Catalysts

Catalysts are substances that speed up reactions by providing an alternative pathway for the breaking and making of bonds.  Key to this alternative pathway is a lower activation energy than that required for the uncatalysed reaction.

Catalysts are often specific for one particular reaction and this is particularly so for enzymes which catalyse biological reactions, for example in the fermentation of carbohydrates to produce biofuels.

Much fundamental and applied research is done by industrial companies and university research laboratories to find out how catalysts work and to improve their effectiveness. If catalytic activity can be improved, it may be possible to lower the temperature and/or the pressure at which the process operates and thus save fuel which is one of the major costs in a large-scale chemical process. Further, it may be possible to reduce the amount of reactants that are wasted forming unwanted by-products.

If the catalyst is in the same phase as the reactants, it is referred to as a homogeneous catalyst. A heterogeneous catalyst on the other hand is in a different phase to the reactants and products, and is often favoured in industry, being easily separated from the products, although it is often less specific and allows side reactions to occur.

Process	Catalyst	Equation
Making ammonia	Iron
Making synthesis gas (carbon monoxide and hydrogen)	Nickel
Catalytic cracking of gas oil	Zeolite	Produces: a gas (e.g. ethene, propene) a liquid (e.g.petrol) a residue (e.g. fuel oil)
Reforming of naphtha	Platinum and rhenium on alumina
Making epoxyethane	Silver on alumina
Making sulfuric acid	Vanadium(V) oxide on silica
Making nitric acid	Platinum and rhodium

General requirements for a heterogeneous catalyst

To be successful the catalyst must allow the reaction to proceed at a suitable rate under conditions that are economically desirable, at as low a temperature and pressure as possible. It must also be long lasting. Some reactions lead to undesirable side products. For example in the cracking of gas oil, carbon is formed which is deposited on the surface of the catalyst, a zeolite, and leads to a rapid deterioration of its effectiveness. Many catalysts are prone to poisoning which occurs when an impurity attaches itself to the surface of the catalyst and prevents adsorption of the reactants. Minute traces of such a substance can ruin the process, One example is sulfur dioxide, which poisons the surface of platinum and palladium. Thus all traces of sulfur compounds must be removed from the petrol used in cars fitted with catalytic converters.   

At high temperatures, the particles of a finely divided catalyst tend to fuse together and the powder may 'cake', a process known as sintering. This reduces the activity of the catalyst and steps must be taken to avoid this. One way is to add another substance, known as a promoter. When iron is used as the catalyst in the Haber Process, aluminium oxide is added and acts as a barrier to the fusion of the metal particles. A second promoter is added, potassium oxide, that appears to cause the nitrogen atoms to be chemisorbed more readily, thus accelerating the slowest step in the reaction scheme.

Aluminium oxide, silicon dioxide, aluminosilicates and zeolites

One of the most important reactions in which aluminium oxide, Al2O3, (often referred to as alumina) takes part in an industrial reaction is in platforming, in which naphtha is reformed over aluminina impregnated with platinum or rhenium. Both the oxide and the metals have catalytic roles to play, an example of bifunctional catalysis. There are hydroxyl groups on the surface of alumina which are, in effect, sites which are negatively charged to which a hydrogen ion is attached that can act as an acid catalyst.

Silicon dioxide (silica) is another acidic oxide used in industry. It becomes particularly active if it has been coated with an acid (such as phosphoric acid), thereby increasing the number of active acidic sites. For example, the manufacture of ethanol is achieved by the hydration of ethene using silica, coated with phosphoric acid:

Aluminosilicates are also used as catalysts when an acid site is required. These are made from silicon dioxide (silica) and aluminium oxide. They contain silicate ions, SiO44- that have a tetrahedral structure which can be linked together in several ways. When some of the Si atoms are replaced with Al atoms, the result is an aluminosilicate. Hydrogen ions are again associated with the aluminium atoms:

Zeolite catalysts

A particular class of aluminosilicates that has excited huge interest in recent years is the zeolites. There are many different zeolites because of the different ways in which the atoms can be arranged. Their structure of silicate and aluminate ions can have large vacant spaces in three dimensional structures that give room for cations such as sodium and calcium and molecules such as water. The spaces are interconnected and form long channels and pores which are of different sizes in different zeolites.

A zeolite which is commonly used in many catalytic reactions is ZSM-5 which is prepared from sodium aluminate (a solution of aluminium oxide in aqueous sodium hydroxide) and a colloidal solution of silica, sodium hydroxide, sulfuric acid and tetrapropylammonium bromide.

It is, for example, a very effective catalyst for the conversion of methylbenzene (toluene) to the three dimethylbenzenes (xylenes). Alas, the mixture produced only contains about 25% 1,4-dimethylbenzene, (p-xylene) the isomer needed for the manufacture of the polyesters and the rest, 1,2- (o-xylene) and 1,3-dimethylbenzenes (m-xylene), is not wanted in such large quantities.

However, if the zeolite is washed with phosphoric acid and heated strongly, minute particles of phosphorus(V) oxide are deposited on the surface making the pores slightly smaller. This restricts the diffusion of the 1,2- and 1,3-isomers and they are held in the pores until they are converted into the 1,4-isomer and can escape (Figure 9).

This remarkable selectivity enables the yield of the 1,4-isomer to be increased from 25% to 97%.

Figure 9 A zeolite acting an a molecular sieve and a catalyst during the formation of 1,4-dimethylbenzene from methylbenzene.

The ability of the zeolite to adsorb some molecules and to reject others gives it the ability to act as a molecular sieve.  For example, in the manufacture of ethanol from ethene or from biomass, an aqueous solution of ethanol is produced, in which there is 4% water still present even after repeated distillations. Further purification of ethanol requires the use of a zeolite which absorbs the water preferentially. Table 2 gives examples of industrial processes involving zeolites.

The branched alkene molecule is desorbed into the gas phase until it is readsorbed on to a metal site where it is hydrogenated to form a branched alkane, 2-methylpropane (isobutane), which is then desorbed into the gas phase.

In the industrial process, naphtha vapour is passed over platinum and rhenium (ca 0.3% each) which are finely dispersed over aluminium oxide.

The rhenium is thought to play an interesting role. If a sulfur compound is allowed to pass over the surface of the catalyst, it is preferentially adsorbed by the rhenium. If sulfur compounds are not removed, reactions occur leading eventually to the formation of carbon which causes the activity of the catalyst to be markedly reduced.

Branched alkanes have a much higher octane rating than straight chain ones. Not only are the alkanes now branched, but cycloalkanes are also formed and, from them, aromatic hydrocarbons. All three classes of hydrocarbon have a higher octane rating than naphtha. Besides aluminium oxide and silicon dioxide, other oxides are important catalysts. For example, in the Contact Process used to manufacture sulfuric acid, the catalyst for the oxidation of sulfur dioxide to sulfur trioxide is vanadium(V) oxide on the surface of silica:

 Potassium sulfate is added as a promoter. Its mode of action is not absolutely clear but it appears to be because its presence lowers the melting point of the catalyst, and allows it to spread out as a very thin layer over the entire surface.

Several important industrial processes are catalysed by mixed metal oxides. The surfaces contain two or more different metal atoms, O2- ions and -OH groups. They are particularly useful in the oxidation of hydrocarbons, where selective oxidation is required. For example, propene can be oxidized to propenal (acrolein) using a mixture of bismuth(III) and molybdenum(VI) oxides.

Without the catalyst, propene is oxidized to a large number of organic compounds, including methanal and ethanal, and eventually forming carbon dioxide. The oxygen atoms on the surface of molydenum(VI) oxide are not very reactive, reacting selectively with propene and breaking the weakest bond in the alkene to form an allyl radical:

The allyl radical is then oxidized on the surface to yield propenal. It is postulated that the allyl radical is oxidized by an oxygen atom that is adsorbed at a molybdenum site. Another oxygen atom, adsorbed on a bismuth site, is then transported to the reduced molybdenum site to replace that oxygen. There is a compensating transport of electrons to complete the cycle.

In the mechanism for this reaction a hydrogen ion is added at the start, and lost at the end. The hydrogen ion functions as a catalyst.

Two other examples are concerned with the production of 2,2,4-trimethylpentane from 2-methylpropene, again using an acid as the catalyst. One uses 2-methylpropane (Table 3) which yields the alkane directly. The other uses only 2-methylpropene.

The mechanism of the reaction also involves the addition of a hydrogen ion to a reactant (Figure 13).

The alkene is then hydrogenated, using nickel as the catalyst, to 2,2,4-trimethylpentane (isooctane):

2,2,4-trimethylpentane is often added to petrol to enhance its anti-knock properties, now that methyl t-butyl ether (MTBE) is being phased out.

Catalysts for polymerization reactions

Ziegler-Natta catalysts

Ziegler-Natta catalysts are organometallic compounds which act as catalysts for the manufacture of poly(ethene) and poly(propene). For their work on the production of polyalkenes, Karl Ziegeler and Giulano Natta were awarded the Nobel Prize in Chemistry in 1965. The catalysts are prepared from titanium compounds with an aluminium trialkyl which acts as a promoter:

The alkyl groups used include ethyl, hexyl and octyl.

The role of the titanium catalyst can be represented as shown in Figure 14.

The alkene monomer, for example ethene or propene, attaches itself to an empty coordination site on the titanium atom and this alkene molecule then inserts itself into the carbon-titanium bond to extend the alkyl chain. This process then continues, thereby forming a linear polymer, poly(ethene) or poly(propene).

The polymer is precipitated when the catalyst is destroyed on addition of water. Because it is linear, the polymer molecules are able to pack together closely, giving the polymer a higher melting point and density than poly(ethene) produced by radical initiation.

Not only do Ziegler-Natta catalysts allow for linear polymers to be produced but they can also give stereochemical control. Propene, for example could polymerize, even if linear, in three ways, to produce either atactic, isotactic or syndiotactic poly(propene).

However, this catalyst only allows the propene to be inserted in one way and isotactic polypropene is produced.

Even greater control of the polymerization is obtained using a new class of catalysts, the metallocenes.

Radical polymerization

Many polymers are produced using radical initiators, which act as catalysts (Table 4). For example the polymerization of chloroethene is started by warming it with a minute trace of a peroxide (R-O-O-R):

In the case of ethene, by using the free radical process, the resulting polymer has a lower density and a lower softening point than the poly(ethene) produced using a Ziegler-Natta catalyst or a metallic oxide. The low density poly(ethene), LDPE, has side chains because the radicals react not only with molecules of ethene, by addition, but also with polymer molecules, by a process known as hydrogen abstraction. The polymer radical can also abstract a hydrogen atom from its own chain:

Both of these reactions lead to side chains so that the molecules of the polymer cannot pack together in a regular way. The polymer thus has a lower melting point and lower density.

Looking forward

The search for catalysts will continue to be one of the highest priorities for the chemical industry as it seeks to run the processes at as low a temperature and as near atmospheric pressure as possible, commensurate with a reasonable rate of reaction.

The gains from improving catalysts are both financial and environmental, leading to lower fuel costs, for example the manufacture of methanol and the reduction of harmful waste gases, for example the manufacture of ethanoic acid. Similarly, benzene and propene are converted into cumene in the manufacture of phenol, using a zeolite catalyst in place of aluminium chloride. This means lower temperatures and pressures are used and the effluent produced is much cleaner.

Further, catalysts are sought which will favour one specific reaction over another, thus again making the process much more economic. There are benefits if a catalyst can be used rather than another chemical that takes part stoichiometrically in the reaction and cannot be recovered and reused. For example, aluminium chloride was used for many years to effect the reaction between benzene and a long chain alkene in the production of alkylbenzene sulfonates, an active surfactant in many detergents. The aluminium chloride could not be recycled and became waste as aluminium hydroxide and oxide. Now a solid zeolite catalyst with acid groups is used and can be reused time and time again with no waste products.

Another similar example is in the manufacture of one of the most important polymers used to make fabrics, polyamide 6 (sometimes known as nylon 6). In this process, cyclohexanone is converted into caprolactam via the oxime (produced by the reaction of the ketone with hydroxylamine hydrogensulfate). The oxime is isomerised by sulfuric acid to caprolactam, and ammonium sulphate is produced as a by-product. However, again a zeolite catalyst, with acidic sites, is now being used to effect the rearrangement. The zeolite is regenerated and saves the use and subsequent waste of sulfuric acid.