Periodic hybrid DFT approach (including dispersion) to MgCl ₂-supported Ziegler-Natta catalysts. 2. Model electron donor adsorption on MgCl₂ crystal surfaces

The adsorption of small probe molecules (H₂O, NH₃ and EtOH) and the small model silane Me₂Si(OMe)₂ on (104) and (110) surfaces of α-MgCl₂ have been studied using periodic DFT calculations including a classical correction (of the type f(R)/R⁶) for dispersion. The results reveal that donors strongly stabilize both crystal surfaces relative to the bulk solid. Moreover, coordination of two donor molecules to the four-coordinate exposed Mg atom of MgCl₂ (110) causes this surface to become preferred over MgCl₂ (104) surface with only a single donor per exposed Mg. However, coverage also plays an important role. The model silane preferentially adsorbs in bidentate mode on MgCl ₂ (110), provided that coverage is 0.5 or lower; at full coverage, there is not enough space for such an arrangement, and only a monodentate binding mode is obtained. Such coverage effects should be even more pronounced for the bulkier silanes used as external donors in real MgCl₂- supported Ziegler-Natta systems, as tailored experiments seem to confirm.