�ݺ�ߣ

CATALYST PREPARATION
A Presentation by
ANOOPAANN THOMAS
M180294CH

CLASSIFICATION OF CATALYSTS
I. Based on physical state
II. Based on chemical nature
III. Based on nature of the reactions they catalyze

GASEOUS CATALYST
eg:- Oxides of nitrogen serve as catalysts for the oxidation of SO2
in the Lead chamber process for producing H2SO4.
LIQUID CATALYST
eg:-Sulphuric acid is used as a catalyst for the formation of
diethyl ether from ethyl alcohol.

CLASSIFICATION OF SOLID CATALYSTS
1. Bulk catalysts
2. Supported catalysts
3. Mixed agglomerates

BULK CATALYSTS
The entire catalyst consists of the catalytically active substance
Eg:-
 Silica –alumina catalysts for catalytic cracking
 Iron-molybdate for oxidation of methanol to formaldehyde
 Iron doped with alumina and potassium oxide for the
synthesis of ammonia

SUPPORTED CATALYST
The catalytically active materials are dispersed over a high surface
area support material.

COMPONENTS OF A SUPPORTED CATALYST
 Catalytic agent
 Support/Carrier
 Promoters and inhibitors

CATALYTIC AGENT
 Catalytically active component in the catalyst.
 Generate the active sites that participate in the chemical reaction.
Activity of any catalyst is proportional to the concentration of active sites.
 Availability of active sites depends mainly on the dispersion of catalytic agent.
What is dispersion??
Total number of exposed atoms of catalytic agent available for reaction
Total number of atoms of catalytic agent present in the catalyst sample

CATALYTIC AGENTS
METALLIC
CONDUCTORS
SEMI-
CONDUCTORS
INSULATORS

METALLIC CONDUCTORS
Metals that have strong electronic interaction with the adsorbates.
Eg:-
 Cu for water gas shift reaction and methanol synthesis
 Au for oxidation of methanol to formaldehyde
 Fe for NH3 synthesis

SEMI - CONDUCTORS
The oxides and sulphides of transition metals that have catalytic
activity.
Eg:-
 CuO for oxidation of nitric oxides
 NiO for dehydration of alkanes
 V2O5 for oxidation of hydrocarbons

INSULATORS
 Functions of insulators are different from that of conductors and
semi conductor materials.
 Insulators have sites that generate protons there by promote
carbonium ion based reactions such as cracking,isomerization or
polymerization.
 Zeolites,Al2O3,SiO2,CaO are few examples of the insulators used as
catalyst.

SUPPORT / CARRIER
 Provides large surface area for dispersion of small amount of
catalytically active agent
 Carrier is important when expensive metals such as
platinum,ruthenium,palladium or silver are used as the active agent.
 Area of the support can range from 1-1000 m2/gm .
 Common supports are alumina, silica, silica- alumina, molecular
sieves etc.
 Support may be inert or interact with the active component.
 Exhibit ability to adsorb reactant.

PROMOTERS
 Substances added during preparation of catalysts that improve
the activity or selectivity or stabilize the catalytic agents.
 Promoters are termed as physical or chemical promoter
depending on the manner they improve the catalyst performance.
 Additives that maintain physical integrity of the support and or
deposited catalytic agents are termed as physical promoters.
 eg:-
In ammonia synthesis,Fe is used as catalyst
Alumina (physical promoter)
K2O (chemical promoter)

3. MIXED AGGLOMERATES
Agglomerated mixture of active substance and support.
 These types of catalysts are used less frequently.

Till now.....
Catalyst
classification
Chemical nature
Nature of the reactions it catalyze
Physical
state
Gaseous
Liquid
Solid
Bulk catalyst
Supported
catalyst
Mixed
agglomerates

UNIT OPERATIONS INVOLVED IN CATALYST
PREPARATION
1. Precipitation
2. Gelation
3. Flame hydrolysis
4. Decantation,filtration,centrifugation
5. Washing
6. Drying
7. Calcination
8. Forming operation
9. Impregnation
10. Crushing and grinding
11. Mixing
12. Activation

PREPARATION OF BULK CATALYST AND SUPPORT
 Precipitation process
 Solgel Process

PRECIPITATION PROCESS
Precipitation occurs in three steps
1. Supersaturation
2. Nucleation
3. Growth

Parameters affecting super saturation

Nucleation
 The nucleus is defined as the smallest solid phase aggregate of
atoms, molecule or ions which is formed during a precipitation
and which is capable of spontaneous growth.
 when the concentration exceeds a critical threshold value a
nucleus will form and the precipitation will begin.
 Nucleation starts with the formation of clusters which are
capable of spontaneous growth by the subsequent addition of
monomers until a critical size is reached.

Growth
 Growth is a function of concentration, temperature and pH.
 Rates of nucleation and growth can be independently controlled
 If nucleation is faster than growth, the system produces a
narrow distribution of small particles. Fast growth results in
narrow distribution of large particles.

SOL GEL PROCESS
SOL GEL PROCESS

 In sol gel process initially a stable colloidal solution called sol is
formed.
 It can be obtained by hydrolysis and partial condensation.
 The condensation of sol particles into a three dimensional network
produces a gel material.
 The gel is a diphasic material in which the solids encapsulate the
solvent.
 The encapsulated liquid can be removed from a gel by either
evaporative drying or with supercritical drying /extraction. The
resulting solid products are known as xerogel and
aerogel respectively.

 Sol-gel synthesis offers better control over surface area , pore
volume and pore size distribution.
 The point at which gelation occurs depends on
Concentration of micelles
Temperature
Ionic strength of the solution
pH

FLAME HYDROLYSIS
 The precursor (metal chloride or carbonyl) is hydrolyzed by water
 Water is formed by hydrogen combustion (hydrogen and air/oxygen
are brought into contact in flame of a torch.
 Used to produce high surface area catalysts

DECANTATION, FILTRATION, CENTRIFUGATION AND
WASHING
 Separate solid phase from mother liquor.
 Washing is required to remove mother liquor completely and to
eliminate impurities.
 The selection of separation method depend on the particle size
of the solids.

DRYING
 Elimination of solvent from the pores of a solid.
 It is important in case of hydrogels that can contain upto 90%
water.
Pore volume reduction on drying a silica hydrogel

CALCINATION
 Heat treatment process beyond drying
 Performed at a very high temperature, higher than those used in
the catalytic reaction and catalyst regeneration.
 Processes occur during calcination
 loss of the chemically bonded water or CO2
 Modification of the texture through sintering
 Modification of the structure
 Stabilization of mechanical properties

Effect of calcination temperature on the pore size of ɤ-Al2O3

FORMING OPERATIONS
 Applied to obtain suitably sized particles of catalyst to be used
in the reactor.
 Importance of forming operations
 Promote catalytic activity
 Strengthen the particle resistance to crushing and abrasion
 Minimize bed pressure drop
 Lessen fabrication cost

1. Spray drying
Spray drying is suitable technique to produce micro-spherical
particles (20-100 μm).

2. Extrusion and wet pressing
 Most commonly applied shaping technique for catalyst and catalyst
supports.
 A wet paste is forced through a die.
 A number of materials need a binder to produce extrudates with
good mechanical behaviour.
 Additives used
 For improving the rheological behaviour of the paste (Clays /
Starch)
 Binders (Alumina , Clays)
 Combustible materials to increase the macroporosity (Carbon
black , Starch)

PREPARATION OF SUPPORTED CATALYST
 The catalyst preparation is characterized by the wetting of the
support with a solution or a slurry of the active phase precursors.
 Precipitation is the preferred deposition route for loading
higher than 10-20%.Below this value , impregnation is usually
practiced.

 Three steps in impregnation
I. Contacting the support with the impregnating solution
for a certain period of time.
II. Drying the support to remove the imbibed liquid
III. Activating the catalyst by calcination or reduction
 Two types of contacting
I. With excess of solution
II. With repeated application of solution

References:-
1. Catalyst preparation methods , Carlo perego & Pierluigi villa,
Politecnico di Milano, Deparhnent of Industrial Chemistry and
Chemical Engineering, Piazza Leonardo da Vinci, 32, 20133
Milano, Italy.
2. J.J. Carberry, Chemical and catalytic reaction engineering,
Dover Publications, 2001
3. G. Ertl, H. Knozinger & J. Weitkamp, Handbook of
Heterogeneous Catalysis, Vol 1, Wiley – VCH, 1997

�ݺ�ߣ

Catalyst preparation methods

More Related Content

Catalyst preparation methods