Ethylene polymerization over novel organic magnesium based V/Ti bimetallic Ziegler-Natta Catalysts

Abstract

A series of novel V/Ti bimetallic polyethylene catalysts were developed successfully through the introduction of vanadium species into the (SiO2/MgR2/MgCl2)·TiClx Ziegler-Natta Catalyst. The organometallic heterogeneous catalysts combined advantages of reducibility of magnesium alkyl on the activation of secondary metal active sites (vanadium active sites), which successfully enhanced average molecular weight in homopolymer products and produced dominantly bimodal molecular weight distribution in copolymer products. Catalyst preparation conditions including calcination temperature and magnesium dosage were investigated to explicit the effect on the relative performance of different metal active centers. Moreover, the factors including hydrogen and ethylene/1-hexene copolymerization were systematically studied. The bimetallic catalysts exhibit 63.9% enhancement on activity, 37.3% better hydrogen response than the original (SiO2/MgR2/MgCl2)·TiClx organic magnesium Ziegler-Natta catalyst. Moreover, a simplified schematic model was proposed to elaborate on the reason why the bimetallic catalysts showed much higher activity than the original one, and each active species performance well. Vanadium active sites react quickly in the initial reaction time on the external surface of the support, which helps titanium active sites crack the catalyst particles together in the fragmentation and activation stage; then the released internal surface active sites, which were accessible to the monomer in the slurry phase, speed up the polymerization reaction during the latter reaction process.