Experimental and first-principles investigation on how support morphology determines the performance of the...

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.