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Zeolites Summer School in Energy and Environmental Catalysis University of Limerick, July 2005

Zeolites Summer School in Energy and Environmental Catalysis University of Limerick, July 2005. Tetrahedra made up of EITHER SiO 4 OR AlO 4 - units. Every unit of AlO 4 - will have an associated cation in order to maintain charge balance, H + , Li + , Na + , K + . NH 4 + etc.

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Zeolites Summer School in Energy and Environmental Catalysis University of Limerick, July 2005

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  1. Zeolites Summer School in Energy and Environmental Catalysis University of Limerick, July 2005

  2. Tetrahedra made up of EITHER SiO4 OR AlO4- units Every unit of AlO4- will have an associated cation in order to maintain charge balance, H+, Li+, Na+, K+. NH4+ etc.

  3. imbalance

  4. SBU of ZSM-5 zeolite

  5. Combination of ZSM-5 SBUs shown along the a axis and as a parallel projection along the b axis

  6. Channel Intersections 0.9 nm Sinusoidal channels (0.54-0.56 nm wide Straight channels (elliptical openings - 0.52-0.58 nm)

  7. Influence of Si/Al Ratio Zeolites with a low [Al] are hydrophobic (and vice versa) Lowensteins' rule, Al-O-Al linkages forbidden (Si/Al must be > or = 1) If the counter ion is a proton then this is hydrogen bonded to the lone pairs of the neighbouring Oxygen bridging atom generating Bronstead Acidity High temperature treatment can de-hydroxylate the zeolite and generate a Lewis acid site (i.e. lone pair acceptor) on Al atoms High concentrations of protons (from a low Si/Al) give a high acidity but lower concentrations of protons yield STRONG acid sites

  8. USES OF ZEOLITES (1) Adsorbents and desiccants- drying agents (2) Separation processes - in gas purification, (3) Animal feed supplements, (4) Soil improvements. (5) Detergent formulations (6) Wastewater treatment, (7) Nuclear effluent treatment, (8) Catalysis

  9. Properties that increase catalytic activity of ZEOLITES. • molecular sieving (for shape selective catalysis) • well defined active sites • cationic exchange capacity, • high surface area, • variable acidity and controllable electrostatic fields (M2+ and M3+), • relatively good chemical and thermal stability. • sites for occluded species – generate “internal” metal particles

  10. Examples of zeolites acting as selective catalysts in ACID CATALYSED reactions

  11. Shape Selective Catalysis (1) Reactant selectivity, (2) Product selectivity, and (3) Restricted transition-state selectivity All these are examples of zeolites acting as selective catalysts in ACID CATALYSED reactions Reactant Selectivity - reactant molecules too large to enter cavities. e.g. Ca / A and Ca / X as catalysts for R-OH  H2O + alkene 1° and 2° alcohols dehydrate on Ca/X only 1° alcohols dehydrate of Ca/A (2° alcohols too large to get into the pores of zeolite A to the active Ca sites)

  12. Ca / X + H2O + H2O Ca / A Active Sites

  13. Product Shape Selectivity; benzene + methanol = xylene Para-xylene is far more valuable than ortho or meta xylene - used in polyester manufacture Only para xylene can diffuse out of the ZSM-5 channel pores

  14. Transition State Shape Selectivity, some transition-state intermediates are too large to be accommodated within the pores/cavities of the zeolites, even though diffusion of neither the reactants nor the products are restricted. transalkylation of dialkylbenzenes meta-xylene, 1,3,5- and 1,2,4-trialkylbenzene.

  15. ZSM-5  Methanol  gasoline catalyst ACTIVE Sites are zeolitic protons ACID catalysis Two intersecting sets of channels. Methanol diffuses in through one set of channels and gasoline diffuses out the second set, thereby avoiding “counter-diffusional” limitations in the reaction rate.

  16. What about the “surface” of the zeolitic particle, i.e. the external surface ? Also has active sites - but no “space” constraints. DURENE (unwanted C10 aromatic) formed on these external sites during MTG. This has been combated by making larger zeolite particles (proportionately less external acid sites) or Selectively poisoning external acid sites with bases too large to enter pores, e.g. tri-methyl phosphine

  17. Bifunctional catalysis on zeolites Ion-Exchanging a H-form zeolite with a metal removes Bronstead acidity, forming sites which may be active for other reactions – Cu2+ in Cu ZSM-5 are active 2NO  N2 + O2 (REDOX SITES) If the system is then reduced with H2 the exchanged metal ions form small metal particles within the zeolite and the Bronstead acidity is restored.

  18. 2 effects – (a) very small (and active ??) metal particles within pores –shape selectivity in metal catalysed reactions and (b) Metal and acid sites in zeolite in very close proximity. Metals very good at promoting hydrogenation / dehydrogenation – Acids very good at promoting isomerisation / cracking. (ALSO More resistant to coking) Methylcyclopentane  cyclohexane 50 times faster on Pd H-Y compared to Pd Na-Y + H-Y close proximity required!

  19. Baku Mosque – Azerbaijan (1086) ZSM-5 (Zeolite Synthesised by Mobil Corp (1974)

  20. Some Characterisation Techniques Temperature Programmed Desorption / Decomposition. Infra Red Spectroscopy of Adsorbed Probe Molecules. X-Ray Techniques

  21. TPD of CO from Pd Temperature Programmed Techniques Temperature Programmed Desorption (TPD) Adsorption of molecular species onto the sample surface at low Temperature Heating the sample with a linear temperature ramp monitoring desorption of species from surface back into gas phase. • area under peak  amount originally adsorbed • peak temperature is related to the enthalpy of adsorption, i.e. to the strength of binding to the surface..

  22. TPD of (basic) NH3 also gives information about the concentration and strength and of surface acid sites. NH3-TPD Weak Strong acidic sites Mordenite ZSM-5 SAPO-11 ALPO-11

  23. VIBRATIONAL SPECTROSCOPY n(C-O) CO ( gas phase ) 2143 cm-1 Terminal CO 2100 - 1920 cm-1 Bridging ( 2f site ) 1920 - 1800 cm-1 Bridging ( 3f / 4f site ) < 1800 cm-1 CO on Pt Very Useful as a “probe” detailing the surface

  24. HOMO -  orbital lone pair (weakly antibonding) LUMO - * orbital (antibonding) CO (g) has a stretching frequency of 2143 cm-1, CO as a ligand stretching 1700 cm-1 to 2200 cm-1 CO ligand bonds metal by (a) donating electron density (from its nonbonding lone pair) into a metal d-orbital And (b) accepting electron density from a filled metal d-orbital of pi symmetry into it's pi* antibonding orbital. (BACKBONDING Stronger CO bond, higher energy stretch Weaker CO bond, lower energy stretch

  25. FTIR of Adsorbed NH3 (or pyridine) on a zeolite gives information about the types and concentrations of acid sites on the surface i.e. adsorbing NH3 onto a Bronstead site  NH4+ads or R-NH3+ which has particular infra red stretching frequencies adsorbing NH3 onto a Lewis acid site  NH3ads or RNH2ads which has different stretching frequencies

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