Alkane Oxidation by an Immobilized Nickel Complex Catalyst: Structural and Reactivity Differences Induced by Surface-Ligand Density on Mesoporous Silica

摘要

Immobilized nickel catalysts SBA*-L-x/Ni (L=bis(2-pyridylmethyl) (1H-1,2,3-triazol-4-ylmethyl)a-mine) with various ligand densities (L content (x) = 0.5, 1, 2, 4 mol% Si) have been prepared from azidopropyl-functionalized mesoporous silicas SBA-N_3-x. Related homogeneous ligand L~(tBu) and its Ni~(II) complexes, [Ni(L~(tBu))(OAc)_2 (H_2O)] (L~(tBu)/Ni) and [Ni(L~(tBu))_2]BF_4 (2L~(tBu)/Ni), have been synthesized. The L/Ni ratio (0.9-1.7:1) in SBA~*-L-x/Ni suggests the formation of an inert [NiL_2] site on the surface at higher ligand loadings. SBA*-L-x/Ni has been applied to the catalytic oxidation of cyclohexane with m-chloroperbenzoic acid (mCPBA). The catalyst with the lowest loading shows high activity in its initial use as the homogeneous L~(tBu)/Ni catalyst, with some metal leaching. As the ligand loading increases, the activity and Ni leaching are suppressed. The importance of site-density control for the development of immobilized catalysts has been demonstrated.