Adsorption of Titanium Tetrachloride on Magnesium Dichloride Clusters

Abstract

Magnesium dichloride and titanium tetrachloride are key components in heterogeneous Ziegler–Natta olefin polymerization catalysis. We have determined the preferred binding modes of titanium tetrachloride on magnesium dichloride clusters at the M06-2X level of theory, thus accounting for dispersion. A systematic study was carried out to locate the lowest energy isomers of (MgCl 2 ) n (TiCl 4 ) m complexes. Altogether ca. 1000 complexes with n ranging from 1 to 19 and m from 1 to 13 were studied. In line with the previous literature, the results consistently show that adsorption of TiCl 4 onto MgCl 2 preferably leads to octahedral six-coordination for both Mg and Ti atoms. This can be achieved by mononuclear binding of TiCl 4 and binuclear binding of Ti 2 Cl 8 on (110) and (104) surfaces of MgCl 2 , respectively. The preferred octahedral six-coordination is also achieved by binding of TiCl 4 on defect sites, of which the most striking example is trinuclear binding mode at corners of the clusters. Overall, the results highlight the relevance of multinuclear binding of TiCl 4 on MgCl 2 .

Preactive Site in Ziegler–Natta Catalysts

“…This is smaller than what we observe experimentally but in the same direction. Indeed, the same mononuclear and binuclear binding modes were observed to lead to preferred octahedral six-coordination in a recent theoretical study on (MgCl 2 ) n (TiCl 4 ) m clusters . Another recent theoretical study, also using clusters, reports the same three structural motifs as in Table .…”

“…Indeed, the same mononuclear and binuclear binding modes were observed to lead to preferred octahedral six-coordination in a recent theoretical study on (MgCl 2 ) n (TiCl 4 ) m clusters. 7 Another recent theoretical study, also using clusters, reports the same three structural motifs as in Table 5. 51 These authors find a substantial fraction of mononuclear 5-104 ("mononuclear {100}" in their study), but that is outweighed by a much larger fraction of 6-110, that is, six-coordinated Ti.…”

Monitoring the Kinetics of Internal Donor Clean-up from Ziegler–Natta Catalytic Surfaces: An Integrated Experimental and Computational Study

“…25,46 This choice was motivated by the need to ensure an effective description, in particular, of AlEt 3 self-dimerization energy, 45,57,58 and is in line with computational protocols for similar model clusters reported by others. 29 In all energy comparisons, the (calculated) dimerization energy of AlEt 3 was taken into account. The activation parameters, ΔH # and ΔS # , were calculated relative to the most stable reference state, i.e.…”

“…Thermal contributions (enthalpy and entropy) were also calculated at said level. Final single point energy calculations, including dispersion corrections, were carried out at the RI-M06-2X/cc-pVTZ − level of theory, preferred to the RI-TPSSTPSS-D2 level used in previous reports. , This choice was motivated by the need to ensure an effective description, in particular, of AlEt 3 self-dimerization energy, ,, and is in line with computational protocols for similar model clusters reported by others . In all energy comparisons, the (calculated) dimerization energy of AlEt 3 was taken into account.…”

“…MgCl 2 -supported Ziegler–Natta catalysts (ZNCs) dominate the ever-growing production of isotactic polypropylene, thanks to an exceedingly high stereoselectivity (isotacticity index I.I. > 98%) and a productivity in the order of several tons of polymer per gram of Ti. − These systems consist of a solid precatalyst and a soluble cocatalyst: the former is typically obtained by reacting a MgCl 2 precursor with TiCl 4 in the presence of a first Lewis base (“internal donor”, ID), whereas the latter consists of AlEt 3 and a second Lewis base (“external donor”, ED). , Surface modification by the electron donors is essential for the stereoselectivity; decades of experimental − and computational modeling − studies demonstrated that ID and ED molecules, chemisorbed in the vicinity of the active Ti sites, shape the catalytic pocket similarly to the ancillary ligands of molecular catalysts.…”