Selectivity to cyclohexene in the gas-phase hydrogenation of benzene over ruthenium

A study has been made of the heterogeneous gas-phase hydrogenation of benzene to cyclohexene and cyclohexane in a continuous-flow apparatus over non-supported Ru, at 0.11 MPa total pressure, in the range of temperatures from 273 to 320 K. When working with highly purified reactants, no formation of cyclohexene is detected. On adding water vapour to the feed a selectivity to cyclohexene is gradually built up over a period of more than 10 h. If Ru is pre-covered with adsorbed water, the selectivity to cyclohexene is effected immediately, and on drying the feed to zero partial water-vapour pressure, the selectivity towards cyclohexene falls away rapidly.At 318 K the maximum cyclohexene selectivity arrived at is 42 %, at 20 % conversion. Reduction of spent Ru catalysts with hydrogen at 473 K leads to total recovery of activity and selectivity. Alloying of Ru with 1 at. % Au improves the catalyst stability but suppresses the cyclohexene selectivity.The preparation of Ru powders with a high surface purity is described. Characterization of the various samples is performed by hydrogen chemisorption, nitrogen physisorption and X.p.s./A.e.s. surface analysis. From adsorption isotherms between 290 and 340 K the enthalpy of adsorption of water on Ru, in the presence and in the absence of chemisorbed hydrogen, is calculated, and the result is compared with the enthalpy of H2O adsorption on Pt.A tentative explanation is presented of the beneficial influence of adsorbed water on the selectivity towards cyclohexene formation.