Catalyst Recovery and Reuse

New approaches to catalyst recovery and reuse have considered the use of membrane systems permeable to reactants and products but not to catalysts (370). In an attempt to overcome the problem of inaccessibility of certain catalytic sites in supported polymers, some soluble rho-dium(I), platinum(II), and palladium(II) complexes with noncross-linked phosphinated polystyrene have been used for olefin hydrogenation. The catalysts were quantitatively recovered by membrane filtration or by precipitation with hexane, but they were no more active than supported... 

This section describes catalytic systems made by a heterogeneous catalyst (e.g., a supported metal, dispersed metals, immobilized organometaUic complexes, supported acid-base catalysts, modified zeolites) that is immobilized in a hydrophilic or ionic liquid catalyst-philic phase, and in the presence of a second liquid phase—immiscible in the first phase—made, for example, by an organic solvent. The rationale for this multiphasic system is usually ease in product separation, since it can be removed with the organic phase, and ease in catalyst recovery and reuse because the latter remains immobilized in the catalyst-philic phase, it can be filtered away, and it does not contaminate the product. These systems often show improved rates as well as selectivities, along with catalyst stabilization. 

Yu K, Sommer W, Richardson JM, Week M, Jones CW. Evidence that SCS pincer Pd(II) complexes are only precatalysts in Heck catalysis and the implications for catalyst recovery and reuse. Adv Synth Catal 2005 347 161-171. 

Finally, the use of the expensive metal palladium as the metal of choice in the telomerization reaction holds obvious disadvantages for the economic feasibility of large-scale processes. Catalyst recovery and reuse should therefore receive further attention in future studies, for instance, by clever reactor design, or heteroge-nization of the catalyst. Alternatively, the use of palladium might be completely avoided if non-noble metals can be prompted to perform the same reactions when a suitably designed ligand environment is offered. 

On the other hand, polymeric beads of supported ILs ( polymer-supported imidazolium salt, PSIS) were also prepared via the covalent anchoring of an imidazolium salt to a polystyrene resin.These PSISs, which have the advantage of significantly enhancing the nucleophilicity of metal salts compared with conventional methods, have been used as efficient catalysts for nucleophilic fiuorination and for other nucleophilic substitution reactions. In particular, the authors found that the applied PSIS had many practical merits product recovery and purification was simple and catalyst recovery and reuse were easy (Figure 4.12). 

For industrial biotransformations, catalyst recovery and reuse are major issues. This may be desirable either for reasons of downstream processing or for repeated use in order to reduce the specific catalyst costs per kg of product produced. A very simple method is the use of membrane filtration. Because of the increasing number of membranes from different materials (polymers, metal or ceramics) this is an attractive alternative. Whereas for whole cells microfiltration or centrifugation can be applied, for the recovery of soluble enzymes ultrafiltration membranes have to be used120-221. Often immobilization on a support is chosen to increase the catalyst s stability as well as to facilitate its recovery. The main advantages of immobilization are ... 

This approach has now been applied to a broad range of catalytic reactions, such as hydrogenation, hydroformyla-tion, hydroboration, and many palladium-catalyzed cross-coupling reactions, and excellent catalyst recovery and reuse have been reported. Although there are many comprehensive reviews of fluorous biphase catalysis,the... 

In addition to traditional organocatalysis, ionic liquids as new catalytic systems have been explored. The first examples used nonimmobilised OTBDPS-L-Ser, protonated arginine or lysine in the presence of ionic liquids based on l-alkyl-3-methyl imidazolium ([bmim], [hmim], [omim]) or JV-butyl-N-methyl pyrrolidinium ([bmpy]) ions. The systems, in addition to giving the aldol adducts with high yields and ee, are efficient for catalyst recovery and reuse. Since 2010 new structures containing a primary amino acid coupled with a 1,2,3-triazolium salt, an acyl group or a polystyrene have been developed. The more effective ones for the aldol reaction depicted Scheme 12.3 (13, 14 and 15, respectively) are represented in Figure 12.3. 

Contained within this book are various chapters that review the possibilities for the sustainable use of catalysts in our chemical industiy. Earth abundant metals are discussed in Sustainable Catalysis With Non-endangered Metals, Parts 1 and 2, while the options for organocatalysis are discussed in Sustainable Catalysis Without Metals or Other Endangered Elements, Parts 1 and 2. The future chemical industiy cannot survive by the use of just one of the above catalyst classes, but will require the flexibility and versatility of both. An important aspect of sustainable catalysis that is also vital for the long-term security of elements is ensuring that we establish improved methods of catalyst recovery and reuse. 

When the coupling reactions are conducted in homogeneous systems in the presence of ligands, separation of the catalysts from the products can often be problematic. However, catalyst recovery and reuse are often highly desirable, especially when precious metal catalysts are used with high loading. It is known that heterogeneous catalysts can be separated from the reaction products more easily, but the catalysts are usually less active. 

Catalyst recovery and reuse Easy, reusability proved but not sufficiently studied Difficult, neutralized by an acid partially lost in postprocessing steps Difficult, the catalyst ends up in the byproducts No reusable catalyst... 

Simplicity of separation of the products from the reaction mixture and the ease of catalyst recovery and reuse... 

---