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

Immobilized nickel catalysts SBA*-L-x/Ni (L=bis(2-pyridylmethyl)(1H-1,2,3-triazol-4-ylmethyl)amine) with various ligand densities (L content (x)=0.5, 1, 2, 4mol% Si) have been prepared from azidopropyl-functionalized mesoporous silicas SBA-N3-x. Related homogeneous ligand and its NiII complexes, [Ni()(OAc)2(H2O)] (/Ni) and [Ni()2]BF4 (2/Ni), have been synthesized. The L/Ni ratio (0.91.7:1) in SBA*-L-x/Ni suggests the formation of an inert [NiL2] 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 /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.