Effect of Mo-modification over Phillips CrOx/SiO2 catalyst for ethylene polymerization

Abstract

Phillips CrOx/SiO2 catalyst is an important catalyst for industrial production of polyethylene. In this work, a Mo-modified Phillips catalyst for ethylene polymerization (defined as CrMo/Si) was developed. The characterizations of the Phillips catalyst (defined as Cr/Si) and the CrMo/Si catalyst by temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) showed an obvious interaction between the chromium and molybdenum components on the silica support. The ethylene polymerization performances at different conditions, in terms of H2 responds, polymerization temperatures, and the copolymerization with 1-hexene were systematically investigated. Although the polymerization activity of the CrMo/Si catalyst was lower, the copolymer presented a higher molecular weight (MW) and a broader molecular weight distribution (MWD). The Mo-modified Phillips catalyst (CrMo/Si) exhibited 67% higher 1-hexene incorporation ability as determined by 13C NMR. Moreover, the short chain branch distribution (SCBD) of the copolymer obtained via the CrMo/Si catalyst was also improved as confirmed by temperature rising elution fractionation (TREF) cross successive self-nucleation and annealing (SSA) methods. The results of TREF and SSA showed that the amount of short chain branches for the copolymer was increased in the high MW part, which would benefit for the long term mechanical property of polyethylene.