Rhodium/silica catalysts

The influence of the support is undoubted and spillover was further confirmed by the excess of hydrogen chemisorbed by a mechanical mixture of unsupported alloy and TJ-A1203 above that calculated from the known values for the separate components. It was also observed that the chemisorption was slower on the supported than on the unsupported metal and that the greater part of the adsorbate was held reversibly no comment could be made on the possible mediation by traces of water. On the other hand, spillover from platinum-rhenium onto alumina appears to be inhibited for ratios Re/(Pt Re) > 0.6. In an infrared investigation of isocyanate complexes formed between nitric oxide and carbon monoxide, on the surface of rhodium-titania and rhodium-silica catalysts, it seems that the number of complexes exceeded the number of rhodium surface atoms.The supports have a pronounced effect on the location of the isocyanate bond and on the stability of the complexes, with some suggestion of spillover. 

To date, reports have involved palladium catalysts for Suzuki and Sono-gashira coupling reactions [63-66], rhodium catalysts for silylations of alcohols by trialkylsilanes [67,68], and tin-, hafnium-, and scandium-based Lewis acid catalysts for Baeyer-Villiger and Diels-Alder reactions [69]. Regardless of exact mechanism, this recovery strategy represents an important direction for future research and applications development. Finally, a particularly elegant protocol where CO2 pressure is used instead of temperature to desorb a fluorous rhodium hydrogenation catalyst from fluorous silica gel deserves emphasis [28]. 

Rhodium immobilized complexes were also found to be effective catalysts of the addition of HSiMe(OSiMe3)2 and HSi(OEt)3 to various allyl ethers. The data presented in Table 7.4 confirm a high catalytic activity of catalysts 1, 3 and 5 in the conversion of allyl ethers into the corresponding silyl derivahves, but, unfortunately, only in the case of allyl phenyl ether did the catalytic achvity remained unchanged up to 10 cycles. ICP analysis of the rhodium solid catalysts after hydrosilylation tests revealed a high concentration of rhodium. Therefore, the decrease in catalytic activity of 1 does not depend only on leaching of rhodium from the silica surface. 

We selected a series of rhodium(II) carboxylates, rhodium(II) carboxamidate [5d] (Doyle catalysts 42h, 42i, 42j), and the bridged rhodium(II) carboxylate (Lahuerta catalyst) 42g, as representatives of the various rhodium(II) catalysts generally utilized. Most of the carboxylate and Doyle catalysts were commercially available and were purified by silica gel chromatography prior to use. The Lahuerta catalyst was prepared according to the literature procedure [23]. 

Acetylene, when adsorbed on active nickel catalysts, undergoes self-hy-drogenation with the production of ethylene [91], although the extent of this process is less than with ethylene. Similar behaviour has been observed with alumina- and silica-supported palladium and rhodium [53], although with both of these metals ethane is the sole self-hydrogenation product some typical results for rhodium—silica are shown in Fig. 21. 

Additional evidence of that hypothesis is given In Tables 4 and 5. The catalysts prepared with carbonyl clusters in n-hexane medium must avoid the MgO hydrolysis. The selectivity patterns for such catalysts show notable differences in comparison with the aqueous Impregnated type catalysts. The carvotanacetone formation is largely diminished and the stereospecificity to axial-equatorial carvomenthol is totaly inhibited. However in Rhodium silica supported catalysts the selectivity to carvotanacetone practically does not change. The effects in stereospecifity towards the carvomenthol product may be due to a small silica hydrolysis effect. 

Rhodium(I) complexes immobilized on silica using 3-(3-silylpropyl)-2,4-pentanedio-nato ligands (38) show good activity in the hydrosilylation of 1-octene with HSi(OEt)3 at 100°C60. The immobilized Rh catalysts are prepared by (i) reaction of (EtO)3Si(CH2)3C(COMe)2Rh(CO)2 with untreated silica (Catalyst A), (ii) reaction of Rh(acac)(CO)2 (acac = acetylacetonato = 2,4-pentanedionato) with silica modified by [(EtO)3Si(CH2)3C(COMe)2] prior to the complexation (Catalyst B), (iii) reaction of [Rh(CO)2Cl]2 with a polycondensate of [(EtO)3Si(CH2)3C(COMe)2] , Si(OEt)4 and water (Catalyst C) and (iv) sol-gel processing of (EtO)3Si(CH2)3C(COMe)2Rh(CO)2 and Si(OEt)4 (Catalyst D). The Catalysts A and B show ca three times better activity than their homogeneous counterparts, while the Catalyst D exhibits only low activity and the Catalyst C is inactive60. 

In a manner similar to the four methods mentioned above, rhodium complex catalysts immobilized on silica modified by 2-(MeO)3Si(CH2)2C5H4N and (Me0)3Si(CH2)30C0CMe=CH2 using [Rh(CO)2Cl]2 as the precursor are prepared61. These immobilized pyridine-Rh complexes are shown to be active catalysts in the... 

In the effort to modify the catalytic properties of rhodium, Wilson and co-workers prepared Rh/Fe/Si02 and other two-metal-silica combinations (74) methanol yields over a Rh/Fe/Si02 catalyst shown in Table XIV were higher than those over the Rh/MgO, Rh/ZnO, and Rh/LaB6 catalysts prepared from cluster carbonyls, but the selectivity of the latter toward methanol was better than that of bimetallic rhodium-iron catalysts. 

FIGURE 31 A noncovalently silica-immobilized rhodium phosphine catalyst (720). 

Platinum-iron on alumina catalysts were characterized by Mbssbauer spectroscopy (Section 4) and their activity tested. Iron in clusters with high Pt Fe ratios, about 5, and fully combined with platinum, was catalytically inert for the CO-H2 synthesis reaction, attributed to a decrease in the electron density of the iron as indicated by the Mbssbauer isomer shift. The direction of electron transfer was opposite to that proposed for alkali-metal promoted iron catalysts. At low Pt Fe ratio, 0.1, ferromagnetic iron as well as Fe " ions and PtFe clusters were produced and dominated the activity/selectivity pattern. Rhodium on silica catalysts produced C2-compounds containing oxygen, specifically acetic acid, acetaldehyde and ethanol, with methane as the other major product. The addition of iron moved the C2-product formation sharply in favour of ethanol and now methanol was also formed. ... 

Platinum and rhodium complex catalysts can be immobilized on organic polymers or on silica. Various organic and inorganic supports have been studied ". ... 

Rhodium-35a and rhodium-35b catalysts in [BMIMKPFg] supported on a silylimidazolium-modified silica have been shown to have activities and regioselectivities comparable to those under ionic liquid-biphase conditions in the... 

Benes prepared some new chiral phosphines suitable as ligands for this reaction.Capka anchored (C2H50)3Si(CH2)3P(Ph)Men to silica (6). Complexed with rhodium, a catalyst for the enantioselective hydrosilylation of ketones like acetophenone and propiophenone was obtained. 

Reversed-flow gas chromatography (RF-GC) has been used to study the kinetics of surface-catalyzed reactions and the nature of the active sites. RF-GC is technically very simple and it is combined with a mathematical analysis that gives the possibility for the estimation of various physicochemical parameters related to catalyst characterization in a simple experiment under conditions compatible with the operation of real catalysts. The experimental findings of RF-GC for the oxidation of CO over well-studied silica-supported platinum-rhodium bimetallic catalysts are in agreement with the results of other workers using different techniques ascertaining that RF-GC methodologies can be used for the characterization of various solids with simplicity and accuracy. 

The intramolecular Buchner reaction of aryl diazoketones has been carried out using both copper(I) and rhodium(II) catalysts. For example, 1-diazo-4-phenylbutan-2-one 27a cyclizes in bromobenzene with copper(I) chloride catalysis, furnishing 3,4-dihydroazulen-l(2//)-one 30 in 50% yield after purification by chromatography over alumina. Trienone 30 is not the primary cyclization product, and the less conjugated isomeric trienone 29a is first produced, but contact with alumina causes isomerization to 30. The yield of this cyclization is further improved when rhodium(II) acetate is used as the catalyst instead of copper(I) chloride. Thus a catalytic amount of rhodium(II) acetate brings about the nearly quantitative conversion of 27a to 29a within minutes in hot dichloromethane. Compound 29a isomerizes to 30 on treatment with triethylamine, and rearranges to 2-tetralone 31a when exposed to silica gel or acid. 

A detailed study of the chemical structure of mesoporous silica catalysts containing rhodium ligands and nanoparticles was carried out by multidimensional solid-state NMR techniques. 

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