Microwave Irradiation for the Facile Synthesis of Transition‐Metal Nanoparticles (NPs) in Ionic Liquids (ILs) from Metal–Carbonyl Precursors and Ru‐, Rh‐, and Ir‐NP/IL Dispersions as Biphasic Liquid–Liquid Hydrogenation Nanocatalysts for Cyclohexene

Abstract

Stable chromium, molybdenum, tungsten, manganese, rhenium, ruthenium, osmium, cobalt, rhodium, and iridium metal nanoparticles (M-NPs) have been reproducibly obtained by facile, rapid (3 min), and energy-saving 10 W microwave irradiation (MWI) under an argon atmosphere from their metal-carbonyl precursors [M(x)(CO)(y)] in the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF(4)]). This MWI synthesis is compared to UV-photolytic (1000 W, 15 min) or conventional thermal decomposition (180-250 degrees C, 6-12 h) of [M(x)(CO)(y)] in ILs. The MWI-obtained nanoparticles have a very small (<5 nm) and uniform size and are prepared without any additional stabilizers or capping molecules as long-term stable M-NP/IL dispersions (characterization by transmission electron microscopy (TEM), transmission electron diffraction (TED), and dynamic light scattering (DLS)). The ruthenium, rhodium, or iridium nanoparticle/IL dispersions are highly active and easily recyclable catalysts for the biphasic liquid-liquid hydrogenation of cyclohexene to cyclohexane with activities of up to 522 (mol product) (mol Ru)(-1) h(-1) and 884 (mol product) (mol Rh)(-1) h(-1) and give almost quantitative conversion within 2 h at 10 bar H(2) and 90 degrees C. Catalyst poisoning experiments with CS(2) (0.05 equiv per Ru) suggest a heterogeneous surface catalysis of Ru-NPs.

Ionic Liquids for the Synthesis and Stabilization of Metal Nanoparticles

“…An elegant route is also the thermal, photolytic or chemical decomposition of compounds with zero-valent metal atoms, such as metal carbonyls M x (CO) y [23,85,95,96] or [Ru(COD)(COT)] (see section Metal nanoparticles from zero-valent metal precursors) [97,98]. Common to the synthesis of M-NPs in ILs is that no extra stabilizing molecules or organic solvents are needed [19,25,27,61,99] …”

“…This MWI synthesis was compared to UV-photolytic (1000 W, 15 min) or conventional thermal decomposition (180 -250 • C, 6 -12 h) of M x (CO) y in ILs. The MWI-obtained nanoparticles have a very small (< 5 nm) and uniform size and are prepared without any additional stabilizers or capping molecules as long-term stable M-NP/IL dispersions [30,85].…”

“…M-NP/IL systems were shown to be quite easily recyclable and reusable several times without any significant changes in catalytic activity [25]. In hydrogenation reactions with Rh-or Ru-NP/IL systems the catalytic activity did not decrease upon repeated reuse [85,96]. A sizable number of catalytic reactions have successfully been carried out in ILs [10,12,221].…”

Ruthenium Nanoparticles in Ionic Liquids – A Saga

“…Ionic liquids containing large ions presenting high polarisability are particularly susceptible to MW radiation and reach high heating rates. Indeed, at a power of only 10 W in a laboratory microwave the suspension reached the 250 °C required in less than 1 minute, presenting a clear advantage over the conventional heating route, and leading to Ru-NPs of an almost identical size and size distribution [81].…”

“…Some examples for catalyst-recycling in batch reactions show the promising potential for their recyclability without significant loss of catalyst activity ( Table 2: Entry A) [55][56][57]81]. It is impor-tant to note that the stability of NPs is superior in ILs in comparison to NPs under solventless conditions.…”