N-heterocyclic carbenes (NHCs) are widely applied ligands to control the reactivity and selectivity of homogeneous transition metal complex catalysts. They are also useful ligands to stabilize nanoparticle catalysts; however, the application of NHCs to modify supported heterogeneous catalysts is very rare. We, for the first time, directly modified the surface of Ru nanoclusters supported on potassium-doped aluminum oxide (Ru/K-Al2O3) with NHCs, and investigated the surface structure (especially the binding mode of NHCs) and the effect of NHC-modification on the activity of the catalyst.[1] The direct impregnation of free NHCs (dicyclohexylimidazolyldiene (1) and dimesitylimidazolylidene (2)) on Ru/K-Al2O3 yielded NHC-modified Ru/K- Al2O3 catalysts (1@Ru/K-Al2O3 and 2@Ru/K-Al2O3). The existence of NHC ligands on Ru/K- Al2O3 was confirmed by FT-IR, N 1s XPS, and TGA analyses. The existence of the metal-carbene bond, the direct evidence that NHCs were coordinated to the surface of Ru nanoclusters, was characterized by the chemical shift of the carbene carbon atom in 13C SS-NMR. For instance, a distinct peak was observed at 164 ppm on 1@Ru/K-Al2O3. This chemical shift was different from free 1 (212 ppm, in solution) and 1 impregnated on only K-Al2O3 without Ru nanoclusters (168 ppm; low intensity), suggesting that 1 prefers to coordinate to Ru nanoclusters on the surface. Chemoselective hydrogenation of phenyl acetylene that contains two different functional groups was performed by the NHC-functionalized Ru/K-Al2O3 catalysts. When the NHC loading was low, the complete hydrogenation of the triple bond and the benzene moiety was observed. When the NHC loading was increased, a dramatic increase in chemoselectivity was observed in that the hydrogenation of the triple bond occurred exclusively. The different behavior in chemoselectivity on two different NHC ligands (1 and 2) will be also discussed.

N-heterocyclic carbene - functionalized Ru/K-Al2O3 catalysts: Preparation, surface characterization, and tunable selective hydrogenation performances

253rd National Meeting (2017)

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