Gold catalysts recycling

Rhodium was about three times the price of gold through 1988—1989 until skyrocketing to 74/g ( 2300/troy oz) in early 1990. Thus precious metal catalyst costs requite an absolute minimum level of use and maximum number of catalyst recycle uses when batch processing is employed. Starting material contaminants may effect catalyst poisoning, though process routes to overcome this by feed stream pretreatment may be devised (37,60). 

Recently, gold has emerged as one of the most active catalysts for alcohol oxidation and is especially selective for poly alcohols. In 2005, Corma [184] and Tsu-kuda [185], independently demonstrated the potential of gold nanoparticles for the oxidation of aliphatic alcohols. For example, in the case of gold nanoparticles deposited on nanocrystalline cerium oxide [184], a TOF of 12 500 h 1 was obtained for the conversion of 1-phenylethanol into acetophenone at 160 °C (Fig. 4.67). Moreover this catalyst is fully recyclable. Another example of a gold catalyst with exceptional activity is a 2.5% Au-2.5% Pd/Ti02 as catalyst [186]. In this case for 1-octanol a TOF of 2000 h-1 was observed at 160 °C (reaction without solvent, Fig. 4.67). 

Catalytic oxidation of vicinal diols to a-hydroxy carboxylates was performed by Prati and Rossi [82] in alkaline aqueous solution with gold-based catalysts prepared by deposition-precipitation with sodium carbonate from HAUCI4 solutions on active carbon suspension. The 1 % Au/C catalysts had remarkable properties compared with conventional Pt/C and Pd/C catalysts in terms of selectivity and stability. Thus, Table 1 shows that at pH 8,1,2-propanediol was very selectively oxidized to lactic acid, which indicates that gold was more selective than platinum and palladium in the oxidation of the primary alcohol function. The activity of gold catalysts was also very stable as a function of conversion or after several recycles, indicating that gold is less sensitive to over-oxidation and/or self-poisoning than platinum and palladium. 

Development of homogeneous gold catalysts with improved stability and recyclability that will render gold catalysis more sustainable and allow its use in industrial applications... 

Similarly, Corma and coworkers showed that gold nanopartides deposited on nanocrystaHine ceria form an excellent recyclable catalyst for the aerobic oxidation of alcohols [97, 98] (Figure 5.18). Another example of a gold catalyst with exceptional activity is a 2.5% Au-2.5% Pd/Ti02 [99] (Figure 5.18). As mentioned above, Auis now... 

As can be seen in the scheme below, the catalytic activity of gold species was also shown in a multi-substituted furan synthesis. Cyclization of allenones in the presence of Au(III)-porphyrin gave rise to the corresponding substituted furan in good to high yields. The catalyst can be recycled several times and still maintain the same catalytic activity <06OL325>. 

Two tetrahalogen-gold(III) compounds (AuCf, AuBrCfi ) in ionic liquids were studied by Raubenheimer et al. for the hydration of phenylacetylene. Although the activity was lower than that reported in previous studies, this strategy described the re-use of the catalyst achieved by recycling the ionic liquid phase [95]. 

Easy recycling of gold hydrosilation catalysts has also been achieved using a fluorous approach.Conversions varied from moderate to excellent for the reaction of dimethylphenylsilane with benzaldehyde. However, the mechanism is not clear at this stage. The catalyst could not be recycled in the absence of fluorous solvents under thermomorphic conditions and the formation of... 

For industrial applications, metal catalysts should be repeatedly recycled or used in continuous mode for a long time. The introduction of gold catalysis in the aerobic oxidation of glucose has opened exciting perspectives Au is a biocompatible, nontoxic metal, which allows even superior productivities with respect to enzymatic catalysis [38], and no leaching problems have been observed using nanometric particles dispersed on different supports [43]. Compared with chemical oxidations, enzymatic catalysis suffers from more plant complexities... 

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