Electronic Properties of Ti Sites in Ziegler–Natta Catalysts

Abstract

Although Ziegler–Natta (ZN) catalysts play a major role in the polyolefin market, a true understanding of their properties at the molecular level is still missing. In particular, there is a lack of knowledge on the electronic properties of Ti sites. Theoretical calculations predict that the electron density of the Ti sites in the precatalysts correlates with the activation energy for olefin insertion in the Ti-alkyl bond generated at these sites after activation by Al-alkyls. It is also well known that the effective charge on the Ti sites in the activated catalysts affects the olefin π-complexation. In this contribution, we exploit two electronic spectroscopies, UV–vis and Ti L2,3-edge near-edge X-ray absorption fine structure (NEXAFS), complemented with theoretical simulation to investigate three ZN precatalysts of increasing complexity (up to an industrial system) and the corresponding catalysts activated by triethylaluminum (TEAl). We provide compelling evidence for the presence of monomeric 6-fold-coordinated Ti4+ species in all of the precatalysts, which however differ in the effective charge on the Ti sites. We also unambiguously demonstrate that these sites are reduced by TEAl to two types of monomeric 5-coordinated Ti3+, either alkylated or not, and that the former are involved in ethylene polymerization. In addition, small TiCl3 clusters are formed in the industrial catalyst, likely due to the occurrence of severe reducing conditions within the catalyst pores. These data prove the potential of these two techniques, coupled with simulation, in providing an accurate description of the electronic properties of heterogeneous ZN catalysts.

Exploring cocatalyst type effect on the Ziegler–Natta catalyzed ethylene polymerizations: experimental and DFT studies

“…Considering that TiCl 4 catalysts supported on (110) termination is more catalytically active than on (104) cut, the effect of Al complexes in the transalkylation process to insert the ethyl on TiCl 4 clusters has been investigated using this particular termination. The preferred choice of (110) termination is not only based on the computational results on bare surface, otherwise it agrees with the recent findings of Taniike-Groppo group which demonstrate that alkylated five-coordinated Ti III centers on (110) termination is mainly involved in ethylene polymerizations using TEA as cocatalyst [37]. The structures have been optimized adsorbing the Al complex on chloride surface atom near to Ti site, as it is exhibited in Fig.…”

Formation of Highly Active Ziegler–Natta Catalysts Clarified by a Multifaceted Characterization Approach

“…The spectrum of activated C is characterized by two very intense and well-defined absorption bands at 38400 and 17800 Finally, the DR UV−vis spectrum of activated F is characterized by the almost complete erosion of the bands at 33600 and 28400 cm −1 that were present before the reaction with TEAl, leaving the place for an intense CT transition absorption band at 40100 cm −1 and a quite weak band at 21700 cm −1 , which is attributed to isolated, 5-fold− coordinated, and alkylated Ti 3+ species (although the broadness of the band reveals a certain heterogeneity of the sites, in terms of slightly different structural parameters and chemical surroundings). 90 According to the data of polymerization activity shown in Figure 1, those species are thus the main responsible for propylene polymerization.…”

Preventing Premature Convergence in Evolutionary Structure Determination of Complex Molecular Systems: Demonstration in Few-Nanometer-Sized TiCl4-Capped MgCl2 Nanoplates

“…This study provides a methodology for structure determination of few-nm sized nanoparticles and a realistic molecular model of the Ziegler–Natta catalyst. The latter is particularly important for comprehending experimental results at a molecular level. , …”