Cocatalysts activity improvement

The use of polystannoxane catalysts with improved cure performance over dibutyltin or other stan-noxane compounds has been claimed with a range of blocked isocyanates iron, zinc, zirconium, nickel, and bismuth salts are claimed as cocatalysts. The improved catalytic activity appears, to some extent, related to increased solubility of the polystannoxanes. 

Cr/aluminophosphate catalysts behave in a similar way. Figure 203 shows the activity improvement obtained with several catalysts with various P/Al ratios. BEt3 is a particularly effective cocatalyst for enhancement of the activity of such catalysts. The data of Figure 203 show that... 

FIGURE 203 Activity improvement of Cr/aluminophosphate catalysts upon addition of 5 ppm BEt3 cocatalyst, as a function of the activation temperature. 

Thus, one might expect the effect of metal alkyl cocatalysts to be less pronounced under high-temperature solution reaction conditions. Nevertheless, polymerizations carried out in solution at higher temperatures, as illustrated by the data of Figure 206, also exhibit an improvement in activity, although the rise in rate is not as great as that in observed slurry polymerizations. The difference is perhaps another indication that cocatalysts activate sites that would not otherwise become active. 

The deposition of noble metals (e.g.. Ft, Rh) or metal oxides (e.g., NiO, RUO2) onto photocatalyst surfaces is an effective way of enhancing photocatalyst activity (Sato and White, 1980 Subramanian et al., 2001). The cocatalyst improves the efficiency of photocatalysts, as shovm in Figure 15, as a result of (i) the capture of CB electrons or VB holes from the photocatalysts (Maruthamuthu and Ashokkumar, 1988), thereby reducing the possibility of electron-hole recombination and (ii) the transference... 

The addition of a third substance must therefore be viewed as a means to improve both on the rate and the continuity of the initiation reaction, but not as it was believed formerly, to determine its occurrence. Of course, the presence of a cocatalyst alters substantially the mechanism through which the active species are generated, but nonetheless in all the systems which will be discussed below, the Lewis acid plays the dominant function of catalyst, without which initiation is impossible. 

The expected cycloaddition of ,[i-unsaturated add chloride and chloral indeed occurred in the presence oftertiary amine. The yields could be significantly improved by adding a Lewis acid cocatalyst, Sn(OTf)2, which would facilitate the deprotonation of acid chloride and activate the aldehyde substrate. More satisfactory results could be obtained when acid chloride was added slowly by syringe pump to avoid massive... 

The fact that recombination of electrons with free holes is about ten times faster than hole trapping is disadvantageous if TiOi is to be used for light-induced water splitting. This also rationalises earlier observations that highly active surface cocatalysts (e.g. Pt and RUO2) are needed on the titania particle surface to improve the... 

Particular interest has been shown in the catalytic oxidation of polyols with dioxygen using supported platinum-group metals as the catalysts. The most studied metals have been palladium and platinum which are, however, often affected by deactivation problems [2]. The introduction of cocatalysts such as bismuth or lead represents an enhancement in the use of these catalysts having the double effect of Increasing catalytic activity and improving catalyst life [3]. 

The behavior shown in Figure 28, a leveling of catalyst activity with increased chromium loading, could be viewed as a consequence of mass transport limitations. However, the activity can still be increased or decreased according to other preparation variables and reaction conditions. For example, activity is improved by increasing the activation temperature, by the addition of cocatalyst, by the reduction of Cr(VI) in CO, or by increasing the ethylene concentration in the reactor. The activity can also be lowered by poisons. 

Reaction variables can also affect the LCB level in the polymer. The addition of cocatalyst to the reactor to improve activity usually also makes the polymer more elastic. An example is shown in Table 25, representing experiments in which two different Cr/silica-titania catalysts were tested with and without 5 ppm of AlEt3 and BEt3 cocatalyst added to the reactor [407], The melt elasticity of the polymer is listed, as determined from the JC-a value in LCB/million carbon atoms. With each catalyst, the use of a... 

According to this view, some sites are more prone to retain the terminated macromer than others, because of greater coordinative unsaturation or higher Lewis acidity. The variables noted above that increase LCB may also improve the tendency of sites to hold on to terminated macromer. For example, raising activation temperature is known to increase electron deficiency on the chromium [65,136,384—386]. Cocatalysts may also create electron deficiency by removing other ligands, and coordinated poisons could reduce coordinative unsaturation or even displace the coordinated macromer. 

Although the term "cocatalyst" usually refers to one of these metal alkyls, in a broader sense anything that improves catalyst activity when added to the reactor has also been termed a cocatalyst. This broad group includes many reducing agents such as complex hydride salts, or even... 

Sometimes the chromium species generated by reaction of the catalyst with the cocatalyst become highly sensitive to H2 as a MW regulator, much like the organochromium catalysts. For example, an attempt was made to minimize the MI (raise the MW) by choosing a combination of catalyst and reaction variables that are all known to raise the polymer MW. A low pore volume Cr/silica was activated at only 600 °C, and the catalyst was treated with fluoride to increase the activity. It was then reduced in CO at 340 °C, again to improve activity and to lower the MI potential of the catalyst. To further lower the MI (actually the HLMI in... 

---