Structure of MgCl2–TiCl4 complex in co-milled Ziegler–Natta catalyst precursors with different TiCl4 content: Experimental and theoretical vibrational spectra

The FT-Raman spectra of a series of mechanically prepared MgCl2–TiCl4 samples with increasing content of TiCl4 have been recorded. Raman scattering lines in the range 600–100 cm−1 reveal the formation of TiCl4 molecules complexed on the lateral cuts of activated MgCl2 crystals. This pattern of lines suggests that atleast three types of complexes are formed during the grinding process and their relative concentration depend on the starting concentration of TiCl4. Two types of these complexes have been shown to be unstable and easily removed when the samples are washed with n-hexane. In the former two cases TiCl4 molecules are seemingly physisorbed and/or weakly complexed on MgCl2. The remaining complex is stable and it is found in every sample also after washing. This stable complex shows Raman lines which can be interpreted as originated from the complexed TiCl4 molecules with the Ti atoms in an octahedral coordination. On the other hands also monomeric TiCl4 species complexed along the (1 1 0) lateral cut or dimeric TiCl4 species complexed along the (1 0 0) lateral cuts of the MgCl2 crystal can take up a geometry with the Ti in an octahedral coordination. Both these structures have been considered in this work. Calculated Raman spectra, molecular energy considerations and Raman lines intensity ratios strongly support the fact that the stable complexes are the monomeric TiCl4 species complexed along the (1 1 0) MgCl2 lateral cuts. A spectroscopic quantitative determination of the complexed TiCl4 for this class of material is proposed.

In this work a series of new data on catalysts precursors relevant in the field of polyolefin catalysis are presented. The experimental FT-Raman spectra of MgCl2–TiCl4 catalyst precursors show clear signals ascribed to three different TiCl4–MgCl2 complexes; the nature of these complexes has been interpreted with the help of quantum chemical calculations. After washing of the precatalyst with n-hexane only one type of complex is found thus it can be proposed as the prototype of TiCl4–MgCl2 complex in the catalyst. Moreover, a quantitative determination of TiCl4 content by Raman spectroscopy is proposed. Figure optionsDownload as PowerPoint slide