Electronic Properties of Ti Sites in Ziegler-Natta Catalysts

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

Period Increase and Amplitude Distribution of Kink Oscillation of Coronal Loop

Coronal loops exist ubiquitously in the solar atmosphere. These loops puzzle astronomers over half a century. Solar magneto-seismology (SMS) provides a unique way to constrain the physical parameters of coronal loops. Here, we study the evolution of oscillations of a coronal loop observed by the Atmospheric Imaging Assembly (AIA). We measure geometric and physical parameters of the loop oscillations. In particular, we find that the mean period of the oscillations increased from 1048 to 1264 s during three oscillatory cycles. We employ the differential emission measure method and apply the tools of SMS. The evolution of densities inside and outside the loop is analyzed. We found that an increase of density inside the loop and decrease of the magnetic field strength along the loop are the main reasons for the increase in the period during the oscillations. Besides, we also found that the amplitude profile of the loop is different from a profile would it be a homogeneous loop. It is proposed that the distribution of magnetic strength along the loop rather than density stratification is responsible for this deviation. The variation in period and distribution of amplitude provide, in terms of SMS, a new and unprecedented insight into coronal loop diagnostics

Energetically accessible structures of xMgCl2/yTiCl4 clusters (x = 6–19, y = 0–4)

Dataset of energetically accessible structures of xMgCl2/yTiCl4 clusters (x = 6–19, y = 0–4) obtained by non-empirical structure determination based on combination of the genetic algorithm and DFT calculations. The dataset was uploaded for a article "Dataset of energetically accessible structures of xMgCl2/yTiCl4 clusters (x = 6–19, y = 0–4) obtained by non-empirical structure determination based on combination of the genetic algorithm and DFT calculations" in Data in Brief

Energetically accessible structures of xMgCl2/yTiCl4 clusters (x = 6–19, y = 0–4)

Dataset of energetically accessible structures of xMgCl2/yTiCl4 clusters (x = 6–19, y = 0–4) obtained by non-empirical structure determination based on combination of the genetic algorithm and DFT calculations. The dataset was uploaded for a article "Dataset of energetically accessible structures of xMgCl2/yTiCl4 clusters (x = 6–19, y = 0–4) obtained by non-empirical structure determination based on combination of the genetic algorithm and DFT calculations" in Data in Brief