Catalytically Active Membranelike Devices: Ionic Liquid\nHybrid Organosilicas Decorated with Palladium Nanoparticles

Abstract

Ionic liquid (IL) hybrid organosilicas\nbased on 1-<i>n</i>-butyl-3-(3-trimethoxysilylpropyl)­imidazolium\ncations associated\nwith hydrophilic and hydrophobic anions decorated with well-dispersed\nand similar-sized (1.8–2.1 nm) Pd nanoparticles (Pd-NPs) are\namong the most active and selective catalysts for the partial hydrogenation\nof conjugated dienes to monoenes. The location of the sputter-imprinted\nPd-NPs on different supports, as determined by RBS and HS-LEIS analysis,\nis modulated by the strength of the contact ion pair formed between\nthe imidazolium cation and the anion, rather than the IL hybrid organosilica\npore size and surface area. In contrast, the pore diameter and surface\narea of the hybrid supports display a direct correlation with the\nanion hydrophobicity. XPS analysis showed that the Pd(0) surface component\ndecreases with increasing ionic bond strength between the imidazolium\ncation and the anions (contact ion pair). The finding is corroborated\nby changes in the coordination number associated with the Pd–Pd\nscattering in EXAFS measurements. Hence, the interaction of the IL\nwith the metal surface is found to occur via IL contact pairs (or\naggregates). The observed selectivities of ≥99% to monoenes\nat full diene conversion indicate that the selectivity is intrinsic\nto the electron-deficient Pd metallic surfaces in this “restricted”\nionic environment. This suggests that IL hybrid organosilica/Pd-NPs\nunder multiphase conditions (“dynamic asymmetric mixture”)\noperate akin to <i>catalytically active membranes</i>: i.e.,\nfar from the thermodynamic equilibrium. Detailed kinetic investigations\nshow that the reaction rate is zero order with respect to hydrogen\nand is dependent on the fraction of catalyst surfaces covered by either\nthe substrate and/or the product. The reaction proceeds via rapid\ninclusion and sorption of the diene to the IL/Pd metal surface saturated\nwith H species. This is followed by reversible hydride migration to\ngenerate a π-allyl intermediate. The reductive elimination of\nthis intermediate, the formal rate-determining step (RDS), generates\nthe alkene that is rapidly expelled from the IL phase to the organic\nphase.