Catalyst deactivation leaching

Supported aqueous phase (SAP) catalysts (16) employ an aqueous film of TPPTS or similar ligand, deposited on a soHd support, eg, controlled pore glass. Whereas these supported catalysts overcome some of the principal limitations experienced using heterogeneous catalysts, including rhodium leaching and rapid catalyst deactivation, SAP catalysts have not found commercial appHcation as of this writing. 

Degradation of the ligand or the linker to the support must be avoided, as this results in metal leaching and catalyst deactivation. For example, phosphorus ligands are sensitive to oxidizing impurities in the feed (peroxides). 

In-situ IR-spectroscopic characterization of the Friedel-Crafts acylation of benzene in ionic liquids derived from AICI3 and FeCl3 showed that the mechanism of the reaction in ionic liquids was the same as that in 1,2-dichloroethane (128). The immobilization of ferric chloride-containing ionic liquid onto solid supports (e.g., silica and carbon) however failed to catalyze the acylation reaction, because leaching was a serious problem. When the reaction was carried out with gas-phase reactants, catalyst deactivation was observed. 

The importance of catalyst stability is often underestimated not only in academia but also in many sectors of industry, notably in the fine chemicals industry, where high selectivities are the main objective (1). Catalyst deactivation is inevitable, but it can be retarded and some of its consequences avoided (2). Deactivation itself is a complex phenomenon. For instance, active sites might be poisoned by feed impurities, reactants, intermediates and products (3). Other causes of catalyst deactivation are particle sintering, metal and support leaching, attrition and deposition of inactive materials on the catalyst surface (4). Catalyst poisons are usually substances, whose interaction with the active surface sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface. Selective poisons are sometimes used intentionally to adjust the selectivity of a particular reaction (2). 

Ardizzone et al. used the esterification of benzoic acid with methanol to test the catalytic performance of different SZ catalysts. " Water had to be continuously removed from the reaction medium to shift the reaction equilibrium to product formation and to avoid catalyst deactivation by sulfate leaching. According to these authors, catalysts with a higher density of acid sites with KdL values in the range —14.2 to -5.6 performed better. Acid sites with pKa. of... 

Some cases of catalyst deactivation by over-oxidation platinum leaching, platinum particle growth and site coverage during reductive pretreatment as well as during reaction were presented for the oxidation of ethanol and methyl-a-D glucopyranoside (MGP) in combination with the use of various catalyst characterization techniques. 

Catalyst deactivation, for example, via leaching, dissolution of the catalyst, or decrease of metal dispersion has to be resolved. 

In summary, this work demonstrates that high selectivities for oxygenated keto-acids derived from glycerol may be obtained by catalytic oxidation on bismuth-promoted platinum under acidic conditions. However, problems of catalyst deactivation by adsorbed acids, overoxidation of targeted products and leaching of the promoter need to be overcome to attain the ultimate goal of theoretical yield. 

The conversion of alcohols in liquid-phase oxidation on metals does not go to completion, or proceeds at a very slow rate, because catalysts deactivate in the course of reaction. Deactivation could be irreversible when the catalyst structure is modified, e. g. by metal-particle growth or metal leaching, or partially reversible when the metal surface is partially blocked by oxygen or reaction products. 

One of the rare examples for the use of immobilized oxynitrilases has been published by Degussa [146]. The company investigated the asymmetric synthesis of (i )-cyanohydrins and used (i )-oxynitrilase, which had been cross-linked and subsequently polyvinyl alcohol-entrapped. The obtained immobilized lens-shaped biocatalysts were much more satisfying in terms of long-term stability and activity compared to the free enzyme and also showed less catalyst leaching than other enzyme supports. Moreover, the immobilization method is cheap, efficient, feasible on an industrial scale, and gives particles of defined size. The utility of these entrapped enzymes could be shown, as indicated in Scheme 57, in the synthesis of (i )-mandelonitrile (R)-175) from aldehyde 174. No catalyst deactivation was observed even after 20 cycles of reuse and yields as well as optical purities of (R)-175 remained constant within normal limits. 

The recyclability of a catalyst is of major concern in the area of Green Chemistry (46, 269-271) and leaching is a major contributor to catalyst deactivation during repeated use. Contamination of the products with dissolved catalyst is also a significant concern since it may add costly purification steps. Because of these concerns, leachproof is currently one of the most common terms used to describe advantageous properties of a supported catalyst. 

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