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Issue 5, 2009
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Regioselective catalytic hydrogenation of citral with ionic liquids as reaction modifiers

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Abstract

Silica and polyaniline supported palladium catalysts prepared by different techniques (incipient-wetness impregnation, deposition precipitation) using Pd(OAc)2 or H2PdCl4 as precursors were studied in the liquid-phase hydrogenation of citral under addition of several ionic liquids ([BMIM][NTf2], [BMIM][PF6], [BMPL][NTf2], [BMIM][DCA], [BMPL][DCA], [B3MPYR][DCA]) either as catalyst coating or as additive. By an incipient-wetness technique, the catalysts were coated with a mixture of IL in acetone. The catalysts were characterized by nitrogen physisorption, whereby a decrease of surface area and pore volume was detected by the IL coated catalysts. Furthermore, ICP-OES, TEM and IR spectroscopy were performed to analyze metal content, particle size and coverage of the catalyst with ionic liquid. Citral hydrogenation was performed at 323 K and under 2.0 MPa H2 in an autoclave with off-line GC analysis of the product mixtures. Beside stirrer speed, catalyst mass and citral concentration, the type and quantity of ionic liquid were also varied to elucidate their influence on activity and selectivity of the Pd/SiO2 catalysed citral hydrogenation. The results show that treatment of the catalyst with ionic liquids—independent of catalyst coating or additive—leads to a selectivity enhancement of the desired product, citronellal. With [PF6] or [NTf2] as the IL anion, maximum selectivities were (60 ± 2)% at 70% conversion. In particular, dicyanamide (DCA) containing ionic liquids allow, under optimised conditions, the quantitative one-pot synthesis of citronellal, at least if the Pd/SiO2 catalyst was coated with 29 wt% [B3MPYR][DCA]. By using polyaniline supported Pd catalysts and [BMIM][DCA] as additive, the consecutive hydrogenation towards dihydrocitronellal was less pronounced and the influence of metal precursor, support material and preparation technique of the catalyst could be excluded. Hydrogenation of pure citral on [BMIM][DCA] coated palladium catalysts offers a solvent-free, green route to citronellal in reasonable selectivities (S = 86%).

Graphical abstract: Regioselective catalytic hydrogenation of citral with ionic liquids as reaction modifiers

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Publication details

The article was received on 24 Dec 2008, accepted on 05 Feb 2009 and first published on 27 Feb 2009


Article type: Paper
DOI: 10.1039/B822992A
Citation: Green Chem., 2009,11, 716-723
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    Regioselective catalytic hydrogenation of citral with ionic liquids as reaction modifiers

    J. Arras, M. Steffan, Y. Shayeghi, D. Ruppert and P. Claus, Green Chem., 2009, 11, 716
    DOI: 10.1039/B822992A

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