文章基本信息

标题：Proton transfers are key elementary steps in ethylene polymerization on isolated chromium(III) silicates
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作者：Murielle F. Delley ; Francisco Núñez-Zarur ; Matthew P. Conley 等
期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2014
卷号：111
期号：32
页码：11624-11629
DOI：10.1073/pnas.1405314111
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Mononuclear Cr(III) surface sites were synthesized from grafting [Cr(OSi(OtBu)3)3(tetrahydrofurano)2] on silica partially dehydroxylated at 700 {degrees}C, followed by a thermal treatment under vacuum, and characterized by infrared, ultraviolet-visible, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopy (XAS). These sites are highly active in ethylene polymerization to yield polyethylene with a broad molecular weight distribution, similar to that typically obtained from the Phillips catalyst. CO binding, EPR spectroscopy, and poisoning studies indicate that two different types of Cr(III) sites are present on the surface, one of which is active in polymerization. Density functional theory (DFT) calculations using cluster models show that active sites are tricoordinated Cr(III) centers and that the presence of an additional siloxane bridge coordinated to Cr leads to inactive species. From IR spectroscopy and DFT calculations, these tricoordinated Cr(III) sites initiate and regulate the polymer chain length via unique proton transfer steps in polymerization catalysis.
关键词：C–H activation ; heterogeneous catalysis