Cr/silica-titania

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.

Partial reduction and poisoning experiments were performed with a Cr/silica-titania activated at 600 °C. The polymerization activity is shown in the upper part of Figure 20 as a function of the reduction temperature in CO. CO treatment at 25 °C had no effect no reduction occurred and full activity was observed. The catalyst remained unchanged from its original orange color. However, when the catalyst was treated with CO at 300 °C,... 

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

FIGURE 34 Kinetics of polymerization on Cr/silica-titania when poisoned by traces of diethyl sulfide. 

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. 

Some acidic catalysts, such as Cr/AlPC>4 and Cr/silica-titania, react with ot-olefins in similar ways. That is, chain transfer may become exaggerated even though there is little reduction in the density of the polymer resulting from incorporation of the comonomer. These points are discussed in Sections 11 and 15. 

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 44 Properties of polymers made (at 95 °C and 0.7 mol 1-hexene L ) with Cr/silica-titania in the presence of various amounts of methanol showing the inhibition of comonomer incorporation by methanol. 

When the polymer becomes partially swollen, the catalyst particle size can also sometimes influence comonomer incorporation [493]. The catalyst particle size determines the polymer particle size, and when the polymer is in a partially swollen state, monomer diffusion can become more important. A typical example is illustrated by the data of Figure 46. A Cr/silica-titania was sized by screening into three narrow portions centered at about 25, 110, and 220 pm in diameter. Each portion... 

FIGURE 46 Densities of polymers made with Cr/silica-titania (650 °C) of varying particle diameters. Under conditions of swelling, larger particle size sometimes tends to encourage 1-hexene incorporation, but lower the activity. 

FIGURE 49 The influence of the mesoporosity of the catalyst on its activity and on the polymer melt index and MW. Cr/silica-titania hydrogel (450 m2g ) was dried by first replacing the pore water with a solvent of varying surface tension to produce catalysts having a variety of pore volumes that were then activated at 800 °C and tested at 105 °C. 

Data from another such series of experiments are presented in Figure 50. Again samples of Cr/silica-titania hydrogel were dried in... 

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. 

Cr/silica-titania, originally having a high pore volume, was compacted at varying pressures to reduce the pore volume and the pore size.Catalysts were activated at 700 °C and tested for polymerization activity at 105 °C. 

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

Treatment of Cr/Silica-titania hydrogel before drying Pore volume (mL g )... 

FIGURE 66 1-Hexene incorporation, as indicated by density and branching measurements, as a function of the catalyst porosity. (Cr/silica-titania, 750 °C, tested at 95 °C with 0.24 mol 1-hexene L 1.)... 

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... 

In another experiment, Cr/silica-titania was activated at 650 °C and then used to polymerize ethylene several times at temperatures varying from 100 to 110 °C—alone, and in the presence of 0.16 and 0.30 ppm of CO in the reactor. Figure 76 is a plot of the response to shear stress (F1LMI/ MI) of these polymers against the MI [407], This plot illustrates a common... 

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). 

Reaction temperature seems to have little effect on the LCB levels in the polymer, which is convenient because reaction temperature is a primary variable used to control MW in the manufacture of polyethylene. Figure 78 shows another Arnett plot representing polymers made from Cr/silica-titania activated at four different temperatures [407]. Each catalyst was tested at a series of reaction temperatures, ranging from 98 up to 110 °C, in order to vary the polymer MW. The polymers were analyzed by rheology, and when plotted on an Arnett graph, the points representing each activation temperature formed a unique line. That is, the variation of reactor temperature resulted in points distributed along a line, and each activation temperature produced a different line. 

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.)... 

TABLE 28 Changes in the Porosity of Cr/Silica-Titania that Accompany Sintering at High Activation Temperatures... 

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... 

Cr/silica-titania hydrogel, with and without aging at pH 10, 80-90 °C. Water = aqueous hydrogel placed in vacuum oven at 60 °C Organic = washed in n-propanol, then dried at 60 °C Activation 800 °C, polymerization at 105 °C, 3.8 MPa. 

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... 

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