Here is a write-up about noble metals on different carriers and what can be done with them.
Activated carbon powder is principally used as a support for catalysts in liquid phase reactions. As carbon is derived from naturally occurring materials there are many variations, each type having it´s own particular physical properties and chemical composition. The surface areas of different carbons can range from 500m2/g to over 1500m2/g.
Trace impurities that might be present in certain reaction systems can occasionally poison catalysts. The high absorptive power of carbons used as catalyst supports can enable such impurities to be removed, leading to longer catalyst life and purer products.
Carbon catalysts are produced in two physical forms, dry powder or paste. The latter contain 50-60% by weight of water which is held within the pores of the carbon. There is no supernatant liquid and the “paste” catalyst has the consistency of a friable powder.
Graphite powder has lower surface area than activated carbon, generally in the range 5m2/g to 30m2/g, although special high surface area graphites are avaliable in the range 100m2/g to 300m2/g. Graphites are made synthetically and are therefore of higher purity than activated carbons, which are manufactured from naturally occurring feedstocks. Graphite is used for selective hydrogenation reactions or when a low porosity support is required to minimise mass transfer problems or absorption of high value products.
Alumina and other oxides
Activated alumina powder has a lower surface area than most carbons, usually in the range 75m2/g to 350m2/g. It is a more easily characterised and less absorptive material than carbon. It is also non-combustible. Alumina is used instead of carbon when excessive loss of expensive reactants or products by absorption must be prevented. When more than one reaction is possible, catalyst supported by alumina may prove to be more selective than the same metal supported on carbon.
Silica is sometimes used when a support of low absorptive capacity with a neutral, rather than basic or amphoteric character is required. Silica-alumina can be used when an acidic support is needed.
Calcium carbonate
Calcium carbonate is particulary suitable as a support for palladium, especially when a selectively poisoned catalyst is required. The surface area of calcium carbonate is low but it finds applications when a support of low absorption or of a basic nature is required. The carbonates of magnesium, strontium, barium and zinc generally offer no advantage over calcium carbonate.
Barium sulphate
Barium sulphate is another low surface area catalyst support. This support is a dense material and requires powerful agitation of the reaction system to ensure a uniform dispersal of the catalyst. A palladium on barium sulphate was traditionally used for the conversion of acid chlorides to aldehydes (Rosenmund reduction) together with an in situ partial poison to improve selectivity. In this application, however, it is being replaced increasingly by palladium on carbon carbonate catalysts for more reproducible results.
Choice of metal
Catalyst performance is determined mainly by the noble metal component. A metal is chosen based both on its ability to complete the desired reaction and its ability not perform unwanted side reactions. Palladium is the most versatile of the noble metals. It is typically the preferred metal for hydrogenation of alkynes to alkenes, carbonyls in aromatic aldehydes and ketones, nitro compounds, reductive alkylations, hydrogenolysis and hydrodehalogenation reactions. Platinum is typically the preferred metal for the selective hydrogenation of halonitroaromatics and reductive alkylations. Rhodium is used for the ring reduction of aromatics while ruthenium is used for the higher pressure hydrogenation of aromatics and aliphatic carbonyl hydrogenation. Mixed metal catalysts often provide benefits.
Choice of metal location
Catalyst performance ca be altered significantly by the appropriate choice of support material, metal location and dispersion within the pore structure of the support. The types of metal location on the support can be divided in three groups; eggshell, intermediate and uniform.
Surface (also known as ‘eggshell’) : the metal is located on the exterior surface of the carrier.
Intermediate: the metal is located on the surface and a bit inside the pore structure.
Uniform: the metal is evenly dispersed throughout the support structure
The catalysts are designed with different metal locations for reactions performed under different conditions of pressure and temperature.
Hydrogenation reactions are generally first order with respect to hydrogen. Thus, the reaction rate is directly proportional to hydrogen pressure. With intermediate and uniform catalyst types, an increasing proportion of the metal becomes accessible as the pressure increases. When all the metal is avaliable, the catalyst properties closely parallel those of an surface catalyst. At higher pressures catalysts with deeper metal location have greater metal dispertion and, hence, higher activity than surface catalysts. Thus, surface catalysts would be chosen for high activity at low hydrogen pressure and uniform at high pressure.
The location of metal deep into the support may lead to large in-pore diffusion resistance to reactants. This will result in increases in residence time and possible changes in selectivity. Thus the variation of the metal location can be used to adjust the selectivity of the catalyst.
Surface catalysts have high activity at low reaction pressures in systems substantially free from poisons. If a catalyst poison is present, this can be overcome by locating the metal deeper into the support structure and hence increasing the metal dispersion. Such a catalyst will exhibit greater poison resistance because;
1. the poison molecules often have high molecular weights and, unlike the smaller reactants, are unable to penetrate the pores where the catalytic metal is located.
2. the increased metal area counteracts the constant area of metal rendered inactive by the poison.
Variation of metal location, metal dispersion, metal loading (0,5-10%), catalyst pH and pore structure of the support has enabled the development of a large range of catalysts which are widely used in numerous reactions.
Which catalyst to chose for which reaction..
Carbon-carbon multiple bonds.
Alkynes and alkenes are readily hydrogenated over supported catalysts with the following general order of activity
Pd > Rh > Pt >> Ru
The ease of hydrogenation decreases with increasing substitution at the double bond. In molecules containing more than one double bond, the least hindered will be hydrogenated preferentially and exocyclic double bonds more easily hydrogenated than endocyclic double bonds. A complication in the hydrogenation of alkenes can be double bond migration and cis-trans isomerisation. The tendency of the noble metals to promote these reactions is generally in the order
Pd > Rh > Ru > Pt = Ir
Hence platinum catalysts are useful when double bond migration is to be avoided.
Alkynes can readily be hydrogenated to alkenes or alkanes under mild conditions using Pt or Pd supported catalysts. Typical operating conditions are temperatures of 20-100 deg C and hydrogen pressures of 1-10 bar. Palladium is the most selective metal for the conversion of alkynes to alkenes without further hydrogenation to the corresponding alkane. Modifiers such as amines or sulphur-containing compounds can be added to the reaction system to improve the selectivity to the alkene. Alternatively, Pd catalysts can be modified with a second metal such as Pb, Cu, or Zn. The best known and most widely used catalyst of this type is Lindlar´s catalyst. Selectivity can also be improved by limiting the hydrogen avalibility.
Alkyne to alkane 50-100 deg C 1-10 bar alcohol solvent or no solvent.
Alkyne to alkene 5-50 deg C 1-3 bar alcohols, HOAc or EtOAc
Alkene to alkane 5-100 deg C 3-10 bar no solvent, alcohols, HOAc or EtOAc
Aldehydes
In general, saturated aliphatic aldehydes can be hydrogenated over Pt, Ru or Ir catalysts to the corresponding primary alcohols at 5-150 deg C and 1-10 bar. Pd tends to be an ineffective catalyst for aliphatic aldehydes but is the metal of choice for aromatic aldehydes, typical reaction conditions are 5-100 deg C and 1-10 bar. Pd will also catalyse the production of hydrocarbon from the hydrogenolysis of the alcohol intermediate. Acidic conditions and polar solvents promote the formation of the hydrocarbon. Ru is the first choice for the hydrogenation of aliphatic aldehydes. Ether formation, which may occur as an alcohol dehydration product, can be reduced using a Ru catalyst.
Aliphatic aldehyde to alcohol 5-150 deg C 1-10 bar alcohols, EtOAc or water
Aromatic aldehyde to alcohol 5-100 deg C 1-10 bar benzene, hexane, DMF or EtOAc
Either to hydrocarbon 5-100 deg C 1-10 bar acidic: alcohols or HOAc
Ketones
Ketone hydrogenation is similar to that of aldehyde hydrogenation, but is usually less facile due to increased steric hindrance at the carbonyl group. Saturated aliphatic ketones can be hydrogenated over Pt or Ru to the corresponding secondary alcohol. Aromatic ketones are best hydrogenated over Pd catalysts.
Ru is the metal of choice for the hydrogenation of water soluble ketones and for hydrogenations in alcohol solvents since ether formation is minimised.
Aromatic ketones can be hydrogenated to the corresponding alcohol or alkyl aromatic (hydrogenolysis) over a variety of catalysts. Hydrogenolysis is favoured by acidic conditions, elevated temperatures and polar solvents. Ketones can be hydrogenated to alcohols using a homogenous catalyst such as dichloro tris(triphenylphoshine) ruthenium(II) [RuCl2(PPh3)3].
The reaction is promoted by alkaline conditions. Homogenous catalysts are particulary attractive for enantioselective hydrogenations of functionalised ketones.
Aliphatic ketones to alcohols 5-150 deg C 1-10 bar alcohols, EtOAc or water
Aromatic ketones to alcohols 5-100 deg C 1-10 bar benzene, hexane, DMF or EtOAc
Either to hydrocarbons 5-100 deg C 1-10 bar acidic: alcohol or HOAc
Nitro and nitroso compounds
Aromatic nitro and nitroso groups are hydrogenated to the corresponding amine over Pd and Pt catalysts. The analogous aliphatic compounds are less easily hydrogenated as the resulting amine tends to inhibit the catalyst. In this case, higher catalyst loadings and more vigorous reaction conditions are required. To some extent the inhibiting effect can be decreased by operating under acidic condtions.
Intermediate oximes, hydroxylamines or azo compounds can be obtained depending upon the reaction conditions. Hydrogenation of nitro compounds is a very exothermic reaction (431 KJ/mol). It is therefore often desirable to use a solvent as a heat sink.
Aromatic nitro to amine 5-50 deg C 1-5 bar neutral or acidic, alcohols or EtOAc
Aliphatic nitro to amine 50-150 deg C 3-25 bar alcohols, preferably acidic conditions
Aromatic nitroso to amine 5-100 deg C 1-10 bar neutral or acidic, alcohols or EtOAc
Aliphatic nitroso to amine 5-100 deg C 1-10 bar alcohol/HCl or HOAc
Reductive alkylations
Reductive alkylations involves the reaction of a primary or secondary amine with an aldehyde or ketone to form a secondary or tertiary amine respectively. Formation of the imine intermediate is favoured by acidic conditions. The imine is seldom isolated. The amines formed are also suitable substrates for further alkylation. Thus, when wanting to produce a secondary amine (with a minimun of tertiary amine) from a primary amine feedstock, the primary amine to carbonyl molar ratio should not exceed one.
In some cases, the amine may be produced in situ from the corresponding nitro or nitroso compound. Similary the carbonyl may, in some circumstances, be produced in situ from the appropriate acetal, ketal, phenol or alcohol.
Aldehydes are generally more reactive than ketones because they tend to be less sterically hindered. Pd or Pt catalysts are preferred for reductive alkylations.
A catalyst of high selectivity is required to minimise the hydrogenation of the carbonyl to alcohol prior to imine formation by condensation. Sulphided platinum catalysts can be used to minimise alcohol formation but generally more severe reaction conditions are required then.
From aldehyde to amine 20-100 deg C 1-50 bar alcohol or no solvent
From ketone to amine 50-150 deg C 1-50 bar alcohol or no solvent
Reductive amination
The reaction is essentially the same as a reductive alkylation except that the amine feedstock is ammonia. The catalyst of choice is almost invaribly Pd. Excess ammonia is employed to suppress hydrogenation of the carbonyl to the alcohol, but also to suppress secondary or tertiary amine formation. A five-fold excess ammonia to carbonyl gives best results.
From aldehyde to amine 50-300 deg C 1-50 bar alcohol or no solvent
From ketone to amine 50-300 deg C 10-50 bar alcohol or no solvent
Nitriles
Hydrogenation of nitriles can be carried out over Rh, Pt or Pd catalysts. Primary, secondary or tertiary amines may be formed via the intermediate imine.
For the formation of primary amines, either acdic solvent conditions (at least 2-3 moles acid/mole nitrile) or excess ammonia (>2 moles/mole nitrile) are required. Formation of secondary amines is facilitated by neutral conditions while tertiary amines are usually produced predominatly only in the presence of a low molecular weight secondary amine.
For aromatic nitriles in ammonia or acidic media, Pd and Pt are preferred for the production of primary amines. Pt and Rh are preferred for the formation of secondary amines in neutral solvents. In neutral media, Pd is also effective for the formation of tertiary amines.
For aliphatic nitriles, with proper selection of conditions either Rh, Pt or Pd may be effective for the formation of primary amines, whereas Rh catalysts yield secondary amines as the predominant product and Pt or Pd catalysts favour the formation of tertiary amines – especially in the hydrogenation of short chain aliphatic nitriles.
In acidic aqueous media, aldehydes and/or alcohols can be made to be the major reaction products by hydrolysis of the imine intermediate in the presence of a Pd catalyst. The aqueous solvent is made acidic with H2SO4, HCl or HOAc. The acid promotes the hydrolysis of the imine and acts as a scavenger for the ammonia.
To primary amine 5-100 deg C 1-10 bar alcohol/ammonia, alcohol/acid or acetic anhydride
To sec-amine 50-100 deg C 2-5 bar alcohol, neutral conditions
To tert-amine 50-100 deg C 2-5 bar alcohol, DMF
To aldehyde 30-100 deg C 1-5 bar alcohol/acid or water/acid
Oximes
Hydrogenation of oximes can give primary, secondary or tertiary amines, hydroxylamines or imines. The imines are rarely isolated as such, since condensation coupling can readily occur as well as the possibility of hydrolysis to the aldehyde or ketone. Acetylation of oximes facilitates the hydrogenation.
Rh is the preferred metal for the formation of primary amines usually producing less secondary amine than Pd. Acidic (H3PO4 or H2SO4) or ammonical solvents favour the formation of primary amines by suppressing reductive coupling side reactions. Acidic conditions are recommended to minimise reaction rate inhibition caused by the amine products. With a Rh catalyst the amount of acid is not critical, but with a Pd catalyst there should be a least 2-3 moles acid/mole oxime.
Pd or Pt catalysts are generally preferred for the partial hydrogenation of oximes to the corresponding hydroxylamine.
Oxime to amine 30-60 deg C 1-5 bar alcohol/acid
