Organochromium catalysts reaction

Organochromium Catalysts. Several commercially important catalysts utilize organ ochromium compounds. Some of them are prepared by supporting bis(triphenylsilyl)chromate on siUca or siUca-alumina in a hydrocarbon slurry followed by a treatment with alkyl aluminum compounds (41). Other catalysts are based on bis(cyclopentadienyl)chromium deposited on siUca (42). The reactions between the hydroxyl groups in siUca and the chromium compounds leave various chromium species chemically linked to the siUca surface. The productivity of supported organochromium catalysts is also high, around 8—10 kg PE/g catalyst (800—1000 kg PE/g Cr). 

Although organochromium catalysts are not well characterized, organochromium compounds are thought to bind to the support by reaction with surface hydroxyls as other types do. When Cr(allyl)3 or Cr(allyl)2 is used, propylene is released (59,60). Chromocene loses one ring (52-55), and / -stabilized alkyls of chromium lose the alkane (81). 

In contrast, the other organochromium catalysts, which terminate mainly by -elimination, produce extremely broad MWD polyethylene. In fact, the range of products is so broad that in addition to high polymers, a good portion of the product is also oligomeric. The activity is not diminished by hydrogen. Side reactions must occur easily on such catalysts because the polymers frequently are considerably branched (all types) and have some internal unsaturation as well. 

The tetravalent chromium alkyl compounds were found to give catalysts that are somewhat more active than the catalyst made from the divalent chromium counterpart, under commercial reaction conditions (90-110 °C, 0.5-1.5 mol ethylene L ). Indeed, they were among the most active organochromium catalysts tested in our laboratory. Their overall 1-h yield was usually also superior to that observed with some of the best chromium oxide on silica-titania catalysts. Even when compared with chromium oxide systems used with a cocatalyst, the catalysts made with tetravalent chromium alkyls were equal or better in activity. Unfortunately, for commercial applications, these catalysts also tend to make some oligomers and wax as well. 

Sometimes the chromium species generated by reaction of the catalyst with the cocatalyst become highly sensitive to H2 as a MW regulator, much like the organochromium catalysts. For example, an attempt was made to minimize the MI (raise the MW) by choosing a combination of catalyst and reaction variables that are all known to raise the polymer MW. A low pore volume Cr/silica was activated at only 600 °C, and the catalyst was treated with fluoride to increase the activity. It was then reduced in CO at 340 °C, again to improve activity and to lower the MI potential of the catalyst. To further lower the MI (actually the HLMI in... 

The Phillips catalyst is not alkylated when it goes into the reactor, and metal alkyl cocatalysts are not normally used. Thus, in contrast to Ziegler, Ballard, or metallocene catalysts, the Phillips catalyst has no Cr-alkyl bond into which ethylene may be inserted. Instead, the chromium somehow reacts with ethylene to generate such a bond. This characteristic is not unique, as many catalyst types also display this ability.8 This issue has been the source of much interest and speculation for half a century. On some catalysts, CO reduction is known to cleanly produce Cr(II). Reaction with ethylene could involve a formal oxidation [52,94,141,250-252,269,322-325,339-345] and many pathways involving Cr(IV) have been proposed, sometimes based on organochromium analogs, such as shown in Scheme 8 [94,250-252,315-319,321-325,342,346-349]. 

However, it is assumed that the active centers are coordinatively unsaturated Cr or Cr centers that are generated by reaction with ethylene (Eq. 8-11). It is also possible to convert the chromate deposited on the silica surface to an active form by high-temperature reduction with CO. In an alternative method of catalyst production, low-valent organochromium compoimds such as chromocene and tris( -allyl)-chromium are used as catalyst precursors. 

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