Preparation And Ethylene Polymerization Of?NH₄?₂SO₄ Modified Cr/V Bemetallic Catalyst

Polyethylene is one of the world's largest resins for consumption,in which high-density polyethylene plays an important role.High-density polyethylene are mainly produced with Phillips chromium catalysts.Based on the Phillips catalyst,our group have introduced a vanadium component on a silica gel support and successfully developed an inorganic chromium/inorganic vanadium bimetallic center catalyst for in-situ synthesis of bimodal polyethylene products.Compared with other systems of bimetallic center catalysts,the catalyst preparation process is more simple and the cost is lower.Its products have high molecular weight,wide molecular weight distribution,and excellent product performance.With its excellent performance,the catalyst system has stepped into the footsteps of industrialization.Based on this catalystic system,this article further investigated the effect of sulfate on this catalyst system:(1)(NH4)2SO4 was used to modify the catalyst in order to explore the effect of(NH4)2SO4 on the performance of the catalyst.(2)Distribution impregnation and co-impregnation of(NH4)2SO4 on silica gel were respectively adopted to identify the influence of different impregnation on catalyst performance.(3)Different calcination temperatures(400?,500?)were chosen to investigate the effect of(NH4)2SO4;(4)To investigate the effect of different(NH4)2SO4 additions on the catalyst.Through rich characterization methods,the effects of all the above conditions on catalyst property and ethylene polymerization were investigated.The experimental results show that the introduction of(NH4)2SO4 can significantly increase the amount of acid on the surface of the support and greatly increase the homopolymerization activity of the original catalyst system.The highest increase is 36.8%.The molecular weight of the product exhibits a more pronounced bimodal distribution,greatly improved Product performance without increasing cocatalyst usage.With the increase of impregnation of(NH4)2SO4,the molecular weight of polyethylene increased significantly,and the polymerization activity of ethylene showed a trend of increasing first and then decreasing.The catalyst prepared by co-impregnation with only one calcination has much more residual SO42-content,higher homopolymerization activity and product molecular weight than that of the two-distribution impregnation method.Moreover,low temperature is more conducive for SO42-loading.Because of the addition of(NH4)2SO4 modifier,the molecular weight of ethylene/1-hexene copolymer didn't change,indicating that(NH4)2SO4 modified catalyst inhibited the copolymerization effect of ethylene/1-hexene.