Periodic Hybrid DFT Approach (Including Dispersion) to MgCl2-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 MgCl2 (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