Bis triphenylsilyl chromate Catalyst

The catalyst based on Bis(triphenylsilyl)chromate, [(CgH5)3Si0]2Cr02, was reported by Carrick and coworkers at Union Carbide [17] and is prepared on a dehydrated (110-800 C) support material of silica/alumina or only silica. Catalysts prepared on silica require the addition of an aluminum alkyl to the catalyst preparation step to achieve high activity, while catalysts prepared on silica/alumina [19] do not require an aluminum alkyl in the preparation procedure. 

A key attribute of the Bis(triphenylsilyl)chromate catalyst deposited into silica is that this catalyst does not require a high temperatme activation (oxidation) step in the catalyst preparation. Consequently, this catalyst is activated in the polymerization reactor by the ethylene, which results in an induction time similar to the Phillips catalyst before the onset of polymerization. However, because the Bis(triphenylsilyl)chromate catalyst on silica utilizes an aluminum alkyl such as diethylaluminum ethoxide (DEAEO), it is believed that the finished catalyst does not contain Cr(VI), but contains chromium in lower oxidation states probably Cr(II), Cr(III) and/or Cr(IV). Tables 3.4 and 3.5 summarize some polymerization data for 

Silyl Chromate Pressure (Psig) Silica/ Alumina (g) Temp (°C) Time (hrs) Yield mg) Ml (12) FI/ MP Activity 

Tlow Index (FI) is melt index with 21.6 kg wt FI/MI is directly proportional to PE molecular weight distribution. 

Silyl Chromate Al Alkyl Temp CQ Time (hrs) Yield PE(g) Flow Index (FI) Activity 

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In another method of contacting catalyst and cocatalyst, the catalyst is treated with concentrated cocatalyst in a batch process. The catalyst is then dried and fed to the reactor as a single powder. This approach is most often used in the fluidized-bed process, in which a solvent is not available and in which the cocatalyst does not have high volatility. Both Cr(VI) oxide and bis(triphenylsilyl) chromate catalysts can be used as the catalyst component (see Section 3.8). Typically the latter is used, and treated with diethylaluminum ethoxide. 

Another important feature of the Bis(triphenylsilyl)chromate catalyst is the broader molecular weight distribution (MWD) of the HOPE produced with this catalyst. A comparison of the MWD of the polyethylene produced with the Bis(triphenylsilyl)chromate catalyst and the Phillips catalyst is shown in Figure 3.15 [17]. 

Figure 3.15 Comparison of the molecular weight distribution for HDPE prepared from the Bis(triphenylsilyl) chromate catalyst and the Phillips catalyst labeled as the commercial polyethylene curve. Reprinted from [17] with permission from John WUey and Sons.

This Cr-based catalyst has several characteristics that make this particular catalyst unique in comparison to the Phillips catalyst and the Bis(triphenylsilyl)chromate catalyst. These unique characteristics relative to these two other Cr-based catalysts include ... 

Garrick WL, Turbett RJ, Karol FJ, et al Ethylene polymerization with supported bis(triphenylsilyl) chromate catalysts, /Polym Sci A Polym Chem 10(9) 2609—2620, 1972. 

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). 

It was reported also that bis(triphenylsilyl) chromate [(Ph3Si)2Cr04], which is closely related to the proposed active species of the Phillips catalyst, polymerises ethylene at high pressure [238]. When supported on silica, it forms a very active catalyst for low-pressure ethylene polymerisation [226]. 

Other chromium catalysts for ethylene polymerization employ chromo-cene [246] and bis(triphenylsilyl) chromate [247] deposited on silica-alumina. The catalyst support is essential for high activity at moderate ethylene pressures (200—600 p.s.i.). The former catalyst is activated further by organo-aluminium compounds. Polymerization rates are proportional to ethylene pressure and molecular weight is lowered by raising the temperature or with hydrogen (0.1—0.5 mole fraction) in the monomer feed wide molecular weight distributions were observed. 

Supported chromium catalysts were developed by Union Carbide Corporation in the 1970s using different chromium precursors than are used in standard Phillips catalysts (6,11). The most important of these are based on chromocene and bis(triphenylsilyl)chromate, depicted in Figure 5.5. These catalysts are used in the Unipol gas phase process for LLDPE and HOPE and are different from standard Phillips catalysts in several respects ... 

Organic sources of Cr(VI) have also been investigated as the chromium source. Baker and Carrick [148] first investigated bis(triphenylsilyl) chromate as a homogeneous model for the surface chromate structures postulated to exist on the Phillips catalyst. This chromate ester is quite stable, but like Cr(VI) /silica, it can also be reduced by olefins under polymerization conditions to give the corresponding aldehyde and Cr(II) or Cr(III). Thus, it mimics the behavior of Cr(VI)/silica in many respects [149]. Bis(triphenylsilyl) chromate does catalyze ethylene polymerization,... 

Supported bis(triphenylsilyl) chromate is widely used as a low-activity substitute for chromium oxide in fluidized-bed reactors with gas-phase reactants. To generate sufficient activity, it is necessary to add an organoa-luminum compound (e.g., AlEt3 or AlEt2OEt) to reduce and alkylate the catalyst. The aluminum alkyl is usually impregnated onto the silica-supported bis(triphenylsilyl) chromate. These catalysts usually provide a broader MW distribution than simple catalysts made from chromium oxide on silica, and the two types are often contrasted with each other [150]. Elowever, catalysts made from chromium oxide on silica can be similarly impregnated with such cocatalysts (Section 17) and they then produce the same broad MW distribution [155-159]. 

FIGURE 38 MW distribution of polymers (HLMI of 10 g (10 min) density of 0.946 g mL ) made with bis(triphenylsilyl) chromate on silica treated with AlEt2OEt. Adding traces of 02 to the reactor poisoned the catalyst and broadened the MW distribution. 

Bis(triphenylsilyl)chromate, [(CgHj)3SiO]2CrO and an aluminum alkyl and the other catalyst is based on chromocene, CrlCjHj). These two Cr-based catalysts, in addition to a Phillips-type catalyst, were used in the 1960s to commercialize the gas-phase process for the manufacture of various grades of HDPE. This process was licensed by Union Carbide around the world under the trade name UNIPOL Process. 

Examination of Table 3.4 shows that the deposition of Bis(triphenylsilyl) chromate into porous silica/alumina support increased catalyst activity by at least a factor of 500. The FI/MI ratio of 86-137 indicates that the molecular weight distribution of polyethylene prepared with this catalyst is relatively very broad compared to the Phillips catalyst, which may provide significant product advantages in certain applications over a similar grade of polyethylene prepared with the Phillips catalyst. 

Examination of Table 3.5 shows that Bis(triphenylsilyl)chromate supported on silica and then treated with various aluminum alkyls is about 100 times more active than similar catalysts prepared on silica/alumina supports without an aluminum alkyl. On a chromium basis, the activity of the silica-supported catalyst is approximately 170,000 g PE/g Cr under the slurry polymerization conditions reported in Table 3.5. Under commercial polymerization conditions in which the effect of impurities is relatively less, the catalyst activity is approximately 1 x 10 g PE/g Cr, assuming a catalyst productivity of approximately 3 Kg PE/g silica-supported catalyst with the catalyst containing 0.3 wt% Cr. 

Consequently, the commercially important Bis(triphenylsilyl)chro-mate catalyst is prepared on silica in a two-step process. First, a solution of Bis(triphenylsilyl)chromate in isopentane is added to the dried silica to produce a slurry in which the Bis(triphenylsilyl)chromate is completely fixed to the silica surface. After the deposition process, an aluminum alkyl such as diethylaluminum ethoxide is added to the first intermediate and the catalyst is dried to produce a free-flowing powder. Unlike the Phillips catalyst. 

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