Catalyst anchored 233 - losses

Chemical anchoring of catalytically active metal clusters onto a support is of practical importance to stabilize catalysts against loss of activity by Ostwald ripening, i.e. metal agglomeration. Documented examples include Pt, Pd, or Rh supported on acidic oxides, in particular zeolites in their H-form. Three types of anchors have been de-... 

We report here a number of examples of the use of this anchored catalyst for the hydrogenation of different substrates at moderate to high substrate/catalyst ratios along with a direct comparison of these results with those obtained using the homogeneous Wilkinson under the same conditions. Also presented will be some examples of the use of the anchored catalyst in long term continuous reactions. Reaction rates, selectivities and the extent of metal loss will be presented where appropriate. 

The reaction of pure silica MCM-48 with dimethyldichlorosilane and subsequent hydrolysis results in hydrophobic materials with still a high number of anchoring sites for subsequent deposition of vanadium oxide structures. The Molecular Designed Dispersion of VO(acac)2 on these silylated samples results in a V-loading of 1.2 mmol/g. Spectroscopic studies evidence that all V is present as tetrahedral Vv oxide structures, and that the larger fraction of these species is present as isolated species. These final catalysts are extremely stable in hydrothermal conditions. They can withstand easily hydrothermal treatments at 160°C and 6.1 atm pressure without significant loss in crystallinity or porosity. Also, the leaching of the V in aqueous conditions is reduced with at least a factor 4. 

Thus, the best result is obtained with (-)-ephedrine anchored on the Al-MTS 2 support (A1-MTS-C1-E 2b) and it is worth noting that ee and activity are close to those obtained in homogenous conditions ((-)-ephedrine, (-)-N-propyl-ephedrine). Moreover, this catalyst can be reused two times without loss of enantioselectivity. 

Anchoring of active catalysts to insoluble materials such as oxides, silicates, and zeolites often reduces the loss of catalyst during the catalytic process (66). The fixation of the active centers can be achieved either by means of their interaction with hydroxyl groups on a solid surface or, alternatively, by means of interactions between the CO ligands of the metal complex and a Lewis acidic center of the surface. Zeolite-supported cobalt catalysts have been reported for hydroformylation reactions (67). 

The homogeneous chiral phosphine/DPEN-Ru catalyst can be immobilized by use of polymer-bound phosphines such as polystyrene-anchored BINAP (APB-BINAP) [57, 98], Poly-Nap [99], and poly(BINOL-BINAP) [100], poly(BINAP) [101]. These complexes hydrogenate T-acetonaphthone and acetophenone with S/C of 1000-10 000 under 8 10 atm H2 to give the corresponding secondary alcohols in 84-98% e.e. The recovered complexes are repeatedly used without significant loss of reactivity and enantioselectivity. Immobilization allows the easy separation of catalyst from reaction mixture, recovery, and reuse. These advantages attract much attention in combinatorial synthesis. 

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