The effect of SiO2 calcination temperature and [Si]/[Mg] molar ratio on the performance of bi-supported Ziegler-Natta catalysts in ethylene polymerizations

In this study, a group of bi-supported Ziegler-Natta catalysts (SiO2@MgCl2/THF/TiCl4/ di-n-butyl phthalate) with various SiO2 calcination temperatures and [Si]/[Mg] molar ratios were synthesized. In this line, SiO2 was calcined in a variety of temperatures (200, 400 and 600 °C) under nitrogen atmosphere to generate different concentrations of hydroxyl groups on its surface, and then used together with MgCl2, THF and DBP to furnish desired catalysts which subsequently employed in ethylene polymerizations. The structural studies on the synthesized catalysts and polymers were conducted using GPC, DSC, SEM, BET techniques, and bulk density measurements. It was found that higher calcination temperatures effectively reduce Mw and narrows MWD. The highest catalyst activity and bulk density was achieved by the catalyst synthesized from the silica particles calcined at 600 °C. The effect of the molar ratio of [Si]/[Mg] was also investigated. Notably, Mw and MWD of polyethylenes were controlled efficiently by optimizing that parameter. The surface area and the catalyst activity showed similar trends, and they were improved by increasing the molar ratio of [Si]/[Mg] from 1 to 2. Interestingly, polymer fibrils have been formed between polyethylene particles when polymerization was catalyzed via synthesized catalyst with [Si]/[Mg] = 2. It is in favor of lower fine production in the industrial plants.