Strategies to developleaching-free heterogeneous catalysts
In order to benefit from the numerous advantages of heterogeneous catalysis (especially the simple removal of the catalyst from the reaction mixture), a wide variety of catalysts supported on different solid supports has been developed over the last decades. Among the different possibilities to attach molecules to solid supports, immobilization using covalent tethering techniques is currently the most promising approach for designing stable immobilized catalysts. Nevertheless, although the obtained catalysts seem to be heterogeneous, the detailed study often revealed that the real nature of the catalyst during the reaction is homogeneous. In this study different methods to immobilize Pd- and Ti-compounds as well as biocatalysts and methods to investigate the leaching behavior are reviewed.
Although homogeneous catalysts are very often applied in industrial processes due to their high activity and selectivity, they have one big disadvantage: they are in the same phase as the educts and products (except for phase-transfer catalysts). Although highly potential methods for metal removal and recycling exist (1), the complete elimination from the reaction mixture to avoid metal carry-over is intricate and costly, reducing the potential for industrial implementation, especially in the field of pharmaceutical synthesis. Furthermore, many homogeneous systems usually consist of the metal and the ligands in a particular stoichiometric ratio. Thus, the removal of the ligands makes the ... ...