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The selective hydrogenation of 1,5,9-cis,trans,trans-cyclo-dodecatriene (1,5,9-ctt-CDT) towards cyclo-dodecene (CDE) depends strongly on the pressure of hydrogen, respectively the hydrogenation rate. High yields of CDE (>90%) can only be reached at extremely low hydrogen pressure. In order to elucidate this exceptional reaction performance the course of reaction has been studied for a wide range of hydrogen pressure, 0.01 > p H 2 > 2.5 MPa , taking into consideration data of other research groups. The CDT hydrogenations were discontinuously carried out in liquid phase on Pd/Al2O3 at T = 353 K. The resulting hypothesis of this study is that the very low reaction rate at low p H 2 is necessary in order to realize a dense surface coverage of 1,5,9-CDT and 1,5-cyclo-dodecadiene (CDD) isomers where these molecules show adsorption on Pd via two double bonds so that readsorption of formed CDE and subsequent hydrogenation to cyclo-dodecane (CDA) is hardly possible. On the whole this new hypothesis on the reaction course of CDT hydrogenation gives a sound and fully consistent view on this rather complicated reaction. |
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