Supercritical fluids in green chemistry

The use of supercritical fluids is often highlighted as an important strategy within green chemistry to replace volatile organic compounds, VOCs, and to enable new, clean technologies. The now classic examples of natural product extraction have been practised in industry for more than twenty years on various scales.¹ In many of these cases, supercritical CO₂, scCO₂, penetrates into a solid substrate to remove a desired product or an unwanted component or impurity. In chemical reactions, scCO₂ is used as a solvent for reagents and/or catalysts to facilitate their intimate contact.² These and many more applications rely on the solvent power of scCO₂ and hence exploit in particular the liquid like properties of scCO₂.

There are, however, also a number of application where the supercritical phase is used mainly because of its properties of a compressed gas phase. Among these applications are processes involving polymeric materials, where the sorption of the compressed gas within the polymer matrix leads to a reduction in the temperatures of the glass transition or melting point. Possible applications of this basic phenomena are found in polymer processing, purification, or coating processes. If the gas phase is released very rapidly from the polymer matrix, foaming occurs allowing generation of polymer architectures and materials ranging from bulk to highly specialised products.

The review by Jacobs, Kemmere and Keurentjes in this issue of Green Chemistry (L. J. M. Jacobs, M. F. Kemmere and J. T. F. Keurentjes, Green Chem., 2008, DOI: 10.1039/b801895b) provides an overview of the state of the art for this area of supercritical fluid research and application. It is interesting to note that the green chemistry aspect of these methods is in fact wider than the actual replacement of undesired VOCs by CO₂ within the process. The materials obtained can display favourable properties, such as enhanced stability or insulation properties at reduced mass. New applications for biodegradable or bio-based polymers can be envisaged if porous materials can be accessed from them in a controlled manner.

In general, the integration of process and product design is an important target for green chemistry, although we have to admit that there is still some way to go before we can really talk about a “designer” process in this context. The 10th anniversary of the launch of this journal is certainly a timely opportunity to encourage further steps in that direction.

Walter Leitner

Martyn Poliakoff

References

1. P. G. Jessop and W. Leitner, in Chemical Synthesis Using Supercritical Fluids, VCH, Weinheim, 1999.
2. E. J. Beckman, J. Supercrit. Fluids, 2004, 28, 121.

---