Zeolites

Summer School in Energy and Environmental Catalysis

University of Limerick, July 2005

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.

Si

O

O OO

Zeolite Chemistry

General formula for the composition of a zeolite is Mx/n[(AlO2)x(SiO2)y] . mH2Owhere cations M of valence n neutralize the negativelycharged zeolite framework.

SiO2 tetrahedra are electrically neutral (e.g., quartz)Substitution of Si(IV) by Al(III) creates an electrical imballanceand neutrality is provided by an exchangeable cation

Al

Si

Na+ Na+

imbalance

SBU of ZSM-5 zeolite

Combination of ZSM-5 SBUs shown along the a axis and

as a parallel projection along the b axis

Sinusoidal channels (0.54-0.56 nm wide

Straight channels (elliptical openings - 0.52-0.58 nm)

Channel Intersections 0.9 nm

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

Acid Sites

Zeolite as synthesized

Bronsted acid form

Lewis acid form

Na+ Na+

H+ H+

+H2O -H2O (500 C)

+

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 (M2+ and M3+),•relatively good chemical and thermal stability.•sites for occluded species – generate “internal” metal particles

Examples of zeolites acting as selective catalysts in ACID CATALYSED reactions

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)

Active Sites

OH

OH

OH

OH+ H2O

Ca / X

OH

OH

OH

OH

+ H2O

Ca / A

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 thoughdiffusion 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 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

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.

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 TechniquesTemperature 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) NH3 also gives information about the concentration and strength and of surface acid sites.

NH3-TPD

Mordenite

ZSM-5

SAPO-11

ALPO-11

Weak Strong acidic sites

Amine-TPD, e.g. isopropyl amine – discriminate between Bronsted and Lewis Acid sites

Isopropyl amine adsorbs on B and Lacid sites

– During the TPD the desorption of propene and ammonia results from the decomposition of the amine occurring ONLY at Brønsted type acid sites, while desorption of isopropyl amine indicate the presence of Lewis type acid sites.

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

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 stretch

Weaker 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

a ba ba ba b

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