Ti-Modified Silica Supported Cr-Mo Hybrid Phillips Catalyst for Ethylene Polymerization

Phillips catalyst (CrO_x/SiO₂) is an important industrial high density polyethylene catalyst. In this work, a Ti-modified Cr-Mo hybrid Phillips catalyst (CrMo/TiSi) was prepared and tested to improve the ethylene polymerization performance. CrMo/TiSi and CrMo/Si catalysts were compared and studied by the temperature-program reduction, X-ray photoelectron spectrometer and density function theory. Due to the Ti-modification, the reduction peak shifted to the lower temperature and the binding energy of the Cr species increased, which reflected that the Cr species were easier to be reduced. The ethylene polymerization performances over the CrMo/TiSi and CrMo/Si catalysts at different conditions were investigated. The Ti-modification increased the activity and the catalyst produced the polymer with higher molecular weight (M_W). Determined by high temperature ¹³C-NMR, the copolymer by the Ti-modified catalyst exhibited higher 1-hexene incorporation. However, the amount of short chain branches for the copolymer decreased in the high M_W part, according to the temperature rising elution fractionation cross successive self-nucleation and annealing (TREF+SSA) results.