RESUMEN
One class of the Ziegler-Natta catalysts (ZNC) - the TiCl4/MgCl2 having triethyl aluminum (AlEt3), has been widely utilized during ethylene polymerization. Although the Ti species plays the role of a major active site, an increase of Ti species does not always improve the activity of ZNC. Herein, investigations of experiments and density functional theory (DFT) elucidate this inverse effect of the increased amount of TiCl4 deposition in ZNC because of the pretreatment process. However, the activity of ZNC on pretreated MgCl2 dropped to 60% of the unpretreated one. The DFT demonstrates that the pretreatment strengthened the interaction between TiCl4 and ZNC, especially on the (104) surface, forming the TiCl4-TiCl4 cluster. The existence of this TiCl4-TiCl4 cluster found on the ZNC (104) surface weakens the adsorption of the first AlEt3 molecule and obstructs further alkylation process, making another Ti site of the alkylated TiCl4-TiCl4 cluster inactive. However, the difficult formation of the TiCl4-TiCl4 cluster found on the ZNC (110) is an important key point that enables the activation of all adsorbed TiCl4 on this surface by facilitating the alkylation process. Moreover, the existence of the MgCl2 (110) surface prevents the formation of the TiCl4-TiCl4 cluster significantly. Hence, it is suggested that the existence of the (110) plane on ZNC plays a key role in controlling the performance of the ZNC, especially the stability via the prevention of deactivation caused by the clustering of TiCl4.