Cocatalysts reaction variables

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... 

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... 

This phenomenon is rather specific. That is, a Cr/silica catalyst is preferred, not a Cr/silica-titania or a Cr/silica-alumina. It must be activated at a high temperature or treated with fluoride, perhaps to reduce potential ligands. Then it must be reduced in CO. When contacted with some cocatalysts, especially aluminum alkyls, the catalyst then becomes highly sensitive to H2. As illustrated in Table 60, in this series of experiments there was a huge jump in MI only when all of these treatments were combined. Activation at 600 °C does not work unless the catalyst also contains fluoride. Activation at 800 °C is effective without fluoride, but the effect is more pronounced with fluoride. The data shown in Figure 214 illustrate the huge shift in the MW distribution resulting from this combination of catalyst and reaction variables. 

Detailed studies of systems involving aluminium-based Lewis acids and hydrogen halides are scarce. Fontana and Kidder investigated the polymerisation of propene initiated by the pair aluminium bromideTiydrt n bromide. The cocatalytic role of the latter acid was clearly proved since no polymerisation could be detected in its absence. The dependence of the rate of polymerisation upon the cocatalyst concentration and the induction periods observed make this system similar to those in which stannic chloride induces the polymerisation of olefins in the presence of variable quantities of water (see Sect. IV-C-3-b). With relatively large quantities of added hydrogen bromide, addition of this acid to the mcmomer to give fso-propyl bromide must have constituted an important side reaction. 

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. 

Both the productivity and stereoselectivity of the oscillating metallocene catalyst (2-PhInd)2ZrCl2 are strongly influenced by the nature of the cocatalyst and reaction conditions, such as temperature, pressure, and monomer concentration. Best performance of the catalyst has been obtained with MAO as the cocatalyst (Wilmes et al., 2002). Both the productivity and [mm/Tzm] are found to increase with decreasing temperature or increasing pressure, with the other variable remaining constant. The effect of monomer concentration [M] on the tacticity of the... 

In conclusion, over 130 semiconductors are known to catalyze the photochemical water-splitting reaction according to eq 1 or either water oxidation or reduction in the presence of sacrificial agents. Even though the principle activitycontrolling factors in semiconductor-heterostructures have been identified, many aspects of the function of inorganic photocatalysts are still unclear. Most importantly, the molecular mechanism of water reduction and oxidation on the semiconductor surface has not yet been elucidated in sufficient detail. ° Many questions about charge transfer between semiconductor and cocatalysts, and its dependence on the structural and electronic features of the interface are still open. The effect of variable material preparations and surface impurities on the catalytic activity of semiconductors (e.g. sulfur and oxide on... 

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