Supported Platinum Metal Catalysts
Supported Platinum Metal Catalysts
Their Selection and Methods of Use in Industrial Processes
A catalyst has two main functions:
By its presence to enable a chemical reaction to proceed much more rapidly, or under milder conditions of temperature and pressure, than would otherwise be possible.
To direct the course of a chemical reaction into the commercially desirable route when two or more routes are possible.
Of the six platinum metals—platinum, palladium, rhodium, ruthenium, iridium and osmium—all except osmium show remarkable catalytic properties and are widely employed. Each of them has some particular catalytic properties that distinguish it from its neighbours. Some of these properties are truly characteristic of the metal and remain unaffected by variation of the support or solvent. Other properties are modified substantially by the method of preparation of the catalyst. It is not possible, therefore, to state with precision the metal of choice for any particular application, but there are several factors that may be outlined to assist the research or development worker to select the metal most likely to suit his needs.
The Choice of Metal
Palladium, platinum and rhodium are comparably efficient metals for the hydrogenation of carbon-carbon multiple bonds; ruthenium is generally less efficient. The hydrogenation of olefinic bonds is readily achieved over rhodium, palladium and platinum catalysts, the reaction usually proceeding faster in the presence of solvents such as the lower alcohols or glacial acetic acid. Migration of the double bond, where this is possible, is most marked with palladium catalysts and least with platinum and iridium catalysts.
Rhodium, palladium and platinum readily catalyse the hydrogenation of the acetylenic triple bond; the corresponding olefin is formed most selectively (and most stereo-specifically) over palladium, and least selectively over platinum.
The rate of hydrogenation of the benzene ring catalysed by any of the platinum metals varies greatly with the number and nature of substituent groups. Thus, for example, toluene and other alkyl benzenes are more easily reduced than benzene. The nature of the solvent used is also very important, acetic acid, methanol or, where possible, water being generally preferred. Reduction of the aromatic hydrocarbons does not usually proceed rapidly at room temperature and atmospheric pressure with supported platinum metal catalysts, although Adams platinum oxide is sometimes satisfactory. At high pressures rhodium and ruthenium are the preferred metals.
Ruthenium, rhodium, palladium and platinum have all been reported as being active for the hydrogenation of heterocyclic compounds such as pyridine and furan, but elevated pressure is usually required.
The hydrogenation of aromatic nitrocompounds to the corresponding amine proceeds readily at room temperature and pressure in the presence of supported palladium catalysts. Platinum and rhodium catalysts are somewhat less effective, while ruthenium catalysts are almost without activity. Aliphatic nitro-compounds are, however, reduced with greater difficulty to the corresponding amine, but the intermediate hydroxylamine can sometimes be isolated.
The phenomenal development of the petroleum, chemical and pharmaceutical industries during the past two or three decades has largely resulted from the introduction of a wide range of catalysts designed to meet specific requirements. Among such catalysts are many containing one or more of the platinum metals, the high intrinsic value of these metals in no way inhibiting their use on a large scale. The successful use of a platinum metal catalyst must necessarily depend upon the very high proportion of its initial cost that may be reclaimed following its almost complete recovery from the spent catalyst. On this basis, the net cost of catalyst required to yield a unit increase in the value of the raw material is remarkably low in most industrial processes. This paper reviews the use of the platinum metals as supported catalysts and discusses the factors governing the choice o f metal and of support for a variety of industrial reactions.
Aldehydes and ketones may be reduced catalytically to primary and secondary alcohols respectively. Ruthenium catalysts are particularly suitable for these processes, and the reduction of polysaccharides (with hydrolysis) to cyclitols using ruthenium catalysts is in commercial operation. Palladium and platinum are also effective in this reaction. In acidic solutions there is sometimes a danger of further reduction of the alcohol to the hydrocarbon, especially when the hydroxyl group is activated, as for example in benzyl alcohol. Such hydrogenolysis is minimised by the use of non-polar solvents or by small concentrations of nitrogen bases such as aniline which act as selective poisons.
Hydrogenation of nitriles is performed in an acid solution when the corresponding primary amine is the desired product, since substantial amounts of secondary and tertiary amines are formed when hydrogenation is carried out in neutral solution. For the hydrogenation of ethyl or benzyl cyanides, or of benzonitrile, palladium catalysts are more effective than platinum catalysts in either acidic or neutral solution.
Hydrogenolysis is the term applied to any process in which the bond between two atoms is cleaved by a molecule of hydrogen. Typical examples are the reductions of benzyl alcohol to toluene, phenol to cyclohexane, alkyl halides to paraffins and halobenzenes to benzene. Conditions of acidity or alkalinity in the solvent, and its polarity, are of great importance to the rate of such processes. Hydrogenolysis often occurs as an undesired side-reaction proceeding simultaneously with another reduction process. Small quantities of alkali are especially effective in arresting the hydrogenolysis of activated hydroxyl groups.
Palladium, platinum and rhodium catalysts are all in some measure effective for hydrogenolysis. Thus palladium on charcoal catalysts will readily dehalogenate bromobenzene and benzyl chloride in methanol solution. Rhodium is the metal of choice for the hydrogenation of the aromatic nucleus where hydrogenolysis of substituents such as the methoxy group is to be kept to a minimum.
The disproportionation of cyclohexene into cyclohexane and benzene and related reactions are readily carried out under mild conditions by palladium on charcoal catalysts. The method of catalyst preparation is most important here, some types being much more efficient than others. Palladium on calcium carbonate is also effective for this reaction.
Palladium catalysts also catalyse hydrogen transfer from a hydrogen donor such as cyclohexene to other hydrogen acceptors. This sometimes forms a convenient basis for small-scale hydrogenations that do not involve molecular hydrogen.
Supported platinum is active for the oxidation of primary alcohol groups to the carboxylic acid group under mild conditions.
The Choice of Support
Once the catalytically active metal has been chosen, it becomes necessary to decide the means by which it can be used most efficiently. The prime function of the support is to extend the surface area of the metal. On a high surface area support such as charcoal the metal may be in the form of islands only one or two atoms thick, thus permitting very effective utilisation of the metal by providing a maximum surface area to weight ratio.
The physical characteristics of the support, and in particular its pore structure, may also modify the role of the metal since the course of a reaction is often greatly influenced by the rates of diffusion of reactants and products within the pore structure. Many of the commonly used catalyst supports (particularly alumina and charcoal) are available in a wide variety of particle sizes, each having a range of surface areas and pore-size distributions. The proposed reaction conditions often impose some limitations on the choice of support. For example, the support has to be stable at the temperature used, and it must not react chemically with any of the reactants, products or solvent.
It is also particularly necessary, when considering the form of support, to decide on the type of process envisaged, since this will determine whether the support needs to be a powder or some coarser material such as granules or pellets. The factors governing the required properties of the support in each case are so different as to warrant separate discussion.
Certain materials, notably charcoal, have been selected as catalyst supports by reason of their high surface areas and their highly adsorptive natures. These can lead to losses of valuable product by adsorption from solution, but as most charcoals are fairly selective this difficulty can often be overcome by changing the grade of charcoal.
It is also usually necessary to remove the catalyst from the reaction mixture after use, so all supports must be easily filtrable. This is particularly important with unstable products which need to be isolated rapidly from the reaction mixture before conversion to a stable derivative.
The first choice of catalyst support in any development work on liquid-phase reactions is almost invariably charcoal because of its wide range applicability. Johnson Matthey catalysts are prepared on specially selected grades of activated charcoal that are often given further purification and activation treatments before being used as catalyst supports. All the charcoals used have relatively high mechanical strength and stable particle size. Although platinum metals supported on charcoal are not pyrophoric, care must be exercised when they are exposed in the presence of hydrogen or organic solvents and vapours. The use of a standard purging procedure with an inert gas such as nitrogen is recommended, but to avoid all risk of conflagration the catalyst may be supplied in the form of a paste. This is in fact a crumbly powder containing about 55 per cent by weight of water, but with the water entirely held within the pores of the charcoal so that no supernatant liquid is apparent. This paste type of catalyst has obvious advantages in ease of handling, and its use is recommended when the presence of water in the reaction system can be tolerated.
Alumina has a much lower surface area than most charcoals, and its use is favoured in reactions where charcoal would give excessive loss of product by adsorption. Alumina-supported metals may, in certain cases, be more selective than the same metal on charcoal in giving the desired product where more than one reaction is possible. The aluminas used are reasonably stable to both dilute acids and alkalis.
Pure precipitated calcium carbonate, if carefully selected for particle size, is a very efficient catalyst support. While charcoal and alumina may both be used for all the platinum metals, calcium carbonate is used mainly as a support for palladium, particularly when a selectively poisoned palladium catalyst is required.
Silica, either as gel or as kieselguhr, is used when it is essential for the support either to have a low adsorptive capacity or to be neutral rather than amphoteric. Silica-alumina is suitable where there is a particular requirement for an acidic support.
The use of granular or pelleted supports facilitates continuous processes using large reactors packed with catalyst. Major industrial applications to date have generally involved vapour phase reactions, but there is an increasing interest in the use of trickle columns to perform continuously reactions that occur essentially in the liquid phase. In these reactions the gas and liquid reactants flow counter-currently through the catalyst bed. A basic requirement for all such catalysts is a high mechanical strength, so that the production of fine particles by attrition is kept to a minimum.
Many supports are available in a variety of forms, including irregularly shaped granules, extrudates and regularly shaped pellets. The packing, fluid flow and heat transfer characteristics of each type largely determine their choice by the chemical engineer for a particular requirement.
Alumina is the most commonly used support but, like charcoal, it exists in many forms. Alpha aluminas of fairly low surface area have been used in reactions where the support is essentially inert and functions only as a means of extending the surface of the active metal. More frequently the support itself plays a vital part in the reaction and in these cases gamma alumina is predominantly employed. Various anions, either occurring naturally in the alumina surface or added deliberately during catalyst manufacture, may influence significantly the overall course of the reaction to be catalysed.
Granular charcoal is not easy to obtain with a mechanical strength sufficiently high for the rigorous conditions of gas phase reactions, but is quite capable of being used in trickle column reactors.
Various other supports—silica-alumina, silica gel, ceramics—may also be used. When low surface area non-porous ceramic supports, such as Raschig rings are used, the amount of metal that can be firmly deposited is rather limited.
Methods of Use
The manner in which a supported platinum metal catalyst is used depends upon its physical form. Powdered catalysts and their paste equivalents (where appropriate) are suitable only for liquid-phase reactions carried out in batches. It is essential in these cases to have effective agitation of the system in order to ensure adequate three-phase contact; otherwise the most efficient use of the catalyst will not be achieved. There are several ways in which good agitation may be attained.
For very small vessels, rapid shaking is probably easiest; if the catalytic reaction is very rapid, frequencies of over 1000 vibrations per minute may be required. Agitation by high-speed stirring is applicable to any scale of operation, and is especially effective if there are baffles in the vessel. The use of a gas stream to agitate the system is also applicable on any scale of operation. This last method is particularly useful where the gas and catalyst are ejected centrifugally by a stirrer. Agitation conditions are often less critical when the reaction is conducted under pressure.
Selectivity in multi-stage processes may also be affected (sometimes advantageously) by inadequate agitation.
Charcoal-supported catalysts are unsuitable for use in fluidised-bed reactors because of the lack of uniformity in particle size. Certain aluminas are, however, satisfactory in this method of application.
Granular and pelleted catalysts are employed in fixed beds either for gas-phase reactions or, in trickle columns, for continuous liquid-phase reactions.
Quality Control of Catalysts
There are many variables that may affect the activity or selectivity of a catalyst in any given reaction. Among these are the nature of the support (depending on its origin, method of manufacture and pre-impregnation treatment), the methods employed in impregnating with the metal, and the drying technique used. There exist many techniques for measuring such parameters as chemical composition, surface area, pore size and pore size distribution, but none is as effective in determining accurately the performance of a catalyst in a particular reaction as an activity check carried out under controlled conditions with the same materials, and in the same manner, as are employed in the reaction to be catalysed.
In order, therefore, that only catalysts known to have acceptable activity and selectivity for the industrial user’s purpose are supplied, it is highly desirable that the reaction in question be carried out on a small scale in the laboratories of the catalyst manufacturer under conditions that approximate as closely as possible to plant operation. A less satisfactory alternative is to use a simpler reaction of the same general type as a quality control test. The hydrogenation of benzene or phenol, for example, may be used for quality control purposes on catalysts for the reduction of unspecified aromatic ring systems. Such compromises may at best, however, offer only a general guide rather than an accurate acceptance criterion.
Catalysts for New Processes
Catalytic processes of all types play an increasingly important role in the continuing growth of the chemical industries. This provides the impetus for widening the present range of standard catalysts and for the development of new ones for novel applications. Such development entails close and confidential collaboration between the user and the specialist catalyst supplier; the extensive experience gained in this field by the Johnson Matthey Research Laboratories has quite often led to the successful solution of such catalyst problems.