Phase equilibrium and kinetics of O2-oxidation of limonene in high pressure carbon dioxide

The effect of phase equilibrium on the kinetics of O2-oxidation of limonene was studied. Vapour–liquid equilibrium measurements of the system (CO2 + O2 + limonene) were performed at 328 K and 8, 10, 10.7 and 11 MPa. In an attempt to establish a relation between vapour–liquid equilibrium data and the kinetics of limonene oxidation, different reactions were performed at 328 K and at different CO2 total pressures of 9.3 MPa, 10 MPa and 10.3 MPa (with the reaction mixture being biphasic) and at 12.2 MPa (in a single supercritical phase). The volumetric concentration of O2 in the liquid phase increased as CO2 was added to the system, although this behaviour was only observed close to the critical region. Kinetics of oxidation reactions performed at biphasic conditions, suggests that is the volumetric concentration of O2 in the liquid phase that is controlling the reaction rates.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► O2-oxidation of limonene can be performed using CO2 as a solvent at high pressure biphasic conditions. ► The CO2 role is simply to enhance the concentration of oxygen in the substrate-rich phase. ► There is a clear relationship between volumetric concentration of O2 and the kinetics of oxidation at biphasic conditions.