Hydroxyl population activation temperature

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. 

Silica and aluminum phosphate have much in common. They are isoelec-tronic and isostructural, the phase diagrams being nearly identical even down to the transition temperatures. Therefore, aluminum phosphate can replace silica as a support to form an active polymerization catalyst (79,80). However, their catalytic properties are quite different, because on the surface the two supports exhibit quite different chemistries. Hydroxyl groups on A1P04 are more varied (P—OH and A1—OH) and more acidic, and of course the P=0 species has no equivalent on silica. The presence of this third species seems to reduce the hydroxyl population, as can be seen in Fig. 21, so that Cr/AP04 is somewhat more active than Cr/silica at the low calcining temperatures, and it is considerably more active than Cr/alumina. 

More reactive compounds, like chromocene, are active on silica but aluminum phosphate is often better. High calcining temperatures, such as 500-800°C, are usually preferred, probably because isolated hydroxyls remain. But this depends to some extent on the particular compound. Alumina is usually the worst choice of support, probably due to its larger hydroxyl population. 

Hydroxyl Population. All of these facts indicate a connection between the hydroxyl population on the silica surface and the catalyst s activity and relative termination rate. Figure 3 plots this decrease in the hydroxyl population. Silica, containing no chromium, was calcined at various temperatures and then reacted with CH3MgI solution. The amount of methane released was taken as an indication of the surface hydroxyl content. As the activation temperature was increased, the hydroxyl population decreased from over 4 OH/nm at 200 C to less than 1 OH/nm at 900 C. However, it never actually reached zero even at the highest temperatures studied, but was always significant compared to the coverage by chromium. 

Figure 3. Dependence of the surface hydroxyl population on activation temperature in different gases.

Whatever the mechanism, why should the behavior of the catalyst be so dependent on the activation temperature From about 400 C to 900 C, where both activity and RMIP dramatically increase, the only significant change on the silica known to occur is the declining silanol population. Perhaps these hydroxyls coordinate to the active centers, blocking ethylene and thus poisoning the catalyst much as free water would do in the reactor In some detailed experiments Krauss et.al. " have indeed found, by measuring the amount and aH of chemisorption by CO, N2, and O2, an inverse correlation between the coordinative unsaturation of Cr(II) centers and the surrounding liydroxyl population. 

Fig. 1. Steps in the formation of an olefin polymerization catalyst. Chromium is thought to bind the high-surface-area carrier by reaction with hydroxyl groups. Activation is accomplished by calcining the support at a temperature of 600° C or higher, which removes much of the excess hydroxyl group population.

The latter result was obtained from several catalytic activity studies for butene isomerization on mixed oxides dehydrated at various temperatures. Double-bond migration of 1-butene requires weak Bronsted acid sites on the surface of solid acid catalysts (with y>K values in the range from about 4 to 8 depending on oxide composition). Most oxides, especially mixed oxides, are active for this simple isomerization reaction if activated by mild dehydration. Infrared spectra show the occurrence of vast populations of surface hydroxyls on the active catalyst surface, which probably mediate the proton transfer from Bronsted acid sites on the catalyst surface to the reactant in the gas phase [140]. On the other hand, too high a dehydroxylation temperature decreases the activity, while rehydration partially restores the catalytic activity [49b,49d]. 

Silicalite is a microporous crystalline silica, i.e., an aluminium-free zeolite, belonging to the MFI-type structure and being the Si/Al oo limit of the ZSM-5 zeohte. The nature, population, and acidic properties of the hy-droxylated species (hydroxyl nests) present in the nanocavities of variously prepared defective silicahtes have been characterized by adsorption of NH3 at room temperature, monitored through the combined use of microcalorimetry and IR spectroscopy [196]. It was foimd that a perfect sihcahte sample eidiibits a very low activity towards NH3, confirming the almost complete absence of defects. The energetics of the interaction indicated that the (mild) acidity of silanols increases as far as the extension of the silanol patches increases. 

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