Reaction pathways and kinetics of cholesterol in high-temperature water

• Cholesterol was reacted in high-temperature water at 300, 325, and 350 °C.
• Cholesta-3,5-diene was the sole primary reaction product.
• Rate constants and Arrhenius parameters for cholesterol dehydration were calculated.
• Products consisted nearly entirely of cholestadienes.
• A reaction network for major product pathways was proposed.

We investigated the reaction of cholesterol, a model compound for sterols in microalgae, in high-temperature water at 300, 325, and 350 °C. Products nearly entirely consisted of cholestadienes, with cholesta-2,4-diene, cholesta-3,5-diene, and cholesta-4,6-diene being the most abundant isomers. Cholesta-3,5-diene was the only primary product, formed via dehydration of cholesterol. Cholesta-2,4-diene and cholesta-4,6-diene likely formed from cholesta-3,5-diene by double-bond migration. We report conversion and product molar yields for each reaction condition. The initial rate of disappearance of cholesterol was first-order with an activation energy of Ea = 127 ± 12 kJ mol−1 and A = 108.35±2.41 s−1. We used a delplot analysis and mechanistic considerations to develop a reaction network for conditions relevant to hydrothermal liquefaction of microalgae.