Selective Hydrogenation of Benzene to Cyclohexene in a Monolithic Reactor

An attractive field of industrial use of a monolithic reactor lies in multiphase catalytic reactions, especially hydrogenation and oxidation of hydrocarbons, due to its superior interphase mass transfer performance. In this paper, liquid-phase selective hydrogenation of benzene to cyclohexene over a Ru/ZrO2 honeycomb monolithic catalyst was studied in detail. The effects of catalyst and reaction variables, such as the support type, ruthenium content, catalyst pretreatment, addition of water or salt in reactants, reaction temperature, hydrogen pressure and volume ratio of water to benzene, on the performance of the reaction were investigated. Good yields of cyclohexene could be obtained using ruthenium particles supported on hydrophilic oxides, like silica, alumina, titania and zirconia. It should be noted that the reaction can occur without catalyst prereduction in hydrogen flow in the monolithic reactor. No cyclohexene was detected in the absence of reaction additives such as water or ZnSO4 solution, but the selectivity for cyclohexene dramatically increased to higher than 20 % when these reaction modifiers were added. There existed an optimum volume ratio of water to benzene for high yield of cyclohexene. Better results were achieved in the presence of 0.1 mol/L ZnSO4 under the reaction temperatures of 413 similar to 443 K and pressures of 3 - 4 MPa.