Activation step, surface chromate

The Phillips catalyst is usually made by impregnating a chromium compound onto a porous, high-surface-area silicate carrier and then calcining in dry air at 500-900° C (14) (Fig. 1). This latter step is an activation that converts the chromium into a hexavalent surface chromate or perhaps dichromate ester. Because each chromium atom is individually attached to the... 

While heat is necessary to effect the esterification, this is probably not the only purpose of the high temperature activation step. Figure 1 shows how the ethylene polymerization rate of Cr03/silica developed as the activation temperature was increased. A respectable activity did not appear until about 500 C, whereas the stabilization of Cr(VI) begins at as low as 200 and the actual esterification has been reported to occur between 150 and 300 C. Furthermore, other sources of chromium behaved similarly in activity to Figure 1 even though the particular mechanism and temperature of binding must vary somewhat. Therefore the activation step must achieve some other necessary effect in addition to formation of the surface chromate ester. 

The surface activation step iii) is optional but is designed to produce a finer particle structure. The sealing process (passivation) in step vi) historically used chromate but environmental pressures have made this an optional step. Some alternative chemical seals must be proved to be suitable for bonding. Sealing is done to improve the water... 

The last step of the activation process, 6 h at 650°C, reflecting industrial conditions, induced also modifications on the surface of the model catalyst. The Cr species were below the XPS detection limit. This can possibly be attributed to two phenomena (i) desorption due to a decrease of chromate stability, as a result of dehydroxylation and strain induced in the surface Si-0 bond surface reorganization of chromate species [45,46], forming Cr203 aggregates, which are known to be inactive versus ethylene polymerization [7]. 

If the silica is treated with fluoride prior to titanation, which converts many of the silanol groups into Si-F surface groups, the reaction with titanium alkoxide is inhibited and the treatment is less effective. The data in Table 34 illustrate this outcome. Silica samples were treated (or not) with two fluoride compounds in aqueous solution, then they were dried at 260 °C in the normal way prior to titanation. Titanium isopropoxide was added to make the catalyst contain 5 wt% Ti. Each sample was then calcined at 815 °C in air. Chromium was applied (0.5 wt%) as bis(f-butyl) chromate) in hexane solution (two-step activation, see Section 12). After final activation in air at 315 °C, each sample was tested at 102 °C, and the polymer MI values obtained are listed in the table. The change in MI shows that the titanium did not attach well to the carrier in the presence of fluoride. As more fluoride was added, the polymer MI dropped. 

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. 

The Phillips catalyst is prepared by impregnating a chromium compoxmd into a high surface area silica, such as Davison Grade 952 silica, with a pore volume of about 1.6 cc/g and a surface area of about 300 mVg. The type of chromium compound used as the chromium source does not affect the behavior of the finished catalyst after the activation (oxidation) step [4]. Chromium(III) acetate, ammonium chromate or dichromate and chromium oxide (CrOj) are possible sources of chromium. Sufficient chromium is used to yield about 0.5 tol.O wt% Cr in the final catalyst. 

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