Effect of aluminum alkyls on a homogeneous and silica-supported phenoxy-imine titanium catalyst for ethylene trimerization

A phenoxy-imine titanium catalyst (FI-catalyst) for selective ethylene trimerization was immobilized on methyl aluminoxane (MAO) pretreated silica and its activity and selectivity was compared with that of the corresponding homogeneous catalyst system. The homogeneous and heterogeneous ethylene oligomerization was conducted in the presence of different aluminum alkyls, commonly used as scavengers during olefin polymerization to remove residual oxygen and moisture from the reaction medium. Both the homogeneous and heterogeneous catalysts were strongly affected by the presence of scavenger in the reaction medium. Upon activation with R3Al/MAO (R= Et, nOct, iBu), the homogeneous catalyst switches selectivity from ethylene trimerization to polymerization. NMR spectroscopic investigations indicate that this change of selectivity can be attributed to ligand exchange between the precatalyst and the aluminum alkyl and reduction of the titanium species. The thereby formed ligand-free and/or reduced titanium alkyls...

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