Formation of Highly Active Ziegler-Natta Catalysts Clarified by a Multifaceted Characterization Approach

Although the formation of nanosized and defective δ-MgCl2 is essential for the performance of Ziegler-Natta catalysts, the process has not sufficiently been elucidated due to certain limitations in characterization. Here, the formation of nanostructures and active surfaces of Ziegler-Natta catalysts was investigated in detail based on a multifaceted set of characterization techniques represented by X-ray total scattering and various spectroscopies in correlation with chemical composition analysis and polymerization tests. Solid samples were extracted in the course of catalyst preparation from Mg(OEt)2 and subjected to the analysis. Several interesting results were found. The addition of TiCl4 almost spontaneously converts Mg(OEt)2 into MgCl2 seeds mainly exposing the {001} basal surface, whose dimensions are below 2 nm; a large Ti amount remains on the material as physisorbed 4-fold-coordinated TiClx(OEt)4-x species. The heating treatment removes the physisorbed TiClx(OEt)4-x and/or convert them into chemisorbed 6-fold-coordinated TiClx(OEt)4-x, while the subsequent addition of an internal donor (here dibutyl phthalate, DBP) promotes a substantial reconstruction and growth of MgCl2 seeds to almost the same size as the final catalyst (ca. 6 nm), with the exposure of the more catalytically relevant lateral surfaces. DBP is in one part adsorbed on MgCl2 surfaces and in the other part complexed with Ti sites. This complex is only partially removed in the following steps of the synthesis. The second TiCl4 addition replaces the chemisorbed TiClx(OEt)4-x with 6-fold-coordinated TiCl4 species, but it also causes side reactions with DBP, as testified by the formation of phthaloyl chloride. After activation by triethylaluminum (TEAl), the activity per Ti for ethylene was almost constant throughout the whole preparation process after the initial TiCl4 addition, whereas the activity for propylene was negligible before the addition of the donor and increased dramatically in the subsequent steps of the preparation. This was further investigated based on spectroscopies for TEAl-activated samples in order to individuate the active Ti species responsible for the catalysis and to monitor the fate of DBP upon TEAl reaction. The multifaceted characterization approach allowed us to integrate information on the formation of δ-MgCl2, their surfaces, and adsorbed species, providing us with deep insights into the meaning of each step within an industrial catalyst preparation method that has been empirically refined over a long history.