| Linear a-olefins (LAOs) are a series of organic raw materials which are widely used in the petrochemical industry for the production of polyolefins, PAO synthetic oils and so on. They have played an important role in the development of national economy and national defense technology. Ethylene oligomerization is the most common method for the production of a-olefins. The resultant olefins by this process are highly linear and can be easily separated. Among various oligomerization catalysts, zirconium-based catalysts can trigger ethylene into well-distributed products under mild reaction conditions, and hence the requirements for industrial plants can be lower. As a consequence, such catalystsare expected to become a potential breakthrough for the development of a China owned novel oligomerization process. Besides, bis(imino)pyridine iron catalysts have been proved to be excellent catalysts with exceptionally high activity and product linearity. In addition, iron is cheap, environmentally friendly and the most abundant transition metal in earth. All of these make the iron catalysts to be a hot research topic. However, there are still some obstacles, e.g. broad product distribution, large amount of wax by-products, serious reactor fouling and large excess of methylaluminoxane (MAO), for such catalysts to commercialization. Since these issues are closely linked to the nature and micro-environment of the active centers, mediation of the micro-environment of the active centers is expected to tune the product distribution and reduce the wax content, thus speeding up the industrialization of the oligomerization process.From the perspective of regulating the micro-environment of the catalytic centers of ethylene oligomerization catalysts, this thesis explored the effects of phenolic compounds on the catalytic behavior of the zirconium and iron-based catalysts, studied their influence and regulation mechanisms on regulating the product distribution and inhibiting the solid wax formation. In addition, we investigated the effects of a new aluminoxane cocatalyst synthesized through the partial hydrolysis of phenol reagents and trimethylaluminum on the catalytic behavior. The main research work and results are as follows:(1) Phenolic reagents were introduced as additives to modify the ZrCl4/AlEt3/EtAlCl2 catalytic system. It was found that the phenol-modification could effectively tune the product distribution and increase the catalytic activity of such zirconium catalyst. The product distribution followed a Schulz-Flory distribution. In this section, the influence of Zr/Phenol molar ratio and the free phenol content on ethylene oligomerization was studied. Among these catalytic systems, Zr(OPh)3Cl·mPhOH/TEA/EtAlCl2 was proved to be highly active with the activity up to 6.8×104g/(molZr·h), while the product consisted of mainly α-olefins (C4-C24) without polymers. In addition, a suitable amount of free phenol was found to be responsible for stabilizing the active species. It was also found that the modification of phenol reagents could result in the formation of a tandem catalytic system, in which the obtained a-olefins were then mostly converted into the corresponding Friedel-Crafts alkylated-toluene products. The effects of the ethylene pressure and the substituents on the phenol on the tandem reactions were then investigated. To the best of our knowledge, such tandem reactions mediated by zirconium catalysts have never been reported previously.(2) The regulating effects of 2-naphthol,1-naphthol, benzoic acid and cyclohexanol on the performance of bis(imino)pyridine iron-based catalysts were also studied. Compared to phenol,2-naphthol acted as a stronger regulating modifier. With larger molecule size, it could effectively promote the separation of MAO anion and iron cation when it was decorated on the MAO surfacecan, thus promoting the P-H elimination to a-olefins and retarding the wax formation The activities of the 1-naphthol-and benzoic acid-modified systems, however, was significantly decreased even the dosage of such modifiers was quite low. This indicated that the interaction between modifiers and the active centers may be closely related to the acidity of the modifiers. Strong acidity of the modifiers would deactivate the catalysts. Therefore, a suitable weak acidity was required for the modifiers to maintain the effective modulating capacity. Finally, this thesis studied the effects of cyclohexanol with a similar structure and lower toxicity compared to phenols. The results showed that the polymer formation could be siginificantly retarded with the treatment of cyclohexanol. But at the meantime, it would also largely deactivate the catalyst system. Due to the lack of delocalized structure of large π bond, negative charges of cyclohexyloxy would localized on the oxygen atoms, which may cause the complexation of the oxygen atoms and the iron centers, leading to the deactivation of the active centers.(3) A new type of phenoxy-aluminoxanes were synthesized through the partial hydrolysis of phenol reagents (Phenol,4-tert-butylphenol and 4-bromophenol) and trimethylaluminum, which were then used as cocatalysts for the activation of bis(imino)pyridine iron catalyst. This hydrolysis process could be carried out under mild conditions, and the resultant phenoxy-aluminoxanes could effectively activate the iron-based catalyst. In addition, the oligomerization product composition could be well regulated by adjusting the ratio of phenol to AlMe3. By comparing the effects of the aluminoxanes when various phenols were used, substituents with larger steric hindrance on the para-position of phenol were proved to favor the shift of the olefin distribution to lower molecular weight direction. On the other hand, when 4-bromophenol was introduced to prepare the corresponding aluminoxanes, excessive concentration of the negative charges in the cocatalyst would be detrimental to the complexation of iron centers and ethylene monomers, leading to a relative decline of the activity. |
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