Surface Species of Titanium(IV) and Titanium(III) in MgCl 2 -Supported Ziegler−Natta Catalysts. A Periodic Density Functional Theory Study

A systematic consideration of different Ti(IV) and Ti(III) species on the (104) and (110) MgCl 2 surfaces has been implemented within DFT using cyclic boundary conditions. Some new mononuclear and dinuclear surface complexes of Ti(IV) and Ti(III) were obtained due to implication of zip coordination mode. A possible spin state of dinuclear Ti(III) species was thoroughly studied: antiferromagnetic (ESR silent) state proved to be the most preferable in a number of cases. The zip antiferromagnetic Ti 2 Cl 6 complexes residing on the dominant (104) MgCl 2 surface make it possible to rationalize the fact that the most part of Ti(III) incorporated in activated MgCl 2 is ESR silent. Besides, these species produce aspecific active sites, thus explaining that aspecific centers significantly prevail over stereospecific one according to kinetic data on the simplest TiCl 4 /MgCl 2 + AlR 3 system.