Metal Nanoparticles Stabilized in Ionic Liquids for Catalytic Multiphase Reactions

Metal nanoparticles (MNPs) have attracted widespread interest as catalytic materials due to their small size and high surface to volume ratio, which lead to a greater catalytic activity. To preserve such features however, agglomeration of MPNs must be prevented by the use of stabilizing agents of which ionic liquids (ILs) are probably the best ever reported compounds. ILs offer unique environments suitable for both the synthesis, the stabilization, and the segregration of highly dispersed MNPs. Such dispersions have been successfully tested for catalysis in a number of different processes mostly including hydrogenations, hydroformylations, and C-C bond forming reactions (Heck, Suzuki, Sonogashira, and Ullman couplings). In this review, an introductory section is focused on major aspects accounting for the stabilizing effects of ILs on MNPs. Then, model examples of catalytic applications are given for Pd-, Rh-, and Ru- NPs in multiphase systems composed of binary and ternary immiscible liquid phases which are able not only to improve the catalytic performance of NPs, but to allow also the recycling of nanocatalysts and the easy separation of reaction products.