Selective Hydrogenation of 1,3-Butadiene to 1-Butene by Pd(0) Nanoparticles Embedded in Imidazolium Ionic Liquids Alexandre P. Umpierre, a Giovanna Machado, a Gerhard H. Fecher, b Jonder Morais, c Jairton Dupont a, * a Laboratory of Molecular Catalysis, Institute of Chemistry, UFRGS, Av. Bento GonÅalves 9500, Porto Alegre, 91501-970, RS, Brazil Fax: ( þ 55)-51-3316-7304, e-mail: dupont@iq.ufrgs.br b Institut für Anorganische und Analytische Chemie, Johannes-Gutenberg Universität, 55099 Mainz, Germany c Institute of Physics, UFRGS, Av. Bento GonÅalves 9500, Porto Alegre, 91501-970, RS, Brazil Received: October 11, 2004; Revised: April 18, 2005; Accepted: April 29, 2005 Abstract: The reduction of Pd(acac) 2 (acac ¼ acetyla- cetonate), dissolved in 1-n-butyl-3-methylimidazoli- um hexafluorophosphate (BMI·PF 6 ) or tetrafluoro- borate (BMI·BF 4 ) ionic liquids, by molecular hydro- gen (4 atm) at 75 8C affords stable, nanoscale Pd(0) particles with sizes of 4.9  0.8 nm. Inasmuch as 1,3- butadiene is at least four times more soluble in the BMI·BF 4 than butenes, the selective partial hydroge- nation could be performed by Pd(0) nanoparticles embedded in the ionic liquid. Thus, the isolated nano- particles promote the hydrogenation of 1,3-butadiene to butenes under solventless or multiphase conditions. Selectivities up to 97% in butenes were observed in the hydrogenation of 1,3-butadiene by Pd(0) nanopar- ticles embedded in BMI·BF 4 under mild reaction conditions (40 8C and 4 atm of hydrogen at constant pressure). Selectivities up to 72% in 1-butene were achieved at 99% 1,3-butadiene conversion, 40 8C and 4 atm of constant pressure of hydrogen. The amounts of butane (fully hydrogenated 1,3-buta- diene) and cis-2-butene products are marginal and the butenes do not undergo isomerisation process, in- dicating that the soluble Pd(0) nanoparticles possess a pronounced surface-like rather than homogeneous- like catalytic properties. Keywords: dienes; hydrogenation; ionic liquids; nano- particles; palladium Introduction The production of polymer-grade 1-butene through the selective hydrogenation of 1,3-butadiene is an impor- tantindustrialprocesssinceeventracesof1,3-butadiene can poison the olefin polymerization catalysis and con- sequently reduce the total polymer production. The hy- drogenationprocessforthegenerationofrichstreamsin 1-butene can be accomplished by a plethora of palladi- um catalysts. Supported palladium catalysts such as Pd/SiO 2 and Pd/alumina, usually in the presence of pro- moters or modifiers, [1] are among the most used and in- vestigated catalysts for the partial hydrogenation of 1,3- butadiene. In this respect, the catalytic activity and se- lectivity can be strongly influenced by the metal disper- sion, the nature of the support and the preparation method.Inparticular,inthesecatalyticprocessesthese- lectivity in 1-butene is reduced due to the parallel iso- merisation of 1-butene to internal butenes and total di- enehydrogenation,whichlimittheoverallyieldin1-bu- tene.Themostacceptedmechanisminvolvesadsorption of the butadiene through di-p-coordination. The 1,2- or 1,4-H addition on the diene produces 1-butene or 2-bu- tenes,respectively.The trans/cis 2-butenes ratios are the direct result of the interconversion between the diene conformer-adsorbed species on the Pd surface. The trans-2-butene is formed preferentially since the S-trans conformerismuchmorestablethanthe S-cis conformer. Moreover, the isomerisation of the monoenes formed is suppressed in the presence of an excess of the diene . [2] An alternative mechanism, involving the formation of carbenes, has been proposed in cases where the ob- served ratios of trans/cis-2-butenes are comparable and n-butaneisformedasdirectproductofthedienehy- drogenation. [3] Transitionmetalcomplexesbasedonpalladiumorco- balt,eitherinhomogenousormultiphaseconditions,are also known to catalyse the partial hydrogenation of 1,3- butadienewithhighselectivityinbutenes,butonlymod- erateyieldsin1-butenewereachieved. [4] Forthesemon- ometallic catalysts the proposed mechanism involves also the formation of a stable p-allyl intermediate (re- gardless of the H 2 cleavage mechanism) through the M  H addition to a conjugated diene. The isomerisation ofthe1-buteneformedintheinternalbutenesischarac- teristicoftheinvolvementof“homogeneous”M  Hspe- FULL PAPERS 1404 # 2005 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/adsc.200404313 Adv. Synth. Catal. 2005, 347, 1404–1412