Hydroxyl population activity

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... 

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. 

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. 

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

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. 

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 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... 

Inoue, K. et al. (1997). Relationship between CYP2C9 and 2C19 genotypes and tolbutamide methyl hydroxylation and S-mephenytoin 4 -hydroxylation activities in livers of Japanese and Caucasian populations. Pharmacogenetics, 7, 103-13. 

Bacterial cell walls contain different types of negatively charged (proton-active) functional groups, such as carboxyl, hydroxyl and phosphoryl that can adsorb metal cations, and retain them by mineral nucleation. Reversed titration studies on live, inactive Shewanella putrefaciens indicate that the pH-buffering properties of these bacteria arise from the equilibrium ionization of three discrete populations of carboxyl (pKa = 5.16 0.04), phosphoryl (oKa = 7.22 0.15), and amine (/ Ka = 10.04 0.67) groups (Haas et al. 2001). These functional groups control the sorption and binding of toxic metals on bacterial cell surfaces. 

The major pathway of coumarin metabolism in most human subjects is 7-hydroxyl-ation to form 7-hydroxy coumarin, which is excreted in the urine as both glucuronic acid and sulfate conjugates. Coumarin 7-hydroxylation activity exhibits a Gaussian distribution in Caucasian populations (Cholerton et al, 1992 Rautio et al, 1992), but some individuals are deficient in this activity. 

Other mutants of CYP3A4 have also been found in a Chinese population, which were associated with a reduction in the ratio of 6- 3 hydroxyl cortisol to cortisol, an indicator of metabolic activity in vivo. 

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