Selective hydrogenation of benzene to cyclohexene using a Ru catalyst suspended in an aqueous solution in a mechanically agitated tetraphase reactor: a study of the influence of the catalyst preparation on the hydrogenation kinetics of benzene and of cyclohexene

The reactivity of Ru based catalysts in the benzene selective hydrogenation to cyclohexene has been studied. The reaction has been carried out in a tetraphase slurry reactor at 423 K, at 5 MPa of pressure, in the presence of two liquid phases: benzene and an aqueous solution of 0.6 mol l(-1) ZnSO4. A detailed kinetic measurement has been carried out in order to study the influence of the preparation procedure and of the treatments on the catalyst activity and selectivity. Moreover, a comparison of benzene and cyclohexene hydrogenation kinetics is presented in order to evaluate the influence of the treatments carried out on the unreduced catalyst on the activity, selectivity and yield to cyclohexene. The kinetics of benzene hydrogenation indicates that the catalyst activity is influenced by the time, during which it remains in the mixture of precipitation for its preparation. The longer the time, the lower the activity is. Selectivity and yield remain practically unaffected after 18 h of permanence in the mixture of precipitation. Treatment of the unreduced catalyst with water depresses both activity and selectivity, while treatment with 1 mol l(-1) NaOH gives the best results. On the contrary, in the hydrogenation of cyclohexene, the NaOH treated catalyst is three times less active than the catalyst treated with water. These results are in agreement with the higher selectivity achieved using NaOH treated catalysts. (C) 2001 Elsevier Science B.V. All rights reserved.