Effect of lewis acid on the activity of ziegler-natta catalyst and the properties of polyethylene

Abstract

This dissertation investigated the effects of Lewis acid compounds on ethylene polymerization with MgCl2-supported Ziegler-Natta catalysts. The study could be divided into three parts. Firstly, the modification of Mg(OEt)2-based Ziegler-Natta catalysts with various metal chlorides were examined on ethylene homopolymerization and ethylene copolymerization with 1-hexene. One of selected metal chlorides (FeCl2, MnCl2, and ZnCl2), was added together with SiCl4 in the chlorination stage of Mg(OEt)2. The result showed that among various metal chlorides, the addition of FeCl2 could enhance both BET specific surface area and the surface atomic ratio of Ti/Mg, leading to the highest activity in both ethylene homo- and copolymerization. The exposure of Ti species on outer surfaces could be thought as a sign of easier mass transfer phenomena leading to the improvement of catalyst performances. Secondly, two methods of catalyst preparations modified with FeCl2 including TiCl4/MgCl2 complexes in tetrahydrofuran (THF) soluble and recrystallization method by ethanol (EtOH), were examined. It was found that FeCl2 modification with both two methods could improve the activity. However, Fe-THF catalyst exhibited higher activity than Fe-EtOH catalyst because of a suitable of interaction between TiCl4 and MgCl2 and the proper location of Ti atoms on the surface as proven by thermogravimetric analysis (TGA) and energy dispersive X-spectroscopy (EDX), respectively. Finally, the effect of the ZnCl2-SiCl4 doped TiCl4/MgCl2/THF catalyst for ethylene homopolymerization and ethylene/1-hexene or ethylene/1-octene copolymerization was also studied. It indicated that the ZnCl2-SiCl4 modified catalyst exhibited very high activities in ethylene homo-and copolymerization and high the viscosity average molecular weight (Mv) of polymer obtained.