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ZEOLITES: Catalytic Nanostructure Materials

CIÊNCIA 2010 - Nanomateriais. ZEOLITES: Catalytic Nanostructure Materials. Carlos Henriques Filipa Ribeiro Auguste Fernandes F. Ramôa Ribeiro. Catalysis and Reaction Engineering Research Group Institute for Biotechnology and Bioengineering

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ZEOLITES: Catalytic Nanostructure Materials

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  1. CIÊNCIA 2010 - Nanomateriais ZEOLITES: Catalytic Nanostructure Materials Carlos Henriques Filipa Ribeiro Auguste Fernandes F. Ramôa Ribeiro Catalysis and Reaction Engineering Research Group Institute for Biotechnology and Bioengineering Department of Chemical and Biological Engineering Instituto Superior Técnico - UTL

  2. CIÊNCIA 2010 - Nanomateriais Zeolite? • uponrapidly heating, the mineralStilbiteproduces large amounts ofsteam, that arises from water,adsorbed by the material • hydrated calcium aluminium silicate • (natural zeolite) • based on this,A.F. Crönstedtnamed the mineral ZEOLITE(zeo – boiling; lithos – stone) A.F. Crönstedt, Akad. Handl. Stockholm,18 (1756) 120

  3. CIÊNCIA 2010 - Nanomateriais CIÊNCIA 2010 - Nanomateriais Zeolites • Crystalline alumino-silicates, with regular open tridimensional nanosized porous framework ZSM-5 (MFI) MORDENITE (MOR)

  4. CIÊNCIA 2010 - Nanomateriais What is special about zeolites? • They have pores with nanosized dimensions(0.3 – 0.8 nm) leading to Shape Selectivity • As crystalline materials, they present a narrow range of pore sizes  gives better selectivity than non-crystalline materials

  5. CIÊNCIA 2010 - Nanomateriais CIÊNCIA 2010 - Nanomateriais Zeolite Framework (1)  Zeolite framework is composed of SiO4 and AlO4tetrahedral units, sharing oxygen between every two consecutive units

  6. CIÊNCIA 2010 - Nanomateriais CIÊNCIA 2010 - Nanomateriais Zeolite Framework (2) Arrangement ofPrimary Building Units oxygen tetrahedral solid tetrahedral negative charge  TO4 O - Al or Si  - oxygen

  7. CIÊNCIA 2010 - Nanomateriais CIÊNCIA 2010 - Nanomateriais Zeolite Framework (3) How zeolites are built

  8. CIÊNCIA 2010 - Nanomateriais CIÊNCIA 2010 - Nanomateriais Zeolite Framework (4) • Cations (Na+, NH4+,H+, transition metals) located inside channels or cavities, to balance negative charges in the framework: Exchange Positions sodium form

  9. CIÊNCIA 2010 - Nanomateriais Zeolite Framework (5) FAU structure (Y zeolite) Exchange sites 1.7 nm Al positions Framework and EFAL Supercage ~ 1.7 nm

  10. CIÊNCIA 2010 - Nanomateriais CIÊNCIA 2010 - Nanomateriais Zeolite Framework (6) Y zeolite (FAU structure) ZSM-5 (MFI structure) 1.3 nm 0.77 nm 0.71 nm

  11. CIÊNCIA 2010 - Nanomateriais F.C. Gulbenkian, 06 Julho 2010 Zeolites - Morphology LTA ANA NAT LTL MOR TON SEM photos W.J. Mortier, Leuven

  12. CIÊNCIA 2010 - Nanomateriais Why Zeolites in Catalysis ? • Acidity  Si/Al ratio (number, density, strength) • Ion-exchange capacity ability to introduce acidity as well as metal species • Stabilization of different types metal species: cations in exchange positions; metal oxide nanoparticles; reduced metal aggregates; isomorphous substituted metal atoms.

  13. CIÊNCIA 2010 - Nanomateriais How nanoporosity influences catalysis ? • Shape selectivity P.B. Weisz et al., J. Phys. Chem. 64 (1960) 382 • Confinement effects E. G. Derouane, J. M. André, A. A. Lucas, J. Catal 110 (1988) 58 Zeolitecages, channels and channels intersections really act like nanoreactors

  14. CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (1) I - Molecular sieving of reactants • Reactant type shape selectivity: competitive cracking of n-octane and 2,2,4- 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

  15. CIÊNCIA 2010 - Nanomateriais 1.5 nm • First Shape Selectivity to be found is in the base of Selectoforming Process (Mobil)–Erionite Zeolite • Selective dehydration of 1-butanol over a 5A zeolite (0.5nm pores), in a mixture with 2-butanol or isobutanol • Selective hydrogenation of n-butene in a mixture with isobutene over a Pt-5A catalyst

  16. CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (3) II - Molecular Sieving of Products Product Selectivity Toluene dismutation Bulkier products can be formed inside pores but their exit is hampered by slower diffusion • Mobil:p-xylenesynthesis, overZSM-5 zeolite Bulkierisomers (o-xylene and m-xylene), can be formed inside the pores but their formation become limited by their desorption

  17. CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (4) II - Molecular Sieving of Products Toluene Ethylene - Product shape-selectivity: acid catalyzed alkylation Toluenewith Ethylene Both reactants are small enough to enter the zeolite pores, but from the potential products (o-, m- and p-ethyltoluene), only the slimp-ethyltoluene is small enough to leave the pore system

  18. CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (5) Transition-State Selectivity Dimethy-benzenedismutationover 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 1,3,5 trimethylbenzene

  19. CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (7) Confinement EffectDue to the strong interaction between zeolite frameworks and moleculeszeolites act as solid solvents • concentration of reactants is much higher inside the structure than it is outside positive effect on reactions rates (Concentration Effect) • Confinement Effect evidences the role of the interaction forces between molecules and zeolite framework

  20. CIÊNCIA 2010 - Nanomateriais Zeolites as Nano- Reactors - Conclusions • Shape Selectivity clearly highlight the important role of the size and shape of pore, cages and channels that constitute thezeolite framework • Zeolite nanopore structure (succession of cages act as nanoreactors) makes zeoliteunique toolsfor the development of selectivity in heterogeneous catalysis • In all processes that use zeolites as catalysts, their activity, selectivity and stabilitydepends not only on the type of active sites,but also on their location inside the zeolite structure

  21. CIÊNCIA 2010 - Nanomateriais Thank you for your attention Structure of ferrierite projected onto (001) plane: Na+ (red circles) low occupancy sites (light blue) Mg2+ (yellow) H2O molecules (dark blue) TO4 tetrahedra (grey)

  22. CIÊNCIA 2010 - Nanomateriais Zeolite Framework (4) • Zeolites accordingly to the number of oxygen atoms in the opening (“ring”) of larger pores • small-porezeolites: 8 - membered oxygen rings and a “free” diameter of 0.3 - 0.45 nm • medium-porezeolites: 10 - member oxygen rings and a “free” diameter of 0.45 - 0.6 nm • large-porezeolites: 12 - member oxygen rings and a “free” diameter of 0.6 - 0.8 nm

  23. CIÊNCIA 2010 - Nanomateriais TO4 tetrahedra T = Si, Al Sodalite cage Final Structure: Sodalite cages linked by hexagonal prisms, in a tetrahedral arrangement, defining supercages (13Å) Zeolites in Industrial Catalysis Catalytic Cracking – Catalyst HY (FAU)

  24. CIÊNCIA 2010 - Nanomateriais F.C. Gulbenkian, 06 Julho 2010 -NH3 (g) Brönsted Acid Sites -H2O Dehydroxylation Basic sites Lewis Acid Sites Active Sites in Zeolites (2) Zeolite as Acid Catalysts ion-exchange thermal treatment Brönsted Acid Sites thermal treatment Lewis Acid Sites

  25. CIÊNCIA 2010 - Nanomateriais Active Sites in Zeolites (5) • Bifunctional Catalysts • different catalytic processes run in the simultaneous presence of different type of active sites (catalytic functions) – multifunctional catalysis • bifunctional catalysts, with both acidic (Zeolites) and hydrogenating (Ni, Pt, Pd, metal sulfides) functions: isomerization and aromatization of light alkanes, hydrocracking, catalytic dewaxing, isomerization of C8 aromatic fraction…

  26. CIÊNCIA 2010 - Nanomateriais Active Sites in Zeolites (6) • Bifunctional Catalysts TEM Pt/ ZSM-5 zeolite

  27. CIÊNCIA 2010 - Nanomateriais n-paraffin products Zeolite cage i-paraffin Shape Selectivity with Zeolites (1) i) Molecular Sieving ( zeolites  molecular sieves) • First Shape Selectivity to be found is in the base of Selectoforming Process (Mobil) – erionite catalysts • Selective dehydration of 1-butanol over a 5A zeolite (0.5nm pores), in a mixture with 2-butanol or isobutanol • Selective hydrogenation of n-butene in a mixture with isobutene over a Pt-5A catalyst

  28. CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (7) 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, but only of the porous structure of thezeolite and the size of transition-state species. • Nevertheless this transition-state effect can co-exist with molecular sieve effects (namely products MolSieve selectivity) • This type of selectivity mainly concerns all transformations that occur by inter-molecular (bimolecular) reactions, as in this case intermediary species are bulkier than with monomolecular reactions, for similar reactants.

  29. CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (8) 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. • 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 unfavours coke formation inside pores

  30. CIÊNCIA 2010 - Nanomateriais Shape Selectivity with zeolites P.B. Weisz et al., J. Phys. Chem. 64 (1960) 382 • Shape Selectivity active sites are included in a nanoporous framework • This framework is constituted by cages, channels and channels intersections (where active sites mainly are) that can really be considered as nanoreactors • Shape and sizeof the inter-connecting rings will determine the selectivity of zeolite catalyzed reactions

  31. CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (8) Confinement Effectzeolites act as solid solvents • This results in an increase of reactants concentrations inside zeolite pores is one of the explanations for the high activity of zeolite catalysts, when compared with amorphous aluminosilicates structures

  32. CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (6) Transition-State Selectivity Transition State Shape Selectivity: m-xylene can undergo acid-catalyzed isomerization into p-xylene and transalkylation into toluene and one of the trimethylbenzene isomers (bimolecular) BUT: no toluene or trimethyl benzenes are observed

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