Mechanisms of Catalytic Reactions and Characterization of Catalysts

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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
Thermodynamic
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 (?G -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 Profile
Reaction path for conversion of A B into
AB
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Activation Energy
Definition
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 Affect

• Catalyst lowers the activation energy for both
  forward and reverse reactions.

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Activation Energy
Catalyst Affect
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
H2 adsorption on palladium
Surface process
bulk process
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Adsorption

• In physisorption
• The bond is a van der Waals interaction
• adsorption energy is typically 5-10 kJ/mol. (
  much weaker than a typical chemical bond )
• many layers of adsorbed molecules may be formed.
  

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Adsorption

• For Chemisorption
• The adsorption energy is comparable to the energy
  of a chemical bond.
• The molecule may chemisorp intact (left) or it
  may dissociate (right).
• 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) lt DE(ads) Physisorption
Chemisorption small minima large
minima weak Van der Waal formation of
surface attraction forces chemical bonds
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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
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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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ExampleThe Reaction A2 2B 2AB
Langmuir-Hinshelwood mechanisms
may have the following mechanism A2
A2A2 2AB BA B AB
AB AB
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Adsorption Mechanisms

• Eley-Rideal mechanism
• Adsorption from the gas-phase
• Desorption to the gas-phase
• Dissociation of molecules at the surface
• Reactions between adsorbed molecules
• 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
A2A2 2AA 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
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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 P-Substituted 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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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

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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

• Electronegativity Effect

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)

• 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)
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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.
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End of the Catalysis and Catalysts
THANK YOU

• References
• A.Corma,Inorganic solid acids and their use in
  acid-catalyzed hydrocarbon reactions. Chem.
  Rev.1995, 95, 559-614.
• Bruce C. gates, Catalytic Chemistry.1992.
• Zaki Seddigi, Characterization of the acidic
  properties of zeolite and their catalytic
  behavior in the synthesis of MTBE. 1994.
• Ali El-Rayyes, Study of the photochemical
  properties of some aromatic compound on molecular
  sieves using a picosecond pulse laser system.
  2001.
• Keith Laidler, Chemical Kinetics, 1987.