There are two types of catalyst- Homogeneous catalyst and heterogeneous catalyst. This difference is based on phase of the reactants and catalyst. If both are in same phase then it is called Homogeneous catalysis and if catalyst and reactants have different phase then it is Heterogeneous catalysis.
Many processes using homogeneous catalyst, which occurs in liquid phase. When a heterogeneous catalyst is used, the catalyst is usually in solid form and the reaction occurs either in liquid or gaseous phase. Heterogeneous catalysts are simple and cheap as compared to homogeneous catalysts.
Definition of Heterogeneous catalysts:
Heterogeneous catalysts relates to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase includes not only solid, liquid and gas, but also includes immiscible liquids, like oil and water.
The large number of heterogeneous catalysts are solids and the bulk of the reactants are gases or liquids.
Types of Heterogeneous catalysts:
1) Supported Catalysts:
A catalyst support is the material, generally a solid with a prominent surface area, to which a catalyst is added. The reaction of heterogeneous catalysts and nano-material based catalysts occurs at the surface atoms.
The support may be inert or it may participate in the catalytic reactions. Support are thermally vaey stable and stand firm against processes required to activate precatalysts.
Examples: Alumina, silica, chromium, various kinds of Carbon etc.
2) Solid acid Catalysts:
Solid acids catalyse many industrial processes. Solid acids do not dissolve in the reaction medium as they act as heterogeneous catalysts. A very important application of solid acid catalyst is alkylation, e.g. benzene + ethylene= ethyl-benzene.
Examples: Zeolites, alumina, many other metal oxides etc.
3) Catalysis by semiconductors:
A sizeable number of transition metal oxides used as catalysts have semi-conducting properties. Semiconductors play a very important role as catalysts. The catalytic reactions take place on the surface, and there is a thin layer of oxides formed on the surface. Semi conducting property of the oxide may control catalysis.
Examples: NiO, ZnO, MnO2, Cr2O3, Bi2O3-MoO3, WS2 etc.
Properties of Heterogeneous catalysts:
1. Heterogeneous catalysts are solid compounds. They are added to liquid or gas reaction mixtures.
2. They cannot be filtered out of a solution.
3. The catalysts are not stable at ordinary conditions.
4. The catalysts can be separated very easily.
5. Operates at high temperature (250-500°C).
6. Activity is very high.
Advantages of heterogeneous catalysts:
1. Avoid formation of inorganic salts.
2. Catalyst is regenerable.
3. It is non-toxic.
4. Easy to handle.
5. Safe to store, long life time.
6. Easy and inexpensive.
7. Tolerates a wide range temperatures and pressures.
8. Easy and safety disposable.
Limitations of heterogeneous catalysts:
1. They cannot be filtered out of a solution.
2. It lowers the activation energy of a reaction.
3. There is limited surface area.
Catalytic reactions:
1. Oxidation:
a) Synthesis of terephthalic acid (TPA) from. xylene using ZSM-5
TPA is also known by other names as PTA (para-phthaluc acid) BDC (benzene-1,4-dicarboxylic acid). Terephthalic acid is an organic compound with formula C6H4(COOH)2, i.e. chemical formula as C8H6O4.
Production:
TPA is manufactured by oxidation of p-xylene using Zeolite Socony Mobile (ZSM-5) as catalyst. The single step reaction can be shown,
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Formation of Terephthalic acid |
Properties of ZSM-5:
1. It is a white solid.
2.It is poorly soluble in water and alcohols.
3. It sublimes without melting when heated.
4. It's density is 1.522 g/cm3, dipole moment is zero.
5. Terephthalic acid has low toxicity.
6. It causes eye irritation and mild skin irritations.
Uses of Catalyst ZSM-5 :
1. It is used in the production of polyethylene Terephalate and several other engineering polymers.
2. In research laboratories, it is used as a component for the synthesis of metal organic frameworks.
3. It is used as a filter in some military smoke grenades, producing a thick white smoke when burned.
4. It is also used in the paint as a carrier.
5. It is mainly used as a pioneer to the polyester. The polyester films are used widely in audio and video recording tapes, data storage tapes, photographic films, labels etc.
Advantages of ZSM-5 catalyst:
1. It acts as a solid acid catalyst
2. Long life of the catalyst is due to low coke formation.
3. The important role of the solid acid catalyst is implementing eco-friendly manufacturing processes.
4. The chemical industry has been benefited tremendously with this catalyst.
5. The catalyst helps to reduce pollution and cuts costs of production etc.
b) Synthesis of benzoic acid from Toluene using KMnO4:
We can split the reactions of arenes inti two groups:
1) those that affect the benzene ring and
2) those that affect the side chain.
We are, at present, interested in the reactions of benzene that affect the side chain.
With the side chain, several things can happen, the most important of which are substitution and oxidation reactions.
Oxidation:
The side chain, but not the benzene ring, is easily oxidized by reagents such as hot potassium manganate(VII), potassium dichromate(VI) or even nitric acid. The main product is benzoic acid, and carbon dioxide will also be given off.
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Formation of benzoic acid |
In the present case we study synthesis of benzoic acid from toluene using the oxidizing agent, KMnO4.
Hydrogenation of alkene to alkane using Raney Ni catalyst:
Hydrogenation is the addition of hydrogen to a multiple bond. It is used in nearly all types of multiple bonds. Hydrogenation is a great importance in synthetic chemistry, especially in the chemical industry.
Example:
4M + C2H4 -----> M2C2H2 + 2MH (where M= Raney Ni)
Synthesis of p-aminophenol from nitrobenzene using Pt/C catalyst:
Catalytic hydrogenation is the best process for the preparation of p-aminophenol from nitrobenzene. The use of Pt/C catalyst has minimized the effluent disposal problems to a great extent and also improved overall economics as well as the quality of the product.
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Synthesis of p-aminophenol |
The catalytic hydrogenation of nitrobenzene to p-aminophenol in a single step is carried out using supported noble metal catalyst (Pt/C) in the presence of H2SO4. Initial reduction of nitrobenzene takes place to form beta-phenyl hydroxylamine as an intermediate, which then rearranges in situ to p-aminophenol (PAP) in the presence of H2SO4. Formation of aniline is the main side product.