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

摘要

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 organoCrm 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 C-13 CP-MAS NMR, IR, and Raman spectroscopies reveal that the resulting sites contain a vinyl ligand, described as ( SiO)Cr-2(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.