Recovery and reuse of the catalyst

A method for the polymerization of polysulfones in nondipolar aprotic solvents has been developed and reported (9,10). The method reUes on phase-transfer catalysis. Polysulfone is made in chlorobenzene as solvent with (2.2.2)cryptand as catalyst (9). Less reactive crown ethers require dichlorobenzene as solvent (10). High molecular weight polyphenylsulfone can also be made by this route in dichlorobenzene however, only low molecular weight PES is achievable by this method. Cross-linked polystyrene-bound (2.2.2)cryptand is found to be effective in these polymerizations which allow simple recovery and reuse of the catalyst. 

The membrane allows not only the recovery and reuse of the catalyst but also the selective separation of the molecules present in the reaction environment. In the conventional photoreactors the molecules and their by-products are freely transported in the final stream giving a not efficient system [73]. In a PMR, if a suitable membrane is used, it is possible to enhance the residence time of the molecules to be degraded or to obtain a selective separation of the products. 

Base-catalysed reactions such as the Knoevenagel, Michael and aldol reactions continue to be of importance in industrial routes to synthetic chemicals and are often inherently clean, with water (or nothing) as the by-product. Traditional homogeneous methods of catalysis often require upwards of 40 mol% catalyst (such as piperidine) with the attendant difficulties in recovery and reuse of the catalyst. They often offer extremely poor selectivity to the desired products, either due to competing processes (side reactions) or further reaction of the first-formed product. 

The subject of alternative reaction media (neoteric solvents) also touches on another issue which is very relevant in the context of this book recovery and reuse of the catalyst. This is desirable from both an environmental and an economic viewpoint (many of the catalysts used in fine chemicals manufacture contain highly expensive noble metals and/or (chiral) ligands. 

The main goals when immobilizing a catalyst on a support are to simplify the reaction work-up, the recovery and hopefully the recycling of the precious chiral catalytic species. However, besides the recovery and reuse of the catalyst, other reasons may lead to the development of a supported version of a catalyhc species. 

Recovery and reuse of the catalyst were also possible in this reaction. After the reaction was completed, the aqueous layer was concentrated to give the catalyst. The recovered catalyst was effective in subsequent Diels-Alder reactions, and it... 

In this case, a heterogeneous catalyst can be used under batch conditions. This would allow removal of fhe cafalysf by filfration or centrifugation, making possible the recovery and reuse of the catalyst and, consequently, an increase in catalyst lifetime. 

Because ofthe presence ofalarge amount of water appears to considerably narrow the field of the previously illustrated resolution system, experiments were started with immobilized enzymes in organic solvents, in which the water concentration was considerably reduced. This proved to be quite an effective strategy, permitting to carry out a nicely working DKR on a representative array of aliphatic N-Boc-amino acid thioesters [62], which could be resolved in high yield and with excellent optical purity. Moreover, the choice of an immobilized form of the enzyme (Alcalase -CLEA ) permitted recovery and reuse of the catalyst for several consecutive batches [63]. The solvent of choice proved to be tert-butanol, which was able to dissolve the hydrophobic substrates, the organic base, and the strictly necessary amount of water (Table 8.6). 

Indeed, some Diels-Alder reactions in water without catalyst have been reported [45].) Thus, naphthoquinone reacted with cyclopentadiene in THF-H2O (9 1) at room temperature to give the corresponding adduct in a 93% yield (endo/exo = 100/0). Recovery and reuse of the catalyst were also possible in this reaction. After the reaction was completed, the aqueous layer was concentrated to give the catalyst. The recovered catalyst was effective in subsequent Diels-Alder reactions, and it should be noted that the yields of the second and even the third runs were comparable to that of the first run. 

Highly cross-linked polymer-supported BINAPHOS ligands were effective for the hydroformylation of styrene and other functionalized olefins (ee s up to 89%). Recovery and reuse of the catalyst was possible at low stirring conditions [28]. 

Catalysts may alter or improve the selectivity of a reaction—particular attention has been paid over the last 30 years to chiral species that can induce the formation of enantiomerically enriched products. Only small quantities of an effective catalyst are required, usually 1 % or less. In some cases, recovery and reuse of the catalyst may be possible. This is only rarely the case with homogeneous catalysts, although every effort is made to recycle those catalysts based on precious metals. Recovery of heterogeneous catalysts is generally easy, requiring only filtration of the reaction mixture. 

Metal nanoparticles are attractive catalysts because of their large surface area and the density of unsaturated surface coordination sites. Fluorous ponytails have recently been incorporated into such metal nanoparticles to enhance their stability and facilitate the recovery of the catalysts. Immobilization of fluorous metal nanoparticles in FSG is a convenient way for the recovery and reuse of the catalyst. 

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