Phillips chromium catalysts ethylene polymerization

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 first chromium-based ethylene polymerization catalyst [1] was discovered by John P. Hogan and Robert L. Banks of Phillips Petroleum Company. On March 4,1958, an historic U.S.Patent 2,825,721was issued to them, and assigned to the Phillips Petroleum Company, in which an olefin... 

M.P. McDaniel, Review of Phillips chromium catalyst for ethylene polymerization, in Handbook of Transitional Metal Polymerization Catalysts, R. Hoff, and R.T. Mathers, Eds., John Wiley Sons, Inc., Chap. 10, pp. 291-446,2010. 

Figure 3.2 Two types of kinetic curves for ethylene polymerization over Phillips chromium catalysts (A) a typical hybrid kinetic curve and (B) a typical single kinetic curve.

Phillips chromium catalyst has been primarily used as ethylene polymerization catalyst in a large industrial scale. However, the same catalyst could cyclotrimerize acetylene and methylacetylene into benzene and tri-methylbenzene (TMB) rather than polyacetylene and polymethylacetylene, respectively (McDaniel, 2010). The mechanism has never been studied up to now, which also own particular academic interests in this field. Hogan and coworkers first reported alkyne cyclotrimerization catalyzed by the Phillips chromium catalyst only a few years later after they invented the Phillips chromium catalyst (Clark et al., 1959). In their report, the acetylene was found primarily cyclotrimerized into benzene, while a ratio of 0.18 of... 

Keywords Chromium Ethylene polymerization Phillips catalyst... 

The Phillips catalyst has attracted a great deal of academic and industrial research over the last 50 years. Despite continuous efforts, however, the structure of active sites on the Phillips-type polymerization systems remains controversial and the same questions have been asked since their discovery. In the 1950s, Hogan and Banks [2] claimed that the Phillips catalyst is one of the most studied and yet controversial systems . In 1985 McDaniel, in a review entitled Chromium catalysts for ethylene polymerization [4], stated we seem to be debating the same questions posed over 30 years ago, being no nearer to a common view . Nowadays, it is interesting to underline that, despite the efforts of two decades of continuous research, no unifying picture has yet been achieved. 

In 1951 Robert Banks and Paul Hogan of Phillips Petroleum discovered that ethylene could be polymerized under rather mild conditions of temperature and pressure to afford high molecular weight polyethylene using chromium trioxide as the catalyst. This invention laid the foundation for both the Phillips and Union Carbide processes for ethylene polymerization (both use heterogeneous chromium catalysts). 

The initiation of polymerizations by metal-containing catalysts broadens the synthetic possibilities significantly. In many cases it is the only useful method to polymerize certain kinds of monomers or to polymerize them in a stereospecific way. Examples for metal-containing catalysts are chromium oxide-containing catalysts (Phillips-Catalysts) for ethylene polymerization, metal organic coordination catalysts (Ziegler-Natta catalysts) for the polymerization of ethylene, a-olefins and dienes (see Sect. 3.3.1), palladium catalysts and the metallocene catalysts (see Sect. 3.3.2) that initiate not only the polymerization of (cyclo)olefins and dienes but also of some polar monomers. 

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... 

A Review of the Phillips Supported Chromium Catalyst and Its Commercial Use for Ethylene Polymerization... 

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,... 

A competitor of Phillips catalyst, based on chromium oxide supported on silica, is the Union Carbide catalyst, which is prepared by the reaction of chromocene with silica. When chromocene, [Cp2Cr ], reacts with SiO2-(800)> it gives [(=SiO)Cr(Cp)] according to mass balance analysis (Scheme 42 and Table 12), and this surface complex is highly active in ethylene polymerization. ... 

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