Organochromium catalysts compounds

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). 

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). 

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. 

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. 

HDPE resias are produced ia industry with several classes of catalysts, ie, catalysts based on chromium oxides (Phillips), catalysts utilising organochromium compounds, catalysts based on titanium or vanadium compounds (Ziegler), and metallocene catalysts (33—35). A large number of additional catalysts have been developed by utilising transition metals such as scandium, cobalt, nickel, niobium, molybdenum, tungsten, palladium, rhodium, mthenium, lanthanides, and actinides (33—35) none of these, however, are commercially significant. 

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. 

It is incorrect to regard only one particular valence state of chromium as the only one capable of catalyzing ethylene polymerization. Active catalysts have been made from organochromium compounds with every valence from Cr(I) to Cr(IV). On the commercial Cr(VI)/silica catalyst the predominant active valence after reduction by ethylene is probably Cr(II), but other states, particularly Cr(III), may also polymerize ethylene under certain conditions. 

The valence of the starting organochromium compound has been varied from Cr(0) to Cr(IV), but seems to make little difference. All species are quite active, and all initiate polymerization rapidly in comparison to the oxide catalysts. There is no induction time, since the chromium is already reduced, and no gradual rise in rate. Polymerization usually starts immediately on contact with ethylene and either holds steady or slowly declines during a 1 hr run. 

Table 8 lists the Mo- and W-catalyzed polymerizations of acetylenes so far reported. The maximum MW s attained are shown in the Table. The polymerization behavior of each monomer will be explained in the following Sections 2.2 and 2.3. Recently it has been reported that bis(benzene)chromium, an organochromium compound, induces oligomerization of certain acetylenes 50). In the future, various Cr catalysts may be found as well as Mo and W catalysts. 

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. 

The variety of kinetics profiles shown in this section illustrates just a few of many diverse examples given. In the next section, examples are given of organochromium compounds that exhibit still further differences (immediate rate), although the catalysts were made from these same supports [63,495]. Thus, all these different behaviors demonstrate that the shape of the kinetics profiles, that is, the initial rise or the slow decay, is not caused by physical effects. The kinetics profiles are not... 

The latter examples of organochromium compounds suggest that reactivity with the support is a necessary requirement for polymerization activity, whereas the acidic character of the support may itself also contribute to the active species. The support acidity thus accounts for large differences in polymerization activity between the various carriers. Indeed, even the catalyst made by depositing dicumenechromium(O) on silica, which became active upon warming to 150 °C, never developed activity comparable to that of dicumenechromium(O) on the acidic supports. 

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... 

Chromium oxide on aluminophosphate produces polymers having a broader MW distribution than its Cr/silica counterparts, which is evidence of greater heterogeneity of Cr species on the catalyst surface. Organochromium compounds on aluminophosphate also produce polymers having broad MW distributions, and with these catalysts these same trends become unusually clear. Perhaps because the chromium tends to bind through only one link to the surface instead of two, it is often possible to obtain more detailed information about the catalyst from the resultant polymer. 

For example, data showing the performance of catalysts made from organochromium compounds are presented in Figure 189 [297,640]. Bis (2,4-dimethylpentadienyl)chromium was deposited onto each of five alu-minophosphates, each calcined at 600 °C, in which the P/Al ratio was varied from 0 (alumina) to 0.9 (just short of stoichiometric AIPO4). Each of these catalysts was used to polymerize ethylene at 90 °C. The resultant GPC curves of these polymers make it clear that two distinct peaks (corresponding to two Cr species) contribute to the MW distributions, and the ratio of the areas of the two peaks parallels the P/Al ratio in the support. 

Cr(DMPD)2 is merely one of many organochromium compounds that, when deposited on aluminophosphate supports, give polymers having a bimodal MW distribution [297,640]. This behavior is typical of catalysts made with all the organochromium compounds we have investigated, with the exception of chromocene (more in Section 16.9). Figure 190 shows the MW distribution of polymers obtained from other... 

The addition of phosphoric acid to alumina, followed by calcination at 600 °C and deposition of an organochromium compound, also resulted in polymers having a bimodal MW distribution like the polymers made from coprecipitated aluminophosphates. In one experiment, even silica was treated with H3PO4 and calcined at 250-500 °C. When this support was treated with organochromium compounds, such as Cr(DMPD)2, the resultant catalyst also produced polymers having two contributions, one of which was the same low-MW product peak associated with phosphate. The data in Figure 191 present an example of this experiment. 

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