Periodic DFT modeling of bulk and surface properties of MgCl2

Abstract

MgCl₂ is the preferred support for the industrial Ziegler–Natta catalysts, and is believed to act as a template for the epitactic chemisorption of the active Ti species. As the first step of a thorough computational modeling of these systems, we studied the bulk and surface structure of the ordered α and β phases of MgCl₂ by means of periodic DFT (B3LYP) methods using localized basis sets. The layer structure of both phases was reproduced satisfactorily with the inclusion of a (small) empirical dispersion correction (“DFT-D”) as a practical method to describe the attraction between the layers. Surface models were studied on slabs with adequate thickness. It appears that various surfaces exposing 5-coordinated Mg are very similar in energy and are the lowest non-trivial surfaces. Cuts exposing 4-coordinated Mg are significantly less stable; both kinetic and equilibrium models of crystal growth indicate that they should normally not be formed to a significant extent. “Nano-ribbons” of single, flat chains of MgCl₂, sometimes proposed as components of the disordered δ phase, were also evaluated, but are predicted to be unstable to rearrangement. Implications for the role of MgCl₂ as catalyst support are discussed.