Toward a Unified Model Explaining Heterogeneous Ziegler–Natta Catalysis

Abstract

We propose a model for MgCl 2 -supported Ziegler−Natta catalysts, which is capable to reconcile the discrepancies emerging in the last 20 years, in which attempts have been made to rationalize experimental data by molecular models. We show that step defects on the thermodynamically more stable (104) facet of MgCl 2 can lead to sites for strong TiCl 4 adsorption. The corresponding Ti-active site is stereoeselective, and its stereoselectivity can be enhanced by coordination of Al-alkyls or Lewis bases in the close proximity. The surface energy of the step defected (104) MgCl 2 facet is clearly lower than that of the well accepted (110) facet.

Synergistic, reconstruction and bonding effects during the adsorption of internal electron donors and TiCl4 on MgCl2 surface: A periodic-DFT investigation

“…Vanka et al have used DFT cluster model approach to show that α (ccp) and β (hcp) phases of MgCl 2 surfaces have different reactivity towards the IDs [35]. Low coordinated sites present along the surface defects have also been thought to play a crucial role in stabilizing the active sites for PP polymerization [36,37].…”

MgCl2‐supported Ziegler‐Natta catalysts: A DFT‐D ‘flexible‐cluster’ approach. TiCl4 and probe donor adducts

“…For a more consistent scenario, it was necessary to wait for the introduction of Grimme's corrections for dispersion, or the implementation of new families of functionals explicitly conceived for solid state applications . Based on the results of periodic DFT‐D studies, it is now generally accepted that TiCl 4 chemisorption on regular MgCl 2 crystal surfaces can only occur in mononuclear form on MgCl 2 (110) or equivalent lateral terminations, exposing tetra‐coordinated Mg.…”

“…While the model has been endorsed by subsequent independent experimental and theoretical studies, we are aware of its limitations. Real ZNC primary particles are small and defective to the point that they hardly deserve the definition of ‘crystals’ .…”

Structural Characterization of the EtOH–TiCl4–MgCl2 Ziegler–Natta Precatalyst

“…10,11 Among the possible terminations of MgCl 2 , the (110) was found to preferentially interact with TiCl 4 . [12][13][14][15] In the preparation of highly active ZN precatalysts for polyethylene synthesis one of the key steps is the introduction of a monodentate Lewis base, typically an alcohol [16][17][18][19][20][21] or an ether, [22][23][24][25][26][27][28] during the preparation of MgCl 2 and its contact with TiCl 4 .…”

“…For this purpose, several experimental [29][30][31][32][33][34][35] and computational 13,15,[36][37][38][39][40][41][42][43][44][45][46] studies were performed. Many features of the catalyst, as the structure of the precatalyst, the activation process, the oxidation state(s) of the active titanium species during the polymerization process were investigated; notwithstanding, there are still several open questions.…”