The representation of tetrahedral

silicates

The chemistry of silicates and aluminosilicates

Silicate : classification

Ortho/ neso

di / soro

Beryl [Si6O18]12-

Ino / meta

cyclo

Many phyllosilicates have Al/Mg in Oh sites

Sheet

silicates

or phyllo

silicates

Kaolinite

China Clay

Al2(OH)4 (Si2O5)

Al Octahedral

Si Tetrahedral

Al

Si

Many phyllosilicates have Al/Mg in Oh sites

Talc Mg3(OH)2Si4O10

Smooth and free flowing

Kaolinite versus Talc: properties

Smooth and free flowing China Clay Al2(OH)4Si2O5

Sticky due to Hydrogen bonding of

OH- groups on the surface

No OH group

on surface

Bone china!

Structure of montmorillonite ( in Fullers earth)

Al4(OH)4 (Si4O10)2

Al Octahedral

Si Tetrahedral

(Si4O10)4−

Fullers earth: montmorillonite,

kaolinite and attapulgite.

Three different forms of tecto silicates (framework silicates) 4 O shared

Pure silicate: quartz, vitreus silica, silica gel, Fumed silica

Aluminosilicates; (having tetrahedral Al replacing tetrahedral Si)

Feldspars Ultramarines ZeolitesFeldspars Ultramarines Zeolites

Sodalite

Na8S2(Al6Si6O24)

Igneous rocks

K(AlSi3O8)

Linde A ( Mol sieves 4A)

Na6(Al6Si6O24). 2H20

Be3Al2(SiO3)6.

(Mg, Fe)2SiO4

asbestos

Talc

Mg3(OH)2Si4O10

aluminosilicates

Mg3(OH)2Si4O10

1. The crown jewels of catalysis

2. Catalyst and adsorbents designed at the molecular scale

3. Can be an effective carrier or support

4. Contain acid sites of strong acidity for catalytic reactions, e.g.

Zeolites or Molecular Sieves

4. Contain acid sites of strong acidity for catalytic reactions, e.g.

catalytic cracking and isomerization

Are porus solids

Have replaceable Na+ cations

Have channels which lead to cavities

Have water molecules in these cavities and channels

The channel and cavity size vary from zeolite to zeolite

Zeolite ChemistryGeneral 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

Zeolite Structure: Framework Alumino silicates

When a tetrahedral silicon (IV) is replaced in a silicate by an

tetrahedral aluminum(III), the framework attains an extra

negative charge. This will be compensated by a cation such as

Na+. Such compounds are termed framework aluminosilicates

Platonic Solids423 BC 347 BC

Archimedian Solids

3 Dimensional Perspective of

Zeolites:

Truncating an Octahedron !

OctahedronTruncated Octahedron

Also known as β cage

8 hexagons and 6 squares

Lowenstein’s Rule:

The inside cavity is known as α

cage having 6 octagons

8 hexagons and 12 squares

Lowenstein’s Rule:The AlO4

- tetrahedra are

always interspersed with

SiO4 tetrahedra

Or

Al-O-Al units does not

occur in zeolites

Influence of Si/Al Ratio in zeolite properties

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 Bronsted 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

Na+ Na+

H+ H+

Bronsted acid form

Lewis acid form

H+ H+

+H2O -H2O (500 C)

+

D4R D6R

β cages – directly linked through square faces = Sodalite

β cages – linked through square faces but with a D4R spacer = Linde A

β cages – linked through hexagonal faces but with a D6R spacer = Faujasite

STRUCTURE OF ZEOLITES

Sodalite Linde A Faujasite

not considered a zeolite (Molecular Sieves 4A)

ZSM-5: Zeolite Socony Mobil–5

is a zeolite belonging to the pentasil family of zeolites.

Its chemical formula is Na3Al3Si93O192·16H2O. Patented

by Mobil Oil Company in 1975, it is widely used in

the petroleum industry as a heterogeneous

catalyst for hydrocarbon isomerization reactions.

ZSM-5 has a high silicon to aluminum ratio.

5.1 x5.5 Å

Most well known

zeolite catalyst

Channel size 5.4 x 5.6 ÅAl: Si ratio 1:31

5.1 x5.5 Å

Synthesis of Zeolites

Template synthesis: The cavity and channel size is controlled and maintained by

building the framework around a specific organoammonim cation. These cations can

ultimately be converted to volatile products at about 500 °C and the cage retains its

framework structure

Zeolite A or Linde-A

Channel size 4.1 ÅAl: Si ratio 1:1 Na12(AlO2)12(SiO2)12. 27 H2O

Used as an ion exchanger for

water softening (in detergents)

due to high Na+ content.

Dehydrated form used for

absorption of moisture and

small volatile molecules.

Reusable and environmentally

safe

Zeolite Y or Faujasite

Channel size 7.4 Å

Al: Si ratio 2:5

Na2(AlO2)2(SiO2)5. 10 H2O

safe

Catalysis of larger molecules.

Metal complexes can be even

made inside the cavities

ZSM 5Si 93 %

non-polar

HydrophobicRing 10

Linde A

Si 50% - Al 50%

Cations Na, Ca

Polar

3-D straight channelsRing 8

Si x% - Al y%

Cations Na, Ca

Polar

3-D entangled

channels

FaujasiteRing 12

Hydrophobic

1-D channels

Examples of zeolites acting as selective catalysts in ACID CATALYSED reactions

+(a)

ZSM-5: Shape Selectivity in Catalysis

CH3 OH +(b)

(a) reactant selectivity for cracking of a straight-chain versus branched C7.

(b) product selectivity for p-xylene over o- and m- forms

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

Note: the benzene in the figure should be

toluene

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 catalyst

SHAPE SELECTIVE CATALYSIS - REACTANT, PRODUCT, TRANSITION

STATE SELECTIVITY