Structure of SiO44- , Classification of silicates based on the structure, Zeolites : their structure and applications.

1. Silicates
Structure of SiO4
4- , Classification of silicates based on the structure,
Zeolites : their structure and applications.
Presented by
G SMITHA
Assistant professor
MES COLLEGE OF ARTS,
COMMERCE AND SCIENCE.
Malleshwaram,
Bengaluru.

2. Structure of silicates.
 Silicates are the compounds in which the anion present are either
discrete SiO4
4- tetrahedra or a number of such units joined together
through corners.
 The basic unit of structure of all silicate crystals is the tetrahedron.
 There are four oxygen atoms, one located at each apex
of a regular tetrahedron.
 A single silicon atom is located at the centred of the tetrahedron.
 This silicon atom has a valence charge of 4, meaning that it is looking to
acquire four electrons through sharing with other atoms to complete
its outermost energy shell, known as the valence shell.

3. An oxygen atom has two electrons in its outermost shell that are
available to bond with the silicon atom.
If four oxygen atoms surround one silicon atom, where each oxygen atom
offers one electron, then the silicon atom’s outermost shell will be complete
and stable. The resulting arrangement comprises a silicate molecule.
One electron remains, allowing those oxygen atoms to search for another
silicon atom to share an electron and form another tetrahedron.
Tetrahedrons are linked together through oxygen bonds.
Atomic number of silicon : 14
Electronic configuration : [Ne] 3s2 3p2
Ground state :
Excited state :
↑↑
↑ ↑ ↑ ↑
Atomic number of oxygen : 8
Electronic configuration : [H] 2s2 2p4
Ground state :
:
↑ ↑

4. Classification of silicates based on structure
Depending on the way the tetrahedral units are linked, the silicates are
classified into the following types.
1. Ortho silicates (or Nesosilicates)
2. Pyro silicate (or Sorosilicates)
3. Cyclic silicates (or Ringsilicates)
4. Chain silicates (or pyroxenes)
5. Sheet or phyllosilicates
6. Three dimensional (or tecto) silicates

5. 1.Ortho silicates (or Nesosilicates)
Ortho silicates (or Neso or Island silicates) are the simplest silicates which
contain discrete SiO4
4- tetrahedral units. Structural unit of ortho silicate unit
is shown below.
For examples : Zircon (ZrSiO4) , Forestrite (Mg2SiO4) , wellimite (Zn2SiO4) ,
Phenacite (Be2SiO4).

6. 2.Pyro silicate (or Sorosilicates)
When two SiO4
4- tetrahedra share one corner, Si2O7
6- anion is formed.
Silicates containing this anion are called pyrosilicates. The structures
possessed by them are also called island structures.
For example : Thortveitite Sc2(Si2O7) , Hemimorphite Zn3(Si2O7)Zn(OH)2·H2O

7. 3.Cyclic silicates (or Ringsilicates)
If two oxygen atoms per tetrahedra are shared to form closed,
the silicate containing these anions are called cyclic silicates. The typical
examples of such anions are Si3O9
6 - and Si6O18
6 - as represented below.
rings such that
the structure with the general formula (SiO3
2-)n or (SiO3)n
2n- are obtained
Example : Benitoite BaTi(SiO3)3 , Beryl Be3Al2(SiO3)6 , Wollastonite (Ca3Si3O9)

8. 4.Chain silicates (or pyroxenes)
If two oxygen atoms per tetrahedron are shred such that a linear single
strand chain of the general formula (SiO3
2-)n or (SiO3)n
2- is formed, then
he silicates containing these anions are called chain silicates.
For example: Spodumene ( LiAl(SiO3)2) , Diopside (CaMg(SiO3)2)
If two chains are cross linked, the resulting double-stranded silicates having the
formula [(Si4O11)n
6-] are called amphiboles.
For ex: Tremolite ( Ca2Mg5(Si4O11)(OH)2) , asbestos (CaMg3O(Si4O11)

9. 5.Sheet or phyllosilicates
The sharing of three corners results in an infinite two dimensional sheet
structure of the formula (Si2O5)n
2n- or (Si2O5
2-)n .
Silicates containing these anions are called sheet silicates.
Clay is an example of sheet silicate.

10. 6.Three dimensional (or tecto) silicates
If all the four corners are shared with other tetrahedra, three-dimensional
structure network is obtained. The general formula for three-dimensional
silicates is ( SiO3 )n .
For example: quartz, tridymite , Zeolites and cristobalite.
Amphibole silicates (chain silicates)

11. Zeolites are three-dimensional, microporous, crystalline solids with well-defined
structures that contain aluminium, silicon, and oxygen in their regular framework;
cations and water are located in the pores.
The silicon and aluminium atoms are tetrahedrally coordinated with each other
through shared oxygen atoms. Compositionally, zeolites are similar to clay minerals.
More specifically, both are alumino-silicates. They differ, however, in their crystalline
structure.
Many types of clay have a layered crystalline structure (similar to a deck of cards)
and are subject to shrinking and swelling as water is absorbed and removed between
the layers.
In contrast, zeolites have a rigid, 3-dimensional crystalline structure (similar to a
honeycomb) consisting of a network of interconnected tunnels and cages. Water moves
freely in and out of these pores but the zeolites framework remains rigid.
Another special aspect of this structure is that the pore and channel sizes are nearly
uniform, allowing the crystal to act as a molecular sieve.
The porous zeolite is host to water molecules and ions of potassium and calcium, as
well as a variety of other positively charged ions, but only those of appropriate
molecular size to fit into the pores are admitted creating the "sieving" property.
Zeolites
Structure

12. Application
Natural zeolite is a new and very good natural filter medium available for the
filtration of water. It offers superior performance to sand and carbon filters,
giving purer water and higher throughput rates with less maintenance
required. It has many advantages over sand and can be used to directly
replace sand in a normal sand filter.
There are three main uses of zeolites in industry: catalysis, gas separation
and ion exchange.
Catalysis
Zeolites are extremely useful as catalysts for several important reactions
involving organic molecules. The most important are cracking,
isomerisation and hydrocarbon synthesis. Zeolites can promote a diverse
range of catalytic reactions including acid-base and metal induced
reactions. The reactions can take place within the pores of the zeolite -
which allows a greater degree of product control.

13. Gas Separation
A widely used property of zeolites is that of gas separation. The porous structure of
zeolites can be used to "seive" molecules having certain dimensions and allow them
to enter the pores. This property can be fine tuned by variating the structure by
changing the size and number of cations around the pores. Other applications that
can take place within the pore include polymerisation of semiconducting materials and
conducting polymers to produce materials having unusual physical and electrical
Attributes.
Ion Exchange
Hydrated cations within the zeolite pores are bound loosely to the zeolite framework,
and can readily exchange with other cations when in aqueous media. Applications of
this can be seen in water softening devices, and the use of zeolites in detergents and
soaps. The largest volume use for zeolites is in detergent formulations where they
have replaced phosphates as water-softening agents. They do this by exchanging the
sodium in the zeolite for the calcium and magnesium present in the water. It is even
possible to remove radioactive ions from contaminated water.