Silica-Supported Chromocene/ImidoVanadium Bi-Metallic Catalysts for Bimodal Polyethylene
Producing polyethylene (PE) with broad or bimodal molecular weight distribution (MWD) in one reactor using bimetallic catalysis is a cost- and energy-effective technique in the polyolefin industry. However, this technique is still faced with great challenges, including the difficulty in molecular weight (MW) regulation and poisonous effect commonly observed between the two metals resulting from strong interactions. In this work, a series of bimetallic catalysts CpCr/imidoV/silica with different Cr/V molar ratios were fabricated by anchoring Cp 2 Cr onto imidoV/silica, and the two mono-metallic catalysts CpCr/silica and imidoV/silica were also prepared for comparison. The results show that not only the activity but also the responses to alkyl-Al cocatalyst, polymerization temperature, and hydrogen were distinct between CpCr/silica and imidoV/silica. The interactions between Cr and V sites seemed to be weak. As a result, their different catalytic behaviors were generally reserved, thus making it possible to regulate the activity and the MW of PE products by adjusting Cr/V molar ratio. Thanks to the distinct hydrogen responses of Cr and V on bimetallic catalysts, bi- or broad-modal PE was successfully synthesized by hydrogen regulation. The relative content of low and high MW fractions could be adjusted by altering both the Cr/V molar ratio and hydrogen partial pressure.
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