Catalyst recovery and recycling

As noted above, recovery and recycling of chemo- and biocatalysts is important from both an economic and an environmental viewpoint. Moreover, compart-mentalization (immobilization) of the different catalysts is a conditio sine qua non for the successful development of catalytic cascade processes. As discussed in Chapter 7, various approaches can be used to achieve the immobilization of a homogeneous catalyst, whereby the most well-known is heterogenization as a solid catalyst as in the above example. 

An approach to immobilization which has recently become popular is microencapsulation in polymers, such as polystyrene and polyurea, developed by the groups of Kobayashi [34] and Ley [35], respectively. For example, microencapsulation of palladium salts or palladium nanoparticles in polyurea microcapsules 

An interesting example of the use of a recyclable, thermoresponsive catalyst in a micellar-type system was recently reported by Ikegami et al. [38]. A PNI-PAM-based copolymer containing pendant tetraalkylammonium cations and a polyoxometalate, PW1204o, as the counter anion was used as a catalyst for the oxidation of alcohols with hydrogen peroxide in water (Fig. 9.25). At room temperature the substrate and the aqueous hydrogen peroxide, containing the catalyst, formed distinct separate phases. When the mixture was heated to 90 °C a 

In Chapter 7 we have already discussed the use of fluorous biphasic systems to facilitate recovery of catalysts that have been derivatized with fluorous ponytails . The relatively high costs of perfluoroalkane solvents coupled with their persistent properties pose serious limitations for their industrial application. Consequently, second generation methods have been directed towards the elimination of the need for perfluoro solvents by exploiting the temperature-dependent solubilities of fluorous catalysts in common organic solvents [42]. Thus, appropriately designed fluorous catalysts are soluble at elevated temperatures and essentially insoluble at lower temperatures, allowing for catalyst recovery by simple filtration. 

Industry employs several techniques for solving these problems [116]. The most common are selective product crystallization, where the catalyst and the excess substrates and reagents are left in the liquid phase, and catalyst precipitation and filtration, where the catalyst is precipitated as a salt from the organic reaction mixture. Other techniques include flash distillation of the product under high vacuum, and liquid/liquid extraction of the catalyst from the reaction mixture. 

Anchoring the catalyst to a solid support is another popular option. Although such systems are no longer truly homogeneous, I include them in this chapter because 

Another option is encapsulating the homogeneous complex in an (inorganic) cage, creating a ship-in-a-bottle hybrid catalyst. Zeolites are often used for trapping large 

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Polymer-supported catalysts incorporating organometaUic complexes also behave in much the same way as their soluble analogues (28). Extensive research has been done in attempts to develop supported rhodium complex catalysts for olefin hydroformylation and methanol carbonylation, but the effort has not been commercially successful. The difficulty is that the polymer-supported catalysts are not sufftciendy stable the valuable metal is continuously leached into the product stream (28). Consequendy, the soHd catalysts fail to eliminate the problems of corrosion and catalyst recovery and recycle that are characteristic of solution catalysis. 

O Dalaigh, C., Corr, S.A., Gun ko, Y. and Connon, S.J. (2007) A magnetic-nanoparticle-supported 4-N, N-dialkylaminopyridine catalyst excellent reactivity combined with facile catalyst recovery and recyclability. Angewandte Chemie International Edition, 46 (23), 4329-4332. 

Various types of POMs are effective catalysts for the H202- and 02-based environment-friendly oxidations. Most of these oxidations are carried out in homogeneous systems and share common drawbacks, that is, catalyst/product separation and catalyst recycling are very difficult. The heterogenization of POMs can improve the catalyst recovery and recycling. This chapter focuses on the development of (1) homogeneous catalysts with POMs and (2) the heterogenization for liquid phase-oxidations. 

A key economic problem in all industrial oxo processes is the recovery of the homogeneous catalysts. It is important in the case of both the original, relatively unstable cobalt and the very expensive rhodium complexes. A number of special procedures were developed for catalyst recovery and recycling.75,79... 

The above processes are only selected examples of a vast number of process options. In the case of carbonylation, the formation of by-products, primarily isocyanate oligomers, allophanates, and carbodiimides, is difficult to control and is found to greatly reduce the yield of the desired isocyanate. Thus a number of nonphosgene processes have been extensively evaluated in pilot-plant operations, but none have been scaled up to commercial production of diisocyanates primarily due to process economics with respect to the existing amine—phosgene route. Key factors preventing large-scale commercialization include the overall reaction rates and the problems associated with catalyst recovery and recycle. 

Furthermore, as mentioned before, organic catalysts may be ready immobilized on a support with the aim of facilitating catalyst recovery and recycling (Benaglia et al. 2003). 

In order to increase the number and a proper spatial arrangement of the catalytic sites the loading expansion of PEG was carried out exploiting the principles of dendrimer chemistry and led to the synthesis of the PEG-supported tetrakis ammonium salt 14 (Tocco et al. 2002). This catalyst displayed a higher catalytic efficiency than 13, while retaining the solubility properties peculiar of the PEG support, that allowed simple catalyst recovery and recycling by precipitation and filtra-... 

Other non-classical reaction media [96] have, in recent years, attracted increasing attention from the viewpoint of avoiding environmentally unattractive solvents and/or facilitating catalyst recovery and recycling. Two examples, which readily come to mind, are supercritical carbon dioxide and room temperature ionic liquids. Catalytic hydrogenation in supercritical C02, for example, has... 

Chapter 7 addresses another key topic in the context of green chemistry the replacement of traditional, environmentally unattractive organic solvents by greener alternative reaction media such as water, supercritical carbon dioxide, ionic liquids and perfluorous solvents. The use of liquid/liquid biphasic systems provides the additional benefit of facile catalyst recovery and recycling. 

Given the prolific use of cross-coupling chemistry in such a wide array of applications, it is easy to see that these reactions have matured remarkably from the academic curiosity they were a mere 20-30 years ago. As industry comes to rely more heavily on this chemistry, future developments will doubtlessly be driven by economics. Strategies for catalyst recovery and recycling have been developed, but there is... 

Catalyst recovery and recycle Amide also gets enriched in its deuterium content. This deuterium and amide must be recovered by evaporating ammonia and stripping off deuterium by scrubbing. 

The discussion in the previous sections has evidenced that the use of biphasic systems has solved, at least in various cases, the problem of homogeneous catalyst recovery and recycle, but there still exists the problem of the cost of recycle and especially of reaction rate per volume of reactor, which derives in large part from mass- and heat-transfer limitations, but also from the low amount of catalytic centers per volume of reactor necessary to avoid side reactions and maintain a high selectivity, and/or limit catalyst deactivation or loss. These aspects often emerge only during the scaling-up and industrialization of the reaction and this is one of the reasons why many interesting reactions at the laboratory scale fail in commercialization. 

Features of the Spherizone process help to reduce both resource consumption and emissions (Figure 17.11). These include use of high yield, highly stereospecific catalysts, recovery and recycle of umeacted monomers, the absence of undesired byproducts from the reaction and the low energy consumption. Owing to the MZCR concept, in the polymerization section of the process the overall energy consumption may be reduced by 0-30%, depending on the type of polymer produced. 

Another method for catalyst recovery and recycling is the use of supported, immobilized, or membrane-trapped (148) catalysts. Examples of... 

The addition of HCN to C=C double bonds can be effected in low yields to produce Markovnikov addition products. However, through the use of transition metal catalysts, the selective anti-Markovnikov addition of HCN to alkenes can take place. The most prominent example of the use of aqueous media for transition metal-catalyzed alkene hydrocyanation chemistry is the three-step synthesis of adi-ponitrile from butadiene and HCN (Eqs. 5-7). First discovered by Drinkard at DuPont [14], this nickel-catalyzed chemistry can use a wide variety of phosphorus ligands [15] and is practiced commercially in nonaqueous media by both DuPont and Butachimie, A DuPont/Rhone-Poulenc joint venture. Since the initial reports of Drinkard, first Kuntz [16] and, more recently, Huser and Perron [17, 18] from Rhone-Poulenc have explored the use of water-soluble ligands for this process to facilitate catalyst recovery and recycle from these high-boiling organic products. 

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