Rhodium silica-supported

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. 

The EXAFS results suggested that the iridium-rhodium clusters dispersed on alumina differed in size and/or shape from those dispersed on silica, based on the result that the total coordination nunbers of the iridium and rhodium atoms in the clusters were very different (7 and 5 in the alumina supported clusters vs. 11 and 10 in the silica supported clusters). These coordination numbers suggested that the clusters dispersed on alumina were smaller or that they were present in the form of thin rafts or patches on the support. The possibility of a "raft-like" structure in the case of the alumina supported clusters suggests an interaction between the metal clusters and the support which is much more pronounced for alumina than for silica. If the clusters on the alumina were present as rafts with a thickness of one atomic layer, one could have a situation in which the rhodium concentration at the perimeter of the raft was greater... 

Hecker, W.C. and Bell, A.T. (1985) Reduction of NO by H2 over silica-supported rhodium Infrared and kinetic studies, J. Catal. 92, 247. 

Other metals on silica supports have been investigated less extensively than platinum and nickel, and average particle diameters have only been estimated by gas adsorption methods, supported in a few cases by X-ray line broadening data. Thus, rhodium, iridium, osmium, and ruthenium (44, 45) and palladium (46) have all been prepared with average metal particle diameters <40 A or so, after hydrogen reduction at 400°-500°C. 

It is evident that the silica support influences the catalytic performance and it is important to understand the details of the processes involved. For the sol-gel material it was shown by 31P NMR spectroscopy that the immobilised cationic complex completely transforms to the neutral rhodium-hydride species under a CO/H2 atmosphere (Scheme 3.3). On dried silica, however, this conversion might not be complete since the dried support is more acidic [32], It is therefore very likely that the neutral and cationic rhodium complexes co-exist on the silica support. 31P NMR measurements on homogeneous rhodium complexes have shown that a simple protonation indeed converts the neutral rhodium hydride species into the cationic complex. 

The existence of two different rhodium species co-existing on the silica support can be used as an advantage by controlling their relative amount. Under standard hydroformylation conditions, the cationic species and the neutral hydride complex are both present in significant amounts. Hence hydroformylation and hydrogenation will both proceed under a CO/H2 atmosphere. Indeed a clean one-pot reaction of 1-octene to 1-nonanol was performed, using the supported catalyst for a hydroformylation-hydrogenation cascade reaction. 98 % of the 1-octene was converted in the... 

For several silica-supported catalysts in condensed phase, including the SAPC system, the rates are disappointing. This can be assigned to slow mass transfer, and perhaps to incomplete rhodium hydride formation as we have discussed and observed. The sol-gel catalyst is relatively fast and is sometimes only a few times slower than the homogeneous one. Since only limited ways of preparation were tested, there is probably more scope for sol-gel catalysts. Space-time yields are promising at the present state of affairs. 

More recently, during research aimed at supporting the highly linear selective hydroformylation catalyst [Rh(H)(Xantphos)(CO)2] onto a silica support, the presence of a cationic rhodium precursor in equilibrium with the desired rhodium hydride hydroformylation catalyst was observed. The presence of this complex gave the resulting catalyst considerable hydrogenation activity such that high yields of linear nonanol could be obtained from oct-1-ene by domino hy-droformylation-reduction reaction [75]. 

Another study on the preparation of supported oxides illustrates how SIMS can be used to follow the decomposition of catalyst precursors during calcination. We discuss the formation of zirconium dioxide from zirconium ethoxide on a silica support [15], Zr02 is catalytically active for a number of reactions such as isosynthesis, methanol synthesis, and catalytic cracking, but is also of considerable interest as a barrier against diffusion of catalytically active metals such as rhodium or cobalt into alumina supports at elevated temperatures. 

Figure 7.1 Transmission electron micrographs of rhodium particles supported on silica spheres (from Datye and Long [7]). 

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 bipodal organotin spedes M2SnBu2 has been prepared on rhodium partides supported on silica [123]. EXAFS studies of this catalyst gave two Sn-Rh bonds of 0.262 nm and two Sn-C bonds of 0.212 nm (Scheme 2.38). 

For this complex, molecular chemistry does not adequately model the surface reactivity and the latter is strongly influenced by the presence of surface hydroxyl groups [22]. The organometallic fragments immobilized on silica have been reacted with trimethylphosphine to afford different silica-supported phosphine complexes of rhodium. The course of the reaction depends strongly on the hydroxyl content of the silica surface [23] (Scheme 7.2). 

We have recently introduced a new, simple and efficient method for preparing of well-defined silica-supported rhodium complexes, based on the reaction of well-defined rhodium organometaUic precursors containing the Rh-O-Si moiety with -OH groups located on the Si02 surface (Scheme 7.5) [36a]. 

Structures of immobilized rhodium complexes on the sihca support have been proposed on the basis of the data obtained from C, P and Si MAS-NMR. NMR spectra of the rhodium-modified solid materials confirmed that trimethylsiloxide ligand was removed from the rhodium coordination sphere during the immobilization process. Formation of a new covalent bond between the rhodium organo-metallic moiety and the silica support occurs, probably with evolution of trimethylsilanol, which is rapidly converted into disiloxane (Me3Si)20. The presence of this molecule in the solution obtained after the silica surface modification process was confirmed by GCMS analysis. 

The silica-supported heterogeneous rhodium catalysts 1-5 were then tested in the conversion of organosilicon compounds, that is in the hydrosilylation of olefins with HSi(OEt)3 [36c,d, 37], heptamethyltrisiloxane and polyhydrosiloxane ]36a, 36c-e, 38]. 

The first fixed-bed application of a supported ionic liquid-phase catalyst was hydroformylation of propylene, with the reactants concentrated in the gas phase (265). The catalyst was a rhodium-sulfoxantphos complex in two ionic liquids on a silica support. The supported ionic liquid phase catalysts were conveniently prepared by impregnation of a silica gel with Rh(acac)(CO) and ligands in a mixture of methanol and ionic liquids, [BMIMJPFg and [BMIM][h-C8Hi70S03], under an argon atmosphere. 

The nature of the support can have a very profound influence on the catalyst activity. Thus, phosphinated polyvinyl chloride supports are fairly inactive (75), and phosphinated polystyrene catalysts are considerably more active (57), but rather less active particularly when cyclic olefins are the substrates than phosphinated silica supports (76). Silica-supported catalysts may be more active because the rhodium(I) complexes are bound to the outside of the silica surface and are, therefore, more readily available to the reactants than in the polystyrene-based catalysts where the rhodium(I) complex may be deep inside the polymer beads. If this is so, the polystyrene-based catalysts should be more valuable when it is desired to hydrogenate selectively one olefin in a mixture of olefins, whereas the silica-based catalysts should be more valuable when a rapid hydrogenation of a pure substrate is required. 

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... 

With alumina-supported palladium, platinum and rhodium and silica-supported platinum [65,66] in the temperature range 20—200°C, no molecular exchange between adsorbed 14C-ethylene and gaseous ethylene is observed, whilst with hydrogen, small quantities of methane are formed at 100°C and above with platinum and rhodium and at 200° C withpallad-... 

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. 

The greater amount of retention observed with acetylene than with ethylene has been ascribed to the ability of the former to polymerise extensively. The existence of surface polymers following acetylene adsorption on alumina- and silica-supported platinum [60], evaporated palladium films [154] and silica-supported rhodium [67] has been demonstrated by thermal desorption studies. 

Khulbe and Mann [155] have obtained infrared spectra of allene adsorbed on silica-supported cobalt, nickel, palladium, platinum and rhodium. The spectra were similar for all the metals, although variations in band intensity from metal to metal were observed. Addition of hydrogen to the allene-precovered surface resulted in similar spectra to those found for chemisorbed and hydrogenated propene in which the surface species was thought to be an adsorbed prop-1-yl group. The authors concluded that the initial allene spectrum was consistent with the adsorbed species being a 1 2-di-o-bonded allene (structure K)... 

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