Theoretical investigation of active sites at the corners of Mgcl 2 crystallites in supported ziegler-natta catalysts

We present a theoretical study on possible models of catalytic active species corresponding to Ti-chloride species adsorbed at the corners of MgCl 2 crystallites. First we focused our efforts on the interaction between prototypes of three industrially relevant Lewis bases used as internal donors (1,3-diethers, alkoxysilanes and succinates) and MgCl 2 units at the corner of a MgCl 2 crystallite. Our calculations show that the energetic cost to extract MgCl 2 units at the corner of (104) edged MgCl 2 crystallites is not prohibitive, and that Lewis bases added during catalyst preparation make this process easier. After removal of one MgCl 2 unit, a short (110) stretch joining the (104) edges is formed. Adsorption of TiCl 4 on the generated vacancy originates a Ti-active species. In the second part of this manuscript, we report on the stereo- and regioselective behavior of this model of active species in the absence as well as in the presence of the three Lewis bases indicated above. Surface reconstruction due to the additional adsorption of an extra MgCl 2 layer is also considered. We show that, according to experimental data, Lewis bases coordinated in the proximity of the active Ti center confer a remarkable stereoselectivity. Moreover, surface reconstruction as well as donor coordination would improve regioselectivity by disfavoring secondary propene insertion. While still models of possible active species, our results indicate that defects, corners and surface reconstruction should be considered as possible anchoring sites for the catalytically active Ti-species. © 2012 American Chemical Society.