Ethylene Polymerization with Supported Vanadium-Magnesium Catalyst: Hydrogen Effect

Abstract

Ethylene polymerization on supported vanadium-magnesium catalysts with the composition VCl4/MgCl2 and VOCl3/MgCl2 was studied in the presence of hydrogen as the chain transfer agent. Vanadium-magnesium catalysts (VMC) are known to exhibit a high reactivity towards hydrogen. This reactivity is manifested as a sharp decrease in polyethylene (PE) molecular weight accompanied by a dramatic (2–6-fold) loss of catalyst activity, when hydrogen is introduced. Catalyst deactivation in the presence of hydrogen is reversible. Catalyst activity is restored, when hydrogen is removed from the reaction medium. With this regard we have studied the effect of the concentration of hydrogen and of Al(i-Bu)3 and Al(i-Bu)2H, used as co-catalysts, on the activity of VMC. We have also identified the reaction products formed after polymerization reaction in the presence of hydrogen. Based on experimental data we suggest reaction the scheme leading to catalyst deactivation in the presence of hydrogen and its reactivation, when hydrogen is removed from the reaction medium. According to this scheme, catalyst deactivation is caused by dialkylaluminumhydride forming in the side reaction of AlR3 co-catalyst with the V-H bonds and then being adsorbed onto the catalyst active sites. It is shown that the deactivation process may be diminished, if some modifiers able to bind with alkylaluminumhydride are introduced into the catalyst composition. Magnesium dichloride introduced into the catalytic system or aluminum trichloride introduced into the catalyst composition may be used for modification. Using this approach we have obtained highly active supported VMC with an activity up to 50 kg/gV·h·bar C2H4 (80°C) at polymerization in the presence of hydrogen.