Surface Species of Titanium(IV) and Titanium(III) in MgCl₂-Supported Ziegler−Natta Catalysts. A Periodic Density Functional Theory Study

Abstract

A systematic consideration of different Ti(IV) and Ti(III) species on the (104) and (110) MgCl 2 surfaces has been implemented within DFT using cyclic boundary conditions. Some new mononuclear and dinuclear surface complexes of Ti(IV) and Ti(III) were obtained due to implication of zip coordination mode. A possible spin state of dinuclear Ti(III) species was thoroughly studied: antiferromagnetic (ESR silent) state proved to be the most preferable in a number of cases. The zip antiferromagnetic Ti 2 Cl 6 complexes residing on the dominant (104) MgCl 2 surface make it possible to rationalize the fact that the most part of Ti(III) incorporated in activated MgCl 2 is ESR silent. Besides, these species produce aspecific active sites, thus explaining that aspecific centers significantly prevail over stereospecific one according to kinetic data on the simplest TiCl 4 /MgCl 2 þ AlR 3 system.

Coadsorption model for first-principle description of roles of donors in heterogeneous Ziegler–Natta propylene polymerization

“…Their adsorption energies (per Ti) were respectively À13 and À11 kcal/mol, both of which were much smaller than that on the (1 1 0) surface ( Table 1). Many of previous calculations reported similar trends for the adsorption energies of TiCl 4 on the MgCl 2 (1 1 0) and (1 0 0) surfaces [34][35][36][37][38], even though energetic magnitudes were more or less dependent on exchange-correlation functionals. Thus, it seems clear that TiCl 4 adsorbs more strongly on the (1 1 0) surface.…”

“…In analogous to the first-and second-generation TiCl 3 catalysts [29,30], the mononuclear species on the (1 1 0) surface and the dinuclear species on the (1 0 0) surface were respectively assigned as aspecific and isospecific active sites [25]. Although the nuclearity of surface TiCl 4 species has not been experimentally concluded yet [31][32][33], most of DFT studies have agreed on the energetic preference of the TiCl 4 mononuclear species on the (1 1 0) surface [34][35][36][37][38][39].…”

“…However, the idea is still evidenced by few aspects such as stronger adsorption of TiCl 4 on the (1 1 0) surface [34][35][36][37][38] and the presence of coordination vacancies at L 1,2 on the same surface. Many other aspects remain unclarified such as energetic feasibility of the coadsorption of donors, electronic and steric interactions upon the coadsorption, and their influences not only on the stereospecificity but also on other catalytic properties such as regiospecificity and molecular weight.…”

Probing the Coordinative Unsaturation and Local Environment of Ti3+ Sites in an Activated High‐Yield Ziegler–Natta Catalyst

“…[5] Thei nfluence of certain Lewis bases and at al ater stage the AlR 3 cocatalyst [1] on the system is still largely unestablished and seminal experimental [3,[6][7][8][9] and theoretical [5,[10][11][12][13][14] investigations have failed to account for all knownf acts.A ctivation is usually performed by reacting the precatalyst with aluminum alkyl compounds (typically triethyl aluminum, TEA). In this process tetravalent Ti 4+ ions assume alower oxidation state [15] and both Ti 3+ and Ti 2+ ions have been proposed.…”

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

“…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.…”