Selective Hydrogenation of 1,3-Butadiene to 1-Butene by Pd(0) Nanoparticles Embedded in Imidazolium Ionic Liquids

Abstract

The reduction of Pd(acac) 2 (acac ¼ acetylacetonate), dissolved in 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI · PF 6 ) or tetrafluoroborate (BMI · BF 4 ) ionic liquids, by molecular hydrogen (4 atm) at 75 8C affords stable, nanoscale Pd(0) particles with sizes of 4.9 AE 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 hydrogenation could be performed by Pd(0) nanoparticles embedded in the ionic liquid. Thus, the isolated nanoparticles 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) nanoparticles 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-butadiene) and cis-2-butene products are marginal and the butenes do not undergo isomerisation process, indicating that the soluble Pd(0) nanoparticles possess a pronounced surface-like rather than homogeneouslike catalytic properties.

Palladium nanoparticle catalysts in ionic liquids: synthesis, characterisation and selective partial hydrogenation of alkynes to Z-alkenes

“…This result is in agreement with those reported for the hydrogenation of 1,3-butadiene catalysed by metal Pd-NPs in IL. 33 In that case, the difference in solubility between the substrate (1,3-butadiene) and the major product 1-butene plays the role for the observed selectivity (up to 72% in 1-butene). Moreover, no isomerisation process was detected at the different stages of the diene consumption, indicating that the NPs dispersed in IL act as heterogeneous catalysts.…”

“…On the other hand, efficient synthesis of Pd-NPs (diameter: 4.9 nm) from the reduction of Pd(acac) 2 (acac ¼ acetylacetonate) by molecular hydrogen using non-functionalised ILs as unique stabilisers was demonstrated earlier. 33 However, these nanoparticles are not stable and the use of phenanthroline as additional capping agent is essential for the NPs stabilisation. 34 The use of external capping agents, or functionalised ILs such as 2,2 0 -dipyridylamine-functionalised imidazolium salts, 35 polymers 36 or ionic carbon nanotubes 37 have been used for the stabilisation and dispersion of Pd-NPs in ILs.…”

Catalytic Applications of Metal Nanoparticles in Imidazolium Ionic Liquids

“…For example, Pd 0 nanoparticles in "classical" ILs such as BMI·PF 6 tend to agglomerate after the hydrogenation of alkenes. [26] However, phenanthrolineprotected palladium nanoparticles in this IL are very active and selective for the hydrogenation of olefins, and the nanoparticles/IL system could be reused several times without a loss of activity. [53] Moreover, elegant investigations by Dyson and co-workers on functionalized ILs [85,86] such as imidazolium or pyridinium salts with the nitrile functional group attached to the alkyl side demonstrated that the ILs could act as both solvent and ligand for metal-catalyzed reactions.…”

“…Indeed, XPS spectra after sputtering displays mainly the M À M component at the 4f region, demonstrating that only the external surface metal atoms are bound to F and O. [24,26,30,33,35] Inasmuch as most of the 1,3-dialkylimidazolium ionic liquids have extremely low vapor pressure [36][37][38] and relatively high viscosity at room temperature, [39][40][41][42] in situ TEM [23,43] and XPS [44] -which require high vacuum-could be performed with the MNPs dispersed in ILs. [37] The size and shape determined by this TEM in situ technique are the same as that obtained with the isolated nanoparticles which were mixed with an epoxy resin distributed between two silicon wafer pieces.…”

Palladium Nanoscale Catalysts in Ionic Liquids: Coupling and Hydrogenation Reactions

“…These include typical homogeneous as well as heterogeneous catalytic reactions such as hydrogenation of multiple bonds, Huang et al, 2003;Scheeren et al, 2003;Kim et al, 2004;Silveira et al, 2004;Umpierre et al, 2005;Migowski & Dupont, 2007;Rossi et al, 2007;Scholten et al, 2007;Hu et al, 2008;Prechtl et al, 2008;Gelesky et al, 2009;Hu et al, 2009;Prechtl et al, 2009;Redel et al, 2009;Rossi & Machado, 2009; and carbon-carbon cross-coupling reactions. Pd-based carbon-carbon cross-coupling reactions were discovered more than three decades ago, and remain under the most investigated transition-metal catalysed reactions.…”