Department of Chemistry and Applied Biosciences, Eidgenössische Technische Hochschule Zürich, CH-8093 Zürich, Switzerland;
Laboratoire de Chimie Catalyse Polymères et Procédés, Unité Mixte de Recherche 5265, Université de Lyon, 69616 Villeurbanne, France; and.
Proc Natl Acad Sci U S A. 2014 Aug 12;111(32):11624-9. doi: 10.1073/pnas.1405314111. Epub 2014 Jul 7.
Mononuclear Cr(III) surface sites were synthesized from grafting [Cr(OSi(O(t)Bu)3)3(tetrahydrofurano)2] on silica partially dehydroxylated at 700 °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.
单核 Cr(III) 表面位是通过在 700°C 下部分脱羟的二氧化硅上接枝[Cr(OSi(O(t)Bu)3)3(tetrahydrofurano)2]合成的，然后在真空中进行热处理，并通过红外、紫外-可见、电子顺磁共振(EPR)和 X 射线吸收光谱(XAS)进行表征。这些活性位在乙烯聚合中非常活跃，可得到具有较宽分子量分布的聚乙烯，类似于通常从菲利普斯催化剂获得的聚乙烯。CO 结合、EPR 光谱和中毒研究表明，表面上存在两种不同类型的 Cr(III)位，其中一种在聚合中具有活性。使用团簇模型的密度泛函理论(DFT)计算表明，活性位是三配位的 Cr(III)中心，而与 Cr 配位的额外硅氧烷桥的存在导致非活性物种。从红外光谱和 DFT 计算可以看出，这些三配位的 Cr(III)位通过聚合催化中独特的质子转移步骤引发并调节聚合物链长。
