Chapter 5 - Reactions in Hydrothermal and Supercritical Water

Abstract

Abstract Chemical reactions in hydrothermal and supercritical water are carried out in a wide range of conditions. The advantage of water as a reaction medium is that it can be adjusted without adding or removing compounds from the reaction mixture. Depending on conditions of state, water thus exerts totally different actions to other compounds. For reactions under hydrothermal conditions, pressure must be higher than the vapor pressure of water to keep the reaction in the liquid phase because gaseous water loses one of the most important properties for a reaction medium: solvent power. An overview on chemical reaction in aqueous systems is presented in this chapter, based on the variability of the properties of water that change with increasing temperature. The ionic product increases up to a temperature of about T = 250 °C by three orders of magnitude, the dielectric constant drops from about 80 D to about 2 D, and density decreases. But up to relatively high temperatures, liquid water maintains essentially the familiar properties of a highly polar solvent. The dramatic change occurs in the neighborhood of the critical point, the so-called critical region. Within about Δ T = 20 K, properties of water change in such a way that familiar properties of water are lost. For example, water loses its solvent power for ionic species and becomes a good solvent for nonpolar components. In the critical region, water has a density that is easily changed by pressure and temperature. While supercritical water largely loses its solvent power at low pressures, it remains a polar solvent. At higher pressures, a good solvent power and higher polarity can be reached for supercritical water. Thus, the near-critical and supercritical region is a working field for realizing different reaction conditions and for recovering products by changing conditions. Furthermore, water at near-critical and supercritical conditions is fully miscible with many organic and inorganic compounds, enabling reactions in a homogeneous medium. Therefore, supercritical water can be a reaction medium for reactions usually carried out in organic solvents. Supercritical water is an excellent reaction medium for reactions requiring heterogeneous catalysts, since high diffusivity avoids mass transfer limitation and high solvent power prevents coke formation and poisoning of the catalyst. The reaction rates of small free radicals are increased due to the high collision frequency. Reactions of high molecular mass free radicals, as they occur during pyrolysis, are slowed down by a cage effect caused by solvent molecules at high pressure. Reactions with many different compounds are described in this chapter, such as hydrothermal reactions of hydrocarbons; hydrolysis reactions with esters, ethers, carboxylic compounds, amines, aldehydes, and others; reactions with acids and bases; condensation reactions; Diels–Alder reactions; and catalyzed reactions like organometallic reactions and oxidation reactions.