Studies on partial hydrogenation of 1,5,9-cyclo-dodecatriene towards cyclo-dodecene

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>pH2>2.5 MPa0.01>pH2>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 pH2pH2 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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► Cyclododecene yields >90% are obtained at very low hydrogenation rates of cyclododecatriene.
► Dense coverage by CDT/CDD prevents readsorption of CDE.
► The preference for CDT/CDD is due to adsorption via two double bonds.
► A fully consistent hypothesis for the reaction course of this complicated hydrogenation is given.