Chromium catalyst, Phillips supported active sites

A supported catalyst for ethylene polymerization which requires no alkyl aluminum for activation was first claimed by the Phillips Petroleum Company (32). It consists of chromium oxide on silica, reduced with hydrogen. Krauss and Stach (93) showed that the active sites are Cr(II) centers. The presence of solvent, or even aluminum alkyls, diminishes... 

Second generation Phillips catalysts involve use of titanium compounds that modify the surface chemistry of the support and enables production of polyethylene with higher MI (lower MW) (12). Titanium tetraisopropoxide, also known as tetraisopropyl titanate (TIPT), is the most commonly used modifier for these catalysts. Hexavalent chromium titanate species are probably formed on the surface as shown in Figure 5.3 (13). Catalyst surfaces contain a diversity of active sites and molecular weight distribution of the polymer is broader than that from generation catalysts. 

Of the many industrial catalysts used for diverse processes, the Phillips catalyst is somewhat unique in that the active sites are not part of a supported crystallite or supported amorphous domain. Although crystallites of a-Cr203 may exist on some Phillips catalysts, they do not contribute to the activity. Instead, each site is individually bonded to the silica support. Therefore, the character of the active site is strongly influenced by the support, which is part of the coordination sphere of the chromium (a ligand), and which participates in the chemistry of polymerization. This role of the support is somewhat unlike those of the other industrial polymerization catalysts, in which silica or alumina is used mostly as just an inert carrier. 

Chromium Catalysts. The Phillips chromium catalyst, which perhaps accoimts for about half of the total HDPE production, is usually made by impregnating a chromium compoimd onto a porous, high surface area oxide carrier, such as silica, and then calcining it in dry air at 500-900°C (25). This latter activation step converts the chromium into a hexavalent surface chromate, or perhaps dichromate, ester. Because each Cr atom is individually attached to the surface, the support is not inert but exerts a strong influence on the polsrmerization behavior of the site. 

Phillips Chromox Catalyst. Impregnation of chromium oxide into porous, amorphous silica-alumina followed by calcination in dry air at 400-800°C produces a precatalyst that presumably is reduced by ethylene during an induction period to form an active polymerization catalyst (47). Other supports such as silica, alumina, and titanium-modified silicas can be used and together with physical factors such as calcination temperature will control polymer properties such as molecular weight. The precatalyst can be reduced by CO to an active state. The percent of metal sites active for polymerization, their oxidation state, and their structure are the subject of debate. These so-called chromox catalysts are highly active and have been licensed extensively by Phillips for use in a slurry loop process (Fig. 14). While most commonly used to make HDPE, they can incorporate a-olefins to make LLDPE. The molecular weight distributions of the polymers are very broad with PDI > 10. The catalysts are very sensitive to air, moisture, and polar impurities. 

It has been calculated that between 0.1 and 0.4 wt% of the total chromium forms active centers [105]. A difficult question relates to the valences of chromium in the active sites. Valences of II, III, IV, V, and VI have been established [106]. Because of the small number of total chromium atoms that are active centers, it has not been possible to unequivocally assign the active valence [107,108]. Krauss and Hums [109] concluded that the reduction of hexavalent chromium centers linked to support produced coordinately unsaturated Cr(II) surface compounds. A speciality of the Phillips catalyst is that there is no influence of hydrogen to control the molecular weight of the polyethylene. Only by higher activation temperatures can the molecular weight be lowered. 

Phillips catalysts are based on Cr(IV) supported on sUica and alumina. The true structure of the Phillips catalyst is not well understood. A mixture of chromium oxide and silicon oxide (Cr03/Si02) is used to create the active sites. The catalyst does not require addition of chemical activators before the polymerization, since the active site is produced prior to the polymerization by thermal activation at high temperatures (600°C, for instance) [21, 22]. Phillips catalysts are used in both gas-phase and slurry processes. Polyethylene made with Phillips catalysts... 

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... 

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