Organochromium catalysts

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). 

Organochloroaluminate ionic liquids as catalysts, 42 495-496 Organochromium catalysis attachment to support, 33 92-93 kinetics of polymerization, 33 93 support effects, 33 94-95 termination mechanism, 33 93-94 Organochromium catalysts, 33 58, 92-95... 

Many other organochromium compounds have since been synthesized and found to be active, including those with chromium exhibiting every valence up to Cr (IV). Chromocene is a well-studied example of an active divalent compound (52-55). Pentadiene-Cr(II) (56) is another, along with allyl-Cr(II) (52, 57). Allyl-Cr(III) is also active (52, 57-61). -Stabilized alkyls of Cr(II) and Cr(IV) such as trimethylsilylmethyl-Cr(IV), which also polymerizes ethylene when supported on an oxide carrier, have been synthesized and tested in this laboratory (57,62). All these organochromium catalysts are comparable in activity to the Cr(VI)/silica standard. 

A number of organochromium compounds also form highly active polymerization catalysts when deposited on an oxide carrier. Usually the carrier does play an essential role, because without it such compounds rarely exhibit any activity. In most respects the organochromium catalysts are quite different from their oxide counterparts. 

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 active site concentration on the organochromium catalysts may be higher than that of the oxide catalysts. The activity usually assumes a more linear increase with chromium loading than on the oxide catalysts, at least up to 2% Cr. Yermakov and Zakharov, studying allyl-Cr(III)/silica catalysts, stopped the polymerization with radioactive methanol, and found that the kill mechanism is different from that on the oxide catalysts (59). The proton of the methanol, and not the alkoxide, became attached to the polymer. This suggests a polarity opposite to that of the oxide catalysts, with the site being more positive than the chain. 

Although most of these organochromium catalysts are comparable in activity to the Cr(VI)/silica standard, they do not resemble Cr(VI)/silica in every behavior. The kinetics of olefin polymerization are understandably quite different, and more importantly, the polymer obtained is also not the same. These differences suggest that organic ligands still exist on... 

Some of the earliest recorded investigations of organochromium catalysts were conducted in our laboratory around 1960 with diarenechro-mium(O) compounds on silica-alumina [280,281]. Another early example came from workers at Union Carbide [301-304,306-310,642], who developed chromocene on silica as a commercial catalyst, which became a well-studied example of an active divalent compound. The so-called... 

Although there are many differences between chromium oxide catalysts and the organochromium catalysts, when they are bonded to the support, organochromium catalysts usually display a similar, but exaggerated, MW response in the polymer produced relative to what is observed with chromium oxide catalysts. For example, the MW of polymer produced with each type of catalyst usually decreased as the support calcination temperature was raised. Similarly, when both chromium oxide and the organochromium compounds were deposited onto aluminophosphate supports, they always yielded lower-MW polymer as the amount of phosphate in the support was raised. 

Furthermore, the polymers produced by the two catalyst types are also different. Indeed, in terms of polymer properties, the organochromium catalysts can be subdivided into two major classes based on their behavior—(a) chromocene and some of its derivatives, and (b) all others. These properties are summarized in Table 54. 

Chromocene catalysts are not very sensitive to the choice of support used. They tend to produce polymers having the same narrow MW distribution. All these characteristics are different from those of the other organochromium catalysts and of chromium oxide catalysts. They are attributed to the influence of the remaining Cp ligand, which probably provides a more crowded and electron-rich environment than is formed on the other catalysts. 

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. 

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. 

Apparently nearly every organochromium catalyst, even those supported on silica treated at 250 °C, contain some amount of the mono-attached chromium species, because a small quantity of the low-MW... 

One of the unique characteristics of most organochromium catalysts is a production of a-olefins, either as the main product or the only product, or in a low yield along with the high-MW polymer. This production of olefins was attributed in Section 16.6 to a mono-attached chromium species (probably of very low coordination number). 

Although there are many similarities between the behavior with cocatalysts and the behavior with organochromium catalysts, including the sometimes intense activity enhancement, use of a cocatalyst with Cr(VI) / silica is not quite equivalent to the use of organochromium catalysts, which usually become active immediately, with full rate instantaneously developed. The organochromium catalysts are usually already reduced and alkylated before entry into the reactor. 

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 initial temperature of catalyst activation can also influence the amount of in situ branching obtained in the polymer. This is in agreement with the olefin-generating behavior of the organochromium catalysts (Figures 185 and 192, Table 55). Table 67 shows an experiment in which Cr/silica-titania was activated at 800 °C or at 650 °C, and then it was reduced and tested for polymerization activity with 5 ppm triethylboron cocatalyst. The 800 °C catalyst resulted in significantly lower polymer density than the 650 °C catalyst. This derives from two causes. The 800 °C... 

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