The phenomenon of changing the rate of chemical reaction by the action of substances that do not alter their own chemical composition during this reaction is called catalysis.
Catalysis plays a very important role in chemical industry. Nowadays, more than 95% of chemicals are produced through a process that includes at least one catalytic step. Catalysis of chain reactions occupies a special place among the various catalytic reactions.
Catalysts are the substances that speed up reactions by providing an alternative pathway for the breaking and making of bonds.
Homogenous catalysis:
Homogeneous catalysis are catalytic reactions in which the catalyst is in the same phase as the reactants.
A proton is the main universal homogenous catalyst. This is because water is the most common solvent, and it forms protons by the process of self-ionization of water.
Homogenous catalyst:
When the catalyst and the reacting substances are present together in a single state of matter, usually as a gas or a liquid.
Types of homogeneous Catalysts:
Type I -
In this type of catalysis, reactants as well as the catalyst are in gaseous state.
For example: NO and NO2 acts as catalysts for the formation of the oxidation of SO2 to SO3 in the lead chamber process for the manufacturing of sulphuric acid.
Type II -
In this type of catalysis, reactants as well as catalyst are in liquid state.
For example: sulphuric acid is used as a catalyst for the formation of diethyl ether from ethyl alcohol. Here the reactant ethyl alcohol, the product diethyl ether and water and the catalyst sulphuric acid all are in liquid state.
Type III-
In this type of catalysis, reactants and catalyst are in solution state.
For example: In the redox reaction, iodide ions are oxidized by persulphate ions and Fe(II) and Fe(III) ions in solution act as catalyst.
Type IV -
In this type of catalysis, acids or bases act as catalyst. According to Bronsted, an acid is a molecule that can give up a proton and base is a molecule that takes up a proton.
Properties of Homogeneous catalysts:
1. They are consumed during a chemical reaction.
2. They increase the energy of the reactants.
3. They offer high degree of interaction between catalyst and reactant molecules.
4. They lower the energy of activation for the reaction.
5. They increase the rate of reaction without increase of temperature.
6. They shift the equilibrium to favour the formation of products.
Catalytic reactions:
A) Wilkinson's Catalyst-
Name of Wilkinson's catalyst is chlorotris(triphenylphosphine) rhodium(I). This complex compound is coordination compound. The formula for this compound is RhCl(PPh3)3, (ph= phenyl). This compound is a square planar, 16 electron complex. It is red violet compound. It is usually obtained from the reaction of rhodium(III) chloride with excess of triphenylphosphine. The synthesis is conducted in refluxing ethanol where triphenylphosphine acts as the reducing agent and gives triphenylphosphine oxide. Thus,
RhCl3(H2O)3 + 4PPh3 ---->RhCl(PPh3)3 + OPPh3 + 2HCl +2H2O
The commonly accepted sequence is as below:
RhCl(L)3 + S ----> RhCl(L)2 S + L
RhCl(L)2 + H2 ----> RhH2Cl(L)2 S
RhH2Cl(L)2S + H2C=CH2 ---->
RhH2Cl(L)2(H2C=CH2) + S
RhH2Cl(L)2 (H2C=CH2) + S -----> RhH(C2H5)Cl(L)2 S
RhH(C2H5) Cl(L)2 S ------> H3C CH3 + RhCl(L)2 S
Where, L= RhCl(PP3)3 and S=solvent. The alkene is ethene.
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Wilkinson's hydrogenation cycle |
Wilkinson's hydrogenation is both stereospecific as well as stereoselective.
Catalytic Applications:
1. The catalytic hydroboration of alkenes with catecholborane and pinacolborane.
2. The selective 1,4- reduction of alpha-beta unsaturated carbonyl compounds in accordance with triethylsilane.
3. When the triphenylphosphine ligands are replaced by chiral phosphines, the catalyst becomes chiral. It then converts prochiral alkenes into enantiometrically enriched alkanes via the process known as asymmetric hydrogenation.
B) Ziegler-Natta catalyst:
A German scientist Kart Ziegler and Italian scientist Giulio Natta introduced a special type of catalyst, with the help of which the vinyl polymerisation could be carried out very effectively and they were awarded Nobel Prize in 1963 for this work.
Thus,
nCH2= CH2 ---> -[CH2-CHR]n-
Ziegler-Natta catalyst consist of titanium trichloride (TiCl)3 and a metal alkyl such as triethyl aluminum, Al(Et)3.
When alkene such as ethylene or ethene is added to such catalyst then pi-cloud of ethylene overlaps the empty orbital of metal. Then ethylene unit inserts itself between metal and ethyl group and hence n-butyl group is formed, which is attached to Titanium.
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| Ziegler-Natta catalyst |
When another ethylene molecule becomes pi- bonded to this new comex, it gives n-hexyl group. Thus, the process continues over again with the alkyl group growing by two carbon atoms in each cycle.
Finally, the long chain separates from the metal and a molecule of polyethylene is formed.
Advantageous of Ziegler-Natta catalyst:
1. It gives linear polymer: The monomer is always attached only at a particular site, so we get completely linear chain and no cross links are developed. The linear chains fit together well in the crystal lattice and the polymer is said to have high degree of crystallinity. It has high melting point, higher density and higher mechanical strength.
2. It permits high degree of stereo chemical control: stereospecific polymerisation is one which results in the production of stereoregular polymerisation, in which all units and sub-units are arranged in space in some definite order.
Monsanto Acetic Acid Synthesis:
A process has been developed by Monsanto, USA, in the late sixties to produce acetic acid by carbonylation of methanol in presence of rhodium / iodide catalyst. This process is called Monsanto Process.
For the manufacture of acetic acid Iridium based catalyst is found superior in recent years.
Monsanto Process may be shown as,
CH3OH + CO -------------> CH3COOH
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Catalytic cycle for Monsanto process |