Study Of The Catalytic Properties And Catalytic Mechanism Of Cr-based Ethylene Oligomerization Catalyst

1-Hexene and1-octene are in high demand due to their use as comonomers for line-low density polyethylene (LLDPE) production. Therefore, the catalysts have good selectivity for ethylene oligomerization toward desirable alkenes have gained special attention. In this thesis, we investigate the catalytic properties of PNP-Cr (PNP=Ph2PCH2CH2N(H)CH2CH2PPh2), SNS-Cr (SNS=EtSCH2CH2N(H)CH2CH2SEt) and PNP-Cr/SiO2catalyst. The reaction mechanism is studied as well.(1) PNP-Cr catalyst was synthesised and used for ethylene oligomerization. The affect of the reaction parametres such as reaction temperature, Al/Cr molar ratio, concentration of the catalyst and the operation sequence was investigated. It was found that, if the reaction temperature was increased from20℃to80℃, the acticity of the catalyst was first enchanced and then decreased. The best results was obtained at60℃. On the other hand, the acticity would be higher if more equivalent of the cocatalyst (MAO) was loaded. Increased the concentration of the catalyst would lead to a higher productivity, but the activity of the catalyst might decrease. The different operation sequence would lead to different results. PNP-Cr/MAO catalytic system showed much better reaction activity than PNP/CrCl3(THF)3MAO catalytic system. Usually, the activity of the PNP-Cr catalyst reached the peak in about5min, and then the activity would decrease.(2) SNS-Cr catalyst which had a similar structure with PNP-Cr was synthesised. The reaction behavior of the catalyst was investigated. Electron spin-resonance spectroscopy was used to study the active species generated during the reaction. The mechanism of selective ethylene trimerization was studied as well. The affect of the reaction temperature for SNS-Cr catalyst was similar with that observed with PNP-Cr catalyst. However, the influence of the Al/Cr molar ratio for PNP-Cr and SNS-Cr was different. Increased the Al/Cr molar ratio from100to700would lead the activity of SNS-Cr first increased and then decreased. The highest activity was observed with Al/Cr molar ratio400. The results of ESR detection showed that, the Cr1species generated may be the active species for ethylene oligomerization. During the reaction, the valence state of the Cr center would convert between monovalence state and trivalence state.(3) PNP-Cr catalyst was loaded to SiO2and the reaction behavior of this supported catalyst was studied then. The reaction results were compared with that generated with PNP-Cr/MAO catalytic system. At last, ionic liquid was used for ethylene biphase oligomerization. It was found that, the selectivity of the supported PNP-Cr catalyst was briefly unchanged while the activity was much lower than that obtained with PNP-Cr catalyst. During the reaction, the activity of the supported PNP-Cr catalyst was more stable than homogeneous phase reaction. The affect of the reaction temperature and Al/Cr molar ratio for PNP-Cr/SiO2catalyst was very similar with PNP-Cr. The study of ionic liquid/toluene biphase reaction system showed that,[BMIM]AlCl4,[BMIM]CH3SO3C1and [BMIM]AlEt3Cl (BMIM=1-butyl-3-methylimidazolium)may be not suitable for ethylene biphase oligomerization when PNP-Cr were used as catalyst.