Olefin hydrogenation by ruthenium nanoparticles in ionic liquid media: Does size matter?

Tailor-made and size-controlled ruthenium nanoparticles, RuNPs, of three distinct sizes between 1 and 3 nm are generated from the decomposition of (η4-1,5-cyclooctadiene)(η6-1,3,5-cyclooctatriene)ruthenium(0) [Ru(COD)(COT)], under H2 in 1-butyl-3-methylimidazolium bis(trifluoromethanesulphonyl)imide, C1C4ImNTf2, by simply varying experimental conditions. Catalytic hydrogenation of 1,3-cyclohexadiene, CYD, and cyclohexene, CYE, in C1C4ImNTf2, has been used as a probe for the relationship between size and catalytic performance (activity and selectivity) of RuNPs. To allow comparison between different reactions, all catalytic reaction mixtures were diligently prepared in order that the parameters such as substrate/catalyst and substrate/ionic liquid ratio, and therefore, viscosity and mass transport factors remained constant. It was found that the catalytic activity increases with the NP size, while high selectivity is only observed with the smaller NPs. In addition, the studied RuNPs exhibit a high level of recyclability with neither loss of activity nor significant agglomeration.