Preparation of Rhodium Nanoparticles in Carbon Dioxide Induced Ionic Liquids and their Application to Selective Hydrogenation

Abstract

There is currently great interest in the generation of metal nanoparticles of controlled size and shape because of their unique properties at the interface between molecular structures and bulk materials. [1,2] Ionic liquids (ILs) have emerged as promising media for the synthesis, stabilization, and utilization of metal nanoparticles for various applications, including catalysis. [3,4] The stabilizing effect of ammonium salts on metal nanoparticles is well-established in conventional solvents [1,2,5] and is of course not restricted to materials with melting points that fall within the definition of ILs (that is, organic salts with melting points, T m , below 100 8C [6] ). Most recently, it has been demonstrated that common organic salts can experience very significant melting point depression in the presence of compressed CO 2 with DT m values around or above 100 8C in certain cases. [7, 8] Herein, we report a method for the generation and entrapment of rhodium nanoparticles in simple solid ammonium salts by exploiting their CO 2 -induced melting to form ionic liquids. The utilization of the resulting materials as selective catalysts for hydrogenation reactions is exemplified, whereby a different catalytic behavior from conventional homogeneous or heterogeneous catalysts was noted for sterically encumbered aromatic olefins as substrates.The process for the generation of the matrix-embedded nanoparticles is depicted in Figure 1. A mixture of the solid ammonium salt [R 4 N]Br and the organometallic complex [Rh(acac)(CO) 2 ] (acac = acetylacetonate) as precursor (Rh loading ca. 1 % by weight) is placed into a window-equipped autoclave. This mixture is then pressurized with CO 2 and H 2 and heated to 40-80 8C for the reduction step. Although most simple ammonium salts have regular melting points around or well above 100 8C, they form liquid phases under the reaction conditions because of the presence of the compressed CO 2 phase. [7, 8] This ensures the dissolution of the molecular precursor and a homogeneous dispersion of the resulting particles. Furthermore, the presence of CO 2 is known to enhance the availability of hydrogen in ionic-liquid phases, which may further facilitate the reduction process.[9] Upon venting the CO 2 /H 2 mixture at the end of the reaction, the nanoparticles are trapped in the solidifying matrix that is left in the reactor.Organic by-products are inevitably formed from the ligands during the preparation of metal nanoparticles by hydrogenation of organometallic precursor complexes. Such side products can be readily removed from the resulting material by extraction with supercritical CO 2 (scCO 2 ). This was demonstrated in the present method by passing a scCO 2 stream through the reactor after reduction and venting through an indicator solution of Fe 2 (SO 4 ) 3 (1 % in H 2 SO 4 (0.1n)). The immediate appearance of the characteristic red color of [Fe(acac) 3 ] proved the presence of acetylacetone in the CO 2 flow.With this simple procedure, well-defined rhodium nanoparticles were obta...

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High-Yield Synthesis of 1D Rh Nanostructures from Surfactant Mediated Reductive Pathway and their Shape Transformation

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