Periodates silica support

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

Activity of the Keggin HPW/Si02 catalyst in terms of the conversions of ethylene and acetic acid and production of ethyl acetate vs. reaction time is displayed in Figure 3. Besides ethyl acetate, ethanol and diethyl ether are also produced. It can be seen that the catalyst is quite stable over the 17 hour period on stream. Activities of the other silica-supported Keggin... 

Scheme 6.11 Deoximation of ketoximes with silica-supported periodate.

Rj = PhCH2 R2 = Ph Rj = Ph, n-C12ttz R2 = Me Scheme 6.34 Oxidation of sulfides to sulfoxides and sulfones by silica-supported sodium periodate. 

Another important highly selective and stable hydroformylation sol gel catalyst is made of silica-supported rhodium covalently bound to supported Xantphos family of ligands.36 By incorporating monoliths of the sol-gel doped material into the paddles of an autoclave stirrer, the catalyst (Rotacat) can be used in a continuous liquid flow process. A single sample of this catalyst was used for a variety of different hydroformylation reactions under widely varying conditions over a period of more than a year, still retaining its selective activity. 

The siloxane (Si—O—SiR) bond hydrolyzes below pH 2, so HPLC with a bonded phase on a silica support is generally limited to the pH range 2-8. If bulky isobutyl groups are attached to the silicon atom of the bonded phase (Figure 25-9), the stationary phase is protected from attack by H30+ and is stable for long periods at low pH, even at elevated... 

The sulfide 1 (0.75 mmol) is dissolved in dichloromethane (2-3 mL) and adsorbed over silica supported sodium periodate (20%, 1.36 g, 1.28 mmol) that is wetted with 0.3 mL of water by thoroughly mixing on a vortex mixture. The adsorbed powdered material is transferred to a glass test tube and is inserted in an alumina bath (alumina 100 g, mesh 65-325, Fisher scientific bath 5.7 cm diameter) inside the microwave oven. The compound is irradiated for the time specified in the table and the completion of the reaction is monitored by TLC examination. After completion of the reaction, the product is extracted into ethyl acetate (2x15 mL). The removal of solvent at reduced pressure affords crude sulfoxide 2 that contains less than 5% sulfone. The final purification is achieved by column chromatography over silica gel column or a simple crystallization. 

A different approach that has been used is a ruthenium-catalyzed Meerwein-Ponndorf-Verley-type reduction of ketones using the silica-supported amino alcohol ligand 22 (Scheme 4.65). It was found necessary to cap the remaining free silica hydroxyl sites to alkylsilane derivatives to prevent catalyst deactivation. Initial studies found that slower flow rates resulted in lower ee because of equilibration back to the starting materials - after optimization, the best conditions were found to be 1400 pl/h providing a 95% conversion and 90% ee. The stability of the catalyst was investigated over time, during which a constant formation of 175 pmol/h was obtained only after a period of 7 days was some decrease in activity observed. The extended lifetime of the... 

Electron microscopy shows that polymerization starts at active centres on the surface of the particle. During this initial stage, a thin polymer cover is formed on and just below the outer surface of the silica support. This thin cover consists of highly crystalline polypropylene, which acts as a diffusion barrier for the monomer. Diffusion of propylene through this layer thus becomes rate-limiting for polymer formation consequently the high initial polymerization activity decreases sharply after a few minutes and a period of relatively low activity is reached. 

The highest propene oxide yields were obtained with both the Ti-SBA-15- and the Ti-silica-supported catalysts, although a higher reaction temperature was needed in comparison to the titania-supported catalyst. The deactivation for these catalysts was also considerably less. At lower temperatures (up to 423 K), all catalysts had an inhibition period for both propene oxide and water formation, which is explained by product adsorption on the support. The side products produced by all catalysts were similar. Primarily, carbon dioxide and acetaldehyde were produced as side products and, in smaller quantities, also propanal, acrolein, acetic acid, and formaldehyde. Propanol (both 1- and 2- as well as propanediol), acetone, carbon monoxide, and methanol were only observed in trace amounts. 

The Cu-Zn catalysts supported on alumina and silica showed different stability performaiice in long runs fFig.l). The stability tests were started at 240 C and LHSV = 2.5 h L After a period of conversion decrease (about 40 and 100 hours for CU-Z11/AI2O3 and Cu-Zn/Si02, respectively) a constant temperature increase of 0.1 C/h was maintained for both catalysts for additional 200-250 hours. Silica supported catalysts deactivated at a faster rate than the corresponding alumina catalysts. After 300-320 hours a fast deactivation rate of both catalysts was recorded. 

Figure 7.1. Dependence of the activity of metals for ethene hydrogenation on Periodic Group number. A. First row metals , second row metals O, third row metals. Open points, condensed metal films filled points, silica-supported metals.

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