Studies on the Characterization of Several Iridium– and Rhodium–clay Catalysts and Their Activity in Imine Hydrogenation

Iridium– and rhodium–clay catalysts were prepared by immobilizing the complexes [Ir(μ-Cl)(COD)]2 [1]+4PPh3, [Ir(COD)(PPh3)2]BF4 [2], [Ir(COD)(PPh3)2]PF6 [3], and [Rh(COD)(PPh3)2]BF4 [4] in montmorillonite K-10 (MK-10), sodium bentonite (Na+-MM), and lithium hectorite (Li+-Hect). Other analogous supports were prepared after thermal and acidic modifications to montmorillonite K-10. For example, MK-10 was calcined at 500°C (MK-10-500) and treated with nitric acid (H+HNO3-MK-10). The supports were characterized using XRD, BET surface area, FTIR, and acidity titration to confirm the properties of smectite–clay materials. The characterization of the immobilized iridium– and rhodium–clays was studied using XRD, FTIR, and conductimetric analyses. From these results, we found that the complexes are adsorbed mainly on the external surface of MK-10 and Li+-Hect. However, the immobilization of the organometallic complexes on Na+-MM is intercalated mainly by the ion-exchange process. Hydrogenation of the aldimine N-benzylidene aniline was studied with the iridium– and rhodium–clay catalytic systems. The aim of our study was to determine how the immobilized system influences the catalytic process, taking into account the metal, the counterion, and the acidity and composition of the support. In some catalytic systems the catalyst was reused for a significant number of consecutive runs. The catalyst 2–MK-10, for example, was reused in consecutive hydrogenations and showed high activity (>97% conversion) for at least 13 runs. The loss of catalytic activity as a consequence of the relative stability of the metallic intermediate species to air is also discussed.