Supported Catalysts for Ethylene

4 Supported Catalysts for Ethylene Polymerisation - Phillips Catalysts 

The use of supports in heterogeneous catalysis was well understood by 1950 and flourished with the discovery in 1954 by Hogan and Banks [220,221] of highly active chromium trioxide catalysts supported on silica, which could 

It should be noted in this connection that hydroxyl groups are not necessarily required to be present on the SiC 2 surface, since completely dehydroxylated silica gel appears still to be an excellent support for active CrC 3-based ethylene polymerisation catalysts [225]. 

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], 

The selection and treatment of the support is fundamental to the process, and a plant may use catalysts made from a variety of supports to produce a whole range of products. Catalyst productivities are of the order of 5 kg of polyethylene per gram of catalyst or higher, with a corresponding chromium content of 2 ppm or less. The percentage of Cr atoms that form active polymerisation centres has been estimated as 12% [43]. Typically, commercial Phillips catalysts contain ca 1 % total Cr and have particle sizes of 30-150 pm [224]. 

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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... 

Montecatini has developed spherical morphology MgCl2 supported catalysts for ethylene and propylene polymerization. The name of this process is spherilene. The major advantage in such... 

A few industrial catalysts have simple compositions, but the typical catalyst is a complex composite made up of several components, illustrated schematically in Figure 9 by a catalyst for ethylene oxidation. Often it consists largely of a porous support or carrier, with the catalyticaHy active components dispersed on the support surface. For example, petroleum refining catalysts used for reforming of naphtha have about 1 wt% Pt and Re on the surface of a transition alumina such as y-Al203 that has a surface area of several hundred square meters per gram. The expensive metal is dispersed as minute particles or clusters so that a large fraction of the atoms are exposed at the surface and accessible to reactants (see Catalysts, supported). 

Fig. 9. Schematic representation of a catalyst for ethylene oxide synthesis (not to scale). The porous support particle consists of microparticles held together...

Jones and co-workers976 have recently reported the use of catalyst 49 (Figure 5.15) with perfluorinated alkanesulfonic acid sites anchored to SBA-15 as a methylaluminoxane-free supported cocatalyst for ethylene polymerization. When catalyst 49 and trimethylaluminum were used in combination with Cp 2ZrMe2 as the metallocene precatalyst, productivities as high as 1000 kg polyethylene molZr-1 h 1 were obtained without experiencing reactor fouling. 

Fig. 13. Preparation of supported alkylaluminum activated chromium catalysts for ethylene polymerization.

McDaniel, M. P., Supported Chromium Catalysts for Ethylene Polymerization , Adv. Catal., 33, 47-98 (1985). 

At the end of considerations of heterogeneous supported catalysts, mention should be made of Ziegler-Natta catalysts for ethylene and propylene polymerisations supported on carbon-containing carriers, although there is rather scant information in the literature concerning this matter. 

Data on the number of active centers, expressed as fractions of total titanium, and the relevant propagation rate constants are available for the most important MgCl2 supported and bulk catalysts for ethylene, as well as for the first and second generation catalysts for propylene (see Table 7 and 8). 

The hydrogenated HTPBs are also used as catalyst supports particularly for ethylene polymerization 184) ... 

Other chromium catalysts for ethylene polymerization employ chromo-cene [246] and bis(triphenylsilyl) chromate [247] deposited on silica-alumina. The catalyst support is essential for high activity at moderate ethylene pressures (200—600 p.s.i.). The former catalyst is activated further by organo-aluminium compounds. Polymerization rates are proportional to ethylene pressure and molecular weight is lowered by raising the temperature or with hydrogen (0.1—0.5 mole fraction) in the monomer feed wide molecular weight distributions were observed. 

This section describes in detail three topics in heterogeneous catalysis to which DFT calculations have recently been applied with great effect, the prediction of CO oxidation rates over RuO2(110), the prediction of ammonia synthesis rates by supported nanoparticles of Ru, and the DFT-based design of new selective catalysts for ethylene epoxidation. All three examples involve the careful application of DFT calculations and other appropriate theoretical methods to make quantitative predictions about the performance of heterogeneous catalysts under realistic operating conditions. 

The complex Cp2TiCl(C=CSiMe3) supported on clay minerals has been used as heterogeneous catalyst for ethylene polymerization.1069 The heterogenization of homogeneous bis-Cp catalysts for olefin polymerization in order to support these sytems for industrial applications has been studied.1499... 

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