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HYDROGENATION
Reaction of Hydrogen with another reactant molecule is known as
Hydrogenation of that molecule.
This reaction may be a simple
addition of Hydrogen atom/ atoms to the molecule or it may cause
substitution/ replacement of another atom or group of atoms
present on or attached to the molecule.
Hydrogenation of a molecule is also known as Reduction of that
molecule. Thus Hydrogenation of Alkenes, Alkynes, Nitro
Compounds, Aromatic Compounds is also termed as reduction of
Alkenes, Alkynes, Nitro Compounds, Aromatic Compounds
respectively.
HYDROGENOLYSIS
When hydrogenation causes knocking out of another atom or
group attached directly to a molecule it is known as
hydrogenolysis of that atom or group. Dehalogenation,
debenzylation etc. denote specific type of hydrogenolysis.
DEHYDROGENATION
Removal
of hydrogen from a molecule is termed as dehydrogenation.
CATALYST
A
Catalyst is a substance which accelerates the rate of reaction
without itself getting consumed theoretically. However in
some reactions the reactants do not react at all unless
appropriate catalyst is present. Appropriate catalyst can also
help in lowering the temperature and pressure condition
requirements of reaction.
Supported
catalyst
This
term is generally used in case of noble metal catalysts.
Catalysis being essentially a surface phenomenon small quantity
of precious noble metal can be used in a manner to get exposure
to disproportionately high surface area with the help of porous
inert support material viz activated charcoal, alumina powder,
calcium carbonate, barium sulphate, silica powder, quartz powder
spherical alumina balls etc.
HOMOGENEOUS
CATALYST
When
Catalyst and the Substrate are of the same phase, the Catalyst
is termed as Homogeneous Catalyst and the process of catalysis
is termed as homogeneous catalysis.
HETEROGENEOUS
CATALYST
When
the Catalyst and the Substrate are in different phases the
Catalyst is termed as Heterogeneous Catalyst and the process of
catalysis is termed as heterogeneous catalysis.
SUBSTRATE
In
the present context the compound to be hydrogenated or
hydrogenolysed or dehydrogenated or desulfurised, etc is termed
as substrate.
HYDROGENATION
CATALYST
A
catalyst which is used in a reaction where one of the reactants
happen to be Hydrogen is known as Hydrogenation Catalyst.
Theoretically
thus a small quantity of hydrogenation catalyst should be
capable of producing infinitely large amount of hydrogenated
product. In actual practice it is not so. Though small quantity
of catalyst does cause proportionately large quantity of a
reactant to react with Hydrogen but it is not an infinitely
large quantity. Factors like deactivation or poisoning or
catalyst do not allow endless/ unlimited use of the catalyst.
Note:
A good hydrogenation catalyst in the absence of external supply
of hydrogen can also act as a catalyst for dehydrogenation.
However, much higher quantity of catalyst is required to be used
for dehydrogenation.
WHY
HYDROGENATION/ HYDROGENOLYSIS/ REDUCTION
When interaction of a molecule with Hydrogen produces another
molecule with higher utility or having a different type of
application, hydrogenation becomes not only a technical need but
a commercial need as well. It is thus a techno-commercial
requirement.
WHY
CATALYST IS REQUIRED FOR HYDROGENATION
The hydrogen available commercially is in molecular form.
The reactivity of molecular form of hydrogen gas is extremely
low. Hydrogen gas in molecular form reacts at extremely slow
rate with organic compounds and the functional groups attached
to the organic molecule even at high temperature and pressure.
However, in presence of suitable catalyst the rate of reaction
becomes appreciably high even at ambient temperature and
pressure. This property of hydrogen gas necessitates and
justifies use of catalyst for hydrogenation.
SELECTIVE
HYDROGENATION
When hydrogenation is
carried out in a manner that hydrogen gets attached to the
molecule in a specified manner only and thereby avoiding or
minimizing addition of hydrogen to the molecule at sites or in
manner other than specified manner, the hydrogenation so carried
out is termed as selective hydrogenation. Conducting a
hydrogenation reaction in a manner so as to get exclusively or
preferentially cis-isomer, trans-isomer, optically active (dextro
/ leavo)
isomers, chloro nitro to chloro amino (i.e. without knocking out
of chloro group) are examples of selective hydrogenation.
Selection of appropriate catalyst and reaction conditions play
vital role in achieving selective hydrogenation.
TYPES
OF HYDROGENATION CATALYST
Numerous types of hydrogenation catalysts have been mentioned in
the technical literature on the subject. Vineeth Chemicals,
however, deal only in two main and versatile types of
hydrogenation catalysts, namely, Raney Nickel Catalyst and Noble
Metal Catalyst. Noble Metal (Palladium, Platinum, Ruthenium,
Rhodium) catalysts are supported on carbon or other inert
materials as well as unsupported like Adam’s Catalyst, PtO2.
Besides, Vineeth Chemicals also manufactures special catalyst
viz Lindlar Catalyst (Palladium on Calcium Carbonate partially
deactivated by Lead).
Whereas Raney Nickel Catalyst can be used for virtually any
types of hydrogenation but has a limitation that it cannot be
used in acidic media, i.e. below pH of 5.5,Supported noble metal catalyst can
be used in acidic media as well. Supported noble metal catalysts
are generally used for selective hydrogenation at relatively
lower temperature and pressure. Noble
metal catalysts are thus very useful for heat sensitive
compounds,
viz.
perfumery compound synthesis and can be used in acidic
conditions as well.
PROMOTERS
A Promotor is a substance which enhances the activity of a
catalyst. Promoters are specific for a specific type of
hydrogenation. What is a promoter for one type of hydrogenation
need not act as promoter for other type of hydrogenation.
INHIBITOR
Inhibitors are substances
which retard activity of a catalyst. Sulphur is inhibitor for
precious metal Catalyst.
CATALYST
POISONS OR DEACTIVATORS
Presence of oxidizing agents or small amounts of deactivators
cause deactivation or poisoning of hydrogenation catalyst.
Sometimes
the product of reaction, reaction intermediates or by products
act as catalytic deactivators and do not allow completion of
reaction. Under such situations user should opt for a different
type of catalyst.
The
user should make a careful study of deactivators and poisons
before conducting large scale hydrogenation. Care should be
taken to minimize presence of catalytic deactivators and poisons
in the reaction mixture so as to prolong life of the catalyst.
Catalyst
manufactured by Vineeth Chemicals are normally resistant to
catalytic poisons if present in small quantities.
REACTION
PARAMETERS
Reaction parameters have great influence on the reaction time,
quality and yield of the reaction product. By suitable
maneuvering of reaction parameters and catalyst type, it is
often possible to influence not only the reaction time but also
the isomer ratio of the reaction product. Reaction parameters
are therefore of vital importance in catalytic hydrogenation.
The parameters of importance are therefore of vital importance
in catalytic hydrogenation.
i.
Solvent
ii.
Catalyst and Catalyst Grade/ Type
iii.
Substrate to Catalyst ratio
iv.
pH of the reaction mixture
v.
Additives/ Promoters
vi.
Temperature
vii.
Pressure
viii.Speed
of Agitator [RPM]/ Quantum of turbulence.
SEPARATION
OF CATALYST AFTER HYDROGENATION
Since Raney Nickel Catalyst and Precious Metal Catalysts
supported on inert materials like Carbon, Alumina, Silica, etc.,
belong to the category of heterogeneous catalysts, at the end of
hydrogenation the catalyst separates out on its own and settle
at the bottom of the reaction vessel and can be easily separated
through filtration.
REUSAGE
OF USED CATALYST
If the catalyst separated
after the reaction has activity sufficient for next batch, the
same can be used. Usually it is necessary to supplement it with
some quantity of fresh catalyst so as to maintain the rate of
reaction. Thus in suitable cases it is possible to reuse the
used catalyst couple of times. This helps in lowering down the
cost of hydrogenation in a significant manner.
Recovery
of precious metal from the spent catalyst and its usage for
making fresh CATALYST
When the
activity of used precious metal catalyst is not sufficient
enough for the next cycle of hydrogenation the spent precious
metal catalyst should be sent to the supplier for recovery of
precious metal, its refining and thereafter its usage for making
fresh catalyst. Vineeth Chemicals is fully equipped for this job
and undertakes the same on commercial terms. Vineeth Chemicals
is able to recovery more then 99% of precious metal from the
spent catalyst. For further details contact General manager,
Vineeth Chemicals.
Reactivation of Spent Raney Nickel catalyst
When the activity of used Raney Nickel
catalyst is not sufficient enough for the next cycle of
hydrogenation a sample of the spent catalyst may be sent to
Vineeth Chemicals for examining a possibility of its
reactivation . If reactivation is found feasible then the entire
lot of the spent catalyst be sent for reactivation. For further
details please contact General manager, Vineeth Chemicals.
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