Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis

Abstract

Interest in catalysis by metal nanoparticles (NPs) is increasing dramatically, as reflected by the large number of publications in the last five years. This field, "semi-heterogeneous catalysis", is at the frontier between homogeneous and heterogeneous catalysis, and progress has been made in the efficiency and selectivity of reactions and recovery and recyclability of the catalytic materials. Usually NP catalysts are prepared from a metal salt, a reducing agent, and a stabilizer and are supported on an oxide, charcoal, or a zeolite. Besides the polymers and oxides that used to be employed as standard, innovative stabilizers, media, and supports have appeared, such as dendrimers, specific ligands, ionic liquids, surfactants, membranes, carbon nanotubes, and a variety of oxides. Ligand-free procedures have provided remarkable results with extremely low metal loading. The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.

Recent developments of task-specific ionic liquids in organic synthesis

“…However, one of the difficulties in using nanoparticles as catalyst is providing stability of small nanoparticles while retaining the activity. ILs have been shown to provide ''electrostatic'' stabilization for metal nanoparticles and more surface area for the reaction to take place (32). So ILs intern controls the stability and activity of nanoparticles.…”

“…Multiphase systems of IL-nanoparticles facilitate easy recovery of nanoparticles. There have been several reviews (32,33) about the preparation and application of nanoparticles in ILs, with the most recent having been published in the 2009 (34). The present review is intended to update the reader about the recent work in the field of combination of two cutting-edge topics Á nanoparticles and ILs.…”

Recent advances in the development of magnetically recoverable metal nanoparticle catalysts

“…68 However, metal NPs have low stability against agglomeration since the bulk metal is the thermodynamic minimum, and therefore organic ligands, surfactants, polymers or inorganic coatings are employed to stop the particle growth process, control the size of NPs and keep them stable by steric or electrostatic stabilization. [69][70][71] These ligands, however, may stabilize the metal NPs in the same phase as the reactants, making it difficult to separate the catalysts. Strategies to facilitate NPs separation include decantation by using biphasic systems, such as the biphasic system water/organic solvent [72][73][74] or two-phase system with ionic liquids, [75][76][77] and filtration or centrifugation by the immobilization of NPs on organic or inorganic supports.…”

An easily recoverable and recyclable homogeneous polyester-based Pd catalytic system for the hydrogenation of α,β-unsaturated carbonyl compounds

“…In the realm of Pd-based catalysts the hydrogenation reaction has been carried out in the presence of numerous systems [7]. Interesting results have been achieved with polymer-bound palladium complexes in heterogeneous reactions [8][9][10]. Recently, an innovative approach to recycle a catalytic system has been proposed.…”