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 many processes, for example, brewing, ammonia synthesis, and combustion); can be easily recycled, and leaves no residues.[1] It is possible to tune the solvent properties simply by optimizing temperature or pressure. The use of small amounts of co-solvents (e.g., MeOH, MeCN, THF) can further modify solvent properties. 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—the 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 its 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.