Active Site Formation in MgCl₂−Supported Ziegler−Natta Catalysts. A Density Functional Theory Study

All types of chemical reactions (reduction, alkylation, and complexation of Ti species by organoaluminum compounds) leading to formation of active sites in heterogeneous Ziegler−Natta catalysts have been studied within density functional theory. A generally accepted mechanism of active site formation was found to be less preferable as compared to the alternative ways considered. Based on the calculation implemented, a whole active site formation scenario was postulated for the different active center precursors on the MgCl2 surface. The mechanism proposed allows us to rationalize the domination of Ti3+ over Ti2+ under reduction of Ti4+ surface species by AlR3, the absence of an electron spin resonance signal for the TiCl4/MgCl2 + AlR3 system with low Ti content, the stronger reduction ability of Al(i-Bu)3 than AlEt3, the deactivation effect of AlR2Cl, and the reactivation ability of AlR3.