Following solid-acid-catalyzed reactions by MAS NMR spectroscopy in liquid phase - Zeolite-catalyzed conversion of cyclohexanol in water

A microautoclave magic angle spinning NMR rotor is developed enabling insitu monitoring of solid-liquid-gas reactions at high temperatures and pressures. It is used in a kinetic and mechanistic study of the reactions of cyclohexanol on zeolite HBEA in 130 °C water. The ¹³Cspectra show that dehydration of 1-¹³C-cyclohexanol occurs with significant migration of the hydroxy group in cyclohexanol and the double bond in cyclohexene with respect to the ¹³C label. A simplified kinetic model shows the E1-type elimination fully accounts for the initial rates of 1- ¹³C-cyclohexanol disappearance and the appearance of the differently labeled products, thus suggesting that the cyclohexyl cation undergoes a 1,2-hydride shift competitive with rehydration and deprotonation. Concurrent with the dehydration, trace amounts of dicyclohexyl ether are observed, and in approaching equilibrium, a secondary product, cyclohexyl-1-cyclohexene is formed. Compared to phosphoric acid, HBEA is shown to be a more active catalyst exhibiting a dehydration rate that is 100-fold faster per proton. Hot analysis: The catalytic conversion of cyclohexanol on zeolite HBEA in hot liquid water leads to dehydration as well as alkylation products. A novel microautoclave suitable for application in magic angle spinning (MAS) NMR at high temperatures has been successfully applied to obtain new insight into the mechanistic pathway leading to an understanding of the reactions under selected experimental conditions.