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

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• 作者：Luigi Brambilla ; Giuseppe Zerbi ; Fabrizio Piemontesi ; Stefano Nascetti ; Giampiero Morini
• 关键词：Ziegler&#x2013 ; Natta catalysts ; TiCl₄&#x2013 ; MgCl₂ complex ; FT-Raman spectroscopy ; DFT quantum chemical calculation
• 刊名：Journal of Molecular Catalysis A ; Chemical
• 出版年：2007
• 期刊代码：57_13811169
• 类别：ch
• 出版时间：14 February 2007
• 卷：263
• 期：1-2
• 页码：103-111
• 文件大小：651 K

摘要

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.