Phillips chromium catalysts

Phillips catalysts, 17 702 20 151-152 active chromium species on, 20 156 Phillips chromium catalyst, 20 152 Phillips Petroleum loop slurry process, 20 168... 

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. 

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. 

McDaniel MP (2009) Review of Phillips chromium catalyst for ethylene polymerisation. In Hoff R, Mathers RT (eds) Handbook of transition metal catalysts. Wiley, Hoboken... 

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. 

McDaniel, M. R, Rohlfing, D. C., Benham, E. A. Long-chain branching in polyethylene from the Phillips chromium catalyst. Polym. React. Eng. (2003) 11, pp. 101-132... 

Spectroscopic investigations on activation of the Phillips chromium catalyst... 

The activation of the Phillips chromium catalyst by Al-alkyl cocatalyst was also systematically studied by XPS and solid state nuclear magnetic resonance (NMR) (Liu et al., 2005 Xia et al., 2006). As indicated by XPS characterizations, there are four oxidation states including +2, +3, +5, and +6 of surface chromium species of triethylaluminum-modified PhiUips catalyst. The observations from above experiments suggested that the active precursor of the chromium site may have the flowing form Cr -2Cr composed of one Cr(II)Ox,surf species and two Cr(VI)Ox,surf species, in which Cr(II)Ox,surf species acted as the real active chromium... 

Before slurry polymerization Phillips chromium catalyst + AIR2OR... 

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.

Scheme 3.2 Heterogeneous model catalysts for Phillips chromium catalyst.

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... 

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... 

The traditional Phillips chromium catalyst was prepared with highly toxic CrOs. Although a lot of efforts had been devoted in replacing CrOs with much less poisonous chromate(lll) acetate, the obtained catalyst after calcination was stiU in the form of highly toxic hexavalent chromate species (McDaniel, 1985, 2010). Furthermore, the chromium compounds contained in wastewater and solid dust produced during the catalyst preparation and the chromium residues in polyethylene products might cause... 

---