Supported ruthenium catalysed selective hydrogenation of citral in presence of [NTf 2] − based ionic liquids

Abstract

The influence of ionic liquids (ILs) based on the bis(trifluoromethanesulfonyl)amide anion ([BMIM][NTf 2] and [HMIM][NTf 2]) on the alumina supported ruthenium catalysed liquid-phase hydrogenation of citral was investigated applying these ILs as additive. For characterisation ICP-OES, hydrogen pulse chemisorption and nitrogen physisorption were applied, the latter was used after reaction to identify the presence of an ionic liquid layer on the catalyst. The catalytic results show that under the chosen reaction conditions (373 K, 7 MPa H 2, 0.3 mol L −1 citral), additives like [NTf 2] − based ionic liquids change the intramolecular selectivity (C O vs. C C hydrogenation) in a way that the desired unsaturated alcohols geraniol and nerol are the main products ( S max = 46% at X = 50% compared to only 25% for neat Ru/Al 2O 3). Concerning the catalyst activity a decrease of the initial turnover frequency (0.048 s −1) in comparison to the IL-free catalyst (0.140 s −1) was observed. The results indicate that in presence of [BMIM][NTf 2] and [HMIM][NTf 2] the site-time yields towards citronellal were more decreased than for geraniol and nerol enhancing the selectivity of the latter ones.