The catalytic function of SiO2-immobilized Mn(II)-complexes for alkene epoxidation with H2O2

Two symmetrical acetylacetone-based Schiff bases were immobilized on a silica surface by grafting and sol-gel procedure. The corresponding supported manganese complexes were prepared and evaluated as heterogeneous catalysts for alkene epoxidation with H2O2. These heterogeneous catalysts show remarkable effectiveness and selectivity towards epoxide formation in the presence of ammonium acetate. Moreover, the developed heterogeneous catalysts preserve the coordination and catalytic properties of the active-homogeneous manganese catalysts for alkene epoxidation vs. the competitive H2O2 dismutation. EPR spectroscopy shows that in heterogeneous manganese catalysts the Mn2+ centers are in a flexible, non-tight, coordination environment, as in the corresponding homogeneous manganese catalysts. However, after a first use of the heterogeneous catalysts, the Mn centers are detached from the ligand and are randomly dispersed on the SiO2 Surface. This is responsible for the loss of catalytic activity. (C) 2009 Elsevier B.V. All rights reserved.