Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Ethylene Activation by Cr(II) and the Structure of the Resulting Active Site,ACS Catalysis

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Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Ethylene Activation by Cr(II) and the Structure of the Resulting Active Site
ACS Catalysis ( IF 11.3 ) Pub Date : 2017-09-29 00:00:00 , DOI: 10.1021/acscatal.7b02677
Carole Brown ₁ , Adrian Lita ₁ , Yuchuan Tao ₁ , Nathan Peek ₁ , Mark Crosswhite ₁ , Melissa Mileham _{2,

3} , J. Krzystek ₄ , Randall Achey ₃ , Riqiang Fu ₄ , Jasleen K. Bindra ₁ , Matthew Polinski ₁ , Youhong Wang ₅ , Lambertus J. van de Burgt ₁ , David Jeffcoat ₁ , Salvatore Profeta ₁ , A. E. Stiegman _{1,

4,

6} , Susannah L. Scott _{5,

6}

Affiliation

The structure and mechanism of the formation of sites which initiate ethylene polymerization in the atomically dispersed Phillips catalyst (Cr/SiO₂) are two of the great unsolved mysteries of heterogeneous catalysis. After CO or C₂H₄ reduction of silica-supported Cr^VI ions to Cr^II ions in the precatalyst, exposure to ethylene results in the formation of organoCr^III sites that are capable of initiating polymerization without recourse to an external alkylating cocatalyst. In this work, a Phillips catalyst prepared, via sol–gel chemistry, as a mesoporous, optically transparent monolith was reduced with CO to the spectroscopically determined Cr^II end point. Ethylene causes rapid reoxidation of these Cr^II sites to Cr^III, even at low temperatures. Solid-state ¹³C CP-MAS NMR, IR, and Raman spectroscopies reveal that the resulting sites contain a vinyl ligand, described as (≡SiO)₂Cr^III–CH═CH₂ although likely with a higher coordination number, which are capable of initiating polymerization. The formation of these vinyl sites is an incommensurate redox reaction involving one-electron oxidation of Cr^II via ethylene disproportionation. The accompanying formation of organic radical intermediates and their characteristic reaction products suggest that the key step is homolysis of a Cr–ethyl bond. Plausible pathways for the initiation mechanism are suggested.

中文翻译：

Phillips乙烯聚合催化剂中引发的机理：Cr（II）对乙烯的活化作用和所得活性位的结构

在原子分散的菲利普斯催化剂（Cr / SiO ₂）中引发乙烯聚合的部位形成的结构和机理是非均相催化的两个未解之谜。在预催化剂中将担载二氧化硅的Cr ^VI离子通过CO或C ₂ H ₄还原为Cr ^II离子后，暴露于乙烯会导致形成有机Cr ^III位，这些位能够引发聚合而无需借助外部烷基化助催化剂。在这项工作中，通过溶胶-凝胶化学法制备的作为介孔，光学透明的整体结构的菲利普斯催化剂用CO还原到光谱确定的Cr ^II终点。乙烯即使在低温下也会导致这些Cr ^II位快速重新氧化为Cr ^III。固态¹³ C ^ CP-MAS NMR，IR和拉曼光谱揭示，所得位点包含乙烯基配体，被描述为（≡SiO）₂铬^III -CH = CH ₂，虽然有可能以更高的配位数，其能够引发聚合反应。这些乙烯基位点的形成是涉及Cr ^II的单电子氧化的不适当的氧化还原反应通过乙烯歧化。伴随形成的有机自由基中间体及其特征性反应产物表明，关键步骤是Cr-乙基键的均质化。提出了引发机制的可能途径。

更新日期：2017-09-30

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