Lindlar catalyst
Encyclopedia
A Lindlar catalyst is a heterogeneous catalyst that consists of palladium
deposited on calcium carbonate
and treated with various forms of lead
. The lead additive serves to deactivate
the palladium sites. A variety of "catalyst poisons" have been used including lead acetate
and lead oxide
. The palladium content of the catalyst is usually 5% by weight. The catalyst is used for the hydrogenation
of alkyne
s to alkene
s (i.e. without further reduction into alkanes).Thus if a compound contains a double bond as well as a triple bond , only the triple bond reduces to a double bond as opposed to both bonds being reduced down to a single bond.[citation needed]
As described by its inventor Herbert Lindlar, the catalyst is prepared by reduction of palladium chloride in a slurry of calcium carbonate followed by adding lead acetate. By this approach, one obtains a catalyst with a large surface area. Further deactivation of the catalyst with quinoline
enhances its selectivity, preventing formation of alkane
s. An example of alkyne reduction is the reduction of phenylacetylene
to styrene. A recent description on how the poisons act on the selectivity of the Lindlar catalyst identifies a complementary role of the poisons while Pb reduces the amount of hydrogen content in the bulk of Pd, quinoline blocks the formation of oligomers.
Alkyne reduction is stereoselective, occurring via syn addition
to give the cis-alkene. An example of commercial use is the organic synthesis
of vitamin A
which involves an alkyne reduction with the Lindlar catalyst.
Other heterogeneous catalysts useful for hydrogenation are Adams' catalyst
, Palladium black
, and Raney nickel
.
Palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...
deposited on calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
and treated with various forms of lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
. The lead additive serves to deactivate
Catalyst poisoning
Catalyst poisoning refers to the effect that a catalyst can be 'poisoned' if it reacts with another compound that bonds chemically to its active surface sites. This effectively reduces the usefulness of the catalyst...
the palladium sites. A variety of "catalyst poisons" have been used including lead acetate
Lead acetate
Lead acetate can refer to:* Lead acetate , Pb4* Lead acetate , Pb2...
and lead oxide
Lead(II) oxide
Lead oxide is the inorganic compound with the formula PbO. Lead oxide occurs in two polymorphs, red, having a tetragonal crystal structure and yellow, having an orthorhombic crystal structure...
. The palladium content of the catalyst is usually 5% by weight. The catalyst is used for the hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
of alkyne
Alkyne
Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature...
s to alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s (i.e. without further reduction into alkanes).Thus if a compound contains a double bond as well as a triple bond , only the triple bond reduces to a double bond as opposed to both bonds being reduced down to a single bond.[citation needed]
As described by its inventor Herbert Lindlar, the catalyst is prepared by reduction of palladium chloride in a slurry of calcium carbonate followed by adding lead acetate. By this approach, one obtains a catalyst with a large surface area. Further deactivation of the catalyst with quinoline
Quinoline
Quinoline is a heterocyclic aromatic organic compound. It has the formula C9H7N and is a colourless hygroscopic liquid with a strong odour. Aged samples, if exposed to light, become yellow and later brown...
enhances its selectivity, preventing formation of alkane
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
s. An example of alkyne reduction is the reduction of phenylacetylene
Phenylacetylene
Phenylacetylene is an alkyne hydrocarbon containing a phenyl group. It exists as a colorless, viscous liquid. In research, it is sometimes used as an analog for acetylene; being a liquid, it is easier to handle than acetylene gas.-Preparation:...
to styrene. A recent description on how the poisons act on the selectivity of the Lindlar catalyst identifies a complementary role of the poisons while Pb reduces the amount of hydrogen content in the bulk of Pd, quinoline blocks the formation of oligomers.
Alkyne reduction is stereoselective, occurring via syn addition
Syn addition
In organic chemistry, syn and anti addition are different ways in which two substituents can be added to a double bond or triple bond. This article will use alkenes as examples....
to give the cis-alkene. An example of commercial use is the organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
of vitamin A
Vitamin A
Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...
which involves an alkyne reduction with the Lindlar catalyst.
Other heterogeneous catalysts useful for hydrogenation are Adams' catalyst
Adams' catalyst
Adams' catalyst, also known as platinum dioxide, is usually represented as platinum oxide hydrate, PtO2-H2O. It is a catalyst for hydrogenation and hydrogenolysis in organic synthesis. This dark brown powder is commercially available...
, Palladium black
Palladium black
Palladium black is a coarse, sponge-like form of elemental palladium that offers a large surface area for hydrogen adsorption. It can be used as a catalyst for hydrogenation reactions....
, and Raney nickel
Raney nickel
Raney nickel is a solid catalyst composed of fine grains of a nickel-aluminium alloy, used in many industrial processes. It was developed in 1926 by American]] engineer Murray Raney as an alternative catalyst for the hydrogenation of vegetable oils in industrial processes...
.