Catalytic dehydrogenation of cyclooctane with titanium, zirconium and hafnium metallocene complexes

Metallocene complexes in combination with cocatalysts like methylalumoxane (MAO) are not only excellent catalysts for olefin polymerization but also appropriate catalysts for the activation of alkanes in homogeneous (autoclave) and heterogeneous (fixed bed reactor) reactions. The activities of the catalysts depend on the temperature, the cocatalysts, additives, the central metal and the ligand structure. Generally, complexes with low steric demands and MAO as cocatalyst gave the highest activities. The comparison of different π-ligands resulted in the following activity order: cyclopentadienyl > indenyl > fluorenyl. The influence of σ-ligands and n-donor ligands gave the following activity order: –Cl > –PMe3 > –CH2Ph > –(CH2)4CH3 > –NPh3. The activities depended on the nature of the cocatalyst and decreased in the following order: MAO ≫ AlMe3 > AlEt3. The addition of aluminum powder and the Lewis base NPh3 increased the activity of the Cp2ZrCl2/MAO catalyst. The Cp2ZrCl2/MAO/NPh3 catalyst showed the highest activity in homogeneous reactions with 458 turnovers in 16 h at 300 °C. The Cp2ZrCl2/MAO/NPh3/SI1102 catalyst gave the highest activity in heterogeneous catalysis with 206 turnovers in 5 h at 350 °C. None of the catalysts required a hydrogen acceptor like an external olefin.