Induction period, Phillips catalysts

Table 4 also summarizes the calculated activation barriers of all the typical reactions (in Scheme 17) during the induction period over catalyst models similar to 4g, 4g-l, and 4g-2 except that both Si atoms within each model were fully fluorinated. Fluorination of the silica support for the F-modified Phillips catalyst showed negligible influence on ethylene dimerization to 1-butene and metathesis to propylene [160], However, the energy barrier was increased significantly in reaction 5 of Scheme 17, in which 1-hexene was formed from the chromacycloheptane species through a one-step intramolecular hydrogen shift. Fluorination showed a positive effect on ring expansion in reaction 4 of Scheme 17. 

Supported CrC>3 catalysts, referred to as Phillips catalysts, are important industrial catalysts and are employed in high-density polyethylene production. Phillips catalysts polymerise ethylene with an induction period, which has been ascribed to the slow reduction of Cr(VI) by the monomer and to the displacement of oxidation products (mainly formaldehyde) from the catalytic species [226]. The prereduction of the catalyst with the use of H2 or CO enables the induction period to be eliminated. Active sites thus formed involve surface low-valence Cr(II) and Cr(III) centres, which can appear as mononuclear (formed from chromate species) and binuclear (formed from dichromate species) [227-232],... 

After activation, the catalyst is intrcxiuced into the polymerization reactor as slurry in a saturated hydrocarbon such as isobutane. The precise mechanism of initiation is not known, but is believed to involve oxidation-reduction reactions between ethylene and chromium, resulting in formation of chromium (II) which is the precursor for the active center. Polymerization is initially slow, possibly because oxidation products coordinate with (and block) active centers. Consequently, standard Phillips catalysts typically exhibit an induction period. The typical kinetic profile for a Phillips catalyst is shown in curve C of Figure 3.1. If the catalyst is pre-reduced by carbon monoxide, the induction period is not observed. Unlike Ziegler-Natta and most single site catalysts, no cocatalyst is required for standard Phillips catalysts. Molecular weight distribution of the polymer is broad because of the variety of active centers. 

Activation of the Phillips catalyst directly by ethylene monomer was further investigated by XPS and TPD-MS methods in order to shed some light on the reaction mechanisms during the induction period. Deconvolution of the XPS spectra for industrial Phillips Cr/silica catalysts treated in ethylene atmosphere at RT for 2 h revealed that surface chromium species presented in three oxidatimi states surface chromate Cr(Vl)0 c,surf species surface-stabilized trivalent Cr(III) species and surface-stabilized Cr(II) species. Compared to the original catalyst before ethylene treatment, about one-third of chromate Cr(VIX) c,surf species (i.e., ca. 22.6% of the whole surface Cr) was reduced to Cr species in lower oxidation states during the ethylene treatment, even under ambient conditions [67]. 

Scheme 6 Plausible mechanistic routes for the formation of the first hydrocarbon species, propylene, during the induction period over the non-pre-reduced Phillips Cr/silica catalyst through interaction with ethylene under various conditions...

Scheme 7 Plausible transformation of metathesis site into polymerization site from induction period to polymerization period on the Phillips catalyst...

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. 

Scheme 15 Plausible monomer reaction mechanism between ethylene and monochromate site on the Phillips Cr/silica catalyst during the induction period of ethylene polymerization...

In the absence of organometallic cocatalyst, the hexavalent chromate species on the Phillips catalyst is first reduced to a lower valence state by ethylene monomers. Experimentally, we found that the exposure of ethylene to Phillips catalyst during the induction period at RT led to the reduction of Cr(VI)0 c surf precursors to Cr(II) 0 c surf species with the simultaneous formation of formaldehyde and unsaturated hydrocarbon species, such as propylene and butene. The proposed reaction mechanisms during the induction period are shown in Scheme 15 [79]. 

In order to elucidate the proposed reaction mechanism for the Phillips catalyst during the induction period, we recently performed a theoretical investigation to study the role of formaldehyde [154]. Through extensive calculations on all the possible configurations, three kinds of stationary complexes were located and are referred to as 4g for a complex without any formaldehyde, 4g-l for a complex with one adsorbed formaldehyde, and 4g-2 with two adsorbed formaldehydes. The optimized geometries are graphically shown in Fig. 24. 

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. 

The Phillips catalyst promotes ethylene polymerization only after an induction period. Obviously, the first step in the activation of a freshly prepared Phillips catalyst is ethylene coordination to chromium. After activation, Cr-H is assumed to act as active catalyst species. However, the presence and relevance of Cr-metallacycle species in the active polymerization systems cannot be ruled out. Productivities of the Phillips catalyst are in the range of 5 kg PE per g of catalyst, resulting in a Cr-content of about 2 ppm in the polymer. 

Liu B, Nakatani H, Terano M Mechanistic imphcations of the unprecedented transformations of ethene into propene and butene over Phillips CrOx/Si02 catalyst during induction period, J Mol Catal A Chem 201(1—2) 189—197, 2003. 

Zhong L, Liu Z, Cheng R, et al Active site transformation during the induction period of ethylene polymerization over the Phillips CrOjc/Si02 catalyst, ChemCatChem 4(6) 872-881, 2012b. 

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