| Popis: |
The catalyst composed of zirconium tetrachloride (ZrCl), triethylaluminum (TEA: Et3Al), and ethylaluminum sesquichloride (EASC: Et2Al2Cl3) was studied for the oligomerization of ethylene. The coordinative interaction of various organoaluminums with Zr and of various additives with Zr and Al was studied by 27 Al NMR, 1 H NMR and 2D NMR. The most active catalyst was obtained when TEA was first added to ZrCl4 and heated above 70 °C for 30 min. Especially the amount of coordinated 2Et3Al to ZrCl4 was increased by this method, and the catalytic activity was enhanced. Because stronger coordination of the organoaluminum led to an increase in the electron density at the Zr atom and weakened the strength of the Zr-alkyl bond, the insertion of ethylene into the Zr-alkyl bond was enhanced. The yield and selectivity of the α-olefin was then increased. An active site for the oligomerization of ethylene is Zr. Therefore, the control of the electron density of Zr is very important. When additives having a small ionization potential (IP) such as tetrahydrothiophene (THT) are coordinated to ZrCl4, the purity of the α-olefin was improved. This is due to the increase in the electron density of Zr, and the steric hindrance of the bulkier additives to the side reactions of the Zr-alkyl bond. On the other hand, a heterocompound additive having a large electron density is coordinated to an Al atom of TEA and EASC co-catalysts. The ability of various additives to be coordinated to the Al atom was proved to increase in the order of tetrahydrofuran (THF)>THT>thiophene. The formation of a solid polymer by-product also increased in the same order. The solid polymer by-product was considered to form on the over-reduced Zr. |
Full Text from ScienceDirect