Structure of MgCl 2–TiCl 4 complex in co-milled Ziegler–Natta catalyst precursors with different TiCl 4 content: Experimental and theoretical vibrational spectra

Abstract

The FT-Raman spectra of a series of mechanically prepared MgCl 2–TiCl 4 samples with increasing content of TiCl 4 have been recorded. Raman scattering lines in the range 600–100 cm −1 reveal the formation of TiCl 4 molecules complexed on the lateral cuts of activated MgCl 2 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 TiCl 4. 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 TiCl 4 molecules are seemingly physisorbed and/or weakly complexed on MgCl 2. 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 TiCl 4 molecules with the Ti atoms in an octahedral coordination. On the other hands also monomeric TiCl 4 species complexed along the (1 1 0) lateral cut or dimeric TiCl 4 species complexed along the (1 0 0) lateral cuts of the MgCl 2 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 TiCl 4 species complexed along the (1 1 0) MgCl 2 lateral cuts. A spectroscopic quantitative determination of the complexed TiCl 4 for this class of material is proposed.