The dirty gas that's cleaning up its act

Carbon dioxide is often blamed for causing global warming. But new research shows that the pollutant may be less harmful than we think, writes Lionel Milgrom

Carbon dioxide has had a bad press – as the climate-change negotiators meeting in The Hague this week can testify – but chemists may have found a new and important use for this waste gas, at least when it can be compressed enough to form an unusual liquid with intriguing properties. Carbon dioxide becomes really interesting under high pressure when it turns into a liquid – called supercritical carbon dioxide, or sc-CO ₂. In this state, CO ₂ has some remarkable properties as a solvent. And with carbon dioxide in plentiful supply – most of it comes from industrial fermentation processes – chemists are cashing in on sc-CO ₂'s solvent properties to turn vital but energy-consuming and polluting industrial processes into environmentally friendly chemical reactions.

Apart from the fact that every breathing creature on earth exhales it, carbon dioxide is best known for two reasons: global warming and rock concerts. Though its growing concentration in the atmosphere from fossil-fuel burning is causing an increased greenhouse effect with rising average global temperatures, carbon dioxide's popularity with rock bands stems from one of its more bizarre properties. At around -80C, carbon dioxide freezes to a white solid, "dry-ice" or "cardice".

However, above this temperature dry-ice does not melt to a clear liquid, like ordinary ice. Instead, it turns directly into a cloudy gas (chemists say it sublimes), and, being heavier than air, it drops rapidly to the floor, spreading like some cold, eerie ectoplasm. Combined with a laser light show, the effect is, well, gothic.

Unlike water, carbon dioxide has no liquid state at ordinary atmospheric pressures, only solid and gas. But under high pressure (just over 70 atmospheres), at 31C carbon dioxide turns into something halfway between a liquid and a gas.

So, like a liquid sc-CO ₂ behaves as a solvent (gases can't do this), but like a gas, it easily spreads over a surface, getting into every nook and cranny much more easily than ordinary liquids.

Both these properties have already been exploited; sc-CO ₂ is being used to decaffeinate coffee and clean intricate machinery of oily grime. The big advantage here, other than its inertness and non-inflammability compared with the usual organic cleaning fluids, is that, when its work is done, easing off the pressure turns sc-CO ₂ back into a gas, leaving behind the dissolved substance (caffeine or grime) and no environmentally-difficult solvent waste to dispose of.

The gas can then be recompressed into a liquid, ready to use again, so it is not added to the CO ₂ already in the atmosphere. Chemists are now finding ways to exploit sc-CO ₂'s solvent properties to make their reactions much more environmentally friendly. Chemistry, because of the industries based around it, is one of the major wealth producers of any industrialised nation. To give some idea how important chemistry is, prior to its break-up, the performance of ICI was a major indicator of the UK economy's health.

The crucial point is that most chemistry is about what happens when chemicals react together in solution. For this, solvents are necessary: they dissolve chemicals, ensuring that they thoroughly mix and react. The problem for chemists is what to do with waste solvents after a reaction. Because they are usually toxic, volatile, sometimes flammable liquids, not easily managed or contained and used in huge quantities, most of the solvents in general use can damage the environment. Consequently, as green issues rise further up the political agenda, cleaner chemical technologies are assuming a much higher priority not just for the chemical industry but also for academia.

Care for the environment is but one factor driving chemists to seek out better solvents. Most chemical reactions, especially in industry, involve catalysts. These are substances that speed up chemistry but also need to be recovered afterwards – for economic as well as environmental reasons: most industrial catalysts are based on precious or semi-precious metals. Not only is it wasteful to throw these away, they can persist in the environment for long periods.

Chemists achieve this recovery by ensuring all the components of the reaction – reacting chemicals, catalyst, and products – are in different phases. For example, the catalyst is usually a solid, while reactants and products are liquids or gases. However, there is a limit to just how quickly and efficiently chemicals and catalyst can be made to interact when in differing phases: the best reaction conditions occur when everything is in the same phase because the chemicals and catalyst can easily get to each other. Then the problem is how to separate everything afterwards, cheaply and cleanly, without having to safely dispose of toxic solvent waste. And that's where sc-CO ₂ comes in.

A team of researchers led by Professor Martyn Poliakoff at Nottingham University has pioneered much of the new chemistry in sc-CO ₂, turning potentially dirty, laboratory-based reactions into environmentally clean potentially continuous chemical processes so sought after by industry. The main problem here, however, is dissolving the catalyst in the sc-CO ₂.

Poliakoff and his group find that while an excellent solvent for gases and non-water-soluble substances, such as hydrocarbons, sc-CO ₂ is not very good at dissolving anything water-soluble, and that can include the catalyst.

His solution to the problem is to bind the catalyst on to a solid support (called immobilising), over which all the reacting chemicals wash. This makes it easy to reclaim and re-use the catalyst after the reaction, while the sc-CO ₂ carries away the reactants and product, which are readily separated simply by reducing the pressure and allowing the solvent sc-CO ₂ to turn back into a gas.

However, although continuous processing is possible using Poliakoff's method, the amount of product at the end of these reactions is not that substantial. Nevertheless, Poliakoff and his team have now successfully reproduced in the laboratory several industrially important chemical processes in sc-CO ₂ by immobilising their catalysts in this way.

Professor David Cole-Hamilton and his team at the University of St Andrews have recently gone one better in sc-CO ₂ chemical processing by ensuring that their catalyst is dissolved in another solvent that also dissolves the sc-CO ₂ and the reacting gases.

This means that all the reactants and the catalyst can come into that intimate chemical contact required for efficient chemical transformations, but still allows for the all-important separation of products from catalyst and reactants. The result is that Cole-Hamilton's process in sc-CO ₂ is far more efficient. "It's early days yet," says Cole-Hamilton, "and we need to be able to make the catalyst last longer. But the work we and others around the world are doing could be the basis for green chemical technologies of the future."