Silica dehydration

VOv supported on ai2o3 VO, supported on silica, ceria, alumina, zirconia, niobia, titania-silica, zirconia-silica VOr supported on alumina, silica Dehydration Dehydration at 773 K in 02/He, methanol adsorption 02/He at 773 K, adsorption of isopropanol... 

The actual decomposition of bonded ligands can be different from what we assume in the above calculation (weight loss), and corresponding bonding density values will be very approximate if based on the data from thermogravimetry or ashing techniques. The formation of trihydroxysilane is highly doubtful, and active silica dehydration could be observed at temperatures above 600°C [69]. 

Thus, it is clear why water cannot be used as a solvent Most titanium esters or halides are too reactive and would react with the water solvent rather than the silica surface. One procedure that permits titanation from an aqueous solution is to use a water-soluble titanium complex that resists hydrolysis until after silica dehydration temperatures are reached, when only silanol groups remain. Titanate complexes of triethanolamine [569], acetylacetonate [570], lactate [569], or even peroxide [571] can be used in this way to perform aqueous titanation. 

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. 

H2SO4 on silica. Dehydration, acetalization, etherification, and many common acid-promoted processes can take advantage of the superb properties of the catalyst. 

Such cyclic structures should have a high reactivity, nevertheless, probability of their realising on pyrogenic silica surface is very small. At the same time, it is assumed in [51] that disiloxane bridges are typical of the surface of disperse silicas dehydrated within wide temperature interval, and they can play a noticeable role in chemical transformation within surface layer, in particular in the reactions with some element halogenides, element organic compounds, in the processes of esterification, rehydroxylation, and so on. 

As it was mentioned above, electron-donor molecules (e.g., amines such as triethylamine, TEA) can strongly promote the Ssi reactions. A similar effect can be also caused by adsorbed water, as elevating temperature of fumed silica dehydration reduces the loading of OSC (Figure 37.3a) as well as the depth of the reaction (Figure 37.3b and 37.3c) [11,12]. Similar effects were observed for many reactions between OSC or organics with fumed oxides [10-13]. 

Table 3.6 The deposition of chromocene into Grade 56 silica dehydrated at 800 C.

Figure 3.19 The proposed structure of chromocene coordinated to a silica siloxane linkage for silica dehydrated at 800°C [25],...

Figare3.20 The e ctofthe silica dehydration temperature on the amount of chromocene fixed to the silica surfiice ( ) toluene at 25°C (A) toluene at 55°C (H) Decane at 25°C. Reprinted from [25] with permission firom Elsevier Publishing. 

Effect of Silica Dehydration Temperature on the Chromocene-Based Catalyst... 

The silica dehydration temperature has a significant effect on catalyst activity with a catalyst prepared on a silica dehydrated at 670 C being approximately 12 times more active than a catalyst prepared on a silica dehydrated at 200 C. This data [22] is summarized in Table 3.7. 

Table 3.7 The effect of silica dehydration temperature on chromocene-based catalyst activity.

Silica Dehydration Temp. (°C) Chromocene (mmol) Catalyst Activity (gPE/mmol CR/100 psi ethylene) Relative Activity... 

Figure 3.23 Infrared spectra of the 0-H stretching region on (A) pure silica and (B) after chromocene deposition with silica dehydrated at (a) 200°C (b) 450°C and (c) 900°C.

Figure 3.24 Proposed configuration of agglomerates of chromocene complexes on silica dehydrated at 900°C so that chromocene reacts with only one hydroxyl group to eliminate one cyclopentadienyl ligand. Reprinted from [30] with permission from the American...

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. 

The deposition of Cr(2-Methyl-allyl)3 onto silica, previously dehydrated at 200,400 or 800°C, was reported in which the stoichiometry of the deposition reaction was monitored by GC analyses of the pentane solution used to carry out the deposition reaction. Ethylene homopolymerization experiments with the catalyst based on the silica dehydrated at 800 C showed that the polyethylene contained a high concentration of short-chain branches, suggesting that this catalyst also produced 1-olefins (primarily 1-hexene) that were incorporated into the growing polymer chain. The catalyst prepared on 400°C silica produced polyethylene with a small amount of short-chain branches, while the catalyst prepared on 200°C silica exhibited no side chains. 

Bade et at postulate that the primary reaction products formed from the reaction of chromium allyls with the surface hydroxyl groups on dehydrated silica are shown in Figure 3.41 A and B. In addition, chromium allyls may also react with a siloxane linkage with silica dehydrated at 800°C, and this reaction is shown in Figure 3.41C. 

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. 

In this catalyst, Cr[CH(SiMe3)2]3 was supported on silica (surface area of 300 m /g and a pore volume of 1.6 cc/g) that was dehydrated at either 200 or 600°C using hexane as the slurry solvent. The catalyst prepared on silica dehydrated at 600°C also produces 1 -hexene in situ, which results in polymer containing a relatively high number of short-chain branches that reduce the density of the polyethylene. 

Figure 3.43 Effect of Cr content on density. ( ) Silica dehydrated at 200 C ( ) Silica dehydrated at 600°C. Polymerization conditions 100°C, ethylene partial pressure 1.4 MPa, slurry solvent 700 ml isobutane, 1 h. Reprinted from [50] with permission from Nature Publishing Group.

Active site 1 in Figure 3.44 is formed with silica dehydrated at 200°C due to the large number of vicinal hydroxyl groups. Active sites 2 and 3 are... 

Table 3.14 Effect of silica dehydration temperature and polymerization characteristics [50],... 

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