Phillips catalysts, activation molecular models

A model Phillips catalyst for ethylene polymerization has been prepared by spin coating of a Cr(III) precursor (Cr(acac)3) on a flat silicon wafer (100) covered by amorphous silica. The spin coating parameters were chosen in order to obtain a homogeneous film. The model catalyst was submitted to an activation process. The surface concentration of Cr decreased from about 0.8 to 0.4 Cr atom/nm as the temperature increased from 150 to 550°C. Direct information concerning the surface molecular species and the environment of Cr was provided by ToF-SIMS and XPS. At 350°C, the catalyst precursor was decomposed Cr species were in the form oxide and surface-anchored chromates. Upon final activation at 650°C for 6 h, Cr species were below the XPS detection limit however the model catalyst was active for ethylene polymerization at 160°C and 2 bar pressure. 

An active Phillips model catalyst has been successfully prepared starting from a Cr(III) precursor. The active phase was deposited homogeneously on a silicon wafer by spin coating, and the model was submitted to the activation process usually applied to real Phillips catalysts. Using complementary surface science techniques, molecular information on the modifications of the state of the Cr during activation was provided. 

Although numerous experiments and spectroscopic characterizations have been conducted on the Phillips catalyst, the precise structure of the active site on the silica surface, reduction of the surface chromate species during the induction period, the formation of the first chromium-carbon bond, and the mechanism for ethylene polymerization still need to be further clarified [11]. In order to achieve more specific information, molecular modeling approaches could provide a useful complement to the experiments and enable us to study these obscure mechanistic problems directly at the atomic and molecular level. In the last decade, very precise mechanistic pictures of the Cr-based polymerization catalysts have been obtained using different theoretical methods, especially through a combination of the experimental findings with theoretical calculations. 

Molecular modeling. To simulate the behavior of the real heterogeneous catalyst, a reasonable molecular model must be first built to mimic the active sites anchored on the support. Figure 3.3 shows some typical molecular models for the active sites of the Phillips chromium catalyst. Espehd and Borve had done a series of systematic density functional theory (DFT) investigations on the active sites of the Phillips chromium... 

In this work, a Phillips model catalyst was prepared by spin coating of a THF solution of Cr(acac)3 on an oxidized silicon wafer. This study was mostly focused on a molecular description of the activation process in conditions similar to those used for industrial catalysts. This model catalyst was characterized by means of SEM, ToF-SIMS and XPS, and tested for ethylene pofymerization. 

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