Supported ruthenium catalysed selective hydrogenation of citral in presence of [NTf2]− based ionic liquids

The influence of ionic liquids (ILs) based on the bis(trifluoromethanesulfonyl)amide anion ([BMIM][NTf2] and [HMIM][NTf2]) 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 H2, 0.3 mol L−1 citral), additives like [NTf2]− based ionic liquids change the intramolecular selectivity (CO vs. CC hydrogenation) in a way that the desired unsaturated alcohols geraniol and nerol are the main products (Smax = 46% at X = 50% compared to only 25% for neat Ru/Al2O3). 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][NTf2] and [HMIM][NTf2] the site-time yields towards citronellal were more decreased than for geraniol and nerol enhancing the selectivity of the latter ones.

Ionic liquids ([BMIM][NTf2], [HMIM][NTf2]) were used as reaction modifiers in the supported ruthenium catalysed liquid-phase hydrogenation of citral enhancing the selectivity towards the preferred unsaturated alcohols geraniol and nerol. The IL inhibits primarily the CC hydrogenation and the decrease of the site-time-yield towards citronellal is more pronounced than for the unsaturated alcohols.Figure optionsDownload as PowerPoint slide