Hot compressed water—a suitable and sustainable solvent and reaction medium?

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Abstract
Hot compressed water in the sub- and supercritical state exhibits exciting physical and chemical properties, which can be varied continuously from gas-like to liquid-like behaviour. Correspondingly, the solvent properties can change from non-polar behaviour as present, for example, in organic solvents to highly ionic characteristics like in salt melts. This opens up several promising opportunities for separation processes and chemical reactions. Under supercritical conditions, substantial amounts of gases and organic substances can homogeneously be mixed with water, which then can be separated by adjusting the subcritical conditions by forming additional phases. This can beneficially be combined with chemical reactions occurring in the homogeneous state leading to integrated processes, which are more effective and competitive. Three approaches to the technical application of hot compressed water are presented to show and discuss the technology, potential, technical hurdles and future research demand in this area of research and development. In supercritical water oxidation (SCWO) water is used as a medium in which organic pollutants are completely degraded under the addition of oxygen, which is completely miscible with water under the process conditions of up to 650 °C and pressures around 25 MPa. Thus, high space–time yields in compact reactor designs can be realized. Hydrogen is produced from biomass by hydrothermal gasification. Here, in an excess of water, the reaction at temperatures up to 700 °C and pressures around 30 MPa directly leads to valuable hydrogen instead of synthetic gas, as in conventional gasification processes, or methane at subcritical conditions in water. After reaction, pressurized hydrogen is obtained and can easily be enriched due to the different partition coefficients of hydrogen and carbon dioxide between the aqueous and gas phase. Even homogeneous catalysis is possible in supercritical water. This has been demonstrated with the cobalt-catalysed cyclotrimerization of acetylenes to form benzene derivatives or hydroformylation to produce aldehydes from olefins. There, only the addition of CO is necessary, the H2 required being formed by the equilibrium of the water–gas-shift reaction. After a homogeneous reaction in the supercritical state, the reaction mixture can be separated at subcritical conditions. In support of the chemical and technical developments and to principally understand the experimental findings fundamental aspects have to be investigated as well. Intensive studies have been devoted to chemical kinetics including the modelling with elementary reaction steps, e.g. to separate ionic and radical reaction pathways. Depending on the reaction conditions, ionic or radical reaction pathways can be favoured or suppressed, allowing for control selectivity. Furthermore, corrosion of relevant reactor materials has been investigated.