=C bond. Carbocationbased mechanism for the formation of olefins W.W. Kaeding, S.A. Butter, J. Catalysis 6 (980) 55 9

93 Zeolites in Industrial Catalysis (5) EUCHEME /08/0 Methanol to Gasoline Mechanism () rd reaction step corresponds to the formation of alkanes and aromatics, from light alkenes by (O) oligomerization-cracking reactions, isomerization reactions, (C) cyclisation reactions (HT) hydrogen transfer reactions, with the participation of carbocations, as intermediate species 9

94 Zeolites in Industrial Catalysis (6) EUCHEME /08/0 Methanol to Gasoline MTG Process Simplified flowsheet of MTG fixed-bed reactor process/mobil 9

95 Zeolites in Green Chemistry () EUCHEME /08/0 Catalysis is the key to waste minimization (R.A. Sheldon, J. Chem, Tech. Biotechnol., 68 (997) 8-88) 88) 95

96 Zeolites in Green Chemistry () EUCHEME /08/0 New Processes Phenol (Production of Bis-Phenol A; Phenolic resins; Caprolactam; Aniline) Direct hydroxylation of Benzene to Phenol with N O: Fe/ZSM /ZSM-5 (Monsanto) Eliminates cumene as intermediate Enables use of N O as oxidant Prevents by-production of propanone 96

97 Zeolites in Green Chemistry () EUCHEME /08/0 Phosphoric Acid AlCl Zeolite Advantages: catalyst can be removed, regenerated and returned for reuse no hazardous waste or acidic emission results from the use of zeolite catalysts they work with lower quality feedstock, yet produce a higher quality product Traditional Process they produce a higher proportion of cumene, and little propylbenzene (by-products) reducing energy used in purification 97

98 Zeolites in Green Chemistry EUCHEME /08/0 New Processes - Cumene (manufacturing of phenol and its co-product acetone) + CH =CHCH Catalyst CH -CH-CH Benzene + Propene Cumene 98

99 Zeolites in Green Chemistry EUCHEME /08/0 Catalyst: : (acid sites) solid phosphoric acid (SPA( SPA) ) (UOP) aluminum chloride (AlCl( ) (Monsanto Kellog) Zeolites: : (Mobil-Raytheon - MCM- ; ; UOP - Beta; Dow-Kellogg - Mordenite ; EniChem - Beta Zeolite) H/Zeolite H + Benzene CH =CH -CH CH -CH + -CH CH CH CH Propene Cumene 99

100 Zeolites in Green Chemistry EUCHEME /08/0 Zeolites Advantages AlCl -based process has the highest total capital investment (additional equipment required for catalyst disposal and the use of more expensive materials of construction) Differences in capital costs between the zeolite- based process (using refinery-grade propylene) and the modern SPA (Solid Phosphoric Acid) - based processes: < 5% 00

101 Zeolites in Green Chemistry EUCHEME /08/0 Zeolites Advantages Because of its high benzene-to-cumene selectivity, the zeolite-based process has the lowest production costs In addition, the zeolite-based process offers higher product purity and uses a regenerable catalyst, eliminating the waste disposal problems associated with the SPA and AlCl catalysts. 0

102 Zeolites in Green Chemistry EUCHEME /08/0 New Processes Caprolactam and Adipic Acid (manufacturing of NYLON polyamide fibers) O NHO Beckmann rearrangement C NH benzene cyclohexanone cyclohexanone caprolactam oxime O 0

103 Zeolites in Green Chemistry EUCHEME /08/0 Benzene Benzene Routes of Synthesis of Caprolactam and Adipic Acid + H + H Ru cat./ Delft Univ. + O Cyclohexane Ciclohexylperoxide Cyclohexanol/-one + O Cyclohexanone + NH OH + H SO -(NH ) SO Adipic Acid + H O Cyclohexene Cyclohexanol + O H-ZSM 5 Asahi Process Cyclohexanone + H O + NH TS- Enichem Cyclohexanone oxime Cyclohexanone oxime Beckmann rearrangment + H SO -(NH ) SO Zeolites Caprolactam Caprolactam W.F. Holderich et al., Catal. Today 7 (997)

104 Zeolites in Green Chemistry EUCHEME /08/0 Synthesis of Caprolactam over Zeolite Catalysts OXIME FORMATION st step ammonia reacts inside TS-, forming the hydroxylamine hydroxylamine ketone oxime nd step hydroxylamine reacts with ketone, giving rise to the formation of the oxime G. Bellusi et al., Cattech (000) -6 0

105 Zeolites in Green Chemistry EUCHEME /08/0 Zeolites Advantages Synthesis of Caprolactam over Zeolite Catalysts Beckmann rearrangement in gas-phase catalized by MFI- Silicalite Time on stream (hr) CHO conv. (%) CPL selectivity(%) Europ. Patent 550 (99) Sumitomo 05

106 Zeolites in Green Chemistry EUCHEME /08/0 Zeolites Advantages mol H is saved in the first step one reaction step less avoid the formation of the toxic hydroxyl amine, NH OH avoid using hazardous and corrosive sulfuric acid in the oxime formation step, as well as in the Beckmann rearrangement avoid the formation of ammonium sulfate (NH ) SO (up to Ton/Ton caprolactame) 06

107 Zeolites as Catalysts EUCHEME /08/0 Conclusions Zeolite structures can be considered as very active nanoreactors,, allowing to get insight reaction mechanisms (molecular level) Zeolite can be shapes hape-selective catalysts Increasing performances due to Confinement effects Possible determination of real active sites for each step of heterogeneous process 07

108 Zeolites as Catalysts EUCHEME /08/0 Conclusions Is possible to achieve the stabilization of well defined metal species Different catalytic functions can be finely tuned Zeolites are well adapted to refining and petrochemical processes, but also to environmental friendly processes 08

109 Catalysis by Zeolites EUCHEME /08/0 Thank You for Your Attention Carlos Henriques 09