Download Catalysis and Catalyst

Mechanisms of Catalytic Reactions
and Characterization of Catalysts
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What is a Catalyst ?
Catalyst is a substance that increases the rate
of the reaction at which a chemical system
approaches equilibrium , without being
substantially consumed in the process.
Catalyst affects only the rate of the
reaction,i.e.Kinetics.
It changes neither the thermodynamics of
the reaction nor the equilibrium composition.
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Chemical Reaction
Thermodynamics says NOTHING about the rate of a
reaction.
Thermodynamics : Will a reaction occur ?
Kinetics
: If so, how fast ?
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Kinetic Vs. Thermodynamic
A reaction may have a large, negative DGrxn, but the
rate may be so slow that there is no evidence of it
occurring.
Conversion of graphite to diamonds is a
thermodynamic favor process (DG -ve ).
C (graphite)  C (diamond)
Kinetics makes this reaction nearly impossible
(Requires a very high pressure and temperature over long time)
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Kinetic Vs. Thermodynamic
Reaction path for conversion of A + B into AB
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Activation Energy
Activation Energy : The energy required to overcome the reaction
barrier. Usually given a symbol Ea or ∆G≠
The Activation Energy (Ea) determines how fast a reaction occurs, the higher
Activation barrier, the slower the reaction rate. The lower the Activation
barrier, the faster the reaction
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Activation Energy
Catalyst lowers the activation energy for both forward and
reverse reactions.
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Activation Energy
This means , the catalyst changes the reaction path
by lowering its activation energy and consequently
the catalyst increases the rate of reaction.
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How a Heterogeneous Catalyst works ?
Substrate has to be adsorbed on the active sites of the catalyst
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Absorption and Adsorption
H H H H H H H H
H
H H H H H H H H
H
H
H HH H
H H
H
H H
H
H
H
H
H
H
H
H
H2 adsorption on
palladium
Surface process
H2 absorption 
palladium hydride
bulk process
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Adsorption
In physisorption
1. The bond is a van der Waals interaction
2. adsorption energy is typically 5-10
kJ/mol. ( much weaker than a typical
chemical bond )
3. many layers of adsorbed molecules may
be formed.
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Adsorption
For Chemisorption
1. The adsorption energy is
comparable to the energy of a
chemical bond.
2. The molecule may chemisorp intact
(left) or it may dissociate (right).
3. The chemisorption energy is 30-70
kJ/mol for molecules and 100-400
kJ/mol for atoms.
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Characteristics of Chemi- and Physisorptions
DE(ads)
<
Physisorption
E(d)
small minima
weak Van der Waal
attraction forces
DE(ads)
Chemisorption
large minima
formation of surface
chemical bonds
CO
physisorption/
desorption chemisorption
physisorption
atomic chemisorption
d
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Adsorption and Catalysis
Adsorbent: surface onto which adsorption can occur.
example: catalyst surface, activated carbon, alumina
Adsorbate: molecules or atoms that adsorb onto the substrate.
example: nitrogen, hydrogen, carbon monoxide, water
Adsorption: the process by which a molecule or atom adsorb onto a surface of
substrate.
Coverage: a measure of the extent of adsorption of a specie onto a surface
H H H H H H H H
H
adsorbate
coverage q = fraction of surface sites occupied
H
H H
H
H
adsorbent
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Adsorption Mechanisms
Langmuir-Hinshelwood mechanisms:
1. Adsorption from the gas-phase
2. Desorption to the gas-phase
3. Dissociation of molecules at the surface
4. Reactions between adsorbed molecules
Two Questions:
• Is the reaction has a Langmuir-Hinshelwood mechanism?
• What is the precise nature of the reaction steps?
Cannot be solved
without experimental or computational studies
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Langmuir-Hinshelwood mechanisms
Example
The Reaction
A2 + 2B = 2AB
may have the following mechanism
A2 + * = A2*
A2* + * = 2A*
B + * = B*
A* + B* = AB* + *
AB* = AB + *
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Adsorption Mechanisms
Eley-Rideal mechanism:
1. Adsorption from the gas-phase
2. Desorption to the gas-phase
3. Dissociation of molecules at the surface
4. Reactions between adsorbed molecules
5. Reactions between gas and adsorbed molecules
The last step cannot occur in a Langmuir-Hinshelwood
mechanism
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Eley-Rideal mechanism
Example
The reaction
A2 + 2B = 2AB
may have the following Eley-Rideal mechanism
A2 + * = A2*
A2* + * = 2A*
A* + B = AB + *
where the last step is the direct reaction between the adsorbed
molecule A* and the gas-molecule B.
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Eley-Rideal or Langmuir-Hinshelwood?
For the Eley-Rideal mechanism:
the rate will increase with increasing coverage
until the surface is completely covered by A*.
For the Langmuir-Hinshelwood mechanism:
the rate will go through a maximum and end up
at zero, when the surface is completely covered
by A*.
This happens because the step
B + * = B*
cannot proceed when A* blocks all sites.
The trick is that the step
B + * = B*
requires a free site.
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Catalyst Preparation
(1) Unsupported Catalyst
Usually very active catalyst that do not require high
surface area
e.g., Iron catalyst for ammonia production (Haber process)
(2) Supported Catalyst
requires a high surface area support to
disperse the primary catalyst
the support may also act as a co-catalyst
(bi-functional)
or secondary catalyst for the reaction
(promoter)
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Supported Catalyst
Highly dispersed metal on metal oxide
Nickel clusters
SiO2
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Molecules in Zeolite Cages and Frameworks
+ p-xylene
ZSM-5
Paraffins
Y-zeolite
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What is ZSM-5 Catalyst ?
 It is an abbreviation for (Zeolite Scony Mobile Number 5 )
 First synthesized by Mobil Company in 1972
 It replaces many Homogeneous Catalysts were
used in
many petrochemical processes
 ZSM-5 has two diameters for its pores : d1= 5.6 Å , d2= 5.4 Å
Where as, Zeolite Y has a diameter = 7.4 Å
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Properties ZSM-5
The ZSM-5 zeolite catalyst is used in the petroleum industry
for hydrocarbon interconversion.
ZSM-5 zeolite is a highly porous aluminosilicate with a high
silica/alumina ratio.

It has an intersecting two-dimensional pore structure.

The aluminum sites are very acidic.

The acidity of the zeolite is very high.

The reaction and catalysis chemistry of the ZSM-5 is due
to this acidity.
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Structure of ZSM-5
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Aromatic Isomerization
Observed by Haag and Co-workers P-Selectivity is
achieved due to the high diffusion coefficient of PSubstituted Molecules Relative to that of Ortho or Meta.
A side reaction in homogenous solution phase
isomerization is the Bimolecular disproportionation of
Xylene.
This Can not occur when using zeolite such
as ZSM-5 since the pore size will not allow
for the bulky bimolecular transition state
necessary for disproportionation.
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+ p-xylene
ZSM-5
Compound
p-Xylene
o-Xylene
m-Xylene
Relative Diffusion
Coefficient
> 10000
1
1
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Zeolite
Catalyst
Type
Effective Diameter of
Kdis/Kiso
ZSM-5
Mordenite
ZSM-4
0.60
1000
0.70
0.80
15000
10000
HY
> 1.2
> 45000
intracrystalline
Cavity (nm)
Selectivities of acidic zeolite for disproportionation and
isomerization of Xylene. The HZSM-5 Catalyst is preferred,
minimizing the bimolecular disproportionation reaction by
virtue of restricted transition state selectivity.
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Schematic diagram of product shape selectivity: Para-xylene
diffuses preferentially out of the zeolite channels
p-xylene
m-xylene
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Acidity of the Catalyst Vs. Acid Sites
Theoretically,
As the Acid Sites increase the Acidity increases
Experimentally,
As the Acid Sites increase the Acidity decreases
How we can understand this behavior?
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Acidity of the Catalyst Vs. Acid Sites
Al is the Acidic Site
But,
Si is more electronegative than Al
B
C
N
O
F
Al
Si
P
S
Cl
•Electronegativity Effect
H+
O
Al
O
Si
Al
Si
Si Makes the release of the H+ faster since it attracts Electrons
toward it which make the proton away of it
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Acidity of the Catalyst Vs. Acid Sites
It is possible to conclude that:
the acid strength of a site will increase when there is a decrease
in the number of Al atoms in the Next Nearest Neighbor
position of the Al atom.
So, the strongest type of framework Bronsted site is
A completely isolated Al tetrahedron which have zero NNN or
(0NNN)
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Acidity Characterization of a Catalyst
Acidity of the catalysts can be assess by:
I. Temperature Programmed Desorption (TPD)
II. Fourier Transformation Infrared spectroscopy ( FTIR)
III. Induced Laser spectroscopy (IL)
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I. Temperature Programmed Desorption (TPD)
1. Pure carrier gas (typically helium) flows over the sample as the
temperature is raised to desorb the previously adsorbed gas e.g.
NH3
2. This characteristic "fingerprint" for each catalyst,used to determine:
the distribution of acid-site strength if ammonia is the sorbed gas,
or the distribution of basic sites if carbon dioxide is the sorbed gas.
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I. Temperature Programmed Desorption (TPD)
Temp.
Time(min)
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II. Fourier Transformation Infrared spectroscopy ( FTIR)
By this method we assess acidic site:
Bronsted acidic site or Lewis acidic site
For example: if pyridine is the probe molecule, It will give peaks at:
1. 1540 cm-1 for Bronsted site
2. 1450 cm-1 for Lewis site
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II. Fourier Transformation Infrared spectroscopy ( FTIR)
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II. Fourier Transformation Infrared spectroscopy ( FTIR)
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III. Induced Laser spectroscopy (IL)
To assess the acidity by IL:
1. Probed molecule has to be prepared in different acidic
concentration solutions
2. Life time measurements has to run onto each concentration
3. Calibration curve between life time Vs. concentration of the acid
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III. Induced Laser spectroscopy (IL)
Life time(ns)
Life time Vs. [HCl]
y = 0.0814x + 1.12
R2 = 0.9627
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
4
5
6
7
8
9
10
11
12
[HCl]
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Conclusion
There is no single method that can be used to determine all
aspects of acidity for a solid i.e. nature, strength and the number
of acid sites
Each method measures only certain aspects and,therefore, the
application of many methods is desirable.
THE END
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End of the Catalysis and Catalysts
THANK YOU
References
1.
A.Corma,Inorganic solid acids and their use in acid-catalyzed hydrocarbon
reactions. Chem. Rev.1995, 95, 559-614.
2.
Bruce C. gates, Catalytic Chemistry.1992.
3.
Zaki Seddigi, Characterization of the acidic properties of zeolite and their
catalytic behavior in the synthesis of MTBE. 1994.
4.
Ali El-Rayyes, Study of the photochemical properties of some aromatic
compound on molecular sieves using a picosecond pulse laser system. 2001.
5.
Keith Laidler, Chemical Kinetics, 1987.
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