Dehydroxylated silica

The same surface species is obtained at ambient temperature by the reaction of Bu3SnH and the silanol groups, suggesting that the Sn-H bond is more reactive in this case than the Sn-C bond. The surface reaction depends upon the degree of dehydroxylation of the surface of silica. On silica dehydroxylated at 500°C the reaction leads to one well-defined surface complex. On the other hand, on silica dehydroxylated at 200°C, the evolution of alkane is continuous. The difference in the latter case is related to the presence of neighboring OH groups, because the number of the surface vicinal OH groups capable of... 

The second approach (Equation(3)) has a number of advantages over the first one (Equation(2)). The alkyl complexes are more reactive than the related alkoxides, the latter being for group 4 elements generally associated into dimers or trimers 48 also, reaction (2) liberates an alcohol which may further react with the surface of silica, whereas the alkane ( Equation(3)) is inert. It was demonstrated by various spectroscopic techniques and elemental analysis that with a silica dehydroxylated at 500 °C under vacuum, the stoichiometry of reaction (3) corresponds to n = 1.45,46 Moreover, a better control of the surface reaction was achieved with the procedure represented in Equation(3). 

We first studied group 4 metals (titanium, zirconium and hafnium) supported on a silica dehydroxylated especially at 700 °C (Table 3.8). Following the laboratory-developed strategy, surface-species have been well-characterized by classical techniques (IR, solid-state NMR gas evolvement, reactivity, etc.). Catalysis results show that titanium is the most active even if its activity is far less than that of homogeneous catalysts. In addition, an important amount of metal was lost by lixiviation even if this phenomenon seemed to stop after a certain time. 

To follow the mode of grafting of an organometallic species onto Si02, the tris (neopentyl)neopentylidene tantalum complex has been used to show the reactivity of its groups with silanol groups of silica (dehydroxylated at 300 to 700 °C)... 

The tris-neopentyl Mo(VI) nitride, Mo(-CH2- Bu)3(=N) [134], reacts with surface silanols of silica to yield the tris-neopentyl derivative intermediate [(=SiO)Mo (-CH2- Bu)3(=NH)] followed by reductive elimination of neopentane, as indicated by labeling studies from labeled starting organometallic complex, to yield the final imido neopentylideneneopentyl monosiloxy complex [(=SiO)Mo(=CH- Bu)(-CH2 - Bu)(=NH)] [135]. The surface-bound neopentylidene Mo(VI) complex is an active olefin metathesis catalyst [135]. Improved synthesis of the same surface complex with higher catalytic activity by benzene impregnation rather than dichlorometh-ane on silica dehydroxylated at 700 °C has been reported [136],... 

Comparison of NMR data between these molecules (solution spectra) and the surface species obtained by the grafting of [W(=Ar)(=CH Bu)(CH 2Bu)2] on silica dehydroxylated at 200 °C (solid-state spectra) allowed the proposal of a grafting reaction sequence for the organometallic W(VI) precursor on the silica surface (Scheme 14.15). 

Scheme 14.15 Possible grafting mechanism of[W(=Ar)(=CH Bu)(CH5Bu)j] on silica dehydroxylated at 700°C vs silica dehydroxylated at 200°C, proposed through analogy with silsesquioxane chemistry. Intermediates that have not been observed...

Summarizing we may state that, as adsorption isotherms revealed the formation of an equilibrium in the initial stage of the reaction, total loading data after two hours of reaction and filtration reveal that specific surface area and mean pore size are the controlling parameters in the loading step. Surface water causes hydrolysis and polymerization. On a dehydrated surface, a surface coverage irrespective of the number of hydroxyls is formed. For silica dehydroxylated at elevated temperature (1073 K) a different behaviour is observed, suggesting the participation of strained siloxanes. 

Sublimation of tetraneopentyl zirconium onto the surface of silica dehydroxylated at 500 °C results in the electrolytic cleavage of one Zr-C bond by a surface proton, with the formation of the tris neopentyl zirconium [Si]-0-Zr(CH2CMc3)3 grafted species. Reaction of this supported Zr alkyl with hydrogen at 150°C leads to the formation of a zirconium hydride. Interestingly, this reaction does not yield neopentane, but rather the formation of methane and ethane are observed. These products are in fact the result of the hydrogenolysis of evolved neopentane catalyzed by the silica-supported zirconium hydride. ... 

In series C of Figure 90, the silica alone was again calcined at 500-950 °C. This time, however, Cr(III) acetylacetonate in toluene was added by impregnation, to avoid major rehydroxylation of the silica surface. The sample was then dried and given a second calcination step only at 500 °C. This time, the polymers were found to have LCB levels very similar to that observed in series A. Thus, it is the state of silica dehydroxylation, and the degree of coalescence, that determine LCB levels. The data from series C show that it is not necessary for the chromium to experience a high... 

Cr/alumina did not respond much to this treatment. Polymer was produced with this catalyst, and the shape of the MW distribution was not changed significantly only a very slight shift to lower MW was observed. Thus, Cr/alumina does not respond to dehydroxylation in exactly the same way as Cr/silica. This could be taken as evidence that, for Cr/silica, dehydroxylation influences the bonding strain or angle, which is relatively fixed on crystalline Cr/alumina. Another possible explanation is that on alumina there is no acidic, low-MW-producing species to embellish with the two-step procedure. 

Mass balance analysis and ESR spectroscopy show that a well-defined Mn(n) complex, Mn(CH2 Bu)2(TMEDA), gives the [(=SiO)Mn(CH2 Bu)(TMEDA)] on SiO2-(700)) while a mixture of [(=SiO)Mn(CH2 Bu)(TMEDA)] and [(=SiO)2Mn(TMEDA)] are obtained on silica dehydroxylated at lower temperature (Scheme 48 and Table 13). It is worth noting that these supported Mn complexes are quite unreactive no reaction is observed with ethene and N2O, but they react violently with O2. 

The reaction of [Rhfallyhs] with silica dehydroxylated at various temperatures ranging from 200 to 500 °C gives... 

FIGURE 34.38 MAS NMR and CP-MAS NMR spectra of silica dehydroxylated at 1023 K (below) and the same silica reacted with ammonia at 823 K (above). Taken from reference 40b. With permission. 

Oxides.—The adsorbent properties of silica gel are known to be highly dependent on the degree of hydroxylation of the surface, and this dependency has been widely investigated in relation to gas adsorption.Khopina and Eltekov have continued earlier work of Kiselev and his collaborators on the influence of the surface chemistry of silica on adsorption from solution. In the present paper a comparison is made of the adsorption of a series of aromatic hydrocarbons (benzene, naphthalene, biphenyl, phenanthrene, o- and m-terphenyl) from n-heptane solution by hydroxylated silica, dehydroxylated silica, and graphitized carbon black, using data obtained in the present work together with earlier published data. The results are analysed in terms of the separation factor(partition coefficient)... 

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