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

2. 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
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ZEOLITE  (zeo – boiling; lithos – stone)
A.F. Crönstedt, Akad. Handl. Stockholm,18 (1756) 120

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

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
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What is special about zeolites?

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

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

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

8. 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)
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sodium form

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

10. CIÊNCIA 2010 - Nanomateriais
Zeolite Framework (6)
Y zeolite
(FAU structure)
ZSM-5
(MFI structure)
1.3 nm
0.71 nm
0.77 nm
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11. CIÊNCIA 2010 - Nanomateriais
LTA
NAT
MOR
ANA
LTL
TON
SEM photos
W.J. Mortier, Leuven
F.C. Gulbenkian, 06 Julho 2010
CIÊNCIA Nanomateriais
Zeolites - Morphology

12. CIÊNCIA 2010 - Nanomateriais
Why Zeolites in Catalysis ?
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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. 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
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14. 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
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Shape Selectivity with Zeolites (1)

15. CIÊNCIA 2010 - Nanomateriais
1.5 nm
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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
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
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Shape Selectivity with Zeolites (3)
II - Molecular Sieving of Products

17. 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

18. 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
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Shape Selectivity with Zeolites (5)
Dimethy-benzene dismutation over HMOR catalysts

19. 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

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 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

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)

25. 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 nm
medium-pore zeolites: 10 - member oxygen rings and a “free” diameter of nm
large-pore zeolites: 12 - member oxygen rings and a “free” diameter of nm

26. 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)

27. 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

28. 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…

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

30. 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

31. 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.

32. 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

33. 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

34. 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

35. 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
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Shape Selectivity with Zeolites (6)
BUT: no toluene or trimethyl benzenes are observed