| Due to the excellent performances, polypropylene has become one of the fastest growing general resins since its firstly commercialization in 1957. Most of polypropylenes are produced by heterogeneous Ziegler-Natta catalyst, which has attracted lots of research works. Review the development of Ziegler-Natta catalyst, it can be found that the researches and developments of the catalyst are focused on carriers and new electronic donors. However, little is known about the adsorption sites, structures and mechanisms of active components such as Ti species and electronic donors, which have become the "bottlenecks" to develop new Ziegler-Natta catalysts. In this work, the structures and function relationships of these key elements in heterogeneous Ziegler-Natta catalyst are studied by molecular modeling method based on Density Functional Theory (DFT). Twelve carrier models of non-defect and defect MgCl2 (110) and (100) lateral cuts are built, and then adsorption of TiCl4 and three different electron donors (ethyl benzoate, diethyl phthalate and 1,3-dimethoxypropane) on these models are studied. Finally, the insertions of propylene monomer on the A5 active site, which is the most probable adsorption sites for TiCl4, are investigated. The results indicate that TiCl4 is unfavorable adsorbed on sites of non-defect and Cl-ionic defect MgCl2 (110) and (100) faces, which are firmly occupied by electronic donor. Contrarily, TiCl4 is more feasible adsorbed on Cl-atomic defects of MgCl2 surfaces, especially for the defects with more atomic Cl loss; the adsorptions of three electronic donors are different with each other on MgCl2 (110) surface, which indicate that MgCl2 (110) lateral cut might be the active face for the propylene polymerization; the active center formed on A5 atomic defect model is the site for insertions of propylene with regiospecificity. The results provided a much deeper understanding of the adsorption sites and structures of Ti active species and electronic donors, and a guidance and direction for further development of new Ziegler-Natta catalysts with high performance as well. |
