Organochromium catalysts group

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). 

One is left to ponder initiation by other organochromium catalysts. Chromium allyls or 2,4-dimethylpentadienylchromium(II) could conceivably rearrange into p-l coordination upon addition of ethylene. However, chromocene must initiate the first chain in some other way, because the site must retain the ring. Thus, for chromocene catalysts, the initiation problem is similar to that described for chromium oxide. The diarene-chromium(O) and Cr(0)(CO)6 catalysts may also have this problem. Perhaps this is why these catalysts sometimes initiate polymerization more sluggishly than the chromium alkyls. However, there is also some evidence that the Cr(0) compounds can be oxidized by surface OH groups to leave a Cr-H group, which could also be considered an alkylated species. 

A research group at Union Carbide developed a very active, supported organochromium catalyst and the UNIPOL gas-phase process for using it to make HOPE and LLDPE [65,66], which is now widely used throughout the world. 

It is curious that during 30 years of interminable debate about valence, almost no mention has been made of organochromium compounds that also make active catalysts. As early as 1961 Walker and Czenkusch at Phillips showed that diarene-Cr(O) compounds polymerize ethylene when deposited on silica or silica-alumina (51). We now suspect that the Cr(0) is oxidized by silanol groups to Cr(I), implying that Cr(I) is also an active valence. Such catalysts, however, do not resemble Cr(VI)/silica. The kinetics and polymer obtained are entirely different. 

In the literature, most of the early discussion of the "active" valence is in reference to silica-supported chromium oxide catalysts. However, many organochromium compounds of widely differing valence are also known to be active upon contact with a support and subsequent exposure to ethylene. For example, as early as 1961, Walker et al. showed that diare-nechromium(O) compounds polymerize ethylene when deposited onto silica or another support [280,281]. The Cr(0) is probably oxidized by silanol groups to Cr(I), consistent with the inference that it too can be an active precursor. 

Therefore, organochromium compounds capable of protolysis by two neighboring silanol groups would be expected to react in this way to varying degrees when the silica was calcined at temperatures below 600 °C. These catalysts can contain both mono- and di-attached chromium... 

SCHEME 39 The similarity of polymers made with organochromium and chromium oxide catalysts suggests some common active species, perhaps like those proposed here. (R is a generic alkyl group, not necessarily the same in each species.)... 

The trimethylsilylmethyl derivative of chromium(II) can even be added to Cr(VI) oxide catalysts, to produce hybrid or "two valent" catalysts that are considerably more active than either component alone [660,682]. Presumably, in addition to reacting with silanol groups, the organochromium component also reacts with and reduces the Cr(VI) oxide to an unknown, but highly active, state. In this way, inactive Cr(VI) oxide sites can react with the organochromium compound to produce a new active site. 

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