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Association of British Science Writers
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These pages were designed, well, cobbled
together, by Michael Kenward on behalf of the ABSW.
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Catalysis*
Setting the scene
Fuel cells - fuelling the future
Auto-exhaust catalysts, new challenges
Ancient catalysts for new drugs
Evolution in a test-tube
Satisfying needs for the future
Summary
Catalysis is one of the most important technologies we have, yet most people are unaware of its wide-ranging significance. It plays a crucial role in many aspects of human progress - in the efficient manufacture of many kinds of materials - from fuels to plastics, in creating new energy sources and protecting the environment, and in developing effective, safer medicines.
Catalysis is the phenomenon by which certain chemicals (catalysts) can speed up a chemical reaction without undergoing any permanent chemical change themselves. They can be recovered after a reaction and used again and again (although most catalysts have finite lifetimes). Without the right catalyst, many reactions hardly go at all. Furthermore, the chemical nature of the catalyst can have a radical effect in selecting reaction pathways leading to different chemical products. Over recent decades there has been enormous progress in understanding the underlying molecular mechanisms, which has had an explosive effect on the development of new catalyst systems.
About 80 per cent of processes in the chemical industry now depend on catalysts to work efficiently, and the number is rising. Since the chemical industry is one of the most important and competitive sectors of all developed economies, particularly the UK, it is not surprising that catalysis is an extremely active research area. New catalysts are being designed and new catalytic processes being devised that aim to produce cleaner chemical processes. These use less energy, and environmentally acceptable agents (for example, air or oxygen as an oxidant instead of hydrogen peroxide) and perhaps water as a solvent, resulting in less noxious waste. Catalyst research is vital in combatting pollution.
Virtually all living processes depend on biological catalysts called enzymes, and understanding their role in molecular biology and human health must not be underestimated. Enzymes and enzyme-like materials are also being increasingly used as catalysts for industrial processes.
There are currently some ingenious, highly innovative developments in catalysis. This briefing highlights some examples that are likely to have an important influence on our daily lives.
* Summary of a media briefing held on 18 May
Setting the scene
Professor Sir John Meurig Thomas of the Davy Faraday Research Laboratory, London, and the Department of Materials Science, University of Cambridge, is a world-leading researcher in catalysis. He introduced the briefing by explaining the central role of the catalyst in the chemical industry. Catalysts are used in three ways. The first is to change cheap and plentiful starting materials such as hydrocarbons in oil and natural gas into chemicals that may serve as fuel or starting materials for other processes. In oil refining, for example, 350 million gallons of oil are converted per day worldwide using 1000 tonnes of catalysts. The catalytic agents may be metals such as platinum, or metal oxides, acids or certain modified minerals. Catalysts are also used to convert the resulting petrochemicals into so-called 'higher added-value' products such as polymers and fine chemicals. Meanwhile, in the pharmaceutical industry, drugs are made via a series of chemical steps from a range of fine chemicals which increasingly involve imaginative, highly selective reactions using catalytic agents with complex structures. These may be metals chemically bound in an organic compound or biological agents called enzymes.
Catalyst requirements
Irrespective of the nature of the catalyst it must have certain properties to be economic. First, it must be highly active. Catalysts work by virtue of possessing certain 'active sites' in their structures. Adsorption or transient bonding at the active site allows the reacting molecules to undergo a particular chemical transformation at much lower energy than would normally be the case in the absence of such sites. The number of active sites and ease of access controls the rate of turnover of reacting molecules. The enzyme catalase, for example, destroys hydrogen peroxide generated in our bodies at a rate of a million million molecules per second at each active site, while the iron catalyst, the foundation stone of the Haber process for making ammonia for the fertiliser industry, turns over only one molecule per active site per second. Such catalysts therefore need to be manufactured so as to contain a large number of active sites per unit surface area. Another important requirement is long lifetime; for instance, the iron catalyst of the ammonia process lasts 15 years!
A third and crucial characteristic is selectivity; catalysts actually direct the course of the reaction as well as speed it up. For example, a copper-based catalyst converts 'syn gas' (a mixture of carbon monoxide and hydrogen produced by the controlled burning of natural gas) into methanol. However, other metal catalysts yield mixtures of hydrocarbons suitable for various fuels. Enzymes are incredibly selective. They are able to recognise a reacting molecule by its shape and transform it in a geometrically specific way.
Zeolites
There are also inorganic catalysts called zeolites which are shape-selective. These are microporous minerals which consist of a framework of pores and channels offering an intricate three-dimensional surface which can be rendered catalytically active. Zeolites are used extensively by the oil industry for many reactions, and their development continues to be an extremely active research area. The pore size and shape are critical - allowing one kind of molecule into the zeolite's interior while excluding a larger species. Professor Thomas' research group is among several which have recently developed new microporous and mesoporous materials that are powerful catalysts. These can be peppered with active sites either during preparation or by anchoring inside them titanium-centred molecular species, or various bimetallic nanoparticles. Such materials can be used partially to oxidise hydrocarbons or to hydrogenate them - challenges that are extremely important in the polymer and pharmaceutical industry.
Fuel cells - fuelling the future
One extremely exciting use of catalysts is in fuel cells. These devices convert the chemical energy in a fuel into electrical energy - as in a battery but with a continuous supply of chemicals from outside. Fuel cells have many applications, including propulsion sources for cars, especially buses and taxis in cities and various electrical appliances. They have considerable benefits, being relatively light for the power they produce with a simple mechanical design (no moving parts) that can be made modular. They respond rapidly but produce little pollution.
Hydrogen fuel cell
The simplest fuel cell converts the fuel hydrogen and oxygen (from the air) into water - a well-known reaction that produces a large amount of energy. Hydrogen is oxidised to water at the anode and oxygen is reduced at the cathode (2H2 + O2 = 2H2O). The two electrodes are coated with a platinum catalyst, and in earlier designs the electrolyte was alkaline.
Diagram of fuel cell
This fuel cell was first developed by William Grove in the 1840s. It works at room temperature. A more modern version was developed by Siemens for powering submarines. Their great advantage is that they have no thermal signature and are almost completely silent so are extremely difficult to detect.
So why aren't we using these fuel cells for cars and other forms of transport? According to Professor Andrew Hamnett of the University of Newcastle, the efficiency of energy conversion needs to be improved to make fuel cells economic. One major challenge is to improve the efficiency of the oxygen reduction step, which requires a large 'overvoltage' (and therefore energy) just to get the reaction started. Four electrons need to be transferred to each oxygen molecule to produce two molecules of water; the three intermediates en route are all rather unstable and must be stabilised by specific bonding to the catalyst surface. This is a very difficult trick to pull off, and chemists have been struggling for more than a century to identify improved catalysts. A second problem with alkaline fuel cells is that carbon dioxide in the air supply reacts with the alkali to form solid carbonate in the electrolyte which impedes catalytic action. The catalyst anode is also readily 'poisoned' by contaminants in the hydrogen. Further improvements depend on finding better catalysts.
PEM fuel cell
One improvement that there has been is to use a solid electrolyte based on a polyfluorotetraethylene (PTFE) membrane that can conduct protons (hydrogen ions) across the cell. These proton exchange membrane (PEM) fuel cells produce decent amounts of power - a 5-kilowatt PEM fuel cell will drive a small car. Again, they are completely silent, so much so, says Hamnett, that artificial noise has to be incorporated into the design!
Methanol fuel cell
Could other fuels be used such as hydrocarbons? So far chemists have not found a way of easily breaking the carbon-carbon links in such materials in an electrochemical cell. However, methanol is a strong candidate. When combined with oxygen in a cell it is converted to carbon dioxide at the anode. Methanol is easier and safer to transport than hydrogen. However, there is a snag: not only is the oxygen reduction reaction at the cathode inefficient, so is the methanol oxidation reaction at the anode. Hamnett and his team have been studying this latter reaction with the aim of designing a better catalyst. By using higher temperatures and pressures and a new platinum-ruthenium catalyst they have achieved respectable power densities of up to 200 milliwatts per square centimetre using air - enough for a car to work.
At the moment fuel cells are more expensive than internal combustion engines, but they are virtually pollution free. Nevertheless, with the continuing downward legislative pressure on vehicle emissions from internal combustion engines, fuel cells will eventually become an economic alternative, says Hamnett.
Auto-exhaust catalysts, new challenges
In the meantime, catalysts have considerably reduced emissions from the internal combustion engine, as Dr Brian Harrison from Johnson Matthey - a leading UK catalyst company - explained. Catalytic converters fitted to vehicles have significantly lowered levels of ozone and photochemical smog, particularly in cities. However, regulations governing emissions are continually being tightened, so catalyst makers are devising new strategies such as improving the efficiency of catalysts under 'cold-start' conditions and developing new catalyst systems to deal with diesel exhaust.
Most vehicles emit pollutants as a result of the imperfect burning of fuel. The pollutants include hydrocarbons, carbon monoxide, nitric oxide and nitrogen dioxide (NOx), and also soot particles (particularly from diesel engines). These pollutants give rise to a phenomenon called photochemical smog by reacting in sunlight to give ozone and unpleasant irritants such peroxylacetyl nitrate.
The three-way catalytic converter
The simplest car exhaust catalysts developed 20 years ago are again based on platinum, spread on an alumina honeycomb support (to give a high surface area). They deal with carbon monoxide and unburnt hydrocarbons which react with oxygen to produce carbon dioxide and water. Today the modern, so-called three-way catalyst - a combination of platinum, palladium and rhodium - also removes (NOx) by promoting the reaction of carbon monoxide with nitric oxide to produce nitrogen and carbon dioxide. However, to be effective, the catalyst works best with standard 'stoichiometric' engine regimes which operate on the basis of a one-to-one air:fuel ratio. If the mixture is too rich there is not enough oxygen to remove the carbon monoxide and hydrocarbons, if too lean (ie excess of oxygen) the nitrogen oxides are unable to react on the oxidised catalyst surface.
Meeting new emission standards
The three-way catalyst has had an enormous impact on improving air quality. Without it it would not be possible to sustain the number of cars we have on the road today. Nevertheless, regulations continue to tighten, and by the early part of the next century all classes of noxious emissions will have to reach ultra-low emission standards. This raises a number of problems. The first is that most hydrocarbons and carbon monoxide are produced when the engine is starting and the catalyst is still cold. One strategy catalyst companies are looking at is to put the catalyst very close to the engine. These 'close-coupled' catalysts, which have to be thermally stable, are starting to be fitted; they work fully about 50 seconds after the engine is switched on with the result that they produce lower emissions than conventional systems.
Lean-burn engines also present a challenge. They do not produce much carbon monoxide (the high air:fuel ratio means it is burnt off), but nitrogen oxides are not removed in the presence of oxygen. Researchers are looking at a number of strategies. A promising one is to include a metal like barium in the catalyst. It stores the oxides of nitrogen as nitrate until they can react with carbon monoxide produced when the car acceleration causes the fuel mixture temporarily to go rich. Another strategy that works for diesel engines, which are also lean-burn, is to inject tiny amounts of fuel into the exhaust which then react with (NOx)
A particular problem for diesel engines is of course particulates. Filters are quite effective, although regenerating them is difficult. Johnson Matthey has been working on a new combined catalyst system in which nitric oxide is first converted to nitrogen dioxide which then oxidises the soot particles collected in a trap.
Ancient catalysts for new drugs
In recent years, chemists have increasingly harnessed enzymes, the biocatalysts used by Nature, to transform simple molecules into more complex fine chemicals for the pharmaceutical and agrochemical industries. Enzymes have the advantage that they are environmentally benign, and they work at room temperature and in water. They are also selective, being able to distinguish between 'enantiomers'. These are molecules that have the same chemical structure but are mirror images of each other; in other words they have 'opposite handedness', or chirality. Many complex organic molecules are chiral or have chiral components, such that the enantiomers have different biological activity. In the drug industry, it is extremely important to prepare molecules in the correct chiral form, as the well-known instance of the thalidomide tragedy reminds us. Thalidomide was sold as a mixture (racemate) of enantiomers. The form that acted as a sedative was safe but its mirror-image is a teratogen.
Making chiral building blocks
Professor Stanley Roberts of the University of Liverpool has been exploiting enzymes to carry out a variety of chemical transformations that involve converting a non-chiral starting material into a chiral building block for making new drugs.
Enzymes derive from a huge variety of microorganisms. The enzyme may be used in its natural state, as a whole cell system, or isolated as the pure compound. A typical whole cell system is baker's yeast. Such enzymes may not only work on the natural substrate but on unnatural ones as well.
Chemists have now identified a diverse range of transformations that can be carried out with biocatalysts. A simple example is the breaking up with water (hydrolysis) of an ester into its constituent carboxylic acid and alcohol using the enzyme hydrolase. What is more, in the absence of water the reaction goes in the opposite direction. Roberts now works with a small chemical company to exploit this reaction in making polyesters.
Identifying a biocatalyst
How are suitable biocatalysts identified? One approach is to screen organisms already available. Working in association with a UK firm Enzymatics (now called Chiroscience) which holds a collection of organisms, Roberts and his colleagues identified an organism containing a biocatalyst that would separate enantiomers of a compound called a gamma-lactam - a key intermediate in the synthesis of a new anti-HIV agent called Ziagen launched by GlaxoWellcome only a few months ago. The team found a biocatalyst that would hydrolyse one enantiomer but not the other, and the chiral material is now being made in tens of tonnes a year.
To find enzyme reactions for completely new transformations may require finding new microorganisms, for example, from the soil. This is what happened when ICI in Billingham wanted to convert benzene into cyclohexadiene diol for synthesising polyphenols. The ICI chemists looked for organisms that had adapted to survive underneath the company's benzene storage tanks by feeding off the drips from the tanks. The organism was able to oxidise benzene in this way, thus carrying out a new type of reaction that could not be done by conventional chemistry.
The origin of life
Chemists would like to extend the scope of biocatalysts. Enzymes are proteins - very long chains of combinations of 20 naturally occurring amino acids. These exist only in one chiral form, the l form. The mirror images, the d forms, do not exist in Nature. This limits the chiral chemistry that can be carried using natural enzymes. An alternative is to synthesise simple enzyme 'mimics' by polymerising one kind of amino acid, say, leucine, which can be prepared in both d and l forms. Roberts has shown that polyleucine is a very effective catalyst for making useful building blocks called epoxides which are used, for example, in the synthesis of the anti-angina agent Diltiazem, the anticancer agent taxol and non-steroidal anti-inflammatory compounds such as Naproxen. Indeed, Roberts suggests that he may be reinventing the wheel. Such simple polypeptides may have been the first biocatalysts on Earth, forming from primordial gases and then playing a crucial part in the molecular evolution of life.
Evolution in a test-tube
Professor Manfred Reetz of the Max-Planck-Institut für Kohlenforschung in Mülheim developed a completely new method to prepare enantioselective catalysts. Specifically, he exploits Darwinian evolution to create new, 'fitter' enzymes as catalysts for making chiral compounds. In the chemistry lab, he says, enzymes do not always work efficiently on every substrate - they do not produce the pure enantiomer but a mixture defined by the 'enantiomeric excess' (ee), the excess of one enantiomer over the other. Equal amounts of enantiomers is represented by an ee of 0, whereas a ratio of 95 to 5 per cent of one to the other would be an ee of 90.
Exploiting random mutagenesis
Reetz's approach is to seek 'mutant' versions of the enzymes that give higher ees than the natural enzyme. The principle is to isolate the gene that codes for the enzyme and make it mutate randomly in a test-tube. This can be done using a form of the polymerase chain reaction (PCR) called error-prone PCR. The technique produces point mutations in the DNA to create a library of mutant genes. The mutant genes are then inserted into host bacteria which are grown on agar plates. Each colony producing a different variant of the enzyme is put in a well on a microtitre array for screening. Reetz says his team can screen up to 800 variant biocatalysts a day per person in this way.
The best enzyme from the first generation of mutagenesis is then subjected to a second round of mutagenesis and the resulting enzymes are screened anew. The process is repeated until the ee approaches 100 per cent. An example Reetz has been working on is the hydrolysis of a racemic ester to a chiral acid using a lipase enzyme (P. aeruginosa). (A coloured version of the compound is used so that its transformation can be monitored visually.) The natural biocatalyst gives an ee of only 2 per cent while the best enzyme from the first generation of mutagenesis gives an ee of 31 per cent. The best mutant of the third round of mutagenesis gives an ee of 57 per cent while that in the fourth generation shows an ee of 81 per cent. Using this technique in combination with another mutagenesis method called saturation mutagenesis the ee value has been improved to more than 90 per cent.
Diagram showing process
Targeting the search
Since the number of possible mutations in the DNA of a gene, and therefore in the protein it encodes for, is vast, Reetz has been looking for ways of speeding up the search. If the three-dimensional structure of the enzyme is known, for example, then by mapping the amino acid sequence of the most effective enzyme, it's possible to identify 'hot spots' where single amino acid changes have had an effect. This information can be used to limit the scope of mutagenesis in the original natural gene and so target the enzyme search. Reetz is also investigating screening methods of distinguishing between the final chiral products without having to separate them.
Satisfying needs for the future
Dr Frank King of ICI Synetix (was called ICI Katalco) concluded the meeting by first reminding people of the economics of the chemical industry and some of the challenges ahead. The value of the chemical industry worldwide is $1.5 trillion (million million) a year - the gross national product of Italy. Catalyst sales are worth $10 billion a year and are growing at between 3 and 5 per cent a year.
The search for cleaner processes
Although virtually all heavy chemical processes such as oil-refining use catalysts, in the fine chemicals sector, only 20 per cent of processes are catalysed. A major challenge is make these smaller scale processes more efficient and less polluting by using a suitable catalyst. One of the most important processes here is oxidation. This is often carried out using osmium tetroxide and chromium vi compounds, which are toxic, so ideally chemists would like to move away from such oxidants.
Companies around the world are looking for economic alternatives. For example, the Italian company Enichem has a process to make propylene oxide from propylene and hydrogen peroxide using a titanium zeolite TS-1. The only waste-product is water. Unfortunately hydrogen peroxide is too expensive for the process to be used commercially. As a result, one of the key aims in this area of chemistry is to find a way of producing hydrogen peroxide more cheaply.
Domestic fuel cells for combined heat and power
King went on to describe a new development that could have enormous impact on our lives, and that is the stationary fuel cell. He suggested that these devices could replace the domestic boiler, generating both heat and electricity, and would remove the need for power stations. According to King, power stations are only about 40 per cent efficient and 30 per cent of electricity is lost in transmission by pylons. Power stations also produce air pollution in the form of oxides of nitrogen and sulphur dioxide, while pylons are both hazardous and an eye sore.
Natural gas of which there is a plentiful supply would be the fuel. It would first be converted into hydrogen which would then react in the fuel cell. Such systems are currently being developed via an EU programme. Current estimates put the cost of having a combined heat and power fuel cell in your house at between £2000 and £3000, eventually coming down to the cost of a typical domestic boiler. King reckons that they could start coming into use in the next 3 to 5 years.
Contacts
Sir John Meurig Thomas FRS
The Master's Lodge
Peterhouse
Cambridge CB2 1QY
Tel: 01223 338202
Fax: 01223 337578
Dr Frank King
ICI Synetix
PO Box 1
Belasis Avenue
Billingham
Cleveland TS23 1LB
Tel: 01642 553601
Fax: 01642 522606
email: frank_king@ici.com
Professor Andrew Hamnett
Department of Chemistry
Bedson Building
University of Newcastle
Newcastle upon Tyne NE1 7RU
Tel: 0191 222 7701/5060
Fax: 0191 222 5489
email: andrew.hamnett@ncl.ac.uk
Dr Brian Harrison
Johnson Matthey PLC
Orchard Road
Royston
Hertfordshire SG8 5HE
Tel: 01763 253000
Fax: 01763 253739
email: HARRIB@Matthey.com
Professor Stanley M. Roberts
Department of Chemistry
University of Liverpool
Crown Street, Liverpool L69 7ZD
Tel: 0151 794 3500
Fax: 0151 794 3587
email: sj11@liverpool.ac.uk
URL: http://www.liv.ac.uk/Chemistry.html
Professor Manfred Reetz
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1
45470 Mülheim/Ruhr
Germany
Tel: 49 208 306 2000
Fax: 49 208 306 2985
email: rathofer@mpi-muelheim.mpg.de
Professor Chris Adams, Director
Institute of Applied Catalysis
PO Box 32
Prenton
Wirral, L43 5XT
Tel: 0151 670 9381
Fax: 0151 670 938
email: chris.adams@iac.org.uk
url: http:/www.iac.org.uk
Further reading
The following offers a simple introduction to catalysis:
The Age of the Molecule, "Chemical marriage-brokers", p73, Royal Society of Chemistry, price £15 for members, from Membership Administration Dept, Thomas Graham House, Science Park, Cambridge, tel: 01223 420066; £19.50 for others, from Turpin, Blackhorse Lane, Letchworth, tel: 01462 672555.
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