Selective hydrogenation of 1,3-butadiene to 1-Butene by Pd(0) nanoparticles embedded in imidazolium ionic liquids

The reduction of Pd(acac)(2) (acac = acetylacetonate), dissolved in 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI center dot PF6) or tetrafluoroborate (BMI-BF4) ionic liquids, by molecular hydrogen (4 atm) at 75 degrees C 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-BF4 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) nanopar-ticles embedded in BMI center dot BF4 under mild reaction conditions (40 degrees C and 4 atm of hydrogen at constant pressure). Selectivities up to 72% in 1-butene were achieved at 99% 1,3-butadiene conversion, 40 degrees C 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.