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
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
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 (M 2+ and M 3+ ), relatively good chemical and thermal stability. sites for occluded species – generate “internal” metal particles
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 H 2 O + 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)
Product Shape Selectivity; benzene + methanol = xylene Only para xylene can diffuse out of the ZSM-5 channel pores Para-xylene is far more valuable than ortho or meta xylene - used in polyester manufacture
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.
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.
What about the “surface” of the zeolitic particle, i.e. the external surface ? Also has active sites - but no “space” constraints. DURENE (unwanted C 10 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
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 – Cu 2+ in Cu ZSM-5 are active 2NO N 2 + O 2 (REDOX SITES) If the system is then reduced with H 2 the exchanged metal ions form small metal particles within the zeolite and the Bronstead acidity is restored.
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!
ZSM-5 (Zeolite Synthesised by Mobil Corp (1974) Baku Mosque – Azerbaijan (1086)
Some Characterisation Techniques Temperature Programmed Desorption / Decomposition. Infra Red Spectroscopy of Adsorbed Probe Molecules. X-Ray Techniques
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. TPD of CO from Pd 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..
TPD of (basic) NH 3 also gives information about the concentration and strength and of surface acid sites. NH 3 -TPD Mordenite ZSM-5 SAPO-11 ALPO-11 Weak Strong acidic sites
CO ( gas phase )2143 cm -1 Terminal CO2100 - 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 VIBRATIONAL SPECTROSCOPY
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 HOMO - orbital lone pair (weakly antibonding) LUMO - * orbital (antibonding) Stronger CO bond, higher energy stretchWeaker CO bond, lower energy stretch And (b) accepting electron density from a filled metal d-orbital of pi symmetry into it's pi* antibonding orbital. (BACKBONDING
FTIR of Adsorbed NH 3 (or pyridine) on a zeolite gives information about the types and concentrations of acid sites on the surface i.e. adsorbing NH 3 onto a Bronstead site NH 4 + ads or R-NH 3 + which has particular infra red stretching frequencies adsorbing NH 3 onto a Lewis acid site NH 3ads or RNH 2ads which has different stretching frequencies