Due to size effects as well as structure/function relationships, metal nanoparticles (NPs) account for such excellent catalytic activity in so many reactions that the nanoparticle catalysts are attracting tremendous interest. 1 However, such NPs having very active surface atoms, can often lead to kinetic liability with respect to inter-particle aggregating to their bulk forms, which undoubtedly are apt to decrease catalytic activity. This problem has been approached by a variety of methods,

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... , the addition of additive stabilizers or supporting the NP catalysts on solid materials. 1d~1f As quasi-homogeneous catalysts, additive-stabilized NP catalysts show high catalytic activity, but difficulty separating them for reuse limits their practical applications. In contrast, solid-supported NP catalysts allow easy recycling as they work heterogeneously, but they often display significant loss of catalytic activity. Ionic liquids (ILs) are attracting considerable attention as an alternative medium for catalysis as well as for the formation of nanomaterials. 2 Since the first report by Dupont et al. on the formation of NP catalysts in ILs, 3 many studies have suggested that imidazolium-based ILs stabilize NP catalysts electrostatically and/or by coordination involving the imidazolium cations. 4 Indeed such imidazolium-based ILs are believed to be effective stabilizers for NP catalysts, but the inevitable self-aggregation has been determined in some cases to result in loss of catalytic activity. One promising solution is immobilization of NP catalysts on high-surface-area nanosupports, which not only stabilize the NP catalysts, but also make easy the recovery and reuse of the NP catalysts. Recently, Kou et al. have reported that NP catalysts supported on vinyl-imidazolium salt/styrene copolymers show increased stability in ILs. 5 Quite recently, we also found that palladium NP catalysts supported on imidazolium-functionalized ionic carbon nanotubes show an extraordinary stability with high catalytic activity for hydrogenations of olefins in an isopropanol (IPA)/IL biphasic solvent system. 6 Since it has been known that imidazolium-based ionic liquids have head-totail arrangements, 7 it would be reasonable to assume that the high stability of these supported NP catalysts in ionic liquid may largely be related with the discriminative formation of micelle-like frameworks via an alternative head-to-tail ar-rangement of ionic liquid over the imidazolium-functionalized nanosupport materials, which could prevent self-aggregation of NP catalysts. Based on these considerations, we reasoned that the palladium NPs supported on the ionophilic spherical magnetite coated with imidazolium-functionalized silica (hereafter denoted as Pd@Fe 3 O 4 /SiO 2 /IL) would invoke secondary interactions with ILs to form micelle-like spherical ionic multilayers. This approach allowed stable catalyst dispersion without aggregation of the supported palladium NP catalysts in a medium of IL. Moreover, we also hypothesized that the micelle-like spherical ionic multilayer would form molecular gates for the substrate's diffusion, making the reaction rate largely dependent on the effective size of the applied substrates. 8 Utilizing a magnetite core in the supported catalysts achieves the additional advantages of highly efficient recovery and reuse. In the present paper, we report the synthesis of the recoverable ionophilic Pd@Fe 3 O 4 /SiO 2 /ILs, showing substratesize-dependant kinetics for the hydrogenations of trans-stilbene and styrene in ILs. Scheme 1 and Figure 1 show preparation and characterization data of palladium nanocatalysts supported on magnetite coated with imidazolium-functionalized silica † This paper is dedicated to Professor Eun Lee on the occasion of his honourable retirement. Scheme 1. Preparation of palladium nanocatalysts supported on magnetic nanoparticles coated with imidazolium salt-functionalized silica.