Elias lecture silicones and silicates part 2 september 30 2015

The chemistry of silicates and aluminosilicates
The representation of tetrahedral
silicates
Silicate : classification
Ortho/ neso
di / soro
Ino / meta
cyclo
Beryl [Si6O18]12-
Sheet
silicates
or phyllo
silicates
Many phyllosilicates have Al/Mg in Oh sites
Kaolinite
China Clay
Al2(OH)4 (Si2O5)
Al Octahedral
Si Tetrahedral
Si
Al
Kaolinite versus Talc: properties
China Clay Al2(OH)4Si2O5
Sticky due to Hydrogen bonding of OHgroups on the surface of the sheets
No OH groups
on surface
Bone china!
Talc Mg3(OH)2Si4O10
Smooth and free flowing
Structure of montmorillonite ( in Fullers earth)
Al4(OH)4 (Si4O10)2
Al Octahedral
Si Tetrahedral
(Si4O10)4
Fullers earth: montmorillonite,
Petrol additive
Methyl-t-butyl ether
kaolinite and attapulgite. Fuller’s
Earth, or Multani mitti, or known as
gopi chandan in India is known to be
an excellent skin-cleansing agent.
Muscovite Mica
The mica group represents 37 phyllosilicate minerals that have a layered
or platy texture. The commercially important micas are muscovite and
phlogopite, which are used in a variety of applications. The crystalline
structure of mica forms layers that can be split or delaminated into thin
sheets usually causing foliation in rocks. A tetrahedral Silicon is replaced
by a tetrahedral Aluminum in mica resulting in an extra charge
compensated by K+ in the interlamellar space leading to sheets
KAl2 (OH)2 Si3AlO10
Oh
Td
Talc Mg3(OH)2 Si4O10
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
Igneous rocks
K(AlSi3O8)
Ultramarines
Sodalite cage
Na8S2(Al6Si6O24)
Blue due to S3 radical anion
Zeolites
Linde A ( Mol sieves 4A)
Na6(Al6Si6O24). 2H20
Zeolites or Molecular Sieves
Catalyst, selective adsorbent, ion exchanger, rigid
support for HT,
1. The crown jewels of catalysis
2. Catalyst and adsorbents designed at the mol ecular scale
3. Can be an effective ca rrier or support
4. Contain acid sites of strong acidity for catalyt ic 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 Chemistry
General formula for the composition of a zeolite is
Mx/n[(AlO2) x(SiO2)y] . mH2O
where cations M of valence n neutralize the negatively
charged zeolite framework.
SiO2 tetrahedra are electrically neutral (e.g., quartz)
Substitution of Si(IV) by Al(III) creates an electrical imbalance
imballance
and neutrality is provided by an exchangeable cation
Na+
Si
Al
Na+
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
3 Dimensional Perspective of
Zeolites:
Truncating an Octahedron !
Octahedron
Truncated Octahedron
Also known as β cage
8 hexagons and 6 squares
Lowenstein’s Rule:
The AlO4- tetrahedra are
always interspersed with
SiO4 tetrahedra
Or
Al-O-Al units does not
occur in zeolites
The inside cavity is known as α
cage having 6 octagons
8 hexagons and 12 squares
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
Na+
Na+
H+
H+
Zeolite as synthesized
Bronsted acid form
+H2O
-H 2O (500 C)
+
Lewis acid form
STRUCTURE OF ZEOLITES
D4R
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
Sodalite
Linde A
not considered a zeolite (Molecular Sieves 4A)
Faujasite
D6R
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
Channel size 5.4 x 5.6 Å
Al: Si ratio 1:31
Most well known
zeolite catalyst
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 Å
Al: 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
Na2(AlO2)2(SiO2)5. 10 H2O
Al: Si ratio 2:5
Catalysis of larger molecules.
Metal complexes can be even
made inside the cavities
Linde A
ZSM 5
Faujasite
Si 50% - Al 50%
Cations Na, Ca
Polar
3-D straight channels
Si 93 %
non-polar
Hydrophobic
1-D channels
Si x% - Al y%
Cations Na, Ca
Polar
3-D entangled
channels
Ring 8
Ring 10
Ring 12
Examples of zeolites acting as selective catalysts in ACID CATALYSED reactions
ZSM-5: Shape Selectivity in Catalysis
(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;
toluene+ 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
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 with zeolite called mordenite
meta-xylene,
1,3,5- and 1,2,4-trialkylbenzene.
ZSM-5 catalyst
SHAPE SELECTIVE CATALYSIS - REACTANT, PRODUCT, TRANSITION
STATE SELECTIVITY
Chemistry of silicon: Challenges and recent developments
Multiple bonds between silicon atoms
ROBERT WEST
1981
UV VIS -* 420 nm
Akiro Sekiguchi
2004
Trans bent structure
Emerald green color
UV VIS -* 328 nm
Reactions of the first disilene
Reactions of the first disilyne
Zerovalent silicon stabilized by NHC
Si(i)
Si(0)
Stabilization of elusive silicon oxides
Science, 2008, 321, 1069, Greg Robinson
Nature, 2015, April, Greg Robinson
A different story for silicon bromides
Silanone
compounds
prepared
recently
having Si=O
but no three
coordinate
silicon
A stable silanone [Cr-(Si=O)-C type ]with a three co-ordinate silicon atom:
Silanones are some of the most sought after compounds of silicon. Unlike ketones,
silanones are very highly unstable and reactive. This is due to (a) a weak  bond
having unfavorable overlap between p (Si) and p(O) orbitals and (b) a strongly
polarized Si +--O- bond.
Road to an ideal silanone [C-(Si=O)-C type ] : where have we reached so far
(till 2015)?