Silicate formation

Several groups have reported deactivation of silica-supported cobalt catalysts. Holmen and coworkers19 30 have reported increased deactivation due to added (external) water in the feed to silica-supported Co catalysts. Kogelbauer et al 1 reported the formation of silicates. Catalysts recovered from FTS as well as catalysts deactivated by steam-treatment both showed fractions of non-reducible cobalt in TPR. The presence of metallic cobalt was a prerequisite for the silicate formation. 

Two patterns are possible in the activation mechanism by simple chiral Lewis base catalysts. One is through the activation of nucleophiles such as aUyltrichlorosilanes or ketene trichlorosilyl acetals via hypervalent silicate formation using organic Lewis bases such as chiral phosphoramides or A-oxides. " In this case, catalysts are pure organic compounds (see Chapter 11). The other is through the activation of nucleophiles by anionic Lewis base conjugated to metals. In this case, transmetal-lation is the key for the nucleophile activation. This type of asymmetric catalysis is the main focus of this section. 

MePOR species and other complexes in cationic clays can be located at the edges of packed platelets, in the interlamellar space or in the mesopores present (Scheme 10.9). A review of the early data in this area is available.[86] The flat metallo macrocycles under clay synthesis conditions help to induce layer silicate formation, the complexes being intercalated between the layers. Whereas with monooxygen atom donors, alkanes can be oxygenated with significantly enhanced activities compared with the homogeneous case, in every case the expected products (ol/on) were obtained. Competitive oxygenation of adamantane and pentane shows lower... 

The comparison between Table 3.1 and Table 3.2 shows that the Gibbs energy and the difference of electronegativities correlate with each other. Fig. 3.1 shows this dependence for the reactions of sulphates and silicates formation. 

Kalinin, Dimitrii, Mechanism and Kinetic of Hydrothermal Reactions of Silicates Formation. Novosibirsk Nauka, 1973. 

The reaction in this mixture was also studied by IR spectroscopy. The formation of the product, calcium silicate, is characterized by the appearance of a new intense band at 1005 cm". A comparison of data on basicity and the IR spectra shows that the silicate formation is accompanied... 

Boyle et al. ( ) measured the enthalpy of combustion of metallic cobalt at 303.16 K. Their analyses of the combustion products indicated that some overoxidation of the metal had occurred. Correction of their results to correspond to CoO was made on the assumption that the excess oxygen was combined as Co O. Also the analyses indicated that the combustion products had attacked the silica-glass capsules, used to hold the metal samples, resulting in appreciable amounts of silicate formation. Corrections for these two side reactions amounted to three percent of the total measured energy of combustion. From these data, we derive A.H (CoO, cr, 298.15 K) = -57.0 0.3 kcal mol". Roth and Havekoss ( ) also investigated calorimetrically the... 

Ca NMR has been used in a study of calcium silicate formation by sol-gel synthesis (Nieto et al. 1995). The initial sol shows a broad symmetric peak at about 50 ppm which, immediately after gelling, becomes too broad to observe. Aging of the gel causes the reappearance of a broad " Ca peak near the initial position. The disappearance of the " Ca peak on gelling and its reappearance in the same position on aging suggest that the Ca environment remains the same throughout the process, but the... 

Farmer, V. C., Krishnamurti, G. S. R., and Huang, P. M., 1991, Synthetic allophane and layer-silicate formation in Si02-Al20j-Fe0-Fe203-Mg0-H20 systems at 23°C and 89°C in a calcareous environment Clays and Clay Minerals, v. 39, p. 561-570. 

The secondary amine of the AEP group is responsible for the supported enamine formation with acetone (aldol reaction), the deprotonation of nitromethane (Henry reaction) and the generation of a potential nucleophile from trimethylsilyl cyanide through hypervalent silicate formation (cyanosilylation reaction). Therefore, the presence of both AEP and UDP groups in close proximity can cooperatively activate the electrophile (through hydrogen bond) and the nucleophile by enamine formation, thus enhancing the reaction rate. 

Eluting anions Borate Silicate Formate Sulfide Chloride Cyanide... 

Layer-silicates Recent studies have also demonstrated the potential microbial influence on clay mineral (layer silicates) formation at hydrothermal vents. Bacterial cells covered (or completely replaced) with a Fe-rich silicate mineral (putative nontronite), in some cases oriented in extracellular polymers (as revealed by TEM analysis), were found in deep-sea sediments of Iheya Basin, Okinawa Trough (Ueshima Tazaki, 2001), and in soft sediments, and on mineral surfaces in low-temperature (2-50°C) waters near vents at Southern Explorer Ridge in the northeast Pacific (Fortin etal., 1998 Fig. 8.6). The Fe-silicate is believed to form as a result of the binding and concentration of soluble Si and Fe species to reactive sites (e.g. carboxyl, phosphoryl) on EPS (Ueshima Tazaki, 2001). Formation of Fe-silicate may also involve complex binding mechanisms, whereas metal ions such as Fe possibly bridge reactive sites within cell walls to silicate anions to initiate silicate nucleation (Fortin etal., 1998). Alt (1988) also reported the presence of nontronite associated with Mn- and Fe-oxide-rich deposits from seamounts on the EPR. The presence of bacteria-like filaments within one nontronite sample was taken to indicate that bacterial activity may have been associated with nontronite formation. Although the formation of clay minerals at deep-sea hydrothermal vents has not received much attention, it seems probable that based on these studies, biomineralisation of clay minerals is ubiquitous in these environments. 

Dolomite, MgC03CaC03, occurs widely on calcining, it yields the mixed oxide. On using this in place of MgO, the activity of the silica product is much reduced by formation of calcium silicate, CaSi03. This helps formation of magnesium, which is gaseous at the temperature of commercial operation, 1450 K. The silicate formation reaction ... 

Table 7.3 shows the composition of the products. It can be seen that potassium and sodium are present in the waste gas in the form of different compounds. The main components of the ashes are considered to be pure oxides. Phosphorus is in the form of sodium phosphate. The total interaction between the oxides (silicate formation etc.) has not been included in the considerations. 

No previous study, however, has adequately addressed the extent and cause of cobalt silicate formation during exposure of Co/silica to steam and its impact on FTS catalytic activity. 

An obvious idea is to use a layer of inert insulating material between the Si and the superconductor, such as Si02- However, here we nm into the problem of silicate formation via the following reaction ... 

The effect of sodium and potassium carbonates can be derived from the effect of their hydroxides which appear as a result of reaction between the carbonates and CaCOH). The calcium carbonate is then precipitated. The 0.01 mol/1 NaOH solution gives the liberation of 43 and 24.4% of CH in case of C3S and P-C2S respectively, after 24 h of hydration [45]. Yoshi and Sudoh [142] attributed the catalytic effect of NaOH to the sodium silicate formation. This sodium silicate is then reacting with Ca(OH)2 giving the C-S-H. The NaOH liberated during this process can react again with the anhydrous phases. 

Cheong, H.M., Choi, S.H., and Han K.S. (1997) Effect of alkahs and SO3 on tricalcium silicate formation and microstructure, in Proceedings 10th ICCC, Goteborg, paper li049. 

The reaction between Si02 and PbO is relevant for PZT films on Si containing substrates like 96% Alumina, Si wafer and LTCC. The existence of silicon oxide in tire substrate and in commercial electrode materials causes the diffusion into the PZT thick film and the reaction to Pb based silicates, which deteriorate ftie ferroelectric behavior of the PZT thick film. Therefore a special Au electrode was developed by Fraunhofer KTS with modified composition preventing silicate formation during firing of the PZT thick film. For ZtOr and AI2O3 substrates a commercial Au electrode (Heraeus 5789) fired at 850°C/ 30 min can bee used. 

Surface diffusion can also provide a fast transport path, and can thus also give rise to rapid growth along the surface. Although this situation is quite different from that of gas transport which was just described, the resultant morphologies can often be indistinguishable. Particularly striking examples of such phenomena are found for silicate formation reactions as illustrated in Fig. 8-12. 

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