Periodic DFT and high-resolution magic-angle-spinning (HR-MAS) 1H NMR investigation of the active surfaces of MgCl2-supported Ziegler-Natta catalysts.: The MgCl2 matrix

Classical MgCl2-supported Ziegler-Natta catalysts (ZNCs) continue to dominate the industrial production of isotactic polypropylene. There is a growing awareness of the inherent competitive edge of these low-cost systems over single-center (primarily metallocene) catalysts and of the potential for further improvement, particularly if deeper insight into the structure of the catalytic surfaces and the mechanisms of their modification by means of electron donors can be achieved. In the framework of a project ultimately aiming at the implementation of ZNCs with known and controlled surface Structures, we are revisiting this whole area by using a combination of advanced computational (periodic DFT) and spectroscopic (high-resolution magic-angle-spinning H-1 NMR spectroscopy) tools. In this article, we report on the neat MgCl2 matrix and on model MgCl2/electron-donor adducts. The results indicate that the (104) surface, with five-coordinate Mg cations, is the dominant lateral termination in well-formed large crystals, as well as in highly activated MgCl2 samples prepared by ball-milling. In the latter case, a minor fraction of surface Mg sites with a higher extent of coordinative unsaturation [e.g., four-coordinate Mg cations on (110) edges and/or at crystal corners or other defective locations] also appear to be present. RMe2Si(OMe) (R = octadecyl) binds to both types of Mg sites, albeit with different strengths resulting in different mobilities. The less-electron-donating RMeSi-(OMe)(2), in contrast, binds to the more unsaturated Mg sites only. The approach described herein is Currently being extended to MgCl2/TiCn systems, as well as to their adductS with internal and external donors of different natures, strengths, and steric demands.