Supercritical fluids: green solvents for green chemistry?

Abstract

Supercritical fluids have a long history. It is over 190 years since Baron Cagniard de la Tour published his historic high-pressure studies [1] and nearly 150 years since Thomas Andrews FRS published his Bakerian lecture [2] that described the phase behaviour of CO2 and defined the critical point for the first time. It was the birth of green chemistry in the early 1990s that gave a new role to supercritical fluids as potentially environmentally more acceptable replacements for conventional solvents in a surprisingly wide range of chemical reactions and processes. Some of these have been scaled up into full-scale industrial processes [3], but not all have proved to be commercially viable. This has most frequently been because the processes have incurred high energy costs in compressing the fluid, most commonly CO2. These costs have been exacerbated by the generally upward trend in the price of oil and the resulting increases in the cost of energy. In the context of green chemistry, the processes contravened the so-called sixth principle of green chemistry, namely that processes should wherever possible be carried out close to ambient pressures [4]. Nevertheless, there remain many areas where super critical fluids offer real advantage and this discussion meeting, Supercritical Fluids and Green Chemistry?, was convened because the organizers felt that the time was ripe to assess progress and the state of the marriage between supercritical fluids and green chemistry. The outcome, as reflected in the papers in this issue, is that the marriage is broadly happy but, as with most marriages approaching their silver wedding, there are some tensions and disagreements, which, as in this case, are not always helped when the partners are of such different ages. Some of the papers presented and discussed at the meeting highlighted the symbiosis of supercritical fluids and another class of alternative reaction media, ionic liquids. These liquids are salts, almost invariably of organic cations and anions, which, despite being ionic in nature, have melting points close to or even below room temperature. Like supercritical fluids, they have a long history, but it was only in the 1990s that their role as solvents for reactions really took off. Their properties could not be more different from those of supercritical fluids, which are highly compressed gases displaying non-ideal behaviour. By contrast, ionic liquids have vapour pressures so close to zero that they can remain under ultrahigh-vacuum conditions for prolonged periods with negligible evaporation [5]. Together, these alternative classes of solvents offer unique and varied opportunities for both chemistry and indeed new methods for chemicals processing. The discussion meeting highlighted cutting-edge examples of the application of these unusual solvents to the fields of polymer science, sustainable energy/catalysis, pharmaceuticals and environmental science. We think it is fair to say that the meeting was a demonstration of vibrant scientific debate and discussion. By the end of the deliberations, it was clear that the magic of these fascinating materials had captured the scientific imagination of another generation of scientists and engineers, now empowered to drive the next phases of development and innovation.