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

Abstract

Hot compressed water in the sub- and supercritical state exhibits exciting physical andchemical 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 opensup several promising opportunities for separation processes and chemical reactions. Undersupercritical conditions, substantial amounts of gases and organic substances canhomogeneously be mixed with water, which then can be separated by adjusting thesubcritical conditions by forming additional phases. This can beneficially be combinedwith chemical reactions occurring in the homogeneous state leading to integratedprocesses, which are more effective and competitive. Three approaches to the technicalapplication of hot compressed water are presented to show and discuss the technology,potential, technical hurdles and future research demand in this area of research anddevelopment.In supercritical water oxidation (SCWO) water is used as a medium in whichorganic pollutants are completely degraded under the addition of oxygen,which is completely miscible with water under the process conditions of up to650 °C and pressures around 25 MPa. Thus, high space–time yields in compact reactordesigns can be realized.Hydrogen is produced from biomass by hydrothermal gasification. Here, in anexcess of water, the reaction at temperatures up to 700 °C and pressures around 30 MPa directly leads to valuable hydrogen instead ofsynthetic gas, as in conventional gasification processes, or methane at subcriticalconditions in water. After reaction, pressurized hydrogen is obtained and caneasily be enriched due to the different partition coefficients of hydrogen andcarbon dioxide between the aqueous and gas phase.Even homogeneous catalysis is possible in supercritical water. Thishas been demonstrated with the cobalt-catalysed cyclotrimerization ofacetylenes to form benzene derivatives or hydroformylation to producealdehydes from olefins. There, only the addition of CO is necessary, theH2 required being formed by the equilibrium of the water–gas-shift reaction. Aftera homogeneous reaction in the supercritical state, the reaction mixture can beseparated at subcritical conditions.In support of the chemical and technical developments and to principally understand theexperimental findings fundamental aspects have to be investigated as well. Intensive studieshave been devoted to chemical kinetics including the modelling with elementary reactionsteps, e.g. to separate ionic and radical reaction pathways. Depending on the reactionconditions, ionic or radical reaction pathways can be favoured or suppressed, allowing forcontrol selectivity. Furthermore, corrosion of relevant reactor materials has beeninvestigated.