Pre-reduction Effects Of Alkyl Aluminum On SiO₂ Supported Silyl-Chromate（Cr）/Imido-Vanadium（V） Bimetallic Catalysts For Producing Bimodal Polyethylene

Polyethylene is the most commonly used polymer in the world,and a significant portion of it(high density polyethylene in particular)is produced by chromium-based catalyst.SiO2-supported Phillips catalyst and S-2 catalyst are mainly used for the production of high density polyethylene.Our group had developed a novel Cr-V bimetallic catalyst for producing bimodal polyethylene on the basis of SiO2-supported S-2 catalyst.Bimodal polyethylene can be used as high-performance pipe materials,which means that Cr-V bimetallic catalyst has a promising market prospect.What’s more,the preparation of this catalyst is quite simple.In this work,the pre-reduction effects of alkyl aluminum on this catalyst was further studied:(1)different amount of triisobutyaluminum(TIBA)was used to pre-reduce the catalyst;(2)Cr-V bimetallic catalyst was pre-reduced by TIBA,triethylaluminum(TEA)and diethylaluminum(DEAC)respectively;(3)TIBA was used to pre-reduce the catalyst supported on different type of silica.Ethylene polymerization performances over these catalysts were studied.The results indicated that the polymerization activity was obviously improved after the catalyst was pre-reduced by TIBA,and the needed dosage of alkyl aluminum also decreased significantly.The alkyl aluminum of the pre-reduced catalyst can also act as a chain transfer agent,significantly reducing the molecular weight of the polymer.The pre-reduced catalyst showed a better copolymerization performances and hydrogen response.And the catalyst pre-reduced by TEA had the higher comonomer insertion rate.The pre-reduction effect of TIBA and TEA was similar,but DEAC was quite different with them.The activity of the catalyst pre-reduced by DEAC was rather low comparing with others,but the molecular weight of the polymers obatained from it was much higher than other polymer coming from catalysts pre-reduced by TIBA or TEA.With the increasing amount of DEAC,the molecular weight of polyethylene decreased.DEAC was mostly deactivated the Cr species,and the polymerization reaction was dominated by V sites.The silica gel can affect the polymerization activity of the catalyst and the properties of the polymer.The pre-reduction step was more effective to improve the activity of the catalyst supported on 955 silica than the catalyst supported on PQ silica.