ROLE OF ADSORBENT IN CATALYTIC REACTIONS

Role of adsorbent in Catalytic reactions

Adsorbent in Heterogeneous catalytic reaction
Adsorption plays a major role inmost of the industries. Various manufacturing industrial chemical process are heterogeneously catalyst reactions. The function of catalyst is to increase the rate of the reaction and it does not affect the equilibrium.

The reaction mechanism includes the following steps.
Step 1 : Adsorption of reactant molecules on the surface of the solid.
Step 2 : Formation of activated complex.
Step 3 : Decomposition of activated complex.
Step 4 : Desorption of products.

1. Synthesis of Ammonia (Haber’s Process)
For synthesis of ammonia iron is used as catalyst with K2O and Al2O3 as promoters. Promoters are the substances which increase the activity of a catalyst. These promoters change the lattice spacing and increase the peaks and cracks on the catalyst surface.
Fe + Promoter
N2 + 3H2 -----------> 2NH3

2. Gas Masks (Purification)
Gas masks are device, which contain activated charcoal as adsorbent. It has high adsorptive capacity and adsorbs all toxic gases immediately (in >0.1 sec) and reduces the concentration of toxic gases from 1000 ppm to 1 ppm. The function of gas mask is to purify the air for breathing by adsorbing the poisonous gases and vapours from the atmosphere.

3. In Oil Industry
For Saturating the unsaturated parts in vegetable oil, nickel catalyst is extensively used.
Vegetable Oil + H2 Ni Vegetable ghee.

Classification of Activated Carbon
Activated carbon is classified into two main classes based on their size and different adsorption capacities.

1. Granular Activated Carbon
These types of carbons have a diameter greater than 0.1 mm. It is used for adsorption of gases and vapours.
Carbon is classified into two main classes based on their size and different adsorption capacities.

2. Powdered Activated Carbon
These types of carbons have a diameter of less than 200 mesh. It is used for purification of liquids.

Activated of Carbon
During activation, charcoal is heated in stream or air atmosphere to about 1500°C. Heating drives out all the impurities as gases and these by leads to a layer surface for adsorption.

a. Direct Method
In this method, activation is carried out by heating carbon in steam or air at about 850°C. Heating drives out all the impurities sticking on the surface and blocking the capillary poles. This leads to a large surface area of carbon.

b. Briquetting Method
The powdered carbon is mixed with wood ear and caustic soda compressed into small briquettes and then heated in steam at 800°C.

c. Chemical Method
The wood is impregnated with a solution of Zinc Chloride or Phosphoric acid and then carbonized.

Role of Activated carbon in waste water treatment
The waste water contains number of compounds which are resistant to bio-degradation and persist in water even after primary and secondary treatments. These impurities are generally carcinogenic and toxic in nature
These compounds may be eliminated by passing over a bed of granulated activated carbon (GAC). These compounds present in waste water are adsorbed by charcoal and removed from water.

a. Treatment of waste water
1. Using a trickling filter method
The small quantities of dissolved organic hazardous waste can be removed by this method. The filter consists of a number rectangular or circular vessels. These vessels are packed with granular activated carbon (GAC). GAC has an enormous surface area (about 1000 m2/g) and is commonly used as adsorbent.

Process
The contaminated water is allowed to trickle down through the GAC packed in a series of vessels. As the trickled waste water starts percolating down wards through the granular activated carbon the hazardous organics in polluted water are adsorbed on the porous GAC matrix. The GAC filter after sometime gets clogged with the adsorbed contaminates and hence must be replaced or regenerated for further use.

Regeneration
When the charcoal becomes saturated, it is heated in vacuum to above 1500°C and the adsorbates are driver off.

Activated carbons used in other industries
1. Refining of sugar, glucose.
2. Removal of taste & colour of drinking water.
3. Removal of metals from waste water
4. Removal of impurities from pharmaceutical.

Role of activated carbon in air
Finally powdered activated carbon is an excellent adsorbent for the removal of toxic gaseous pollutants of impure air. It is used to remove impurities like NO2, SO2, CO and CO2 from impure air.

a. Treatment of polluted air
Using charcoal adsorption tube - In this technique, the polluted air is sucked by the tube and is passed through a glass wool. The pollutants in air are adsorbed on the granular activated charcoal.
The amount of pollutant adsorbed is proportional to the surface area of the adsorbent and the Physics, Chemical characteristics of the adsorbent. The temperature are pressure are maintained during the process. The GAC filter after sometime gets clogged with the adsorbed contaminates. The adsorbed pollutants can be leached with a suitable solvent such as carbon disulphide and the solution is analysed by gas chromatography.