Studies On The Ionic Liquid-Stabilized Metal Nanoparticles For Hydrogenation

In this work, two kinds of functionalized ionic liquids-[2,3-dimethyl-1-(3-N, N-bis(2-pyridyl)-propylamido) imidazolium hexafluorophosphate ([BMMDPA]PF6) and 1-(3-aminopropyl)-2,3-dimethyl-imidazolium salts [AMMIM]X (X=Br-, OAc-)] were first synthesized and utilized for stabilizing palladium and nickel nanoparticles. The nanoparticles have been applied to selective hydrogenation in ionic liquids and aqueous phase, respectively. The relationships of the preparation conditions, physico-chemical characteristics (especially the particle size, surface electronic character) with the performance of the catalyst were discussed and analyzed by UV-vis, XRD, TG, TEM, XPS methods.(1) Suitable preparation and mild reaction conditions were favorable to enhancing the activity and selectivity of the catalyst.As stabilizers, the functionalized ionic liquids could be specially designed to increase the nanocatalysts solubility in reaction media (avoiding the mass transfer limitations), to control the nanoparticles size and enhance the stability of nanocatalysts from being aggregated. They could also alter the surface electronic characteristics of the nanocatalysts, and thus leading to the higher activity and selectivity. In the control experiments, it was found that the excess stabilizer would block and deactivate the active centre of the catalysts and seriously suppressed the activity of the hydrogenation. However, the purification steps seemed to be an effective way to remove the excess ligands adsorbed on the surface of the catalyst and the activity could be recovered. Besides, suitable pre-reduction conditions were critical to obtaining small, well-dispersed and stabilized nanoparticles and thus kept their high activity and selectivity.(2) XRD analysis had confirmed that both of the Pd(0) and Ni(0) crystallites were indexed as face-centered cubic (fcc) structure. UV-vis, TG, ICP-AES, TEM and XPS analyses showed that even if the nanocatalysts had been underwent the purification steps, there was still ca.24 wt.% of ionic liquid remaining on their surface.(3) The different anions in functionalized ionic liquids-[AMMIM]X utilized for stabilizing Ni(0) nanocatalysts significantly affected their activity towards selective hydrogenation of C=C double bonds of citral:BF4- and [N(CN)2]- were easy to hydrolyze under reaction conditions, which would induce the highly active Ni(0) nanocatalysts to be oxidized into unreactive Ni(II) species; the anions containing more -OH or -COOH groups with strong coordination ability would lead the precursor hard to be reduced to active metal nanocatalysts; Ni(0) nanocatalysts stabilized by [AMMIM]X (X=OAc-,NH2CH2CH2COO-and CH3CH(OH)COO-) had showed comparable performance under optimized reaction conditions. The activity difference of OAc- and Br- anions was unexpected:although the size of Ni(0) nanoparticles stabilized by [AMMIM]OAc was much bigger, they owned higher activity and stability. This could be ascribed to stronger hydrophility of Br- anion and thus some of the active centers were blocked by Br- and became inaccessible to substrates.(4) The strategy in this work was to utilize biphasic catalytic system (organic/ionic liquid, organic/aqueous phase), which was convenient for the recycling of the catalysts. The present catalytic systems had showed higher activity and selectivity towards many substrates than commercial catalysts.