A Lindlar catalyst is a heterogeneous catalyst that consists of palladium deposited on calcium carbonate which is then poisoned with various forms of lead or sulphur. It is used for the hydrogenation of alkynes to alkenes (i.e. without further reduction into alkanes) and is named after its inventor Herbert Lindlar.
Lindlar catalyst is commercially available but may also be prepared by the reduction of palladium chloride in a slurry of calcium carbonate followed the addition of lead acetate. A variety of other "catalyst poisons" have been used, including lead oxide. The palladium content of the supported catalyst is usually 5% by weight.
The catalyst is used for the hydrogenation of alkynes to alkenes (i.e. without further reduction into alkanes). The lead serves to deactivate the palladium sites, further deactivation of the catalyst with quinoline or 3,6-dithia-1,8-octanediol enhances its selectivity, preventing formation of alkanes. Thus if a compound contains a double bond as well as a triple bond, only the triple bond is reduced. An example being the reduction of phenylacetylene to styrene.
Computational chemistry indicates that the poisons may act in a complimentary manner. Thus, 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.
Researchers are currently developing novel Lindlar-type catalysts in order to avoid the environmental challenges of working with lead. In this context, BASF recently commercialized a hybrid Pd-based catalyst by modifying the palladium surface with an organic ligand (trademark name: NanoSelect, Strem Chemicals); this material is as stereoselective as the Lindlar catalyst.
- Rosenmund reduction, a reduction using palladium on barium sulphate, poisoned with sulphur compounds.
- Urushibara Iron, an iron based catalyst used to hydrogenate alkynes to alkenes.
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- NanoSelect catalyst
- Vilé, G.; Almora-Barrios, N.; Mitchell, S.; López, N.; Pérez-Ramírez, J. (17 April 2014). "From the Lindlar catalyst to supported ligand-modified palladium nanoparticles: selectivity patterns and accessibility constraints in the continuous-flow three-phase hydrogenation of acetylenic compounds". Chemistry - A European Journal 20: 5926–5937. doi:10.1002/chem.201304795.