| Linear a-olefins (LAOs) are the building blocks for a range of industrial and consumer products, such as polyolefin and PAO synthetic oils, which are crucially important for the development of domestic economy and national defense. Ethylene oligomerization is currently the most predominant method for a-olefin production and bis(imino)pyridine iron complexes have been proved to be excellent catalysts with exceptionally high activity and 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 highly interested and a potential breakthrough for us to develop a new oligomerization process. However, there are still some obstacles, e.g. broad product distribution, simultaneous production of insoluble polymers, severe fouling and large excess of methylaluminoxane (MAO), for such catalysts to commercialization.The nature of the catalyst active species can be a decisive factor for the reaction behaviors and product properties. It thus becomes important to solve the above-mentioned problems by mediating the nature or the micro-chemical environment of the active species. Aiming at such modulation in iron catalysts, several methods, e.g. optimization of process conditions, introduction of modifiers, modification of MAO, heterogenization of homogeneous catalysts or development of novel alternative activators would be studied in this thesis. In addition, the fouling mechanism during ethylene oligomerization would be discussed. The main work and results are as follows:(1) Iron acetylacetonate was mainly used as the iron source to form a homogeneous pre-catalyst with a bis(imino)pyridine (BIP) ligand. The influence of reaction time, temperature, ethylene pressure and hydrogen on the activities and product distributions were studied. It was suggested that shortening the reaction time or increasing the temperature would reduce the mass fraction of insoluble polymers in the total products, while increasing the pressure or introducing hydrogen would cause an increment in activity and polymer formation. In addition, the Mw and MWD of the insoluble polymers were increased with the treatment of hydrogen, which could be ascribed to the enhanced chain propagation rate. The MAO concentration appeared to be a more important factor than the Al/Fe ratio. The utilization efficiency of MAO could be largely increased by suitable combination of catalyst addition strategies, and the Al/Fe ratio could be reduced to 200-300, which was very close to the theoretical analysis data.(2) A series of Lewis acids or bases were introduced as modifiers to tune the system L-Fe(acac)3/MAO, which would selectively or non-selectively interact with the iron center, MAO, the ligand or other catalytic components, thus mediating the micro-chemical environment of the active species. Trifluoroacetylacetone and phenyl trichlorosilane were found to mainly interact with AlMe3, while exert little impact on the active centers, thus enhancing the reaction activity with little effect on the product composition. Phosphine compound, triphenylphosphine, was proposed to tightly coordinate with the iron center, occupying the coordination sites for ethylene. The polymer formation was then reduced, while the activity decreased more pronounced. Aluminum ethyl dichloride (AlEtCl2) was a highly reactive compound, which could cause the decomposition of the BIP ligand and strongly react with the iron centers. It was also proposed that the bridged methyl of MAO could be replaced by the chlorine atom from AlEtCl2. All of these resulted in the remarkable decrease in activity and product molecular weight. Based on these observations, requirements for a suitable modifier were proposed.(3) Alkoxysilanes were tested to tune the product distribution, and the mediating mechanism was studied. It was suggested that alkoxysilanes could modify the structure and property of MAO, modulate the micro-chemical environment of active species and finally reduce the polymer formation. A suitable alkoxysliane (e.g. tetraethyl orthosilicate (TEOS)) could "trap" the AlMe3 containing in MAO and give a further modification of the MAO structure. The former effect would cause an increment in activity, while the later effect would break the MAO clusters into smaller aggregates and decorated TEOS on the surface of MAO, which may further lead to a better separation of the cationic iron centers and the anionic MAO clusters. With this modification, the additional steric effect that MAO exerted on the active species could be reduced, and more soluble olefins were thus produced by β-H elimination. With such effects, a pronounced reduction of the polymer share in the total products from above 30 wt% to less than 3 wt% could be realized. Based on these findings, a more effective modifier, dicyclopentyldimethoxysilane (DCPDMS), was discovered. To achieve the similar retarding effect toward polymers, the needed [Si]/[Al] molar ratio in DCPDMS-mediated system was only 0.2, while in the case of TEOS, the adopted molar ratio must be 3.0-5.0.(4) A series of para-alkyl and para-halogen substituted phenolic compounds were adopted to mediate ethylene oligomerization. The steric and electronic effects of the p-position on the reactivity and product distribution, along with the mediating mechanism were investigated. It was proposed that the hydroxyl of a phenol modifier could react with MAO and modify the MAO structure, thus tuning the micro-chemical environment of active species and the product composition. The polymer formation was largely reduced with the enlargement of the p-alkyl size. And introduction of a halogen on the p-position could increase the reactivity of the hydroxyl, thus enhancing the MAO modification. 1H NMR studies showed that the phenolic compounds could scavenge AlMe3 and gave rise to larger MAO aggregates. The phenoxy groups could also be decorated on the surface of MAO. A larger MAO would impose strong steric effect on the active centers, inhibiting the chain termination, while the decorated phenoxy groups would lead to a less tightly bound ion pair, enhancing the β-H elimination. Therefore, the final influence of phenolic compounds could be a comprehensive result of the above-mentioned effects.(5) Starting from the modification of the activator or the pre-catalyst, alternative activation methods were studied in order to reduce the cost of MAO. Firstly, a novel type of phenoxy aluminoxane was prepared according to the preparation of MAO. Such phenoxy aluminoxane could serve as an effective activator when a small amount of extra AlMe3 was added. The activity was found to be 10-fold of the AlMe3-activated system with no polymer formation. Increasing the amount of added AlMe3 would cause the formation of some polymers, which may be due to the decrease of phenoxy groups on the activator. Secondly, a supported catalyst was prepared from the point of catalyst modification. Activating the adduct of MgCl2 and 1,4-butanediol with AlEt3 would result in a highly active support, which was then used to immobilize L-Fe(acac)3. This supported catalyst could better stabilize the active species and be activated by alkyl aluminums instead of MAO with high activities. The thermo stability of the MgCl2-supported catalyst was also found to be better than the homogeneous L-Fe(acac)3/MA.O system.(6) The fouling mechanism during ethylene oligomerization was preliminarily studied. There existed physical and chemical reasons for the reactor fouling. The cold model experiments showed that the physical fouling was mainly happened at the gas-liquid phase interface on the inner wall and the mixer shaft. Fouling in the actual situation would occur on the whole wall under the liquid. In addition, the fouling polymer could be divided into two layers. The molecular weight of the polymer from the inner compact layer was much higher than that of the outer loose layer and the slurry polymer, which could be ascribed to the in situ chain propagation on the wall when the active species were "anchored" on the wall. Furthermore, the amount of polymers sticking on the non-metal sheets was less than that on the metal sheets, which again approved the existence of chemical fouling. Some methods were proposed to reduce or prevent such fouling process. |
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