Titania site distribution

FIGURE 105 Three-dimensional plots representing the minimum number of Schulz-Flory site distributions needed to reproduce the MW distributions of polymers made with the two low-temperature activated catalysts in Table 35. On the left (Cr/silica), about half of the branches are in an island at low MW and high branch concentration, which has been eliminated on the right (Cr/silica-titania). Adapted with permission from Ref. [435]. 

As noted in Section 6.9, when added to the reactor in trace amounts, some polar compounds can selectively inhibit some sites in preference to others. In addition to affecting the MW distribution, they sometimes have an influence on the branch content and distribution as well. An example of this behavior is shown in Figure 44. Ethylene-hexene copolymers were produced with a Cr/silica-titania catalyst activated at 850 °C. Methanol was added to the polymerization reactor in incrementally greater amounts to "titrate" the activity of the catalyst. As expected, the activity declined as methanol was added, reaching nearly zero at about three CH3OH molecules/Cr atom. The average polymer MW increased as methanol was added. The results imply that some sites, those more... 

The presence of titania on Cr/silica also has a strong and beneficial effect on the polymer s MW distribution. Titania tends to broaden the MW distribution by contributing large amounts of low-MW material, which, as already noted, tends to lower the overall average MW. An example is shown in Figure 99. The addition of titania generates a new class of active sites that terminate chains easily and thus produce mainly low-MW polymer. 

This inhibition is actually a useful trait. As noted in Section 7.6, Cr/ silica tends to concentrate branching into the low-MW part of the MW distribution, which can impair some polymer properties. Because the sites associated with titania tend to produce low-MW polymers [435], inhibition of comonomer incorporation by these sites tends to place proportionally more of the branching into the other (higher-MW) regions of the MW distribution for a given density. Thus, Cr/silica-titania catalysts tend to produce better polymer properties than Cr/silica, because they create first, a broader MW distribution with more long chains, and second, a flatter branch profile, in which the long chains receive more of the branches. 

The data of Table 35 indicate that the sites producing low-MW polymer on Cr/silica-titania are not the same after activation at 500 °C as after activation at 900 °C. After activation at 900 °C, they are more like those on Cr/silica. As in Figure 101, this again suggests that the sites associated with titania are not thermally stable. Upon heating they are redistributed (perhaps to silica) and lose their unique character. In this example, the unique characteristic of titania is the rejection of branches in the low-MW side of the distribution. It is this trait, few branches at the low-MW side of the distribution, that makes Cr/silica-titania such a valuable commercial catalyst. 

The data of Figure 112 provide an example, in which a Cr/silica-titania (5 wt% Ti) was activated at 550 °C and then treated in CO at 100 °C. After the catalyst was cooled in CO, copolymers were made with the catalyst at 95 °C. The MW distribution is shown of copolymers obtained with the catalyst before and after CO treatment at 100 °C. The CO treatment of the catalyst caused a loss of the low-MW part of the polymer MW distribution, resulting in considerable narrowing. This finding is another confirmation that the low-MW titania-associated sites are most easily reduced. 

FIGURE 112 Change in MW distribution of polymer made with Cr/silica-titania that was partially reduced in CO at only 100 °C. The reduced sites were then poisoned by exposure to CO at 25 °C, leaving only the unreduced sites to produce polymer. (Tested at 95 °C with 0.50 mol 1-hexene L-1). 

A similar experiment was conducted without 1-hexene added to the reactor and those results are also listed in Table 36. The same effect on MW distribution was observed. The incorporation of 1-hexene increased significantly when the titania-associated sites were selectively poisoned. This is another indication that titania inhibits branching in the low-MW side. Table 36 shows the drop in the polymer density, and the rise in the amount of 1-hexene incorporated, when the catalyst was treated in CO at only 100 °C. 

Most cocatalysts continue to influence the polymer MW distribution even after the optimum ratio of cocatalyst to chromium (maximum activity) has been reached. This behavior indicates that some of the lower pathways in Scheme 45 also contribute. Either sites are changed, or even destroyed, by attack on the Cr-O-Si bonds, or the alkyl exchange reaction occurs. This latter pathway tends to enhance the low-MW side of the MW distribution by introducing another chain transfer mechanism. Only some very specific, perhaps the most acidic, sites are probably involved, such as those sites producing low-MW polymer on Cr/silica-titania or Cr/AIPO4. These sites already have a tendency to produce low-MW polymer [52,332,681],... 

The broadening of the MW distribution on the low-MW side is not as prominent when the Cr/silica-titania catalyst is calcined at 800-900 °C. Neither is it observed much when the catalyst is Cr/silica, containing no titania. The effect is observed mainly when some low-MW, acidic, Ti-associated sites are present. 

Cr/silica-titania, activated at various temperatures from 426 to 871 °C, was tested with and without 8 ppm of BEt3. In each case, the addition of BEt3 broadened the MW distribution. However, the broadening on both sides, especially the low-MW side, diminished as the activation temperature was raised. This result may again reflect the loss at higher temperatures of those more reactive sites associated with titanium. 

In some cases, the cobalt precursor tends to interact with the support. This interaction impedes the generation of active cobalt sites during reduction. Normally, it leaves a fraction of the cobalt chemically inactive. According to Jacobs et al. [1], the strength of the interaction for the three most common supports follows the order y-alumina > titania > silica. The presence of a promoter such as rhenium facilitates reduction of cobalt species interacting with the support [2-4]. However, cobalt is usually not completely reduced after the normal reduction procedures. The effect of rhenium for Fischer-Tropsch synthesis selectivity was recently described in detail by Storsaeter et al. [5]. It was concluded that presence of rhenium shifts the product distribution to heavier compounds, quantified by the C5+ selectivity. 

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