Zeolites - Properties and applications and

zeolites

1. ZEOLITES
DR.KRISHNAN SRINIVASAN
Assistant Professor
Dept. of Chemistry
TVU, Vellore-632 115

2. Azeolitemineralisacrystallinesubstancewithastructure
characterizedbyaframeworkoflinkedtetrahedra,eachconsistingoffourO
atomssurroundingacation.Thisframeworkcontainsopencavitiesintheform
ofchannelsandcages.TheseareusuallyoccupiedbyH
2Omoleculesand
extra-frameworkcationsthatarecommonlyexchangeable.Thechannelsare
largeenoughtoallowthepassageofguestspecies.Inthehydratedphases,
dehydrationoccursattemperaturesmostlybelowabout400°Candislargely
reversible.Theframeworkmaybeinterruptedby(OH,F)groups;these
occupyatetrahedronapexthatisnotsharedwithadjacenttetrahedra
Inorganic, crystalline,aluminosilicate materials –3D network-cavities and
cages
Consist of interconnected SiO
2 and AlO
2tetrahedra , Cations and H
2O
Chemically and thermally stable
Tetrahedra form 3D frameworks with linked channels down to molecular level
Usually microporous
Recommended Nomenclature for Zeolite Minerals: Report of The Subcommittee on
Zeolites of the International Mineralogical Association, Commission on New Minerals and
Mineral Names.The Canadian Mineralogist35 (1997) 1571 –1606.

3. +
(SiO4)
4-
(AlO4)
5-
Al
3+
Si
4+
Sharing of each oxygen
by two Si atoms
Tetrahedrally shared
3D network

4. Zeolite Building blocks
Schematically, oxygen are shown as
straight lines (bonds) and Si at the
corners
All corners are connected→ 4 lines from
each Si
O
SiSi
O
Si
Si
O
O
Si
Zeolite -Silicalite

5. APPLICATION:
-cationexchange
-adsorptionandrelatedmolecularsieving
-dehydrationandrehydration
-biologicalactivity
-catalytic
.
*** the ability to exchange inherent cations for
other cations on a basis of ion selectivity-used to
reomve the heavy metals from effluents
zeoliteshavedifferentdimensionsofporesandchannelsresponsiblefor
theirphysicalandchemicalproperties

6. Product Shape Selectivity;
benzene + methanol = xylene
Para-xylene is far more valuable than ortho or meta xylene -
used in polyester manufacture

7. ZSM-5
zeolites A, X, Y and ZSM-5
Natural
Synthetic
SBU

8. Adsorption and related molecular sieving
Theadsorptiononmolecularsievesisthereforedependentonthe
followingphysicalmolecularproperties:
-SizeandShape;moleculesorionslargerthantheporeopeningof
thezeolitecannotbeadsorbed,smallermoleculesorionscan.
-MolecularPolarity;duethechargeonouterandinnerzeolitic
surfaceparticle,moleculeswithlargepolarityorpolarisabilitycan
beadsorbedpreferentiallyunderidenticalconditions.
Theadsorptionprocessisfullyreversibleandofpurelyphysical
nature.Thestructureofthezeoliteisunchangedduringthe
adsorptionprocessanditslaterregeneration.

9. differencebetweennaturalzeolitemineralsismanifestedincontentofhydrated
cationswhichoccupiedporesandchannels,ratioofSi/Alinstructureandnumber
ofwatermolecules
•Trapselectivelygasmoleculesoftherightsize
•Trapselectivelygasmoleculesoftherightshape
•Trapselectivelygasmoleculesoftherightpolarity
•Highlypolarmaterials,ionconductorsandionexchangers
•Al
+3
acidicsite:catalyzeanumberofchemicalreactions
•Roomtemperatureoperation
•Highspecificsurface
Use of Zeolites for sensing