Abstract
This paper presents the results of study of titanium–magnesium catalysts often used in polymerization processes, by photoluminescence spectroscopy (PL) in combination with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The interaction of dibutyl phthalate (DBP) with MgCl2 was studied at DBPadded/Mg = 0–1 (mol/mol). The luminescence spectra with excitation at 278 nm and the excitation spectra for main emission bands were recorded. It was shown that DBP adsorbed on magnesium chloride, both in the form of donor–acceptor complexes (D+A–) and in the form of molecular complexes. At DBPadded/Mg <0.15, the formation of D+A– complexes occur predominantly; with an increase in DBPadded/Mg, the fraction of molecular complexes increases. Molecular complexes are destroyed during the treatment of the support by TiCl4. In this case, the structure of magnesium chloride is disordered and new coordination–unsaturated sites are formed. This work is a first attempt to apply PL spectroscopy in combination with DRIFTS spectroscopy to study titanium–magnesium Ziegler–Natta catalysts. The application of PL spectroscopy to such systems made it possible to detect interactions within and between donor molecules, which would be particularly challenging to achieve using other spectroscopic methods. Both spectroscopic methods provided crucial information about the existence of two types of complexes on the sample surface which is important for tuning the synthesis procedure of the titanium–magnesium catalysts for olefin polymerization.
Highlights
Titanium–magnesium catalysts (TMCs) for the stereospecific polymerization of ethylene have been known for over 60 years
MgCl2/nDBP samples were prepared by applying dibutyl phthalate (DBP) on magnesium chloride at DBPadded/MgCl2 equal to 0.1, 0.15, 0.4, and 1
It can be seen that the amount of adsorbed donor in MgCl2/nDBP samples increases with an increase in DBPadded/MgCl2
Summary
Titanium–magnesium catalysts (TMCs) for the stereospecific polymerization of ethylene have been known for over 60 years. TMCs are usually composed of magnesium chloride, electron donor base (phthalates, for example, dibutyl phthalate (DBP), malonates, alkoxysilanes, succinates, 1.3-diesters), and TiCl4 (Scheme 1). The formation of TMCs is still debatable, and this is primarily due to unresolved questions about the nature of the interaction of the Lewis bases (LB) with magnesium chloride and TiCl4. The specificity of the catalysts for olefin polymerization does not allow the contact of samples with air at any stages of their preparation, and the catalysts themselves operate at temperatures of 40–90 C. Infrared (IR) spectroscopy and in particular diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is one of the common physicochemical methods
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