Hydroxyl population

Several factors must be taken into account when the dispersion of iron catalysts prepared by carbonyl complexes is compared to that of conventionally prepared catalysts. The iron loading and the possible formation of irreducible iron phases (by the interaction of Fe or Fe with the support) can determine a low reduction degree for conventionally prepared catalysts with low iron content and a support with high ability to react with the iron cations. In contrast, when catalysts prepared from carbonyl complexes are considered, for a given support the temperature of pre treatment which defines the hydroxyl population of the surface is a main aspect to be taken into account. For Fe/Al203 catalysts prepared from iron carbonyls and reduced after impregnation at a moderate temperature (573 K), the extent of... 

Several researchers have measured the change in hydroxyl population on binding of the chromium. A chromate species should react with two hydroxyls per chromium, whereas dichromate displaces only one. Results... 

Fig. I. Aqueous impregnation of Cr03 on silica. Chromium lowers the hydroxyl population found on silica after calcining. The number of hydroxyls lost per chromium attached is not constant but depends on the calcining temperature.

Fig. 8. The polymerization kinetics of Cr/silica depends on the temperature of calcining, which controls the surface hydroxyl population.

All of these facts indicate a strong reverse correlation beteen the hydroxyl population on the silica surface and the catalyst activity and termination rate. Possibly these hydroxyls coordinate to the active center and kill or at least retard it. Groeneveld et al. have reported that on barely activated samples, protons from surface hydroxyls later appear in the polymer (5,9). This may be evidence of interference by hydroxyls. Or perhaps the hydroxyls are not directly involved at all, but merely reflect some other important change such as the strain introduced onto the surface by their condensation. Whatever the reason, this relationship is used commercially to control MW and many other important polymer properties. 

As noted in Section III,E the hydroxyl population on the catalyst seems to exert a powerful influence on the activity and melt index potential, which are... 

Fig. 17. The hydroxyl population on silica depends on the temperature and atmosphere during calcining. Reducing agents like CO and carbon-sulphur compounds tend to depress the OH level further.

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. 

Fic,. 21. Hydroxyl population on silica, y alumina, and aluminum phosphate having similar porosity. (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. 

Figure 5.22 Hydroxyl population with temperature of silica gel Kieselgel 60. aOH + free OH bridged OH (Gillis-D Hamers model).

The activity and termination rate of the Phillips Cr/silica polymerization catalyst have been examined in relation to the surface hydroxyl population. 

The chromium loading is always low enough that only a few of these hydroxyls are occupied by chromium. The others remain as a background hydroxyl population, some of which condense during activa-... 

Therefore in this paper we have tried to determine if these hydroxyls could be directly involved in the polymerization. The hydroxyl population has been studied under various activation conditions to see how it correlates with the overall activity of the catalyst and also with the termination-initiation rate. 

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. 

A further decrease 1n the hydroxyl population was obtained when the silica was calcined, not in air or nitrogen, but in carbon monoxide. This is also shown in Figure 3. The curve representing the OH population in carbon monoxide split away from that in air at 600 C, and the separation increased up to sintering at 925 C. We believe that this is primarily due to a water gas shift mechanism in which surface moisture is removed by conversion to CO2 and H2- The CO could act through a direct attack on... 

Even more effective than carbon monoxide for removal of hydroxyl groups, was sulfur in the presence of carbon monoxide. Figure 3 also plots the hydroxyl population on silicas treated with C 2 vapor and carbon monoxide. The OH level separates from that of the air treated samples at as low as 400 C, and by 950 C very few hydroxyls remained. The exact mode of action of the sulfur in these experiments is unknown, but one possibility is that it somehow affects the water gas shift equilibrium. Sulfur was not detected on these samples. 

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

Halides. Another treatment which can lower the hydroxyl population, or even eliminate it altogether, is halogenation of the silica surface.This removes hydroxyls, not by condensation as with CO and sulfur, but by replacement with halide, which prevents later attachment by Cr. The presence of halide probably also changes the electronic environment on the silica. Thus, fluoriding has long been used to increase activity but decrease RMip.i l Chloride also depresses RMIP, as well as the surface bromide and iodide of silica. However, these latter two have recently been studied, and it was possible to burn off most of the iodide or bromide with oxygen above 600 C, leaving a partially dehydroxylated surface. 

In this section, we wiU consider the hydroxyl population on some materials of practical interest including oxides, zeolites, and porous phosphates, which have found application as catalysts and catalyst supports as well as metal organic frameworks. 

There are many investigations of the hydroxyl population on various aluminas because of the extraordinary importance of this oxide in catalysis. Noticeably, the hydroxyl spectra hardly depend on the polymorph form, although variations in the relative intensities of the OH bands are observed even with a single polymorph. Compared to oxides like MgO or Zr02,... 

Data regarding the hydroxyl population on mtile are less abundant and controversial. It appears that, for this titania modification, the sample morphology very strongly affects the surface properties. Various OH groups were reported 3718, 3694, and 3673 cm (270) 3685, 3655, and 3410 cm (678) 3650 and 3410 cm (682) and 3670 cm (683). A model of the mtile surface considering the (110), (101), and (100) planes was proposed more than 40 years ago (682). 

In conclusion, partially dehydroxylated oxide surfaces exhibit a large inventory of surface OH groups and water molecules together with Lewis acidic and Lewis basic sites with coordinative unsaturation (structures II and III of Scheme 1). The hydroxyl population is the souree of protons that cause enhanced surface electrical conductivity and catalytic activity. It is significant that the increase in the conductivity value is paralleled by increases in either the amount of weakly bound protons or their mobility [48]. Almost all metal oxides are active in catalytic isomerization of alkenes, which is one of the least demanding reactions in terms of the requirements for the acid strength of active sites [34]. Studies on several oxide systems show that the activity is lost after extensive dehydration and is partially restored by... 

In order to reduce the deactivating interaction between the coordination catalyst and the clay surface, Huang and coworkers [112] proposed an indirect supporting method in which a common support, such as MgCl2 or Si02, is deposited onto the clay surface to increase the hydroxyl population on the clay surface where the loading of active catalyst occurs. It is well known that MgCl2 dissolves in alcohols to form... 

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