Chromium/silica catalyst dehydration

Karol and coworkers [34] reported silica-supported chromium-based catalysts based on Bis(indenyl) chromium (II), Bis(fluorenyl) chromium (II) and Bis(9-methyIfluorenyI) chromium (II) in place of chromocene. These catalysts were prepared at room temperature by reacting the dehydrated silica with a hexane solution containing one of the Cr(II) compounds. The Bis(indenyl)-based catalyst exhibited good activity under some polymerization conditions, while the Bis(fluorenyl)-based catalyst was significantly less active than the chromocene-based catalyst. Table 3.8 summarizes some polymerization data for these catalyst systems. 

Ordinarily the chromium binds to the silica by reacting with hydroxyls on a fully hydrated surface, because chromium is impregnated aqueously onto the silica and then calcined. However, a different catalyst results if the chromium attaches instead to a surface already dehydrated by calcining. A large promotional effect, particularly on the termination rate, is obtained (76). To do this the silica is first dehydrated at 900°C, for example, then impregnated with chromium anhydrously so that the surface is not rehydrated. A secondary calcining step at some lower temperature such as 300-600°C then fixes the chromium to the silica. The method is especially effective if the support also contains titania. 

Again (as mentioned in Section V,C) sulfur compounds perform better than CO, as can be seen in Fig. 20, because they are better dehydrating agents. When Cr/silica is reduced by COS or CS2 a black chromium sulfide forms. Reoxidation then converts it back to the hexavalent oxide. The catalyst retains no sulfur, but it often takes on a new reddish hue and the activity is greatly improved. This is probably an extension of the trend already observed in Fig. 10, which shows both activity and termination to increase as the catalyst is dehydrated. Perhaps the color change from yellow to orange, and finally to red for sulfided catalysts, indicates a transition from chromate to dichromate, or maybe just less coordination to hydroxyls. Adding water vapor to a sulfided catalyst completely reverses the benefit. 

The reaction is catalyzed by all but the weakest acids. In the dehydration of ethanol over heterogeneous catalysts, such as alumina (342—346), ether is the main product below 260°C at higher temperatures both ether and ethylene are produced. Other catalysts used include silica—alumina (347,348), copper sulfate, tin chloride, manganous chloride, aluminum chloride, chrome alum, and chromium sulfate (349,350). 

Phillips catalysts are produced by reaction of a chromium compound (usually CrO ) with dehydrated silica ... 

The hold time at maximum temperature can also be important in determining catalyst behavior [75,235,734,735]. The conversion to Cr(VI) occurs very rapidly, even before the maximum temperature is reached, and so there is no need to wait for the oxidation. The esterification of Cr(VI) to anchor the chromium to the silica surface is also complete even before the hold temperature is reached. Rather, it is the silica dehydration reaction that can be time dependent, because it involves a rearrangement and annealing of the silica structure. 

However, more recently Bade and coworkers [49] reported that the deposition of Cr(2-Me-allyl)j onto silica forms an active catalyst for ethylene polymerization in which the silica dehydration temperature used to dry the silica prior to the deposition of the chromium allyl affects the catalyst activity, the molecular weight of the polyethylene and the density of the polyethylene produced with the finished catalyst. 

Species A in Figure 3.41 requires adjacent silica hydroxyl groups and would most likely be formed on silica dehydrated at 200°C, while species B requires isolated silica hydroxyl groups that are the predominate species on silica dehydrated at 400 and 800°C. Bade et al. found infrared evidence for the reaction illustrated in Figure 3.41C and suggested that this type of reaction takes place on silica dehydrated at 800°C where strained siloxane linkages may be present. The catalyst prepared on 800°C silica was unique in that it was the only catalyst that exhibited infrared bands associated with a q -bonded allyl group attached to the chromium center. 

The catalyst used was nominally a mixture of 75% magnesia and 25% silica or sihceous earth, which are well known as dehydrogenation and dehydration catalysts respectively. Up to 3% chromium oxide was included to inhibit toe formation of magnesium silicate. 

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