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Reaction kinetics of supercritical water oxidation (SCWO) of ammonia/methanol mixture was investigated at 530 °C, 25 MPa, and [NH 3 ] 0 = 2.9–3.0 mmol/L, both experimentally and computationally. Ammonia conversion increased with an increase in the initial methanol concentration. Ammonia oxidation was retarded after complete oxidation of methanol. The major product from ammonia oxidation was N 2 O. Methanol decomposition and CO oxidation were accelerated in the ammonia/methanol mixture under the conditions used. Calculations using an elementary reaction model show that ammonia and methanol mutually promote their oxidation through radical intermediates. The mixture effect in the SCWO of ammonia/methanol can be explained by a radical chain mechanism in which ammonia and methanol share the radicals generated and promote mutual oxidation reactions. The ammonia oxidation cycle initiated by methanol co-oxidation was maintained by nitrogen-containing species even after methanol had been completely oxidized.

Current research analyzing the effects of water in the field of homogeneous and heterogeneous reactions of organics in sub- and supercritical water are reviewed in this article. Since the physical properties of water (e.g., density, ion product and dielectric constants) can affect the reaction rates and mechanisms of various reactions, understanding the effects that water can have is important in controlling reactions. For homogeneous reactions, the effects of water on oxidation, hydrolysis, aldol condensation, Beckman rearrangement and biomass refining were introduced including recent experimental results up to 100 MPa using special pressure-resistance equipment. For heterogeneous reactions, the effects of ion product on acid/base-catalyzed reactions, such as hydrothermal conversion of biomass-related compounds, organic synthesis in the context of bio-refinery, and hydration of olefins were described and how the reaction paths are controlled by the concentration of water and hydrogen ions was summarized.

An acid-catalyzed dehydration mechanism was investigated for formic acid decomposition through calculations based on density functional theory. In previous experimental investigations, formic acid dehydration in hot compressed water was reported to proceed faster at pressures >30 MPa. Higher concentration of hydrogen ions because of the large ion product of water at high pressure was believed to contribute to the acceleration of the dehydration reaction. In this study, the structures and energies of the transition states and intermediates were determined through calculations based on density functional theory with the B3LYP/6-311+G(2d,p) level of theory. A comparison of their threshold energies indicated that the dehydration proceeded via the protonation of hydroxyl oxygen, and that the acid-catalyzed dehydration was energetically more favored than the water-catalyzed mechanism. These results suggested that the abundant hydrogen ions in hot compressed water at high pressure accelerated the dehydration occurring via an acid-catalyzed formic acid dehydration mechanism.

The effects of pressure on formic acid decomposition in sub- and super-critical water from near the critical pressure to extremely high pressures (20–100 MPa) at temperatures of 380 and 400 °C were investigated. Near the critical pressure, the dehydration and decarboxylation rates decreased with increasing pressure. The apparent activation volumes near the critical pressure were high where the isothermal compressibility of water was large, implying that formic acid attracts water molecules better than the transition state does, and that the partial molar volume of the transition state is larger than those of the reactants. At higher pressures, dehydration rate increased with increasing pressure. Comparisons with experimental results for HCl addition at constant pressure suggest that dehydration proceeds via an ionic reaction at high pressures. The pressure may affect formic acid decomposition in supercritical water negatively through solvation effects near the critical pressure, and positively through ionic reactions above 30 MPa.

Reaction kinetics of methanol oxidation in supercritical water at high pressure condition (420 °C; 34–100 MPa; ρ = 300–660 kg/m 3 ) was investigated. Pseudo-first order rate constant for methanol decomposition increased with increasing water density. Effects of supercritical water on the reaction kinetics were investigated using a detailed chemical kinetics model. Incorporating the effect of diffusion in a reduced model revealed that overall kinetics for SCWO of methanol is not diffusion-limited. Roles of water as a reactant were also investigated. The dependence of sensitivity coefficient for methanol concentration and rate of production of OH radical on water density indicated that a reaction, HO 2 + H 2 O = OH + H 2 O 2 , enhanced the OH radical production and thereby facilitated the decomposition of methanol. It is presumed that concentration of key radicals could be controlled by varying pressure intensively.

Reaction kinetics of methanol and ethanol oxidation in supercritical water at 520–530 °C and 24.7 MPa were investigated both experimentally and by computational simulation. Furthermore, studies were performed on the oxidation of the two alcohols in binary mixtures. For the methanol system, experimental data showed that the methanol conversion decreased with increasing initial methanol concentration in the low concentration range (from 6.48 × 10−6 to 3.94 × 10−5 mol/l), whereas the conversion increased for initial concentrations in the high concentration range (from 2.23 × 10−4 to 1.55 × 10−3 mol/l). Kinetic analyses based on the elementary reaction model showed that production of OH from the reaction of H2O with HO2 seemed to play an important role at the low methanol concentrations and that the characteristic dependence of methanol conversion on initial methanol concentration was due to the very high concentration of H2O in supercritical water oxidation of methanol. For the binary system, it was found that methanol conversion was accelerated by ethanol addition whereas ethanol oxidation was slightly retarded by the presence of methanol. Calculation with an elementary reaction model could reproduce the phenomenological mutual effects of alcohols with respect to reaction rates, and it was found that the acceleration/retardation effect of conversions could be well characterized by the time profile of OH radical, rather than HO2 radical.