Influence of Phase Behavior on the Selective Hydrogenation of Cinnamaldehyde with Ruthenium Complexes in Supercritical Carbon Dioxide

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The phase behavior has significant effects on the activity and selectivity of Ru complexes for cinnamaldehdye (CAL) hydrogenation in supercritical carbon dioxide (scCO₂). A single phase formed in scCO₂ is more effective for selective hydrogenation of C=C bond, while the presence of two phases (CO₂ gas and CAL liquid phases) benefits the hydrogenation of C=O bond. In this two-phase hydrogenation, the reaction rate increases with increasing CO₂ pressure and the selectivity to cinnamyl alcohol (COL) depends on both CO₂ pressure and CAL concentration. Visual observations show that the CAL liquid phase expands at high CO₂ pressures due to dissolution of CO₂ and H₂, and this contributes to the enhancement of the reaction rate and the COL selectivity.