Behavior of Cholesterol and Catalysts in Supercritical Water

Cholesterol reacted over 5 wt % Pt/C, 5 wt % Pd/C, and HZSM-5 in supercritical water at 400 °C. The major products with the Pt/C and Pd/C catalysts are cholesterol-derived steroids, polynuclear aromatic compounds, aliphatic hydrocarbons, and gases (H₂, CH₄, and C₂H₆), resulting from hydrogenation, dehydrogenation, and cracking reactions. HZSM-5 favored initial dehydration of cholesterol to form cholesta-3,5-diene and then catalyzed further isomerization and cracking reactions. The presence of H₂ had little or no effect, except when the catalyst was absent, in which case added H₂ led to higher cholestadiene yields and lower cholesterol conversion. Increasing the catalyst loading resulted in higher yields of lower molecular weight products but also significantly lower carbon recoveries. Characterization of the Pt/C and Pd/C catalysts showed carbon support gasification and particle growth after exposure to supercritical water.