Homogeneous catalytic reactions catalyst recovery

An important additional interest of the heterogeneous catalysis compared to homogeneous one is the recovering and the recyclability of the porphyrin catalysts after the epoxidation reactions. For the homogeneous catalytic reactions, the recovery of the porphyrin is a difficult task as its separation from the other reactants is very difficult. In the case of the heterogeneous catalytic reactions, the insoluble polymers can be recovered by simple filtration, washed, dried and involved in a new catalytic cycle. This is a very important feature in heterogeneous catalysis. 

In many homogeneous catalyst-based industrial processes efficient recovery of the metal is essential for the commercial viability of the technology (see Section 1.4). This is especially true for noble metal-based homogeneous catalytic reactions. Apart from economic reasons spent catalyst recovery is also essential to prevent downstream problems, such as poisoning of other catalysts, deposition on process equipment, waste disposal, etc. Several different techniques are being followed industrially for the recovery of the catalyst from the reaction medium after the end of the reaction ... 

Ionic liquids used as the green solvents could avoid the utilization of volatile hazardous organic solvents, simplify the product isolation in the homogeneous catalytic reaction systems, and provide the access to the recovery and reuse of the catalytic systems as well besides, it can avoid the pollution. Furthermore, the ionic liquids could act as the catalysts, ligands, or supports of reagents through the functionalization of the ionic liquids. 

What can drive the switch from existing homogeneous catalytic processes to novel ionic liquids technology One major point is surely if a higher cost-effectiveness can be obtained in some cases. This can be attributed to an improved reaction rate and selectivity, associated with more efficient catalyst recovery and better environmental compatibility. 

One of the most important difficulties in the use of homogeneous catalysis for technical applications is the recovery and reuse of the expensive metal catalyst. Various concepts for catalyst recychng are apphed to overcome this problem in current technical applications. A simple method is to perform the catalytic reaction in a single phase and to separate the product and the catalyst afterwards by distillation of the product, as reahzed, for... 

Catalysts immobilized on resins are quite often used for catalytic reactions. Phase-transfer conditions, guaranteeing separation and recovery of the catalysts by use of water/organic solvent biphasic systems, are another possibility. Unlike homogeneous processes, these kind of catalytic processes take account of product loss and substrate selectivity. Furthermore,... 

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. 

Literature is abundant in data concerning homogeneous catalytic performances of colloids and triflates [1-2]. Although these catalysts exhibit very high activity and selectivity in a large number of reactions of practical interest, they are rather non-stable and their recovery from the solution is difficult. Therefore the preparation of hybrid catalysts preserving these species unaltered is of current interest. 

Traditionally, the Michael reaction is catalyzed by mild to moderately strong bases such as potassium t-butoxide, diisopropylamine [38], and tetramethylguani-dine [39] as homogeneous catalysts. The main disadvantages of this type of catalyst are the production of significant amounts of multiple Michael adducts, which are difficult to separate, and problems associated with catalyst recovery. These problems, plus recent interest in environmentally friendly solid catalysts, has led to the development of heterogeneous catalytic systems for this reaction. Examples of heterogeneous catalysts are those based on CsF and KF on alumina... 

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