Zeolites

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Zeolites Summer School in Energy and
Environmental Catalysis University of Limerick,
July 2005
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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.
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imbalance
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SBU of ZSM-5 zeolite
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Combination of ZSM-5 SBUs shown along the a axis
and as a parallel projection along the b axis
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Sinusoidal channels (0.54-0.56 nm wide Straight
channels (elliptical openings - 0.52-0.58 nm)
Channel Intersections 0.9 nm
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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 gt
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
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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
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• 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

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Examples of zeolites acting as selective
catalysts in ACID CATALYSED reactions
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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)
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Active Sites
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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
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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. transa
lkylation of dialkylbenzenes meta-xylene,
1,3,5- and 1,2,4-trialkylbenzene.
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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.
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What about the surface of the zeolitic
particle, i.e. the external surface ? Also has
active sites - but no space constraints. DUREN
E (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
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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.
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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!
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Baku Mosque Azerbaijan (1086)
ZSM-5 (Zeolite Synthesised by Mobil Corp (1974)
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Some Characterisation Techniques Temperature
Programmed Desorption / Decomposition. Infra Red
Spectroscopy of Adsorbed Probe Molecules. X-Ra
y Techniques
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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..

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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
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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 ) lt 1800
cm-1
CO on Pt
Very Useful as a probe detailing the surface
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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
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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 ? NH4ads 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