Heterogeneously Catalysed Oxidation in Other Supercritical Fluids

Heterogeneously Catalysed Oxidation in Other Supercritical Fluids 

Due to the absence of stable alternatives to SCCO2 and SCH2O, beyond these solvents the oxidation of hydrocarbons under SCF conditions is possible almost exclusively in neat substance, comparable to solventless technical oxidation processes like cyclohexane oxidation. This implies that the starting material itself is in a supercritical 

The most pronunent example is the partial oxidation of sc isobutane (Tc = 134.7 C,pc = 36.3 bar. Pc = 0.225 g/cm ) towards tert-butanol via hydroperoxides. The reaction is autocatalytic and follows a similar mechanism as in cyclohexane oxidation. Isobutane oxidation has gained importance because of the applications of the oxidation products, tert-butyl hydroperoxide and tert-butyl alcohol, in the manufacture of important chenucals like propylene oxide and methyl tert-butyl ether (MTBE). A comparative study between supercritical oxidation and liquid-phase oxidation with air, but without a catalyst, provides thermodynamic and kinetic data on isobutane oxidation. Supercritical conditions provided higher rates and selectivities than in liquid phase, because a liquid phase-like mechanism runs at 

Another example is known for the partial oxidahon of sc propene with O2 (Tcrit = 92.4 C, Peril = 46.6 bar, perit = 0.223 g/cm ) at mole frachons of propene from 20-90% over Ag catalysts. In the ophmvun temperature range of 180-220°C, conversion reaches 3.6% and propylene oxide selechvity is 36%. 

One example is reported in which the oxidant N2O was in a supercriheal state (Tcrit = 36.4°C, Peril = 71.5 bar, perit = 0.452 g/cm ). Its critical data are similar to those for CO2 and N2O might be an attrachve oxidant as N2 is the only by-product. It was possible to oxidise phosphines to phosphine oxide without a catalyst below 100 C. Compounds with other funchonal groups remain unoxidised while in SCN2O soluhon. With a Pt/C catalyst, it became feasible to oxidise secondary alcohols to ketones, e.g. isobomeol to campher with 100% yield.  

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