Electronic Properties of Ti Sites in Ziegler–Natta Catalysts

Alessandro Piovano,Piero Torelli,Gentoku Takasao,Elena Groppo,Toshiaki Taniike,Andrea Martini,Matteo Signorile,Toru Wada,Luca Braglia

doi:10.1021/acscatal.1c01735

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.

Highlights

Ziegler−Natta (ZN) catalysts are at the heart of the polyolefin production, affording at present almost 80 million tons of polypropylene per year, with a worldwide economic turnover exceeding 100 billion dollars,[1] and their great properties are recognized as a fundamental benchmark for the whole chemical industry
Their extraordinary success in terms of activity and selectivity is due to the perfect combination of four indispensable components, namely, a titanium chloride precursor, a high surface area MgCl2 support, organic molecules acting as Lewis bases, and an aluminum alkyl activator.[2−5] The first three components constitute the precatalyst, which can be prepared following different routes that have been optimized in decades of industrial research[6,7] to generate multigrain and porous spherical particles as a result of aggregation of so-called primary particles.[8−14] This hierarchical structure is fundamental to guarantee controlled fragmentation during olefin polymerization and to provide a polymer with the desired morphology
Since the structure and the morphology of the δ-MgCl2 primary particles are retained during the catalyst formation in the presence of an aluminum alkyl activator, the synthetic protocol drives the distribution of the active sites and their stereospecificity.[33−41] Albeit this concept is widely accepted based on the analysis of the polymer produced, so far direct experimental evidence on the different properties of the

Summary

Introduction

Ziegler−Natta (ZN) catalysts are at the heart of the polyolefin production, affording at present almost 80 million tons of polypropylene per year, with a worldwide economic turnover exceeding 100 billion dollars,[1] and their great properties are recognized as a fundamental benchmark for the whole chemical industry Their extraordinary success in terms of activity and selectivity is due to the perfect combination of four indispensable components, namely, a titanium chloride precursor, a high surface area MgCl2 support, organic molecules acting as Lewis bases (namely, the electron donors), and an aluminum alkyl activator.[2−5] The first three components constitute the precatalyst, which can be prepared following different routes that have been optimized in decades of industrial research[6,7] to generate multigrain and porous spherical particles as a result of aggregation of so-called primary particles.[8−14] This hierarchical structure is fundamental to guarantee controlled fragmentation during olefin polymerization and to provide a polymer with the desired morphology. Since the structure and the morphology of the δ-MgCl2 primary particles are retained during the catalyst formation in the presence of an aluminum alkyl activator, the synthetic protocol drives the distribution of the active sites and their stereospecificity.[33−41] Albeit this concept is widely accepted based on the analysis of the polymer produced, so far direct experimental evidence on the different properties of the

Methods

Results

Conclusion