Leaching metal from solid catalyst

Finally, we note that caution should be exercised, because many of the solid catalysts reported in the literature have not been subjected to rigorous proof of heterogeneity under oxidizing conditions [31]. Both alkyl hydroperoxides and, especially, hydrogen peroxide can readily leach metal ions from solid catalysts. 

A simple but effective means of preparing supported metal ion catalysts is to employ ion exchange resins. For example, a cobalt-exchanged H-type resin (Dowex 50) was shown43 to be an effective solid catalyst for the autoxidation of acetaldehyde to acetic acid at 20°C. No leaching of cobalt ions from the resin was observed and the catalyst was used repeatedly (5x) without any significant loss of activity. More recently the use of weak acid resins exchanged with cobalt ions as catalysts for the autoxidation of cyclohexane... 

As mentioned previously, the role of the metal is not specific. Even transition metal free layered double hydroxides are suitable catalysts for the olefin epoxidation with O2 and a sacrificial aldehyde (205). Leaching of the metal from the solid catalyst is a serious problem since organic acids that are potential metal ligands accumulate during the reaction. Leaching occurs, for example, with the polybenzimidazole-supported Ni2+ catalyst (199). 

Consequently the mechanism would be a homogeneous one, where the solid catalyst serves only as a reservoir for active Pd species in solution as already proposed by Mizoroki. This general problem of metal leaching from supported catalysts in liquid-solid phases is well known and e.g. illustrated by Sheldon et al. [202]. 

There is evidence in some cases that C-C coupling by heterogeneous catalysts also occurs by soluble metal colloids (or smaller species) leached from solid supports, such as silica. In the 1970s, the catalytic activity of palladium black for C-C coupling was already reported by Mizoroki et al. [11,12], Various examples of Heck reactions with heterogeneous catalysts have been reported since then [13]. Therefore, while heterogeneous catalysts are not the main focus of this section, they are addressed where appropriate. 

Metal oxides are usually utilized in their pure form or in different mixtures as solid catalysts in fine chemical preparation since they are commercially available or easily synthesized, they are stable toward moisture, and their properties can be tailored by doping with convenient metal ions [74]. Dimethylbenzophenones can be obtained in high yield (88-97%) by reaction between benzoyl chloride and the three isomeric xylenes (xylene/benzoyl chloride ratio=1.5) in the presence of iron(III) oxide. Various researchers have observed leaching of FeClj (produced by reaction of iron oxide with HCl) into solution, suggesting a possible catalytic contribution from this homogeneous iron-based Lewis acid. 

The procedures which allow for the immobilization of a metal complex on or in a solid are numerous and well described they result from more than thirty years of continuous and imaginative efforts. More difficult, and consequently less successful, work has been described regarding the syntheses of efficient catalysts from these precursors. An immobilized complex must show high activity and selectivity for the target reaction, must be easily recovered intact, and must be stable towards metal leaching under the reaction conditions. These two latter requirements are particularly important where asymmetric catalysis is the goal, because the metal and the ligands... 

Immobilization is a popular means of simplifying separation of a catalyst from the reaction mixture. In contrast with immobilized metal complexes (via a solid-support-bound ligand) leaching problems are a less critical issue when using organocatalysts immobilized by covalent bonding to the solid support. 

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