An NMR study of the species produced by the reactions of dialkyl magnesium and HCl with calcined silica and the polymerisation performance of Ziegler–Natta catalysts produced from this support material

The effect of calcinations on the silica surface groups and thereby on the activity of Ziegler–Natta catalysts in ethylene homopolymerisation has been studied. Silica was calcined at different temperatures and treated with MgR2 and HCl. Silica surface groups were identified by using 1H MAS NMR and 13C and 29Si CP MAS NMR techniques. Magnesium, titanium and chlorine were measured by elemental analysis. Ziegler–Natta catalysts were prepared from these supports and subsequently used in ethylene homopolymerisation. Maximum activity was obtained with the catalyst based on 590 °C calcined silica. The results indicate that MgR2 reacts with siloxane-groups (Si–O–Si) in the 300 °C calcined silica, leaving the hydrogen-bonded hydroxyl-groups unreacted. Low activity Si–O–Ti(Cl)2–O–Si species are formed after reacting with TiCl4. The higher activity in the catalyst based on 590 °C calcined silica can be explained by the formation of –Si(R)–O–Si–O–TiCl3 groups, originating from the siloxane bridges which cannot form in 300 °C calcined silica. Other explanations for the higher activity are a higher Mg/Ti ratio or small amounts of crystal water formed in the 590 °C calcined silica.