Toward the Optimization of a Cr-V Bimetallic Catalyst for Producing Bimodal Polyethylene: Effect of Vanadium Content and Calcination Temperature

This work covers the synthesis of bimodal polyethylene with a SiO2-supported vanadium-oxide/silyl-chromate (Cr-V) bimetallic catalyst, which is prepared by using the residual hydroxyl groups on the VOx/SiO2 catalyst precursor to support chromium species. The effects of vanadium content and calcination temperature of the support on catalyst activity and polymer properties are investigated in detail. It is found that increasing vanadium content in the Cr-V catalyst leads to the decline of catalyst activity, while the molecular weight of the resulting polyethylene is raised. On the other hand, the decline of catalytic activity at high vanadium content in the Cr-V catalyst can be remedied by decreasing the calcination temperature of the support, and the molecular weight of the polymers is reduced at the same time. However, too low a calcination temperature is not beneficial for catalyst activity due to the poisoning effect of the excess hydroxyl groups on the active sites, and this would cause the increase of molecular weight as well.