Ti-and Al-modified Supported Cr-V Catalysts For Ethylene Polymerization

Phillips catalyst as an important catalyst for commercial high density polyethylene (HDPE) production has an important market with wide application of pipe material. Compared to PE pipe material with bimodal molecular weight distribution (MWD) from Ziegler-Natta catalyst by dual-reactor process. PE pipe material with unimodal MWD made from Phillips catalyst by single-reactor process have a lower cost and simpler production procedure, and its product shows features of high molecular weight (MW), broad molecular weight distribution and long chain branches. However, HDPE from Phillips catalyst have the demerit of unsatisfactory short chain braches distribution, which is unfavorable for further improvement of its properties as a high grade pipe material. In this work, new types of Ti-and Al-modified silica supported Cr-V polyethylene catalysts were developed in order to further improve the performance of HDPE as pipe material. After structure characterization, the modified catalysts were tested in a slurry polymerization process under different polymerization conditions to investigate the kinetics of ethylene homopolymerization and ethylene/α-olefin copolymerization. The properties of the HDPE polymer, such as melting point, crystallinity, molecular weight and molecular weight distribution were also investigated. In order to get a better understanding of Al effect on short chain branches (SCB) distribution, copolymer from Al-modified Cr/V catalysts were analyzed by means of temperature rising elution fractionation (TREF) combined with successive self-nucleation and annealing (SSA).The results showed that, Ti-modified Cr/V catalysts with small amount of V showed higher activities. V species as an active site tended to increase high molecular weight fraction of polymer. PE from Al-modified Cr/V catalysts had a bimodal MWD and MW significantly moving to higher MW compared to the Cr/V catalyst.1%Al-modified Cr/V catalyst exhibited10%higher polymerization activity than that of the Cr/V catalyst. With Al content increasing, homopolymerization and copolymerization activities of Al-modified Cr/V catalysts gradually decrease by using higher concentration of cocatalyst. MW gradually increased with Al content. TREF-SSA analysis showed that HDPE from Al-modified Cr/V catalyst had an increased SCB content in the high MW portion compared to the Cr/V catalyst, which might improve the long term performance of HDPE as pipe material.