The partial hydrogenation of benzene to cyclohexene by nanoscale ruthenium catalysts in imidazolium ionic liquids

The controlled decomposition of an Ru-0 organometallic precursor dispersed in 1-n-butyl-3-methylimidazolium hexafluorophosphate ((BMIPF6)-P-.), tetrafluoroborate ((BMIBF4)-B-.) or trifluoromethane sulfonate ((BMICF3SO3)-C-.) ionic liquids with H-2 represents a simple and efficient method for the generation of Ru-0 nanoparticles. TEM analysis of these nanoparticles shows the formation of superstructures with diameters of approximate to57 nm that contain dispersed Ru-0 nanoparticles with diameters of 2.6 +/- 0.4 nm. These nanoparticles dispersed in the ionic liquids are efficient multiphase catalysts for the hydrogenation of alkenes and benzene under mild reaction conditions (4 atm, 75degreesC). The ternary diagram (benzene/cyclohexene/(BMIPF6)-P-.) indicated a maximum of 1% cyclohexene concentration in (BMIPF6)-P-., which is attained with 4% benzene in the ionic phase. This solubility difference in the ionic liquid can be used for the extraction of cyclohexene during benzene hydrogenation by Ru catalysts suspended in (BMIPF6)-P-.. Selectivities of up to 39% in cyclohexene can be attained at very low benzene conversion. Although the maximum yield of 2% in cyclohexene is too low for technical applications, it represents a rare example of partial hydrogenation of benzene by soluble transition-metal nanoparticles.