The properties which make supercritical carbon dioxide an attractive solvent for extraction also apply to its use as a medium for reaction chemistry. A fluid’s most important physical and transport properties that influence the kinetics of a chemical reaction are intermediate between those of a liquid and a gas in the supercritical carbon dioxide. The reactants and the supercritical carbon dioxide frequently form a single supercritical fluid phase. Supercritical fluids share many of the advantages of gas phase reactions including: miscibility with other gases, low viscosities, and high diffusivities, thereby providing enhanced heat transfers and the potential for fast reactions. Supercritical fluids are especially attractive as a reaction medium for diffusion-controlled reactions involving gaseous reagents such as hydrogen or oxygen.
An example of using supercritical fluids as a reaction medium is the hydrogenation of pharmaceuticals to promote enantio selective hydrogenation to favor a cis or trans version of a molecule during hydrogenation. By performing the reaction in two, instead of three phases, the rate of hydrogenation reactions can be increased over 1,000 times. As a results, the size of the reactor and the associated equipment is less than 1/10th that of conventional autoclave systems. Oils and fatty acid esters, as well as hydrogenare soluble in supercritical carbon dioxide. The reaction rate is increased because excess hydrogenis always available for reaction, and the catalyst pores are not filled with stagnant liquid.