Hydrogenation of cinnamaldehyde over Pt/RHCs in supercritical carbon dioxide – Influence of support pretreatment and phase behavior

The selective hydrogenation of cinnamaldehyde (CAL) was investigated using rice husk-based porous carbon (RHCs) supported platinum catalysts in supercritical carbon dioxide (scCO2). The effects of surface chemistry treatment of the support and the reaction phase behavior have been examined. The Pt/H–RHCs (HNO3-pretreated) was more active for CAL hydrogenation compared with Pt/NH3–RHCs (NH3 · H2O-pretreated). The Pt/RHCs catalyst exhibited a higher selectivity to cinnamyl alcohol (COL) compared with commercial catalyst of Pt/C, which is relative to the micro–mesoporosity structure of the RHCs. Under the reaction conditions, the product distribution changes markedly with the variation of the phase behavior. A higher selectivity to hydrocinnamyl alcohol of 90% was obtained in two-phase reaction system of CO2–riched gas phase (CO2, H2, dissolved cinnamaldehyde) and solid phase (catalyst); while, the main product changed to cinnamyl alcohol with a selectivity of 88% in three-phase reaction system of CO2–riched gas phase (CO2, H2, dissolved cinnamaldehyde), cinnamaldehyde–riched liquid phase (cinnamaldehyde, dissolved CO2 and H2) and solid phase (catalyst). It is attributed to the influence of the molecule interactions and the substrate concentration effects.