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

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• 作者：Vincenzo Busico ; Mauro Causà ; Roberta Cipullo ; Raffaele Credendino ; Francesco Cutillo ; Nic Friederichs ; Raffaele Lamanna ; Annalaura Segre ; Valeria Van Axel Castelli
• 刊名：Journal of Physical Chemistry C
• 出版年：2008
• 出版时间：January 31, 2008
• 年：2008
• 卷：112
• 期：4
• 页码：1081 - 1089
• 全文大小：528K
• 年卷期：v.112,no.4(January 31, 2008)
• ISSN：1932-7455

文摘

"Classical" MgCl₂-supported Ziegler-Natta catalysts (ZNCs) continue to dominate the industrial productionof isotactic polypropylene. There is a growing awareness of the inherent competitive edge of these low-costsystems 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 modificationby means of electron donors can be achieved. In the framework of a project ultimately aiming at theimplementation of ZNCs with known and controlled surface structures, we are revisiting this whole area byusing a combination of advanced computational (periodic DFT) and spectroscopic (high-resolution magic-angle-spinning ¹H NMR spectroscopy) tools. In this article, we report on the neat MgCl₂ matrix and onmodel MgCl₂/electron-donor adducts. The results indicate that the (104) surface, with five-coordinate Mgcations, is the dominant lateral termination in well-formed large crystals, as well as in highly activated MgCl₂samples prepared by ball-milling. In the latter case, a minor fraction of surface Mg sites with a higher extentof coordinative unsaturation [e.g., four-coordinate Mg cations on (110) edges and/or at crystal corners orother defective locations] also appear to be present. RMe₂Si(OMe) (R = octadecyl) binds to both types ofMg 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 currentlybeing extended to MgCl₂/TiCl_n systems, as well as to their adducts with internal and external donors ofdifferent natures, strengths, and steric demands.