Rhodium zeolites specific activity

Encapsulated rhodium complexes were prepared from Rh-exchanged NaY zeolite by complexation with (S)-prolinamide or M-tert-butyl-(S)-prolinamide [73,74]. Although these catalysts showed higher specific activity than their homogeneous counterparts in non-enantioselective hydrogenations, the hydrogenation of prochiral substrates, such as methyl (Z)-acetamidocinnamate [73] or ( )-2-methyl-2-pentenoic acid [74], led to low... 

The optimum rhodium level, with respect to specific activity, for RhNaY catalysts was found (209) to be about 0.6 wt. % Rh (4% replacement of Na+ assuming Rh3+), which corresponds to only approximately one rhodium site per unit cell of zeolite Y [i.e., an approximate stoichiometry of RhNa50(AlO2)56(SiO2)136]. This implies that the average distance between... 

The addition of trimethylphosphine to these rhodium/zeolite catalysts destroyed all catalytic activity because the phosphine was small enough to fit into the zeolite cavity and could deactivate all of the rhodium in the catalyst. The bulky tributylphosphine, however, could not enter the cavity and, thereby, only blocked the external rhodium from further reaction. This specific blocking enhanced the selectivity in the hydrogenation of a mixture of cyclopentene and 4-methylcyclohexene over a Rh/ZSM-11 catalyst. After treatment of the catalyst with tributylphosphine to block the external catalytically active sites, only the... 

The Role of Zeolites Where zeolites seem to play a more specific role appears probably when activities and selectivities are examined. Though few comparative studies were made possible upon examination of the litterature, it would appear that both rhodium and iridium exhibited lower activities than in the homogeneous reaction systems but far larger activities than exhibited when supported over traditional carriers (A1 0, Al 0 -Si0 , carbon, etc...). This is probably due to the better molecular dispersion of the rhodium and iridium catalytic precursors in zeolite media than over any of these carriers and still more important diffusion limitations than in solution. 

Because the accelerating effect of the solid catalyst comes from the surface atoms, it is desirable to have the largest surface possible, meaning a high degree of dispersion of the substance in question. In most cases, very small particles of catalytically active material such as platinum or rhodium are applied for stability to highly porous carrier materials with specific surfaces of several hundred square meters per gram. Aluminum oxide, silicon dioxide, activated carbon, as well as zeolites (crystalline aluminosilicates with numerous submicroscopic pores and canals) are all suitable for this purpose. Examples of such supported catalysts are... 

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