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.