Ruthenium catalysts, silica-supported

Proven, industrially used catalysts are mostly based on either iron or cobalt. Ruthenium is an active F-T catalyst but is too expensive for industrial use. Both Fe and Co are prepared by several techniques including both precipitation and impregnation of (e.g. alumina or silica) supports. The more noble Ni catalyst produces nearly exclusively methane and is used for the removal of trace of CO in H2. 

Figure 1. X-ray absorption spectrum of a silica supported ruthenium-copper catalyst at 100 K In the vicinity of the K absorption edge of ruthenium. Reproduced with permission from Ref. 8. Copyright 1980, American Institute of Physics.

Supported (alumina, silica) Ru catalysts The Mossbauer data show that RuCl3 (l-3)H20 reacts chemically when supported onto alumina, but does not when impregnated on a silica support. The study further shows that a supported ruthenium catalyst converts quantitatively into RUO2 upon calcination, and that the reduction of a supported ruthenium catalyst converts all of the ruthenium into the metallic state... 

The co-existence of at least two modes of ethylene adsorption has been clearly demonstrated in studies of 14C-ethylene adsorption on nickel films [62] and various alumina- and silica-supported metals [53,63—65] at ambient temperature and above. When 14C-ethylene is adsorbed on to alumina-supported palladium, platinum, ruthenium, rhodium, nickel and iridium catalysts [63], it is observed that only a fraction of the initially adsorbed ethylene can be removed by molecular exchange with non-radioactive ethylene, by evacuation or during the subsequent hydrogenation of ethylene—hydrogen mixtures (Fig. 6). While the adsorptive capacity of the catalysts decreases in the order Ni > Rh > Ru > Ir > Pt > Pd, the percentage of the initially adsorbed ethylene retained by the surface which was the same for each of the processes, decreased in the order... 

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... 

Irrespective of the reaction conditions used (/. e. ultrasound, microwave, changing reaction times, temperature and solvents), the maximum turnover number (TON) that was achieved was 75. In principle, second generation Grubbs-type initiators immobilized on non-porous silica should behave similar to those immobilized on monolithic supports[16]. In fact, catalysts immobilized onto monolithic supports give similar maximum TONs (< 65) in the absence of any chain transfer agent (CTA). Ruthenium measurements by means of ICP-OES revealed quantitative retention of the original amount of ruthenium at the support within experimental error ( 5%), thus otfering access to metal free products. 

Dispersion is defined as the ratio of surface atoms to total atoms in the metal crystallites, and it is determined from chemisorption measurements (26,29). A typical hydrogen chemisorption isotherm is shown in Figure 2.4 for a silica-supported ruthenium catalyst containing 5 wt% ruthenium. The quantity H/Ru in the right-hand ordinate of the figure is the ratio of the number of hydrogen atoms adsorbed to the number of ruthenium atoms in the catalyst. The catalyst was treated with a stream of hydrogen in an adsorption cell at 500°C, after which the cell was evacuated and cooled to room temperature for the determination of the isotherm. The adsorption is... 

Figure 2.4 Typical hydrogen chemisorption isotherm at room temperature for a silica-supported ruthenium catalyst containing 5 wt% ruthenium (28). (Reprinted with permission from Academic Press, Inc.)...

Figure 4.7 Normalized EXAFS data at 100°K (ruthenium AT-absorption edge), with associated Fourier transforms and inverse transforms, for silica-supported ruthenium and ruthenium-copper catalysts (31). (Reprinted with permission from the American Institute of Physics.)...

Jessop and co-workers have pointed out that homogeneous catalysis in supercritical fluids can offer high rates, improved selectivity, and elimination of mass-transfer problems.169 They have used a ruthenium phosphine catalyst to reduce supercritical carbon dioxide to formic acid using hydrogen.170 The reaction might be used to recycle waste carbon dioxide from combustion. It also avoids the use of poisonous carbon monoxide to make formic acid and its derivatives. There is no need for the usual solvent for such a reaction, because the excess carbon dioxide is the solvent. If the reaction is run in the presence of dimethy-lamine, dimethylformamide is obtained with 100% selectivity at 92-94% conversion.171 In this example, the ruthenium phosphine catalyst was supported on silica. Asymmetric catalytic hydrogenation of dehydroaminoacid derivatives (8.16) can be performed in carbon dioxide using ruthenium chiral phosphine catalysts.172... 

In recent years the Asahi Corporation has developed a benzene-to-cyclohexene process involving a liquid-liquid two-phase system (benzene-water) with a solid ruthenium catalyst dispersed in the aqueous phase. The low solubility of cyclohexene in water promotes rapid transfer towards the organic phase. An 80000 t annum plant using this process is in operation. Another way to scavenge the intermediate cyclohexene is to support the metal hydrogenation catalyst on an acidic carrier (e. g. silica-alumina). On such a bifunctional catalyst the cyclohexene enters catalytic alkylation of the benzene (present in excess) to yield cyclohexylbenzene [19], which can be converted, by oxidation and rearrangement reactions, into phenol and cyclohexanone. 

A key question is whether the diatomic molecule in its interaction with metal surfaces remains molecular or dissociates into carbon and oxygen. Broden et al. (3) predicted, by the perturbation of molecular orbitals for CO adsorbed, that only iron could dissociate CO. However, other metals in Group VIII such as nickel (A) ruthenium (5) and rhodium (6) can dissociate CO. Recently Ichikawa et al.(7) observed that disproportionation of CO to CO2 and carbon occurs on small particles of silica-supported palladium. These results show that carbon deposition phenomena may occur via either dissociation of CO on the metals used or disproportionation of CO to CO and carbon on small platinum particles. Cant and Angove (8) studied the apparent deactivation of Pt/Si02 catalyst for the oxidation of carbon monoxide and they suggested that adsorbed CO forms patches and that oxygen atoms are gradually consumed. 

MC Shoenmaker-Stolk, JW Verwijs, JA Don, JJF Scholten. The catalytic hydrogenation of benzene over supported metal catalysts. 1. Gas-phase hydrogenation of benzene over ruthenium-on-silica. Appl Catal 29 73-90, 1987. 

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