Solvents

Carbon dioxide

The use of supercritical CO2 (carbon dioxide gas above its critical temperature, 31.1°C, and critical pressure, 73.8 bar) as a solvent has many advantages: it is non-flammable; non-toxic (although is an asphyxiant at high concentrations); readily available (it is a by-product of for example brewing, ammonia synthesis and combustion); can be easily recycled and leaves no residues.[1]  Supercritical CO2 is currently used in applications such as the decaffeination of coffee, extracting hops for use in brewing and for dry cleaning clothes.  Carbon dioxide has the advantage over other supercritical fluids that its critical temperature is remarkably low at only.  It can be used as a reaction medium and also has potential for product processing (e.g. extraction, particle formation and as a mobile phase for chromatography).  Supercritical fluids have advantageous solvent properties as they are effectively intermediates between gases and liquids: their densities are nearer to liquids and their viscosities are nearer to gases.[2]   It is possible to tune the solvent properties simply by optimising temperature or pressure.  The use of small amounts of co-solvents (e.g. MeOH, MeCN, THF) can further modify solvent properties toluene.  High diffusion rates offer potential for increased reaction rates and scCO2 is also inert to oxidation and resistant to reduction.

Supercritical carbon dioxide is however not without drawbacks, capital cost of high-pressure equipment and energy costs for compression of carbon dioxide (particularly for large scale work) can be disadvantageous.[2]  Other potential issues include is reactivity in the presence of some nucleophiles and the fact that it is a relatively weak solvent, although as mentioned earlier this can be modified via the use of co-solvents.

Recommended reading:

W. Leitner and P. G. Jessop, Green Solvents, Supercritical Solvents, Handbook of Green Chemistry (ed. P. T. Anastas) vol. 4, 2013.

  1. N. Tanchoux and W. Leitner, Supercritical Carbon Dioxide as an Environmentally Benign Reaction Medium for Chemical Synthesis, in Handbook of Green Chemistry and Technology, Blackwell Science Ltd, 2007, pp. 482-501.
  2. N. Winterton, Chemistry: Necessary but not Sufficient, in Chemistry for Sustainable Technologies : A Foundation, Royal Society of Chemistry, Cambridge, UK, 2011, ch. 8, pp. 194-252.