Revisiting the identity of δ-MgCl₂: Part II. Morphology and exposed surfaces studied by vibrational spectroscopies and DFT calculation

The activation of MgCl₂ is an essential step for preparing performant Ziegler-Natta (ZN) catalysts for olefin polymerization, either by mechanical grinding of pristine MgCl₂ or by chemical conversion of a precursor. Depending on the adopted activation process, different nanostructures are obtained in terms of disorder and morphology, which in turn influence the overall catalytic performance. In this work, we focused on the morphology of δ-MgCl₂ nanoparticles (i.e. the relative extension of the exposed surfaces), by investigating with vibrational spectroscopies a series of mechanically and chemically activated MgCl₂ samples, and by building up a comprehensive set of model structures (both ordered and disordered) and relative surfaces in order to highlight nanosizing effects and achieve an accurate description of MgCl₂ particles at an atomistic level. We found that both mechanical and chemical activation promote the expression of MgCl₂ surfaces exposing strongly acidic Mg²⁺ sites, which are those mostly involved in the catalysis, together with an increase of inter-surfaces edges, which have been recently considered as the main responsible for the ZN catalysts stereo-selectivity.