Preparation And Modification Of Novel Si/Mg Supported Ziegler-Natta Polyethylene Catalysts

As one of the five general resins,polyethylene?PE?is widely used in our daily life,industrial and agricultural production etc.Nowadays,the domestic polyethylene industry is still in short supply.On one hand,the plant capacity of the domestic polyethylene industry is unable to meet the actual demand.On the other hand,with the rapid development of the national economy,the demand for the high-performance,value-added and functional polyethylene is high,which still relies manly on imports.As the core of the polyethylene industry,the catalyst technology has been a significant factor for enhancing the plant capacity of polyethylene and improving the macro and micro structures of the polyethylene materials.Therefore,the development of novel high-efficiency and high-performance polyethylene catalysts and their modification research are imminent.Based on the vanadium-modified?SiO₂/MgO/MgCl₂?TiCl2 Ziegler-Natta catalyst system developed in our lab,following achievements were made:?1?The imido ligand?organic isocyanate?was used to modify the catalyst to obtain novel imido-modified SiO₂-supported Ti/Mg ZN catalysts,and the effects of the catalyst components,Al/Ti molar ratio,comonomer and hydrogen on the catalyst performances and polymer properties were investigated.Meanwhile the influences of the imido ligand on the catalyst system were also studied;?2?Various vanadium-modified ZN catalysts were prepared by changing the pH value?pH = 5,7 and 9?of the co-impregnation solution,and UV-Vis,ICP,IC and NH₃-TPD etc.methods were used to characterize the catalysts.The influences of Al/Ti molar ratio,comonomer and hydrogen on the catalyst performances and polymer properties were investigated;?3?A series of the third metal element modified?SiO₂/MgO/MgCl₂?·TiCl_x ZN catalysts were prepared using different metal salts?including Zr,Al,V,Cr,Fe,Ni,Mo and W salts?under the optimum pH value.The correlation between the third metal element and catalyst performances as well as the effects of Al/Ti molar ratio,comonomer and hydrogen on the catalyst performances and polymer properties were investigated;?4?Novel SiO₂/MgCl₂ bi-supported titanium/vanadium hybrid ZN catalysts were prepared by the refluxing method using VOCl₃ as vanadium sources.This work focused on the effects of the reaction sequences with TiCl₄ and VOCl₃?including the one-step co-reaction method and the two-step reaction method?on the catalyst system.For the novel imido-modified SiO₂-supported Ti/Mg ZN catalyst systems,the imido ligand can not benefit the catalyst activity and the 1-hexene incorporarion,but it is advantageous for the stability of the catalyst activity.The results of hydrogen response experiments show that the novel catalyst systems exhibit a good hydrogen response without affecting the ethylene polymerization activity,showing a potential for industrial application compared with the traditional ZN catalysts.The pH value of the impregnation solution has a great influence on the catalyst performances.An appropriate pH value?pH = 7?can maximize the catalytic activity?59.7 g PE/g Cat·h-1?and the hydrogen response,and the optimal Al/Ti molar ratio is only 2.5,which is lower than those of the other two catalysts?pH = 5 and pH = 9?and the industrial value,significantly reducing the cost.With the decrease of the pH value,the 1-hexene incorporation shows an increasing tendency from 4.48?mol%??pH = 9?to 8.43?mol%??pH =5?.Modification with the third metal elements is an effective method for the?SiO₂/MgO/MgCl?·TiCl_x ZN catalyst system to modulate the catalyst performances:Compared with the unmodified catalyst,the ethylene homo-polymerization activity and the 1-hexene incorporation are enhanced by introduction of zirconium,vanadium,aluminum and chromium,while deteriorated by ferrum,nickel,molybdenum and tungsten;The correlations of the catalyst activity and 1-hexene incorporation ability with the electronegativity of the third metal elements are discovered.It's found that the lower the electronegativity of the third metal elements leads to the higher activity and the higher a-olefin co-polymerization ability of the catalyst;Compared with the unmodified catalyst,the third metal element modified catalysts show the hyrogen response results:obvious advantage?V,Cr?>slight advantage?Fe?>no difference?Zr,Al,Ni and W?>disadvantage?Mo?;The effects of ferrum and nickel on the polymers properties are more pronounced.The polyethylene obtained with Fe-modified catalyst has the highest molecular weight,reaching the ultra-high molecular weight region.Ni-modified catlayst produces the lowest molecular weight and the widest molecular weight distribution polyethylene.For the novel MgCl₂/SiO₂ bi-supported titanium/vanadium hybrid ZN catalysts,the one-step co-reaction method with TiCl₄ and VOCl₃?the MgTiV/Si catalyst?is superior to the two-step reaction method?the MgTi-V/Si and MgV-Ti/Si catalysts?in both the catalytic activity and the 1-hexene incorporation.And the titanium/vanadium hybrid catalysts through two-step reaction method are inclined to exhibit more traits of the second active species supported during the catalyst preparation.