Solid Acid/Base Catalysis in Sub- and Supercritical Water

Solid acid/base catalysis in sub- and supercritical water (Sub&SCW) is a promising method to control organic reactions such as biomass conversions to chemicals and fuels, organic synthetic reactions, and degradation reactions. Synergistically using the unique characteristics of Sub&SCW and the catalytic properties of solid acid/base compounds, high reaction rates and/or high product selectivity can be achieved. This Review provides an overview of the use of solid acid/base catalysts for Sub&SCW reactions. Recent progress in the elucidation of the acid/base catalytic properties of solid catalysts in Sub&SCW and the effect of water properties on the catalysis is introduced. In addition, the application of solid acid/base catalysts in several research fields and the stability of these catalysts during reactions is discussed

Kinetic Study on Ammonia Oxidation Acceleration by Multi-Injection of Methanol in Supercritical Water

The effectiveness of multistage methanol injection in supercritical water oxidation (SCWO) of an ammonia/methanol mixture as a method for utilizing the alcohol co-oxidation effect was studied at 530 °C and 25 MPa. A simple global reaction rate model was developed based on the results of kinetic experiments performed at various ammonia, methanol, and oxygen concentrations, yielding excellent agreement with experimental values. Ammonia conversion was predicted when the same molar flow rate of methanol was split between two points: a first injection at the entrance of a reactor and a second injection midway in the reaction stream. Two-stage methanol injection resulted in higher ammonia conversion than in the model calculation when methanol was injected once at the reactor entrance. Model calculations under the same experimental conditions showed that two-stage injection improves ammonia conversion, which aligns closely with predicted values. Moreover, our calculations suggest that ammonia conversion depends on injection timing and on methanol concentration of the flow after injection as well

Isomerization of α‑Pinene to Monocyclic Monoterpenes in Hot Compressed Water Using TiO₂ and WO_<i>x</i>/TiO₂ Catalysts

The isomerization of α-pinene in hot compressed water (HCW) at 250 °C and 7 MPa using TiO₂ and WO_<i>x</i>/TiO₂ catalysts was investigated. In HCW, α-pinene mainly converts into monocyclic monoterpenes rather than into the polycyclic monoterpenes that form in He gas or water vapor. A plausible explanation for this is that Lewis acid sites on the surfaces of the catalysts convert to Brönsted acid sites in HCW owing to the stabilization of H⁺. In HCW, α-pinene is converted into acyclic monoterpenes less with the catalysts than without. Furthermore, the selectivities of the TiO₂ and the WO_<i>x</i>/TiO₂ catalysts for monocyclic monoterpenes were quite different, which was attributed to the rapid formation of α-terpinene and γ-terpinene from α-pinene and isoterpinolene from terpinolene over the strongly acidic WO_<i>x</i>/TiO₂ catalyst. No significant coking of the TiO₂ and the WO_<i>x</i>/TiO₂ catalysts occurred in HCW, and both retained their activity for 6 h

Catalytic Effect of the SUS316 Reactor Surface on the Hydrolysis of Benzamide in Sub- and Supercritical Water

The catalytic effect of an SUS316 reactor surface on the hydrolysis of benzamide in sub- and supercritical water was investigated at 350–450 °C and 25–65 MPa. Tube reactors having different surface area to volume ratios were used to distinguish the effect of the homogeneous reaction and the surface-catalyzed reaction. The surface of an SUS316 reactor acted as a heterogeneous catalyst for the hydrolysis reaction. Kinetic analysis indicated that surface Fe₂O₃ mainly contributes to the catalytic effect of SUS316. Both homogeneous and surface-catalyzed reactions occurred, and the contribution of each reaction depended on water density. The surface-catalyzed reaction was dominant at low water density, and the contribution from the homogeneous reaction became larger with increasing water density. Furthermore, at 400 °C and 25 MPa, hydrolysis products promoted the reaction

Kinetic Analysis of a Solid Base-Catalyzed Reaction in Sub- and Supercritical Water Using Aldol Condensation with Mg(OH)₂ as a Model

The kinetics of aldol condensation between acetone and benzaldehyde catalyzed by Mg­(OH)₂ in sub- and supercritical water at various temperatures (250–450 °C) and pressures (23–31 MPa) was investigated to elucidate the effects of water properties on solid base catalysis. The kinetics obey the Eley–Rideal (ER) mechanism, in which the reaction occurs between acetone enolate adsorbed on the catalyst and benzaldehyde in the bulk phase. Minimal benzaldehyde is adsorbed on the catalyst because of competitive adsorption with water; thus, benzaldehyde in the bulk phase mainly precipitates. The pressure dependence indicates that the reaction rate is affected by the competitive adsorption of water and acetone, as expected for the ER mechanism, and by changes in the solvent properties of supercritical water. The increase in the activation energy and the decrease in the amount of adsorbed acetone with increasing pressure are plausible effects of the solvent properties