Engineering Chemistry S Datta

140 ENGINEERING CHEMISTRY
(v) Dehydrogenation C 6
H 5
C 2
H 5
⎯⎯→ C 6
H 5
CH = CH 2
+ H 2
Cr 2
O 3
/MoO 3
(Styrene)
Friedel– Craft’s reaction
(vi) Alkylation C 6
H 6
+ RX ⎯⎯→ C 6
H 5
R + HX Anhydrous AlCl 3
, BF 3
, HF
(vii) Isomerisation Normal paraffin ⎯⎯→ isoparaffin Anhydrous AlCl 3
(viii) Cracking Long chain paraffin ⎯⎯→ branched Bauxite,
chain alkanes MoO 3
(ix) Halogenation Benzene + Cl 2
⎯⎯→ Chlorobenzene Fe or anhydrous FeCl 3
(x) Polymerisation Ethene ⎯⎯→ Polythene Zeigler catalyst or
Styrene ⎯⎯→ Polystyrene
Peroxides or
Vinyl chloride ⎯⎯→ PVC
Persalts
Phase Transfer Catalyst
This type of catalytic reaction may be considered as a combination of homogeneous and
heterogeneous catalyses. If we take water and a non-aqueous solvent like benzene and if the
reactants remain in the benzene phase and the catalyst in the water phase the reaction is
accomplished at the interface provided a little surface active agent (detergent) is added to the
system. This type of catalytic reaction is now widely used.
Highlights:
Two other types of catalytic reactions are to be noted.
(i) Autocatalysis—Examples of such autocatalytic reaction are:
(a) Hydrolysis of ester—water formed in the reaction catalyses the reaction.
(b) Decolourisation of KMnO 4
by oxalic acid. Mn +2 ion formed initially catalyses
the reaction.
(ii) Induced catalysis—Sodium arsenite solution is not generally oxidised by O 2
but sodium sulphite is oxidised. If air is passed through a solution containing
the two salts, both of them undergo simultaneous oxidation. This type of catalytic
reaction is known as induced catalytic reaction.
Catalyst Poison
Certain foreign substances, when present in very trace amount, may damage the
efficiency of the contact catalyst and may even destroy the catalytic activity. These substances
are called catalyst poisons or anti-catalysts. As 2
O 3
acts as a catalyst poison in the contact
process for the manufacture of H 2
SO 4
. The poison acts very likely by blocking the active sites
of the catalyst.
Catalyst Promoters
Promoters are the substances which promote the catalytic activity or enhance the activity
of a catalyst by its presence. An important catalyst promoter is Al 2
O 3
and alkali metal oxide,
when used with iron in the Haber’s process, the catalytic action increases.
Highlights:
Characteristics of catalytic reactions:
(i) Catalyst remains unchanged in mass and chemical composition at the end of
the reaction.
(ii) Always a small quantity of catalyst is required.
(iii) A catalyst is more effective in a finely divided state.
(Contd.)