CATALYST PRODUCT SEPARATION TECHNIQUES IN HECK REACTION

The Heck reaction finds several applications in industry because it is one of the effective tools for the formation of a new C─C bond. In addition to the catalytic activity and selectivity, catalyst–product separation strategies are very important for the industrial application. There are various methods of interest ranging from conventional heterogeneous catalysts to heterogenization of homogeneous catalysts. The heterogeneous catalysts are classified into supported metal catalysts, zeolite-encapsulated catalysts, colloids–nanoparticles, and intercalated metal compounds. The homogeneous metal complexes catalysts are heterogenized using modified silica catalysts, polymer-supported catalysts, biphasic catalysts, supported liquid-phase catalysts, nonionic liquids solvents, perfluorinated solvents, and reusable homogeneous complexes. In general, heterogeneous catalysts are effective and stable at higher temperatures, which may be important for the activation of less reactive but less expensive chloroaryls substrates. However, the heterogeneous catalysts have a major drawback of poor selectivity toward Heck coupling products. The heterogenized metal complexes catalysts operate under relatively mild conditions as compared with heterogeneous catalysts, and so they can be applied to the production of pharmaceuticals and fine chemicals. Catalysis using supercritical solvents with catalyst separation techniques is promising for the development of green chemistry processes. Although the concepts described in this article have been reviewed mainly for Heck reactions, they should be applicable to a wide range of other chemical transformations (hydrogenation, carbonylation, hydroformylation, and so on) that, currently, are homogeneously catalyzed reactions.