Kinetic study of heterogeneous catalytic hydrogenation of cyclohexene to cyclohexane in ionic liquid–alcohols mixtures

Heterogeneous catalytic hydrogenation of cyclohexene, catalyzed by Pt/Al2O3, was carried out in solutions of 2-hydroxyethylammonium formate (a room temperature ionic liquid, RTIL) mixed with methanol, ethanol and propan-2-ol at 25 °C. The rate constants of the reaction in ionic liquid alcohol mixtures were higher than alcohol alone. First-order rate constant of the reaction in the RTIL relative to propan-2-ol is approximately 28. Furthermore, the rate constant of the reaction increases with the mole fraction of the ionic liquid. Single-parameter correlations of log k   vs. normalized polarity parameter (ETN), hydrogen-bond acceptor basicity (β) and hydrogen-bond donor acidity (α) do not give acceptable results in the solutions. In addition, log k does not show an acceptable dual-parameter correlation with π* (dipolarity/polarizibility, one of the Kamlet–Taft parameters for solvent that shows the dipolarity of the solvent) and α, π* and β   and ETN and β. However single-parameter correlation of log k vs. π* gives reasonable results. The increase of the reaction rate with π* is attributed to the non-polar nature of the reactants.

Heterogeneous catalytic hydrogenation of cyclohexene by Pt/Al2O3, was carried out in solutions of 2-hydroxyethylammonium formate (a room temperature ionic liquid) mixed with methanol, ethanol and propan-2-ol. The rate constant of the reaction increases with the mole fraction of the ionic liquid. Increase of the reaction rate with π* (dipolarity/polarizibility of solvent) is attributed to the non-polar nature of the reactants.Figure optionsDownload as PowerPoint slide