Oxide on Other Supports

Unlike Ziegler catalysts, chromium oxide based catalysts are extremely sensitive to minor changes in the preparation or calcining history. The active sites no doubt respond to the local electronic environment, which determines the molecular weight distribution of the polymer. Therefore, replacing the 

Alumina will also bind Cr03 and stabilize it to 900°C, and it can polymerize ethylene when reduced to Cr(II). High surface area y alumina can be made having the porosity necesssary for good activity. Besides the electronic differences between Si—O—Cr and A1—O—Cr bonds, such alumina catalysts typically have 50-100% more hydroxyl groups than silica at normal calcining temperatures. This is clear in Fig. 21, which shows the hydroxyl populations of three different supports. The hydroxyl concentration was measured by reaction with methylmagnesium iodide. 

The polymerization behavior of Cr/alumina seems to reflect the higher hydroxyl population. More surface hydroxyls also means more sites available to support chromium, and alumina does stabilize about twice as much Cr(Vl) as silica. However, the higher chromium levels do not yield a more active catalyst. Cr/alumina is typically only one tenth as active as Cr/silica. Termination rates are also extremely depressed on Cr/alumina. Both effects could be attributed to the extra hydroxyls, which are thought to interfere with polymerization. 

However, the depressed termination rate is not always a disadvantage. For example, Cr/silica is not well suited to make ultrahigh molecular weight polymer. Molecular weight can be increased by lowering the calcining temperature, reactor temperature, and porosity, etc., but even under the best conditions it is difficult to achieve an inherent viscosity (IV) much higher 

Hydroxyl population on silica, y alumina, and aluminum phosphate having similar porosity. (Measured by reaction with methylmagnesium iodide.) 

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