Spectroscopic Investigation of Heterogeneous Ziegler–Natta Catalysts: Ti and Mg Chloride Tetrahydrofuranates, Their Interaction Compound, and the Role of the Activator

Abstract

X-ray powder diffraction (XRPD), Infrared, Raman, and UV/Vis spectroscopy have been used to investigate the structural, vibrational, and optical properties of Ti and Mg chloride tetrahydrofuranates as precursors of heterogeneous Ziegler-Natta catalysts for polyethylene production; as well as their interaction compound (pro-catalyst) and the final catalyst obtained after interaction with the AlR(3) activator. Although the structure of the precursors and of the pro-catalyst were well known, that of the catalyst (obtained by reaction of the pro-catalyst with AlR(3)) was not easily obtainable from XRPD data. IR and Raman spectroscopy provided important information on tetrahydrofuran (thf) coordination and on the ν(M-Cl) region; whereas UV/Vis spectroscopy gave the direct proof on both the formal oxidation state and the coordination environment of the active Ti sites. Those presented herein are among the first direct experimental data on the structure of the active Ti sites in Ziegler-Natta catalysts, and can be used to validate the many computational studies that have been increasing exponentially in the last few decades.

Silica-supported Ti chloride tetrahydrofuranates, precursors of Ziegler–Natta catalysts

“…In the frame of a wider work devoted to the physical-chemical characterization of silica-supported Ziegler-Natta catalysts based 50 on tetrahydrofuranates of TiCl 4 and MgCl 2 , 17 we performed a detailed spectroscopic investigation on the reactivity of titanium chloride tetrahydrofuranates (TiCl 4 (thf) 2 and TiCl 3 (thf) 3 , thf = tetrahydrofurane) towards a polymer-grade silica and on the structure of the resulting Ti-grafted sites. Reactivity of surface 55 hydroxyl groups of silica towards titanium chloride and other metal chlorides was studied in the past by several research groups, mainly by FT-IR spectroscopy and chemical analysis, and was used as a mean of determining the surface structure and the number of isolated and H-bonded hydroxyl groups of silica.…”

“…The UV-Vis spectra of the two Ti IV and Ti III chloride tetrahydrofuranates precursors (grey spectra in Figure 3a and b) were already 55 discussed in our previous work. 17,43 Both precursors display a well defined colour: Ti IV is bright yellow, as a consequence of the intense Cl(2p) →Ti(3d) charge-transfer transition having the edge (arbitrarily defined as the maximum of the first derivative) at around 22600 cm -1 (2.80 eV). On the contrary, the pale blue 60 colour of Ti III takes origin from the double and well defined d-d absorption band centred around 14000 cm -1 (1.74 eV), which is typical of Ti III species (d 1 transition metal) in a six-fold coordination;…”

Titanium(III, IV)-Containing Catalytic Systems for Production of Ultrahigh Molecular Weight Polyethylene Nascent Reactor Powders, Suitable for Solventless Processing—Impact of Oxidation States of Transition Metal

“…Catalytic system III was fairly active without the Et 3 Al cocatalyst (entries [18][19][20], but, in most cases, it was inferior to system II (Figure 4). In some cases, the ZN catalysts formed in situ (systems I and II, entries 1-16, in our opinion) showed superior results to systems obtained from single components [37,38].…”

“…Reaction of MgCl 2 with a Lewis base, typically alcohols or ethers such as THF, is a generally known method of its activation [17][18][19][20]. Alternatively, THF can serve as a ligand interacting with TiCl 4 .…”

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

“…10,11 Among the possible terminations of MgCl 2 , the (110) was found to preferentially interact with TiCl 4 . [12][13][14][15] In the preparation of highly active ZN precatalysts for polyethylene synthesis one of the key steps is the introduction of a monodentate Lewis base, typically an alcohol [16][17][18][19][20][21] or an ether, [22][23][24][25][26][27][28] during the preparation of MgCl 2 and its contact with TiCl 4 .…”