Thermodynamics of formation of uncovered and dimethyl ether-covered MgCl₂ crystallites. consequences in the structure of Ziegler-Natta heterogeneous catalysts

In this work, we report on the structure and formation energy of uncovered MgCl₂ crystallites of different shapes (hexagonal and square), sizes (up to crystallites composed of 157 MgCl₂ units), and edges (crystallites presenting the (104) and (110) edges). Both uncovered crystallites and crystallites covered by dimethyl ether were considered. Our results indicate that the formation energy of uncovered crystallites, irrespective of shape, size, and edges, linearly depends on the density of vacancies (measured as the ratio between the number of Mg vacancies and the number of MgCl ₂ units in the crystallite) and that larger crystallites that present (104) edges are favored. In the case of crystallites completely covered by dimethyl ether, our results indicate that the formation energy of crystallites, again irrespective of shape, size, and edges, inversely depends on the dimethyl ether/Mg ratio. As opposed to uncovered crystallites, in the presence of dimethyl ether, smaller crystallites presenting (110) edges are favored. The knowledge acquired with both uncovered and dimethyl ether-covered crystallites was used to achieve insight into the behavior of carbon monoxide-covered crystallites by performing calculations on a limited number of small crystallites.