Hydrogenation catalysts

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Hydrogenation catalysts are catalysts that enable the addition (addition) of hydrogen, so that hydrogenation , i.e. the reduction of organic compounds, occurs. These are usually subgroup elements . In particular, the platinum metals such as palladium and platinum are of great importance in the activation of molecular hydrogen, which precedes the attachment. The great importance of the platinum elements results from the fact that although they interact with molecular hydrogen, they do not form hydrides , but instead store the hydrogen unbound in their octahedral or tetrahedral holes. The cheaper elements such as nickel or iron are of particular technical importance here . Hydrogenations play a major role in organic synthesis but also in the food industry (e.g. margarine production ).

A distinction is made between homogeneous and heterogeneous catalysis. In homogeneous catalysis, the hydrogenation catalyst is completely dissolved in the solvent. This is achieved in through the use of ligands . In heterogeneous catalysis, an insoluble mixture of hydrogenation catalyst and solvent with the compound to be hydrogenated is present. When using salts as hydrogenation catalysts, the salt is generally reduced first and the finely divided element forms the active hydrogenation catalyst.

Basics

The non-activated hydrogen molecule is very inert as a reducing agent because the bond between the two hydrogen atoms is quite stable. The task of the hydrogenation catalyst is to weaken this bond and thereby increase the reactivity of the hydrogen. This enables the hydrogenation or reduction of functional groups with multiple bonds .

Catalytic hydrogenation scheme

In order to be catalytically active, the catalyst metals must be in a finely divided form. Depending on the manufacturing process, this leads to different catalysts with different reactivities and selectivities with regard to substrate and reaction conditions.

Heterogeneous hydrogenation catalysts

The heterogeneous hydrogenation catalysts are all insoluble in the hydrogenation solvent (organic solvents such as methanol , dioxane or glacial acetic acid ), which is why this process is a heterogeneous catalysis. Heterogeneous catalysis catalysts have the advantage that they can be easily removed from the reaction mixture by a simple filtration.

Platinum and palladium

Palladium and platinum are the hydrogenation catalysts of choice, as metals or oxides, on various support materials, as they are very versatile. They are stable and lose little of their activity over time.

The most common catalysts are commercially available today. Palladium and platinum differ little in their activity, but palladium is more modulatable than the comparable platinum catalysts due to different manufacturing methods. The following is a list of the various catalysts that differ in their method of manufacture and the carrier material.

The Lindlar catalyst should be highlighted from the list. The catalyst is poisoned with lead acetate. This makes it possible to hydrogenate an alkyne so that it is only reduced to an alkene and not to an alkane . As a result of the syn addition of the hydrogen during the hydrogenation, Z -configured alkenes can be prepared.

Lindlar Alkin.svg

Rhodium and ruthenium

The catalysts based on rhodium or ruthenium are much less common than palladium or platinum, primarily for economic reasons.

Rhodium catalysts are particularly suitable for the hydrogenation of aromatic ring systems, since here the hydrogenation often takes place at room temperature and normal pressure.

Ruthenium catalysts are mainly used for the reduction of aldehydes and ketones , since here carbon-carbon multiple bonds are not attacked at normal pressure and room temperature .

The production of the catalysts is analogous to the palladium or platinum catalysts, depending on the use.

The Nishimura catalyst, which is a rhodium-platinum mixed oxide, should be mentioned as a single catalyst.

nickel

Nickel is one of the inexpensive hydrogenation catalysts. As a rule, Raney nickel is used here. Due to its manufacture, the catalyst already carries hydrogen on the metal surface. Depending on the method of preparation, more or less basic nickel catalysts are obtained, which are referred to as Raney nickel W 2, Raney nickel W 6 or Raney nickel W 7. Another known nickel hydrogenation catalyst is nickel, which is absorbed onto kieselguhr .

Cobalt

Cobalt catalysts are produced in a manner comparable to Raney nickel and are therefore also referred to as Raney cobalt.

Cobalt catalysts are more unreactive than nickel catalysts, but are particularly suitable for the hydrogenation of nitriles , since the poor reactivity here means that carbon-carbon multiple bonds are not attacked and unsaturated amines can be produced.

iron

Iron catalysts are generally used on an industrial scale as a fixed bed and are obtained, for example, by reducing magnetite . They have a lower reactivity compared to cobalt. On an industrial scale, iron catalysts are of great importance in the hydrogenation of adiponitrile to hexamethylenediamine . They are rarely used in the laboratory.

Copper chromite and zinc chromite

Copper chromite (CuCr 2 O 4 ) ( Adkins catalyst ) is an oxidic catalyst, since finely divided cupric oxide is likely to be formed when the catalyst and hydrogen come into contact. However, the copper chromite appears to protect the oxide from further reduction to elemental copper, since pure copper (II) oxide is otherwise reduced to copper under hydrogenation conditions.

Copper chromite catalysts are used in the reduction of carbonyl groups because of their selectivity . The ability to reduce carbon-carbon double bonds is low, but it does exist. In the case of α, β-unsaturated carbonyl groups, the carbon-carbon double bond is also attacked. However, zinc chromite catalysts are more suitable for this purpose. In the case of α, β-unsaturated esters, however, both fail. However, due to developments such as the Luche reduction , these selectivity differences have lost their significance today.

Homogeneous catalysis

The homogeneous catalysis in hydrogenations is particularly important in the enantioselective reduction of prochiral groups such as suitably substituted alkenes , unsymmetrical ketones or imines.

Alkenes

The best known and most widespread catalyst in the homogeneous catalytic hydrogenation of alkenes is the Wilkinson catalyst , which, due to its own achirality, does not have an enantioselective effect. However, most chiral catalysts are based on it. The chirality is usually determined by the choice of chiral ligands . The importance of homogeneous catalysis grew with the work of the later Nobel Prize winner William Standish Knowles , who chirally modified the Wilkinson catalyst by using chiral phosphine ligands instead of triphenylphosphine ligands. Most chiral catalysts are based on this work. With this modification, the enantiomeric ratio of 15: 1 has been achieved. That was very good in 1968. A short time later, Leopold Horner achieved a similar result. An important discovery, which made homogeneous catalysis valuable for organic chemistry, was the discovery of an asymmetric hydrogenation of the carbon-carbon double bond in an N -acyldehydroamino acid to a chiral intermediate of the Parkinson's drug L -DOPA by Knowles. Another advance in homogeneous catalytic hydrogenation in enantioselective synthesis was the discovery of the axially chiral 2,2'-bis (diphenylphosphino) -1,1'-binaphthyl ligand (BINAP) by Ryoji Noyori , who was also awarded the Nobel Prize, along with Knowles .

Ketones

The enantioselective reduction of unsymmetrical ketones is of considerable importance. The enantiomeric excess achieved is often over 90% or even 99%.

Imine

There are also established processes for the enantioselective reduction of the carbon-nitrogen double bond in prochiral imines .

literature

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