Selective hydrogenation of cyclohexenone on iron–ruthenium nano-particles suspended in ionic liquids and CO2-expanded ionic liquids

Abstract

Iron and ruthenium mono- and bimetallic nanoparticles (NPs) with different metal ratios (Fe–Ru 9 : 1, Fe–Ru 3 : 1, and Fe–Ru 1 : 1) have been prepared in imidazolium ionic liquids (ILs) via decarbonylation of the corresponding metal carbonyl precursors. The as-prepared ionic liquid suspended nanoparticles were tested as catalysts for the selective hydrogenation of cyclohexenone to cyclohexanone at 50 °C in a high pressure batch reactor equipped for in situ Attenuated Total Reflection (ATR) infrared spectroscopy. After reaction, supercritical carbon dioxide was applied to extract the products from the reaction mixture, thus facilitating a green approach to solventless transition metal catalysis. The course of the reaction and extraction was followed by ATR-IR spectroscopy. From the IL suspended monometallic NPs, Ru showed much higher activity than Fe. Addition of 50 at% iron to Ru (Fe–Ru 1 : 1) resulted in a remarkable decrease of the average particle size of the NPs from 2.9 nm to 1.6 nm. Among the different NPs prepared, the bimetallic Fe–Ru 1 : 1 was most active followed by the pure Ru. Addition of CO₂ to the reaction mixture greatly increased the reaction rate (4 times faster with 60 bar of CO₂ than without), as demonstrated for the highly active Fe–Ru 1 : 1 NPs suspended in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF₄]). At 60 bar of CO₂ high selectivity >95% to cyclohexanone, at a TOF of ca. 300 h⁻¹, was achieved and the catalyst could be reused 5 times without noticeable deactivation.