Heterogeneous Catalysis in Supercritical Media:2.Near-Critical and Supercritical Water

摘要

It was demonstrated in part I,also published in this journal,that supercritical carbon dioxide may be advantageous for the use as a reaction medium in hetero-geneously catalyzed reactions.Due to its miscibility with hydrogen,hydrogena-tions of increased selectivity can be carried out or carbonization of catalysts can be reduced by the dissolution and removal of coke precursors.The drawback of supercritical H2O(T_(crit)=374°C)as compared to supercritical CO2(T_(crit)=31°C)is its relatively high critical point.At these temperatures,organic substances decompose within minutes.Consequently,many reactions are carried out below this critical point,in the so-called"near-critical water"or"hot compressed water."Reaction conditions in near-critical and supercritical water are also referred to as"hydrothermal."In spite of the relatively high pressures and temperatures required and the associated high experimental expenditure,many types of reactions have already been studied,as the strong dependence of physical and chemical properties on pressure and temperature makes near-critical and supercritical water a unique and extraordinary solvent.