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

Zeolite? A.F. Crönstedt, Akad. Handl. Stockholm,18 (1756) 120 upon rapidly heating, the mineral Stilbite produces large amounts of steam, that arises from water, adsorbed by the material hydrated calcium aluminium silicate (natural zeolite) based on this, A.F. Crönstedt named the mineral CIÊNCIA 2010 - Nanomateriais ZEOLITE  (zeo – boiling; lithos – stone) A.F. Crönstedt, Akad. Handl. Stockholm,18 (1756) 120

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

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 CIÊNCIA 2010 - Nanomateriais What is special about zeolites?

CIÊNCIA 2010 - Nanomateriais Zeolite Framework (1) CIÊNCIA 2010 - Nanomateriais  Zeolite framework is composed of SiO4 and AlO4 tetrahedral units, sharing oxygen between every two consecutive units

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

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

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

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

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

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

CIÊNCIA 2010 - Nanomateriais Why Zeolites in Catalysis ? CIÊNCIA 2010 - Nanomateriais 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.

How nanoporosity influences catalysis ? CIÊNCIA 2010 - Nanomateriais 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 Zeolite cages, channels and channels intersections really act like nanoreactors CIÊNCIA 2010 - Nanomateriais

CIÊNCIA 2010 - Nanomateriais 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 CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (1)

CIÊNCIA 2010 - Nanomateriais 1.5 nm CIÊNCIA 2010 - Nanomateriais 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

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

CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (4) II - Molecular Sieving of Products - Product shape-selectivity: acid catalyzed alkylation Toluene with Ethylene 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 Toluene Ethylene

Shape Selectivity with Zeolites (5) 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 diphenyl - methane intermediates Transition-State Selectivity CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (5) Dimethy-benzene dismutation over HMOR catalysts

CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (7) Confinement Effect  Due to the strong interaction between zeolite frameworks and molecules zeolites 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

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 the zeolite framework Zeolite nanopore structure (succession of cages act as nanoreactors) makes zeolite unique tools for the development of selectivity in heterogeneous catalysis 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

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)

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

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

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

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…

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

CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (1) i) Molecular Sieving ( zeolites  molecular sieves) Zeolite cage n-paraffin i-paraffin products 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

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 the zeolite 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.

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

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 size of the inter-connecting rings will determine the selectivity of zeolite catalyzed reactions

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

Shape Selectivity with Zeolites (6) 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) Transition-State Selectivity CIÊNCIA 2010 - Nanomateriais Shape Selectivity with Zeolites (6) BUT: no toluene or trimethyl benzenes are observed