Subcritical and supercritical water have attracted attention as tunable reaction media for organic reactions because their properties can be adjusted by controlling temperature and pressure. It is expected that the appropriate solvent properties will be used for each step of a multi-step reaction process by changing only the temperature and pressure. In this study, α -pinene was converted to p -cymene using a two-stage flow reactor where subcritical and supercritical water were used as solvents. The synthesis involved acid-catalyzed isomerization and oxidative dehydrogenation. For determining appropriate reaction conditions for acid-catalyzed isomerization with WO X /TiO 2 as a solid catalyst (first stage) and oxidative dehydrogenation (second stage), the temperature, pressure, initial concentration of oxidant (second stage), and starting compound (second stage) for each stage were investigated. The results confirm that the maximum yield of p -cymene exceeded 0.4 when the first stage temperature was set to 250 °C or 300 °C; the second stage temperature was set to 400 °C, and the pressure for both stages was set to 30 MPa. This yield value was approximately twice as large compared to the yield values of p -cymene synthesized in a single-stage flow reactor in supercritical water at 400 °C and 30 MPa. The first stage involved ion-mediated reactions, while the second stage involved radical-mediated reactions. Since the reactivity of these reactions differs between subcritical and supercritical water, and the reactivity can be adjusted by pressure, combining subcritical and supercritical conditions in two-stage reactors increased the target product yield. ● Subcritical water offers high α -terpinene yield in α -pinene reaction with WO X /TiO 2 ● Supercritical water is preferred for oxidative dehydrogenation to p -cymene ● More p -cymene is produced from α -terpinene than limonene or α -pinene ● Two-stage reactions provide twice p -cymene yield than single-stage reaction
OKITA et al. (2026) studied this question.