Supported Rhodium Catalysis

Polymer-supported catalysts incorporating organometaUic complexes also behave in much the same way as their soluble analogues (28). Extensive research has been done in attempts to develop supported rhodium complex catalysts for olefin hydroformylation and methanol carbonylation, but the effort has not been commercially successful. The difficulty is that the polymer-supported catalysts are not sufftciendy stable the valuable metal is continuously leached into the product stream (28). Consequendy, the soHd catalysts fail to eliminate the problems of corrosion and catalyst recovery and recycle that are characteristic of solution catalysis. 

Richardson, J.T. and Paripatyadar, S.A. Carbon dioxide reforming of methane with supported rhodium. Applied Catalysis, 1990, 61 (1), 293. 

Kieffer, R., Kiennemann, A., and Rodriguez, M. Promoting effect of Lanthana in the hydrogenation of carbon monoxide over supported rhodium catalysts. Applied Catalysis. A, General, 1988,... 

In a further variation, the PVP-supported rhodium catalyst was used for methanol carbonylation in supercritical carbon dioxide [100]. This reaction medium has complete miscibility with CO and dissolves high concentrations of methanol and methyl iodide, while being a poor solvent for ionic metal complexes. Catalytic reaction rates up to half of those obtained in conventional liquid-phase catalysis were achieved with minimal catalyst leaching. 

The concept of supported IL catalysis depicted in Figure 1 has recently been introduced [22]. Preparation of the supported IL catalyst involved dissolving [Rh(nbd)(PPh3)2][PFg] in a mixture of [BMlM][PFg] and acetone, followed by addition of silica gel. The acetone was removed under vacuum, leaving the catalyst in which the IL and rhodium catalyst were immobilized on the silica support. The catalyst was used in a Robinson-Mahoney reactor to hydrogenate 1-hexene, cyclohexene and 2,3-dimethyl-2-butene. Moderate to excellent yields could be obtained and 18 batch runs without significant loss of activity were achieved. Compared with liquid-liquid biphasic catalysis the amount of IL required was drastically reduced. 

Although the oxidation of tertiary phosphines by these catalytic processes has minimal useful application, it needs to be considered as a problematic side reaction in homogeneous catalysis. Much effort is being currently expended to immobilize platinum metal phosphine complexes on heterogenized tertiary phosphine supports, and irreversible oxidation at phosphorus on these supports effectively destroys the supported catalyst. Recent observations that the compound Rh6(CO)i6 catalyzes the oxidation of tertiary phosphines correlate with the report that phosphine oxidation occurs with molecular oxygen on Rh6(CO)i6 bound to diphenylphosphino-functionalized poly(styrenedivinylbenzene). Thus, in order to use these phosphinated polymer-supported rhodium catalysts, one needs either to rigorously exclude oxygen, or to find a way to inhibit the simultaneous catalyzed phosphine oxidation. 

Using supported rhodium as catalyst, 2-nitrostyrene is converted in benzene into skatole in 70y. yield, with CO/H2 (160 atm) at 160"CD2ll. However this reaction, conducted under hydrotormi1 ation conditions, involves -formation o-f 2-homogeneous catalysis, reduction o-f the nitro group by heterogeneous catalysis, then ring closure and thermal dehydration ... 

Oh, S. H. Eickel, C. C. Influence of metal particle size and support on the catalytic properties of supported rhodium CO—O2 and CO—NO reactions. Journal of Catalysis 128, 526-536 (1991). 

Halabi, M. H., De Croon, M. H. J. M., Van Der Schaaf, J., Cobden, P. D., Schouten, J. C. (2010). Low temperature catalytic methane steam reforming over ceriaezirconia supported rhodium. Applied Catalysis A General, 389, 68—79. 

M. Ichikawa. Catalysis by supported metal crystallites from carbonyl clusters. II. Catalytic ethanol synthesis from carbon monoxide and hydrogen under atmospheric pressure over supported rhodium crystaUites prepared from Rh carbonyl clusters deposited on titanium dioxide, zirconium oxide, and lanthanum oxide. Bull. Chem. Soc. Jpn. 51, 1978, 2273-2277. 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

Products of a so-called vinylogous Wolff rearrangement (see Sect. 9) rather than products of intramolecular cyclopropanation are generally obtained from P,y-unsaturated diazoketones I93), the formation of tricyclo[2,1.0.02 5]pentan-3-ones from 2-diazo-l-(cyclopropene-3-yl)-l-ethanones being a notable exception (see Table 10 and reference 12)). The use of Cu(OTf), does not change this situation for diazoketone 185 in the presence of an alcoholl93). With Cu(OTf)2 in nitromethane, on the other hand, A3-hydrinden-2-one 186 is formed 160). As 186 also results from the BF3 Et20-catalyzed reaction in similar yield, proton catalysis in the Cu(OTf)2-catalyzed reaction cannot be excluded, but electrophilic attack of the metal carbene on the double bond (Scheme 26) is also possible. That Rh2(OAc)4 is less efficient for the production of 186, would support the latter explanation, as the rhodium carbenes rank as less electrophilic than copper carbenes. 

Rhodium Catalysed Hydroformylation Using Supported Ionic Liquid Phase SILP) Catalysis... 

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

Electron spin resonance (ESR) signals, detected from phosphinated polystyrene-supported cationic rhodium catalysts both before and after use (for olefinic and ketonic substrates), have been attributed to the presence of rhodium(II) species (348). The extent of catalysis by such species generally is uncertain, although the activity of one system involving RhCls /phosphinated polystyrene has been attributed to rho-dium(II) (349). Rhodium(II) phosphine complexes have been stabilized by steric effects (350), which could pertain to the polymer alternatively (351), disproportionation of rhodium(I) could lead to rhodium(II) [Eq. (61)]. The accompanying isolated metal atoms in this case offer a potential source of ESR signals as well as the catalysis. 

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