Cr/silica-titania catalysts

Fig. 16. The relative melt index potential (RMIP) of a series of cogelled Cr/silica titania catalysts rises and then falls with calcining temperature, indicating first dehydroxylation then sintering. However, the more titania in the catalyst, the more easily it sinters and therefore the lower the temperature at which peak RMIP develops.

Fig. 18. A drop in surface area marks the onset of sintering in a series of Cr/silica-titania catalysts calcined in dry air or CO. Sintering is less severe in CO.

Fig. 19. The termination rate, plotted here as relative melt index potential (RMIP), reflects the extent of surface dehydroxylation in two series of Cr/silica-titania catalysts, calcined in (Y) air or ( ) CO and then air to reoxidize the chromium, both at the temperatures shown. The third series ( ) shows the additional benefit of low-temperature attachment. It was calcined in CO at the temperatures shown, then air at a lower temperature (760°C).

Fig. 20. After being reduced at 870°C, three series of Cr/silica-titania catalysts yield highest termination rates (RMIP) after reoxidation at 600°C. Catalysts reduced in CS2 display best results because CS2 is the most effective dehydroxylating agent. Carbon monoxide is second best. Trivalent samples calcined in N2 also show the benefit of low-temperature reoxidation, but without the effect of increased dehydroxylation.

FIGURE 21 Polymerization kinetics at 110 °C of Cr/silica-titania catalysts (1 wt% Cr) activated at various temperatures. 

FIGURE 40 Melt index of polymers made at 105-109 °C with Cr/silica-titania catalyst (870 °C) in the presence of various amounts of butene isomers. 

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... 

FIGURE 50 Pore volume distribution of Cr/silica-titania catalysts dried by various methods and then activated at 800 °C. The mesoporosity of the catalyst influences its activity and the polymer melt index (tested at 105 °C with 1.5 mol C2H4 L ). 

FIGURE 58 Pore volume distributions of Cr/silica-titania catalysts that were compacted to varying degrees. 

FIGURE 59 Mw and melt indices of polymers made at 105 °C with compacted Cr/silica-titania catalysts, after activation at 700 °C. 

TABLE 23 Seemingly Contradictory Trends in the Properties of Polymers Made at Constant Ml with Cr/Silica-Titania Catalysts Indicate that LCB Levels Increased with Rising Activation Temperatures... 

Reaction variables can also affect the LCB level in the polymer. The addition of cocatalyst to the reactor to improve activity usually also makes the polymer more elastic. An example is shown in Table 25, representing experiments in which two different Cr/silica-titania catalysts were tested with and without 5 ppm of AlEt3 and BEt3 cocatalyst added to the reactor [407], The melt elasticity of the polymer is listed, as determined from the JC-a value in LCB/million carbon atoms. With each catalyst, the use of a... 

TABLE 25 Properties of Polymers Made Under Three Conditions with Two Cr/Silica-Titania Catalysts, One Alkaline-Aged and the Other Not, Showing LCB Levels Increased by Alkaline Aging and the Choice of Cocatalyst... 

FIGURE 77 Impact resistance of films as a function of the polymer melt index. The addition of poisons to the reactor affects LCB levels in the polymer. In contrast to 02, CO diminishes elasticity, which in turn results in less orientation in the blown film, and therefore improved impact and tear resistance. (Cr/silica-titania catalyst, activated at 650 °C, polymer density of 0.938 g mL, film thickness 25 pm). 

FIGURE 78 Arnett plot of polymers made with Cr/silica-titania catalysts. Raising the activation temperature from 700 to 982 °C resulted in greater deviation from the linear reference line, indicating increased LCB. However, raising the reaction temperature from 98 to 110 °C caused points to move parallel to the JC gridlines, indicating no effect on LCB. 

FIGURE 82 Response to shear stress, shown here as the polymer HLMI/MI ratio, as a function of catalyst activation temperature for polymers made in the slurry process with Cr/silica-titania catalyst. Reaction temperature was varied (102-110 °C) to produce three series of polymers of constant melt index. (Compare with Figure 83.)... 

FIGURE 86 LCB content in polymers made with aged Cr/silica-titania catalysts. Longer alkaline aging time for the hydrogel lowered the catalyst surface area and raised the polymer melt elasticity. 

TABLE 29 Influence of Alkaline Aging and the Drying Method on the LCB Level in Polymer Made with Cr/Silica-Titania Catalyst... 

FIGURE 87 LCB of polymers made with Cr/silica-titania catalyst that was physically compacted at various high pressures to lower its pore volume. Compaction increased the polymer melt elasticity, as indicated by increasing JC-a and a broadening of the relaxation time distribution (decreasing CY-a). 

FIGURE 93 Response to shear stress of polymers made with Cr/silica-titania catalyst that was sprinkled through a high-temperature flame to create localized sintering. Higher HLMI/MI ratio at a given Ml indicates increased levels of LCB. 

An example of this behavior is shown in Figure 94 [407,521], In multiple slurry polymerization rims, a Cr/silica-titania catalyst was used to make different yields of polymer. In each ran, polymerization was stopped at different times so that the polymer yield was varied from as little as 30 to more than 5000 g of polymer g 1 of catalyst. Reaction temperature and... 

Many commercial recipes exist for the manufacture of Cr/silica-titania catalysts, and they can be grouped into two general techniques, both of which have advantages and drawbacks. In the most commonly used method, a commercial silica is treated or impregnated with a reactive... 

FIGURE 96 Polymerization kinetics (105 °C) of Cr/silica-titania catalysts varying in titanium content, each activated at 650 °C. 

Figure 101 shows the GPC curves obtained from these polymers. Very low-MW polymer was produced at 400 °C, but the MW shifted to higher values as the activation temperature was raised. This trend is opposite to the normal trend expected with Cr/silica or Cr/silica-titania catalysts, in which MW declines at higher temperatures (see Figure 22). This trend indicates that the sites associated with titania are not stable at higher temperatures, and that these Cr(VI) species probably rearrange or migrate into a more stable bonding configuration that may not involve titania, or at least that inhibits its low-MW contribution.

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. 

FIGURE 107 Fluidity (inverse of melt viscosity) of polymers made with cogelled Cr/silica-titania catalysts of varying titania contents. In this plot, the fluidity (similar to melt index) is increased (lower MW) by titania, but titania also promotes sintering at high temperatures. (Ti02 listed in mol%.)... 

FIGURE 114 MW and branch distributions of polymers made with Cr/silica-titania catalyst (5 wt% Ti) activated at 700 °C (parent) followed by CO at 350 °C (CO-reduced). 

FIGURE 123 Melt index potentials of Cr/silica-titania catalysts that were activated in various gases by the R R process. Catalysts were reduced 3 h in the gas shown at the temperature shown, then reoxidized 2 h in air at the same temperature. Reduction in CS2 produced polymers of the highest melt index. 

FIGURE 128 Ml values of polymers made with R R-activated Cr/silica-titania catalysts that were reduced 3 h in CO and then reoxidized 2 h as shown. The reoxidation step is more efficient (requires lower temperatures) if done in an atmosphere of high 02 concentration. 

Fluoride ruins the MI enhancement resulting from the two-step activation of Cr/silica-titania catalysts. This tendency probably indicates that fluoride binds to surface titania to displace chromium on the more reactive sites that produce low-MW polymer. An example is shown in Table 42. A silica-titania (5 wt% Ti) was calcined at 820 °C, impregnated with 0.5 wt% Cr as bis(f-butyl) chromate in hexane, and then activated in air at 315 °C. It produced polymer with a MI of 77. It was then dry mixed with 1 wt% ammonium hexafluorosilicate and calcined again at 315 °C. When retested, it produced polymer having a MI of only 0.5, which is comparable to that of Cr/silica activated at 820 °C. This comparison suggests that fluoride displaced chromium from the titania, leaving a catalyst comparable to Cr/silica. 

It is well established that the presence of fluoride on a Cr/silica-titania catalyst increases its 1-hexene incorporation efficiency, and lowers the amount of very low-MW material [605]. Table 43 shows one example of this. Two catalysts, Cr/silica and Cr/silica-titania, were activated at 600 °C, with and without fluoride. These catalysts were tested for polymerization activity in the presence of 0.5 mol L 11-hexene. In the absence of titania, no difference was observed in the amount of 1-hexene incorporated into the polymer. However, Cr/silica-titania exhibited a major increase in 1-hexene incorporation when it contained fluoride. 

A) Two-Step Activation of Cr/Silica-Titania Catalyst (B) Then Fluoride-Treated ... 

This tendency is seemingly an advantage to polyethylene manufacturers. However, the titania tends to absorb the fluoride, perhaps selectively, to form Ti-F surface groups. Consequently, the fluoride displaces chromium from the titania. It converts the Cr/silica-titania catalyst back to one resembling Cr/silica, which is known to concentrate the branching mostly in the low-MW side of the distribution (see Figure 103). 

An example is shown in Table 44 [613]. A typical Cr/silica-titania catalyst was impregnated to incipient wetness with concentrated, aqueous solutions of the nitrate salts of the alkaline earth metals. Then ammonium hydroxide was added to precipitate the alkaline metal hydroxide within and around the pore structure of the catalyst. After a few hours of aqueous alkaline aging, the catalyst was washed and dried. 

Figure 201 shows the kinetics of polymerization with a Cr/silica-titania catalyst activated at low temperature (538 °C). Two curves are shown, representing polymerization first without a cocatalyst, and then with 10 ppm of triethylboron in the reactor [689]. The polymerization rate develops more rapidly in the latter case, which means that the average activity is much higher. 

FIGURE 201 Polymerization kinetics on Cr/silica-titania catalyst activated at 540 °C and tested with and without BEt3 cocatalyst. 

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