Preperation of the catalyst

2. Basic unit operations in solid catalyst preparation
Industrial Catalysts
Industrial catalysts can be broadly grouped into three
categories:
1. Bulk Catalyst: When the entire catalyst consists of the
catalytically active substance then the solid catalyst is
called a bulk catalyst. Examples are:
 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.
2. Supported catalysts: In supported catalysts, the
catalytically active materials are dispersed over the high
surface area support material. Examples are:
 hydrodesulphurization is carried over molybdenum oxide
supported on alumina.
3. Mixed agglomerates: These catalysts are
agglomerated mixture of active substance and support.
These type of catalysts are used less frequently.

3. 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. Solgel 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 involve s 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.

4. Precipitation and co-precipitation
 desired component is precipitated from the solution.
 Mainly used when active components are cheap.
 Co-precipitation is used for simultaneous precipitation of more than one component.
 Catalysts based on more than one component can be prepared easily by co-precipitation.
 The precipitation process is used for preparation of bulk catalysts and support material such as Al2O3,
SiO2, TiO2, ZrO2 etc.
 the metal hydroxides are precipitated from their precursor salt solution because of their low solubility.
 The precipitation of hydroxides can be performed from an acidic solution by the addition of an alkaline
precipitating agent.
 Usually, ammonia or sodium hydroxide is used as the precipitating agent.
 Highly soluble inorganic salts such as nitrates, carbonates or chlorides are generally used as metal
precursors.
 For example, preparation of alumina is done by precipitating aluminium hydroxide from aluminium nitrate
solution by addition of ammonium hydroxide
Process

5. Basic Steps of Precipitation Process

6.  For precipitation the solution must be super saturated
 In supersaturated region the system is unstable and
precipitation occurs with any small disturbance.
 The supersaturation region is approached
• by increasing the concentration through evaporation
• lowering the temperature
• by increasing pH (mostly used)
 The solubility curve is also function of pH.
 As pH increases solubility decrease and curve shift from 1 to
position 2. Then the point which was initially in solution
region becomes in supersaturate region.
 Commonly used reagents are NaOH, KOH, NH4OH, carbonates
and bicarbonates.
 Particles within supersaturated region develop in two steps :
nucleation and growth.
Parameters affecting supersaturation

7. Major steps during precipitation:
1. liquid mixing / supersaturation
2. nucleation
3. crystal growth to form primary products
4. aggregation of the primary particles
• Initial mixing or inter-dispersing of components in the solution has a significant effect on the
precipitation.
• Good mixing result in a more homogeneous product particularly in case of co- precipitation.
• Rate of stirring primarily affects the nucleation whereas growth rate is function of solution
concentration.
• Stirring rate also affect the aggregation.
• Aggregate size can be influenced by changing the stirring rate and the manner of mixing.

8. Precipitation process can be carried out in different ways.
 Batch mode
 Sequence of addition of the starting materials.
 Continuous mode.
Batch process:
• The metal salt solution is taken in a vessel and
the precipitating agent is added.
• The advantage of this method is its simplicity.
• variation of batch composition during
precipitation which leads to changes in the
properties of the precipitate.
Continuous process:
• Continuous addition of salt solution and precipitating
agent to a vessel with simultaneous withdrawal of
precipitate.
• This process has a higher demand on process control.
• All the parameters (pH, temperature, concentration,
residence time) can be controlled as desired.
a. Homogeneous mixture (large amount precipitating agent)
useful for co-precipitation
b. hydroxide with lower solubility tends to precipitate first
i.e.formation of non-homogeneous product
c. better homogeneity and process control but Aging is also
longer for final products
Aging results in change in structure and properties of
hydroxide network. Aging leads to more crosslinked
network.
Sequence of addition of the starting materials:

9. Process parameters

10. Preparation of dual oxides catalysts by coprecipitation
Mixed oxide support and catalyst can be prepared by coprecipitation method.
the solubility of the two components should be in similar range for simultaneous precipitation
resulting in homogeneous product.
Otherwise the precipitation will be sequential resulting in non-homogeneous product.
1. SiO2-Al2O3
SiO2-Al2O3 is used in catalytic cracking process and is also used as support for active metals in various
applications. Preparation of dual oxides by coprecipitation is similar to precipitation of single oxide. At
pH 6 (at 500C) the precipitation of both silica and alumina sols begins and gelation takes places.
2. NiO-Al2O3
NiO-Al2O3 is used for hydrogenation and methanation reactions. Although this catalyst can be
produced by other route, coprecipitation method of preparation is also done to increase the intimate
interaction between active metal and support. The sodium bicarbonate can be used as precipitating
agent for formation of nickel aluminium hydroxyl carbonate with good homogeneity of final product.

11. Sol gel method
 a stable colloidal solution called sol is formed.
 The sol is a liquid suspension of solid particles(1 nm to 1 micron)
 It can be obtained by hydrolysis and partial condensation of
precursors such as an inorganic salt or a metal alkoxide.
 The further 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 molecular weight of the oxide species produced continuously
increases.
 aqua sol or aqua gels when water is used as a solvent and
aquosol or alcogel when alcohol is used.
 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.
 When gels are dried by evaporation, the dried product is called
xerogel.
 When the gels are dried by supercritical drying, the dried gel is
called aerogels.
 The aerogel retains high porosity and has very high pore volume. General scheme of preparation by solgel method

12. Impregnated Catalyst/ Supported Catalyst
 One of the best known methods for producing catalysts is the impregnation of porous support
materials with solution of active components.
 Impregnation as a means of supported catalyst preparation is achieved by filling the pores of a
support with a solution of the metal salt.
 Especially, catalyst with expensive active components such as noble metals are employed as supported
catalysts.
 A widely used support is Al2O3, activated carbon and silica gel.
Industrial examples:
 Ethylene oxide catalyst in which a solution of a silver salt is applied to Al2O3
 Catalyst in the primary reformer of ammonia synthesis with 10-20% Ni on Al2O3

13. Impregnation Process:
 The support is immerses in a solution
of the active component under precisely
defined conditions (concentration,
mixing, temperature, time)
 Depending upon the production
conditions, selective adsorption of the
active component occurs on the surface
or in the interior of the support. The
result is non uniform distribution.

14. Washing and filtering
 Washing can be done by decantation. This method is time consuming.
 In this method the precipitate or gel is added to a large volume of distilled water and the suspension is
thoroughly stirred. Then, the suspension is allowed to settle.
 The foreign undesirable ions are desorbed from particles as they settle down slowly at the bottom.
 When a clear interface is visible, the water is removed by decantation and the process is repeated.
 The number of washings required is determined by checking the impurity level of the decanted water.
 After washing, the precipitate or gel is filtered. The process can be reversed.
 That is the filtration is done first and the precipitate or gel is washed with distilled water in the subsequent
step. This method takes less time.
 Impurity level in the wash water is checked to determine the required number of washings.
Drying
Drying is described as the elimination of water or solvent from the pores of the precipitate or gel. It can be done
in two ways:
 Solvent evaporation
 Super critical drying

15. Calcinations or sintering
After the removal of pore liquid, further heat treatment is necessary to convert the precipitate or dry gel to
catalytically useful form. After drying, the next step of heat treatment is known as calcination. Often the heating is
done in the presence of flowing air or oxygen to burn any residual organics or to oxidize the sample. Multiple
changes occur during this process including:
1.Active phase generation: The hydroxide form is converted to oxide form.
2. Stabilization of mechanical properties:
The catalysts sample is subjected to a more severe heating treatment than that is likely to encounter in a
reactor to ensure the stability of its textural and structural properties during reaction.
3. Loss of chemically bound water: The chemically bound water is removed at higher temperature.
4. Changes in pore size distribution and surface area due to sintering: Exposing the sample to high temperature
over an extended period of time leads to some sintering and consequently decrease in surface area.
5. Change in phase distribution: Higher temperature cause material to crystallize into different structural forms.

16. Extrusion:
 suspension or paste of the catalyst powder is passed through a profiled die that determines the shape of
the body.
 Screw extruders are very common in use.
 Slurry of the catalyst is fed to the extrudate at one end and the screw forces the slurry through the holes
at the other end.
 As the ribbon of slurry emerges from the holes, a knife is arranged at the end to cut it to the required
size.
 Particles of narrow size distribution can be obtained by this method.
Spray drying:
 This process involves atomization of slurry feedstock into a spray of droplets and contacting the droplets
with hot air in a drying chamber.
 Particle sizes are determined by the size of droplets, which is controlled by design of spray nozzles, slurry
flow rate, slurry viscosities.
 Products in a spray dryer are spheres of diameters in the range of 0.05 to 0.5 mm.

17. Schematic diagram of different catalysts formulation techniques