The Partial Hydrogenation of Benzene to Cyclohexene by Nanoscale Ruthenium Catalysts in Imidazolium Ionic Liquids

Abstract

The controlled decomposition of an Ru⁰ organometallic precursor dispersed in 1‐n‐butyl‐3‐methylimidazolium hexafluorophosphate (BMI⋅PF₆), tetrafluoroborate (BMI⋅BF₄) or trifluoromethane sulfonate (BMI⋅CF₃SO₃) ionic liquids with H₂ represents a simple and efficient method for the generation of Ru⁰ nanoparticles. TEM analysis of these nanoparticles shows the formation of superstructures with diameters of ≈57 nm that contain dispersed Ru⁰ 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, 75 °C). The ternary diagram (benzene/cyclohexene/BMI⋅PF₆) indicated a maximum of 1 % cyclohexene concentration in BMI⋅PF₆, 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 BMI⋅PF₆. 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.