Optimising physical properties and phase behaviour

Supercritical fluids at conditions well removed from the critical region can enhance the rate of a reaction by promoting the effective concentration of a key reagent, X, accessible to the substrate. Thus, for a bimolecular reaction 

An increase in rate in this simple reaction, as well as in more complex schemes both homogeneous and heterogeneous, arises from improved accessible concentration (solubility) of X in the substrate-containing phase (section 3.1.1.1) and/or superior diffusivity of X in the vicinity of the substrate, which can cause an increase in k (section 3.1.1.2). These effects may allow reactions to 

1 Supercritical fluids as phase homogenisers. Applications of supercritical fluid reaction media which arise from the superior solvation of some species in this phase in comparison to either liquids or gases are considered in this section. Cases in which the concentration of key reagents or catalysts is boosted or controlled at a specified level are discussed, as is the removal of unwanted by-products, which would otherwise poison or foul a reaction catalyst. 

The hydrolysis of aniline to phenol in supercritical water by Patat [5] is accredited as the earliest reported study of a simple reaction in a supercritical medium. The reaction is catalysed by hydrogen ions formed from dissociation of phosphoric acid. Close to the critical point of water the catalyst has limited solubility, but as the conditions of the flow reactor were raised towards 450°C and 710 bar, the reaction rate was observed to increase by an order of magnitude. The mechanism was unaffected by the phase change it was from the better contact between reactants and catalyst that the rate improvement was derived. Townsend has taken advantage of homogeneity to study the reaction of a series of coal model compounds (alkyl-aryl hydrocarbons and ethers) in supercritical water [6]. For the hydrocarbons a free-radical pyrolysis route does not take advantage of the medium. However, for the ethers enhanced rates of reaction through a hydrolysis route occurs. 

Poliakoff and coworkers have used a high pressure cell [11] to study nitrogen and hydrogen half-sandwich complexes from the photolysis of metal arene tricarbonyl compounds [(r - C H ) M (CO)3], where for M = Cr, w = 6 for M = Mn or Re, w = 5 for M = Fe, = 4. Complete infrared invisibility of xenon assisted in spectroscopic identification of the product species. 

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