Phillips chromium/silica polymerization catalyst

The Phillips Cr/silica polymerization catalyst is prepared by impregnating a chromium compound onto a wide pore silica and then calcining in oxygen to activate the catalyst. This leaves the chromium in the hexavalent state, monodispersed on the silica surface. Chromium trioxide (Cr03) has been impregnated mast commonly, but even a trivalent chromium salt can be used since oxidation to Cr(VI) occurs during calcining. 

The Phillips Cr/silica catalyst is prepared by impregnating a chromium compound (commonly chromic acid) onto a support material, most commonly a wide-pore silica, and then calcining in oxygen at 923 K. In the industrial process, the formation of the propagation centers takes place by reductive interaction of Cr(VI) with the monomer (ethylene) at about 423 K [4]. This feature makes the Phillips catalyst unique among all the olefin polymerization catalysts, but also the most controversial one [17]. 

Therefore, in such heterogeneous polymerizations, almost all industrial catalysts are supported, for example on silica, whereas the typical Ziegler s titanium catalysts are by definition supported on magnesium chloride. These catalysts are adsorbed at the surface or incorporated into the crystal structure of the support. Other catalysts, such as Phillips chromium catalysts, can be coupled at the support surface by a chemical bond. 

Chromocene (CrCp2) supported on silica is used to generate certain chromium-based catalysts for the polymerization of ethylene (e.g., Phillips and Union Carbide catalysts). The nature of the organometallic species responsible for the catalysis is not known with certainty, though it is noteworthy that some Crm alkyls such as [Cp Cr(CH2Ph)(THF)2]+BPh catalyze the polymerization of ethylene.19... 

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. 

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. 

Supported metal complex catalysts for alkene polymerization. Supported chromium complexes on silica have been used for many years in the Phillips process for ethylene polymerization, and promoters are not required. Like these supported complexes, the classical TiClj Ziegler polymerization catalysts have also long been viewed as presenting surface catalytic sites that are well described as molecular analogues. 

The Phillips catalyst is prepared from relatively inexpensive chromium salts it is robust, but structurally complex, and the catalytic sites are not identified. To make a structurally simpler silica-supported alkene polymerization catalyst, Ajjouet al. used the precursor bis(neopentyl)chromium(IV). The synthesis chemistry was represented as follows ... 

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. 

Other Early Developments. In addition to the breakthrough by Ziegler, two other discoveries of ethylene polymerization catalysts were made in the early 1950s. A patent by Standard Oil of Indiana, filed in 1951, disclosed reduced molybdenum oxide or cobalt molybdate on alumina (13). At the same time, Phillips discovered supported chromium oxide catalysts, prepared by impregnation of a silica-alumina support with Cr03 (14 16). Both the Phillips catalyst and titanium chloride based Ziegler catalysts are widely used in the production of high density polyethylene (HDPE). 

Another type of olefin polymerization catalyst system, often referred to as a Phillips type catalyst, is based upon chromium trioxide, typically impregnated on a solid support such as silica or alumina [4]. 

The Nobel Prize in Chemistry was awarded to Karl Ziegler and Giulio Natta in 1963 for their research in developing olefin polymerization catalysts primarily based on titanium compounds and aluminum alkyl compounds required for the catalyst initiation process. However, by 1963 the details on the discovery of the Cr-based catalyst system in which the chromium compound was supported on amorphous silica were widely published and commercially important for the manufacture of HDPE. In the view of this author, the 1963 Nobel Prize awarded in chemistry should also have included two additional scientists, John P. Hogan and Robert L. Banks from Phillips Petroleum. 

Chromium trioxide based catalysts supported on silica were developed by Phillips Petroleum at the same time as the original work of Ziegler and Natta. These catalysts polymerize non-polar olefins by mechanisms which are similar to those involved in Ziegler-Natta polymerization but do not give such good stereochemical control and are used principally for the preparation of linear polyethylene. More recently, supported catalysts of very high activity for the polymerization of ethylene have been prepared from chromates and also from chromacene. 

Qiu P, Li X, Zhang S, et al Supporting mechanism of non-toxic chromium(III) acetate on silica for preparation of Phillips ethylene polymerization catalysts, Asia Pac J Chem Eng 4(5) 660-665, 2009. 

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... 

The original recipe involved the aqueous impregnation of chromic acid on silica, although nowadays less-poisonous chromium(III) salts are used. Over the years, a family of Phillips-type catalysts has emerged producing no less than 50 different types of polyethylene, and this versatility is the reason for the commercial success of the Phillips ethylene polymerization process. The properties of the desired polymer product can be tailored by varying parameters such as calcination temperature, polymerization temperature and pressure, by adding titania as... 

Commercial linear polyethylene, the most commonly used type of plastic, was bom more than half a century ago with the accidental discovery at Phillips Petroleum Company that chromium oxide supported on silica can polymerize a-olefins.1 The same catalyst system, modified and evolved, is used even today by dozens of companies throughout the world, and it accounts for a large share of the world s high-density polyethylene (HDPE) supply, as well as some low-density polymers. The catalyst is now more active and has been tailored in numerous ways for many specialized modem applications. This chapter provides a review of our understanding of the complex chemistry associated with this catalyst system, and it also provides examples of how the chemistry has been exploited commercially. It is written from an industrial perspective, drawing especially on the commercial experience and the research of numerous scientists working at Phillips Petroleum... 

Organic sources of Cr(VI) have also been investigated as the chromium source. Baker and Carrick [148] first investigated bis(triphenylsilyl) chromate as a homogeneous model for the surface chromate structures postulated to exist on the Phillips catalyst. This chromate ester is quite stable, but like Cr(VI) /silica, it can also be reduced by olefins under polymerization conditions to give the corresponding aldehyde and Cr(II) or Cr(III). Thus, it mimics the behavior of Cr(VI)/silica in many respects [149]. Bis(triphenylsilyl) chromate does catalyze ethylene polymerization,... 

Another example of promotion by an added metal oxide is Cr/silica incorporating Sn(IV) ions [548,594], Like TiC>2, SnC>2 contains a tetravalent metal ion that can exist in tetrahedral coordination, and has a similar ionic radius. Indeed, SnC>2 and T1O2 are isomorphous. Mixed oxides of SnC>2 and SiC>2 are known to exhibit acidity [595-597], Figure 131 shows the result of adding SnC>2 to the Phillips catalyst. Silica was dried at 200 °C and then treated with an excess of SnCLi vapor. The support was then calcined at 500 °C to remove chloride. It was impregnated anhydrously with chromium and then activated at 500 °C in air. It was quite active in polymerization tests at 105 °C, and the MW distribution of tire polymer is shown in Figure 131. 

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