catalysis and heterogeneous catalysis,
types of catalysis; difference between homo and hetero catalysis;
heterogeneous catalysis; preparation, characterization, supported catalysts, deactivation and regeneration of catalysts, example of drug synthesis
1. CATALYSIS
AND
HETEROGENEOUS CATALYSIS
PRESENTED BY : AZMIN M. MOGAL
( 2ND SEMESTER M.PHARM)
ENROLLMENT NO : 172540822003
GUIDED BY: Dr. Uttam A. More
( Assistant professor )
Department : Pharmaceutical chemistry
Subject : Advanced organic chemistry- II
Topic: Catalysis
Shree Dhanvantary Pharmacy College, kim-surat
2. CONTENT
• INTRODUCTION
• TYPES OF CATALYSIS
• DIFFERENCE BETWEEN HETEROGENEOUS
AND HOMOGENEOUS CATALYSIS
• ADVANTAGES AND DISADVANTAGES
• HETEROGENEOUS CATALYSIS
2
3. CATALYSIS
The substances that alter the rate of a
reaction but itself remains chemically unchanged
at the end of the reaction is called a Catalyst.
• The process is called Catalysis.
OR
Ostwald (1895) redefined a catalyst as, “A
substance which changes the reaction rate
without affecting the overall energetics of the
reaction is termed as a catalyst and the
phenomenon is known as catalysis’’.
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5. Characteristics of catalyst
A catalyst remains unchanged in its mass and chemical composition during
the reaction.
Only a small quantity of the catalyst is required.
A catalyst does not change the value of equilibrium constant. It only
hastens the attainment of equilibrium.
Catalysts are specific in nature. This means that every reaction is catalyzed
by a specific catalyst.
Catalyst exhibit maximum activity at a particular temperature which is
known as optimum temperature.
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6. TYPES OF CATALYSIS
1. Homogeneous catalysis :
When the reactants and the catalyst are in the same phase (i.e. solid, liquid or gas). The catalysis is said
to be homogeneous.
A)In Gas Phase :
Oxidation of Sulphur (SO2)to Sulphur trioxide(SO3)with nitric oxide (NO) as catalyst.
B)In Solution Phase:
I. Hydrolysis of cane sugar in aqueous solution in the presence of mineral acid as catalyst.
C12H22O11+ H2O C6H12O6 +C6H12O6
Glucose Fructose
II. Decomposition of Hydrogen peroxide (H2O2) in the presence of Iodine (I-) as catalyst.
2H2O2 2H2O+O2
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H2SO4
iodine
7. 2. Heterogeneous catalysis : The catalytic process in which the reactants and
the catalyst are in different phases is known as heterogeneous catalysis.
Example;
Combination of nitrogen and hydrogen to form ammonia in the presence of
finally divided iron(Haber process for ammonia).
N2 + 3H2 + Fe 2NH3 + Fe
2. Positive catalysis : When the rate of the reaction is accelerated by the
foreign substance, it is said to be a positive catalyst and phenomenon as
positive catalysis.
3. Negative catalysis : There are certain, substance which, when added to
the reaction mixture, retard the reaction rate instead of increasing it.
These are called negative catalyst or inhibitors and the phenomenon is
known as negative catalysis.
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gas gas solid
8. 5. Auto-catalysis : In certain reactions, one of the product acts as a catalyst. In
the initial stages the reaction is slow but as soon as the products come into
existences the reaction rate increases. This type of phenomenon is known as
autocatalysis.
6. Acid-base catalysis : In acid-base catalysis, a chemical reaction is catalyzed
by an acid or a base. The acid is the proton donor and the base is the proton
acceptor. Typical reactions catalyzed by proton transfer are esterification and
aldol condensation reactions.
7. Enzyme catalysis :Enzyme catalysis is the increase in the rate of a chemical
reaction by the active site of a protein. The protein catalyst (enzyme) may be
part of a multi-subunit complex, and/or may transiently or permanently
associate with a co-factor. (E.g. Adenosine Triphosphate). It is also known as
the Bio-catalysis.
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10. THEORY OF CATALYSIS
(i) Intermediate compound formulation theory:
According to this theory one of the reactants combines with catalyst to form intermediate
product, which carries out the reaction
(ii) Adsorption theory:
According to this theory, reactants are adsorbed on the surface of the catalyst and form a
film. Due to high concentration of the reactants on the film, reaction proceeds at a faster rate.
(iii) Modern adsorption theory:
According to this theory, reactants are adsorbed at the active centers i.e. free valencies
etc. on the solid surface and form activated complex which under strain forms new molecules
and leaves the surface. This explains, why the finely divided catalyst has greater activity.
(iv) Energy activation theory:
According to this theory, catalyst changes the value of activation energy which can be
crossed by the reactants easily and consequently products are formed.
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17. HETEROGENEOUS CATALYSIS
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The catalysis in which the catalyst is in a
different physical phase from the
reactants is termed heterogeneous
catalysis, most important of such reaction
are those in which the reactants are in the
gas phase while the catalyst is a solid the
process is called Contact Catalysis.
18. In heterogeneous catalysis, solids catalyze reactions of molecules in gas
or solution. As solids – unless they are porous – are commonly
impenetrable, catalytic reactions occur at the surface. To use the often
expensive materials (e.g. platinum) in an economical way, catalysts are
usually nanometer-sized particles, supported on an inert,porous
structure
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19. Examples of Heterogeneous Catalysis
• Heterogeneous Catalysis with gaseous reaction is (contact catalysis)
Combination if Sulphur dioxide (SO2 ) and oxygen in the
presence of finally divided platinum or vanadium pentoxide(V2O5) (contact
process for sulphuric acid).
2SO2 + O2 + [Pt] 2SO3+Pt
Gas Gas solid
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21. • As per the modern adsorption theory, which is a combination of the old
theory of adsorption and the intermediate theory of compound formation,
the process of heterogeneous catalysis takes place with these five steps,
1. The reactants diffuse to the surface of the catalysts. In this process, the
reactants first get in contact with the external surface, out of which some
of them cross the barrier and enter the interior exposed surface that
includes paths and cracks on the external surface.
2. These molecules then get adhered to the suitable sites available for
adsorption.
3. The reactants, when bound to the surface have a higher probability of
reacting with each other, and after the reaction, they form an
intermediate compound.
4. After this process, the intermediate compound gets desorbed from the
surface, which again becomes available for adsorption for other
molecules to come.
5. The intermediate compound then disintegrates to form the final
products, which then diffuse out of the internal pores and the external
surface of the catalyst. 21
22. 22
Here, we see that the catalyst remains
unchanged and is obtained in its original
form once the reaction is over. The mass
and the chemical composition of the
catalyst are not altered throughout the
process. We cannot explain the concept
of catalytic promoters and inhibitors
through the modern theory of adsorption.
25. HETEROGENEOUS CATALYST PREPARATION
Contemporary solid catalysts are rather refined and sophisticated
materials derived from commercially available chemicals. The variety of such
chemicals for the preparation is quite wide, indeed some catalysts can be
prepared by many different routes but, in general, some general elementary
steps or operations have to be followed. These steps or operations are based on
two approaches,
(a) Detailed knowledge of the scientific laws which govern chemical and
physical transformations based on the fundamentals of inorganic or solid
state chemistries, or
(b) empirical observations related to carefully guarded know-how.
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27. Heterogeneous catalysts preparation :
27
1. Preparation of Bulk catalysts
a. Precipitation method
b. Sol-gel method
c. Other methods
i. Hydrothermal synthesis
ii. Flame hydrolysis
iii. Other methods
2. Preparation of Supported catalysts
a. Simple Impregnation
b. Co-impregnation
3. Oxide catalysts preparation
a. Single oxides
b. Mixed oxides
4. Metal catalyst preparation
a. Reduction of oxides
b. Unsupported metal catalysts
c. Supported metal catalysts
5. Acid-base catalysts
30. IMPREGNATION METHOD
IMPREGNATION also called capillary impregnation or dry impregnation, is
a commonly used technique for the synthesis of heterogeneous catalysts.
Typically, the active metal precursor is dissolved in an aqueous or organic
solution.
Then the metal-containing solution is added to a catalyst support containing the
same pore volume as the volume of the solution that was added.
Capillary action draws the solution into the pores. Solution added in excess of the
support pore volume causes the solution transport to change from a capillary
action process to a diffusion process, which is much slower.
The catalyst can then be dried and calcined to drive off the volatile components
within the solution, depositing the metal on the catalyst surface.
The maximum loading is limited by the solubility of the precursor in the solution.
The concentration profile of the impregnated compound depends on the mass
transfer conditions within the pores during impregnation and drying.
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31. 31
• Dry Vacuum Pressure Impregnation
• Wet Vacuum Pressure Impregnation
• Wet Vacuum Impregnation
Dry Vacuum Pressure Impregnation
This process requires two process tanks. The advantage to this type of process is
there is no liquid present during the initial vacuum stage to interfere with pulling
gases from the pores. The pressure step is beneficial in forcing the sealant into the
pores. This type of process is excellent in sealing extremely small porosity.
Wet Vacuum Pressure Impregnation
This process only requires one process tank. Parts are submerged in the sealant
which remains in the process tank at all times. The vacuum is applied to the parts and
sealant together, followed by pressurization with air. The process retains the pressure
step, but does not use the dry vacuum.
Wet Vacuum Impregnation
This is the simplest and fastest of the casting impregnation processes. It is similar to
the VP method, except the tank is not pressurized. Penetration of the resin into the
parts takes place at atmospheric pressure.
37. Solid catalysts:
Catalyst components
A solid catalyst consists of mainly three components :
1. Catalytic agent
2. Support /carrier
3. Promoters and Inhibitors
Support or carrier :
• Support or carrier provides large surface area for dispersion of small amount of catalytically active
agent. This is particularly important when expensive metals, such as platinum, ruthenium, palladium
or silver are used as the active agent. Supports give the catalysts its physical form, texture,
mechanical resistance and certain activity particularly for bifunctional catalysts. Area of the support
can range from 1 - 1000 m2/gm. Common supports are alumina, silica, silica-alumina, molecular
sieves etc. The surface area of α - alumina is in the range 1-10 m2/gm whereas the surface area for γ
or η - alumina can be in the range 100 – 300 m2/gm.
• Support may be inert or interact with the active component. This interaction may result in change in
surface structure of the active agent and thereby affect the catalyst activity and selectivity. The
support may also exhibit ability to adsorb reactant and contribute to the reaction process.
• Supported catalysts: In supported catalysts, the catalytically active materials are dispersed
over the high surface area support material. For example, hydrodesulphurization is carried
out over molybdenum oxide supported on alumina.
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38. Preparation of solid catalyst
The catalyst preparation methods can broadly categorized as follows :
1. Bulk preparation process:
Bulk catalysts and supports are prepared by this method. Bulk preparation is mainly done
by the following methods :
a. Precipitation process
b. Sol gel process
2. Impregnation process:
Supports are first prepared by bulk preparation methods and then impregnated with the
catalytically active material. The active materials can be deposited on the supports by
various methods. Most of the methods involve aqueous solutions and liquid solid
interface. In some cases, deposition is done from the gas phase and involves gas- solid
interface.
3. Physical mixing :
Mixed agglomerated catalysts are prepared by this method. These catalysts are prepared
by physically mixing the active substances with a powdered support or precursors of
support in ball mill. The final mixture is then agglomerated and activated.
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40. DEACTIVATION
Loss in catalytic activity due to chemical, mechanical or thermal processes.
Heterogeneous catalysts are more prone to deactivation.
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41. Poisoning
Not only blocks the active sites, but also induce changes in the electronic or geometric structure of
the surface.
Poisons mainly include;
Groups VA and VIA elements (N, P, As, Sb, O, S, Se, Te)
Group VIIA elements (F, Cl, Br, I )
Toxic heavy metals and ions (Pb, Hg, Bi, Sn, Zn, Cd, Cu, Fe)
Molecules, which adsorb with multiple bonds(CO, NO, HCN, benzene)
Types:
Selective
Anti-selective
Non-Selective
Reversible
Non- reversible 41
43. Advantages of poisoning :
Pt-containing naphtha reforming catalysts are often pre-sulfided to
minimize unwanted cracking reactions.
S and P are added to Ni catalysts to improve isomerization selectivity in the
fats and oils hydrogenation industry.
V2O5 is added to Pt to suppress SO2 oxidation to SO3 in diesel emissions
control catalysts.
S and Cu added to Ni catalyst in steam reforming to minimize coking.
For selective hydrogenation from alkynes to alkenes, Lindlar catalyst
Pt/CaCO3) is partially poisoned with Pb and quinoline.
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44. Fouling / coking
Physical deposition of species from the fluid phase onto the catalyst
surface is fouling
Fouling of catalyst due to carbon deposition is coking. coke may contains
soot, produced in gas phase (non-catalytic carbon),
ordered or disordered carbon, produced on an inert surface (surface carbon),
ordered or disordered carbon, produced on surface which catalyses formation of
carbon (catalytic carbon),
condensed high molecular weight aromatic compounds which may be liquid or
solid (tar).
Coking can be studied under two headings:
coke formation on supported metal catalysts
Coke formation on metal oxide and sulphide catalysts
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48. Sintering
Support Sintering
Driving force is to lower the surface energy and the transport of
material
Coalescence of particles, particle growth and elimination of the pores.
Reaction atmosphere also promotes sintering.eg. Water vapour
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A model representing surface dehydroxylation from
contact region of two adjacent particles of alumina
γ-Alumina to δ-alumina to
α-phase via θ-phase
49. Metal sintering
Temperature: Sintering rates are exponentially dependent on T.
Atmosphere: Decreases for supported Pt in the following order: NO,
O2, H2, N2
Support: Thermal stability of supports Al2O3 > SiO2 > carbon for
given metal
Pore Size: Sintering rates higher in case of non-porous materials
Additives: C, O, CaO, BaO, CeO2 decrease atom mobility
Promoters: Pb, Bi, Cl, F, or S; oxides of Ba, Ca, or Sr are trapping
agents that decrease sintering rates.
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50. Chemical transformation / phase transformation
Reactions of gas/vapour with solid to produce volatile compounds
Direct volatilization temperatures for metal vaporization exceed
1000°C
metal loss via formation of volatile metal compounds can occur at
moderate temperatures (even room temperature)
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51. Reactions of gas/vapour with solid to produce inactive phases
Chemical modifications are closely related to poisoning
But the loss of activity is due to the formation of a new phase
altogether.
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53. Crushing of granular, pellet or monolithic catalyst forms due to a load.
Attrition, the size reduction and/or breakup of catalyst granules or
pellets to produce fines, especially in fluid or slurry beds.
Erosion of catalyst particles or monolith coatings at high fluid velocities.
collisions of particles with each other or with reactor walls,
shear forces created by turbulent eddies or collapsing bubbles
(cavitations) at high fluid velocities
gravitational stress at the bottom of a large catalyst bed.
Thermal stresses occur as catalyst particles are heated and/or
cooled rapidly
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Mechanical degradation
54. Regeneration procedure
The spent catalyst was solvent washed with heptane at 100 o C to remove excess wax.
The catalyst sample was subsequently subjected to a calcinations (i.e. oxidation) step in
a fluidized bed calcination unit, using an air/N2 mixture and the following heating
program: 2 ◦C/min to 300oC, 6–8 h hold at 300 o C.
The oxygen concentration was gradually increased from 3 to 21% O2/N2 to control the
exotherm.
The oxidized catalyst sample was subsequently subjected to a reduction in pure
hydrogen in a fluidised bed unit using the following heating program: 1 oC/min to 425 o
C, 15 h hold at 425 ◦C. The reduced catalyst was off loaded into wax.
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REGENERATION
59. 59
Worldwide, more than 85% of all chemical
products are manufactured with the help of
catalysts. Virtually all transition metals of the
periodic table are active as catalysts or catalyst
promoters. Catalysts are divided into
homogeneous catalysts, which are soluble in the
reaction medium, and heterogeneous catalysts,
which remain in the solid state. A heterogeneous
metal catalyst typically consists of the active
metal component, promoters, and a support
material.
