Total Oxidation in Supercritical Fluids

During the degradation of tert.-butlybenzene only nonpolar free radicals are formed with low interaction with the solvent. Oxygen-containing radicals can interact much more with water, which is important, for example, for oxidation reactions. In these cases specific solvent effects are of more importance and significant difference in the chemical kinetics of the reaction in water compared with nitrogen as reaction medium could be assumed. 

This study concerns only very small conversions. Longer reaction times would lead to more intermediates, which may react with water or add OH free radicals to form a large variety of products. It is likely that after longer reaction times significant amounts of oxygen-containing compounds, e.g. alcohols or ketones, are found. 

Another C—C-bond splitting reaction or pyrolysis study [67] also shows no significant change when the reaction occurs in argon instead of in SCW. The pyrolysis rate of hexadecane in SCW was almost the same as that in 0.1 MPa argon. 

In pyrolysis reactions with nonpolar intermediates the main role of water is solvation and dilution. Dilution leads to a preference of unimolecular reactions by which, e.g., tar formation is avoided. The important free radicals of the pyrolysis chain reactions are large free radicals, which react with a reaction rate in the high-pressure plateau even at ambient pressure. At higher pressure the reaction rate could decrease by diffusion limitation. Water can participate in the chain reaction, but without significant influence on the overall kinetics. 

This does not mean that no solvent effects during pyrolysis reactions exist. Troe and coworkers [68] report a unique increase of the reaction rate of the recombination of benzyl radicals in a high-pressure medium (Ar, N2, CO2) due to solvent effects. This effect was more strongly accentuated in CO2. It has to be considered that any solvent effect in single reaction steps of pyrolysis reactions does not necessarily mean that a significant influence on the overall reaction rate is to be found. 

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