Selective hydrogenation of 1,5,9-cyclo-dodecatriene towards cyclo-dodecene Original Research Article

The liquid phase hydrogenation of 1,5,9-c,t,t-cyclo-dodecatriene (CDT) and of cyclododecadienes (CDD) towards cyclo-dodecene (CDE) using a Pd/Al2O3 catalyst of egg-shell type has been studied in discontinuous operation at different hydrogen pressures and for stepwise reduced hydrogen pressure. The yield of CDE is considerably increased if the hydrogen concentration and consequently the rate of hydrogenation are markedly reduced. By this means a CDE yield of 93% at a (CDT + CDD) conversion of 98% could be reached. Because of the low hydrogenation rate the conversion is not influenced by mass transfer effects.For discussion the partial hydrogenation of CDT is compared with the hydrogenation of cyclo-octadiene for which a cyclo-octene yield of >99% is attainable at a much higher reaction rate. In order to understand the partial hydrogenations of cyclo-octadiene and of CDD ab initio Hartree–Fock calculations have been carried out to study the structure of these molecules. On this basis a hypothesis for the lower selectivity of CDT-hydrogenation towards CDE compared to that of cyclo-octadiene towards cyclo-octene is presented. The increase of CDE selectivity with decreasing hydrogen pressure is traced back to the increased rate of the isomerization of t,t- and c,t-CDDs towards the more strongly adsorbed c,c-CDDs.