Heterogeneous catalysis in supercritical media: 2. Near-critical and supercritical water

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 heterogeneously catalyzed reactions. Due to its miscibility with hydrogen, hydrogenations 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 H(2)O (T(crit) = 374 degrees C) as compared to supercritical CO(2) (T(crit) = 31 degrees 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.