Clean speed ahead with catalysts
Key text
This topic is sponsored by the bequest of J R Anderson, FAA.
Catalysts could help solve some of the world’s biggest pollution problems.
Catalysts are like super-heroes: they dramatically change the course of events, but are themselves unchanged. A new generation of catalysts might even help save the planet.
What is a catalyst?
A catalyst is any substance that causes or speeds up a chemical reaction and which is not itself permanently changed. It works by lowering the energy required for a reaction to take place (Box 1: Catalyst chemistry).
Catalysts are accustomed to playing heroic roles. They are, in fact, essential for life. Almost all reactions that take place in biological cells, for example, require a type of catalyst called an enzyme.
A catalyst is even credited with changing the course of world history. In the early 1900s, a German chemist, Fritz Haber, and an industrialist called Carl Bosch, used a simple catalyst to greatly improve the efficiency of ammonia production, an essential ingredient in many agricultural fertilisers, pharmaceuticals, cleaning agents and explosives. What became known as the Haber-Bosch process made possible a huge increase in agricultural production and thus underpinned the 20th century's massive population boom (Box 2: The Haber-Bosch process).
The importance of catalysts
Haber and Bosch both won the Nobel Prize, but they were not the only scientists to be recognised for their work on catalysts. Wilhelm Ostwald won a Nobel Prize in 1909 for his study of reactions catalysed by acids and bases. French scientist Yves Chauvin and US researchers Richard Schrock and Robert Grubbs jointly won a Nobel Prize in 2005 for their development of catalysts for metathesis, an important process in the manufacture of plastics. And in 2007 Gerhard Ertl won a Nobel Prize for, among other things, experiments that shed further light on the role of catalysts in the Haber-Bosch process.
Yet many scientists believe that the full potential of catalysts is still far from realised. The race is on to discover catalysts that will further improve the efficiency of industrial production, clean up polluting processes, and even help provide the fuels of the future.
A new breed of catalyst
Breakthroughs are increasingly common, even for relatively old processes. Ethanol (C2H5OH), for example, is widely used as a solvent and increasingly as a fuel. In developed countries, about 95 per cent of all ethanol is produced by adding water (H2O) to ethene (H2C=CH2), which is usually derived from natural gas or crude oil, in the presence of an acidic catalyst. For example, in the presence of sulfuric acid (H2SO4) ethanol is made according to the following equation:
The use of powerful acids has several drawbacks: they are hazardous to workers and the environment and because they are corrosive they are difficult to store, transport and handle. In recent years, catalysts called zeolites have been used instead of acids to produce ethanol. Zeolites are a group of crystalline minerals, generally made of aluminium, silicon and oxygen (aluminosilicates), with a highly porous structure; they have been called molecular sieves because they can be used to 'strain out' larger molecules.
Zeolites occur naturally but many forms have also been created artificially. They act as catalysts for a wide range of chemical reactions that take place inside their cavities. Hydrogen-exchanged zeolite is a form of zeolite that has protons bound to its framework so it acts as an acidic catalyst for the hydration of ethene. Its high porosity provides a large surface area on which the reaction can take place eliminating the need for large volumes of acid. Once the reaction is complete, the zeolite, which is unchanged, can be re-used.
The high porosity of zeolites provide a large surface area for chemical reactions
(© Karl Harrison, 3DChem.com)
Transcript of an interview in which catalysts for cleaner production of nylon are discussed.
(ABC Radio National, 12 January 2006, Australia)
Nylon
The process for producing nylon is changing. Nylon is a family of synthetic polymers used in hundreds of products, from women's pantyhose, to parachutes, to carpets, to drumstick heads. The traditional production of a common form of nylon, nylon-6, generates large quantities of ammonium sulfate, a waste product, and also requires volatile solvents and reagents that have an unfortunate tendency to explode. British scientists have streamlined the production of nylon-6 by using a form of zeolite made of aluminium phosphate as a catalyst. The process relies on air as an oxidant rather than highly reactive solvents and reagents and does not generate ammonium sulfate. The net result is a safer and much more environmentally friendly process.
Production of another common form of nylon, nylon-6,6 releases large quantities of the greenhouse gas nitrous oxide. But a more environmentally friendly technique is now available thanks to the Draths-Frost method developed in America. Genetically modified strains of bacteria can be used to convert plant carbohydrates into an organic acid. In the presence of a catalyst this acid can be converted to a key ingredient for nylon production without the emission of large quantities of nitrous oxide.
Catalytic converters
The potential of catalysts as environmental saviours is huge – as the experience with catalytic converters has already demonstrated. Catalytic converters are devices that sit on a car's exhaust pipe near the engine manifold. Installed in all new cars in Australia since 1986, they have played a major role in reducing air pollution. Most comprise three stages. The first stage involves a reduction catalyst made of platinum and rhodium, which assists in converting nitric oxide and nitrogen dioxide – two gases involved in global warming, smog and lung disease – to nitrogen and oxygen. The second stage of the catalytic converter has an oxidation catalyst, made of platinum and palladium, which assists in the conversion of carbon monoxide, a highly toxic gas, and hydrocarbons such as benzene, a known carcinogen, to carbon dioxide. The third stage consists of a control system that monitors the flow of gases and adjusts the car's fuel injection system accordingly.
Paving the way
Catalysts could also be used to reduce the concentration of toxic particles in air pollution hotspots, such as city centres. Scientists in the Netherlands and Japan, for example, recently started an experiment to test the use of roads to remove pollution from the atmosphere. A titanium dioxide-based catalyst has been added to concrete street pavers with the idea that, when the pavers bind nitrogen oxides from the atmosphere, the catalyst will assist in the conversion of these gases to nitrates, which have a much lower toxicity.
Another newly discovered catalyst, a combination of silicon dioxide and phosphonium, has been shown to assist in the manufacture of dimethyl carbonate, a preliminary product in the manufacture of a class of plastics used to make, among other things, iPod cases, compact disks and DVDs. The process consumes carbon dioxide, a greenhouse gas, instead of needing the very toxic chemical phosgene. Because the process fixes the carbon in plastic, some of its steps would remove carbon dioxide from the atmosphere while, at the same time, generating useful products.
Biofuels
But arguably the most important front on which catalysts will be deployed in the future will be the production of renewable energy. With limited global supplies of fossil fuels, the search is on for environmentally friendly energy sources to replace petrol in cars, diesel in trucks and ships, and kerosene in aeroplanes. Catalysts are already being used for solar energy as well as for the production of biofuels and hydrogen.
Biofuels are fuels such as bioethanol or biodiesel made from plant or (less commonly) animal material and food waste. They are promoted as a renewable energy source without the environmental impacts of fossil fuels but using annual land-based crops such as sugar cane and corn has drawbacks. Their use for biofuel production can be relatively energy inefficient and also competes with food production. Algae have been suggested as an alternative source of bioenergy which could overcome these problems.
Trees are also efficient bioenergy producers, partly because they are only harvested every decade or so and the energy inputs for cultivation and harvesting are therefore much lower than for annual crops. A drawback, though, is that trees contain large amounts of cellulose and lignin which are both difficult to break down. Again, novel catalysts are coming to the rescue. Research at the University of Sydney is focused on the development of catalysts to convert fibrous plant matter into biofuels. And Chinese scientists have devised a process using hot, pressurised water and a platinum-carbon catalyst to convert lignin to products which can be used to make biofuels. Such a process, if it can be scaled up, could greatly increase the role of tree plantations in energy production.
To produce biodiesel, plant oils are converted to esters using a process called transesterification. The catalysts traditionally used in this process can themselves have environmental effects as well as being expensive. Japanese scientists recently announced the discovery of a new catalyst made from glucose, which some say will make the production of biodiesel even cheaper and 'greener'.
Superhero of the future?
In the face of declining fossil fuel resources, degradation of our environment and the growing risk of climate change, the world's industries need to become more efficient and less polluting. Catalysts can help by reducing energy needs, solvent use and waste production. The search for the chemical superheroes is on.
Boxes
1. Catalyst chemistry
2. The Haber-Bosch process
Related Academy Links
External sites are not endorsed by the Australian Academy of Science.
Posted November 2008.